Experimental and numerical study on laminar burning characteristics of premixed...

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 4 8 7 6 – 4 8 8 8

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Experimental and numerical study on laminar burningcharacteristics of premixed methane–hydrogen–air flames

Erjiang Hu, Zuohua Huang*, Jiajia He, Chun Jin, Jianjun Zheng

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 15 January 2009

Received in revised form

25 March 2009

Accepted 26 March 2009

Available online 25 April 2009

Keywords:

Methane

Hydrogen

Laminar burning velocity

Markstein length

Sensitivity analysis

Flame structure

* Corresponding author. Tel.: þ86 29 8266507E-mail address: [email protected]

0360-3199/$ – see front matter ª 2009 Interndoi:10.1016/j.ijhydene.2009.03.058

a b s t r a c t

An experimental and numerical study on laminar burning characteristics of the premixed

methane–hydrogen–air flames was conducted at room temperature and atmospheric

pressure. The unstretched laminar burning velocity and the Markstein length were

obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for

further understanding of the effect of hydrogen addition on the laminar burning velocity,

the sensitivity analysis and flame structure were performed. The results show that the

unstretched laminar burning velocity is increased, and the peak value of the unstretched

laminar burning velocity shifts to the richer mixture side with the increase of hydrogen

fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend.

They are: the methane-dominated combustion regime where hydrogen fraction is less

than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the

methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than

80%. In both the methane-dominated combustion regime and the methane-inhibited

hydrogen combustion regime, the laminar burning velocity increases linearly with the

increase of hydrogen fraction. However, in the transition regime, the laminar burning

velocity increases exponentially with the increase of hydrogen fraction in the fuel blends.

The Markstein length is increased with the increase of equivalence ratio and is decreased

with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen

addition is regarded as the increase of H, O and OH radical mole fractions in the flame.

Strong correlation is found between the burning velocity and the maximum radical

concentrations of H and OH in the reaction zone of the premixed flames.

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction engines have been realized in the spark-ignition engines.

With the depletion of crude oil reverses and the strengthening

of automotive emission legislations, the development of

alternative fuel engines has attracted more and more atten-

tion in engine community. Natural gas is a clean fuel in which

methane is its major component, is considered to be one of

the favorable fuels for engines. The natural-gas-fuelled

5; fax: þ86 29 82668789.n (Z. Huang).ational Association for H

However, due to the slow burning velocity of natural gas and

its poor lean-burn capability, the natural gas spark-ignited

engine still remains its disadvantages like low thermal effi-

ciency, large cycle-by-cycle variation, and poor lean-burn

capability, and these will decrease the engine power output

and increase the fuel consumption [1,2]. One of the effective

methods to solve the problem of slow burning velocity of

ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

mailto:[email protected]

http://www.elsevier.com/locate/he

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 4 8 7 6 – 4 8 8 8 4877

natural gas is to mix the natural gas with a fuel that possesses

the high burning velocity. Hydrogen is regarded as the best

gaseous candidate for natural gas due to its very high burning

velocity, and the combination of natural gas with hydrogen is

expected to improve the lean-burn characteristics and

decrease the engine emissions [3–6].

Flame propagation is strongly related to improvement of

combustion in spark-ignited (SI) engines. In conventional SI

engine, combustion initiates from the spark plug where the

turbulent flame front will develop and propagate to the

combustion chamber volume [7]. By the time when 5–10% of

fuel/air mixture is burned, the combustion process in SI

engines is well established and the flame front moves quickly

in the engine cylinder [8]. The burning velocity directly affects

the flame propagation speed and hence, the operation of the

SI engine. Faster burning in SI engines leads to a more robust

and repeatable combustion and permits engine operation

with substantially larger amount of exhaust gas recirculation

(EGR), bringing the reduction in NOx emission. Fast burning

results in the decreased combustion duration and the

compact combustion process will improve engine thermal

efficiency and decrease fuel consumption [7].

The laminar burning velocity is the fundamental param-

eter in combustion and it is base for turbulent burning

velocity. Laminar burning velocity can be used to validate the

chemical reaction mechanisms [9,10] and is of practical

importance in the design and optimization of internal

combustion engines and power plant burners [11]. There are

three approaches in measuring the laminar burning velocity.

They are the stagnation plane flame method [12,13], the heat

flux method [14,15] and the combustion bomb method

[10,16,17]. The stagnation plane flame method can establish

different flame configurations, but it is difficult to draw a clear

flame front and to stabilize the flame under the high-pressure

conditions. The heat flux method needs to determine the heat

loss as a function of inlet velocity and to extrapolate the

results to zero heat loss to get the adiabatic burning velocity.

The combustion bomb method utilizes the prototypical

propagating spherical flame configuration and has drawn the

particular attention due to its simple flame configuration,

well-defined flame stretch rate and well controlled experi-

mentation [18,19]. In this study, the laminar burning velocities

of the methane–hydrogen–air mixtures were measured by

using the spherically expanding flame.

Many previous researches on burning velocities concen-

trated on the methane–air flames [13,16,18,20,21] and/or the

hydrogen–air flames [9,19,22–26]. Recently, some experi-

mental studies reported the measurement of laminar burning

velocity for the methane–hydrogen–air flames [11,13,27–34].

Scholte and Vaags [28] firstly reported their measurements by

means of the tube burner method. Liu et al. [30] and Huang

et al. [31] conducted more extensive experimental studies over

wide range of equivalence ratios and hydrogen fraction in

methane–hydrogen–air flames. The studies can be classified

into two categories. One is from view of enhancement of

methane flame by adding the hydrogen [11,13,32–34] and

another from the view of inhibition of hydrogen flame by

adding the methane [27,29]. Yu et al. [13] studied the laminar

burning characteristics of methane–hydrogen–air flames with

the assumption that the stoichiometrically small amounts of

hydrogen in the mixture were completely consumed and

found a linear correlation between laminar burning velocity

and hydrogen fraction. Law and Kwon [27] studied the

potential of hydrocarbon addition to suppress explosion

hazards and found that a small or moderate amount of

methane addition could remarkably reduce the laminar

burning velocities and would suppress the propensity of onset

of both diffusional–thermal instability and hydrodynamic

cellular instability in hydrogen–air flames.

In numerical study, simulations of the premixed hybrid

flames have been performed extensively [27,32–40]. Most of

these computations were conducted with the CHEMKIN [41]

and/or the COSILAB [42] laminar premixed flame codes where

detailed kinetic schemes can be implemented [43,44]. Similar

to those in experimental studies, these simulation work and

chemical kinetics analysis mostly focused on methane-rich

flames [32–36,38–40] or hydrogen-rich flames [27,37].

The objectives of this study are the experimentally

measurement combining with simulation on burning veloci-

ties of methane–hydrogen–air mixtures at various hydrogen

fractions and equivalence ratios, and provides the chemical

kinetics of the flames. The study will deepen the under-

standing of methane–hydrogen–air mixture flames.

2. Experimental method

2.1. Experimental setup and procedures

Fig. 1 shows the experimental setup. It includes a constant

volume combustion chamber, and the systems for heating,

ignition, data acquisition and high-speed schlieren photog-

raphy. The combustion chamber is a cylindrical type with an

inner diameter of 180 mm and a volume of 5.5 L as shown in

Fig. 2. The centrally located electrodes are used to ignite the

combustible mixtures. Pressure transmitter, thermocouple,

pressure transducer, inlet and outlet valves are mounted on

the chamber body. Two quartz windows with 80 mm diameter

are located at two sides of the vessel for optical access. A high-

speed digital camera (HG-100K) operating at 10,000 frames per

second is used to record the flame photos during flame

propagation. Fuel and dry air are supplied into the chamber

through the inlet valve corresponding to the given equiva-

lence ratio and dilution ratio. Ten minutes waited before the

ignition start to ensure the homogeneity and motionless of

the mixtures.

Experiments were conducted at initial pressure of 0.1 MPa

and initial temperature of 303 K. Purities of methane and

hydrogen in the study are 99.9% and 99.995%, respectively.

The volumetric percentage of hydrogen in the fuel blends

(XH2 ) is defined as,

XH2¼ VH2

VCH4 þ VH2

(1)

where VCH4 and VH2are the volume fraction of methane and

hydrogen in the fuel blends, respectively.

The equivalence ratio ðfÞ is defined as,

f ¼ F=AðF=AÞst

(2)

Fig. 1 – Experimental setup.


where F/A is fuel–air ratio and (F/A)st refers to the stoichio-

metric value of F/A. For stoichiometric methane–air and

hydrogen–air mixture combustion, the chemical formulas are

as follows:

CH4 þ 2ðO2 þ 3:762N2Þ ¼ CO2 þ 2H2Oþ 2� 3:762N2 (3)

H2 þ 0:5ðO2 þ 3:762N2Þ ¼ H2Oþ 0:5� 3:762N2 (4)

Mixture can be expressed as,

�1�XH2

�CH4þXH2

$H2þ�

2f

�1�XH2

�þXH2

2f

�ðO2þ3:762N2Þ (5)

2.2. Laminar burning velocity and Markstein length

For an outwardly-propagating spherical flame, the stretched

flame velocity, Sn, reflecting the flame propagation speed, is

derived from the flame radius versus time [25],

Sn ¼dru

dt(6)

Fig. 2 – Schematic of constant v

where ru is the radius of the flame in schlieren photograph and

t is the elapsing time from spark ignition.

A general definition of stretch at any point on the flame

surface is the Lagrangian time derivative of the logarithm of

the area A of any infinitesimal element of the surface,

a ¼ dðlnAÞdt

¼ 1A

$dAdt

(7)

For the outwardly-propagating spherical flame, the flame

stretch rate can be deduced in the following form,

a ¼ 1A

$dAdt¼ 2

ru$dru

dt¼ 2

ru$Sn (8)

In respect to the early stage of flame expansion, there exists

a linear relationship between the flame speeds and the flame

stretch rate [18],

Sl � Sn ¼ Lb$a (9)

where Sl is the unstretched flame propagation speed, obtained

as the intercept value of Sn at a¼ 0 in the plot of Sn against a.

olume combustion vessel.

Fig. 3 – Computational burning velocities by using GRI-

Mech 3.0 versus experimental results of methane–

hydrogen–air mixtures.


Burnt-gas Markstein length Lb is the negative value of the

slope of Sn versus a curve. The diffusional–thermal instability

of the flame front is dependent upon Lb. Positive values of Lb,

which correspond to Lewis numbers larger than unity, indi-

cate that the flame speed decreases with the increase of the

stretch rate, while a negative value of Lb corresponds to

a Lewis number smaller than unity, indicating that the flame

speed increases with the increase of flame stretch rate.

Negative values of Lb are associated with more unstable

flames.

The characteristics of the igniter can influence the

measured value of burning velocity. Previous study showed

that the flame speeds were independent of ignition energy

when flame radius is greater than 5 mm. This phenomenon

was also observed by Bradley et al. [18], Huang et al. [31] and

Lamoureux et al. [25]. Their studies gave an approximate value

of 5 mm so as to avoid possible effect exerted by the spark-

ignition disturbance completely. On the other hand, when the

flame radius is smaller than 25 mm, the pressure in this

constant volume combustion chamber varies little; thus the

combustion process can be regarded as a constant-pressure

one. By taking into account of the effect of ignition energy and

pressure rise in the combustion chamber, flame photos in the

range of 5 mm–25 mm are used in the analysis. In addition,

the data were also restricted by the occurrence of cellular

structure due to the obvious increase in flame speed caused by

the increased flame front area [45].

Fig. 4 – Schlieren photographs

In the early stage of flame propagation, the flame

undergoes an isobaric developing process, the unstretched

laminar burning velocity, ul, is related to Sl from mass

conservation across the flame front,

Aruul ¼ ArbSl (10)

where A is the flame front area, ru and rb are the unburned and

burned gas density, respectively. The unstretched laminar

burning velocity, ul, can be obtained from Eq. (10)

ul ¼ rb$Sl=ru (11)

In this study, repeated experiments were conducted in

three times and the averaged data are used in the analysis.

This can ensure repeatability of the results within the exper-

imental uncertainty (95% confidence level). The accuracy of

thermocouple is 1 K and the variation in initial temperature is

303� 3 K. Thus, the relative error in initial temperature is

0.99%. Initial pressure is measured by a mercury manometer

and its accuracy is 1 mm, thus, the variation in initial pressure

is 0.1 MPa� 133.28 Pa and the relative error in initial pressure

is 0.133%. Mixture was prepared according to the partial

pressures of the constituents. For methane–air mixture, the

relative error of fuel–air equivalence ratio gives its minimum

value of 1.35% at fuel-lean conditions ðf ¼ 0:6Þ and the

maximum value of 1.44% at fuel-rich conditions ðf¼ 1:3Þ.While for hydrogen–air mixture, the relative error of fuel–air

equivalence ratio has its minimum value of 0.371% at fuel-

lean conditions ðf ¼ 0:4Þ and the maximum value of 0.917% at

fuel-rich conditions ðf ¼ 4:5Þ. The resolution of digital flame

photographs taken is 352 pixel� 352 pixel. We use Photoshop

to measure the diameter of the flame. The ratio of picture

scale and real scale is (5.93� 0.01):80 mm. So the relative error

of measured radius has its maximum value of 1.35% at small

radius ðru ¼ 5 mmÞ and the minimum value of 0.27% at large

radius ðru ¼ 25 mmÞ. The experimental standard errors in the

measurements are determined based on the method of Kline

and McClintock [46]. The maximum standard errors are 16.6%

for Markstein length, 4.5% for stretched flame propagation

speed, 5.5% for unstretched flame propagation speed, 7.3% for

stretched laminar burning velocity, 7.8% for stretched mass

burning velocity, and 8.6% for unstretched laminar burning

velocity.

3. Numerical methods

A freely propagating adiabatic, premixed, planar flame was

simulated using PREMIX [41], Sandia’s steady state, laminar,

of methane–air mixtures.

Fig. 5 – Flame radius versus time at different hydrogen

fraction.


one-dimensional flame code. PREMIX uses a hybrid time-

integrating/Newton iteration technique to solve the steady-

state mass, species and energy conservation equations and

can simulate the propagating flame. Equations were solved by

using the TWOPNT, a boundary value problem solver in the

CHEMKIN package [47]. Also built in the CHEMKIN package are

a transport property processor and a gas-phase interpreter

which provide the species transport properties and process

the chemical reaction mechanism.

Fig. 6 – Stretched flame propagatio

One of the critical elements needed for the simulation is

a proper reaction mechanism which describes the essential

fundamental reaction paths followed by the overall reaction.

GRI-Mech is an optimized detailed chemical reaction mecha-

nism for the calculation of natural gas chemical reaction

process and the latest version is GRI 3.0 [43]. GRI 3.0 consists of

325 elementary chemical reactions with associated rate

coefficient expressions and thermochemical parameters for

the 53 species. It includes the detailed combustion reaction

mechanism for hydrogen. The ranges of GRI 3.0 are 1000–

2500 K in temperature, 10 torr–10 atm in pressure and 0.1–5 in

equivalence ratio.

To simulate and interpret the effect of hydrogen addition

on methane–air chemical reactions, the chemical kinetics

mechanism used in the calculation must be capable of the

calculation of the pure methane and methane–hydrogen

fuel blends. Prior to the calculation, the GRI 3.0 mechanism

needs to be validated by the experimental results. Ren et al.

[32,33] implemented the GRI-Mech in the PREMIX code and

validated this kinetic mechanism for hydrogen–methane

blends by comparing the computed values of the laminar

burning velocity with experiments at low values of equiv-

alence ratio (0.63–0.73) and hydrogen mole fractions in the

fuel less than 8%. In this study, the validation of reaction

mechanism was attempted for stoichiometric mixture and

rich mixture as well as in high hydrogen fraction in the

hybrid mixtures.

Fig. 3 shows the computational laminar burning velocities

by using GRI-Mech 3.0 versus the experimental results. A good

agreement is found except at high hydrogen fraction. This is

n speed versus flame radius.

Fig. 7 – Stretched flame propagation speed versus stretch rate.


probably related to the used kinetic scheme. Maximum devi-

ation of 16% is presented in the stoichiometric hydrogen

flame. However, this error will not significantly affect the

results and conclusions of the analysis. The results show that

the GRI 3.0 can well reproduce the laminar burning velocity of

methane–hydrogen–air mixtures at stoichiometric mixture

combustion and rich mixture combustion as well as wide

range of hydrogen fractions. GRI 3.0 was also validated in

calculation of laminar flame speed [32–34,48] and ignition

delay time [49,50] for methane–hydrogen–air flames in the

previous literatures.

Fig. 8 – Markstein length versus equivalence ratios at

different hydrogen fractions.

4. Results and discussion

4.1. Flame propagation speed and burned gas Marksteinlength

Fig. 4 shows the schlieren photograph of methane–air

flames at the stoichiometric equivalence ratio. The

smoothly and spherically expanding flame propagates

outwardly from the electrode gap in the center of chamber

at the stoichiometric equivalence ratio. At the early stage of

flame development, the cooling effect of the electrodes on

the flame propagation is observed and this leads to a slower

flame propagation speed in the direction of the electrodes

than that in the vertical direction. When the flame radius

has developed to a certain value, the electrodes’ cooling

effect becomes trivial. To avoid the influence from the

electrodes, the radius in the vertical direction is used in the

calculation.

Fig. 5 gives the flame radius versus the time for the

stoichiometric methane–hydrogen–air mixtures. There

exists a linear relationship between the flame radius and

time. The slope of the radius–time line is increased with the

Fig. 9 – Unstretched flame propagation speed versus

equivalence ratio at different hydrogen fractions.Fig. 10 – Unstretched laminar burning velocity versus

equivalence ratio at different hydrogen fractions.


increase of hydrogen fraction, and this indicates the

increase of flame propagating speed as hydrogen fraction

increases.

Fig. 6 shows the stretched flame propagation speed (Sn)

versus the flame radius. The stretched flame propagation

speed is the derivative of flame radius with respect to time,

and it reflects the flame moving speed relative to the

motionless combustion wall. With the propagation of flame,

the flame propagation speed shows different trends at

different equivalence ratios and hydrogen fractions. Fig. 6a

and b give the stretched flame propagation speed of

methane–air flames and hydrogen–air flames versus the

flame radius at different equivalence ratios, respectively. In

the case of f < 1:0 for hydrogen combustion, the stretched

flame propagation speed decreases slightly with the increase

of the flame radius. For stoichiometric hydrogen–air flames,

little variation of stretched flame propagation speed versus

flame radius is presented. For hydrogen–air flames with

equivalence ratios between 1.0 and 3.0 and for methane–air

flames with equivalence ratios between 0.6 and 1.1, the

stretched flame propagation speed increases slightly with

the increase of flame radius. In the case of high rich flames

(f ¼ 4:0 for hydrogen–air flames and f ¼ 1:2 and f ¼ 1:3 for

methane–air flames), the stretched flame propagation speed

shows a large increase with the increase of flame radius at

the early stage of flame propagation and a slight increase

with the increase of the flame radius at the subsequent stage

of flame propagation. The maximum value of stretched

flame propagation speed is presented at the equivalence

ratio of 1.8 for the hydrogen–air flames. Fig. 6c plots the

flame propagation speed versus flame radius at the stoi-

chiometric equivalence ratio. For all hydrogen addition

flames, Sn increase slightly as flame propagates. The flame

propagation speed increases and the increment of flame

propagation speed will increase with the increase of

hydrogen fraction especially when hydrogen fraction in the

fuel blends is larger than 80%.

Fig. 7 shows the stretched flame propagation speeds

versus flame stretch rate at different equivalence ratios and

hydrogen fractions. In the early stage of flame propagation

where the flame radius is small, the stretch rate of flame

front surface is large. Removing the data affected by the

ignition energy and electrodes during the early stage of

flame development and large radius where pressure is

increased, a linear correlation between the stretched flame

propagation speed and the flame stretch rate is

Fig. 11 – Comparison between the measured laminar

burning velocities with those in literatures.

Fig. 12 – Burning velocity versus hydrogen fraction at

different equivalence ratios.


demonstrated. The unstretched flame propagation speed is

obtained as the intercept value of Sn at a¼ 0 in the plot of Sn

versus a. Importantly, this gives the value of laminar

burning velocity ul from Eq. (11). For methane–air combus-

tion, the stretched flame propagation speeds decrease with

the increase of stretch rate at all equivalence ratios, and the

gradients of the Sn� a lines take the negative values, rep-

resenting the positive values of Markstein length. For

hydrogen–air combustion, in the case of f > 1:0, the

stretched flame propagation speeds are increased as the

flame propagates outwardly, and the gradients of Sn� a

lines take the negative values, indicating the positive values

of Markstein length. However, in the case of f < 1:0, the

stretched flame speeds are increased with the increase of

the stretch rate, and the Markstein length gives a negative

value. Fig. 7c shows the Sn� a curve for the mixtures with

different hydrogen fractions at the stoichiometric equiva-

lence ratio. The results show that the stretched flame

propagation speeds are increased and the burned gas

Markstein lengths (Lb) are decreased with the increase of

hydrogen fraction, and this indicates that the flame stability

will decrease with the increase of hydrogen fraction.

Fig. 8 gives the Markstein lengths (Lb) versus the equiva-

lence ratio at different hydrogen fractions. For all methane–

hydrogen–air mixtures, the Markstein length shows an

increase trend with the increase of equivalence ratio, and the

behavior becomes more remarkable at high equivalence

ratio. However, the Markstein lengths of the methane–

hydrogen–air flames with large hydrogen fraction and the

hydrogen–air flames show a slow increase with the increase

of equivalence ratio, and the curve of Lb � f tends to be a flat

one in rich mixture regions. Moreover, the Markstein length

is decreased with the increase of hydrogen fraction. This

suggests that the flame front stability is enhanced with the

increase of equivalence ratio but the addition of hydrogen

into the methane–air mixtures will decrease the stability of

the flame front. This behavior may connect to the large

cycle-by-cycle variations of hydrogen engine at lean mixture

operation [51]. This phenomenon can be explained with the

classical models of flame instability due to effects of pref-

erential diffusion proposed by Manton et al. [52] and

Markstein [53]. The laminar premixed flames will tend to be

unstable due to the effects of preferential diffusion under the

conditions where the fast-diffusing component (CH4 and H2

in the present instance) is deficient (corresponding to fuel-

lean combustion in this study). The Markstein lengths of

methane–air flames obtained by Gu et al. [20] are also plotted

in Fig. 8, and close results are demonstrated between liter-

ature [20] and this study.

Fig. 9 shows the unstretched flame propagation speeds

(Sl) versus the equivalence ratios at different hydrogen

fractions. For a given equivalence ratio, the unstretched

flame propagation speeds are increased with the increase of

hydrogen fraction and the behavior becomes more obvi-

ously for methane–hydrogen–air flames with large hydrogen

fraction and the hydrogen–air flames. Maximum value of Sl

shifts to the richer mixture direction (high equivalence

ratio) with the increase of hydrogen fraction in the fuel

Fig. 13 – Sensitivity factors of methane molar fraction as function of hydrogen fraction.


blends. Maximum value of the unstretched flame propaga-

tion speed of methane–air flames is presented at the

equivalence ratio of 1.1 and it is 1.8 for hydrogen–air flames.

Small increase in Sl is observed when hydrogen fraction is

less than 60%, and this means sufficient addition of

hydrogen can remarkably increase the flame speed for

methane–hydrogen–air flames.

4.2. Laminar burning velocity

Fig. 10 shows the unstretched laminar burning velocity versus

the equivalence ratio at different hydrogen fractions. Similar

to the unstretched flame propagation speed, when XH2 is less

than 60%, with the increase of equivalence ratio, the curves

show similar pattern with the peak value of ul at the equiva-

lence ratio of 1.1. When XH2 is greater than 60%, the maximum

value of ul will shift to the richer mixture direction with the

increase of hydrogen fraction. For hydrogen–air flames

ðXH2 ¼ 100%Þ, the maximum value of the unstretched laminar

burning velocity is presented at the equivalence ratio of 1.8.

Remarkable increase in unstretched laminar burning velocity

is observed when XH2 is larger than 60%.

Fig. 11 plots the unstretched laminar burning velocities of

methane–hydrogen–air mixture both this study and the data

in literatures for the comparison. The results show that the

data of the present study agree well with those of literatures in

both methane–hydrogen–air flames and hydrogen–air flames.

The laminar burning velocity (ul) versus the hydrogen

fraction in the fuel blends at three equivalence ratios (f¼ 0.8,

1.0 and 1.2) are plotted in Fig. 12. The results clearly show that

the values of the laminar burning velocity are below the

averaged values estimated by molar proportions. This implies

the presence of strong non-linear effects in chemical kinetics

and reflects in the values of unstretched laminar burning

velocity. As shown in Fig. 12, at all the equivalence ratios, it is

possible to identify three regimes in the hybrid flame propa-

gation depending on the hydrogen mole fraction in the fuel

blends. In the case of hydrogen fraction less than 60%, it is the

regime of methane-dominated combustion. Here a linear and/

or a slight increase in laminar burning velocity is presented

with the increase of hydrogen addition. In the case of

hydrogen fraction larger than 80%, it is the regime of

methane-inhibited hydrogen combustion. Here a linear and

sharp decrease in laminar burning velocity is presented with

the increase of methane fraction in the fuel blends. In the case

where hydrogen fractions are between 60% and 80%, it is the

transition regime. Here the laminar burning velocities

increase exponentially with the increase of hydrogen fraction

in the fuel blends.

4.3. Sensitivity analysis

To deepen the understanding which reaction has a large

effect on the flame propagation in the methane–

Fig. 14 – Predicted flame structure of premixed stoichiometric methane–hydrogen–air flames.


hydrogen–air system, the sensitivity analysis is performed.

In particular, the methane mole fraction sensitivity factors,

defined according to [41], are calculated and used to eval-

uate the major reactions affecting the combustion of the

blends,

SFi ¼ki

xvxCH4

vk; (12)

CH4 i

The first-order sensitivity analysis of methane is given in

Fig. 13 with the factors lower than �2 and larger than 2 for the


flames with different hydrogen fractions at the three equiva-

lence ratios (0.8, 1.0, 1.2). It is suggested that the following

elementary reaction steps mainly contribute to the kinetic

control of the hybrid mixture combustion:

OþCH4 5 OHþCH3 (R11)

Hþ O2þH2O 5 HO2þH2O (R35)

HþO2 5 OþOH (R38)

HþCH3þ (M) 5 CH4þ (M) (R52)

HþCH4 5 CH3þH2 (R53)

OHþCH4 5 CH3þH2O (R98)

OHþCO 5 HþCO2 (R99)

It is found that methane is mainly consumed via reactions

(R11), (R52), (R53) and (R98). The main consumption reactions

of CH4 in the flame are the abstraction reactions attacked by H,

O and OH and form CH3. However, the above listed steps are

not simultaneously involved at all equivalence ratios. Reac-

tions (R11) and (R35) do not play any role in the stoichiometric

mixture combustion. Meanwhile, for rich mixture combus-

tion, only steps (R38), (R52) and (R53) are presented.

At three equivalence ratios, the chain branching step (R38)

has the highest sensitivity for CH4 conversion whose rele-

vance in hydrocarbons combustion has been well docu-

mented in the literatures [35,58–60]. Methane combustion

mostly proceeds through reactions with OH radicals (R98) and

H radicals (R53) for lean and rich conditions, respectively

[33,60]. While both (R53) and (R98) dominate the stoichio-

metric mixture combustion. Increasing H2 fraction will

generate more H radicals, accelerating the reaction (R38) and

enhancing the burning intensity. On the other hand, hydrogen

radicals also participate in the dominant termination reac-

tions (R35) and (R52) in lean mixture combustion and (R52) in

the stoichiometric and/or rich mixture combustion,

competing for H radical with the chain branching reaction

(R38). The sensitivity factors of both reactions (R35), (R52) are

positive, as shown in Fig. 13, since they obstruct the methane

conversion through subtraction of H atoms.

4.4. Flame structure

As discussed above, measurements and predictions of

laminar burning velocities are in reasonably good agree-

ment over the present test range. Therefore, the detailed

flame structures from predictions were studied to gain

a better insight of the effect of hydrogen addition on

laminar burning velocities. Flame structures of freely

propagating methane–air flames with 0%, 40%, 80% and

100% hydrogen in the fuel blends for the stoichiometric

mixtures are presented in Fig. 14. The final burned gas

temperature and the maximum values of H, O, OH radical

mole concentration are plotted in Fig. 14. Comparison of the

temperature distributions shows that the final burnt-gas

temperature is increased with the increase of hydrogen

fraction. The results show that for the stoichiometric

methane–air flames, the radical OH has the largest

maximum concentrations in the flames, with H has some-

what small value in maximum concentration, roughly 10%–

30% lower than that of OH. Concentrations of O and CH3 are

less than half of OH concentration. However, for the

hydrogen–air flames, the largest concentration is H radical.

The maximum concentrations of OH, H and O in flames are

increased with the increase of hydrogen fraction, and this

will promote the combustion of methane–air flames. A

strong correlation is existed between the burning velocity

and the maximum radical concentrations of H and OH

radicals in the reaction zone of premixed flames [61,62]. It is

expected that the increase in maximum concentration of H

and OH would lead to the corresponding increase of the

laminar burning velocity of flames.

5. Conclusions

An experimental and numerical study on the laminar pre-

mixed methane–hydrogen–air flames was conducted at room

temperature and atmospheric pressure. The main conclu-

sions are summarized as follows.

1. The laminar burning velocity increases with the increase of

hydrogen fraction and the peak value of the laminar

burning velocity shift to the rich mixture side. Three

regimes are identified depending on the hydrogen fraction

in the fuel blend. They are: the methane-dominated

combustion regime where hydrogen fraction is less than

60%; the transition regime where hydrogen fraction is

between 60% and 80%; and the methane-inhibited

hydrogen combustion regime where hydrogen fraction is

larger than 80%. In both the methane-dominated combus-

tion regime and the methane-inhibited hydrogen combus-

tion regime, the laminar burning velocity increases linearly

with the increase of hydrogen fraction. However, in the

transition regime, the laminar burning velocity increases

exponentially with the increase of hydrogen fraction in the

fuel blends.

2. For methane–hydrogen–air mixtures, Markstein length

shows an increase with the increase of equivalence ratio,

and the behavior becomes more remarkable at high

equivalence ratio. Markstein lengths of methane–

hydrogen–air flames with large hydrogen fraction and

hydrogen–air flames show a slow increase with the

increase of equivalence ratio. Markstein length is decreased

with the increase of hydrogen fraction.

3. Enhancement of chemical reaction with hydrogen addition

is due to the increase of H, O and OH mole fractions in the

flame as hydrogen is added. A strong correlation is existed

between burning velocity and maximum radical concen-

trations of H and OH radicals in the reaction zone of


premixed flames. High burning velocity corresponds to

high radical concentration in the reaction zone.

Acknowledgments

This study is supported by National Basic Research Program of

China (Grant No. 2007CB210006) and National Natural Science

Foundation of China (Grants No. 50636040 and No. 50821604).

r e f e r e n c e s

[1] Rousseau S, Lemoult B, Tazerout M. Combustioncharacteristics of natural gas in a lean burn spark-ignitionengine. Proceedings of the Institution of MechanicalEngineers, Part D: Journal of Automobile Engineering 1999;213(D5):481–9.

[2] Ben L, Dacros NR, Truquet R, Charnay G. Influence of air/fuelratio on cyclic variation and exhaust emission in natural gasSI engine. SAE paper no. 992901; 1999.

[3] Blarigan PV, Keller JO. A hydrogen fuelled internalcombustion engine designed for single speed/poweroperation. International Journal of Hydrogen Energy 2002;23(7):603–9.

[4] Akansu SO, Dulger A, Kahraman N. Internal combustionengines fueled by natural gas–hydrogen mixtures.International Journal of Hydrogen Energy 2004;29(14):1527–39.

[5] Hu EJ, Huang ZH, Liu B, Zheng JJ, Gu XL. Experimental study oncombustion characteristics of a spark-ignition engine fueledwith natural gas–hydrogen blends combining with EGR.International Journal of Hydrogen Energy 2009;34:1035–44.

[6] Hu EJ, Huang ZH, Liu B, Zheng JJ, Gu XL, Huang B.Experimental investigation on performance and emissions ofa spark-ignition engine fuelled with natural gas–hydrogenblends combined with EGR. International Journal ofHydrogen Energy 2009;34:528–39.

[7] Bilgin A. Geometric features of the flame propagation processfor an SI engine having dual ignition system. InternationalJournal of Energy Research 2002;26(11):987–1000.

[8] Pulkrabek WW. Engineering fundamentals of the internalcombustion engine. Prentice-Hall; 1997.

[9] Aung KT, Hassan MI, Faeth GM. Flame stretch interactions oflaminar premixed hydrogen/air flames at normaltemperature and pressure. Combustion and Flame 1997;109(1–2):1–24.

[10] Huzayyin AS, Moneib HA, Shehatta MS, Attia AMA. Laminarburning velocity and explosion index of LPG–air andpropane–air mixtures. Fuel 2007;87(1):39–57.

[11] Milton BE, Keck JC. Laminar burning velocities instoichiometric hydrogen and hydrogen–hydrocarbon gasmixtures. Combustion and Flame 1984;58(1):13–22.

[12] Vagelopoulos CM, Egolfopoulos FN. Direct experimentaldetermination of laminar flame speeds. In: Symposium(international) on combustion; 1998: 27 (1): p. 513–19.

[13] Yu G, Law CK, Wu CK. Laminar flame speeds ofhydrocarbonþ air mixtures with hydrogen addition.Combustion and Flame 1986;63(3):339–47.

[14] Bosschaart KJ, de Goey LPH, Burgers JM. The laminar burningvelocity of flames propagating in mixtures of hydrocarbonsand air measured with the heat flux method. Combustionand Flame 2004;136(3):261–9.

[15] Bosschaart KJ, de Goey LPH. Detailed analysis of the heat fluxmethod for measuring burning velocities. Combustion andFlame 2003;132(1–2):170–80.

[16] Tseng LK, Ismail MA, Faeth GM. Laminar burning velocitiesand Markstein numbers of hydrocarbon/air flames.Combustion and Flame 1993;95(4):410–26.

[17] Marley SK, Roberts WL. Measurements of laminar burningvelocity and Markstein number using high-speedchemiluminescence imaging. Combustion and Flame 2005;141(4):473–7.

[18] Bradley D, Gaskell PH, Gu XJ. Burning velocities, Marksteinlengths, and flame quenching for spherical methane–airflames: a computational study. Combustion and Flame 1996;104(1–2):176–98.

[19] Sun CJ, Sung CJ, He L, Law CK. Dynamics of weakly stretchedflames: quantitative description and extraction of globalflame parameters. Combustion and Flame 1999;118(1–2):108–28.

[20] Gu XJ, Haq MZ, Lawes M, Woolley R. Laminar burningvelocity and Markstein lengths of methane–air mixtures.Combustion and Flame 2000;121:41–58.

[21] Liao SY, Jiang DM, Gao J, Huang ZH. Measurements ofMarkstein numbers and laminar burning velocities fornatural gas–air mixtures. Energy and Fuels 2004;18:316–26.

[22] Hermanns RTE, Konnov AA, Bastiaans RJM, de Goey LP.Laminar burning velocities of diluted hydrogen–oxygen–nitrogen mixtures. Energy and Fuels 2007;21:1977–81.

[23] Dowdy DR, Smith DB, Taylor SC, Williams A. The use ofexpanding spherical flames to determine burning velocitiesand stretch effects in hydrogen/air mixtures. Proceedings ofthe Combustion Institute 1991;23:325–32.

[24] Kwon OC, Faeth GM. Flame/stretch interactions of premixedhydrogen-fueled flames: measurements and predictions.Combustion and Flame 2001;124:590–610.

[25] Lamoureux N, Djebayli-Chaumeix N, Paillard CE. Laminarflame velocity determination for H2–air–He–CO2 mixturesusing the spherical bomb method. Experimental Thermaland Fluid Science 2003;27(4):385–93.

[26] Qiao L, Kim CH, Faeth GM. Suppression effects of diluents onlaminar premixed hydrogen/oxygen/nitrogen flames.Combustion and Flame 2005;143:79–96.

[27] Law CK, Kwon OC. Effects of hydrocarbon substitution onatmospheric hydrogen–air flame propagation. InternationalJournal of Hydrogen Energy 2004;29(8):867–79.

[28] Scholte TG, Vaags PB. Burning velocities of mixtures ofhydrogen, carbon monoxide and methane with air.Combustion and Flame 1959;3:511–24.

[29] Miller DR, Evers RL, Skinner GB. Effects of various inhibitors onhydrogen–air flame speeds. Combust Flame 1963;7:137–42.

[30] Liu Y, Lenze B, Leuckel W. Investigation on the laminar andturbulent burning velocities of premixed lean and richflames of methane–hydrogen–air mixtures. Prog AstronautAeronaut 1991;131:259–74.

[31] Huang ZH, Zhang Y, Zeng K, Liu B, Wang Q, Jiang DM.Measurements of laminar burning velocities for natural gas–hydrogen–air mixtures. Combustion and Flame 2006;146:302–11.

[32] Ren JY, Qin W, Egolfopoulos FN, Tsotsis TT. Strain-rateeffects on hydrogen-enhanced lean premixed combustion.Combustion and Flame 2001;1(24):717–20.

[33] Ren JY, Qin W, Egolfopoulos FN, Mak H, Tsotsis TT. Methanereforming and its potential effect on the efficiency andpollutant emissions of lean methane–air combustion.Chemical Engineering Science 2001;56:1541–9.

[34] Halter F, Chauveau C, DjebaIli-Chaumeix N, Gokalp I.Characterization of the effects of pressure and hydrogenconcentration on laminar burning velocities of


methane–hydrogen–air mixtures. In: Proceedings of thethirtieth symposium (international) on combustion; 2005. 30:p. 201–8.

[35] Sher E, Refael S. A simplified reaction scheme for thecombustion of hydrogen enriched methane/air flame.Combustion Science and Technology 1988;59:371–89.

[36] Refael S, Sher E. Reaction kinetics of hydrogen-enrichedmethane–air and propane–air flames. Combustion andFlame 1989;78:326–38.

[37] Kunioshi N, Fukutani S. Fuel mixing effects on propagationof premixed flames. II. Hydrogenþmethane flames. Bulletinof the Chemical Society of Japan 1992;65:2573–7.

[38] Gauducheau JL, Denet B, Searby G. A numerical study of leanCH4/H2/air premixed flames at high pressure. CombustionScience and Technology 1998;137:81–99.

[39] El-Sherif SA. Control of emissions by gaseous additives inmethane–air and carbon monoxide–air flames. Fuel 2000;79:567–75.

[40] Uykur C, Henshaw PF, Ting DS-K, Barron RM. Effects ofaddition of electrolysis product on methane/air premixedlaminar combustion. International Journal of HydrogenEnergy 2001;26:265–73.

[41] Kee RJ, Grcar JF, Smooke MD, Miller JA. PRMIX: a FORTRANprogram for modeling steady laminar one-dimensionalpremixed flames. Report. Sandia National Laboratories; 1985.SAND 85–8240.

[42] Rogg B. RUN-1DL: the Cambridge universal laminar flamecode. Report. University of Cambridge (Department ofEngineering); 1991. CUED/A-THERMO/TR39.

[43] Smith Gregory P, Golden David M, Frenklach Michael,Moriarty Nigel W, Eiteneer Boris, Goldenberg Mikhail,Bowman C Thomas, Hanson Ronald K, Song Soonho, GardinerWilliam C Jr., Lissianski Vitali V, Qin Zhiwei. Available from:http://www.me.berkeley.edu/gri_mech/.

[44] Qin Z, Lissianski V, Yang H, Gardiner Jr. WC, Davis SG, WangH. Combustion chemistry of propane: a case study ofdetailed reaction mechanism optimization. In: Proceedingsof the twenty-eighth symposium (international) oncombustion; 2000. 28: p. 1663–69.

[45] Bradley D, Lawes M, Kexin Liu, Verhelst S, Woolley R. Laminarburning velocities of lean hydrogen–air mixtures at pressures upto 1.0 MPa. Combustion and Flame 2007;149:162–72.

[46] Kline SJ, McClintock FA. Describing uncertainties in singlesample experiments. Mechanical Engineering 1953;75:3–8.

[47] Kee RJ, Rupley FM, Miller JA. CHEMKIN-II: a fortran chemicalkinetics package for the analysis of gas-phase chemicalkinetics. Technical report SAND89-8009. Sandia NationalLaboratories; 1989.

[48] Di Sarli V, Di Benedetto A. Laminar burning velocity ofhydrogen–methane/air premixed flames. InternationalJournal of Hydrogen Energy 2007;32(5):637–46.

[49] Safta C, Madnia CK. Autoignition and structure ofnonpremixed CH4/H2 flames: detailed and reduced kineticmodels. Combustion and Flame 2006;144(1–2):64–73.

[50] Chaumeix N, Pichon S, Lafosse F, Paillard CE. Role ofchemical kinetics on the detonation properties of hydrogen/natural gas/air mixtures. International Journal of HydrogenEnergy 2007;32(13):2216–26.

[51] Kim YY, Lee JT, Choi GH. An investigation on the causes ofcycle variation in direct injection hydrogen fueled engines.International Journal of Hydrogen Energy 2005;30:69–76.

[52] Manton J, Elbe G, Lewis B. Burning-velocity measurements ina spherical vessel with central ignition. Proceedings of theCombustion Institute 1953;4:358–63.

[53] Markstein GH. Non-steady flame propagation. New York:Pergamon; 1964. p. 22.

[54] Tanoue K, Goto S, Shimada F, Hamatake T. Effects ofhydrogen addition on stretched premixed laminar methaneflames. Transactions of the Japan Society of MechanicalEngineers, Part B 2003;69:162–8.

[55] Takahashi F, Mizomoto M, Ikai S. Alternative energysources III. In: Nejat Veziroglu T, editor. Nuclear energy/synthetic fuels, vol. 5. New York: McGraw-Hill; 1983. p.447–57.

[56] Hassan MI, Aung KT, Faeth GM. Measured and predictedproperties of laminar premixed methane/air flames atvarious pressures. Combustion and Flame 1998;115:539–50.

[57] Davis SG, Quinard J, Searby G. Markstein numbers incounterflow, methane- and propane–air flames:a computational study. Combustion and Flame 2002;130:123–36.

[58] Law CK. Combustion physics. New York: CambridgeUniversity Press; 2006.

[59] Dagaut P, Dayma G. Hydrogen-enriched natural gas blendoxidation under high-pressure conditions: experimental anddetailed chemical kinetic modeling. International Journal ofHydrogen Energy 2006;31(4):505–15.

[60] Dagaut P, Nicolle. An experimental and detailed kineticmodeling study of hydrogen-enriched natural gas blendoxidation over extended temperature and equivalence ratioranges. Proceedings of the Combustion Institute 2005;30(2):2631–8.

[61] Butler CJ, Hayhurst AN. Measurements of theconcentrations of free hydrogen atoms in flames fromobservations of ions: correlation of burning velocities withconcentrations of free hydrogen atoms. Combustion andFlame 1998;115:241–52.

[62] Padley PJ, Sugden TM. Chemiluminescence and radicalre-combination in hydrogen flames. In: Seventh symposium(international) on combustion, vol. 7(1). Pittsburgh: TheCombustion Institute; 1958. 235–42.

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