Push-pull ventilation system: a CFD approach for the performance analysis

International Journal of Ambient Energy, Volume 28, Number 3 July 2007

Push-pull ventilation system: a CFD approach for the performance analysis V. Betta*, F. Cascetta*" and A. Palombo ***

Vittorio Betta, DETEC, University of Naples, Federico 11, Piazzale Tecchio 80, 801 25 Naples, Italy. " Furio Cascetta, DIAM, Second University of Naples, Via Roma 29, 81031 Aversa, Italy. *** Adolfo Palombo, DETEC, University of Naples, Federico II, Piazzale Tecchio 80, 80125 Naples, Italy. (To whom all correspondence be addressed) e-mail: [email protected] O Ambient Press Limited 2007

SYNOPSIS In this paper the results of a CFD investigation for a push-pull local ventilation system are presented. The velocity field and the capture efficiency prediction are investigated for different system arrangements. The analysis is carried out for systems respectively with and without fluid- dynamic obstacle in the working plane and for dust pollutants with particle diameters ranging from 1.0 to 100 pm and emission velocities between 1.0 and 7.0 mls. The analysis, carried out using the commercial code FLUENT, shows that the system performances are dependent on operating conditions and are influenced by the presence of the above-mentioned obstacle on the working plane. In the case of working plane emission without obstacle the system capture efficiency is in general very low never exceeding 25%. Higher efficiencies are instead achieved for particle emission from a vessel placed in the system's working plane centre. In this case even unitary peak efficiencies are reached for particular operating conditions.

INTRODUCTION In modern industrial processes different chemical compounds and toxic substances are often utilised. Besides the possible health hazards, such pollutants (gases, dusts or vapours) can be dangerous for processing facilities, equipment and products as well [ I ] . For these reasons, in industrial work spaces, the adoption of suitable ventilation systems is recommended. The best way to safeguard the operators is to adopt a Local Exhaust Ventilation (LEV) system that typically consists of a suction hood [2-41.

In this paper the performance of a push-pull LEV system is dealt with. Such a device, in addition to a suction hood, consists of a jet nozzle that pushes the airborne contaminant from the source where it is generated towards the system suction opening (Figure 1). Push-pull systems are typically adopted for surface metal treating in open wide vessels but can be conveniently utilised in other industrial processes too. Some examples are the control of dust in the mining industry [5] and of solder fume in the electrical and electronics industry [6] where the solder pyrolysis products, that contain volatile and particulate compounds, cause sensitisation of the operator's respiratory tract. With push-pull systems the capture efficiency is up to 15 - 20% higher than that of conventional hoods for the same operating conditions. Alternatively, for the same capture efficiency, push-pull systems are 25 - 30% more cost-effective due to lower airflow [7].

The first studies in this field dealt with push- pull system design [8] and the relative developments [9] by experimental approach. More recent analyses investigated the reverse flow problem generated by obstacles between the push

Push-pull ventilation system: a CFD approach for the performance analysis Betta, Cascetta, Palombo

\ working plane

jet and the suction hood, the operator's exposure to the contaminant [ I 01 and airflow visualisation by smoke injection methods [ I 11. Other researchers investigated the velocity field in order to provide guidelines for design improvement [12] and the system capture efficiency for gas pollutants in chemical treatment tanks using numerical simulations [1 31.

The aim of this paper is, by numerical approach, to analyse the system velocity field in different types of system operating conditions and to assess the capture efficiency for different types of dust pollutant emissions. In addition, the particle flight-paths are investigated with particular attention to eventual cross drafts in the operator's breathing zone (height from the working plane where the operator could be breathing).

Figure 1 The local ventilation push- pull system.

SYSTEM MODELING AND SIMULATION The simulation model consists of a push-pull ventilation system having a working plane of 2.0 m x 1.0 m. The push jet nozzle and the suction hood are both rectangular and measure 0.0050 rn x 2.0 m and 0.040 m x 2.0 m respectively (Figure 2). The working plane is located 0.10 m under the openings. The suction conduit is endowed with a superior flange having a height of 0.40 m and extending to the entire width of the system (2.0 m). The working plane is bounded by an edge of 0.10 m in height on its lateral sides (Figure 1). Reference is made to an orthogonal three-dimensional frame located with the xz plane coincident with the plane symmetry, with the x-axis origin positioned at the base of the push jet conduit and directed outbound and the z-axis going up (Figure 2). Such system configuration is

Figure 2 Dimensions of the considered push-pull system.

I push jet suction h


obtained following the recommendations of the ACGlH ventilation manual [14]. Note that in the manual the design criteria refer to a push-pull system for gaseous contaminants. In the present analysis the same geometry is considered in order to test the relative performance for dust contaminant control.

Two different operating conditions are considered. The first one refers to a push-pull system having the working plane free from fluid- dynamic obstacles. By such configuration the system plane with its lateral edges can also simulate an open tank containing dust contaminants. In the second operating condition a cylindrical obstacle (in the following also called vessel), of 0.050 m in height and a 0.20 m in diameter, is located in the centre of the working plane. This obstacle simulates a contaminant vessel, a workpiece or a simple foot block. From the fluid-dynamic point of view this solid object, located between the push jet and the suction opening, represents a constraint that modifies the velocity field and thus the push-pull system performance.

Both configurations were created using the grid generator GAMBIT 2.1 [15]. For each model a simulation control volume was defined. The relative dimensions were in both cases designed to obtain still air in the investigated domains despite the velocity field produced by the ventilation system. Since the latter is physically and fluid-dynamically symmetric, only half of the above-mentioned control volumes were modelled and considered in the simulation. The relative heights from the working plane, the widths from the symmetry plane and the lengths were respectively 2.0 m, 2.0 m and 3.0 m. On both domains, a structured computational mesh of about 300,000 cells was generated. The size of the basic three-dimensional cells was taken to resolve the velocity gradients of air due to the

push jet and suction hood interactions. The higher these gradients in the investigated region, the smaller the cells and the closer the mesh design. In Figure 3 the mesh of the working plane zone of the system without obstacle is shown. As to the boundary conditions the control volumes were designed in analogy to industrial semi-closed workrooms. In order to comply with the mass balance on the domain (the suction flow rate is higher than the push jet flow rate) its surface at the back of the push conduit was modelled as open (pressure-inlet condition). Through such a surface the air flow rate passages, necessary to close the mass balance on the considered domain, were assessed. The remaining surfaces were instead modelled on a wall boundary condition. The control volume extends to a portion of the suction and push-jet conduits. Here air velocity outlet and inlet boundary conditions were respectively assigned.

The numerical simulation on finite volumes [16] was carried out using the commercial CFD code FLUENT 6.1 [ I 71. Here, the air handled by the push-pull system is assumed to be an incompressible Newtonian fluid. Isothermal airflow without buoyancy effects was considered. The Renormalization Group (RNG) k-E turbulence model [I 71 was adopted. The theoretical model is based on the fully turbulent flow assumption and originates from the Reynolds-averaged Navier- Stokes equations. The continuity time-averaged equation in a Cartesian reference system (using tensor notation) is:

- Ui - = O

Xi (1

while the time-averaged momentum one is:

Figure 3 Computational mesh for a push-pull system without obstacles in the working plane.

Push-pull ventilation system: a CFD approach for the performance analysis Betta, Cascetta, Palornbo

Direct reference can be made to the FLUENT manual [I 71, so all model details can be omitted, for the sake of brevity.

The previous equations of mass conservation and momentum (Equations (1) and (2)), and the transport equations of turbulent kinetic energy and dissipation of turbulent kinetic energy are discretised on each elementary cell of the flow calculation domain [ I 71. The discretised equations are then solved for each variable iteratively until a convergent solution is obtained. It is assumed that sufficient convergence is achieved when the dimensionless residual term of each system equation is less than

RESULTS AND DISCUSSION The 3D CFD simulation of the modelled systems is referred to push-jet and exhaust air flow rates taken from the ACGlH ventilation manual [14]. The corresponding average velocities are respectively 9.6 and 10 m/s while the Re numbers (based on the openings' hydraulic diameters) are respectively 0.65 x 1 04, and 5.3 x 1 04.

Velocity field. The first step of the investigation is focused on the air velocity fields produced by the push-pull system respectively for the system work plane without obstacles and with the above- mentioned cylindrical object located in the plane centre. The simulation results can be applied to other push-pull systems having similar geometries as well [18]. Therefore the dimensionless air velocity, V*, presented in the following figures is referrej to the average velocity inside the exhaust duct, V:

where A is the suction hood area, V is the exhaust air volume flow rate, and V is the velocity module. For the same reason, the distances from the origin of the considered Cartesian frame are made dimensionless with reference to the system plane length, L, between the push-jet and the exhaust hood:

(a) Push-pull system without obstacles in the work plane. In Figure 4, as to the system symmetry plane (y* = O ) , the contours of the air velocity module, V*, are reported. As in any push-pull ventilation system, except for the air regions close to the push jet and suction hood, the velocity field is mostly uniform and the air flow tends to stick to the working plane. For z* 0, i.e. in the air region included in the plane edges, significant velocity values are always reached, in particujar for z* - 0.05, V* is higher than 15% of V. This means that for this particular simulated system geometry the air velocity is higher than 1.5 m/s. By contrast with a simple slot suction system the air velocity in front of the exhaust opening decreases to 10% of that of the conduit average at one equivalent hydraulic diameter distance [7]. By increasing z* it is possible to observe that the push jet influence progressively decreases. For z* = 0 the velocity field is practically determined only by the exhaust hood.

velocity contours for a push-pull system without obstacles in the working plane.


(b) Push-pull system with a cylindrical obstacle in the work plane centre. Under the same boundary conditions, the velocity field of the system symmetry plane, shown in Figure 5, is much higher than that of the previous case a). This is due to the fact that the obstacle does not allow the flow to develop as a classical backward-facing step [19]. Between the plane edge under the push jet opening and the cylindrical object an air recirculating zone is generated. Here a normal pressure gradient pushes up the flow, preserving its directionality and a certain amount of velocity. At the exit of the obstacle high velocities and a significant field directionality are still maintained. As to the former case, a wider uniform velocity field region is obtained in which about double air velocity modules are detected. In particular, in the air region up to-the obstacle (z* > -0.05), V* reaches 40% of V (Figure 5). This means that in this zone for the particular simulated system geometry the air velocity exceeds 4.0 m/s. Note that significant velocities are, in this case, achieved also for z* > 0. Shifting to the vertical planes parallel to the symmetry plane and far from the obstacle, the observed velocity fields are similar to those of the former case a), (Figure 4).

Discrete phase and capture efficiency modelling. The behaviour analysis of the emitted particles is carried out following an uncoupled approach, and therefore it is assumed that the air velocity field is not disturbed by the motion of the emitted particles. In order to realistically simulate the particles spreading due to the turbulence, a stochastic model is adopted. The instantaneous

velocity obtained by the sum of stationary and turbulent fluctuation terms is introduced in the equation of motion. The analysis refers to inert solid dusts having constant physical properties. Furthermore, the following assumptions are also taken into account for the considered particles: (i) spherical shape i.e. unitary dynamic shape factor [20]; (ii) diameter, D, defined by the relative aerodynamic value [21] (diameter of a spherical particle having: 1000 kg/m3 density, final velocity of the same particle, the relative motion governed by gravity in still air at the same conditions of temperature, pressure and relative humidity); (iii) unitary Cunningham correction factor (slip factor) [20]. The simulation results are referred to particle diameters ranging from 1.0 to 100 pm (by an incremental step of 10 pm) so an approxi- mation of the Cunningham correction factor is carried out only in the case of 1 pm particle diameter [20]. The behaviour analysis of particles having diameters greater than 100 pm is not reported because the considered ventilation systems are not suitable to control such particle diameters: the flight-path deviations due to the system velocity field are always very slight.

The considered parameter for the system performance analysis in relation to the emitted particle capture is the capture efficiency, defined as the fraction of generated contaminant that is directly captured by the hood:

n q (%) = - X 100

m (5)

where n is the number of particles captured by the exhaust hood while m is the number of the emitted

Figure 5 Velocity contours for a push-pull system with a cylindrical obstacle in the working plane.


ones. The aim is to investigate the capture efficiency of the push-pull system in a general case with some initial simplifications such as the absence of external disturbances. The non- captured particles, i.e. the ones that land on the system surfaces, represent a potential health hazard in that they might be transferred, by a subsequent external disturbance, from the working plane to the work room air. In particular, this could happen when the push-pull system is turned off. For this reason it is important to know the percentage of captured particles that can be considered as ultimately eliminated from the working space.

The capture efficiency is assessed for the following simulated cases:

(I) singular vertical emission from the system work plane;

(11 ) multiple distributed vertical emissions from the system work plane;

(Ill) singular vertical emission from the above- mentioned cylindrical vessel;

(IV) multiple emissions from the cylindrical vessel.

In all cases the considered particles emission velocities, w, range between 1.0 and 7.0 m/s. For particle dispersion a stochastic tracking technique for turbulent flow was adopted considering a suitable "number of tries". In FLUENT the latter represents the number of the simulated particle emissions from each point. Each trajectory calculation includes a new stochastic represent- ation of the turbulent contributions to the trajectory equation. For this reason, when the

particle injection number from the same point changes, different capture efficiencies are obtained. Only for a sufficiently high "number of tries" the computed capture efficiency becomes a steady average value. For the simulations, 1000 emissions from each point were considered and thus 1000 different particle trajectories were computed. Note that the cohesion forces between particles during their trips were not modelled since these were judged to be negligible.

(I) Singular vertical emission from the system work plane. The particles emission was carried out from the centre of the system working plane (Figure 6). In general for velocities, w, lower than 5.0 m/s the capture efficiency is always lower than 20%. Such a result can easily be explained if the imposed boundary conditions on the solid system surfaces are considered. From the computational point of view it is in fact assumed that the particle reaching one of these surfaces ends its trip exactly at the impact point (surface escape condition in FLUENT). The capture efficiency does not take them into account disregarding the possibility that they might rebound, or slide, or be dragged towards the capture hood. This assumption can be justified by the typical nature of the industrial environment work surfaces: they are often wet, dusty, or strewn with grease and thus adhesive. Other conditions also account for such behaviour: (i) the particles rarely have a perfectly spherical shape; (ii) the real particles and the system surfaces on which they impact are made of deformable materials (inelastic collision behaviour).

-.-.-.- o = 1.0 mls - - ..- - - rn = 3.0 mls n

100 - Figure 6 Capture efficiency for a singular vertical emission from the system work plane.

11 (%)

80 -- particle 1

T pull


For particles having the features considered in this analysis the surface adhesion effect is always two orders of magnitude higher than the take-off effect due to an air flow of 10 m/s.

For very small particles, inertia and gravity are negligible. Therefore they could continuously follow the streamlines of the system velocity field. In spite of this, the obtained capture efficiency is very low (Figure 6) since for these particles the stop distance [21] (maximum distance covered by a particle released in still air with an emission velocity w and in the absence of external forces) is also very low. In fact, the deviation of their aerodynamic trajectories caused by the velocity field action begins when particles are still very close to the work plane surface. In such conditions and because of turbulence, the particles cannot easily move away from the surface without colliding against it. For example for a particle of 1.0 pm released with an emission velocity of 10 m/s, the stop distance is about 0.035 mm. Note that for such a particular particle diameter the capture efficiency does not depend on the relative emission velocity, w (Figure 6). Maintaining the latter constant and increasing particle diameter, higher particle heights from the work plane and consequently longer stop distances are obtained. The probability that particles collide, in such conditions, on the system plane becomes gradually lower. It is noteworthy to observe that in the case of particles having the same density the height achieved during their flight and their stop distance are increasing functions of particle speed and diameter.

For particle diameters greater than 20 pm the efficiencies decrease for the following reasons. The emission velocity fixes the height reached by the particles before the deviation caused by the velocity field. From here onwards the particles are transported by the air flow and their motion is governed by two forces: aerodynamic resistance (drag force) and gravity. In the area of the exhaust opening, the streamlines transporting the particles can enter straight into the hood or deviate significantly. In the latter case the larger the particle the lower the system capture efficiency. In fact, when particle inertia increases it is less likely that the particles will deviate with the velocity field and more likely that they will collide against the hood flange. All these observations explain the efficiency trend between 20 and 60 pm. Beyond 60 pm, for emission velocities of 5.0 and 7.0 m/s (Figure 6), a new increase due to the particular flight-paths of particles, that drive the latter directly into the suction hood, is detected. Finally, for even greater particle diameters the efficiency decreases again since a collision of such particles against the inferior edge of the hood occurs.

( 1 1 ) Multiple distributed vertical emission from the system work plane. The considered emission is in this case distributed on 100 release points uniformly located on the working plane (Figure 7). Each point is positioned at the corner of a square having a 10 cm side. The capture efficiency is in this case calculated on 100,000 particle emissions.

Note (Figure 7) that the capture efficiency

Figure 7 Capture efficiency for distributed vertical emission from the system work plane centre.


trends are very similar to those obtained for the singular vertical emission from the work plane. On the other hand, because of the current emission typology, the maximum efficiency values for w > 5.0 m/s are here detected for greater particle diameters. In this case all the above-mentioned observations on the adhesion effect between particles and system surfaces remain valid.

On the other hand, the whole system working plane and its edges can be considered as an open dust pollutant emitting tank. In this case the above-mentioned adhesion phenomenon has to be taken into account in a different way. In fact, the particles that, during their motion, impact on a surface made of the same dust pollutant, do not have to be considered as emitted in one place and deposited in another one, but simply as not emitted. The simulated flight paths show that, in the absence of external interferences, all the non- captured particles fall into the tank from which they were emitted. In such a case, since no particle "escaped" from the tank (in the absence of any external disturbance), a unitary system capture efficiency has to be considered. In Figure 8 particle flight paths for distributed emission (D = 100 vm, w = 7.0 m/s) from the working plane are displayed. Here it is possible to observe that for the considered operating conditions the particle collisions against the working plane or the vertical system edges generate a lot of pollutant deposit on such surfaces.

(I I I) Singular vertical emission from the cylindrical vessel. In this case it is assumed that the above- mentioned cylindrical obstacle functions as a contaminant vessel, a workpiece or a simple foot block from which an airborne dust pollutant is emitted. The vertical emission is carried out from a single point located on the cylindrical obstacle's

Figure 8 Particle flight paths for distributed emission (D = 100 pm, o = 7.0 m/s) from the working plane.

superior base diameter parallel to the system's symmetry plane. Starting from the obstacle edge closest to the push nozzle and moving on this diameter, keeping constant the particle diameter and the velocity emission, the capture efficiency trend initially decreases reaching a minimum value (between 10 and 15% of the diameter length) and then increases to a maximum located at the vessel edge closest to the hood opening. The capture efficiencies on the edge closest to the push-jet are high because the streamline directions in this air region raise the particles and so the adhesion effect on the superior surface of the obstacle is avoided. For the sake of brevity, in the following only the results obtained for the particles emitted from the obstacle centre and from the vessel edge closest to the suction hood are reported.

The results obtained for the particles emitted from the obstacle centre are reported in Figure 9. The capture efficiencies are clearly higher than those shown in the previous cases (Figures 6 and 7). This is due to: (i) the velocity field in the air region close to this emission point and in the area near the suction hood which is significantly directional and stronger; (ii) the smaller particles have a lower probability to impact on the horizontal system surfaces because the emission point is located at a higher position over the working plane; (iii) the larger particles which have a lower probability to impact on the flange or on the plane edge under the hood since an almost directional velocity field in proximity of the suction opening is in this case obtained.

The increasing and then decreasing trends of the capture efficiency can be explained by the same observations mentioned for the previous case I). In particular, it is possible to observe (Figure 9) that for D = 1.0 vm the efficiency is about 30%, higher efficiencies are obtained when the particle diameters increase, even reaching


-V- co = 7.0 m/s vessel I I

r 1 I 1

unitary values (except for w = 1.0 m/s). Once the maximum efficiency range is exceeded (for velocity emission between 3.0 and 5.0 m/s), for increasing particle diameters the trends decrease reaching a new minimum value, never lower than 50%. Furthermore in the right-hand part of Figure 9 a constant unitary efficiency for w = 3.0 m/s is achieved for 50 D 100 pm. By contrast, for

Figure 9 Capture efficiency for singular vertical emission from the vessel centre.

lower or higher emission velocities the efficiency decreases when the particles reach a certain height and then in their descent collide respectively against the plane edge under the hood or against the flange.

The particle emission from the cylindrical vessel edge closest to the suction hood (Figure 10) is simulated also so that the results are not

Figure 10 Capture efficiency for singular vertical emission from the vessel edge.


affected by the presence of the vessel's superior surface and thus by the particles impacting on it. The numerical analysis results are reported for this case in Figure 10. As expected, the efficiency is on average higher than in the previous case. In particular it has a unitary value for particle diameters up to 50 pm for all the investigated emission velocities. For particle diameters greater than 50 pm, the efficiency trends are similar to those reported for the case of emission from the centre of the cylindrical vessel and likewise explainable.

(IV) Multiple emissions from the cylindrical vessel. In this case an emission of particles from 10 points located on a virtual circle positioned on the superior vessel's surface is considered. The shape described by the particle flight-paths (before the deviations due to the air velocity field), is in general a truncated cone with its minor base placed on the vessel's superior surface. The simulations carried out are for different emission angles, a, formed by the particle trajectories and the truncated cone-axis and for the different radii, R, of its minor base. In these simulations the considered angles and base radii are respectively: 0°, 30°, 60" and 3.0 x m, 2.0 x m, 6.0 x m. In general the results show that the capture efficiency trends are similar to those of the previous case (Ill) and likewise explainable. For the sake of brevity only the following cases are described:

a = 0°, R = 3.0 x m. In this case the shape traced by the particle flight-paths (before their deviations due to the air velocity field) is cylindrical. The capture efficiencies have practically the same values shown in the previous case (Ill) (Figure 9);

a = 30°, R = 3.0 x m. In this case, while for particle diameters between 0 and 50 pm, the efficiency trends are similar to those reported for the previous case (Ill), for particle diameters greater than 50 pm the capture efficiencies are always higher than 70% for w 2.0 m/s and higher than 85% for w 3.0 m/s;

a = 60°, R = 3.0 x m. In this case the capture efficiencies are in general significantly lower than those obtained in the previous cases for a = 30" and 60". Anyway, the values are in all cases higher than 30% for all the investigated emission velocities and particle diameters;

a = 60°, R = 2.0 x m. In this case the capture efficiencies never reach the unitary value (in contrast to what had been observed before). The relative trend shapes are the same as in the previous cases but for particle diameters close to 100 pm and for 2.0 w 5.0 m/s the efficiencies are lower than 30%.

a = 60°, R = 6.0 x lo-' m. For this case the results are displayed in Figure 11. They are the

worst obtained for the multiple conic emission. The capture efficiencies start from lower values than the previous ones, they never exceed 90% and for w 3.0 m/s they fall below 20%. Particle flight paths for D = 100 pm, w = 7.0 m/s from the superior base of the cylindrical vessel are displayed in Figure 12. Here it is also possible to observe the particle collisions against the working plane edge under the hood.

CONCLUSIONS In this paper the results of a numerical simulation performance for a push-pull local ventilation system are presented. The system design configuration and the basic operating conditions are obtained following the ACGlH [14] recommendations. The simulations are carried out for a system: (a) without obstacle in its system working plane; (b) with a cylindrical vessel located in the system's plane centre. Other causes of disturbance such as external airstreams, operator movements and air motions due to convective thermal phenomena are not taken into account.

According to the standards, when no obstacle is placed on the working plane a uniform velocity field is detected for a wide air region. The influence of the push-jet is in this case limited to the zone under the push and suction openings. Here the air velocity reaches 15% of the average value in the exhaust hood conduit. Up to this air region remarkable velocities are achieved just in front of the hood face. As to the case with the cylindrical obstacle between the push jet and the suction opening, in the area where the obstacle is positioned, a directional velocity field is observed with higher air velocities in its upper region. Here, the air velocity reaches 40% of the average value in the exhaust hood conduit.

For these two system configurations the capture efficiency for dust particle diameters ranging from 1.0 to 100 vm and for emission velocities between 1.0 and 7.0 m/s is assessed in the following cases:

(I) singular vertical emission from the system work plane;

(11 ) distributed vertical emission from the system work plane;

(Ill) singular vertical emission from the cylindrical vessel ;

( IV) multiple emissions from the cylindrical vessel.

In the first two cases the efficiencies are in general very low never exceeding 25%. Although the flight-paths of the investigated dust pollutants never reach the operator's breathing zone the considered system's operating conditions generate a lot of pollutant deposit on the working plane. For the other two cases the capture efficiencies are vice versa on average higher, being in general never lower than 15% and with


Figure 11 Capture efficiency for multiple conical emission from the vessel.

Figure 12 Particle flight paths for multiple conical emission (D = 100 pm, o = 7.0 mls) from the superior base of the cylindrical vessel.

unitary peak values for particular operating NOMENCLATURE conditions. The presence of the obstacle located A area of the suction opening (m2) in the system working plane centre allows full control D particle diameter (m) of pollutants. Also here, for all the simulated D, hydraulic diameter of the slot opening (m) particle diameters and for all the considered k specific kinetic energy (m2/s2) emission velocities, the particle flight-paths never L system plane length between the push jet and reach the operator's breathing zone. the hood (m)


m number of emitted particles (-) n number of captured particles (-) p pressure (Pa) R radius of the truncated cone minor base (m) Re Reynolds number (-) Sij mean strain-rate tensor (I/s) ui air velocity component in the direction xi (m/s) V air velocity (m/s) V exhaust air volume flow rate (m3/s) x, y, z coordinates (m)

Greek symbols a angle of particle emission (deg) E turbulence dissipation rate (m2/s3) -q capture efficiency (-) Y air kinematic viscosity (m2/s) p air density (kg/m3) w particle emission velocity (m/s)

Superscript - average * dimensionless

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Push-pull ventilation system: a CFD approach for the performance analysis

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