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In this paper, a ferrofluid-based cooling technique is proposed for solar photovoltaic (PV) systems, where ferrofluid flow can be easily altered by the application of an external magnetic field leading to enhanced heat transfer from the hot surface of PV systems. The effect of both constant and alternating magnetic field on ferrofluid flow through a minichannel is explored numerically in the present work. A detailed parametric study is performed to investigate the effect of actuation frequencies of alternating magnetic field (0.5–20 Hz) and Reynolds numbers (Re = 24, 60, and 100) on heat transfer characteristics of ferrofluid. An overall enhancement of 17.41% is observed for heat transfer of ferrofluid in the presence of magnetic field compared to the base case of no magnetic field. For the case of alternating magnetic field, a critical actuation frequency is observed for each Reynolds number above which heat transfer is observed to decrease. The enhancement or decrease in heat transfer of ferrofluid is found to depend on several factors such as actuation frequency of alternating magnetic field, Reynolds numbers of ferrofluid flow, and formation/dispersion of stagnant layers of ferrofluid at the magnet location. Preliminary visualization of ferrofluid flow is also carried out to provide a qualitative insight to the nature of transportation of ferrofluid in the presence of an alternating magnetic field.

Experimental and analytical investigation was carried out to show the effect of surface tension on the heat transfer coefficient of gas flow inside micropipe heat exchangers during the condensation process. An empirical correlation was formulated for the coefficient of the convective heat transfer. A comprehensive comparison between the experimental results with published correlations in the literature and that obtained by the developed correlation was conducted. The results showed that the predicted heat transfer coefficient of the proposed correlation was closer to the experimental results than any other correlation, a finding related to the consideration of the surface tension.

An experimental study on single-phase laminar forced convection in a single porous tube heat exchanger is presented. Parametric studies are conducted for different inlet pressures, different mass flow rates, and different porosities to evaluate the effects of particle diameter and Reynolds number on the heat transfer and friction factor. The Nusselt number and friction factor are developed for efficient design of a porous heat exchanger based on the present configuration. Heat is transferred to the walls of the heat exchanger by natural convection mode. Gravel sand with different porosities is used as a porous medium during the tests. The flow of carbon dioxide as a working fluid in the porous medium is modeled using the Brinkman–Forchheimer-extended Darcy model. A dimensionless performance parameter is developed in order to be used in evaluating the porous tube heat exchanger based on both the heat transfer enhancement and the associated pressure drop. The study covers a wide range of inlet pressures (Pi), mass flow rates ( ), porosity of gravel sand (e), and particle diameters (dm) which ranged 34.5 ≤ Pi ≤ 43 bars, 8 ∗ 10−5 ≤ ≤ 16 ∗ 10−5 kg/s, 34.9% ≤ e ≤ 44.5%, 1.25 ≤ dm ≤ 5.15 mm, respectively. This study revealed that a smaller particle diameter can be used to achieve higher heat transfer enhancement, but a larger particle diameter leads to a more efficient performance based on heat transfer enhancement. The average heat transfer coefficient of carbon dioxide decreases when the porosity increases. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21059

Three different types of heat exchangers were tested experimentally to investigate two-phase heat transfer coefficient and pressure drop during the condensation process of CO2 gas. Experimental results revealed that the convection heat transfer coefficient was enhanced by a factor of four due to the existence of porous media and by a factor of seven due to the use of micro-pipes when compared to the normal macro-tubes. The pressure drop was measured and noticed only in porous tubes and micro-pipes, reaching about 17.5 kPa/m and 8.4 kPa/m, respectively. Comparisons between experimental and correlated results were conducted.