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1、Customer Training Material Lt4Lecture 4 Natural Convection Heat TransferHeat Transfer Modeling using ANSYS FLUENT L4-1 ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 ANSYS FLUENT Customer Training Material Heat Transfer Modeling using ANSYS FLUENT Outline 1.
2、Theory/Definition a. Phenomena bTransition to turbulent flow 4. Model setup in FLUENT a. Reference density and temperature b. Transition to turbulent flow 2.Turbulence a. Dynamic vs. thermal boundary temperature b. Boussinesq vs. incompressible ideal gas 5El yy layer b. Full buoyancy effect 3 M d li
3、i 5. Examples a. Validation case Tall cavity b. Baseline cases i.Closed domain with high Rayleigh 3.Modeling tips a. Pressure discretization b. Time stepping for unsteady simulation C osed doatgay e g number ii.Plume with high Rayleigh number c. Industrial case Glass tank TC21 simulation 6. Referenc
4、es 7. Appendix L4-2 ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 pp Customer Training Material Natural convection: Theory Top= 298 K; gx= -9.81 m/s2Conditions:Pop 101325 Pa; Top 298 K; gx 9.81 m/s Models:Standard k with enhanced wall treatment, Energy acti
5、vated Solver Controls: Default under-relaxation SIMPLE scheme for pressure-velocity coupling PRESTO! pressure discretizationPRESTO! pressure discretization 2ndOrder Upwind discretization for u, v, T, k, Fluid Properties: DensityBoussinesq; 0= 1020 kg/m3 Thermal Expansion Coefficient = 0 000294 K-1Th
6、ermal Expansion Coefficient = 0.000294 K-1 Cp (J/kg-K)4000 J/kgK Thermal conductivity0.5 W/mK Viscosity0.0009 kg/ms Boundary Conditions Hot wall set to 45.5 C Cold wall set to 4.3 C L4-36 ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 Customer Training Mater
7、ial Heat Transfer Modeling using ANSYS FLUENT Subsea Connection Box Results Bottom of the geometry (adjacent to hot wall) development of a bdlhhdliboundary layer on the heated steel pipe. Heat transfer from the bottom of the internal pipe to the fluid zone of the connection box. Top of the geometry
8、(adjacent to cold wall)Laminar boundary layer Top of the geometry (adjacent to cold wall) Laminar boundary layer appears near the wall. Inner (Hot) WallOuter (Cold) Wall L4-37 ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 Customer Training Material Heat Tra
9、nsfer Modeling using ANSYS FLUENT Subsea Connection Box Using the steady solver for this problem is unstable!p No possibility to converge. Large residuals Predicted flow field is not Predicted flow field is not physical (see below) Heat transfer fluxes not balancedbalanced L4-38 ANSYS, Inc. Propriet
10、ary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 Velocity Vectors Customer Training Material Heat Transfer Modeling using ANSYS FLUENT Subsea Connection Box Transient approach Convergence achieved Low residuals Flow physical Heat transfer fluxes well balanced L4-39 ANSYS, Inc. Pr
11、oprietary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 Velocity Vectors Customer Training Material Heat Transfer Modeling using ANSYS FLUENT Subsea Connection Box Transient term acts like an under- relaxation factor for the coupling between flow and energy equationsbetween flow a
12、nd energy equations (we are not interested in the transient nature of the flow). Transient solution approachpp Start with a large time step size (10,000 seconds). Not realistic but allows information to convect rapidlyconvect rapidly. Perform only one iteration per time step. Gradually decrease the
13、time step size tfi/ithl tito refine/improve the solution. Successively decrease by one order of magnitude until t 1 second. This is close to a realistic time step size L4-40 ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 for this Rayleigh number. Customer Tr
14、aining Material Example Case #3 High Rayleigh NumberHigh Rayleigh Number Natural Convection Past a Cylinder L4-41 ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 Customer Training Material Heat Transfer Modeling using ANSYS FLUENT High Rayleigh Number Flow Pa
15、st a Cylinder Computational strategy Grid 6,877 cells Solver Pressure-based, double precision Both steady and unsteady Turbulence Models RNG kd lithhdll ttt RNG k model with enhanced wall treatment L4-42 ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 Custome
16、r Training Material Heat Transfer Modeling using ANSYS FLUENT High Rayleigh Number Flow Past a Cylinder The steady approach method described is not applicable in the case of natural convection around an obstacle. Time step evaluation for unsteady simulation Estimate the time constant from Tg LL U L = = PrRa 2 Use a time step of t = / 4 L4-43 ANSYS, Inc. Proprietary 2
