جستجوی مقالات مرتبط با کلیدواژه "natural convection" در نشریات گروه "مکانیک"

In this research, the combined heat transfer of natural convection, conduction, and radiation has been investigated in a solid block including two cavities filled with gas (air), where the left wall of the block has a temperature of Th, the right wall of the block has a temperature of Tc. The upper and bottom walls of the block are thermally insulated. The block contains two cavities, whose internal surfaces are diffuse gray in terms of radiation, and the gas inside the cavity is a nonparticipating medium. The research is carried out by the finite element method and Comsol Multiphysics software version 6.1. The results show that with the increase of the Rayleigh number, the convective Nusselt number increases while the radiative Nusselt number decreases. But due to the low contribution of surface radiation in the heat transfer rate, the total heat transfer rate increases with increasing Rayleigh number. Increasing emissivity reduces the convective Nusselt number and increases the radiative Nusselt number so that it decreases the total heat transfer rate. Increasing the thermal conductivity coefficient ratio also strengthens the natural convection while it deteriorates the radiation exchange.

The present research aims to investigate the combination of natural convection and surface and volumetric radiation in wateraluminum oxide nanofluid with non-Newtonian effects in a square cavity with a conductive baffle on its hot wall. The internal walls are radiatively active. The mass, momentum, and energy conservation equations are solved using the control-volume method and the SIMPLE algorith in the Fortran software for the non-Newtonian nanofluid. The volumetric radiation term in the energy equation is approximated using the Rosseland method. The effect of the baffle position and size, the volumetric radiation parameter on the flow field and the heat transfer rate are investigated. In lower Rayleigh numbers (Ra=103 ), increasing baffle length improves thermal performance by about 80%, but in high Rayleigh numbers (Ra=105 ) the average Nusselt number decreases by about 16%. Accordingly, there is a critical Rayleigh number in which increasing the length of the baffle does not affect the average Nusslet number. A rise in the volumetric radiation parameter in the range 0 ≤ ≤ 2 increases the average Nusselt number by about two times at Ra=105

In this study, the effect of DBD plasma actuator on natural convection heat transfer enhancement in four vertical channels with the smooth wall, rectangular corrugation, trapezoidal corrugation and triangular corrugation has been numerically studied. In order to investigate the effect of using a plasma actuator and channel geometry, local Nusselt number, variations of average Nusselt number ratio, and performance evaluation criterion on the flow field and heat transfer are studied. The results show that increasing voltage increases the average Nusselt number for all channels. For example, in Rayleigh number 105 , by increasing voltage from 6 kV to 10 kV, the average Nusselt number ratio is 89.11, 96.47, 102.50, and 96.62% for vertical channels with smooth, rectangular corrugation wall, trapezoidal corrugation wall, and triangular corrugation wall, respectively. Also, the performance evaluation criterion in the presence of the DBD plasma actuator for all vertical channels has a value higher than 1.25 in all conditions, which indicates the effectiveness of the DBD plasma on the thermal efficiency of the natural convection heat transfer in smooth and corrugated channels

In this study, the natural convection in a square cavity filled by two immiscible Newtonian and non-Newtonian liquids heated partially from bottom wall was numerically investigated. The heat source placed on the bottom of a cavity produces a uniform heat flux of q˳". The remaining parts of the bottom wall of the cavity are insulated and the vertical and top walls are kept at cold temperature of TC. The study investigates the effects of relevant parameters such as the Non-Newtonian to Newtonian volume ratio (0≤H≤0.7), the power-law index (n=0.6 and 1.4), Rayleigh number (103 ≤Ra≤106 ) and the Non-Newtonian Prandtl (Pr2=100 and 1000) on flow and temperature fields and the rate of heat transfer. The governing dimensionless equations for the power-law and Newtonian fluids flow, based on properties of Newtonian fluid with Pr1=100, are solved with the numerical finite difference method based on the control volume formulation and SIMPLE algorithm. In order to evaluate the code, its results were compared to another paper in the field of non-Newtonian fluids. Increasing the average Nusselt number for heat source means that the source temperature difference with the cold walls decreases. The results show that increasing the non-Newtonian fluid Prandtl number due to decreasing the thermal conductivity of the non-Newtonian fluid increases the heat source temperature and decreases the average Nusselt number. Also with stronger natural convection in the square enclosure, the average Nusselt number for non-Newtonian fluid increase with decreased power law index.

In the present paper, mixed convection of water-copper nanofluid in a double lid-driven square cavity consisting of two insulated rotating cylinders is studied numerically. To solve two phase modeling of the nanofluid, the Buongiorno model has been implemented. The governing equations have been solved utilizing the Finite Element Method. The parameters studied in this paper include the effect of different configurations of the cylinders, the rotation direction of the cylinders, the angular speed of the cylinders (Ω), the distance of the cylinders from the moving walls (xc), and the Reynolds number (Re). According to the presented results, the horizontal positioning of the rotating cylinders is the optimal configuration. The rotation direction of the cylinders has an important effect on the heat transfer rate from the hot surface and according to the results, the optimal direction depends on the distance of the cylinders from the walls. Furthermore, with the increase of Re, the heat transfer rate also enhances, so by increasing this number from 50 to 200, the heat transfer rate augments by about 105%.

In this study, a new desalination system, called an improved stepped solar still, is proposed to desalinate saline water using solar energy. This desalination system is created by adding a separate but integrated condenser to a standard stepped solar still. The condenser space is connected to the desalination section by two air gaps at the bottom and top of the absorber plate. Air circulation between the two parts occurs naturally (free flow). To compare and evaluate the performance of the new desalination systems, a standard stepped solar still was built with the same specifications and dimensions. The two solar stills are tested and compared in a few days of spring (June 11 and 12) and in a few days of autumn (October 10 and 11). The experimental results show that the improved stepped solar still has a better performance than the standard ones so that the freshwater produced by improved solar still is between 76 to 127% more than the freshwater produced by the standard ones. The maximum daily amount of freshwater produced by the improved solar still per square meter of occupied area of solar still area equals 4.57 liters.

Due to the increasing importance of using clean and renewable energies, one of the simplest mechanisms for converting solar radiant energy into thermal energy, on non-industrial applications, is the use of solar air heaters, which has always been an attractive idea for researchers. Therefore, the present study investigates the effect of air gap on the performance of plane solar air heaters with two glass covers by means of numerical simulations. A similar study has not been yet reported in various research resources. For this purpose, the equations governing the forced convection flow of air passing through the heater duct and also the natural convection flow caused by the buoyancy force in the air gap between two glass covers are numerically solved by applying the finite element method and using COMSOL Multiphysics software. In addition, in order to determine the temperature field in the solid elements of the air heater (including glass cover, absorber plate and insulation layer), conduction heat transfer equations are simultaneously solved with the flow equations. The accuracy of the numerical simulations performed in this research has been evaluated by comparing those with the theoretical and experimental findings of previous researches, and an acceptable consistency and agreement has been obtained in this regard. The results demonstrate the desired effect of the presence of air gap between the two covers on the performance of plane solar air heaters, such that changing the air flow rate from 2 to 14 g/s, the performance of this type of air heaters at a radiant heat flux of 1000 W/m2 compared to the simple model (without air gap) has shown an increase in efficiency about 4 to 6 percent. Also, in the conditions of inlet flow rate of 9.6 g/s and radiant heat flux varying between 400 to 1200 W/m2, the use of two glass covers has increased the outlet air temperature about 2 to 6 °C, compared to the simple model.