Correlation of underground car park impulse ventilation control and realistic fire scenarios Aleš Jug*, Faculty of Chemistry and Chemical Technology, Aškerčeva street 5, 1000 Ljubljana, Slovenia, Stojan Petelin, Faculty of Maritime Studies and Transportation, Pot pomorščakov, 6320 Portorož, Slovenia, Peter Bukovec, Faculty of Chemistry and Chemical Technology, Aškerčeva street 5, 1000 Ljubljana, Slovenia, *Corresponding author: E-mail:
[email protected]
Abstract Among smoke control systems, impulse ventilation control is the newest one. Ventilation of covered car parks is essential for removing harmful vehicle exhaust fumes and providing assistance to fire fighters by clearing smoke in the event of fire. Impulse ventilation has been used in Europe tunnels for more than 10 years. Nowadays it becomes popular and effective in underground car parks as well. The effectiveness of impulse ventilation can be measured in two ways: the effectiveness of ventilation, regarding to building size and openings in a first stage and effectiveness of ventilation during a fire. In the article efficiency of impulse ventilation will is analyzed on a realistic case fire scenario. Keywords: fire safety, underground garage fires, fire scenario, impulse ventilation systems 1.
Introduction
Ventilation systems of underground car parks are important for control and removing harmful vehicle exhaust fumes. It may provide assistance to building users and fire fighters by clearing smoke in the event of a fire. The system consists of special designed impulse ventilators supported with fire detection modules. Impulse ventilation in general is based on a number of high velocity ventilators in place of the distribution ductwork and located in a place where accumulation of smoke is expected. Impulse fans operate on the same method as well proven tunnel ventilation principles. When operate, they produce a strictly oriented and high velocity jet. It pushes air in the direction of smoke exhaust system and therefore cleans the surrounding air. The impulse fans can be used as a support to classical ductwork smoke control systems or can be installed as an independent smoke control system. Hence the impulse ventilation system has a new application in underground car parks it has to be tested and analyzed on theoretical and experimental basis. Several studies on use of impulse ventilation systems are known. It was found that some of the previous studies analyses jet fans used in tunnels but there were not many analysis of impulse ventilation systems in underground car parks. Impulse ventilation systems are also defined in two standards: British Standard BS7346-7 and standard EN 12101-3. Both standards specify the basic requirements for impulse ventilation systems but don’t define detailed design specifications. However relatively new standards signifies that impulse ventilation is becoming the norm for car parks in the several EU countries.
An impulse ventilation is a system intended for the ventilation of enclosed and basement car parks without the use of ducting within the body of the car park. Impulse ventilation differs from the conventional ducted ventilation system in three principle ways: - ducting is not used. Impulse fans take the place of ducting in providing control of the airflow within the car park, - smoke management and control, which is not usually possible with a ducted system, is a key feature of the Impulse system, - in larger car parks the system is designed for smoke control with the airflow based on a design fire size rather than simply using the air change rates which are referred to in the current building regulations. 2.
Fire Simulations
Although the fire safety computer models are going nowadays through the “judgment” phase, they can still represent good and economically qualified engineering tool. However, when fire safety models are used, a designer must know the model capabilities, limitations and uncertainties. All models are based on physical phenomena; they are abstractions and idealizations with inherent uncertainties that are both qualitative and quantitative. Models can not “predict” the phenomenon as such. They can be used to highlight the trend of certain dependent parameters that in themselves are abstractions and were manifestations of reality. Validation in its simplest form involves a comparison of model calculations with measurements of quantitative variables in a real case. A validation process should determine the degree of accuracy of fire calculation of fire parameters for hazard analysis purposes. It is clear that validation results will be better and closer to a real case if the model input parameters will address the real situations and will represent the “most likely” outcome. The computer code Fire Dynamics Simulator (FDS) was used to carry out CFD simulations. FDS uses large eddy simulation turbulence model. The present fire scenario based on a real building where underground car park (Fig. 1) is in use for building employees and nearby shopping mall users. Car park is equipped with impulse fans.
Figure 1: Garage floor plan
3.
Defining car park fire scenarios
A fire scenario is a generalized, detailed description of an actual or a hypothetical, but credible, fire incident.1 Such scenarios identify chains of events leading to deaths and other fire losses. The fire scenario is mainly just a set of fire conditions. The building fire safety design concept is the solution of more or less well defined predefined variables. The basic geometry simulated in this step is a space 40 m long, 35 m wide and 2,4 m high. Twelve jet fans are placed at the ceiling. Each fire scenario includes same fire load of approximately 4 MW. There were two main fire load locations (Fig. 2). Fire scenario A predict fire location at the wall, while fire at fire scenario B was simulation of an axisymetric plume.
Figure 2: Fire location
An analysis of the expected fire scenario is the initial focus of a performance evaluation. We may identify several steps of an underground garage fire scenario evaluation: a. identify an area or room of origin to start the process of understanding the building – fire in a present fire scenario was located near the walls and in the central part of the garage area. b. assess the barrier properties – garage was assumed to be wide open, with no barriers or smoke curtains. c. select a fire growth hazard for the assessed area or room (Tab. 1, Fig. 3) – fire growth was assumed to be the same as predefined in several car burning analysis. Car as a characteristic fuel was defined separately. Subsequently, a plot of the steady state mass loss rate data as a function of the external flux has utility in determining the heat of gasification, L and the total flame heat flux, q fl . Table 1: Mass Loss Rate for a Typical Car Fire HF, kW/m 20 30 40 40 50 60
2
2
Mass Loss Rate, g/m .s 17 21 24 24 27 32
Steady state mass loss rate 35
SS-Mass loss rate,g/m^2.s
30 25 20 15 10 5 0 0
10
20
30
40
50
60
70
External heat flux, kW/m2
Figure 3: Steady State Mass Loss Rate
d. estimate a fire duration from established burning to full room involvement for the single garage level – only first 10 minutes of fire development were analyzed. Full room fire involvement in large car parks initially starts with a fire in a single car. The fire size at this stage involves initial car and the proximity of an adjacent fuel package such as nearby cars. The process continues spreading to other cars until conditions reach a level where full room involvement or its equivalent is present. While hot layers do form at the ceiling and back radiation does pyrolize unburn fuels, the principal mechanism of fire growth is a direct radiation introduced fire propagation throughout the arrangement of cars in underground garage floor. Some of the car fire test reports are summarized in several articles and analyses.3 Following the measurements and experimental results we can assert that car burning heat release rates are in upwarding trend (Fig. 4). The statement supports Schleich test report where he stated that the amount of heat of combustion from the vehicle made in 1995 doubled in relation to older vehicles.7 Car burning heat release rates 9
8,5
8
7,5
Heat release rate (kW)
7 6 5 5 4 2,66
3 2
1,75 1,33
1 0 a
b
c
d
e
f
Car fire test
Figure 4: Summary of car fire HRR rates.
Heat release rates influence significantly fire growth from ignition to flashover. During this period, many complex phenomena occur. The rates of flame spread, heat release, smoke formation, and flow of fire gases depend on fire environment characteristic. The compartment
effect, size of the compartment, ventilation opening, nature of combustibles, furnishings, and finishing all play important roles in the growth of fire. The dominant mechanism for reaching garage floor involvement is flashover. Flashover in a closed underground car park building is characterized by following: -
Fire starts somewhere underneath a car or in a car. A fire plume forms and a hot layer of heated gases of combustion and soot develops at the generally low ceiling. Ventilation provides sufficient air to support combustion. Hot layer radiation and some radiation from the flames pyrolize other unburned fuels (adjacent cars) in the garage floor. Ignition of adjacent cars in the garage floor occurs.
The described fire development requires some time, depending on floor size and fire load. A system designed for smoke clearance ensures that the air and smoke throughout the car park are mixed, and flow generally towards the extract points. This means that the whole car park may become filled with a diluted smoke/air mixture, just as it can with traditional, natural or mechanical ventilation. 4.
Jet fan location in influence on fire growth
In a typical underground car park building structure common fire scenario can be a car ignition, where we may assume that gasoline or diesel fuel will present one of the primary fuels. Fire growth to the ceiling point generally depends on fuel, room or floor characteristics and ventilation. The important room and floor details are ceiling height, proximity of flames to walls and insulation. Ventilation factors are size and location of openings and active fire protection measures such us heating, ventilation and air-conditioning operation. Fuel factors of major importance are fuel package size and fuel location and arrangement. Environment factors are smokiness and radiant heat transfer. Floor details that influence fire growth to fool room involvement are ceiling height, length to width ratio and room insulation. Ventilation as the last factor is influenced by openings (location and size), heating, ventilation and air- conditioning operation and venting in case of fire. For the present computer CFD analysis all together 12 jet fans were used (Fig. 5). A jet fan is represented as a constant volumetric flow rate device. The flow induced by the jet fan in the car park was held constant.
Figure 5: Summary of car fire HRR rates.
Detailed input parameters for a typical car park garage fires can be obtained from building fire design documents, statistical data and experienced engineer. 5.
Influence of impulse ventilation and fire
In the present fire scenario jet fans were located as shown in figure 5. Though cold test was performed in different underground car park, only one row of jet fans was used during a calculation and analysis. An impulse system designed to assist fire service access will keep part of the car park free of smoke. The control system detects where the fire occurs and starts only those impulse fans and extracts fans that will move the smoke directly to the nearest extract point and remove it, without recirculation through the car park. The accuracy of a fire model may be assessed by its ability to predict the results of actual experimental data.2 It was shown many times that some correlations over predict temperature rise while others tend to under predict it. It is important to understand how variation between predicted and measured values affects the use of correlations and models for performancebased design or evaluation. In the present analysis and scenario fire detection and therefore starting up process for jet fans wasn’t analyzed. Fans started automatically at point where fire produced energy level reaches 10 kW/m2. Despite of model accuracy, several key factors must still be developed: standard fire scenarios with more cars, different ways of fire detection (for ex. CO detection) etc. 6.
Results and Validation of Simulations
Cold Flow Tests During the analysis, a number of cold flow tests were performed in which different combinations of jet fans were operated and bulk flow measurements at each loop were recorded once the flow stabilized. In tests with the same number of operating fans, different combinations of fans were used to identify any differences associated with location of jet fans or between fans operating in parallel or series arrangements. Some fans were located close to instrument loops and it was conceivable that their performance could be adversely affected by local obstructions. Cold test were performed in different location (Fig. 6), but with the same
jet fans. For the cold test, four jet fans were running together and analyzed. Underground car park with jet fans was equipped also with classical ducted smoke control.
Figure 6: Cold smoke test
Temperature profiles Cold tests showed a substantial influence of jet fans while they operate as an addition to a regular smoke control. Smoke behavior during a cold tests was similar (Fig. 7) than a smoke flow calculated with CFD computer codes.9
Figure 7: Comparison of a smoke velocity
CFD code was validated using an Alpert correlation, where user can calculate the temperature and velocity field of an unconfined ceiling jet from a turbulent fire plume. 2 Tmax. - T 3 6.18Q h T
Q Tmax. - T 1.97 h r T h
, 2
r 0.18 h
3
,
r 0.18 h
In the present equations T max. is the ceiling temperature, T∞ is the ambient temperature and Qh is non-dimensional heat release, defined as:
* Q h
Q
c pT gh h 2
Comparison between calculated and modeled temperature profiles is shown in figure 8.
Figure 8: Comparison of a temperature profiles
It is shown that horizontal cross sectional calculated temperature is consistent with modeled temperature profile. 7.
Conclusions
At the present analysis several assumptions were made. Fire ceiling jet develops and fills underground car park area that can be real scenario as well. Due the simplification and complexity of the problem, only one row of jet fans were calculated and analyzed. It may be expected, that analysis under predict fire load. Smoke will probably create back layers that will influence on jet fan optimization and efficiency. Cold test results were compared with calculations where it was shown some similarity. It may be expected that cold smoke will developed in a different way than a real hot smoke, produced from car combustion. There results are consistent with the results obtained experimentally and with CFD models. Where at some point results were inconsistent (for example CO formation at higher fire loads), the most obvious reason for inconsistency were over-predicted fire loads in CFD calculations.
8.
References 1. J. M.Watts, Fire Scenarios, Fire Technol., 1991, Vol 27, 289-290 2. J. Mangs, O. Keski-Rahkonen, Characterisation of the Fire Behaviour of a Burning Passenger Car, Fire Safety J., 1994, Vol 23, 17-35 3. European Commission, Directorate General for Research, Recherches, Development of Design Rules for steel Structures Subjected to Natural Fires in Closed Car Parks, Profil ARBED, 1996, 51-94 4. Fire Incidents within General Vehicle Parking Areas 1998-2000, Metropolitan Fire & Emergency Services Board, Melbourne, 2001, 45-73 5. Y. Li, M. Spearpoint, Analysis of vehicle fire statistics in New Zealand parking buildings, Fire Technol., 2007, Vol. 43, 93 - 106 6. B. Zhao, J. Kruppa, Structural Behavior of an open car park under real fire scenarios, Fire mater. ,2004, Vol. 28, 269 - 280 7. A. Jug, Ocena zgorevalne toplote in količine zgorevalnih produktov ob gorenju osebnih vozil ,14. mednarodni simpozij o elektroniki v prometu - zbornik referatov, Elektrotehniška zveza Slovenije, 2006, Ljubljana, 68-82 8. X.G. Zhang, Y.C. Guo, C.K. Chan, W.Y. Lin, Numerical Simulations on fire spread and smoke movement in an underground car park, Build. Environ., 2007, Vol. 42, 3466-3475 9. R. G. Gann, J. D. Averill, N. D. Marsh, M. R. Nyden, Assessing the Accuracy of a Physical Fire Model for Obtaining Smoke Toxic Potency Data, International
Interflam Conference - Interflam '07)., 11th Proceedings. Vol. 2. September 3-5, London, England, 2007, 1021-1032
