Natural Convection over Surface (Thermodynamics)?

Question

Consider a walled mounted power transistor that dissipates 0.18W of power in an environment at 35 degree Celsius. The transistor is 0.45cm long and has a diameter 0.4cm. The emissivity of the outer surface of the transistor is 0.1, and the average temperature of the surrounding is 25 degree Celsius. Disregarding any heat transfer from the base surface, determine the surface temperature of the transistor. Use air properties at 100 degree Celsius.

Answer 1

Much of this type of work on Convection Heat Loss has been modeled in the past few years using the concept of : Dimensionless Constants similar to Mach's Number, examples being Nusselt's; Reynold's; Prandtl; & Grashof's Numbers. Look these names up in your Library.
The combination of these Dimensionless Numbers which model Heat Loss from various types of surface are uncovered by experiment. These Empircal Formulae can be found in various books.


When working with Transistors, however, we tend to use the data on heat sinks given by manufacturers in terms of Temperature Rise per Watt dissipated on the heat sink - a type of thermal resistance.

The figures you give are not related to Convection however. They are relevant to Radiation from the transistor body.
The rate of radiation is proportional to the difference of the 4th power of the temperatures (in Kelvins), the emissivity and the surface area of the emitter.
It is a standard ALevel problem. Look it up in a senior school Physics Book.]
Good Luck

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