Numerical study on heat transfer enhancement characteristics of tube inserted with centrally hollow narrow twisted tapes

In this study, a new tube insert, named centrally hollow narrow twisted tape, is developed and its effect on the heat transfer enhancement performance of a tube under laminar flow conditions is numerically simulated. The model defines two study variables-hollow width and clearance-and then incorporates the concept of a unilateral twisted tape; the effect of the number of unilateral twisted tapes on the tube heat transfer performance is then examined. The results show that the tube with cross hollow twisted tape inserts has the best overall heat transfer performance for different hollow widths of the tape. Compared with the conventional twisted tape, the optimum overall heat transfer performance of the new type of tape increases by 28.1%. Clearance, which is defined as the width between the tube and twisted tape, also affects the heat transfer performance. The smaller the clearance, the better is the overall heat transfer performance. For different Reynolds numbers, the number of unilateral twisted tapes that gives the optimum overall heat transfer performance is different. If the Reynolds number increases over 600, the tube can achieve the best overall heat transfer performance when the number of unilateral twisted tapes is 4. Empirical formulas for Nu and f are obtained based on calculation results. The results show that under laminar flow conditions, the cross hollow twisted tape is a very promising high-performance tube insert.