الفهرس | Only 14 pages are availabe for public view |
Abstract The present work is devoted to experimentally investigate the characteristics of the convective heat transfer and pressure DROP of pure water in the annulus side of horizontal double pipe heat exchangers (DPHEs) with and without continuous Helical Fins (HFs) conducted on the outer surface of the internal tube as one of the passive heat transfer enhancement techniques. This work is performed at different geometrical parameters of the HFs and at different operating conditions for the annulus side of the DPHE since they are one of the main parameters that affect the thermal performance of that type of the heat exchangers. Ten DPHEs of counter-flow configurations are constructed; one is without any fins, while the other nine heat exchangers are with continuous HFs fabricated with different HFs height ratio () and pitch ratio (), with ranges of 0.275 ≤ ≤ 1, and 0.333 ≤ ≤ 1. While the annulus-side operating conditions are of volume flow rates of 6.01 to 18.26 l/min; corresponding to annulus-side Reynolds number (Rean) of 2050 ≤ Rean ≤ 15925, with three inlet temperatures, 15, 20 and 25C; corresponding to annulus-side Prandtl number (Pran) of 5.8 ≤ Pran ≤ 8. In the experiments, hot pure water, with constant inlet temperature of 50C and flow rate of 8.07 l/min, is passed through the internal tube while cold pure water is passed through the annulus. Totally, a series of 210 experiments is carried out on the ten heat exchangers; 189 runs are for the heat exchangers with HFs and 21 runs are for case of no fins. The thermal performance results in terms of annulus average Nusselt number, average heat transfer coefficient and Fanning friction factor, in addition to the overall heat transfer coefficient are presented for the different governing parameters. For all experiments, the results showed that installing continuous HFs around the outer surface of the internal tube of DPHEs increases the heat transfer rate in addition to the pressure DROP in the annulus-side when compared with that in the un-finned heat exchangers. Moreover, the annulus average Nusselt number (Nuan) and Fanning friction factor (fan) increase with increasing HFs height ratio, and with decreasing HFs pitch ratio. Compared with plain annulus-case, the results showed that using the HFs increases both Nuan and fan with average increases of 69.4164.4% and 48.6113.1%, respectively, when increases from 0.275 to 1, and with average increases of 78.1183.2% and 67.699.2%, respectively, when decreases from 1 to 0.333. Furthermore, there is a slight increase in annulus average Nusselt number with decreasing the annulus-fluid inlet temperature, while its effect on the annulus-side friction factor can be neglected. Additionally, the hydrothermal performance index (HTPI) is determined to compare the performance of the helically finned double pipe heat exchangers (HFDPHEs) with that of plain annulus case. The results demonstrated that HFs of = 0.667 provides the highest HTPI, while HTPI increases with decreasing . Finally, correlations for Nuan, fan, and HTPI for DPHEs with HFs in the annulus as a function of the investigatedparameters are proposed |