Jet fire studies sharpen hazard assessment techniques
Dr. Andrew JohnsonUsing a combination of large-scale experiments and computational fluid dynamics modeling, Shell Research in the UK is investigating hazards from jet fires in a wide range of scenarios. Knowledge gained from these studies is being used to formulate new, simple-to-use hazard protection tools and to improve testing of passive fire protection materials.
Shell Research and Technology Centre
These materials have traditionally been specified on the basis of fire resistance tests performed in a furnace operating under time-temperature conditions defined by a fire curve. The tests are based on furnace temperature rather than heat flux, and conditions may differ significantly from those in a real fire in terms of the balance of radiative and convective heat transfer, gas velocities and thermal shock.
All of these are major factors that can affect the performance of the protective material, particularly in jet fires resulting from high pressure gas leaks. Understanding of the real conditions to which such materials would be subjected is therefore necessary for the design of improved fire resistance tests.
The hazards posed by jet flames are scale dependent. Therefore, Shell Research has been using large-scale experimental facilities to assess the consequences for the size of jet fires that might occur on offshore installations. The measurements have provided unique information about the extent of flame engulfment and the heat loading to structures. The data also provides a validation of models being developed to predict jet fire hazards for a wider range of accident scenarios.
By simulating the complex physics of jet fires using CFD techniques, Shell is developing a tool which provides a comprehensive description of the flowfield at every location in the calculation domain including temperature, velocity pressure, soot formation, combustion products and heat transfer. CFD involves the numerical solution of the equations governing the flow process, which in the case of combusting jets includes the momentum of the jet, the combustion and convective and radiative heat transfer.
Shell Research chose CFX, a general-purpose commercial CFD code developed by AEA Technology in Harwell, UK to provide the backbone of complex geometry definition, solution of the equations and results visualization. To this Shell Research has added a body of new models for combustion, soot formation and radiative and convective heat loading. The data provided by the large-scale experiments enabled the validation of the models to produce an accurate simulation off turbulent gas jet flames.
In the simulation of combusting jets, the first task of the CFD model is to predict the jet shock structure accurately. Since the emerging jet is at high pressure, it forms series of complex shock structures as it expands down to atmospheric pressure. The model then predicts the position of the flame lift-off point, when the jet starts to combust, and the flame impingement on specific objects. It also calculates the soot formation which is the dominant source of thermal radiation in most hydrocarbon jet flames.
The model has provided reliable predictions for the jet structure, jet flame trajectory, flame lift-off position, flame temperatures, soot formation and external thermal radiation for both high pressure sonic releases, typical of accidents offshore and low pressure flames such as flares.
Shell Research is now using the model to calculate flame engulfment and the preliminary predictions of heat fluxes to objects inside the flame show the correct trends. The model is now being developed further to obtain accurate heat transfer rates.
With CFX, Shell Research is developing an efficient tool for the prediction of jet flames impinging on surfaces. This will permit hazards to be assessed for a wide range of jet fire scenarios of relevance to offshore facilities. The simulations will also provide accurate and comprehensive data to help ensure the survivability of passive fire protection materials under any circumstances.
For more information contact Dr Andrew Johnson, Shell Research and Technology Centre on +44 151 373 5865 or CFX marketing department, AEA Technology on +44 1235 433771.
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