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March 3, 2009, 10:39 |
Mass Transfer & VOF
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#1 |
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Dear all,
I'm trying to solve a condensation problem. To do this, I use the VOF model and have written a complex condensation model. There are several problems: - 1) The Absolute Pressure drops BELOW zero. This should not be possible ofcourse. My operating pressure is 4 bar abs. - 2) The mass transfer works not like it should: I've tested this with a very simple condensation model (UDF): from species = water. to species = steam. DEFINE_MASS_TRANSFER(Condensation,cell,mix,from_in dex, from_species_index, to_index, to_species_index) { Thread *gas = THREAD_SUB_THREAD(mix, from_index); Thread *liq = THREAD_SUB_THREAD(mix, to_index); real T_sat = 406; real m_lg; if (C_T(cell,mix)>T_sat) m_lg = 5000; else m_lg=-5000; return m_lg; } This means that if the cell temperature exceeds the saturisation temperature, evaporation should occur. Otherwhise evaporation occurs. But if I solve the problem (unsteady) this is not the case. In my simulation, at one place the mass transfer is 5000 kg/(m3-s) from water to steam. This is true indeed if I look to the temperatures, but if I look to the phase, the opposite occurs (condensation instead of evaporation). I use the explicit VOF sceme, no turbulence models, energy equation. Unsteady with dt=1e-6 sec. Solution controls: p-v coupling: coupled pressure: Presto momentum: Quick VOF : modified HIRC energy: Quick. Please, can anyone help me?? Thanks in advance! R Liew, Technical University of Eindhoven |
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