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1、Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,1,New Initiatives at Fluent Inc.,Phase Change in Heat Exchangers,Brian Bell, Fluent Inc.,UGM 2001,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,2,Motivation,Demonstrate the use of Fluent to model phase change
2、 in heat exchangers Processes of interest Condensation Evaporation Boiling Illustrate how to model one such process through use of a detailed example Shell-and-tube condenser Provide motivation for users to begin developing models of their own devices,Company Confidential Copyright 2001 Fluent Inc.
3、All rights reserved.,3,Outline,Problem Description Shell-and-tube condenser Pure vapor condensation Non-condensable gases Modeling Approach Porous medium Heat and mass transfer modeling Model Implementation User-Defined Functions and User-Defined Memory Results Steam condenser with non-condensable g
4、ases Commercial chiller condenser,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,4,Description of Problem,Shell-and-tube condenser,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,5,Goals of CFD Modeling,Condenser performance characterized by heat and mass t
5、ransfer rate CFD allows evaluation of factors affecting heat and mass transfer in condenser Tube bundle configuration Tube arrangement Number of passes Location of inlet ports Baffles Pressure drop Velocity field Non-condensables Location and configuration of purge system Results allow identificatio
6、n of potential design improvements,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,6,Film Condensation Process,Driving potential for condensation is the temperature difference between vapor and cooling water Driving potential variation caused by Pressure drop Rise of cooling wat
7、er temperature Non-condensables,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,7,CFD Modeling Theory,Porous medium approach Tube bundle treated as porous medium Enables computationally efficient modeling of entire condenser Comparison with detailed modeling approach In 2-D, O(1
8、00)-O(1000) control volumes per tube versus more than one tube per control volume Heat and mass transfer models Condensation rate calculation Condensation rate determined from local flow field and cooling water temperature Liquid film flow rate tracked in bundle from top to bottom Cooling water temp
9、erature tracked from inlet to outlet,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,8,Porous Medium Approach,Representation of tube bundle as porous medium Porosity is only required parameter Porosity defined as ratio of fluid volume to total volume,Example: staggered tube bund
10、le with equilateral triangular layout,Porosity, b, expressed as:,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,9,Transport Equations,Generic transport equation for porous medium approach,Distributed resistance takes form of source terms that model details of the flow that are
11、not resolved by the grid Porosity in convection and diffusion terms not modeled in Fluent Distributed resistance terms most significant in tube bundle region,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,10,Evaluation of Modeling Approach,Advantages Computationally efficient D
12、oes an alternate, tractable approach exist? Approach demonstrated to give meaningful data by several authors Disadvantages Loss of some flow details due to averaging Can be overcome by detailed modeling of small regions of condenser,Company Confidential Copyright 2001 Fluent Inc. All rights reserved
13、.,11,Heat Transfer Process,Film condensation on horizontal tube,Cooling Water,Tube Wall,Condensate Film,Liquid-vapor Interface,Refrigerant Vapor,Latent heat released at liquid-vapor interface transferred to cooling water,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,12,Heat Tr
14、ansfer Model,Heat transfer is modeled by coupling of thermal resistance network with CFD code,Tcw,Tt,i,Tt,o,Ti,Rcw,Rtube,Rcond,Cooling Water,CFD code provides interface temperature, Ti Cooling water and tube thermal resistances are generally well-known Film heat transfer coefficient is required for
15、Rcond,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,13,Film Heat Transfer Coefficient,Critical component of heat transfer model Obtain from experiment,Or obtain from literature Steam condensation on smooth tubes,Figure courtesy of Kansas State University, Professor Steve Eckle
16、s, and Duane L. Randall,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,14,Modeling Assumptions,Effect of liquid on flow field is neglected Approach can also be implemented in Eulerian-Eulerian multiphase framework Satisfactory model for liquid phase representation not currently available Published results of this type of model do not appear to show significant advantage Vapor is assumed to be saturated No supe
