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Thermal Considerations in Engineering Design: Understanding Heat Transfer Mechanisms

Learn about heat flow mechanisms (conduction, convection, radiation), thermal modeling with electrical circuits, and factors affecting thermal resistance in electronic components. Explore concepts like forced convection and radiation in designing thermal systems.

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Thermal Considerations in Engineering Design: Understanding Heat Transfer Mechanisms

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  1. ECE2799 - Engineering DesignThermal Considerations Prof. Mazumder Prof. Bitar Updated 11/16/2017

  2. Heat Flow • Heat (Power) flows from an area of higher temperature To lower temperature • 3 Heat flow mechanisms • Conduction transferring heat through a solid body • Power easily flows thru high thermal conductivity material (e.g. copper, aluminum) • Convection heat is carried away by a moving fluid • Free convection • Forced convection  uses fan or pump • Radiation • Power is radiated away by electromagnetic radiation

  3. Conduction • Heat is transferred through a solid from an area of higher temperature to lower temperature • Good heat conduction  need large area, short length and high thermal conductivity • Example: aluminum plate, l= 10 cm, A=1 cm2, T2 = 25C (298K), T1= 75C (348K), k = 230 W/(m-K) Thermal Modeling, Marc Thompson, 2000

  4. IC Mounted to Heat Sink Thermal Modeling, Marc Thompson, 2000

  5. New Japan Radio Ver.2014-09-04

  6. Heat flow model by electrical circuit Heat flow  Current ; Temperature  Voltage ; Heat source  Current Source Thermal resistance  Resistor ; Thermal capacitance  Capacitor

  7. Heat flow model by electrical circuit Heat flow  Current ; Temperature  Voltage ; Heat source  Current Source Thermal resistance  Resistor ; Thermal capacitance  Capacitor IS Q : Power dissipated by device  IS TJ : Junction Temp of Device  Voltage at node Tj Rtheta-J-C : Thermal resistance from junction to case TC : Case temp  Voltage at node Tc Rtheta-C-H : Thermal resistance from case to heatsink TH : Temp where heatsink is attached  Node voltage at TH Rtheta-H-A : Thermal resistance of the heatsink TAMB : Ambient temp  Node voltage at Tamb. New Japan Radio Ver.2014-09-04

  8. Thermal Resistance of Electronic Components http://pdfserv.maximintegrated.com/en/an/AN4083.pdf Junction Temperature: TJ = TA + (ΘJA × P) Where: TJ = junction temperature TA = ambient temperature, and P = power dissipation in Watts Maximum Allowable Power Dissipation: Pmax = (TJ-max - TA) / ΘJA Maxim Deration Function: describes how much the power dissipation must be reduced for each °C of ambient temperature over +70°C. The deration function is expressed in mW/°C. Deration function = P / (TJ - TA) Where: TA is typically +70°C (commercial) and TJ is the maximum allowable junction temperature, typically +150°C. To find the maximum allowable power when the ambient temperature is above +70°C (for example, +85°C in the extended temperature range), proceed as follows: Pmax85C = Pmax70C - (Deration Function × (85 - 70))

  9. New Japan Radio Ver.2014-09-04

  10. New Japan Radio Ver.2014-09-04 New Japan Radio Ver.2014-09-04

  11. Convection Heat Transfer Coefficient • Convection can be described by a heat transfer coefficient h and Newton’s Law of Cooling: • Heat transfer coefficient (h) depends on properties of the fluid, flow rate of the fluid, and the shape and size of the surfaces involved, and is nonlinear • Equivalent thermal resistance: Reference: B. V. Karlekar and R. M. Desmond, Engineering Heat Transfer, pp. 14, West Publishing, 1977

  12. Free Convection • Heat is drawn away from a surface by a free gas or fluid • Buoyancy of fluid creates movement • For vertical fin: • Example: square aluminum plate, A=1 cm2, Ta = 25C (298K), Ts= 75C (348K) Thermal Modeling, Marc Thompson, 2000

  13. Forced Convection • Need to use when heat sinks can not dissipate sufficient power by natural convection and radiation • In forced convection, heat is carried away by a forced fluid (moving air from a fan, or pumped water or coolant etc.) • Forced air cooling can provide typically 3-5 increase in heat transfer and 3-5 reduction in heat sink volume • In extreme cases you can do 10x better by using big fans, convoluted heat sink fin patterns, etc. Thermal Modeling, Marc Thompson, 2000

  14. Thermal Performance Graphs for Heatsinks • Curve #1: natural convection (P vs. DTsa) • Curve #2: forced convection curve (Rsa vs. airflow) Thermal Modeling, Marc Thompson, 2000 http://www.electronics-cooling.com/Resources/EC_Articles/JUN95/jun95_01.htm

  15. Radiation • Energy is lost to the universe through electromagnetic radiation •  = emissivity (0 for ideal reflector, 1 for ideal radiator “blackbody”); s = Stefan-Boltzmann constant = 5.6810-8 W/(m2K4) • Example: anodized aluminum plate,  = 0.8, A=1 cm2, Ta = 25C (298K), Ts= 75C (348K) Thermal Modeling, Marc Thompson, 2000

  16. Comments on Radiation • In multiple-fin heat sinks with modest temperature rise, radiation usually isn’t an important effect • Ignoring radiation results in a more conservative design • Effective heat transfer coefficient due to radiation for ideal blackbody ( = 1) at 300K is hrad = 6.1 W/(m2K), which is comparable to free convection heat transfer coefficient • However, radiation between fins is usually negligible (generally they are very close in temperature) Thermal Modeling, Marc Thompson, 2000

  17. Cost for Various Heat Sink Systems • Note: heat pipe and liquid systems require eventual heat sink http://www.electronics-cooling.com/Resources/EC_Articles/JUN95/jun95_01.htm

  18. Comparison of Heat Sinks STAMPED “CONVOLUTED” EXTRUDED FAN http://www.ednmag.com/reg/1995/101295/21df3.htm

  19. Liquid Cooling • Advantages • Best performance per unit volume • Typical thermal resistance 0.01-0.1 C/W • Disadvantages • Need a pump • Heat exchanger • Possibility of leaks • Cost

  20. Heat Pipe • Heat pipe consists of a sealed container whose inner surfaces have a capillary wicking material • Boiling heat transfer moves heat from the input to the output end of the heat pipe • Heat pipes have an effective thermal conductivity much higher than that of copper Thermal Modeling, Marc Thompson, 2000

  21. Thermoelectric (TE) Cooler • “Cooler” is a misnomer; a TE cooler is a heat pump • Peltier effect: uses current flow to pump heat from cold side to warm side • Pumping is typically 25% efficient; to pump 2 Watts of waste heat takes 8 Watts or more of electrical power • However, device cooled device can be at a lower temperature than ambient Thermal Modeling, Marc Thompson, 2000

  22. Other Important Thermal Design Issues • Contact resistance • How to estimate it • How to reduce it • Thermal pads, thermal grease, etc. • Geometry effects • Vertical vs. horizontal fins • Fin efficiency (how close together can you put heat sink fins ?) Thermal Modeling, Marc Thompson, 2000

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