Numerical investigation on heat transfer and flow characteristics in helically coiled mini-tubes equipped with dimples

Abstract

Heat transfer and flow in dimpled helically coiled mini-tubes are numerically examined. The effects of Reynolds number Re, ratio of tube diameter to coil diameter t, dimple density d and dimple size s are examined. It is shown the dimple case provides a higher Nusselt number Nu upon the smooth case. The highest and lowest dimensionless heat transfer coefficients Nu/Nu0 are 2.281 and 1.788, respectively. The Nu/Nu0 firstly rises and then declines against the increase in Re, producing a peak value. Greater Nu/Nu0 is achieved when a lower t is used. A decrease in d or s results in a higher Nu/Nu0 with Re lower than particular-Re, while an opposite trend occurs with Re higher than particular-Re. The highest and lowest evaluation parameter (Nu/Nu0)/(f/f0)1/3 are 1.830 and 1.437, respectively. A better performance from energy saving viewpoint is achieved with a lower t. Besides, great reduction in wall temperature level is achieved due to dimples. With the increase in Re, the flow pattern develops accompanied with more obvious separation and reattachment behavior. Utilization of dimples produces significant promotion in velocity level close to the boundary layer, which is principally responsible for heat transfer enhancement. The boundary velocity coefficient shows great similarity to Nu/Nu0. Tangential velocity is greatly promoted when dimples are fixed, which is believed to generate intensified secondary flow. The thickness of the low velocity layer and the high temperature region in the fluid domain are greatly reduced due to dimples. Furthermore, a lower temperature gradient level and a more uniform fluid temperature field are achieved owing to dimples. The dimple case brings about great reduction on average heat transfer entropy generation level, with enhanced effect at a lower t. An evident decrease in maximum wall temperature is identified when dimples are used. Correlations for predicting Nusselt number and friction factor are obtained.