Heat transfer and pressure drop correlations of microchannel heat exchangers with S-shaped and zigzag fins for carbon dioxide cycles

A new microchannel heat exchanger (MCHE) with S-shaped fins was developed using the three-dimensional computational fluid dynamics (3D CFD) FLUENT code. The MCHE provided 6–7 times lower pressure drop while maintaining heat-transfer performance that was almost equivalent to that of a conventional MCHE with zigzag fins. This study was done to confirm the simulation results of thermal-hydraulic performance using a supercritical carbon dioxide loop, and to propose empirical correlations of Nusselt numbers and pressure-drop factors for a new MCHE with S-shaped fins and a conventional one with zigzag fins. This study is also intended to confirm the independence of Pr obtained in the previous study by widely varying Pr from 0.75 to 2.2. Experimental results show that the pressure-drop factor of the MCHEs with S-shaped fins is 4–5 times less than that of MCHE with zigzag fins, although Nu is 24–34% less, depending on the Re within its range. The Nusselt number correlations are expressed, respectively as Nu S-shaped fins = 0.1740 Re 0.593 Pr 0.430 and Nu zigzag fins = 0.1696 Re 0.629 Pr 0.317 for the MCHE with S-shaped and zigzag fins, and their pressure-drop factors are given as f S-shaped fins = 0.4545 Re −0.340 and f zigzag fins = 0.1924 Re −0.091. The Nu correlation of the MCHE with S-shaped fins reproduces the experimental data of overall heat transfer coefficients with a standard deviation (1 sigma) of ±2.3%, although it is ±3.0% for the MCHE with zigzag fins. The calculated pressure drops obtained from pressure-drop factor correlations agree with the experimental data within a standard deviation of ±16.6% and ±13.5% for the MCHEs with S-shaped and zigzag fins, respectively.