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In-cylinder Steam Generator - Please Help (Interesting Case)!

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Old   September 10, 2012, 09:55
Default In-cylinder Steam Generator - Please Help (Interesting Case)!
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Problem Definition
The problem faced is to simulate an in-cylinder steam generator. The cylinder has been drilled in a heated block of aluminum with initial temperature of 523K. The cylinder can be assumed to only contain steam prior to liquid water injection. Water at a flowrate of 0.003 kg/s is injected into the cylinder as in the picture below. All injected water is evaporated and the generated steam exits at the outlet. Thus, there is no gaseous inlet. The internal walls have been treated with a calcium hydroxide solution so as to improve heat transfer (no boiling in the film-boiling regime but the nucleate/transition-boiling regime). Initially, a satisfactory steady-state solution is desired.

Geometry and Data
See attached jpeg!

Previous Attempts
The problem has been simulated using the discrete particle method (DPM) in FLUENT. The built-in hollow cone spray injector with a Rosin-Ramler size distribution injector has been applied. The wall temperature has been fixed to 523 K for steady-state simulations and a pressure-outlet of 1 bar. For particle-wall interaction the Lagrangian Wall-Film has been applied (various number of splashed particles have been tested). Also a fixed heat-flux boundary condition has been applied in order to transfer sufficient heat, however FLUENT raises the wall temperature to more than 10 000 K in this case which gives exceptionally unrealistic results.

Problems During Previous Attempts
The heat needed to be transferred from the wall in order to heat and evaporate the water has been estimated to 7.5 kW or an average surface heat flux of 600 kW/m2. The heat transfer has been the real challenge here as the total surface heat flux from the walls in simulations only reaches approximately one tenth of the required heat flux. The reason for this has been investigated and it is believed that it is caused by the Lagrangian Wall-Film physics which only considers heat transfer from the wall through conduction and the enthalpy required for vaporization is instead taken from the cell into which the vapor is added and not from the wall. During nucleate-boiling the heat transfer is governed by more mechanisms than just wall-liquid conduction. Because of the short-comings of the Lagrangian Wall-Film model, the FLUENT built-in Eularian Boiling model (RPI) is now being considered. Or possibly writing a UDF.

Questions
It seems as though the RPI-model is developed for sub-cooled boiling with a liquid bulk but in this case we have a dispersed flow situation with droplets injected into a gaseous bulk, is the RPI-model applicable in this case?

Does the RPI-model describe the physics of this problem better than the Lagrangian Wall-Film?

Is it possible to use the Eularian Wall-Film with DPM particles? Any experience here?
Attached Images
File Type: jpg Problem Description.jpg (56.1 KB, 14 views)
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boiling, evaporation, spray, steam generator, wall-film

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