Thermo-Hydraulic Performance of a Fin-and-Tube Heat Exchanger with Differently Configured Curved-Rectangular Vortex Generators

Abstract

In this study, the thermal and hydraulic performances of three curved-rectangular vortex generators (VG_1, VG_2, and VG_3) in a compact fin-and-tube heat exchanger were numerically investigated. The secondary flow theory and field synergy principle were employed to characterize the effects of VGs on the flow characteristics and hence the heat transfer enhancement. The thermal and volume goodness enhancement factors were utilized to evaluate the overall performance of VG configurations on the engineering aspects. Results reveal that the secondary flow and synergy between the flow and temperature fields are significantly promoted by applying VGs, by which the heat transfer performance is enhanced. By increasing the tube row number from 3 to 5, the global Nusselt number increases for all the VG cases due to the improved thermal mixing of air flow and fin efficiency. However, in terms of the per-tube heat rejection enhancement, the configuration with tube row number of 3 is most effective for all the VG designs. Results indicate that VG_2 has achieved the best balance between the thermo-hydraulic and volume goodness criterions. The obtained thermal and volume goodness enhancement factors by VG_2 reach as high as 1.42 and 1.24, respectively, manifesting its potential for the development of high-performance and compact fin-and-tube heat exchangers.

Notes

1 The installation of VGs has indicated significant influence on the flow structure and thermal mixing within the flow channels. VG_1 and VG_2 which have the VG height equal to 0.6 time of the fin spacing generate longitudinal vortices to suppress the recirculation zones, whereas VG_3 which has the VG height equal to the fin spacing generates dual transverse vortices behind each VG to form recirculation zones. As a result, VG_1 and VG_2 have achieved much better thermal mixing and thinner thermal boundary layers than that in VG_3. As compared to VG_1 that has the VGs punched out from the plate fins, VG_2 which fabricates the VGs directly on the plate fin surfaces yields much stronger longitudinal swirl flows within the flow channel.

2 By applying VGs, the secondary flow is significantly enhanced, while the synergy angle between the velocity vector and temperature gradient is decreased. Results indicate that the higher secondary flow intensity while smaller synergy angle is beneficial for the convective heat transfer performance. Among the VG cases, VG_2 has achieved the highest secondary flow intensity while smallest synergy angle, which offers VG_2 the best heat transfer performance. Moreover, it is found that the local synergy angle has shown direct impact on the local heat transfer performance while the local secondary flow intensity has inconspicuous influence.

3 As N increases from 3 to 5, the maximum enhancement of the global $\overline{Nu}$ is increased from 37.2% to 49.1% for VG_1, 38.4% to 54.8% for VG_2, and 32.8% to 48.0% for VG_3. The better heat transfer performance at a larger N is attributed to the further improved fin efficiency and thermal mixing of air. However, in terms of the per-tube heat rejection enhancement, N = 3 is the most effective configuration for all the VG cases.

4 According to the thermal enhancement factor ($\mathit{PEC}/PE{C}_0$) and volume goodness enhancement factor ($(Q/V\Delta T)/{(Q/V\Delta T)}_0$), VG_2 has achieved the best balance between the thermo-hydraulic performance and per-volume heat rejection capability, which makes it the best option in developing the high-performance and compact fin-and-tube heat exchanger in this study.

Additional information

Funding

This work was supported by The Natural Science Foundation of Guangdong Province, China (2018A030313878, 2019A1515012223) and The Natural Science Foundation of China (51908142).

Notes on contributors

Jinlong Xie

Jinlong Xie is an Associate Professor in the School of Mechanical and Electrical Engineering, Guangzhou University, China. He received his Ph.D. degree from Nanyang Technological University, Singapore, in 2014. He worked as a research scientist in Temasek Laboratories@NTU, Singapore, from 2015 to 2018. His main research interests include heat transfer enhancement, energy storage system, thermal management of power electronics, and computational heat transfer.

Hsiao Mun Lee

Hsiao Mun Lee is an Associate Professor in the School of Mechanical and Electrical Engineering, Guangzhou University, China. She received her Ph.D. degree from Nanyang Technological University, Singapore, in 2014. She worked as a research scientist in Temasek Laboratories@NTU, Singapore, from 2014 to 2015, and then worked as research fellow in National University of Singapore from 2015 to 2018. Her main research interests include computational fluid mechanics and heat transfer, energy storage system, and acoustic and vibration control.