Heat transfer

There are two definitions of “heat transfer”. The phenomenon of heat transfer is the transfer of thermal energy (heat) in physical systems. Heat transfer as a field of study is an important aspect of the field of thermodynamics dealing with the transfer of thermal energy in systems. Transfer of thermal energy can only be described statistically, and is still not fully understood. In many commonly encountered systems, the only currently accurate descriptions of heat transfer relationships are entirely empirical. This makes scientific study of heat transfer rather complex. However, heat transfer is hardly a minor field. Accurately predicting the transfer of thermal energy is important in almost every aspect of engineering, and there are several engineering disciplines devoted exclusively to the study and manipulation of heat transfer (thermal engineering, HVAC engineering, etc.). Heat transfer is arguably the single aspect of thermodynamics most commonly dealt with by the general public.

Most heat transfer can be described according to the means of energy transmission. There are three:

Conduction, in which heat is transferred via direct contact
Convection, in which heat is carried by fluids in motion
Radiation, in which heat is transferred by the emission and absorbtion of radiation

According to the first law of thermodynamics, net heat transfer always tends from regions of high temperature toward regions of low temperature. The rate of heat transfer between two regions (such as the inside and outside of a greenhouse) depends on the temperature difference between the two. Heat transfer always occurs across some area, and the rate of energy transfer is proportional to the area. The rate of conductive heat transfer is determined by the thermal conductivity of the materials. The rate of convective heat transfer is described by a heat transfer coefficient. The rate of radiative heat transfer is described by the Stephan-Boltzman constant. The contributions of each of these heat transfer methods can be combined to yield an overall heat transfer coefficient for a given system.