Experimental investigation of energy conversion efficiency using nanofluids in mini-channel heat exchanger

Hybrid (combined) heat transfer enhancement methods (mini-channel & nanofluids) are utilized in heat exchangers (e.g. chemical, food, petroleum, nuclear, automotive, electronics, mechanical, aerospace and military defense industries), and other thermal devices (miniature Polymerase Chain Reaction-PCR devices, radars, high-temperature thermo-photovoltaics, thermal cameras, electronic processors, etc.) within thermal systems. These methods must also increase energy conversion efficiency (ECE) between required pumping power (PP) and transferred thermal power (TP). This study designed a compact shell-and-tube heat exchanger (STHE) by reducing the hydraulic diameters of the shell and tube sides using mini-channels. Unlike the literature, this experimental study focused on the overall ECE of water and different low volumetric concentrations (VCs) of Al 2 O 3 -W nanofluids (0.2 %, 0.4 %, 0.6 %, and 0.8 %) through a tube bundle on the shell side with three ECE metrics. In the experiments carried out at fifteen volume flow rates (60–500 L/h), contrary to the expectation, the Nusselt ( Nu ) number of base fluid (BF) water was on average 34.8 %, 19.8 %, 29.9 %, and 36 % higher than the nanofluids at VCs of 0.2 %, 0.4 %, 0.6 %, and 0.8 %, respectively. The pressure drops measured with nanofluids were 1.2, 1.4, 1.6 and 1.9 times as high as BF water. The balance between the TP transferred in the tube bundle flow and the PP was negatively affected compared to the BF water. The PP of nanofluids increased by 17.1 %–79.4 % on average compared to water. On the other hand, the TP decreased by 10.1 %–22.4 % on average. Therefore, the reduction in the ECE of the mini-channel heat exchanger was 1.5–5.7 times. The thermal enhancement factor (TEF) results of 0.2 %, 0.4 %, 0.6 %, and 0.8 % concentrated nanofluids averaged 0.64, 0.74, 0.62, and 0.55, respectively. The efficiency evaluation criterion (EEC) results averaged 0.73, 0.71, 0.57, and 0.49, respectively. The performance evaluation criterion (PEC) results of the same nanofluids were 27 %, 29 %, 43.1 %, and 50.8 % lower than BF water. The results calculated from both this study and the literature show that nanofluids are futile.