Characteristics of the Absolute Vorticity Flux along the Main Flow Direction on the Cross Section of the Channel Formed by Oval Tube Bank Fins

Abstract

Secondary flow as a heat transfer enhancement strategy is frequently used to increase heat transfer coefficients, but its mechanism of heat transfer enhancement is not well known. There are two important steps to understand this mechanism: 1) to specify the intensity of the secondary flow in more general case; and 2) to relate it with heat transfer intensity. It has recently been found that the absolute vorticity flux along the main flow direction can be used to specify the intensity of the secondary flow in more general cases. This article reports the quantitative relationship between the intensity of secondary flow and the heat transfer enhancement for the oval tube bank fin heat exchanger. The relationship between the strength of secondary flow and the intensity of convective heat transfer is studied numerically; the effects of the geometrical parameters such as eccentricity, fin spacing on the absolute vorticity flux, and thermo-hydrodynamic characteristics are considered. The results reveal that the production, development, and dissipation of the horseshoe vortices can be exhibited by the local distribution of the absolute value of the absolute vorticity flux in the main flow direction on the cross section of the flow passage; except at the beginning region of boundary layer and the wake regions behind the tubes, there is a close relationship between the absolute vorticity flux along the main flow direction and the span-averaged Nusselt number; this means that the absolute vorticity flux can only account for the contribution to convective heat transfer made by the secondary flow. The reported results can explain the mechanism of heat transfer enhancement by the secondary flow, because when the secondary flow plays a major role in convective heat transfer, the larger the absolute vorticity flux in the main flow direction, and the stronger the heat transfer intensity. For real application, the absolute vorticity flux can be used to select the optimal geometrical parameters for good heat transfer performance when the secondary flow is used to enhance convective heat transfer.

L. B. Wang acknowledges support from the National Natural Science Foundation of China (No. 50876040).