B1 Fundamentals of Heat Transfer

The term heat is defined by the first law of thermodynamics as the energy that is transported across the boundary of a thermodynamic system due to a temperature difference between the system and its surroundings. The first law of thermodynamics in a general form can be written as follows: (1) $$\Delta E = W + Q + E_{\rm{M}}.$$ The right hand side of this equation summarizes the three different forms of energy that can be transported across the system boundary: heat Q, work W, and energy E _M that is tied to a mass transport. As a result of such energy transfer across the system boundary the energy inside the system changes by ΔE, written on the left hand side of the equation. The transport process related to the transfer of heat is called heat transfer. Applying the second law of thermodynamics, one can derive that heat is always transferred in the direction of decreasing temperature. But thermodynamics does not tell us how the amount of heat transferred depends on this driving temperature difference or temperature gradient. Nor does it tell us how it depends on the geometry of a heat exchanger, or on material or process properties, including the duration of the process. Before these dependencies can be described in detail some general relations and definitions must be given.