STUDIES ON HYDROCARBON FUEL FIRES IN A COMPARTMENT

Abstract

Modern buildings are typical and involves lots of complexity; examples are residential complexes, office buildings, educational facilities and industrial units etc. In the past, there have been a number of fire accidents resulting in loss of lives and valuable assets, and have raised concerns regarding fire safety standards in buildings. Fire occurring in compartment possess a terrible threat. Fire hazards in a compartment is caused by high temperature in compartment, generation of toxic gases such as CO and smoke. Inhalation of smoke and toxic gases is the main reason of casualties in fire accidents. For designing a fire protection system, it requires a precise understanding of behavior of different fuels under different ventilation conditions, and the interaction between fire and its surroundings. This can be achieved through extensive experiments and better measurement techniques. In the last decades, a number of experiments have been conducted in full scale and reduced scale compartments using solid, liquid and gaseous fuels. Characteristics of pool fires have been studied by various researchers (McCaffrey, 1979; McCaffrey et al., 1981; Heskestad, 1984; Bouhafid et al., 1988) and the correlations for flame height, plume centerline temperature, velocity and hot gas layer temperature have been presented. Hayasaka (1997) performed experiments on small pool fire of heptane, methanol and kerosene. It was found that there are different processes associated with the change in fuel temperature and these are: preheating, transition and boiling. Experiments have also been conducted to study the effect of fire source location inside a compartment. In this regard, recent work has been performed by Parkes et al. (2005) using heptane pool fire, located at rear, center and front locations in the compartment. The scenario involving fire in a ventilated compartment is one of the key issues for fire safety assessment in a plant. Ventilation inside the enclosure is an important parameter since ventilation affects the burning rate and burning rate affects the type of scenario in the enclosure (back flow, overpressure etc.). One of the earlier works for ventilated compartment fire has been performed by Kawagoe (1958), and they gave a relation for mass flow rate of air (𝑚̇ 𝑎𝑖𝑟 = 0.5𝐴√𝐻0), applicable for post flashover fires. Steckler et al. (1982) also conducted experiments in a compartment of size 2.8 m (L) × 2.8 m (W) × 2.18 m (H) using methane burner as fuel source. They presented the mass loss rate as a function of opening geometry, fire strength and fire location. Lock et al. (2008) experimentally investigated the effect of wall ventilation in an ISO 9705 enclosure of size 2.4 m (L) × 2.4 m (W) × 3.6 m (H) with a door ventilation of size 0.8 m (𝑊0) × 2 m (𝐻0) using a heptane spray burner. It was found that for the same heat release ii rate, similar temperatures were observed for each ventilation case, however different gas compositions were observed. Studies have also been conducted on blended fuels. Tran et al. (2014) have studied the combustion characteristics of biofuel and their blends with petroleum diesel in pan diameter of 0.042 and 0.057 m. Biofuels selected were canola methyl ester, soya methyl ester and their blends with diesel. The blends were tested for biofuel concentration of 25, 50 and 75% by volume. Experimental investigations revealed radiative fraction was comparable for all the flames. Yamali (2015) have also studied blended pool fire of heptane and 10 % to 30 % volumetric fraction of ethanol in square and rectangular pans under different ventilation conditions in a tunnel and found that with increasing ethanol content, the quasi-steady HRR of ethanol pool fire increases. Moreover, steady burning rate increases with ethanol ratio due to higher combustion temperature related to ethanol. Several CFD software packages have also been used in the past for the fire modeling purpose. Some of these software’s includes CFX, Phonics, Fluent, Fire Dynamics Simulator (FDS), JASMINE, FIREFOAM and SMARTFIRE. Motivation for the present study Based on the extensive literature review studied, it has been found out that significant research on pool fires of petroleum fuels has been performed in the past, however there is shortage of data on the burning characteristics for biofuels. Moreover, in today’s world, energy demand is growing at a fast pace and most of the energy demand are met by fossil fuels namely, petroleum and natural gas whose reserves are constantly depleting. Therefore, there has been growing interest on the use of renewable and eco-friendly sources of energy, and biofuels are one of them. Previously there has not been any study conducted on crude jatropha oil and its biodiesel with respect to its behavior during fire in a compartment. Research work presented in the thesis tries to study the burning behavior of different fuels (crude jatropha oil, diesel, biodiesel and blended fuel), both experimentally and numerically inside a compartment. Experimental set up A fire research facility is developed in fire research laboratory of the Mechanical and Industrial Engineering Department, IIT Roorkee. Experimental test compartment is comparatively large and is of internal size 4 m (L) × 4 m (W) × 4 m (H). A door opening of 1 m width by 2 m height has been provided, and it can be changed with the help of sliding panels placed on either side of door. In all, thirteen experiments have been carried out based on different iii fuels, different pan sizes and different ventilation sizes. For each of these experiments, fuel has been placed at the center of the floor and its flash point is achieved by using oil preheater assembly. Fuel pan along with oil preheater assembly is placed at weighing platform. In these experiments, various parameters have been measured such as: heat release rate (HRR), mass loss rate (MLR), exhaust gases concentration (O2, CO2 and CO), profiles of flame temperature, flame height, room corner temperature, door centerline temperature and velocity, upper zone gas temperature and heat flux at different wall locations (ceiling, floor and wallls). Heat release rate has been measured by large scale heat release calorimeter which works on the principle of oxygen depletion. Finally, numerical simulations have been performed using the Fire Dynamics Simulator (FDS), for two of these experiments. Results and Discussion In view of the above, experimental studies on jatropha oil pool fire has been performed with pan diameters of 0.2, 0.3, 0.4, 0.5 and 0.6 m. Analyzing results, it has been found that like other fuel fires, the whole combustion process can be divided into four stages: ignition, growth, steady and decay stage. Experimentally obtained results of mass loss rate have been validated with those available in literature for pool fires of hydrocarbons. Plume centerline temperature has also been compared with the available correlations, and results are found to be in agrement for the intermittent and plume region. An attempt has also been made to calculate the flame height, and for that images taken for the fuel burning have been processed in MATLAB. For the pool fires of 0.3 and 0.4 m, results are found to be promising and in agreement with the available correlations while for 0.5 and 0.6 m pool fires, either the flame height is over predicted or under predicted. Experimental layer height is found to be approximately at 1 m from the floor for all pool fires (i.e. 0.2 to 0.6 m diameter). It means that layer height is independent of pool size in the case of present compartment size. After complete burning of fuel, some amount of residue has been observed in the fuel pan and its amount is in the range of 22 to 47 g for the pool fires of 0.2 to 0.6 m diameter. Radiative fraction has also been calculated based on point source method and solid flame model, and it has been found to be in range of 0.15 to 0.25 for the pool size of 0.2 to 0.6 m diameter. To study the effect of ventilation, different ventilation opening sizes have been created by varying the door opening from full door to quarter door. Experiments have been performed in a jatropha oil pool of 0.6 m diameter. It has been found that ventilation size affects the mass loss rate, heat release rate and thermal environment inside the compartment. Plume centerline temperature and hot gas layer temperature are found to be less in full door ventilation size due to iv the effect of cooling of plume gases caused by the presence of excess air. Reduction in ventilation size from full door to quarter door restricts the availability of oxygen for combustion. This is also reflected from the results of combustion efficiency wherein, it decreases from 0.91 to 0.85. Moreover, the amount of residue leftover increases from 31 to 49.2 g. Assessment of heat release rate in a compartment fire is a major concern for fire investigators to predict the fire behavior. An attempt has been made to validate the experimental results of 0.6 m pool fire by making an energy balance. Experimental results obtained for full door ventilation have been used and various heat loss terms (boundary heat loss, doorway heat loss and gas heating) are calculated to predict the heat release rate. Analyzing the results, it has been found that the predicted heat release rate is close to the heat release rate by mass loss method while percentage difference between predicted and oxygen depletion heat release rates are 13.8 %. Major portion of heat release by burning of jatropha oil goes to heat up the compartment boundaries and, nearly 82 % of the energy has been utilized to heat compartment boundaries, 14 % of energy has been lost through doorway opening and the remaining 4 % has been used for gas heating. Experiments have also been performed in 0.6 m pool diameter with diesel, biodiesel and blended fuels (blend of diesel with biodiesel). Biodiesel is mixed with diesel and blends have been prepared with 20 and 50 % volume concentrations of biodiesel. It has been found that the burning of biodiesel increases the total burning duration for the similar quantity of diesel burned. The average burning rates for diesel and biodiesel are found to be 4.9 and 7.1 g/s respectively, and biodiesel burned 140 second more as compared to the similar quantity of diesel fuel. This is due to the formation of heavier components in biodiesel as it oxidizes. With the increase in biodiesel concentration, combustion efficiency decreases. Moreover, radiative fraction of fuels are comparable having highest fraction obtained for diesel and the lowest for biodiesel. Numerical simulations have been performed using using Fire Dynamics Simulator (FDS), version 6.2.0 developed by NIST, USA. FDS solves numerically a large eddy simulation form of Navier Stokes equations which are based on the assumptions of low speed and thermal driven flow; with focus on smoke flow and heat transport, to describe the fire behavior. Combustion is not modeled and instead fire is represented as a volumetric heat source. Heat release rate measured by oxygen depletion calorimeter is specified as the input in FDS. Simulations have been performed on two different radiative fractions i.e. 0.35 (default) and 0.18. In earlier experiments on jatropha oil pool fire, radiative fraction has been found to be 0.18 corresponding v to 0.6 m pool diameter. Computational domain has been extended to the size 6 m (L) × 6 m (W) × 6 m (H) to capture the fire behavior at wall surface and door. Simulations have been carried out for full door open and half door open experiments and the total number of cells in the model are in the range of 512000 to 1360800. It has been found that simulations for full door ventilation at radiative fraction of 0.35 results in under prediction of results at all locations of the thermocouples. Simulation with radiative fraction of 0.18 provides better results, and are in agrement with experimental values. Moroever, with the refinment in mesh size, predicted layer height approaches the experimental layer height. It can be further concluded that the radiative fraction of 0.18 provides better results as compared to the default value of 0.35, and the value of 𝐷∗⁄Δ can be taken in the range of 8.34 to 12 for accurately resolving the events of compartment fire. It is our view that the present study will be helpful for taking decisions regarding safe handling of fuels considered in the study (jatropha oil, diesel, biodiesel and blended fuels) in chemical process plants and ware houses. These studies will also be useful for designing fire safety methods, and consequently for designing fire safety standards