The present study is devoted to experimentally investigate the effect of alumina (Al2O3) nanoparticles concentrations in water, Twisted Tape Insert (TTI) twist ratio and the operating conditions on the saturated flow boiling in a uniformly heated vertical tube. The experiments are carried out for saturated pure water or A2O3//water nanofluid at atmospheric pressure (inlet temperature is 100±2C), flow rate ranges from 1.88 L/min to 9.39 L/min, and for applying heat flux from 27 to 114.5 kW/m2 on the tube surface. In addition, the nanoparticles volume concentration varies from 0 to 1%, and two TTI twist ratios are used; 2 and 4. Totally, 450 test runs are performed; 25 runs are for the tested tube (employed with pure water), 50 runs are for the tested tube fitted with a TTI (employed pure with water), 125 runs are for the plain tested tube (employed with Al2O3/water nanofluid), and 250 runs are for the tested tube fitted with a TTI (employed with Al2O3/water nanofluid). The thermal performance results are presented for the different governing parameters in terms of the average wall superheat (〖T〗_sat), average HTC, average Nusselt number, Fanning friction factor and the hydrothermal performance factor (HTPF) to compare the thermal performance of the flow boiling for employing a TTI with/or Al2O3/water nanofluid in the tube with the thermal performance of the flow boiling for pure water in a plain tube. The experiments demonstrated that increasing both the wall heat flux and the TTI twist ratio in addition to decreasing both the working fluid Reynolds number and Al2O3 nanoparticles loading augment the wall superheat. In addition, compared with pure water in a plain tube, employing Al2O3/water nanofluid and/or a TTI provides a lower wall superheat. The results illustrated also that increasing the wall superheat significantly improves the flow boiling average Heat Transfer Coefficient (HTC) and average Nusselt number, while it has an insignificant effect on the flow friction factor. Additionally, increasing the working fluid Reynolds number significantly augments the flow boiling average HTC and average Nusselt number, while it reduces the flow friction factor. Besides, compared with the pure water flow boiling in a plain tube, employing the Al2O3 nanoparticles and/or TTI increases the average Nusselt number and flow Fanning friction factor. In addition, increasing the Al2O3 nanoparticles loading in the water and/or decreasing the TTI twist ratio significantly augment the flow boiling average HTC and average Nusselt number in addition to the flow friction factor. Moreover, the results indicated that decreasing both the working fluid flow rate and TTI twist ratio and increasing both the Al2O3 nanoparticles loading and the wall heat flux augment the HTPF. Also, employing the Al2O3/water nanofluid provides a higher HTPF than that employing a TTI individually, while employing the Al2O3/water nanofluid and the TTI simultaneously provides the maximum average Nusselt number, flow friction factor and HTPF. Over the studied range of the operating conditions, applying a TTI of y = 2, = 1% and wall heat flux of 114.5 kW/m2 at flow rate of 1.88 L/min (corresponds to Reynolds number based on the swirl velocity of Resw 18070.8) provides the maximum value of the HTPF; 2.22. Finally, set of correlations are proposed to predict the wall superheat, the saturated flow boiling average Nusselt number, Fanning friction factor and the HTPF as a function of the investigated parameters. |
