Experimental and theoretical investigation of the performance of an air to water multi-pass heat pipe-based heat exchanger

In this paper, the performance of a multi-pass heat pipe-based heat exchanger (HPHE) is investigated experimentally and theoretically. The heat pipe system consists of copper heat pipes in a specific equatorially staggered configuration to facilitate heat transportation from a hot gas (air) to a water flow, which cools the condenser section of these heat pipes. The effect of the Reynolds number on the heat transfer rate was studied by altering the number of passes for the evaporator section for the same system by the incorporation of various baffles and by varying the water flow rate. The experimental results have highlighted the strong correlation between heat exchanger performance and the Reynolds number. By increasing the number of passes from one to five, the effectiveness of the HPHE was improved by more than 25%. It has been demonstrated that increasing the number of passes increases the Reynolds number of the flow, leading to higher heat transfer coefficients and lower thermal forced convection resistances. The HPHE overall performance, as well as, the outlet temperatures of the fluids were predicted through two theoretical models, based on the Log Mean Temperature Difference (LMTD) method and the Effectiveness-Number of Transfer Units (ε-NTU) method. The predictions were compared with experimental results and the accuracy of the models reported. The validation showed that the developed iterative LMTD model predicted the performance of the HPHE within ±15.5% error. In comparison, the ε-NTU model predicted the total effectiveness with a maximum error of 19% and was able to predict the outlet temperatures of both air and water streams within an accuracy of ±0.7 °C. The reported research is of importance for the application of heat pipe heat exchangers in waste heat recovery. Finally, knowledge is provided on the accuracy of the available prediction models.