Forced convection heat transfer of concurrent Al2O3/PCM nanofluid flows in a concentric double tube

A trend toward high power and relatively small package sizes in electronic components has resulted in a dramatic increase in the corresponding heating flux; thus, the enhancement of internal convection has been a focus of attention. In this study, to improve the thermal efficiency of the internal flow, the forced-convection heat transfer of concurrent flows of functional working fluids through the outer annulus/inner tube of a concentric double tube was investigated experimentally and compared to that of an identical double tube filled with water, for which limited information is available. The fluids in the outer annulus/inner tube were 0.5% Al2O3-water nanofluid/pure water, 1.0% Al2O3-water nanofluid/pure water, and 1.0% Al2O3-water nanofluid/4.63% phase-change nanofluid. The nominal values of the main parameters were as follows: heating power = 120 W, 160 W, and 200 W; Reynolds number = 800, 1300, and 1700; and concurrent flow ratio = 0.1, 0.29, 0.45, 1.0, 1.6, and 2.38. The results indicated that when the outer annulus and the inner tube were filled with a 1.0% Al2O3-water nanofluid and a 4.63% PCM emulsion/or water, respectively, a high total flow rate and high flow ratio could suppress the rise in the wall temperature and increase the average heat transfer coefficients. Although using a functional working fluid may enhance heat transfer, this gain is considerably smaller than the increase in the pressure drop, resulting in the figures of merit for nearly all the considered cases being lower than 1.