The usage of heat exchangers in numerous fields creates a need to lower their energy and material use as well as increase their efficiency. There are many ways to enhance heat transfer required to design heat exchangers that have high performance. This has given rise to intense investigations on heat transfer enhancement. Generation of structured secondary flow by longitudinal vortices results in heat transfer enhancement with lower energy consumption. This technique has been given more thought because of a relative reduction in friction loss of fluid flow with higher heat transfer. It is attributed to the third generation heat transfer enhancement methods because it does not rely on the steepness of the velocity components made by secondary flow.
Qiang Zhang, Liang-Bi Wang and Yong-Heng Zhang and Lanzhou Jiaotong University in China, investigated the mechanisms of heat transfer enhancement in a laminar channel flow when using longitudinal vortex generators from another view point. The team also purposed to find out how flux with velocity gradient and velocity with flux gradient together contributed to heat transfer in a channel. Their research work is now published in the International Journal of Thermal Sciences.
The scholars began their research by analyzing the mechanisms of heat transfer enhancement by basing it on convective transport equation of heat flux which describes the convective process of heat transfer in a parameter of heat flux. They then carried out a numerical study on how velocity with flux gradient and flux with velocity gradient contributed to heat transfer in a channel with vortex generators and one without. The differences under the varying conditions were noted. The researchers then drew out comparisons based on the convective transport equation of heat flux.
The authors observed that transport of heat flux along the wall normal direction and main flow direction is enforced by local transport of heat flux in span direction. While, the local transport of heat flux in span direction is promoted by longitudinal vortices through enlargement its convection contribution terms. Which in turn enlarges the convection contribution terms in the convective transport equation of heat flux along other two directions. This resulted in the increase of difference of heat flux per unit temperature on both top and bottom surfaces. Therefore, heat transfer enhancement is enforced greatly by longitudinal vortices. The factors that affect the velocity contribution terms are the height and attack angle of the vortex generators which affects the heat transfer enhancement. It was discovered that along the main flow direction there was a decrease in intensity of longitudinal vortices. This showed that when the value of vorticity was large the heat transfer was strong and vice versa.
In general, it is thus noted that the authors have proven their view point of the mechanisms of heat transfer enhancement produced by longitudinal vortices. The reported convective transport characteristics of heat flux in a channel with and without vortex generators shows that the velocity, velocity gradient, heat flux and the gradient itself contribute differently to convective heat transfer. Without helping of the convective transport equation of heat flux, and then screening such detailed different roles of the convection contribution terms regarding the transport of heat flux in different direction, it is impossible to clear the mechanism of heat transfer enhancement by longitudinal vortices. Therefore, it is eminent to give spacial attention to the role of the convective transport equation of heat flux. With helping of this equation, it is expected to find more effective technologies in enhancing convective heat transfer, and decrease the running cost of heat exchangers.
Qiang Zhang, Liang-Bi Wang, Yong-Heng Zhang. The mechanism of heat transfer enhancement using longitudinal vortex generators in a laminar channel flow with uniform wall temperature. International Journal of Thermal Sciences Volume 117 (2017) page 26-43.
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