الفهرس 
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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