ANSYS, Inc. March 31, Phase Change Models in FLUENT

Transcription

1 ANSYS, Inc. March 31, 2016 Phase Change Models in FLUENT

2 Phase Change Models in ANSYS FLUENT In ANSYS Fluent the following are available for modeling phase change Boiling models RPI Non equilibrium Critical Heat Flux Evaporation Condensation Thermal Phase Change Lee In addition there are Cavitation and Wet steam models also available Cavitation model is suited for predicting cavitation Wet steam model is suited for applications such as rapid expansion of steam which causes condensation This document discusses the applicability of Wall boiling models and Evaporation Condensation models alone ANSYS, Inc. March 31, 2016

3 Heat Flux Phase Change Models in ANSYS FLUENT Wall boiling models Wall boiling model is applicable for subcooled nucleate boiling in vertical channels. RPI Boiling model Applicable to subcooled nucleate boiling Non-equilibrium Boiling Extension of RPI to take care of saturated boiling Critical Heat Flux Extension of RPI to take care of boiling crisis Lee Model Lee Model is a simplified approach to model volumetric phase change where user needs to tune the coefficients to match the results with the known experimental results. Even though this model can be used with the Eulerian model, it is recommended to be used with the Mixture multiphase model. Thermal Phase Change Model Thermal Phase Change Model is also a volumetric phase change model. However, unlike Lee Model, there is no need to tune the coefficients for mass transfer. Mass transfer is governed by the chosen heat transfer mechanism. This model is available with the Eulerian multiphase model. Single Phase Subcooled Nucleate boiling Critical Heat Flux Saturated Minimum Heat Flux Transitio nal or Unstable Wall Superheat (T wall - T sat ) Stable Film boiling ANSYS, Inc. March 31, 2016

4 Sample Case A sample case where liquid at saturation conditions enters an annular pipe and flows vertically upwards. The inner surface of the pipe is heated which causes evaporation of the fluid RPI wall boiling model is suited for such conditions. However for explanation purposes, the same case is run using Thermal Phase model, and Lee model to explain the comparative behavior. This document does not explain the theory behind each of the models and users are requested to refer to documentation section for details. Please note that this is not a validation study or a best practice document ANSYS, Inc. March 31, 2016

5 Sample Case Annulus geometry ID (mm): 141 OD (mm) : 182 Height (mm) : 200 Inside pipe surface area (m2) = πx0.141x 0.2 = Heat flux provided on wall (w/m2) = Heat supplied (w) = x = Heat balance Let m be mass of steam generated M x Latent heat = heat supplied M x (1262.3x 103)= Mass of steam = kg/s Theoretically maximum mass flow rate of steam at outlet = Kg/s ANSYS, Inc. March 31, 2016

6 Input Data Sl. No Parameter Value 1 Mass flow rate at inlet (Kg/s) 3 2 Vapor Volume inlet 0 3 Liquid inlet Saturation inlet Latent Heat of Vaporization (Kj/Kg) Latent Heat entered as Standard State Enthalpy(J/Kgmol) 2.28e+07 7 Heat flux on wall (W/m 2 ) Operating Pressure 108 bar ANSYS, Inc. March 31, 2016

7 Domain for CFD Analysis gravity inlet outlet Heated wall ANSYS, Inc. March 31, 2016

8 RPI Boiling Model Report -> Fluxes Overall mass balance Contours of Volume fraction of Steam Phase Interaction: Drag: Ishii Lift: Moraga Heat transfer: Ranz Marshall Steam flow rate at outlet ANSYS, Inc. March 31, 2016

9 Thermal Phase Change Model Report -> Fluxes Overall mass balance Contours of Volume fraction of Steam Steam flow rate at outlet ANSYS, Inc. March 31, 2016

10 Lee Model Report -> Fluxes Contours of Volume fraction of Steam Overall mass balance Steam flow rate at outlet ANSYS, Inc. March 31, 2016

11 Lee Model Residuals (Varying Evap frequency) With increasing evaporation frequency, convergence will be difficult to achieve ANSYS, Inc. March 31, 2016

12 Lee Model Varying Evaporation Frequency Volume outlet With varying evaporation frequencies, vapor volume fraction at outlet changes and hence, it has to be fine tuned to match expected results ANSYS, Inc. March 31, 2016