Thermal performance enhancement of a double-tube heat exchanger with novel twisted annulus formed by counter-twisted oval tubes.

A novel twisted annulus between two counter-twisted oval tubes is proposed for a double-tube heat exchanger. The oval tubes of the annuli have same twist pitch, but twist with opposite twisting directions. The thermohydraulic performance of the annuli with different aspect ratios and twist ratios is numerically presented. The results indicate that strong secondary flow is generated in the twisted annulus, which can significantly promote the heat transfer. The maximum Nu and f of the twisted annuli are separately 157% and 118% larger than those of the corresponding straight annuli. The largest value of the thermal performance factor is 1.98 in the studied range of geometrical parameters. Correlations of Nu and f as a function of Re , twist ratio and aspect ratio are proposed with the deviations within ±10% and ±8%, respectively. [ABSTRACT FROM AUTHOR]