Experimental Study On Enhancing Convective Heat Transfer In The Core-flow

Based on the analysis of convective heat-transfer enhancement theory inside tubes, a new theory to improve the overall performance of heat exchangers has been proposed in the article, which investigates the core-flow area inside the tube instead of the boundary-layer area. To achieve a high convective heat transfer coefficient and reduce the drag force, a kind of large length-to-diameter ratio rod insert is used in the core-flow area inside the tubes. Furthermore, it can reduce the size of the heat exchanger, thus reducing the costs associated with both material and manufacturing of the heat exchanger.In the present study, the experiment set-up has been determined,and water is used as the working fluid. The experiments are carried out in square and circular tubes, respectively. In order to check the effect of the internal rods on both the heat transfer rate and pressure drop across the heat transfer test section, flow rates are varied such that the Reynolds number could vary from 1,000 to 8,500. The first test is conducted with a plain tube without a rod insert. The results using the plain tube serve as a basis for comparison for the cases with rod inserts.The results show that the average overall heat transfer coefficients in square and circular plain-tubes are enhanced with rod inserts by as much as 220% and 190% at a constant heat flux, respectively. Although the percentages increase in pressure drop are larger than the percentages increase in heat transfer rate, the overall performances are better compared with those of plain-tubes.That is because a core-flow area with more uniform temperature profile and more obviously varied velocity profile has been formed, and great changes in both velocity and temperature profiles have been taken place in the boundary layer area. This is the essential cause of the increase in the value of PEC (Performance Evaluation Criterion), and the PEC>1 in the experiments, which means a considerable enhancement in heat transfer for the given pumoing power. However, it appears more clearly in the laminar-flow than in the turbulent-flow.At last, the statistical correlations with a high accuracy between the heat transfer coefficient and the Reynolds number are got by treating the great amount of experimental data. So dose the friction factor. The overall performances are analyzed, and a scientific basis for the core-flow area heat transfer enhancement theory is provided. As the flow in modern industrial heat exchangers are confined in a closed space, the method to improve the performance of heat transfer devices presented in the article can be expected to put into common use.