MODELLING OF HYDROGEN JET FIRES USING CFD

The computational fluid dynamics (CFD) software FLACS has primarily been developed to model dispersion and explosion phenomena; however models for the simulation of jet fires are under development. The aim is to be able to predict industrial fires efficiently and with good precision. Newly developed models include e.g. flame models for non-premixed flames, discrete transfer radiation model as well as soot models. Since the time scales for fire simulations are longer than for explosions, the computational speed is important. The recent development of non-compressible and parallel solvers in FLACS may therefore be important to ensure efficiency. Hydrogen flames may be invisible, will generate no soot and tend to radiate less than hydrocarbon fuels. Due to high pressure storage the flame lengths can be significant. Simpler jet flame relations can not predict the jet flame interaction with objects and barriers, and thus the heat loads on impacted objects. The development of efficient and precise CFD-tools for hydrogen fires is therefore important. In this paper the new models for the simulation of fire are described. These models are currently under development and this manuscript describes the current status of the work. Jet fire experiments performed by Health and Safety Laboratories (HSL), both free jets and impinging jets, will also be simulated to evaluate the applicability and validity of the new fire models. 1.0 INTRODUCTION AND MOTIVATION It is evident that hydrogen can be an attractive future energy carrier due to its low-level pollution effects and the fact that hydrocarbon-based fossil fuels have a limited lifetime. However, the perceived increased risk level in society following the transition to an eventual “hydrogen economy” remains a concern. The safety characteristics are especially worrisome, primarily due to its wide flammability range and high burning velocity. Even if hydrogen fires are expected to be less destructive than hydrogen explosions, they present their own challenges. A hydrogen fire is often difficult to detect (due to the absence of any soot and visible radiation) without a thermal imaging camera or flame detector. A leak in a pressurized hydrogen storage system will result in a jet that may extend for several meters. If ignited, the jet flame can cause serious damage to anything it encounters. Due to a much lower ignition energy than traditional fuels and also the possibility for “self-ignition” due to shock-structures in a high pressure release heating and igniting flammable pockets of hydrogen-air, the likelihood for ignition of hydrogen jets is significantly higher than for e.g. natural gas jets. Further, hydrogen fire hazards are unique because the thermo-physical properties of hydrogen (e.g., density, mass diffusivity, flammability, detonability) are sufficiently different from those of other fuels that it is unclear whether existing measures for handling fuels such as natural gas or propane can be simply extended to hydrogen. Understanding the sources of and conditions that can lead to ignition is important to advancing safe operating criteria and application of design standards. The computational fluid dynamics (CFD) software FLACS has primarily been developed to model dispersion and explosion phenomena; however models for the simulation of jet fires are under development. The aim is to be able to predict industrial fires efficiently and with good precision. Newly developed models include e.g. flame models for non-premixed flames, direct transfer radiation models as well as soot * Email: olav@gexcon.com models. Since the time scales for fire simulations are longer than for explosions, the computational speed is important. The recent development of non-compressible and parallel solvers in FLACS may therefore be important to ensure efficiency. Simpler jet flame relations can not predict the jet flame interaction with objects and barriers, and thus the heat loads on impacted objects. The development of efficient and precise CFD-tools for hydrogen fires is therefore important. One main motivation for the development of FLACS-FIRE is also an observed increasing interest for probabilistic fire risk assessments in the oil and gas industry, likely influenced by a recently adopted standard ISO19901:3 [1]. According to this standard, oil and gas installations shall evaluate the accidental risk from explosion and fire, and if worst-case design is not feasible, it shall be demonstrated that the frequency for escalation (loss of main safety barrier) is less than once every 10,000 years. Since fire modelling is a complex phenomena involving complicated interaction between turbulence, chemistry, flow, etc., it is important to carry out extensive validation of the CFD models to ensure reliable results. In this paper, we have used recent jet fire experiments performed by Health and Safety Laboratories (HSL), both free jets and impinging jets, to evaluate the applicability and validity of the new fire models [2]. This paper is organized as follows: The fire model in FLACS is presented in Section 2. In Section 3, the experiments used for validation in the current study are briefly described. The results and discussion are presented in Section 4 and the conclusions in Section 5.