فهرست مطالب محمد کلته

Thermoelectric generators are a sustainable and environmentally friendly technology that can recover wasted heat energy and convert it to electricity. Meanwhile, integrated thermoelectric generators have been able to significantly increase the performance of thermoelectric generators. In this paper, the effect of flow channel cross-sections on integrated thermoelectric power generator performance is investigated numerically using the finite volume method. In this regard, various flow channel configurations including circles, trapezoids, squares, and rectangles have been taken into account and the effect of cross-sectional area ratio, semiconductor length, and Reynolds number on the performance of the device has been evaluated. In this study, the top and bottom of conductor surfaces are exposed to a cold temperature and a hot fluid with a constant velocity and temperature enters the channel. The results show that the power output, voltage, and thermal efficiency of 36 rectangular configurations are higher than other flow channels. Also, the heat input, power output, and thermal efficiency at a cross-sectional area ratio of 0.28 are respectively found to be 1.68, 1.77, and 1.52 times higher than at a cross-sectional area ratio of 0.68. In addition, an optimal length for a semiconductor is determined, in which the maximum output power is achieved.

Gasification technology is an important part of the clean coal technology. For further development of this technology, the processes inside the gasifier and the interactions of injected gas and solid fuel particles need to be understood. In the current study, a numerical simulation of an entrained flow coal gasifier is investigated using experimental operating conditions. The reactions and kinetic parameters of the gasification process are fitted using coal gasification data obtained from similar published papers. Comparison of the simulation results, experimental data and two other similar studies confirm the accuracy of the developed model. The focus of this study is on the accuracy of the models presented for the devolatilization process and the effect of the oxidizer change from oxygen to air on the gasifier performance. Four devolatilization models are investigated including chemical percolation devolatilization, single rate, Kobayashi and constant rate models. The predicted trends of species changes are similar in different devolatilization models but the amount of produced syngas is somewhat different depending on the accuracy of each model. The Kobayashi and constant rate models predicted the devolatilization rate lower than the other two models. The results obtained from the chemical percolation devolatilization model are more consistent with experimental data compared to other models, but require higher computational times. The use of air oxidizing agents rather than oxygen reduces the species mole concentration of produced syngas and hence reduces the gasifier efficiency.

In the present study, the electroosmotic and pressure driven flow of nanofluid in a microchannel with homogeneous surface potential is investigated by using the Poisson-Boltzmann equation and the flow field is assumed to be two-dimensional, laminar, incompressible, and steady. Distribution of nanoparticles in the base fluid is assumed to be homogeneous; therefore the nanofluid flow is modeled as a single phase. The thermal conductivity of the nanofluid is modeled by using the Patel model to account for temperature dependency. In order to validate the numerical solution, the results are compared with available analytical solutions and the comparison shows a good match with the results. Then, the effects of different parameters such as ion molar percentage, volume fraction, and nanoparticles’ diameter on the flow field and heat transfer are examined. The results show that by fixing the electric field and increasing the pressure gradient, the local Nusselt number will decrease, and by fixing the pressure gradient and enhancing the electric field, the Nusselt number increases. The average Nusselt number increases about 45, 35 and 25% while nanoparticles’ diameters are 100, 110 and 120nm, respectively. For Γ=0.05, the average Nusselt number increases 10% while ion concentration changes from 10-4 to 10-2. Furthermore, the direction and magnitude of velocity and concavity of the velocity profile can be controlled by choosing a suitable phase angle between electrical and pressure driven flow parameters.

In this article, the effect of velocity slip and temperature jump on the flow and heat transfer characteristics of Al2O3 – Water nanofluid in a microchannel with insulated upper wall and constant heat flux on the lower one, is investigated using the lattice Boltzmann method. The problem is solved at Re equal to 5, for base fluid and nanofluid with 0.02 and 0.04 volume fractions, no-slip and slip conditions with 0.04 and 0.1 slip coefficients and also at 5 to 50 nm nanoparticle diameters. The results show that, in general, using the hydrophobic surfaces in addition to making a considerable reduction in wall shear stress, somewhat increases the heat transfer efficacy at uniform wall heat flux condition that can not be seen in the constant wall temperature situations. Also, it is shown that the effect of temperature jump on the average Nusselt number, is more for base fluid than the nanofluid and increases for higher slip coefficients. For nanofluid with 0.04 volume fraction, the average Nusselt number increases continuously with slip coefficient but, for base liquid, firstly it increases and then decreases.

The present paper studies the effect of nanofluids on natural convection in an assumed Non-Darcy, porous medium on vertical wall embedded with MHD field. The wall temperature and free stream temperature are assumed constant. The base fluid is water that contains 〖Al〗_2 O_(3 )¡CuO_and Cu as nanoparticles. We assumed that the flow is laminar and in no slip and thermal equilibrium condition. Partial differential equations are transformed to ordinary differential equations that are solved by Differential Transform Method (DTM). The current semi-analytic solution is compared with 4th order Runge-Kutta results and a good agreement is achieved. The obtained results show that adding nanoparticles to water, increases the heat transfer amount that is also increased with nanoparticle concentration. On the other hand, increasing the magnetic field decreases the nondimensional heat transfer coefficient.

According to the ever-increasing need for low-cost and unlimited energy sources، renewable energies have been taken into consideration. Solar energy is one of the main sources of renewable energies. In this article، with the use of solar energy، conventional and sloped solar chimney power plant performance in different climates of Iran has been examined. To do this، the appropriate mathematical models for the radiation of the Sun، solar collector and chimney are used. For a better analysis، three types of horizontal collectors and 30 ° and 60 ° sloped ones are considered and their received radiation and electric power outputs are compared together. The results show that the horizontal collectors produce more power in the summer، while in the winter، power output increases with an increase in collector slope. Also، 300 sloped collectors get the most Sun''s radiation، while 60° sloped ones produce maximum electrical power. Finely، among the considered cities، Yazd has the best performance.