4. Heat Transfer. Training Manual 001419 15 Aug 2000 4-2 Heat Transfer Enhancements Radiosity method...

4. Heat Transfer

Training Manual 00141915 Aug 2000

4-2

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.7Heat Transfer Enhancements

• Radiosity method extended to 2-D

• Transient thermal analyses are much faster

4.1 2-D Radiosity

Heat Transfer Enhancements


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2-D Radiosity

• Extends ANSYS 5.6 new feature to provide for efficient handling of 2D radiation problems

• Uses Radiosity method introduced in ANSYS 5.6

• View factors can be queried

• Supports temperature-dependent emissivity


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.7

• Supports open and closed radiating systems

• Supports 2d plane and axisymmetric problems

• Supports multiple enclosures

• Applicable to: PLANE13, PLANE35, PLANE55, PLANE67, PLANE77


… 2-D Radiosity


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.7

• New V2DOPT command activates 2-D radiosity:

V2DOPT, GEOM, NDIV, HIDOPT, NZONE

GEOM: Choice of geometry

0 = planar (default)

1 = axisymmetric

NDIV: Number of divisions for axisymmetric geometry

HIDOPT: View factor option

0 = hidden (default)

1 = non-hidden

NZONE: Number of zones for view factor calculation,

default = 200.


… 2-D Radiosity


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.7

• GUI implementation of V2DOPT:

Main Menu > Radiation Opt > -Radiosity Meth- View Factor . . .

– Set Plane or Axisymmetric

– Select Hidden (default)or Non-hidden method


… 2-D Radiosity

4.2 Fast Transient Solver



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Fast Transient Solver

• Dramatically reduces transient thermal solution time, especially for nonlinear analyses, while maintaining solution accuracy.

• Improved tabular boundary condition efficiency

• Increased element calculation speed


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… Fast Transient Solver

• Speed-up achieved by:

– Direct matrix assembly

– Selective reform strategy

– Improved solver strategy


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Direct Matrix Assembly

• Formulate and assemble element matrices in memory

• Split [K] matrix into sub-[K] matrices as needed:

– [K]solid for temperature-dependent solids

– [K]nl for highly nonlinear elements

– [K]surf for surface boundary conditions

• Similarly, split [C] is split into sub-[C] matrices for in-memory storage

• Assemble sub-matrices and solve equations


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Selective Reform Strategy

• Decision to reform [K] and [C] matrices is based on maximum change in the temperature-dependent material property between current time point and previous reform time point.

– Controlled by Reform Tolerance (default is 5%)

– Entire matrix is reformed if any element’s material property evaluation exceeds tolerance


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Selective Reform Strategy (cont'd)

• Used for temperature-dependent solid matrices [K]solid and [C]solid

• Nonlinear matrices and surface boundary condition matrix terms are re-computed every substep

• Supported by ICCG and JCG solvers


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Improved Solver Strategy

• Both ICCG and JCG now use the previous solution as a starting point for pre-conditioning

– Drastically reduces the number of iterations to converge

• Auto time-stepping now considers three criteria for controlling time step size

– Maintain time-step such that the maximum property change (since the last reform step) stays at the Reform Tolerance

– Evaluate Rayleigh criteria and adjust time step (pre-existing criteria)

– Heuristic criteria based on maximum temperature change per time step (implemented at Release 5.4)


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Improved Solver Strategy (cont'd)

• No equilibrium iterations are performed

• Memory guidelines: 1 GB per 1 Million DOF’s


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Other efficiencies gained:

• Tabular boundary conditions now processed more efficiently

• Temperature-dependent material property tables combined into a single table look-up

• OUTRES with the “none” option now more efficient

• Surface boundary condition processing is now more efficient


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Procedure

• THOPT, Option, ReformTol, nTabPoints, TempMin, TempMax

– Option:• NONLIN - Use Full reform Strategy (default)

• QUASI - Use Selective Reform Strategy

• LINEAR - Do not reform element matrices

– ReformTol - Material property change tolerance for matrix reform. Defaults to 0.05 (5%).

– nTabPoints - Number of temperature points for the Fast Material table. Defaults to 64.

– TempMin, TempMax - Min and Max temperatures for Fast Material table, obtained from MPTEMP command.


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Procedure (cont'd)

• Requires either ICCG or JCG solver

• Typical command sequence

antype,trans

thopt,quasi,.05,64,

eqslv,iccg


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Procedure (cont'd)

• For non-zero initial conditions, set NSUB > 1 to resolve temperature dependencies. Typical command sequence:

antype,trans

thopt,quasi,.05,64,0,1200

autots,off

timint,off

nsubst,3 ! Resolve temperature dependencies

eqslv,iccg

time,...

...

solve

timint,on

...


4-21

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• Example

– Transient non-linear thermal analysis (benchmark performed by a large customer)

– Statistics• 119,600 SOLID70 tetrahedral elements

• 34,588 SURF152 surface-effect elements

• 9 MASS71 lumped thermal elements

• 8 FLUID116 thermal-flow elements

• 29,300 DOF’s

– Unsymmetric matrices

– Tabular boundary conditions

– Mission duration - 1200 sec.


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Compaq Alpha 4100 5/533: Performance increase (45/2) = 22.5

Benchmark results

4. Heat Transfer. Training Manual 001419 15 Aug 2000 4-2 Heat Transfer Enhancements Radiosity method...

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Transcript of 4. Heat Transfer. Training Manual 001419 15 Aug 2000 4-2 Heat Transfer Enhancements Radiosity method...