Validation of flacs code for risk analysis of hydrocarbon pool fires
Tutor / director / evaluatorPlanas Cuchi, Eulàlia
Document typeMaster thesis
Rights accessOpen Access
Fires have been object of study over the last decades due to their destructive power. Fire’s hazardous nature and its ability to inflict damage to property, the environment and people, has produced a need to understand how it works in every aspect. Currently, the main focus is to estimate the fire characteristics and main effects, in order to accurately design emergency plans and prevention measures. Due to the needs previously stated, fires have been studied and analyzed mainly from an experimental point of view. However, experimental data is arduous and extremely expensive to obtain due to the amount of resources needed. Additionally, small-scale models, which are generally easier to be undertaken, cannot be extrapolated to full-scale models. Considering this, semi-empirical methods were developed, but can only be applied to simple scenarios and they cannot fully model them. To achieve complete models of fires, CFD (Computational Fluid Dynamics) modeling has been recently used as a way to achieve a cheaper and easier method to study the fire development of full-scale fires in a wide range of conditions. Nevertheless, CFD models require a huge validation effort before they could be widely applied. The main objective of this thesis is to analyze the performance and if possible validate the CFD code FLACS-Fire v10.5 (Flame Accelerator Simulator) for pool-fires. FLACS is a Computational Fluid Dynamic (CFD) program, which solves the compressible conservation equations for mass, momentum, enthalpy, and mixture fraction using a finite volume method. To model a fire it is necessary to include, among others, processes that involve submodels for: turbulence, combustion, thermal radiation, and soot generation. It is of utmost importance, while developing fire models, to validate them against experimental data in pursuance of being able to conclude whether the simulation is valid or not, and to determine the inherent error in comparison with reality. This process consists in a replication of the experimental setup in the CFD, in this case FLACS, and compare it with experimental data previously available. In the present work, gasoline and diesel fuel experimental pool fires were modeled with FLACS-Fire v10.5 code. Simulations considered different pool fire experiments with diameters ranging from 1.5 to 4 meters. In addition, simulations were run with the Eddy Dissipation Concept (EDC) as combustion model; with the κ-ε model as turbulence model; and with the Discrete Transfer Model (DTM) as the radiation model. The predicted results of temperature’s evolution at different heights, burning rate, and thermal radiation were compared with experimental measurements. The results for gasoline and diesel pool fires indicate that FLACS-Fire v10.5 is able to model pool fires. Pool model 3 (PM3) was able to run all simulations, and Pool Model 1 (PM1) does not perform well with pool diameters higher than 1.5m. Predicted values of the proposed parameters are in a fair concordance with experimentally obtained values. Temperatures measured at the centerline of the flame are in most cases overestimated. Burning rates are well approximated with small and large pool fires (0.15 kg/s-0.5kg/s) but largely over predicted in gasoline pool fires of medium size. Thermal radiation is also forecasted with values larger than their experimental counterparts. Chapter 1, contains a brief introduction to the master thesis. It gives a general understanding of the importance of pool fires in the industry. It also gives a global introduction to Computational Fluid Dynamics (CFD), and its relevancy in the study of accidents, especially in the case of pool fires. Chapter 2, consists of a theoretical background of fire phenomena and the combustion process, with a special focus on pool fires. First, a brief and simple explanation of the combustion process is given. Then, an introduction to heat transfer is provided, in order to show the essentials of how thermal energy is transferred and how it affects pool fires. Finally, an introduction to pool fires characteristics and their mechanisms is given, with an emphasis on the zones composing the fire as well as its main features. Chapter 3, mainly covers the existing work concerning the ongoing topic. It covers authors who have worked with pool fires, especially in the validation of FLACS-Fire; as well as others who gather experimental data. Chapter 4, comprises the crucial elements in fire modeling using FLACS-Fire v10.5. Principally, it contains the submodels FLACS uses for: fluid flow, turbulence, radiation, combustion, soot formation, and pool modeling. This chapter shows a theoretical understanding and the basis from which the simulations are later performed. Chapter 5, is constituted by a detailed explanation of the experimental data used in the present thesis. Instrumentation used in the experiments is thoroughly analyzed, as well as the fuels used and the experiments performed. Chapter 6, includes the simulations performed in the present thesis, as well as, a comprehensive analysis of the data obtained. Initial simulations studying various variables such as grid, radiation model and pool model are studied. Final simulations are also evaluated, which especial emphasis on the discrepancies with the experimental data.