Maximizing Performance by Minimizing Thermal Resistance
The task of optimizing performance and minimizing thermal resistance can be best demonstrated by a theoretical example. Consider a heat transfer process where 50/50 ethylene glycol and water (EGW) is cooled by ambient air in a plate-fin heat exchanger. Figure 2 illustrates the heat flow path through the heat exchanger using an electrical analogy.
In this example, heat flows by convection between temperatures TH and T1, then by conduction between temperatures T1 and T2, and finally by convection between T2
. The total thermal resistance is then equal to the sum of the three thermal resistances in series.
By comparison, a cold plate typically has only one coolant flowing through it. As a result, heat flows by conduction from the heat-dissipating electronic device mounted on the cold plate through the thermal interface material and cold plate materials. Heat then flows by convection from the internal surface of the fluid path material to the coolant.
As shown in the example above, if we want to maximize heat transfer we must minimize thermal resistance. To accomplish this, we must increase the corresponding heat transfer areas, the film coefficients, or both. Increasing the heat transfer area is relatively easy in concept, though sometimes constrained by application requirements such as weight, size, and pressure drop. An effective way to increase the heat transfer area is to increase the fin density (fins per unit length). Increasing the film coefficient is more complicated, however, because the film coefficient is dependent upon the properties of the fluid being considered, the fluid velocity, and the fin geometry.