Heat exchangers are devices widely used in the industrial and civil fields, from air conditioning to the chemical sector. Heat exchangers allow heat rejection from a hot fluid to a cold fluid without any mixing process.
The ever growing demand of energy efficiency requires the optimisation of such devices. As a consequence, once the type of heat exchanger is chosen during the sizing process, the designer must deal with an optimisation problem with several degrees of freedom.
If, for example, we take into account a finned cross-flow heat exchanger, we should determine the material and the dimension of pipes and fins, the number of columns, rows, fins, and the distance among them. The only constraints of this problem are mass flow rates, desired outlet temperature and global size.
Technical literature offers several correlations, such as Zukauskas correlation, that can help in the design process for standard working condition. Anyway, in most cases working conditions are far from being standard; therefore, correlations offer poor results. For this kind of situation, Phi Drive can give a great contribution with CFD analysis, thus offering the most efficient solution from the technical and economical points of view.
An example from the CFD analysis
Below is shown a cross-flow heat exchanger. Inside the pipes there is a flow of hot water, while outside them flows cold air. From the image we can observe the temperature increment of air due to heat exchange with water that, in turns, cools down.
Fin implementation allows the increment of exchanging surface and therefore of heat exchanged. In the following image, taken from another CFD analysis, we can observe the same cross-flow heat exchanger of before but finned. Fins increase heat exchange: air reaches a higher temperature while air a lower one.
Heat exchangers are also used to make a flowing liquid evaporate, or make a gas flow condensate. Such phenomena are complex as they involve phase change, hence properties change. Below is shown the phase change (in terms of volume fraction) of water that turns into steam.