Exploitable results
The aim of the this part in the FLIE project was to undertake a systematic validation of the predictive models developed during the project in order to provide recommendations for best practice in using CFD predictive tools for hazard assessment in situations where flashing release presents serious safety issues.
- To undertake critical review of the performance of existing modelling techniques for modelling accidental atmospheric 'flashing' jet releases and subsequent dispersion - including possible rain-out using commercially available CFD tools.
- To have main focus on the two-phase aspects, with emphasis on flash atomisation and droplet dynamics downstream from other industrial applications such as combustion chambers and refrigerants and investigate the applicability of such sub-models to simulate mechanisms in dispersing flashing jet.
- To evaluate various existing theoretical approaches that can be applied to calculate flashing conditions and provide guidance for selecting appropriate model based on experimental data generated during the project.
A Computation Fluid Dynamics (CFD) model that can be used to simulate the flow from a well-defined droplet jet has been developed, consolidated and to a certain degree validated. The important effects due to interaction with the surrounding gas and collisions between droplets (drag, break-up, coalescence, evaporation, condensation) are modelled. The model is also able to represent a discrete size distribution of droplets. Further modelling of the flashing source is needed to complete the CFD model.
The work on the CFD model was done within an existing code called FLACS (Flame Acceleration Simulator). The FLACS code was initially developed at CMI (Chr. Michelsen Institute), and has been maintained and further developed at CMR (Chr. Michelsen Research) and GexCon. It can be used to simulate ventilation, gas release and dispersion, and gas explosions in complex geometries like offshore platforms and onshore industrial plants.
FLACS is a commercial product, and several licensing arrangements ranging from fully paid commercial to almost free academic licenses are available. More information may be obtained from GexCon's web site http://www.gexcon.com. A general model for handling of droplets or small particles in FLACS has been implemented. The model handles particular entities in conjunction with release, dispersion and explosion phenomena as a continuous phase.
Several existing developmental models in FLACS were used as the starting point for a new unified model:
-Water-spray flow and droplet break-up model (Anders Hallanger, CMR)
-Dust flow and combustion/explosion model (Bjørn J. Arntzen, GexCon)
-Pool evaporation model (Hans-Christen Salvesen, GexCon)
-Radiation model (Idar E. Storvik, GexCon and Ivar Øyvind Sand, CMR)
-Local multilevel grid refinement technique (Thor Gjesdal, GexCon)
The new development of the FLIE related part of the FLACS code has been financed through two projects with sponsors from the industry and 50% financing from the EC. There are still problems with the model for evaporation and boiling of droplets in the code, further work is required to bring this part to function properly.
A large number of tests have been performed in order to verify that the particle/droplet models in FLACS are correct. Comparison with analytical where that is possible and with available experimental data are important parts of the model development and validation. An advanced prototype CFD tool was developed, and this has a potential of being the basis for a commercially available tool in the coming years. GexCon has the intention to develop this prototype further to a commercial grade tool, and to make it available to the industry and consultants under the existing license model used for the FLACS package.
The development of improved models for flashing liquids releases in industrial environments would be achieved by a concerted action comprising the design and execution of sensible experimental programmes. INERIS performed large-scale releases under realistic conditions to provide validation data for the developed models and simulation tools.
The project has provided high-quality data sets, both at laboratory scale and at large scale, of the flashing expansion in the near field. The experiments have employed advanced measurements techniques such as laser techniques and fast response probes, as well as conventional techniques. The obtained data have been used to assist in the development and validation of detailed models of the source processes during accidental release of pressurised liquids.
A variety of experimental conditions were studied:
- Type of LPG (propane, butane)
- Pressure in the tank
- Nozzle configuration (size and shape)
-Obstacle configuration (free jet, impinging jet, distance of impact)
The task of INERIS for FLIE project was to carry out large-scale experiments to collect data useful for the improvement of the existing dispersion models. These tests made it possible to clarify the weaknesses of the calculation models of source terms (evaluation of the mass-flow, of the quality of release, the phenomenon of the aerosol creation, pool evaporation). In spite of the many practical and technical difficulties related to the implementation of this type of tests, we realised nearly a hundred experiments and recorded sufficient exploitable data to be able to improve the dispersion models in the near field as well as the sources terms models. However, the various accumulated delays did not make it possible to exploit these measurements ahead. Therefore it is now important to devote time to the fine exploitation of these results in order to concretise this three years investment by the creation of a calculation model.
The work was dedicated to the design of the experimental facility to perform laboratory-scale liquid-flashing tests. Preliminary tests were performed with conventional and novel laser-based measurement techniques. These tests allowed:
- To design the instrumented lab-scale liquid-flashing source, which resembles as much as possible a true hazardous situation in the case of accidental release of pressurized liquids caused by equipment or operator failure.
- To evaluate the applicability of standard and novel laser-based measurement techniques such as standard Particle Image Velocimetry (PIV), Multi-Intensity Layer Particle Image Velocimetry (MIL-PIV), Particle Tracking Velocimetry and Sizing (PTVS), Phase-Doppler Anemometry (PDA) and Global Rainbow Thermometry (GRT). Standard PIV, MIL-PIV and PDA are applicable to a flashing two-phase jet, except for regions of extremely high droplet density. GRT and PTVS are being further developed in order to make them suitable for the harsh flashing environments. Measurements were also performed by thermocouples, showing the problem of ice formation on this intrusive instrument, leading to erroneous measurements. This had a feedback on the design of the flashing facility, which should allow operation at 0% humidity, thus preventing the ice formation.
It is expected that the results of the FLIE project will lead to an improved understanding of the governing phenomena, and thus improve the safety of existing and future industrial plants. The present work on small-scale experiments addressed the influence of the initial parameters of the flashing liquid jet on the two-phase characteristics downstream of the orifice exit in case of a sudden release of pressurized liquefied R134a (refrigerant). Due to the non-equilibrium nature of the near field regions, conducting accurate data measurements for droplet size and velocity is a challenging task.
Laser-based optical techniques like Particle Image Velocimetry (PIV), Phase Doppler Anemometry (PDA) were used to obtain information for particle diameter and velocity evolution in this harsh environment. Moreover, high-speed video photography presented the possibility to understand the break-up patterns of the R134a liquid jet as function of driving pressure, superheat level, and discharge nozzle characteristics. Temperature measurements with an intrusive technique such as thermocouples, and non-intrusive measurements with Infrared Thermography were also performed. Cases for different initial pressure, temperatures and orifice diameters were studied. The break-up pattern, droplet size, velocity distribution and temperature evolution along the radial and axial directions have been presented as function of initial parameters.
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