Experimental thermal and flow characteristics in a traverse corrugated tube fitted with regularly spaced modified wire coils

The work presents experimental investigations on turbulent thermal and flow characteristics in a traverse corrugated tube (TCT) fitted with regularly spaced wire coils with gradually varying width (WCs-GVW). The experiments were performed in the TCT with WCs-GVW having constant heat flux conditions at Re = 6000 to 18000 by using air as working fluid. The goal motivated by this compound heat transfer enhancement technology was to induce transverse vortices by ribs and to generate swirl flows by WCs, aimed to impart flow separation and reattachment and flow mixing, and hence improvement in heat transfer. Three arrangements including the diverging arrangement of WCs (DWC), the converging arrangement of WCs (CWC) and the diverging and converging arrangement of WCs (DCWC) and five space ratios at S/D = 0.60, 1.55, 2.97, 5.34 and 10.09 were considered. Effects of those parameters on Nusselt number (Nu), friction factor (f), performance evaluation criterion (PEC), Bejan number (Be) and augmented entropy generation number (φs) were analyzed. The main findings are that the CWC arrangement possesses the lowest PEC and the Nu and f in the DWC arrangement increase with decreasing the space ratio. In contrast to the plain tube, the Nu in the TCT with WCs-GVW increases nearly 1.74 to 2.26 times while the f is in a range of about 4.18-10.68 times. The obtained results show that the maximum PEC of about 1.13 is achieved by the WCs-GVW with DWC arrangement at S/D = 10.09 and Re = 14102. However, all compound ones exhibit lower PEC values than the alone use of the TCT. In addition, it is found that the Be in the baseline of the plain tube is decreased via the employment of the alone TCT or co-use of the TCT and WCs-GVW, nevertheless, the total entropy production is dominated by the entropy production due to heat transfer under considered parameters. The φs in the plain tube is reduced to be less than unity by using the enhanced devices indicates the thermodynamic advantage of those TCT and TCT/WCs-GVW, and the lowest φs of about 0.5 is gained by utilizing the WCs-GVW with DWC arrangement at S/D = 0.60 and Re = 6517. At last, the empirical formulas of the investigated TCT and TCT/WCs-GVW in terms of Nu and f are proposed.