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March 8th, 2010 at 11:49 am

Heat transfer in evaporators mode

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The literature of heat transfer in general recognize three distinct modes of heat transfer: conduction, convection and radiation. Strictly speaking, conduction and radiation should be classified as heat transfer processes, because only these two mechanisms depend for their operation on the mere existence of a temperature difference. The last of the three, convection leves, is not strictly conform to the definition of heat transfer because it depends for its operation on mechanical mass transport. But because the convection fulfills even the transmission of energy from regions of high temperature regions to lower temperatures, the term “transfer of heat from the convection” has become generally accepted in most situations heat flows not one, but many of these mechanisms at the same time.

Conduction is the transfer of heat from one part of a body in another part of the same body or a body more on physical contact with it, without appreciable particle displacement of the body. Conduction can occur in solid, liquid or gas.

Radiation is the transfer of heat from one body to another, not in contact with it, by means of electromagnetic waves moving through space, even when a gap that exists between them.

Convection is the transfer of heat from one point to another within a fluid, gas or liquid, the mixing of an aliquot with another. In natural convection, the movement of the fluid entirely is the result of density differences resulting from differences in temperature; in convection, the movement is produced by mechanically. Forced when the speed is relatively low, it should be realized that “freeconvection” factors such as the density and temperature difference, can have a major influence.

In solving problems of heat transfer, it is necessary not only for the heat transfer mode which play a role to recognize, but also to determine whether a process is constant or Unsteady. When the rate of heat flow in a system does not vary with the time-when is constant-temperature anywhere not edit conditions prevail and stationary. At steady state conditions, the rate of heat input at any point of the system should be exactly equal to the rate of return of heat and no change in internal energy can take place. Most of the problems of transfer of heat engineering concern steady state systems.

The flow of heat in a system is temporary or unstable when temperatures at various points of the system change with time. Since a change of temperature indicates change internal energy, we conclude that the accumulation of energy is associated with unstable heat flux. Problems of heat-irregular are more complex than are those of steady-state and often can only be solved by methods approximate.

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