Flow & Heat Transfer Calculation in Heat Pipe
Abstract: The calculation of heat pipe includes the in-compressive vapor flow, the working fluid in porous media, the interconnection of the two parts, vapor/liquid phase change, couple of velocity and temperature and so on complicated physical phenomenon. Using FEPG to simulate heat pipe flow/heat transfer calculation shows the convenience and advantage of its handling multi-field coupling problems.
1. Background
Heat pipe is one of ideal method for heat transfer. It is widely used in aerospace, energy, electricity, nuclear and solar energy and other engineering areas due to easily design and manufacture.
Fig 1. Zalman (Korean) heat pipe hard drive disk
Fig 2. Construction of heat pipe
Fig 3. Comparison of different cooling methods
2. Heat Pipe Calculation Overview – Interconnection of Multi-field
Heat Source (High Temp) Heat Source (Low Temp) Heat Conduction within the Wall Variables Variables Variables Temperature, Input/Output Heat
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Heat Conduction within the Wall |
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Variables |
Variables |
Variables |
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Temperature, Input/Output Heat |
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Flow/Heat Transfer within the Wick |
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Variables |
Phenomenon |
Control Equations |
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Temperature, Velocity u, v, Pressure drop |
Phase Change, Flow in porous media… |
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Interface of Fluid and Vapor |
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Variables |
Phenomenon |
Control Equations |
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Evaporator, Condenser, Capillary Pressure |
Mass, momentum, energy balance between two phases, Saturated Vapor |
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Flow/Heat Transfer within the Core of HP |
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Variables |
Variables |
Variables |
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Temperature, Velocity u, v, Pressure drop |
Temperature, Velocity u, v, Pressure drop |
Temperature, Velocity u, v, Pressure drop |
3. Equations:
3.1 Continuity Equation
3.2 Momentum Equation
For vapor, the void coefficient is 1.0, and its permeability coefficient can set a large number such as 1.0E10.
3.3 Energy Equation
In vapor and porous media:
In wall area:
3.4 Boundary Condition
3.5Coaxial Symmetry
3.6 Condition of Vapor-Liquid Interface
Mass continuity condition:
Saturated Vapor at the interface:
Condition at Wick-Wall Interface:
4. Key Issues for Heap Pipe Calculation
1. Solve flow and temperature field of vapor in core part and water in porous media at different place and with different material;
2. Coupling of momentum equation and energy equation
3. Momentum jump at vapor and liquid interface;
4. Convection item
4.1 Pressure fluctuating at large Re
4.2 Iteration of non-linear equations
5. Flow chart of solving
Heat pipes are passive devices of very high "effective" thermal conductivity. The heat /mass is transferred by evaporation (in the evaporator section), convection (in the adiabatic section), condensation (in the condenser) or by wicking in the porous medium. The presence of condensation requires the modeling of a phase change in the condenser.
Designers face the challenge of allowing the proper operation of the heat pipe with the minimum temperature drop, without superseding any of the heat pipe's "limits". This involves avoiding the capillary limit, boiling effects, reaching the sonic or entrainment limits or inducing a flood risk. The task of the designer is further complicated by the dynamic behavior of the heat pipe under operating conditions, including the interaction with the surrounding system.
The Fluent solution allows the designer to accurately model the heat and mass transfer in the heat pipe. Beyond the typical heat exchange phenomena (conduction, convection and possible radiation), phase change can accurately be modeled by the Fluent CFD. Accurate modeling of the operating window and the resulting performance of the heat pipe can thus be obtained faster than ever before.
Schematic of a heat pipe
Temperature variation
Pressure variation