Energy production generates large volumes of hot exhaust gases. By using heat exchangers and evaporators, the thermal energy from these gases can be recovered, and gases can be prevented from entering the environment at excessively high temperatures. In a heat exchanger, thermal energy is transferred across a solid partition from a hot medium (gas or liquid) to a cold one.
Most heat exchangers are equipped with fins to improve the transfer of total thermal energy. There are appropriate heat exchanger geometries for a wide variety of applications. Regardless of the different designs, the goal in developing heat exchangers is, on the one hand, to maximize the amount of heat transferred and, on the other, to minimize flow resistance. Flow resistance is expressed as a pressure drop across the heat exchanger.
The fluid mechanical processes in the boundary layer along the flow channels formed by the fins are primarily responsible for the temperature increase and pressure drop. In the velocity boundary layer, the local velocity increases from zero at the wall to a specific value. The velocity profile at the wall determines the flow resistance. Equivalently, the temperature profile at the wall in the thermal boundary layer determines the amount of heat transferred. Since experimental measurement of the boundary layers inside the heat exchanger is extremely difficult, the development of heat exchangers is primarily based on a global experimental approach. This means that the pressure drop and temperature increase across the heat exchanger are measured for various operating states. Information about local flow effects in the boundary layers cannot be determined in this way.
However, fluid flow simulations can be used to attempt to gain a deeper insight into the internal processes of the heat exchanger. The following two illustrations present the velocity and temperature distribution in a cross-section between the heat exchanger fins (blue = low values, red = high values).
Using these numerical results, it was possible to identify potential for optimization and to develop new types of heat exchangers.
