<P> A thermal diode or thermal rectifier is a device that causes heat to flow preferentially in one direction . </P> <P> A heat exchanger is used for more efficient heat transfer or to dissipate heat . Heat exchangers are widely used in refrigeration, air conditioning, space heating, power generation, and chemical processing . One common example of a heat exchanger is a car's radiator, in which the hot coolant fluid is cooled by the flow of air over the radiator's surface . </P> <P> Common types of heat exchanger flows include parallel flow, counter flow, and cross flow . In parallel flow, both fluids move in the same direction while transferring heat; in counter flow, the fluids move in opposite directions; and in cross flow, the fluids move at right angles to each other . Common constructions for heat exchanger include shell and tube, double pipe, extruded finned pipe, spiral fin pipe, u-tube, and stacked plate . </P> <P> A heat sink is a component that transfers heat generated within a solid material to a fluid medium, such as air or a liquid . Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems or the radiator in a car . A heat pipe is another heat - transfer device that combines thermal conductivity and phase transition to efficiently transfer heat between two solid interfaces . </P>

What are the different types of heat transfer