No abstract

In this study, natural convection of heat transfer in various channel geometries with a constant surface area under laminar flow condition has been investigated numerically. Various hot surface temperatures (Th = 35-95°C) have been applied on the surfaces of the channels to investigate four different geometries of annular channels (Circular (C), Square (S), Elliptic (E) and Airfoil (F)) on the heat transfer rate. Once the optimum geometry was exhibited, the effect of three nanofluids (Al2O3/water, CuO/water and SiO2/water) is investigated in the analysis and compared to pure water to enhance the convective heat transfer of the base fluid. Moreover, with these nanofluids, analysis has been performed for three different volume concentrations of nanoparticles of Ø = 2%, 4% and 6% along with 0% (pure water). Porous foams (ε = 0.9 to 0.99) were used in addition to nanofluids to see if heat transfer could be improved. Results indicate that the heat transfer rate was greatly increased when the airfoil geometry was used, with a maximum and minimum increase in heat transfer coefficient of 60% and 46%, respectively. Also, higher nanoparticle of Al2O3 dispersion to the base fluid enhances the heat transfer rate by 15% compared to other nanofluids.

In order to maximize the thermal efficiency of shell and coil heat exchangers, substantial research has been done and geometrical modification is one way to improve the exchange of thermal energy between two or more fluids. One of the peculiar features of coiled geometry is that the temperature distribution is highly variable along the circumferential section due to the centrifugal force induced in the fluid. Moreover, most researchers are concentrated on using a shell and single helical coil heat exchanger to enhance the heat transfer rate and thermal efficiency at different operating parameters. Therefore, the aim of this study is to investigate temperature variation ((T-1, T-2, T-3 and T-4) across a shell and single/double coil heat exchanger at different coil pitches, hot water flow rate, and cold-water flow rate along the outer surface of the coil using experimental and numerical analysis. For single and double coil heat exchangers, Computational Fluid Dynamics (CFD) is carried out using pure water with a hot water flow rate ranging between 1-2 l/min for the coil side heat exchanger. For single coil heat exchangers, the numerical analysis findings showed a good agreement with experimental four-temperature measurement results (T-1, T-2, T-3 and T-4) with an error rate of 1.80%, 3.05%, 5.34% and 2.17% respectively. Moreover, in the current double coil analysis, the hot outlet temperature decreased by 3.07% compared to a single coil (baseline case) at a 2.5L/min hot water flow rate. In addition, increasing the coil pitch will increase the contact between the hot fluid and the coil at a constant hot water flow rate and thereby decrease the hot fluid outlet temperature. Finally, a computational analysis was carried out to examine the flow structure inside single and double coil heat exchangers, and the findings indicated that the effect of centrifugal forces in double coil heat exchangers at various coil pitches caused the secondary flow to be substantially reduced.