• Energy Research

Local thermal non-equilibrium (LTNE) effects in the developed region of the forced convection in a circular tube filled with saturated porous medium are analytically studied at the constant wall-temperature boundary condition, as well as at the iso-flux boundary condition. The flow in the pipe is described by the Brinkman-Forchheimer-extended Darcy equation. A two-equation model is used for the energy balance. Profiles describing the velocity field obtained by perturbation techniques are used to find the temperature distributions using the successive approximation method. Moreover, the velocity and temperature fields are simulated numerically to validate the results of the analytical part. A fundamental relation and a new dimensionless number, ΔNE, for the temperature difference between the fluid and solid phases (LTNE intensity) are established based on a perturbation analysis. It is found that the LTNE intensity (ΔNE) is proportional to the product of the normalized velocity and the dimensionless temperature at LTE condition and depends on the conductivity ratio, Darcy number, and the porosity of the medium. Finally, the proposed relation for the LTNE intensity is simple and fundamental for estimation of the importance of LTNE condition.

The analytical heat transfer performance study (i.e. HTP, introduced as the ratio of the heat transfer rate enhancement to the pressure drop increment) for microchannels enhanced by a porous material is performed. The Darcy–Brinkman equation of motion and the two-energy equation model for heat transfer within the porous material (i.e. the local thermal non-equilibrium condition, LTNE) are used to find the Poiseuille (Po) and Nusselt (Nu) numbers in the slip-flow regime (i.e. when the Knudsen number, Kn, < 0.1). To cover a wide range of heat exchangers, effects of hydrodynamical characteristics of the porous material (i.e. porosity, ϕ, and Darcy number, Da), the effective thermal conductivity ratio (k), the interphase convective heat exchange between the two phases (Bi), the velocity-slip and temperature-jump coefficients (α and β), the dimensionless thermal resistance of the walls (Rw), and the internal heat generations within the solid and fluid phases of the porous material (ωf and ωs) are considered. HTP maps are presented for a wide variation range of all pertinent parameters to design or to select a micro-heat exchanger according to the heat transfer enhancement with respect to the pressure drop cost. Results reveal that the HTP may effectively be increased by inserting rarefied porous inserts. The present analysis suggests the implementation of porous inserts within flow passages especially when 0.001 < Kn < 0.1 (the slip-flow regime).

Abstract Constrained fluid flow and conjugate heat transfer in microchannel heat sinks (MCHS) with converging channels are investigated using the finite volume method (FVM) in the laminar regime. The maximum pressure of the MCHS loop is assumed to be limited due to constructional or operational conditions. Results show that the Poiseuille number increases with increased tapering, while the required pumping power decreases. Meanwhile, the Nusselt number increases with tapering as well as the convection heat transfer coefficient. The MCHS having the optimum heat transfer performance is found to have a width-tapered ratio equal to 0.5. For this tapering configuration and at the maximum pressure constraint of 3000 Pa, the pumping power reduces by a factor of 4 while the overall heat removal rate is kept fixed in comparison with a straight channel.

The convection heat transfer inside a tube filled with a porous material under the constant heat flux thermal boundary condition which is widely used in practical applications is studied in the present article. The Darcy–Brinkman–Forchheimer model is used to cover a wide range of working mediums from clear fluid flow to slug flow (Darcy flow). The case of temperature-dependent thermal conductivity is considered in the present study and the corresponding Nusselt number is analytically obtained using perturbation techniques for the first time. The change in thermal conductivity with respect to temperature occurs at high-temperature applications wherein high-temperature variations exist as well as the radiation heat transfer. A linear model for the thermal conductivity variation with temperature is considered in the present study. The obtained profile for the Nusselt number can be used for quick calculations as well as validation of numerical and experimental studies, especially at high temperatures wherein the experimental studies are accompanied by higher uncertainties. The results show that the Nusselt number increases linearly with the linear increase in the thermal conductivity and as well the heat transfer rate. Furthermore, results show that the Nusselt number (and the heat transfer rate as well) shows more augmentation to the thermal conductivity enhancement due to the temperature-dependent nature of thermal conductivity (especially arising from the radiation heat transfer) in comparison with the clear fluid flow case.

In this paper, thermal performance of a room heated with an attached sunspace has been experimentally investigated and has been modeled using the EnergyPlus software. A fixed porous bed was placed beneath the room's floor to store the heat. To increase the heat storing capacity, water tanks was also used. The room with attached sunspace was built and tested in Karaj (Iran). Results illustrated that applying water tanks in the sunspace had a noticeable role in heat storing capacity. For example, the porous bed together with water tanks saved at least 37% (up to 87%) of energy cost compared to 10% (up to 15%) saved energy cost without water tanks. In addition, the simulation of annual thermal performance of the room by Energy-Plus software confirmed the applicability of the used configuration of sunspace along with added water tanks and porous bed made form arranged bricks as heat storing media a step forward to a completely green room (net zero energy building). The sunspace with water tanks showed a more stable condition (lower temperature variations) which is a base of thermal comfort.

Abstract The cooling of photovoltaic panels plays an important role in improving electrical efficiency and increasing the lifetime. In this paper, a radiation shield for filtering the thermal part of solar irradiance has been proposed as a passive cooling method. A 3D model of a PV module with all consisting layers has been numerically simulated. The radiative transfer equation has been solved by discrete ordinates (DO) method. The numerical results of the uncooled PV module have been compared with the experimental data. The radiation shields with 40% and 80% reflections in the wavelength interval between 1 μm and 2.4 μm have been performed to reduce the thermal effect of solar irradiation over the PV module. The effects of radiation shield on the instantaneous and monthly average surface temperature of the PV module have been investigated. It has been found that the effect of radiation shield on temperature reduction of the PV module is more considerable in seasons with high solar irradiation. The numerical results show that the radiation shield with 80% reflectivity has more effect on temperature reduction and the electrical efficiency of the PV module. The numerical results reveal that, for solar irradiation between 1025 and 1142 W/m2, the radiation shield with 80% reflection can increase the electrical efficiency by about 5.14%–5.72%.

Abstract The continuous and the binary techniques are the main types of genetic algorithms that could be employed for an optimization problem. The study investigates the robustness and accuracy of each technique for the wind turbine blades design and optimization problem. For that purpose, the geometry of the blade was designed for the maximization of the output power, the desirable goal of the blade design. The design variables consist of the distribution of the chord and twist along the blade. Results indicate that the continuous genetic algorithm outperforms the binary one from the standpoint of the accuracy and also the computational time. However, the use of uniform crossover could improve the convergence rate of the binary genetic algorithm. Moreover, the sensitivity analysis of the applied genetic algorithms with respect to the population size and the mutation rate was performed to find the appropriate values for those parameters. The outcomes emphasized that adopting a small number of population size together with a large number of generations could speed up the convergence rate of the problem. Also approved in the study was the efficiency of the so-called “Superblade” operator which improved the convergence rate of the algorithm and resulted in the powerful blades.

Abstract This paper is concerned with measurement of buoyancy-driven air flows between the two compartments of a half-scale model of a stairwell. The flow was maintained by a heater placed in the lower compartment. The results of earlier investigations are extended by considering a through-flow via two small openings, one in the lower and the other in the upper compartment. The stairwell model and its instrumentation are described. The results include velocity and temperature measurements, volume and mass flow rates of air circulating between the lower and upper compartments and rate of heat loss from the stairwell; all are presented for various heat input rates. For the range of experimental conditions and sizes of the openings considered, the through-flow mass flow was about 15% of the circulation flow rate. The results also include the rate of heat and mass transfer via the openings.