NUMERICAL SIMULATION OF A FLOW INDUCED BY A FIRE IN A COVERED CAR PARK. Author: Neidy G. Veiga 1. Thesis Supervisor: Pedro J.

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1 NUMERICAL SIMULATION OF A FLOW INDUCED BY A FIRE IN A COVERED CAR PARK Author: Neidy G. Veiga 1 Thesis Supervisor: Pedro J. Coelho 1 1 Dep. de Eng. Mecânica, Instituto Superior Técnico, Universidade Técnica de Lisboa Av. Rovisco Pais, Lisboa, Portugal s: neidy.galvao@ist.utl.pt, pedro.coelho@ist.utl.pt Keywords: Car parks, jet fan ventilation, smoke control, thermal plume, numerical simulation, Fluent. Abstract. In the present article, results are presented in the frame of a project about the analysis of the jet fans efficiency inside covered car parks. Normal conditions are tested, in which the pollutants made by the internal combustion engines of the vehicles circulating in the park is evacuated, as well as fire conditions, in which the smoke evacuation is simulated. Three fundamental cases were studied: free jet; deflected and not deflected in order to analyse the influence of the deflectors in the flow; wall jet; and the interaction of the wall jet with the thermal plume resulting from a fire. In the two first cases, the flows are isothermal and steady while the third case concerns an unsteady and non-isothermal flow. In all cases, the flows are turbulent. Numerical simulation with the commercial software Fluent was compared to the experimental data available. These were provided by the National Laboratory of Civil Engineering (LNEC), which is in partnership for the project with the Instituto Superior Técnico (IST) and various companies. From this project, the following results are expected: The software validation for the flow prediction inside a covered car park; Proposition of guidelines for eliminating some eventual risks caused by this ventilation systems; Evaluation of the smoke emission impact from a fire which will allow to predict with a better precision the eventual risks for the occupants; Analysis of the assumptions which can be made for optimizing the numerical simulations; The main objective of this work is the optimization of the jet fans systems for the smoke evacuation and for increasing its safety. The key point is to ensure the tightness of the smoke without creating physical barriers which difficult the occupants evacuations and the emergency services actions. 1. Introduction The covered car park configuration frequently requires that the enclosures are ventilated mechanically, since the natural ventilation may not be enough for ensuring safety conditions and space habitability. The jet fans systems in covered car parks represent an attractive alternative which is more and more used by the designers and the engineers. However, it is a relatively recent model, and some factors must be taken in consideration to choose this configuration instead of the traditional system. The air quality inside the covered car park is an important parameter in this project, and the regulation limits the maximum level of carbon monoxide (CO ) which is a function of the sampling period considered, i.e., the averaged value of 5 ppm in 8 hours period, 1 ppm in minutes periods, and the instantaneous value of ppm. In some cases, the percentage limit of CO present in the air and limits concerning the concentration of others pollutants are also imposed, like for instance for the nitrogen oxides ( NO x ). Moreover, the installation of sensors able to detect other prejudicial substances for the human health may be necessary, mainly when the diesel vehicles concentration is above 3% of the total area of each floor of the park. The experimental data available for that kind of flow are rare, so the LNEC initiative for the development of this project is important for a better understanding of the behaviours of the fire

2 and the flow which may be helpful to the competent authority to establish better regulation norms. Actually, there are no jet fans project laws, neither for the pollution control, or for the smoke control. The technical norm for safety in fire situation scenario (Port. 153/8, December 9), in the frame of the occupation type II (parking), some requirements are imposed, like the partitioning of the park area. However, this norm does not concern the jet fans, and the conception and design rules of this kind of systems are not clearly defined. So there is a need to create a set of rules validated by the scientific community and that can be used by the designers required. In normal condition, the jet fan flow system objective is to avoid the accumulation of toxic gases emitted by the vehicles, which otherwise may reach a high and dangerous level for the human health. For the dilution, new air is inserted inside the park, and is mixed with the gases emitted by the internal combustion motors. In fire scenario, a bigger air flow is blown and the jet fans should work at their maximum speed. However, a bigger velocity of the flow doesn t always mean better operation conditions. More air could feed and increase the fire. On the other side, if the longitudinal air velocity is lower than the critical velocity, the smoke could propagate to the upstream, causing a dangerously increase of the air/gases temperature in the vicinity of the roof with the time and with the fire spread (S.M. Li, J.;Chow, W.K, 3). One main advantage of the jet fans system is its flexibility. The fans can be activated individually such that a rapid and efficient answer can be made, according to the local and instantaneous necessity of each section of the park. Nevertheless, with the use of that type of system, it is possible to prescind the fire partitioning, like regularly imposed in the traditional ventilation system, and so using all its advantages. However, if it is necessary to use expensive solution for the mitigation of the fire risk, the system is no more economically attractive compared to the traditional ventilation system. Another point to be considered is the bigger costs for the maintenance. The system efficacy depends on various factors. For instance: The potential stagnation zone and the fluid recirculation, caused by the interaction between the jet fans end with the obstacles present in the park and/or by the interaction between the thermal plume, should be treated very carefully; Constructive solution allowing a better smoke evacuation of the disaster area by significantly bigger mass flow than those implied by the usual ventilation shall be adopted. The possibility of jets that create positive pressure on the peripheral walls of the car parks must be avoided. Sometimes, it may be necessary to maintain the jets fans close to these walls inactive. These positive pressure may overlap the depression caused by the exhaustion fans, reducing its efficiency The fans arrangement should be carefully studied, such that the impulse action of the fan over the air mass doesn t create vortices, which in case of fire imply unpredictable behaviour of the flow and the smoke may contaminate the upstream zone as well as the downstream fans zone.. Theoretical Formulation.1. Conservation equations for a turbulent jet The study of the flow in a car park and the simulation of the interaction between the thermal plume and the wall jet imply a three-dimensional flow, with heat transfer, smoke propagation and various other physical processes. It is a complex flow and the relevant quantities may be described by the transport equations for the mass, momentum and energy. The Reynolds transport theorem is applied in the formulation of the mass, the momentum and the energy conservation equations. The theorem expresses the conservation of a variable φ according to the following equation, applied to a fixed arbitrary control volume:

3 dφ = d r r ( dv) + ρφ( V n)ds ρφ (1) vc sc In Eq. (1), Φ is an extensive quantity, φ the corresponding intensive quantity ( φ = dφ dm or Φ = ρφ dv ), ρ is the density, V r the flow velocity at the surface s, dv is an elementary control vc volume, and n r is the normal vector of the elementary surface ds. vc and sc represents the control volume and the control surface, respectively. Replacing Φ by the mass (), the linear momentum (3), or the energy (4), it is possible to express the basic laws of the fluid dynamics in a control volume. In the same way, the conservation equations for the species are obtained by replacing Φ by the mass fraction of each species (5). dm r r ( dv) ( V. n)ds d = = ρ + vc ρ () sc r dmv d r r r r = F = ( Vdv) V ( Vrel n)ds ρ + vc ρ. (3) sc de = dq dx = dw d = d r r ( edv) ρ e( V. n)ds + ρ (4) vc sc r r ( xdv) ρ x( V. n)ds + ρ (5) vc sc The equations which describe the turbulent flow are formally the same as for the laminar flow, except that the resolution becomes much more difficult to handle. The turbulence is a very complex physical phenomenon, and consequently, its modelling is a very large problem. However, in this study, the turbulence is modelled by two parameters, the kinetic turbulent energy k and its dissipation rate ε defined below, i.e., In this project, the turbulence model applied is k ε. This model points relative advantage over the k ω model, when the velocity profile at the jet fan outlet is similar to an experimental profile (Berg, 6). However, both models require approximately the same computational cost, implied by two additional equations. ( u + v + w ) 1 k (6) ε ν ϕ i u ϕi + ϕ i v ϕi + ϕ i w ϕi, ϕ = x, y, z i (7) 3

4 .. Numerical model The software Fluent solves the equations with the finite volume method. The mesh is generated by Gambit. The second-order upwind scheme is used for the discretization of the convective term. The algebraic equations, deduced from the discretization of the differential equations involved in the problem studied, were solved by using a corrective pressure algorithm. SIMPLE has been used in the case of isothermal steady flows, while PISO has been applied in the case of unsteady non isothermal flows. The jet fans are composed by a cylindrical body, an axial fan localized inside the body, and in the middle of the body, by acoustic attenuators which are placed on both sides of the axial fan, by a protective grid, by flow deflectors located immediately to the exit and by an electric motor. Since the flow details in the neighbour of the fan are not relevant for the objective of this work, namely the study of the smoke dispersion in a car park, the fans were simulated by two simplified methods. In the first method, more rigorous, the fans are assumed as circular pipes of thickness 5 cm, the blades crown are represented by a circular flat surface where the momentum is imposed for the used fan, i.e. the change in air pressure due to the fan. This assumption allows to reduce significantly the complexity of the mesh without compromising the results. The flaps are represented by flat surfaces without thickness, with the only function of deflecting the jet. The rotating air developed by the fan, even if it can be easily simulated, was not considered, since the protective grid of the exit fan and the flaps substantially reduce the swirl. In the detailed method, the mesh required to simulate the conduct and the blade is too much refined in the fan zone, which complicate the simulation for a domain with big dimensions, like a car park. So, the second method, more simple, replaces the jet fan by a momentum source, which allows to use a coarser mesh in the fan zone. The numerical value of this source will have to be obtained by trial and error, in order to reproduce as far as possible, the smoke dispersion through the car park in case of fire. This process of trial and error hasn t been applied yet, and the results presented in the following section for the second method, may be seen as a first approximation which may be improved when others values of the source momentum will be tested. Total pressure conditions were imposed at the entry of the simulation domain, and the static pressure has been assumed equal to the atmospheric pressure at the exit. In the solid surface, wall laws were used. In the case of a fire outbreak, it was defined by specifying the released power and the volume which generates the corresponding energy. 3. Results In the current project, the following cases were studied: free jet without deflectors (JL), in which the flow is axisymetric and the mesh is two-dimensional; deflected free jet (JLD), in which the fans have at the exit of the cylindrical body three flow deflectors; deflected wall jet (JTD) in which the fans are represented like in the preceding case, i.e., a detailed model which presents three deflectors at the exit, and in this case the fan axes being at.3m of the park roof; interaction between a wall jet and the thermal plume resulting from a fire (JTD+PT) in which the flow is unsteady and non-isothermal. At the initial instant, the fan is off. After a few time, 8s, when the sensor detects the fire (simulated by a prescribed heat source), send a signal to start the fans and, the wall jet flow induced by the fan will restrict the smoke flow propagation; Deflected free jet using a momentum source which replaces the fan (F-JLD). The adoption of a source momentum to represent the fan has also been tested for the deflected wall jet and for the interaction between the deflected wall jet and the thermal plume. They are a set of simplified models and the corresponding acronyms are identical to those used for the detailed models, except that they are preceded by the letter F. In the Figure 3.1, the mesh used to represent the fan is displayed. The fan axe is localized along the x coordinate, and the vertical is along the y coordinate. The flow is symmetric relatively to the vertical plane because the swirl, caused by the rotating movement of the blades can be neglected. 4

5 Plano do ventilador Deflectores y z x Figure 3.1 Mesh used for the wall jet simulation The first simulation has been done for a free turbulent and isothermal jet, caused by a jet fan of 54 N (fan B), without deflector. The mesh is axisymetric (two-dimensional). The results show that the dispersion rate of the jet, S, is.17 using the standard k-ε model and.95 for the realizable k-ε model. The second value is in good agreement with the experimental values:.96 (Panchapakesan, 1993),.1 for the hot-wire data (Hussein, 1994) and.94 for the laser-doppler data (Hussein, 1994). It was also seen that in the selfsimilar region, the non-dimensional axial velocity profile overlapped, as expected. In agreement with the axisymetric turbulent jet theory, without vortices, in the self-similar region the averaged value of the maximum velocity, u max, normalized by the averaged value of the velocity in the exit section of the jet, u o, may be expressed as: umax ( x) ko = u x ro (8) ou umax ( x) ko u ( x xv ) ro (9) where k is decaying velocity constant at the jet axe and r is the fan radius. Eq. (9), taken from (B. Pope, ), is similar to Eq. (8), except that a corrective term, x v, is introduced. It represents the longitudinal gap of the jet virtual origin and the exit of the fan. The next simulation was the flow induced by a jet fan with a deflector at the exit, turning the simulation in three dimensions. The results are presented in Table 3.1. In Figure 3., the decaying of the jet velocity along the axe for the fan A is displayed (for the fan B, the results obtained was similar). The decaying of the jet has been predicted with precision, although the theoretical curve is obtained without deflector. This agreement is probably because the small deflector angle,θ, has a marginal influence on the velocity profile. 5

6 1..8 Teórico Numérico Umax/U x/r Figure 3. Profiles of the x-component of the velocity along the jet axe. Fan A Fan B M1 M M1 M Re r.1575m.m θ.5º 5 º S k x m m v Table 3.1 Jet characteristics M1: Based on Eq. (8) M: Based on Eq. (9) The experimental value of k varies between 6.6 (Panchapakesan, 1993) and 5.8 for the laser-doppler data (Hussein, 1994). In Table 3.1, it can be seen that the numerical values for the fan A and B are in good agreement with the experimentals data. The angles of the jet deflection are 1.9º and 3.91º for the fan A and B, respectively (lower than θ as expected). The results in obtained, show that the velocity profiles are still axisymetric, even with the presence of the deflectors. This behaviour can still be explained by the small angle of the deflectors. The simulations made also shows that the jet dispersion rate is a little higher than the values obtained when there is no deflector. Figura 3.3 displays a comparison between the radial profiles obtained numerically and the experimental data obtained for the fan A at the beginning of the project. The velocity profiles, normalized by the maximum velocity, function of the radial coordinate normalized by the half-wih of the jet, b, are identical because they are in the self-similar region of the flow, and they are in good agreement with the experimental data. 6

7 U/Umax x=16m r/b Figura 3.3 Radial profile of the x-component of the velocity normalized by the maximum velocity, at the planes normal to the jet fan A axe (line numerical prediction; symbol experimental data). The comparisons between the detailed case and the simplified case are displayed in Figure 3.4 and Figure 3.5. Results obtained with the JLD model (more precise) are very accurate when compared to the characteristics given by the manufacturer, namely the draining mass flow and the velocity profile at the fan exit. The velocity field obtained with the simplified model, F-JLD, shows in general, higher values than those predicted by the detailed model. However, for the fan A, the predictions obtained with the two models are very close. This coincidence is a little fortuitous, given that only one simulation has been made for the simplified case, without evaluating the momentum source by a trial and error technique. Although the rate of flow is similar in both cases, the agreement between the obtained predictions is not guaranteed, like in the predictions concerning the fan B (not shown). There are others parameters which guide the flow and that have to be taken into account for a better coherence of the simplified model results. The analysis of these parameters, like for instance the influence of the intensity and the direction of the momentum source, must be evaluated in future work relative to the project. This is necessary to ensure that the jet dispersion rate, as well as the dragged fluid flow, remain similar to the prediction made by the detailed model y (m) u (m/s) 1 1 x=m x=4m x=8m x=1m x=16m x=m Figure 3.4 Profile of the x-component of the velocity of a free jet for the fan A (line JLD; symbol F- JLD)

8 y (m) u (m/s) 1 1 x=m x=4m x=8m x=1m x=16m x=m Figure 3.5 Profile of the x-component of the velocity of a wall jet for the fan A (lines JTD; symbols F-JTD). A typical situation of of fire scenario, i.e., the interaction between the thermal plume and the flow induced by the fan, has then been simulated. The regime is unsteady, and the thermal plume is originated by a heat source of power kw. The volume of this source is.8 m 3 and is located at 1 m fron the fan. In Figure 3.6 and Figura 3.7, temperature and x-component velocity profiles, respectively, are displayed along various parallel lines to the jet axe, in the same vertical plane, after a time allowing the steady regime to be reached. The temperature predictions with the simplified method (F-JTD+PT) present significant difference with the detailed method, especially for the profiles closer to the fan, which reveals the need to adjust the momentum source value. Regarding the velocity profiles, the predictions obtained by both methods are close. The discontinuity observed close to x = m, in the profile along the fan axe, is due to the presence of a deflector. Consequently, it is observed only in the first case. It can be also seen a local increase of the velocity in the heat source zone for the profile close to the ground, which indicates a small deflection of the thermal plume induced by the wall jet flow. 8

9 y=-m y=-1m y=m y=1m eixo ventilador (y=1.7m) T (K) x (m) Figure 3.6 Temperatura profiles along the x direction, in the vertical plane, for the fan A (lines JLD+PT; symbols F-JTD+PT) y=-m y=-1m y=m y=1m eixo ventilador (y=1.7m) u (m/s) x (m) Figura 3.7 X-component velocity profiles along the x-direction, in the vertical plane, for the fan B (lines JLD+PT; symbols F-JTD+PT)

10 4. Conclusion This work takes place in the frame of a project about the efficiency of the jet fans in covered parks in case of fire. In parallel, experimental works are in progress at the National Laboratory of Civil Engineering. Numerical simulation of a turbulent and isothermal jet as well as the flow resulting from the interaction of a wall jet with a thermal plume due to a hypothetical fire has been studied. Two distinct methods were tested to simulate the jet fans. One, more rigorous, simulates the cylindrical surface, where can be found the axial fan and the deflectors at the exit, and provides the variation of pressure in the fan. This first method was used as reference for the second one, which simulates the fan and the flaps through the imposition of a momentum source. The simulations gave good results for the jet dispersion rate and the decaying of the velocity along the jet axe, and they were in good agreement with the velocity profiles obtained from experimental results. These results shown that the theoretical values for an axisymetric turbulent jet without deflectors are close to the computational models predictions, which means that the deflectors, due to the very small angle, have a poor influence on the velocity profile, even though they modify the flow. The simplest method, which has also shown satisfying results, can still be improved. Although there is some quantitative disparity between the detailed and the simplified models, it is quite clear that the results are qualitatively similar, which turns the simplified model to be a possible option, if the momentum source is prescribed appropriately. Moreover, the interaction between a wall jet and a thermal plume has been simulated. In this case, there is no theoretical reference values, neither experimental result which allow to evaluate the results precision. That s why an experimental study is in progress in the frame of this project. Consequently, only a comparison of the results between the two methods used to simulate the fans could be developed. The predictions obtained with the simplified method are not as accurate as in the case of an isothermal jet, but the systematic study of the intensity of the momentum source should improve the agreement between the two models. Therefore, the simplified methodology allowed an adequate evaluation of the fans systems, based on jet fans, avoiding the use of more complex and computational demanding methods. REFERENCES Berg, J.R.; Ormiston, S.J.; Soliman, H.M. - Prediction of the flow structure in a turbulent rectangular free jet, International communications in heat and mass transfer, Elsevier 33, (6). B. Pope, S. - Turbulent Flows, Cambridge University Press (). Godinho Viegas, J.C - Utilização de ventilação de impulso em parques de estacionamento cobertos, LNEC (7). Hussein, H.J.; Capp, S.P.; George, W.K. - Velocity measurements in a high-reynolds-number, momentum conserving, axisymmetric, turbulent jet, Journal of Fluid Mechanics, Vol. 58, pp (1994). Panchapakesan, N.R; Lumley, J.L - Turbulence measurements in axisymmetric jets of air and helium. Part 1. Air jets, Journal of Fluid Mechanics, Vol. 46, pp (1993). S.M. Li, J. e Chow, W.K - Numerical studies on performance evaluation of tunnel ventilation safety systems, Pergamon, Tunnelling and Underground Space Technology, Elsevier Science Ltd. 18, (3). 1