A review of investigations on liquid fuel combustion in porous inert media

lable at ScienceDirect

Progress in Energy and Combustion Science 35 (2009) 216–230

Contents lists avai

Progress in Energy and Combustion Science

journal homepage: www.elsevier .com/locate/pecs

A review of investigations on liquid fuel combustion in porous inert media

M. Abdul Mujeebu a,*, M.Z. Abdullah a, M.Z. Abu Bakar b, A.A. Mohamad c, M.K. Abdullah a

a Porous Media Combustion Laboratory, School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysiab School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysiac Department of Mechanical and Manufacturing Engineering, CEERE, The University of Calgary, Calgary, Alberta T2N 1N4, Canada

a r t i c l e i n f o

Article history:Received 5 June 2008Accepted 10 November 2008Available online 30 December 2008

Keywords:Porous mediaLiquid fuel combustionNumerical modelingSpray combustionLeidenfrost temperatureOptical thickness

* Corresponding author. Tel.: þ60 1 4305 1476; faxE-mail address: [email protected] (M.A. Mu

0360-1285/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.pecs.2008.11.001

a b s t r a c t

Utilization of a porous medium for combustion of liquid fuels is proved to be a promising approach forfuture applications. The porous medium burner for liquid fuels is more advantageous than theconventional open spray flame burner for several reasons; these include enhanced evaporation of dropletspray owing to regenerative combustion characteristics, low emission of pollutants, high combustionintensity with moderate turn-down ratio and compactness. This article provides a comprehensivepicture of the global scenario of research and developments in combustion of liquid fuels within a porousmedium that enable a researcher to determine the direction of further investigation. Accordingly,a glossary of the important terminology, the modeling approach, advances in numerical and experi-mental works and applications are included. The papers published in standard journals are reviewed andsummarized with relevant comments and suggestions for future work.

� 2008 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2172. Liquid fuel combustion in porous media (LFCPM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2173. Governing parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .218

3.1. Peclet number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2183.2. Equivalence ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2183.3. Turn-down ratio (TDR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2183.4. Optical thickness (OT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2183.5. Boiling, Nukiyama and Leidenfrost temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2193.6. Weber number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2193.7. Stokes number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

4. Numerical investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2194.1. Arrhenius equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2204.2. The governing equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

4.2.1. Energy equation for gas phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2204.2.2. Energy equation for solid phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2214.2.3. Energy equation for liquid phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2214.2.4. Mass conservation equation for liquid phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2214.2.5. Mass conservation equation for fuel vapor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

4.3. Description of liquid droplets motion and their collection on the skeleton of solid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2214.4. The heat transfer coefficient between liquid and solid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2224.5. Simulation results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2224.6. Further advances in modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

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M.A. Mujeebu et al. / Progress in Energy and Combustion Science 35 (2009) 216–230 217

5. Experimental investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2245.1. Application based studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

1. Introduction

Porous media combustion has some unique characteristics. It istotally different from conventional combustion devices; it gives riseto high radiant output, low NOx and CO emissions, high flamespeed and higher power density and modulation. The differencebetween combustion in porous medium (PM) and a conventionalsystem arises because of better and more efficient heat transferfrom burned gases to unburned mixture. In a conventionalcombustion system, convection is the dominating mode of heattransfer (since gases have very low thermal conductivity and areless participating in radiation) from the burned to the unburned. Onthe other hand, in porous media combustion, the conduction andradiation modes of heat transfer are also significant. In addition, theconvective heat transfer is also improved, because of increasedsurface area within the porous matrix. There is a better homoge-nization of temperature across the porous matrix and the presenceof a significant amount of radiation helps to preheat the incomingair–fuel mixture upstream, thereby improving the combustionefficiency. Fig. 1 (a) is the schematic of a two-layered porous burner.In order to have better control over flame stabilization, PM burnersare constructed with two different materials, forming two zones[1]. The first is the preheating zone, made of low porosity and lessconducting material and the second is the combustion zone, madeof highly radiating and conducting material. The pore size is alsolarge in combustion zone. The reason for choosing a material of lowthermal conductivity and small porosity in preheating zone is toavoid combustion and the resulting flashback. Proper selection ofporous material (both for preheat and combustion zone) allows thecombustion start at the interface of the two zones and spread overthe entire volume of the combustion zone.

Porous burner can contribute to complete combustion due toenergy feedback by thermal radiation and conduction throughporous matrix. The heat from exhaust gas is recirculated to preheatthe unburned mixture by a combined mode of heat transfermechanism (conduction and radiation) yielding ‘‘excessenthalpies’’ or ‘‘super adiabatic flame’’ which has a peak tempera-ture much higher than the corresponding adiabatic flame temper-ature of the mixture. This energy feedback leads the stabilizedflame to have higher burning speed and a wide lean flammabilitylimits than free flame. Fig. 1(b) shows the comparison of enthalpywith and without heat recirculation. Details of the combustionwithin PM and its practical applications are available in literature[1–4]. However, an exclusive survey on liquid fuel combustion inporous media (LFCPM) is still lacking. In this paper, a detailedreview of all documented works in this area is presented.

2. Liquid fuel combustion in porous media (LFCPM)

The combustion of liquid fuel is widely applied in industrialfurnaces. The actual combustion phenomenon for liquid is complexdue to different modes of heat exchange including phase change;for instance, evaporation rate determines the intensity of heattransfer as well as burning. The problem of liquid droplets hasa background based on the theoretical and experimental investi-gation of vaporization and flaming of liquid droplets [5–8]. InRef. [5], different vaporization models are compared for singledroplet and also for spray. The classical droplet vaporization model

was re-examined by Abramzon and Siringano [6] in order todevelop a model for the spray combustion calculation, which tracedthe life histories of many individual droplets. Elperin and Krasovi-tov [7] developed the quasi-steady model of combustion andevaporation of a large and moderate size single droplet in a quies-cent atmosphere.

Although past investigations have dealt with droplet combus-tion of liquid fuel principally in the free space, there are a fewarticles devoted to the interaction of surface of solid and thecombustion of liquid fuel. Boyarshinov et al. [8] reported theexperimental results of ethanol evaporation from a porous surfaceand its combustion in an air flow. They found that variations in theflow velocity in the flow core weakly affected the temperature andconcentration of substances on the reactor wall. The flametemperature and the distribution of mass flows over the walldepended essentially on the flow velocity. Also, heat- and mass-transfer coefficients decreased in combustion.

Michaelides et al. studied particle sublimation or dropletvaporization in a channel with constant wall temperature [9], andalso under turbulent flow [10]. They developed separate models forboth the cases and the effects of parameters such as particle sizes,physical properties, size of the wall temperature step and turbu-lence intensity, on the rate of evaporation of the condensed phasewere analyzed. The temperatures and velocities of the two phasesduring the phase change were also calculated and presented.

The application of porous media to liquid fuel combustion hasbeen introduced only in the recent past. Typical applications of thistechnology are quite new and have had a strong impact on indus-trial and domestic applications. A potential application of a liquidfuel porous ceramic burner is the incineration of liquid hazardouswaste. Liquid hazardous wastes are difficult to incinerate inconventional burners because they are often low energy contentfuels and contain chlorinated species. These wastes can be effi-ciently burned in porous ceramic burners owing to the highervolumetric heat release. Also, the soot formed in conventionalincinerators often acts as condensation sites for hazardous mate-rials, becoming hazardous itself when emitted to the atmosphere.This problem can be eliminated by porous ceramic burners as theliquid waste is vaporized before combustion. The radiant preheat-ing of the fuel–air mixture, which is inherent to its design, mayalso aid the combustion of mixed fuels containing less reactivespecies [2].

Liquid fuel-fired porous combustors (LFFPC) are classified into 1)fuel-vaporizing type and 2) fuel-spraying type. Commonly, spray-ing type is used for industrial and civil applications such as boilersand furnaces. In spraying type, homogeneous combustion in largerange load is little obtained because the flame is relatively large andthe stability of the flame is affected by aerodynamics among the airand the droplets.

Compared with the spraying type, the vaporizing type has theadvantages such as high turn-down ratio, compactness and theeasy proportional control that matches a gaseous fuel-fired boiler.Its combustion becomes close to homogeneous and has smallerflame and less soot emission by mixing the vaporized fuel with air.But it has the disadvantage of consumption of much electric powerfor fuel vaporization. For instance, the power consumption ofkerosene vaporizing type boiler is 800 W at ignition, 470 W at thecombustion on main burner for a 55 kW class boiler. Therefore total

Nomenclature

Cp specific heat at constant pressured diameter of liquid dropletD mean size of element of porous material structureE activation energyh heat transfer coefficientH combustion heatk absorption coefficientL latent heat of evaporationMf evaporation rate of spray in the specific volumeNu Nusselt numberr radius vector of the dropletR radius of spherical obstacleRg universal gas constantStk Stokes numbert timeT temperatureV dimensionless velocity vector of inviscid flowXL thickness of porous medium

xd, yd dimensionless space coordinates of liquid droplet inSection 4.3

ydc distance from the centerline of the flow to outermosttrajectory of droplet colliding with obstacle.

Greek symbolsa collision probabilityad collection efficiency of droplets on a single obstacled Kronecker symbol4 polar angleg volumetric heat transfer coefficient between different

phases3 porosity of porous mediuml thermal conductivitym dynamic viscosityq dimensionless timer densitys optical coordinate (¼kx)sc optical thickness of porous medium (¼kxl)x dimensionless radius vector of the droplet

M.A. Mujeebu et al. / Progress in Energy and Combustion Science 35 (2009) 216–230218

thermal efficiency of the boilers may be decreased due to the muchamount of energy loss during the conversion process from fossilfuel energy to electricity in power generation [11].

3. Governing parameters

While dealing with this topic, knowledge of some parametersthat influence the performance of liquid fuel combustion in porousmedia is desirable. A brief account of some basic parameters ispresented as follows.

3.1. Peclet number

The modified Peclet number Pe is the deciding factor to specifythe combustion region. If the Pe< 65, the flame is unable to prop-agate and quenching occurs. Alternatively, for Pe� 65 flame prop-agates [1]. The modified Peclet number is defined as,

Pe ¼�SLdmCpr

�=k (1)

where SL is the laminar flame speed, dm is the equivalent diameterof the average hollow space of the porous material, Cp is the specificheat of the gas mixture, r is the density of the gas mixture and k isthe thermal conductivity coefficient of the gas mixture. The equa-tion shows that the conditions for the development of the flame areessentially dependent on the equivalent diameter dm of the meanhollow space or on the mean pore diameter of the porous material.The Peclet number is an indirect measure of porosity also.

Fig. 1. (a) Schematic of a two section PM. (b) Comparison

3.2. Equivalence ratio

The equivalence ratio is defined as the ratio of the actual fuel/airratio to the stoichiometric fuel/air ratio. Stoichiometric combustionoccurs when all the oxygen is consumed in the reaction, and thereis no molecular oxygen (O2) in the products. If the equivalence ratiois equal to one, the combustion is stoichiometric. If it is <1, thecombustion is lean with excess air, and if it is >1, the combustion isrich with incomplete combustion.

3.3. Turn-down ratio (TDR)

It is the measure of the capability of a burner to modulatethrough a firing range or it is the measure of flame stability ofa burner and is defined as the ratio of maximum firing rate tominimum firing rate of a burner. Typical values of TDR for industrialheating operations are in the range of 3:1 to 6:1 [12].

3.4. Optical thickness (OT)

Defined as the product of refractive index (n) and thickness (t),i.e.,

OT ¼ nt (2)

Refractive Index (n) of a medium is the ratio of the velocity ofpropagation of an electromagnetic wave in vacuum to its velocity inthe medium [13].

of enthalpy with and without heat recirculation [1].

Fig. 3. The relation between Wein and Weout (Naber and Farrel) [16].


Higher value of n indicates higher optical thickness and a lowerspeed of propagation of UV radiation. Hence an optically thinmedium can permit fast propagation of radiation compared to thethick one.

3.5. Boiling, Nukiyama and Leidenfrost temperatures

A Leidenfrost drop forms when a volatile liquid is brought incontact with a very hot solid. Then a vapor film forms in betweenthe solid and the drop, the drop appears like a liquid pearl. Uponimpinging on a hot surface, the fuel droplets experience differentphysical processes depending on the surface temperature, resultingin wetting or non-wetting without film formation on the surfaceand varied droplet life times. As shown in Fig. 2, point b is theboiling point of the fuel droplets, point c is the so-called Nukiyamapoint and point d is the Leidenfrost point of the fuel droplets. Whenthe surface temperature is lower than the boiling point, the fueldroplets stick on the surface in the shape of a lens and graduallyevaporate. In the region b–c, the lens shaped droplets are destroyedand disappear at the highest rate due to the maximum rate of heattransfer. At temperatures above the Nukiyama point, the fuel filmdisintegrates and some satellite droplets form that bounce from thesurface. At and above the Leidenfrost temperature, due to theimmediate fuel vaporization, the surface is not wetted, and thereexists a thin layer of fuel vapor formed between the surface and thedroplets. Thus, the fuel droplets do not directly adhere to thesurface and a spherical vaporization takes place [14].

3.6. Weber number

The Weber number is the ratio between the inertial force andthe surface tension force, and can be expressed as

We ¼ rv2l=s (3)

where, We¼Weber number (dimensionless), r¼ density of fluid(kg/m3, lb/ft3), v¼ velocity of fluid (m/s, ft/s), l¼ characteristiclength (m, ft) and s¼ surface tension (dyne/cm). Since the Webernumber represents an index of the inertial force to the surfacetension force acting on a fluid element, it can be useful analyzingthin film flows and the formation of droplets and bubbles [15].

Weber number is one of the critical parameters that influencesthe change of the fuel droplets’ movement. According to Naber andFarrell [16], fuel droplets with Weber number less than 80 willreflect from the hot surface, and those with Weber number largerthan 80 will breakup into several smaller droplets that alsorebound from the hot surface after incident droplets impinging onthe hot surface. A fuel droplet with Weber number less than 80rebounds from a hot surface after impinging on it and loses some of

Fig. 2. Schematic of droplet lifetime on a hot surface [14].

its kinetic energy. Experimental results of Senda et al. [17] showthat the kinetic energy loss of a droplet in the tangential directioncan be ignored compared with that in the vertical direction.

A relation between the incident droplet Weber number Wein

and the reflection droplet Weber number, Weout, had beenproposed by Naber and Farrel as shown in Fig. 3. They also sug-gested a droplet reflection model as shown in Eq. (4) that candetermine the vertical component of the droplet’s velocity afterimpingement on a hot surface [16].

Weout ¼ 0:678Wein exp�� 4:415� 10�2 �Wein

�(4)

According to Eq. (4), the vertical direction velocity of the dropletafter collision with the hot surface can be derived from thefollowing equation

Vnew ¼ �Weouts

rf din(5)

3.7. Stokes number

The Stokes number, named after Irish mathematician GeorgeGabriel Stokes, is a dimensionless number corresponding to thebehavior of particles suspended in a fluid flow. Stokes number isdefined as the ratio of the stopping distance of a particle toa characteristic dimension of the obstacle, or

Stk ¼ sUo

do(6)

where s is the relaxation time of the particle, Uo is the fluid velocityof the flow well away from the obstacle and do is the characteristicdimension of the obstacle. For Stk[1, particles will continue ina straight line as the fluid turns around the obstacle thereforeimpacting on the obstacle. For Stk� 1, particles will follow thefluid streamlines closely.

Apart from the aforementioned, many other parameters alsoinfluence the performance of liquid fuel combustion in porousmedia, for instance, type of the PM material, configuration of theburner, fuel droplet size, distance between the spray nozzle and theupstream burner, the size and shape of the porous medium, rate ofcombustion heat feedback, residence time of the mixture, rate ofheat transfer between the fuel droplets and the PM and so on.

4. Numerical investigations

The interaction between the fuel spray and a porous mediumis the key factor for homogenization of the fuel–air mixture in the


porous medium. Unfortunately, the complicated process andpatterns of fuel spray impingement on the porous medium, aswell as the spray spreading in it, could not be observed andmeasured with a conventional experimental facility because thecomplex porous medium matrix does not allow optical accessinto it. However, numerical simulation provides an alternative,and in fact, simulation is the only way to perform the analysis atpresent [14].

In the open literature, there are few articles concerning theinteraction between a fuel spray and a porous medium. As far as theauthors are aware the first liquid fuel combustion model for porousmedium is formulated by Martynenko et al. [18]. The porousmedium under study was of high porosity with uniformly distrib-uted spheres and with uniform distribution of cavities with equalmean pore size in the porous medium. The physical model used bythem is shown in Fig. 4. Droplets with initial diameter 10–50 mmwas considered and were monodispersed. The interaction betweendroplets during evaporation was neglected. Physical properties offuel such as latent heat of evaporation and healing value wereconstant. Pseudosteady-state conditions were assumed for theevaporation. The temperature distribution inside the fuel dropletwas uniform, but time-varying. Laminar isobaric flow consisting ofair, fuel droplets, gaseous combustible mixture and hot products ofcombustion with constant velocity u0 was assumed. After evapo-ration the fuel was assumed to be mixed immediately with air toform a homogeneous combustible mixture. Overall density andthermo-physical properties of the gaseous phase were treated asconstant. Mass fraction of the liquid was negligible, and the gas wasoptically transparent. Accordingly, radiative heat transfer betweenthe skeleton surfaces of the porous medium was considered,omitting the radiative heat transfer around liquid droplets. Themost probable modes of heat exchange between different phaseswere assumed to be like in Fig. 5. The heat transfer between gas andsolid was assumed to be dominant.

On the basis of the above assumptions, the governing equations(the energy conservation equations for the gaseous, solid and liquidphases) were formed. Combustion was modeled by using the so-called Arrhenius equation for a single-step chemical reaction ofa premixed flame with relevant kinetic constants of reactions asproposed by Aronowitz et al. [19]. Models for droplets motion anddroplets heat transfer were also proposed.

4.1. Arrhenius equation

The Arrhenius equation is expressed as

k ¼ A expð�E=RTÞ (7)

Fig. 4. The physical model used

where E is defined as the apparent activation energy, and A is the pre-exponential or frequency factor. The activation energy E is usuallyidentified as the energy barrier (or threshold) that must be sur-mounted to enable the occurrence of the bond redistribution stepsrequired to convert reactants into products. The pre-exponentialterm or frequency factor, A provides a measure of the frequency ofoccurrence of the reaction situation, usually envisaged as incorpo-rating the vibration frequency in the reaction coordinate [20].

One of the applications of the Arrhenius equation is the deter-mination of the activation energy for a reaction. Taking the naturallog of the Arrhenius equation gives a linear equation:

lnðkÞ ¼ Ea

R

�1T

�þ ln

�fp�

(8)

A graph of ln k versus 1/T should give a straight line whose slope is�Ea/R. By measuring the rate constant at a range of differenttemperatures, a graph can be plotted to determine the activationenergy of a reaction.

4.2. The governing equations

The one-dimensional governing equations proposed by Marty-nenko et al. are listed below.

4.2.1. Energy equation for gas phase

3v

vt

h�Cara þ Cvprrvpr

�Tg

iþ v

vx

h�Cara þ Cvprrvpr

�uoTg

i

¼ lgv2Tg

vx2þ ggs

�Ts � Tg

�� ð1� aÞð1� dÞggl

�Tg � Tl

�þ rvHA exp

�� E

RgTg

�� ð1� aÞd _Mf L (9)

where

rvprhrfo � rf

d ¼�

0; for T1 < Tsat1; for T1 ¼ Tsat and Tg � Tsat

H is the combustion heat, L is the latent heat of evaporation, and Tsat

is the saturation temperature of solvent–fuel mixture. The right-hand side terms of the equation describe thermal conduction,convective heat transfer among phases (gas solid and gas liquid),combustion, and vaporization, respectively.

by Martynenko et al. [18].

Fig. 6. Model of liquid droplet motion used by Martynenko et al. [18].

Fig. 5. Scheme of the most probable convective heat transfer modes between threephases: liquid, gas and solid [18].


4.2.2. Energy equation for solid phase

Csð1� 3ÞrsvTs

vt¼ �ggs

�Ts � Tg

�þ að1� dÞgslðTl � TsÞ

þ ð1� 3Þ2lsv2Ts

vx2 �vqr

vx� ad _Mf L (10)

where the radiative term is written as follows

vqrðsÞvx

¼ �2pk

"IoE2ðsÞ þ IeE2ðsc � sÞ � 2Ib

þZ sc

oIbðs0ÞE1ðjs0 � sjÞds0

#(11)

4.2.3. Energy equation for liquid phase

3Clv

vt

�rf Tl

�þ Cl

v

vx

�rf uoTl

�¼ ð1� aÞð1� dÞggl

�Tg � Tl

�� að1� dÞgslðTl � TsÞ (12)

where rf ¼ rlðpd3=6Þ; this equation is applicable only for theregion Tl� Tsat.

4.2.4. Mass conservation equation for liquid phase

3vrf

vtþ v

vx

�rf uo

�¼ �d _Mf (13)

where

_Mf ¼ð1� aÞggl

�Tg � Tsat

�þ agslðTs � TsatÞ

Lþ Cg�Tg � Tsat

� (14)

Equation (14) means that the total convective heat flux underconstant saturation temperature of the liquid causes the evapora-tion of droplets and contributes to the change of the temperature ofthe gaseous phase.

4.2.5. Mass conservation equation for fuel vapor

3vrv

vtþ v

vxðrvuoÞ ¼ d _Mf � rvA exp

�E

RgTg

�(15)

4.3. Description of liquid droplets motion and their collection on theskeleton of solid

Variation of a in equations (9)–(12) and (15) within range [0, 1]allows to simulate the different regimes of fuel vaporization. Toimprove understanding of the collision dynamics and to providethe most probable value of a, the model involving dropletimpingement on the skeleton of the porous medium was suggested.The applied model was mainly based on the trajectory analysis ofthe droplets captured by immersed solid obstacle of simple shape[21]. As shown in Fig. 6, a trajectory of moving droplet follows thestreamline of gas flow, but deviates from it in the neighborhood ofthe obstacle because of inertia.

Depending on starting conditions, the droplet either is inter-cepted by an obstacle or moves away from it. If ydc denotes thedistance between the outermost trajectory for which the particlesstill collide with the obstacle and the centerline of the flow,collection efficiency of droplets on sphere is defined as ad¼ (ydc/R).Here, it was assumed that the interaction force of the droplet withthe flow is only the ordinary Stokes drag, and the variation of thedroplet diameter during the motion around a single obstacle isnegligible. Consequently, the equation of droplet motion at r waswritten in the form:

p6

d3rld2rdt2 ¼ 3pmgd

�v� dr

dt

�(16)

In the dimensionless form,

Stkd2x

dq2 ¼ V � dx

dq(17)

using the following variables

x ¼ rR; V ¼ v

uo; q ¼ uot

Rand Stk ¼ rld

2uo

18mgR

In the case of inviscid flow around an obstacle of sphericalshape, substituting explicit expression for the velocity component,equation (9) could be rewritten for xd¼ z cos 4 and yd¼ z sin 4

projections, respectively:

Stkd2xd

dq2 ¼ 1�2x2

d � y2d

2�

x2d þ y2

d

�5=2� dxd

dq(18)


Stkd2yd ¼ 1� 3ydxd� � � dyd (19)

Fig. 8. Temperature distribution along combustion section: (B) experimental datafrom Kaplan and Hall [27], (d) numerical simulation (Martynenko et al. [18]).

dq22 x2

d þ y2d

5=2 dq

4.4. The heat transfer coefficient between liquid and solid

The expression for specific heat transfer from the solid to theliquid droplets used by Martynenko et al. is

qsl ¼ uorlL exp

"1�

�Tw

Tsat

�2#

(20)

Thus, heat transfer coefficient between solid and liquid is

hsl ¼ qsl=ðTs � TlÞ (21)

4.5. Simulation results

Figs. 7–10 show some findings from the study of Martynenkoet al. They compared the simulation results with the experimentaldata of Kaplan and Hall [27]. Fig. 7 shows the variation of ydc and ad

with Stk. The computational result of ad was fitted well by thepolynomial equation (22) using a least-square method for Stkwithin range [0.5–4]:

ad ¼ 0:093þ 0:387Stk� 0:054Stk2 (22)

The calculated temperature profile and experimental axialtemperature distributions along the combustion section of theporous ceramic burner at equivalence ratio 0.64 are shown in Fig. 8which indicates a good agreement between the simulation andexperimental results. The predicted steep temperature gradient atthe entrance was attributed to the vaporization process.

The temperature distributions of solid, liquid and gaseous pha-ses, mass fraction profiles of liquid phase and combustible vapor,and reaction rate within the PM are shown in Fig. 9. The amountof liquid diminished by evaporation only and the mass fraction ofgaseous fuel increased in the vaporization region and quicklyvanished by combustion. The differences between solid and gastemperatures were small except in the flame zone. The distributionsof volumetric heat fluxes and evaporation rate are shown in Fig. 10.

4.6. Further advances in modeling

The model introduced by Martynenko et al. was highly simpli-fied, and was valid within a relatively narrow range of geometrical

Fig. 7. Deposition efficiency for single obstacle of spherical shape (Martynenko et al. [18]).

parameters. The effects of solid-phase properties, fluid-phasemixing and diffusion were not analyzed. The vaporization of liquidby radiation in the entrance region of the porous medium and thedynamics of the droplets and their accumulation in the pre-evap-oration region were also not addressed. Park and Kaviany [22] alsohad adopted the collision probability to study the interaction of fueldroplets with a PM regenerator in a regenerative engine. Sankaraet al. used a local thermal non-equilibrium model to simulate fueldroplets’ evaporation in a heated porous medium [23].

All the researchers did not, however, take into account thedetailed interactions between the fuel spray and the porousmedium with high porosity. This problem was specificallyaddressed in the work of Zhao and Xie [14]. They did numericalsimulation on the interaction of a pressure swirl spray and a hotporous medium to analyze the process of the fuel/air mixtureformation and the role of the PM in mixture homogenization andcombustion in a PM engine. They used the modified KIVA-3V codein which an improved spray/hot wall interaction model wasincorporated. They claimed that the improved model fits into theregime above the Leidenfrost temperature, determines the prop-erties of post impingement fuel droplets and the quantity of heattransfer between the fuel droplets and a hot surface. An evapo-rating fuel spray impingement on a hot plane surface was simulatedfor validating the model. Numerical results compared well withexperimental data for spray radius in the liquid and vapor phases.The linearized instability sheet atomization (LISA) model was usedto describe the atomization and breakup processes of the spray

Fig. 9. Temperature and mass fraction distributions within porous medium (Marty-nenko et al. [18]).

Fig. 10. Heat flux distribution within porous medium (Martynenko et al. [18]).

Fig. 12. Physical model used in Ref [25].


from the pressure swirl atomizers. The structure of a hot porousmedium with porosity of 0.88 was established using a simplemodel. The injection, movement and vaporization of the fueldroplets inside the PM and their impingement on the block edgeswas computed to obtain the spatial distribution and time evolutionof the temperature and fuel concentration inside the PM. Theinfluences of the operating parameters, including ambient pressureand spray cone angle, on the characteristics of the fuel spray andmixture formation were discussed based on the numericalsimulations.

Kayal and Chakravarty [24] presented a numerical analysis ofcombustion of liquid fuel droplets suspended in air inside an inertporous media. A one-dimensional heat transfer model (physicalmodel shown in Fig. 11) was developed assuming completevaporization of oil droplets prior to their entry into the flame. Theeffects of absorption coefficient, emissivity of medium, flameposition on radiative energy output efficiency and optimum oildroplet size at the entry, defined as the maximum size for completevaporization before entering the combustion zone, were presented.It was shown that the inert porous medium with low absorptioncoefficient will produce high downstream radiative output withlarge oil droplet sizes.

In their extended numerical work [25] the liquid fuel wasconsidered to be added in a drop wise fashion uniformly over thetop surface of the matrix and allowed to trickle through the system.The schematic of the physical model is shown in Fig. 12 whichillustrates that the liquid (kerosene) fuel and air both uniformlydistributed over top horizontal surface of the PM of length L, flowvertically downward through an adiabatic duct in x-direction. The


air with mass velocity ru along with liquid, both in laminar flow,enter medium (x¼ 0) at temperature Ti. Vaporization of liquidoccurs at x¼ x0 over thickness d and mixture of liquid vapor and airburns near the region at x¼ x* and flows out at x¼ L at temperatureTe. The porous medium was divided into three regions where A, Band C represented pre-combustion, combustion and post-combustion regions, respectively. The interfacing planes which areperpendicular to x-direction are separating the regions and desig-nated by 1, 2, 3 and 4. The radiative heat fluxes in forward andbackward directions in the porous medium are qþ and q�,respectively.

A one-dimensional heat transfer model was developed understeady state conditions using a single-step global reaction mecha-nism. The effects of optical thickness, emissivity of medium, flameposition and reaction enthalpy flux on radiation energy outputefficiency as well as the temperature, position and thickness ofvaporization zone were presented using kerosene as fuel. Theyfound that low values of optical thickness and emissivity of porousmedium will ensure efficient combustion, maximize downstreamradiative output with minimum upstream radiative loss.

Further contribution of their research [26] was to developa model based on a combined self-sustained liquid fuel vapor-ization–combustion system, where the liquid fuel vaporizationoccurs on a wetted wall plate with energy transferred through theplate from the combustion of vaporized oil. The vaporizationenergy was derived through the radiative interaction of thevaporizing plate and an upstream end surface of the porousmedium. The inert porous medium, used in the flow passage ofcombustion gas, was allowed to emit and absorb radiant energy.The radiative heat flux equations for the porous medium werederived using the two-flux gray approximation.

The schematic diagram representing two-dimensional modelunder steady state condition is shown in Fig. 13. The system isdivided into three regions A, B (free spaces) and C (porousmedium). The vertical plate 1 separates region A and B. The fuel oil(Tl¼ Ti) enters at x¼�D1, y¼ 0 and moves from y¼ 0 to y¼D2

under gravity in the form of a thin film along the vertical plate 1.During this passage, the liquid fuel is heated from Ti to boiling pointTb and vaporized completely at y¼D2. A suitable air ejector sucksthe fuel vapor from region A through bottom opening(height¼D3�D2) of plate 1 to region B. The air ejector is at thebottom of region B at y¼D3 between x¼ 0 and x¼�D1. This notonly prevents the flashback of flame from combustion zone ofregion B to region A but also uses region B as a well-stirred


Fig. 14. The burner setup used by Kaplan and Hall [27].


combustion reactor having high swirl and back mixing of gaseouscomponents for completion of combustion at Tg¼ T 0. In region B,the combustion zone was assumed to be in the space between x¼ 0to �D1 and y¼ 0 to D2. Heat of vaporization of fuel was transferredfrom combustion zone at region B to region A through the plate 1.The combustion products move from combustion zone in x-direc-tion in between y¼ 0 and y¼D2 through the porous medium inlaminar flow in region C and leaves the region at Tg¼ TL. Initiallyregion B space is heated through combustion of a fuel gas(methane) in air where both gas and air enter through airport. Aftersufficient temperature of combustion is reached in region B, theradiative heat becomes sufficient in region B to heat plate 1 forvaporization of fuel oil. Then the fuel gas is switched off. The systembecomes self-sustained for vaporization and combustion of fuel oil.The system was insulated and assumed to have no heat loss.

The effects of emissivities of vaporizing plate and porousmedium, the optical thickness of medium and equivalence ratio onthe kerosene vaporization rate, combustion temperature andradiative output of the system were analyzed. They concluded thatthe combination of low and high emissivities of vaporizing plateand porous medium, respectively, with low optical thickness ofmedium makes the system suitable over a wide range of power.

5. Experimental investigations

An experimental setup for liquid fueled porous ceramic burnerwas devised by Kaplan and Hall [27] using three types of porousceramics namely magnesia-stabilized zirconia, silicon carbide, andyttrium-stabilized zirconia. Heptane, the fuel used, was impingedon the combustion section using a pressure atomizer with a fixedflow rate of approximately 0.025 lpm and with the fuel dropletdiameters in the range 50–100 mm. The experimental setup isshown in Fig. 14. A porous ceramic section capable of beingremoved and altered was fit onto the experimental apparatus asshown. Several porous ceramics (three or four), each 2.5 cm thick,were stacked inside a quartz cylinder or an alumina insulationsleeve, which formed the 10 cm-diameter combustion section. Thecombustion section was fixed to the downstream end of a 10 cm I.D.7.5 cm long quartz cylinder that acted as a window for viewing theupstream end of the combustion section. The quartz was connectedto the downstream end of a 10 cm diameter stainless steel tube. Air

entered the 52 cm long tube at its upstream end through two1.25 cm diameter openings. An oil burner spray nozzle was used tointroduce the liquid fuel. It was found that by using magnesia-stabilized zirconia ceramics encased in an alumina insulatingcylinder, complete combustion was achieved at a fuel flow rate of0.025 lpm for fuel/air equivalence ratios of 0.57–0.67 with theceramic configuration 4 ppcm–10 ppcm–4 ppcm–4 ppcm–10ppcm. Emission concentrations for the alumina-insulated PIMburner were very low at a heptane flow rate of 0.025 lpm. Correctedfor 3% oxygen, CO varied from 3 to 7 ppm and NOx from 15 to20 ppm. Run with an increased heptanes flow rate of 0.032 lpm, thecorrected emissions for this burner were still low: CO varied from 1to 4 ppm and NOx varied from 15 to 25 ppm. Two critical aspects ofburner design, such as insulation of the combustion section andpre-vaporization of the fuel were examined. Alumina insulationwas replaced with the quartz to determine the effects of radiativeheat loss from this section. For the quartz-enclosed burner, the COand NOx emissions were lower, owing to lower temperaturesresulted from radiative heat loss through the quartz. Regarding pre-vaporization a quartz-enclosed burner was used to combust firsta nonvaporized heptane spray and then pre-vaporized heptane. Theemissions for both CO and NOx were similar for the two conditions:the CO varied from approximately 1 to 4 ppm and the NOx variedfrom approximately 11 to 15 ppm. This indicates that a significantfraction of the nonpre-vaporized heptane spray eventually vapor-ized and mixed sufficiently well with the air to react as a premixedflame.

A similar type of study was reported by Tseng and Howell [28]on zirconia porous burner using heptane as fuel. A one-dimensionallaminar flow model was used in this work. Combustion wasmodeled using a multistep reaction mechanism. Non-local thermalequilibrium between the gas, the liquid, and the solid phases wasaccounted for by considering separate energy equations for thethree phases. Nongray solid radiation and droplet radiationabsorption were included in the model. The equations for the liquidphase are formulated using a Lagrangian description and theequations for the gas and solid phases were formulated using anEulerian description. A burner that facilitated effective control ofthe equivalence ratio and the burning rate was built and burningrates, temperature, and emissions were measured. Numericalresults were obtained for droplet size and temperature history aswell as for flame speed and temperature profile within the burner.Stable combustion was still realized at a small equivalence ratio 0.3with an average droplet diameter of about 10 mm. Emission levelswere similar to those found by Kaplan and Hall. Liquid fuel dropletsexperienced early complete evaporation in the low temperatureregion before the flame front due to the small droplet size (<25 mm)and high volatility of the fuel. It was revealed that energy feedbackmechanism by combined mode of heat transfer (i.e., heat transfer

Fig. 16. Fuel injectors: (a) air-assist injector and (b) swirling-air injector (Vijaykant andAgrawal [29]).


from gas to liquid droplets and from solid (porous medium) toliquid droplets via convection and thermal radiation absorption bythe droplets) was not large enough to effectively enhance theenthalpy required for droplet evaporation. This, according to theauthors, was because interactions of the droplets with the solidporous medium or droplet collisions, which could help to enhanceevaporation, were not taken into account in the model. This mightbe a critical problem when using relatively large droplets or usingother fuels having higher heats of vaporization than that ofheptane. The higher the heat of vaporization of the fuel, the finerthe droplet spray that was needed; otherwise, stable combustionflame of the liquid fuel spray within the PIM might no longer bepossible. To eliminate the need of using very fine droplet spray anda fuel atomizer, especially when using fuel having a relatively highheat of vaporization, a special vaporization method of the fuel wasneeded.

As extension to the work of Kaplan and Hall, Vijaykant andAgrawal [29] had investigated combustion of kerosene insideporous medium to reduce the NOx, CO and soot emissions. Siliconcarbide (SiC) coated carbon foam was used as PM to attain highstructural strength. The two-zone porous burner design as shownin Fig. 15 consisted of preheat and combustion sections. Differentconfigurations were tested by stacking together square porouspieces of 2.5 cm thickness. Two types of fuel injectors wereconsidered: (i) the air-assist injector where approximately 5% of thecombustion air is used for atomization and the remaining air entersas the primary co-flow around the injector (Fig. 16a), and (ii) theswirling-air injector (Fig. 16b) where all of the combustion airenters the injector to create a swirling flow around the fuel jet toenhance atomization and fuel–air premixing. The distance betweenthe injector and PM inlet is a key operational parameter, which wasvaried in experiments with both injectors over a range of equiva-lence ratios and heat release rates. The NOx and CO emissions weremeasured to optimize the PM configuration with minimum emis-sions. Three combustor operational regimes were identifieddepending upon the injector location: (a) when the injector waslocated far upstream of the PM, the fuel pre-vaporized to produce

Fig. 15. Schematic of the experimental setup used by Vijaykant and Agrawal [29].

a homogeneous reactant mixture and the emissions were thelowest, (b) when the injector was at an intermediate location, thefuel pre-vaporized to form a non-homogeneous reactant mixtureand emissions increased slightly, and (c) when the injector wasclose to the PM, the residence time was insufficient to fully pre-vaporize the fuel droplets, and hence, the combustion occurred inthe diffusion mode. Consequently, emissions of CO, NOx and sootincreased significantly. The swirling-air injector reduced emissionsby promoting premixing between the fuel and air. Emissions werealso reduced by a careful selection of the pore size and thickness ofthe PM section. For the optimized configuration, the pressure dropin the PM section was less than 0.7% of the total pressure. Theauthors concluded that the combustion of liquid fuels using porousinert media could be a viable approach for gas turbines and othersimilar applications.

Takami et al. [30] had attempted to develop liquid fuel porousceramic burner without using a fuel atomizer, as extension to thework of Tseng and Howell. The burner setup is shown in Fig. 17.Kerosene was supplied (instead of heptane) drop wise (instead of indroplet spray) to the top surface of a horizontal porous ceramicplate burner through a steel wire net which served for uniformdistribution of fuel over the surface of the burner. Stable combus-tion flame, which is similar to a pool of fire, was realized on thelower surface of the burner where the fuel vapor was ignited uponwhich it mixed with the swirling combustion air suppliedtangentially from the wall of the combustion chamber. Thermalstructures in terms of temperature distribution and various speciesconcentration profiles such as CO, CO2 and NOx along the centerlineof the combustion chamber were measured to evaluate the burn-er’s performance. Almost complete combustion in the equivalenceratio ranging from 0.5 to 0.9 was achieved. A favorable flammabilityregion having the minimum equivalence ratio 0.1 and maximumturn-down ratio of 5.8 at the range of input load of about 670–3880 kW/m2 was also reported. Effects of kerosene input andequivalence ratio on the thermal structures were clarified.However, only the temperature profiles within the combustion

Fig. 17. The experimental burner setup used by Takami et al. [30]

Fig. 18. The experimental setup used by Jugjai et al. [31].


chamber were measured and discussed, whereas those of theporous burner were not. Thus, the interaction between the liquidfuels and the porous burner (evaporation mechanism) thatoccurred inside the porous burner together with interactionbetween the porous burner and the adjacent combustion chamberwere not discussed and well understood.

A step further to the work of Takami et al., a more detailedevaporation mechanism inside the porous burner was consideredby Jugjai et al. [31]. A simpler porous burner system for burningkerosene as shown in Fig. 18 was proposed. The heat transferphenomena, the evaporation mechanism, and the combustioncharacteristics, which occur simultaneously in the burner system,were studied by measuring thermal structures in terms oftemperature profiles along the centerline of both the porous burnerand the combustion chamber. Also radial temperature profiles atany location along the centerline of the combustion chamber weremeasured to evaluate the performance of the downstream porousemitter. The effect of parameters such as thermal input, equivalenceratio, installation of the downstream porous emitter and its opticalthickness on the combustion temperatures and emission charac-teristics of the burner system were clarified.

The extension to this study was reported by Jugjai and Polmart[32] in which they focused on the introduction of a packed bedemitter downstream of the porous burner as shown in Fig. 19.Enhancement of evaporation and combustion was evaluatedthrough the measured thermal structures in terms of temperaturedistribution along the burner length and emission characteristics atthe burner exit. Stable combustion with low emission of pollutantswas realized even though the combustion flame was confinedbetween the porous burner and the packed bed emitter with anincrease in the back-pressure. The effects of various parametersincluding heat input and equivalence ratio on the combustioncharacteristics were clarified to confirm improvement in mixing ofthe fuel vapor/air mixture and turn-down ratio of the burner. It was

found that the introduction of the packed bed emitter was anefficient method for enhancement of evaporation and combustionof the liquid fuels without a spray atomizer. Again in 2007 Jugjaiand Pongsai [33] presented a similar type of experimental study onthe so-called LFFPC.

Improvements to the works of Takami et al. and Jugjai et al. havebeen achieved by Fuse et.al [11]. Their focus was on developinga burner without using electrical heater for vaporization, therebysaving a large amount of electricity. The concept of self-sustainedcombustion by the enhancement of fuel vaporization by the radiantheat flux from the flame, high temperature walls and from theporous medium itself was introduced. They proved that an opticallythin (high porous) PM could achieve this goal compared with theoptically thick (low porous) one used by the previous researchers.

Fig. 20 shows the concept of the combustion self-sustained byan enhancement of fuel vaporization. This work was aimed tosubstantiate the concept of self-sustained combustion by oilvaporization enhanced by radiation through the higher porousmedia, for the purpose of the development for electric powersaving burners for home use. The applicatory scale combustor wasmanufactured with the larger pore sized ceramics than that used inthe previous works. The stable combustion region, the temperaturedistribution in combustor and the concentration of NOx emissionfrom the combustor were measured. The effectiveness of the largepore sized ceramics for reflux of radiation heat was also discussed.

As outcome of their continued effort in this area for furtherimprovements in self – sustainable vaporizing combustion a fuel-vaporizing combustor equipped with porous burner made of Al2O3

and 1.7 MHz-ultrasonic oscillator was manufactured in 2005 [34].The concept of combustion in the new combustor is shown inFig. 21. It was supposed that the fuel-penetration was derived from

Fig. 19. The experimental setup used by Jugjai and Polmart [32] with three-way swirling air and packed bed emitter.

Fig. 20. Comparison of combustion concept employed by Fuse et al. [11] with that ofprevious workers. a) Low-porous ceramic [30]. b) High-porous (optically thin) [11].

Fig. 21. Concept of the improved combustion system developed by Fuse et al. [34].a) Ignition startup. b) Self-sustainable vaporizing combustion during the stable state.


Fig. 22. Schematic of the burner used by Kamal and Mohamad [36].


a capillary effect and the local pressurization of ultrasonic influ-enced by pore size and ultrasonic propagation. Combustion char-acteristics of ethanol fuel with manufactured combustor wereinvestigated experimentally. Vaporizing combustion was sustainedwith recovering a part of its thermal radiation heat through theporous burner because Al2O3 has transparency for light which hasnear IR wavelength. Complete combustion was achieved withinAl2O3 burner under the equivalence ratio ranged from 0.63 to 0.80,which showed that the combustion mode was similar to a pre-mixing combustion. However, the NOx concentration wascomparatively high in the stable combustion region because theheat value per surface area of the flame was increased due to a localhigh temperature zone caused by a shortened flame.

Periasamy et al. [35] conducted experiments on spray combus-tion by using an open-cell, silicon carbide-coated, carbon–carbonceramic foam as a porous medium. Aviation-type kerosene wassprayed into a co-flowing, preheated air environment using an air-blast atomizer, and the spray subsequently entered the porousmedium. The minimum combustion heat feedback rate required for

Fig. 23. Sequence of motion of the regenerator and piston and physical rendering

complete vaporization and the vapor concentration downstream ofthe porous medium were measured. It was found that the surfacetemperature of the porous medium was uniform and the minimumheat feedback rate required for complete vaporization increased asthe distance between the porous medium and the injector wasdecreased. With porous media and combustion heat feedback,complete vaporization was achieved at a co-flow air temperature of400 K. Without porous media, however, a minimum co-flow airtemperature of 500 K was required to achieve the same quality ofevaporation. It was also revealed that a combustion heat feedbackrate of 1% produced average vapor concentrations of 63% and 43%more than that with no heat feedback at equivalence ratios of 0.3and 0.6, respectively.

Another breakthrough in spray combustion has been reportedrecently by Kamal and Mohamad [36] who employed a rotary swirlburner for combustion in a cross-flow steam of air. The objective ofthe study was to analyze the effect of fuel spray orientation onmixing dynamics with the swirled air. The effect of swirl on CO andNOx emissions was found to be dominated owing to improvedmixing and increased kinetic rates. The burner setup for nozzleangles of �45� and 45� is shown in Fig. 22.

5.1. Application based studies

The concept of the cool flame vaporizer had been developed byTrimis et al. [37] and Brehmer et al. [38]. They developed liquid fuelporous burners on this concept for industrial applications. It offereda clear separation between vaporization and combustion zoneswhich is highly desirable for combustion of liquid fuels. An existingin-cylinder thermal regeneration concept for diesel engines wasexamined by Park and Kaviany [22] for the roles of the porous insertmotion and the fuel injection strategies on the fuel evaporation andcombustion and on the engine efficiency. They claimed thata regenerative diesel engine using an in-cylinder reciprocating,porous regenerator (as shown in Fig. 23) has the potential toimprove fuel–air mixing and combustion. The porous insert isattached to a rod and moves in the cylinder, synchronized, but outof phase with the piston. During the regenerative heating stroke,

of fuel injection and air blowing during the regenerative heating stroke [22].


the regenerator remains just beneath the cylinder head for most ofthe period and moves down to the piston (as it approaches the TDCposition). During the regenerative cooling stroke, the regeneratormoves up and remains in the original position until the nextregenerative heating stroke. Following the combustion andexpansion, the products of combustion (exhaust gases) retain anappreciable sensible heat. During the regenerative cooling stroke,the hot exhaust gas flows through the insert and stores part of thissensible heat by surface-convection heat transfer in the porousinsert (with large surface area). The super adiabatic flametemperature (due to the thermal regeneration of the combustionheat) and the fuel droplet-regenerator interaction, enhance the fuelevaporation. A uniform fuel vapor distribution is possible due to thedeflection of fuel droplets by the air flow emanating from theporous regenerator and this can improve combustion. A two-gas-zone and a single-step reaction model were used with a Lagrangiandroplet tracking model that allowed for filtration by the insert.A thermal efficiency of 53 percent was predicted, compared to43 percent of the conventional diesel engines. Further, the appli-cation of highly porous open-cell structures to internal combustionengines for supporting mixture formation and combustionprocesses was introduced by Weclas [39–42]. Novel concepts forinternal combustion engines based on PMC technology were pre-sented and discussed. His study proved that gas flow, fuel injectionand its spatial distribution, vaporization, mixture homogenization,ignition and combustion could be controlled or positively influ-enced with the use of porous media reactors.

Recently, the porous medium combustion has been applied tomicro- and meso-scale applications. A meso-scale liquid fuel filmcombustor with a central porous inlet was devised by Li et al. [43].The effects of porous material type and bead size on the flamestructures and combustion characteristics were examined. Porousmedia made of stainless steel and bronze were tested with differentfuel and air flow rates, equivalence ratios, and bead sizes. The flamestructure and its corresponding stabilization mechanism weredifferent between the stainless steel and the bronze porous mediacombustor. In the stainless steel case, the high specific heat capacityenhanced fuel vaporization and fuel–air mixing, and the flameanchor located on the surface of the porous cap. In the bronze case,due to its low heat capacity, the flame was swept downstreamwhere the recirculation zone above the porous cap offered a lowvelocity field to help anchor the flame. Few more interestingfindings have also been presented.

6. Concluding remarks

A thorough review of published works on LFCPM has beenmade. Definitely there are many excellent published works leftwithout citation due to page limitation, but without any intention.This technology is still not fully adopted in practical applicationsowing to its complexity in operation. Hence there is a wide scopefor researchers in this area to develop energy efficient and eco-friendly combustion techniques. If properly engineered, this can bean excellent candidate to be applied in automobiles, stirlingengines, burning of hazardous liquid wastes as well as in domesticand industrial burners. The recently introduced micro- and meso-scale applications have to be explored further. The combustionmodeling approach can be improved further by incorporatingmultistep reaction kinetics, and more parameters that criticallyinfluence the combustion in a practical perspective.

Acknowledgement

The authors would like to thank the Ministry of Technology andInnovation, Malaysia and Universiti Sains Malaysia for the financial

support for the ongoing research on porous media combustiontechnology.

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A review of investigations on liquid fuel combustion in porous inert media

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