Study On Fire Behavior Of The Typical Oil In The Engine Compartment And Flame Thermal Radiation Model Of The Car

Cars are a common means of transportation in our daily life.While the number of cars is increasing,the fire problem brought by cars cannot be ignored.The engine compartment of the car is a unified entity with complex structures.In a limited space,each functional component,such as oil system,electrical system and pipeline system et al.needs to work in complex conditions,resulting in different failure modes,and each part has different fire risks to varying degrees.Car fire is characterized by various fire origins,rapid fire spread,explosive combustion and releasing a large amount of toxic gases.Once a fire occurs,it will bring great fire loss.Therefore,it is the basis of car design,car fire investigation and fire fighting and rescue to fully understand the scientific car fire problems due to main failure modes in the engine compartment.The research of this paper is based on the car fires caused by the typical fault mode of the oil system in the engine compartment.The thermal oxidation,ignition and combustion characteristics of typical oil materials in the engine compartment are discussed.By simulating the engine compartment fire situation,the fire spreading process was reconstructed,and the relevant flame radiation model was established to quantitatively evaluate the fire risk of car burning.Specific research contents are as follows:(1)The quality and heat change of automatic transmission oil under air atmosphere were studied under different heating rate conditions.The result shows that the thermal oxidation process of oil can be divided into two stages:low temperature oxidation(LTO)and high temperature oxidation(HTO).In the LTO stage,the mass loss is about 90%,which is an important stage to measure the thermal oxidation stability of the oil.The activation energies during the reaction at different heating rates were obtained by using model-free methods.And the results show that the activation energy is basically unchanged before the conversion degree is 0.6.This process involves relatively simple reactions,mainly involving the evaporation of water and the degradation of additives(such as alcohols,lipids and other oxygen-containing compounds).Then,the activation energy increased with the increase of conversion degree.The average activation energy in LTO stage is 71.78±13.12 kJ/mol.Furthermore,the variation range of pre-exponential factors and the reaction model function at different heating rates were obtained by means of Kissinger and Criado methods,respectively.An optimal fitting method based on the principle of least square method is adopted,and the optimized activation energy and pre-exponential factor in LTO stage are 69.33 kJ/mol and 1.28×106min-1,respectively.And the reaction model follows the deceleration model curve.(2)The oil phase temperature,ignition time,mass loss rate and heat release rate of automatic transmission oil under the coupling conditions of external radiation and the initial oil thickness were studied.The experimental results show that the ignition time decreased significantly with the increase of external radiation,but as the initial oil thickness is increasing,the decreasing trend gradually weakened.Furthermore,the ignition time increases with the increase of the initial oil thickness,but with the increase of the external radiation,the gap of ignition time tends to narrow.In addition,the heat transfer model of the liquid phase was established,and the comparison with the experimental data showed that the variation of the ignition time was essentially related to the one-dimensional heat transfer in the liquid phase before ignition.The increase of the initial oil thickness can promote the heat transfer between the upper and lower liquid layers,which leads to the decrease of surface heating rate and finally prolongs the ignition time.However,the effect of initial oil thickness is not obvious at higher external radiation.Furthermore,the quasi-steady mass burning rate of a fixed area oil pool fire is almost constant,independent of the initial oil thickness,but linear with the external radiation.In addition,by introducing thickness coefficient,the correlational relationship between initial oil thickness and peak heat release rate was established in the exponential form.When the initial oil thickness exceeds the threshold value of 8 mm,the increase of peak heat release rate tends to decrease.(3)The fire spreading process,heat release rate and flame radiation variation after engine compartment fire were studied.The result shows that the fire spread from the engine compartment to the passenger compartment,then to the back of the car,and the dropping of the front bumper and windows caused the engine compartment and passenger compartment to burn at a much higher intensity.The burning was restriced in the engine compartment in the first 15 min after ignition.When fire overflowed from the air conditioning grille,the HRR curve shows t2 fast fire with a fire growth coefficient of 0.084 kW/s2.Then,when the bumper burns down,the HRR increases at a rate of 15.273 kW/s to reach a peak heat release rate of 3.06 MW.After the gasoline leak,the peak heat release rate reached 3.38 MW.In addition,the cuboid flame radiation model of engine compartment and passenger compartment during the intense combustion stage was established.Based on the vehicle size,radiation heat flux and heat release rate data,the flame radiation fractions of engine compartment and passenger compartment in the intense burning phase were obtained and verified.Therefore,the radiation field around the car fire was quantified,and the criterion of fire safety distance was proposed.In this study,the minimum recommended values of fire safety distance for persons without any protective measures,adjacent cars placed in parallel and adjacent cars placed head to head with burning cars are 7.3 m,2.1 m and 2.2 m,respectively.