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This numerical investigation aims to study the turbulent characteristics and thermal enhancement parameter of CuO–water nanofluids through heat exchangers enhanced with double V-cut twisted tapes. The twist ratio of the twisted tapes is 5.25, and the cut ratio (b/c) is varied from 0 (conventional twisted tape) to 1.8. The Reynolds number is in the range of 5000–15,000, and nanoparticles volume fraction is in the range of 0 < ϕ < 1.5 % . The flow is fully turbulent, and (RNG) k − ϵ turbulent model is used for the numerical analysis. The results reveal that strong turbulent kinetic energy and additional vortex flow through the cuts of the modified twisted tapes are the main reason for better fluid mixing and heat transfer enhancement. The heat transfer enhances about 14.5% for the case of ϕ = 1.5 % . Furthermore, using double V-cut twisted tapes improves the Nusselt number of the nanofluid flow inside heat exchangers about 138% compared to conventional twisted tape without cuts. The maximum value of the thermal performance η = 1.99 is achieved by using nanofluid with ϕ = 1.5 % and b/c = 1.8 at Re = 5000. [ABSTRACT FROM AUTHOR]

The present work investigates the natural convection of nanofluids in Rayleigh-Benard problem. New models, particularly some enhancement factors, are presented in this research to calculate the heat transfer characteristics of nanofluids around critical Rayleigh numbers, including the criteria in which the creation of Rayleigh-Benard and transition to turbulent flow occurs. The present models are compared with available models in the literature to reveal discrepancies of theoretical models in prediction of nanofluids heat transfer characteristics. This research is conducted for SiO2, Cu,CuO and Al2O3 as nanoparticle and water as the base fluid and discussed the SiO2-water nanofluid in more detail. Results indicate that the Nusselt number and convective heat transfer ratio are reduced for SiO2-water nanofluid for almost all volume fractions and as a result using SiO2 as nanoparticle into water (the base fluid) is not desired for a natural convection in Rayleigh-Benard problem. Moreover, the onset of natural convection is delayed due to the reduction in enhancement factor. [ABSTRACT FROM AUTHOR]

This paper presents numerically an appropriate position of a porous insert to get a better thermohydraulic performance from a porous heat exchanger. The simulation is based on the Darcy-Brinkman-Forchheimer model in the porous field. Two-dimensional continuity, momentum, and energy equations with incompressible, laminar, steady assumptions have been solved using a finite volume approach. The analysis is performed for different values of porous layer thickness, length, and porosity at a fixed value of Reynolds number (Re = 100) and thermal conductivity ratio (Rc = 5). The results showed that there is about a 48% and 13% reduction in pressure drop and Nusselt number, respectively, by decreasing horizontal porous substrate thickness from 1 to 1/2 for δv = 1/3 at ε = 0.7. As a result, the pressure drop reduces considerably with a reasonable reduction in heat transfer rate by decreasing horizontal porous substrate thickness from 1 to 1/2. [ABSTRACT FROM AUTHOR]

The present work investigates an analytical study on the problem of laminar film condensation on a nanosphere. Due to the microscale interaction, the problem is analyzed by taking into account the effects of slip in velocity and Jump in temperature. A relation is derived for the liquid film thickness in the form of a nonlinear differential equation which is solved numerically using the fourth order Runge-Kutta method. Finally, the effect of velocity slip and temperature jump on different condensation parameters including the liquid film thickness, velocity and temperature profiles, Nusselt number, and liquid mass flow rate is discussed. It is found that the increase in the velocity slip and temperature jump results in a thinner liquid film and therefore increases the heat transfer coefficient. [ABSTRACT FROM AUTHOR]

The present work studies heat transfer of the gas fluid in open microchannels at constant wall heat flux boundary conditions. The proposed model assumes the fluid is a continuum, but employs a slip boundary condition on the channel wall. Using appropriate similarity variables, the fundamental equations of the boundary layer are transformed to ordinary differential equations containing a Prandtl number. These ordinary differential equations are solved numerically using a fifth order Runge-Kutta and shooting method. Consequently, the velocity profiles, the temperature profiles, the wall shear stress and the local Nusselt number exhibit a dependence on the slip coefficient. The results illustrate that the thermal creep term has a very small effect on the solution. It is shown that laminar boundary layer of gas fluid with slip flow tends to increase near wall fluid axial velocity and temperature jump on the wall as the flow becomes more rarefied. It will be seen that heat transfer in open microchannels is affected with temperature jump more than slip velocity under constant wall heat flux boundary condition. [ABSTRACT FROM AUTHOR]

In this paper, an optimization procedure is performed by response surface methodology to optimize the position and size of the partition inside a single slope solar still. The partition is installed separately at bottom surface and glass cover of the still to improve the performance. The optimization procedure is performed to determine the maximum Nusselt number as a response. Two-dimensional steady equations with laminar assumption have been solved using a finite volume approach. Analysis is performed for a fixed value of Rayleigh number. Beside this, a sensitivity analysis is performed to calculate the sensitivity of the response (Nusselt number) to the position and size of the partition. Results show that the real optimized parameters for the maximum normalized Nusselt number of bottom installed partition are X′ = 0.23 and Y′ = 0.18. These parameters for the the maximum normalized Nusselt number of top installed partition are X′ = 0.53 and Y′ = 0.56162. Also, the installation of partition leads to an increase in vortices number with smaller sizes. Smaller vortices provide sufficient pathways to heat exchange and increase the still efficiency. [ABSTRACT FROM AUTHOR]