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Design of Heat Sinks -- II

Design of Heat Sinks -- II. P M V Subbarao Professor Mechanical Engineering Department IIT Delhi. Power Consumption For Cooling may be Higher than Power for Computing !!!!!!. Steps in Design of Forced Convection Heat Sinks. Analytical modeling Maximization of heat dissipation

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Design of Heat Sinks -- II

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  1. Design of Heat Sinks -- II P M V Subbarao Professor Mechanical Engineering Department IIT Delhi Power Consumption For Cooling may be Higher than Power for Computing !!!!!!

  2. Steps in Design of Forced Convection Heat Sinks • Analytical modeling • Maximization of heat dissipation • Least-material optimization • Design for manufacture

  3. Design Calculations for Fin Arrays – Thermal Resistance In order to select the appropriate heat sink, the thermal designer must first determine the maximum allowable heat sink thermal resistance. To do this it is necessary to know the maximum allowable module case temperature, Tcase, the module power dissipation, qmod, and the thermal resistance at the module-to-heat sink interface, Rint. s d b b Tbase Tcase The maximum allowable temperature at the heat sink attachment surface, Tbase, is given by Rint Tcase

  4. The maximum allowable heat sink resistance, Rmax, is then given by • The thermal resistance of the heat sink is given by

  5. The gap, S, between the fins may be determined from

  6. Structure of Heat Sink : Constant air velocity

  7. Structure of Heat Sink : Constant volumetric flow rate

  8. Cooling Medium Addition of a fan to any heat sink changes passive cooling to active cooling. Addition of a fan to a typical heat sink almost always improves thermal performance. Thermal resistance of typical heat sink 5 °C/W with natural convection can change to 1.2 °C/W with addition of fan. Most demanding applications use liquid cooling in place of air cooling further improves heat sink performance. To dissipate 1000 watts of heat with a 10 °C temperature rise would take thousands of times less volumetric water flow than airflow. Liquid cooling can dissipate more heat with considerably less flow volume, maintain better temperature consistency, and do it with less local acoustic noise.

  9. Fan Characteristic Curves

  10. Selection of Fans : Heat Sink Pressure Drop To determine the air flow rate available in a heat sink and fan system, it is necessary to estimate the heat sink pressure drop as a function of flow rate. This is called as system curve. Match system curve to fan characteristic curve (pressure drop versus flow rate).

  11. Effect of Changing A Fan Fan A Heat Sink A Fan B Heat Sink B

  12. Pressure Drop Curves

  13. Effect of number of fins and fin height

  14. Thermal Resistance

  15. Junction Temperature Vs Number of Fins

  16. Closure A fan with a different fan curve is employed, the flow rates will change and the optimum heat sink design point may change as well. The important point is that to determine how a heat sink will perform in a given application both its heat transfer and pressure drop characteristics must be considered. It should also be noted that an underlying assumption is that all the flow delivered by the fan is forced to go through the channels formed between the heat sink fins. Unfortunately this is often not the case and much of the air flow delivered by the fan will take the flow path of least resistance bypassing the heat sink. Under such circumstances the amount of flow bypass must be estimated in order to determine the heat sink performance.

  17. Design Strategies • One modify several independent variables to improve heat sink performance with respect to your design criteria. • A larger heat sink surface area, for example, will improve cooling, but may increase the cost and lead-time. • Using the entire volume available will provide maximum cooling, but if this provides more cooling than necessary, you can reduce the heat sink volume to reduce its cost and its weight. • Increasing the base thickness distributes heat more uniformly to the fins if the package is smaller than the heat sink, but increases weight. • The interface material can have a significant affect on assembly costs as well as on thermal resistance. • Thicker fins, on the other hand, provide more structural integrity and may be easier to manufacture, but increase the weight for a given thermal resistance.

  18. Design Optimization • A heat sink that cools adequately fulfills only part of the objective. • Optimizing the design creates the best available heat sink solution for the application and benefits the overall system design. • Some advantages of optimization include: • minimization of thermal wake effects: the impact of heat sinks on downstream components • accommodation of changes in system CFM due to additional heat sink pressure drop • weight reduction to pass shock and vibration tests • elimination of extra heat sink support (additional holes and real estate in the PCB) • reduction of costs gained from use of the same heat sink for multiple components and use of off-the-shelf catalog parts.

  19. Homogeneous ODE How to obtain a non-homogeneous ODE for one dimensional Steady State Heat Conduction problems? Blending of Convection or radiation effects into Conduction model.

  20. Define: How to get strictly non-homogeneous Equation?

  21. One Dimensional Non-Homogeneous Conduction Equation P M V Subbarao Professor Mechanical Engineering Department IIT Delhi A truly non-homogeneous ODE….….. A Basis for Generation of Tremendous Power….

  22. Conduction with Thermal Energy Generation A truly non-homogenous ODE. Consider the effect of a process occurring within a medium such as thermal energy generation, qg, e.g., Conversion of electrical to thermal energy in an electric rod. Curing of concrete brides and dams. Nuclear fuel rod. Solid Propellant Rockets.

  23. Heat Transfer in Rocket Solid Propellent

  24. Fast Construction of Bridges : RCC Technology Convection & Radiation Convection & Radiation The term ‘curing’ is used to include maintenance of a favorable environment for the continuation of chemical reactions, i.e. retention of moisture within, or supplying moisture to the concrete from an external source and protection against extremes of temperature.

  25. Plane Wall with Thermal Energy Generation For one-dimensional, steady-state conduction in an isotropic medium properties: q’’’ homogeneous medium with constant properties:

  26. The temperature distribution is parabolic in x. Using Fourier’s law: Heat transfer rate: Thus, dT/dx is a function of x, and therefore both the heat transfer rate and heat flux are dependent on x for a medium with energy generation.

  27. Boundary Conditions Solution : Case 1: Simple Dirichlet Boundary Conditions: & q’’’ Maximum temperature occurs inside the slab close the higher temperature surface.

  28. Boundary conditions Solution : Case 1: Convection Boundary Conditions: At x = -L : q’’’ At x = L :

  29. heat transfer rate:

  30. with

  31. Boundary Conditions Solution : Case 3: Symmetric Dirichlet Boundary Conditions: q’’’ Maximum temperature occurs at the center of the slab. The left and right parts of the slab are isolated at the axis.

  32. Modified Boundary Conditions ``` Solution : Case 3: Symmetric Dirichlet Boundary Conditions:

  33. Boundary Conditions At x = L : Solution : Case 3: Symmetric Convection Boundary Conditions:

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