fluent 13.0中的自然对流问题

Default treatment
T = 2 °C V = 1.2 m/s
Gb is a sink term for stable stratification Turbulent mixing decreases stratification.
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∂ ∂ (ρ k ) ∂ (ρ ui k ) = + ∂ xj ∂ xi ∂t ⎡⎛ μ ⎞ ∂k ⎤ ⎜μ + T ⎟ ⎢⎜ ⎥ + Gk + Gb − ρ ε − YM + Sk σk ⎟ ∂ x j ⎥ ⎢⎝ ⎠ ⎣ ⎦
g
T = 100 °C V = 1 m/s
Gb set to 0 in the k Equation
Velocity Velocit field Initial temperature (K)
Temperature field
Expected flow pattern during cool down in pipe cross section due to buoyancy forces
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L4-5
Release 13.0 December 2010
Customer Training Material
Turbulent Boundary Layers
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L4-6
Release 13.0 December 2010
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L4-4
Release 13.0 December 2010
Heat Transfer Modeling using ANSYS FLUENT
Laminar to Turbulent Transition
5. Examples 5 E l
a. Validation case – Tall cavity b. Baseline cases
i. ii. C osed do a Closed domain with high Rayleigh t g ay e g number Plume with high Rayleigh number
τ yx
y
Γx
τ yx +
∂τ yx ∂x
dy
T = Tf
P
• For this class of problems, flow and energy are strongly coupled.
Forces acting on a fluid particle in natural convection.
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L4-2
Release 13.0 December 2010
Customer Training Material
Natural convection: Theory & Definitions
– Experiments show that the critical Rayleigh number, Rac, is around 109. – The transition zone is quite large as Ra varies between 106 and 1010.
Buoyancy force Kinemati c viscosity
3.Modeling i 3 M d li tips
a. Pressure discretization b. Time stepping for unsteady simulation
c. Industrial case – Glass tank TC21
6. References pp 7. Appendix
– Typically gravitational – Centrifugal (rotating machinery) – Coriolis (atmospheric and oceanic vortical motion)
Customer Training Material
P+
T = Tw
∂P dx ∂x
L4-8
Release 13.0 December 2010
Customer Training Material
Turbulent Flow Heat Transfer
The Full Buoyancy Effects Option
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Heat Transfer Modeling using ANSYS FLUENT
Boundary wenku.baidu.comayers
• Impact on numerical modeling for turbulent flow
– Energy and momentum equations are strongly coupled.
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L4-3
Release 13.0 December 2010
Heat Transfer Modeling using ANSYS FLUENT
Phenomena
• In natural convection, fluid motion is generated due to density difference (buoyancy) in d it diff (b )i the fluid caused by temperature gradients. • Body forces
L4-1
Release 13.0 December 2010
Heat Transfer Modeling using ANSYS FLUENT
Outline
1.Theory/Definition
a. Phenomena b. b Transition to turbulent flow
Customer Training Material
Customer Training Material
Lecture 4 L t Natural Convection
Heat Transfer Modeling using ANSYS FLUENT
ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved.
Release 13.0 December 2010
Heat Transfer Modeling using ANSYS FLUENT
Boundary Layer Grid Generation
Customer Training Material
• Start with a 2D test case – This is a good way to confirm what are the characteristic integration time steps and mesh size required for the d i d h i th desired physics.
Customer Training Material
• In natural convection, the Reynolds number no longer characterizes the flow. • With an appropriate reference velocity, it is possible to determine a critical value of the Rayleigh number (RaL).
4. Model setup in FLUENT
a. Reference density and temperature b. Boussinesq vs. incompressible ideal gas
2.Turbulence
a. Dynamic vs. thermal boundary y y layer b. Full buoyancy effect
Customer Training Material
Vertical Wall Thermal Boundary B d T Layer
w
Free Stream
L
x
T∞
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L4-7
L4-10
Release 13.0 December 2010
Heat Transfer Modeling using ANSYS FLUENT
The Full Buoyancy Effects Option
• To include buoyancy effects on ε, you must enable the Full Buoyancy Effects option under B Eff t ti d Options in the Viscous Model panel. • This option is available for the three k–ε models (SKE, RKE, RNG) and for the Reynolds stress model (RSM). t d l (RSM) • Available for k–ω models via UDF (shown in the Appendix).
• It is recommended to construct the mesh such that y+ < 1 in order to correctly resolve both the momentum AND thermal viscous sublayers. • This is straightforward for Pr ~ 1 or Pr < 1. • When Pr > 1 the thermal sublayer is 1, much thinner than the momentum viscous sublayer. • This behavior is relatively insensitive to grid resolution provided that the resolution, momentum boundary layer structure is accurate (y+ ≤ 1 for the first cell layer and at least 10 cells between 1 < y+ < 30).
β g L3 ΔT Ra L = GrL Pr = να
Kinematic viscosity Thermal diffusivity
wher e
Pr =
ν α
Thermal diffusivit y
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L4-9
Release 13.0 December 2010
Heat Transfer Modeling using ANSYS FLUENT
Turbulence Generation due to Buoyancy
Customer Training Material
• The importance of the buoyancy term (Gb) can be seen in a mixing layer example using the standard k–ε turbulence model.