Numerical Simulation of Critical Velocity in Ventilation

Given the importance of the safety and health of passenger’s in underground tunnels, analysis and simulation of fires in tunnels is to design a ventilation system. Longitudinal ventilation systems are widely used in tunnel ventilation and are one of the most important parameters for fire safety in this type of critical velocity system. Critical velocity of ventilation is minimum longitudinal velocity flow of air that prevents the smoke from the fire to the upstream flow. In this study, the effects of the distance between the fire source of the tunnel, obstacle before and after the fire, several sources of fire in the tunnel and ventilation shaft tunnels will be discussed on the critical velocity. Each effect parameter according to a correlation analysis of numerical results obtained using the solver FLUENT software Airpak and then the relationship between dimensionless critical velocity and dimensionless heat release are stated.


Introduction
Due to increasing population growth and urban and inter-urban roads need to use the tunnels to reduce traffic and shortening the path.There are 1,500 kilometers of tunnels used in Europe.(Haack, 2002) Although the issue of tunnel safety is always an important issue in the design of tunnels.There will always be a risk that the issue of fires that because of lack of oxygen may cause serious consequences in case of fire (Nfpa, 2003;Rahmanian et al., 2014) This phenomenon is causing hazards to life and property in many years.Events such as this can be a fire in the Baku and Daegu underground tunnels South Korea that resulted in 289 deaths and 189 people.(Carvel et al., 2005) Hence safety of the tunnel at the time of the fire and smoke from a fire flow control is very important.
Fire a complex distribution of energy as light and heat that field position is examined in two models.By starting a fire in the tunnel, a column of smoke due to the buoyant force moves towards the roof of the tunnel and then expands symmetrically on both sides of the tunnel.(Hurley et al., 2015;Moghaddam et al., 2016) This includes non-linear interaction phenomena of turbulence, chemical reactions, heat and light in the area of fire.(Hanson et al., 2000;Shayesteh, 2015) Fire in a closed area is advanced form of multiple types and amounts of fuel, and the amount depends on the level of emissions.Fire in closed environment factors influencing the growth can be mainly two parameters, environmental characteristics and fuel parameters.Stages of fire is divided., according to fire the combustion curve, area growth, fully developed fire and the collapse of the course (Karlsson & Quintiere, 1999) The worst conditions in terms of safety and complexity, is the fire fully developed.At this point the heat release rate is the maximum rate of smoke production and thus the highest content of its own and to prevent back flow smoke ventilation rate is needed.So if the ventilation rate is determined based on the maximum heat release rate, another heat release rates also flowing back.So this scenario for a fire in the tunnel was considered in this study.
In longitudinal ventilation, smoke exit strategy creates a flow of air in the tunnel is long, so that smoke and hot gases forced to move in the direction of the tunnel and is blown in the direction of flow.Thus upstream from the pollutant source of fire and hot gases and toxic secure and safe way for passengers departing and arriving officers caused the fire source.So determining the capacity of the system is important, because if system capacity is low smoke and polluted air in the opposite direction of the air flow created by the ventilation system and fire source moves towards upstream.This phenomenon is called smoke streaming minimum ventilation flow rate is called critical velocity, so that the smoke flowing back upstream toward the source of fire prevention.Ventilation critical velocity the rate of heat release from the fire source, intake air temperature, intake air density, heat capacity of air depends on the geometry of the tunnel and tunnel slope.(Li, Lei & Ingason, 2010;Elahinia, 2016) In recent years, several studies have been done in the field of numerical simulation and experimental modeling of fire.Fire phenomena tunnel with mass transfer is a disturbing phenomenon.Physically combustion phenomena due to the complexity of modeling is not very full, for this reason, different research methods have been proposed to test this complex phenomenon, in general, three categories of full-scale, small-scale numerical simulation is divided.
Heat release rate can be associated with a third critical velocity based on the Froude number is expressed as an empirical relationship.(Thomas, 1968) Following the large heat release rate in terms of critical velocity which leads to the correct empirical relationship was examined.(Oka & Atkinson,1995) Tilt impact on critical velocity was investigated and finally a corrective relationship was provided for critical velocity.(Atkinson & Wu, 1996;Moghaddam et al., 2016) The study was conducted due to the geometry of the tunnel on critical velocity that finally critical velocity of the hydraulic diameter as the characteristic length between the known ventilation and the relationship was critical velocity in the tunnel with the effect of different geometry.(Wu & Bakar, 2000) Also, ambient occlusion index closed the tunnel using a small tunnel models and numerical models to calculate critical velocity was investigated.(Kang, 2010) The impact of smoke spread in a tunnel blockage and experimental results on a small scale was reported.Results showed that the presence of obstruction, obstruction tunnel almost as much critical velocity is reduced.(Lee & Tsai, 2012) The results of the study during the critical velocity and the smoke and ventilation shows that length of the tunnel is directly proportional to the size of the fire and ventilation speed and altitude varies inversely tunnel.(Hu, Huo & Chow, 2008) Effect of the fire source was investigated on critical velocity ventilation, so that the results showed that the critical velocity to stretch the fire (gas burner) along the length of the tunnel first increased and then reduced and also the maximum speed fire-square crisis does not happen.(Li & Chow, 2008) The effect of aspect ratio on the temperature distribution of smoke from fire on an experimental basis in the tunnel with a scale of 1 to 20 reviews and also FDS fire tunnel is simulated.(Lee & Ryou, 2005) Studies show that the effects of ventilation rate on the rate of burning of these two parameters are directly related to each other.(Roh et al., 2008;Raad, Moghaddam & Elahinia, 2016) Also, the effect of the fire source was investigated on critical velocity in the tunnel near the critical velocity is increased, (Hu, Peng & Huo, 2008) and near the outlet tunnel decreases rapidly.(Tsai, Lee & Lee, 2011) Also, if a fire occurs by two separate fires source, critical velocity decreases with increasing distance between them.(Tsai, Chen & Lee, 2010) Temperature distribution under the roof of the tunnel by twelve test (Hu et al., 2007) and was just a simulation.(Hu et al., 2006) (using the code FDS) so that the results show that temperature changes exponentially over and under the roof of the tunnel is reduced.The results of the fire simulation with experimental data comparing the accuracy of the FDS code in this case are measured.
Smoke from a fire in floor height of the house and a fire in the tunnel looked was critical velocity, shows good agreement between the simulation results with experimental data.(Tilley, Rauwoens & Merci, 2011) Then, the temperature distribution in a road tunnel ventilation and determine critical velocity FDS codes were examined.(Niknam, Madani & Salarirad, 2012) The effect of the slope and lateral position on the source of the fire in the tunnel under the roof of the tunnel was investigated how the temperature distribution.(Fan et al., 2013;Hu et al., 2013) In a study, fire plume is devoted as a source of input current to this area.Low control volume, is input source.Due to the small number of areas in the computing equation modeling (heat and mass transfer) assuming a uniform and ideal gas is used for each region.(Novozhilov, 2001) Optimum ventilation system in case of fire by computational fluid dynamics simulation performance was evaluated.(Yuan & You, 2007) Fluent in simulation accuracy Commercial Code longitudinal ventilation compared with experiments performed in a road tunnel were approved.(Vega et al., 2008) Numerical simulations of two road tunnels with a central route for escape in case of fire were studied.(Modic, 2003) The rate of heat generation in a fire in the tunnel New York was studied using numerical calculations.(Willemann & Sanchez, 2002) Comparison of one-dimensional model for areas where the developed, to assess critical velocity air flow ventilation was provided.In this model one-dimensional method is developed for areas where heat flow is used and in other three-dimensional fluid flow equations solved in detail.(Colella et al., 2009) Critical velocity air conditioning and heat release rate of two parameters on the distribution of smoke were investigated.This study shows that contrary to the critical velocity, the heat release rate has very little effect on the distribution of smoke.(Kunsch, 1998) In reviewing that factors slope and it was in tunnel area is shown, how to spot the source of the fire cannot correctly predict the spread of smoke.Hence fire was considered as areas for the location of the fire were distinguished in different cars.(Chen et al., 2003) In a study on the efficiency of the ventilation system in fire, found when the fire exits in smoke at the top of the fire should be opened, Smoke outlets and opening fire on both sides of the output of fresh air in the bottom of the fire and ventilated system performance.(Ballesteros et al., 2006) Researchers provide a way to slow down the critical velocity of ventilation, smoke extraction points around the tunnel was built that these points of smoke extraction in the truth as an intermediate shaft tunnels that makes smoke come out of the shafts embedded in the roof of the tunnel.Finally, using numerical methods to reduce the amount of critical velocity depends on the number of points and their distance to the source of the fire.[42] The dynamic effect of floating in a tunnel with longitudinal ventilation in order to study the effect of the fire source was investigated on critical velocity ventilation.
Results in such a way that non-dimension critical velocity has any clear dependence on the diameter of the fire source, the current density Buoyant and not ambient air.(Le et al., 2014) Driven flow profile of the output of longitudinal ventilation in tunnel roof to improve the performance of the fire was investigated.The numerical results of this study suggest that the use of exit points in roof of the tunnel efficiently reduce air pollution through the tunnel.(Harish & Venkatasubbaiah, 2014) The use of hybrid ventilation to create a safe environment in the upstream and downstream of the fire in the tunnel tested experimentally.If the proper distance and shafts in the ceiling there is a fire place, there will be conditions downstream of the press.As a result of smoke in the tunnel height even with the use of different shafts in roof of the tunnel remains constant.
Hence experimental value for predicting the amount of smoke in conditions of high heat release rate was presented.(Tanaka et al., 2015) The critical velocity and Back flow by one-tenth-scale analysis of results and solving computational fluid dynamics were studied.Hydraulic diameter parameter as a variable was chosen to identify different tunnels.(Chen et al., 2015) In this study, the effect parameters, away from the fire source of the tunnel, obstacle before and after the fire, existence are several sources of fire in the tunnel and ventilation shaft between tunnels will discuss on the critical velocity.Each effect parameter according to a correlation analysis of the numerical results and the relationship between dimensionless critical velocity and dimensionless heat release stated.It is worth noting that in this study only investigated the effects of fire and combustion phenomena phenomenon is not modeling.

The Governing Equations
Due to costly real-scale studies are usually performed in very special cases.That's why researchers emergency ventilation system design, use of numerical simulation.Field models as models Computational Fluid Dynamics (CFD), also known as computers with high computing capacity they need.
In this models, first the modeling area to be determined.Then boundary conditions are applied to restrict the solution.Governing equations, the equations of conservation of mass, momentum, energy and species that are solved together in pairs.Thus, time averaging is done on equations (RANS) or location (LES).
Depending on which of the above two approaches used in discrete equations, algebraic equations of the form is changed.In the first approach in the entire range of computing and in computational cells of equations of mass, momentum, energy and species distribution modeling solution and are including Reynolds stresses.In the second approach, conservation equations in the computational domain larger than the size of the filter cells directly solved and in smaller the size of the filter cells, Reynolds stresses are modeled.
Governing equations using time averaging in k-ε turbulence model are as follows.In this study was to investigate the phenomenon of fires in tunnels and the use of computational fluid dynamics Conservation equations (momentum, energy and mass) using FLUENT solver Airpak is done in software.

Conservation of mass equation:
(1-3) Momentum conservation equation: (2-3) In the above equation, such as the Reynolds stresses are calculated according to the following equation using the approximate Bouzinsk: In this equation, k represents the kinetic energy of turmoil and turbulence kinetic energy and ε represents the dissolution rate are defined in accordance with the following formula: turbulence viscosity and equation expresses the conservation of energy as: (5-3) In the above equation (T ij ) eff represents the effective stress tensor is expressed according to the following equation: ( ) K is coefficient of thermal conductivity and Pr isturbulence Prandtl number and amount is 0.85.As well as = + represents the total energy per unit mass in which the internal energy per unit mass u and v is velocity.Also effective viscosity expressed as μ = + .Turbulent kinetic energy equation and its demise rate is expressed as follows.(8-3) In the above equation, D represents mass distribution and S c represents the source and species including the penultimate equation for the following models: In  The fire sourc nd it is equal to om the walls of      Results can be expressed using the critical velocity without a chimney ventilation in the tunnels (0.48 meters per second) and non-dimensional equation , = −0.0152+ 0.19 + 0.5.This equation is important in terms of analysis, because it can be based on the distance at which the slowest speed required ventilation is critical, according to acquire the base tunnel.If that is the least critical velocity ventilation, energy consumption is also reduced.

Conclusion
According to the simulation results, the effect parameters of the tunnel away from the fire source, existence barrier before and after the fire, existence several sources of fire in the tunnel and ventilation shaft of the tunnel on critical velocity was investigated.The results are the effect of the fire on the wall shows the critical speed, the distance of the fire source from the wall is less, more critical velocity ventilation and ventilation rate with increasing distance from the wall of a critical source of fire is reduced.The temperature distribution in side view of the fire source in a manner that is visible in higher altitudes.
In other words flowing moves toward higher altitudes and upstream.Also numerical results show that the effect of the source of the fire source, existence barrier causing ventilation flow resistance and require more ventilation flow.The speed amount is equal to 0.7 (to prevent fire four times) and 0.62 (To prevent fire twice) meters per second, respectively.Then, when the fire was prevented after the critical speed of 0.62 and 0.6 respectively ventilation is achieved.
existence two sources of fire in the tunnel was the result of the review, existence two sources of fire near each other to strengthen the fire source and to overcome the crisis quickly ventilation should be increased.Whatever ignition sources are to be cut apart is required critical velocity.Finally, existence a chimney near the fire caused smoke and heat conditions at the exit of the tunnel and the distance from each source is high chimney fire, the chimney effects existence reduced.
is the kinetic energy produced and constants in the equation just = 1.44 ، = ، = 0.09 ،σ = 1 ‫ﻭ‬ σ = 1.3 are expressed.Such contamination is the transfer equation as follows. Fig

F
Figure 6.Temp

Table 3 .
Existence effect of two sources on critical velocity in tunnel ventilation