aminreza noghrehabadi

The purpose of this paper is to analyze the effects of structural and mechanical characteristics of metal foam on the melting behavior of phase change materials under the influence of different heat fluxes. To this aim, a two dimensional numerical model considering the non-equilibrium thermal factor, non-Darcy effect and local natural convection was used. The governing equations of PCM and metal foam are discretized using a finite volume method with a collocated grid arrangement. To simulate the melting of PCM, the enthalpy-porosity method is applied which computes the liquid fraction at each iteration, based on the enthalpy balance. The effect of metal foam characteristics (porosity, pores size and base material) and wall heat flux on the PCM melting time were investigated. The result showed that for both wall heat fluxes (4000 W m-2 and 8000 W m-2), foam structure and its mechanical properties have significant influence on the PCM melting time which these effects should be considered.

Effects of different volumetric fractions and Reynolds number on forced convection heat transfer through water/aluminum oxide nanofluid in a horizontal tube are investigated. The flow regime is laminar and the method of simulation is the axisymmetric lattice Boltzmann method (ALBM). The profiles of velocity and temperature were uniform at the input section, on the tube walls the uniform heat flux was considered; moreover, hydrodynamic, and thermal development conditions at the output section were applied. It was observed that an increase in the volumetric concentration of the nanoparticles added to the forced convection heat transfer coefficient and Nusselt number of the nanofluid, as compared to the base fluid. For a volumetric fraction of 5% and Reynolds number of 100 at the input section of the tube (0.1≤X/D≤7) the forced convection heat transfer coefficient increased by 24.165%, while an average increase of 21.361% was observed along the entire length of the tube (0≤x/D≤30). A comparison between the improvements in heat transfer at the two input temperatures, it was found that the forced convection heat transfer coefficient and Nusselt number will increase further at the lower input temperature; Moreover, with increasing the Reynolds number, the percent improvements in forced convention heat transfer coefficient and Nusselt number increased.

This study aimed at linear stability analysis of the stratified two-phase liquid-gas flow in a horizontal pipe. First, equations governing the linear stability of flow in each phase and boundary conditions were obtained. The governing equations were eigenvalue Orr Sommerfeld equations which are difficult and stiff problems to solve. After obtaining the velocity profiles of the gas and liquid phases in the pipe, the instability equations for each phase with related boundary conditions were coupled and simultaneously solved by using the Chebyshev Tau - QZ polynomial method. The instability spectra for some points has been plotted and some curves about instability conditions the same as neutral stability curve which shown stable and unstable region respect to Reynolds number had been drown. According to the neutral stability curve for each phase, the liquid phase is more exposed to instability than the gas phase. The liquid phase was unstable in low Reynolds numbers and a large amplitude of the wave velocity α but gas was unstable in higher Reynolds number and small amplitude of α.

The low conversion efficiency of solar cells produces large amounts of thermal energy to the cells, and with an increase in the temperature of solar cells, their electrical efficiency decreases. Therefore, a hybrid photovoltaic thermal system improves the overall efficiency of the system by adding thermal equipment to the solar cell and removing excessive heat from these cells. In this paper, we study the effect of SiO2/water nanofluids on thermal and electrical efficiency of domestic photovoltaic thermal systems (DPVT) theoretically and experimentally. In the theoretical part, based on the control-volume finite-difference approach, an explicit dynamic model was developed for a single-glazed flat-plate water-heating photovoltaic thermal collector with closed loop cooling system with withdrawing urban water from the storage tank. The model accuracy was verified in comparison with the measured experimental data. Experimental results show that by increasing concentrations of nanofluid, the thermal and electrical performance has improved and overall efficiency decreased by increasing the diameter of the nanoparticles. The overall efficiency of the DPVT for 0 and 3 weight percent of SiO2/ water nanofluids with a diameter of 11-14 nanometers increased to 5.4% and 7.76% compared to base fluid, respectively.

In this article the instability of single phase flow in a circular pipe from laminar to turbulence regime has been investigated. To this end, after finding boundary conditions and equation related to instability of flow in cylindrical coordination system, which is called eigenvalue Orr Sommerfeld equation, the solution method for these equation has been investigated. In this article Chebyshev polynomial Tau-QZ algorithm has been selected for the solution technique to solve the Orr Sommerfeld equation because in this method some of complex terms in the instability equation in cylindrical coordination will be appeared. After finding Orr Sommerfeld parameters related to Chebyshev polynomial Tau-QZ algorithm the solution have been done for Re=5000 and Re=1000, then the results had been compared with the results of valid references where other methods had been used in them. It have been observed that the use of Chebyshev Tau-QZ algorithm has higher accuracy concerning the results and it also has a higher accurate technique to solve the Orr Sommerfeld instability equations in cylindrical coordination system.

Solar collectors play key role in solar thermal systems and their efficiency has more effect in the system performance. The shape and geometry of solar collectors is an important factor to increase their efficiency. In this paper, two collectors, a 3-D stationary solar collector which is called solar conical collector and flat plate collector has been investigated. These two solar collectors have the same absorber area and designed and tested in the same conditions. The performance of solar collectors was experimentally studied by the use of ASHRAE standard. Experiments were performed with water as a working fluid in the south of Iran with deference outdoor conditions and the best data has been selected. The results show that the average of thermal efficiency of a solar 3-D collector is about 59% while this value was about 53% for flat plate collector. The performance of both collectors versus radiation, flow rate, temperature variation was studied and the experiments indicated that the efficiency of the 3-D conical collector has better than the flat plat collector.

This study numerically investigated heat transfer and fluid flow characteristics in a novel cylindrical heat sink with helical minichannels for the laminar flow of fluid with temperature-dependent properties. A finite volume method was employed to obtain the solution of governing equations. The effects of helical angle, channel aspect ratio, and Reynolds number, which were regarded as main parameters, were determined. The overall performance of the heat sink was also analyzed on the basis of the thermal performance factor and the augmentation entropy generation number. Results showed that a decrease in the channel helix angle and an increase in the channel aspect ratio and Reynolds number enhance the average heat transfer coefficient and pressure drop in the heat sink. The thermal performance factor and entropy generation minimization method revealed that an aspect ratio of 1.2 enables the best heat sink performance at all helix angles. When the helix angle decreases, performance increases, especially at low aspect ratios.

In this paper, the modified couple stress theory is used to study static and dynamic pull-in instability of a general model of a nano-cantilever under a sudden applied DC voltage in the presence of the surface effects. A partial part of the nano-cantilever is subject to the electrostatic and capillary forces. Euler-Bernoulli theory is used to model the beam and the equation of motion is derived by using Hamilton’s principle. The governing equations are transformed into a non-dimensional form and then solved using finite element method (FEM). The results, obtained using FEM are compared with the data available in the literature and found in good agreement. Basic parameters for engineering design at the nanoscale, such as deflection and pull-in voltage have been calculated for both of the dynamic and static modes. The results of dynamic analysis of the beam show that as the voltage increases, the beam goes into an oscillating mode with large amplitudes just before pull-in phenomenon occurs and the beam collapses into the substrate (fixed electrode). Moreover, it is found that a decrease in the length of the fixed electrode (increase of the partially affecting parameter), and the increase of the fringing field effect, the size effect and the surface effect increases the pull-in voltage of the nano-cantilever beam.

The geometry of a collector is one of the important factors that can increase the incident radiation on the collector surface. In the present study, the incident radiation for a stationary collector with cone geometry, i.e. a conical collector, is theoretically and experimentally investigated. This type of collector is always stable and does not need a fixture to install. Moreover, it has a symmetric geometry, with all its sides facing the sun. The main advantage of this collector is its ability to receive beam, diffuse, and ground-reflected radiation throughout the day. The variation of the incident radiation is theoretically estimated by using an isotropic sky model based on the available data. The theoretical data are validated by an experimental test of a conical collector of a specific size. The results show that the conical solar collector is more operative in receiving total solar radiations than a horizontal plate such as a flat-plate collector and can be a suitable option for solar water heating. A calculation of the incident radiation shows that the incident radiation is maximized when the cone angle of the conical collector is equal to the latitude of the site test.

The shape of a solar collector is an important factor in solar-to-thermal energy conversion. Conical shape is one of the stationary and symmetric shapes that can be employed as a solar water heater. Flow rate of working fluid on the solar collector has an important effect on the efficiency of the collector. The present study is an experimentally investigated of the performance of the solar conical collector with 1m2 of absorber area at different volumetric flow rates. Water was used as the working fluid with the volumetric flow rate between 0.35-2.8 lit/min and the experiment was held in the ASHRAE standard conditions. The results demonstrated that the efficiency of the conical collector is increased by increasing the flow rate of the working fluid; in addition, the difference between inlet and outlet temperatures is decreased. The maximum recorded outlet-temperature of the collector during the experimental tests was 77.1 0 C and the maximum value of thermal efficiency was about 60%.

A two-dimensional-in-space mathematical model of amperometric micro biosensors with selective and perforated membranes has been proposed and analyzed. The model involves the geometry of micro or nano meter holes partially or fully filled with an enzyme. The model is based on a system of the reaction-diffusion equations containing a nonlinear term related to the Michaelis-Menten enzymatic reaction. In this study, in order to generate general equation, first, dimensionless parameters are introduced and then by replacing them into governing equation are converted to dimensionless equations.The general equations have been solved numerically in 2D space.. Using numerical simulation of the biosensor action, the influence of the geometry of the holes as well as of the filling level of the enzyme in the holes on the biosensor response was investigated. For this purpose three different geometries including cylindrical, upright circular and downright circular cone for cavities are considered and the impact of these geometries on the response of the biosensor in different levels of enzyme are obtained. Biosensor's respond based on rate of enzyme level variations to slope of the cone variations are determined. In the biosensor, as the level of enzyme rises in all three geometries, the biosensor output current increases. Under the same conditions, the sensitivity of biosensor in upright circular cone is more than the other two geometries and increases with a decrease in conical gradient. As long as the enzymatic properties are the same, the more biosensor's number, the more sensitivity.Moreover, a concept known as reduced dimensionless current is introduced by providing and calculating dimensionless current in the biosensor.

In this paper, gas-liquid two-phase flow in the annulus of a real well during under-balanced drilling operations is simulated numerically. Oil and gas flow from the reservoir in to the annulus is considered due to under-balanced drilling condition. A numerical code based on one-dimensional form of steady-state single pressure two-fluid model in the Eulerian frame of reference is developed and its results are validated using experimental data from two real wells. The results of numerical simulation show better accuracy in comparison with other researches. Given the importance of prediction and control of the bottom-hole pressure and the amount of oil and gas production during the drilling operations, the effects of controlling parameters such as liquid and gas injection flow rate and choke pressure are discussed. Also, the effects of different controlling parameters on the characteristics of two-phase flow pattern, including liquid and gas void fractions, liquid and gas velocities and pressure distribution along with the annulus are discussed. According to the results, the effects of choke pressure and injected liquid flow rate on the production of the oil from the reservoir are independent of the values of each other and are dependent on the injected gas flow rate.

The effect of a transverse magnetic field on the boundary layer flow and heat transfer of an isothermal stretching cylinder is analyzed. The governing partial differential equations for the magnetohydrodynamic, temperature, and concentration boundary layers are transformed into a set of ordinary differential equations using similarity transformations. The obtained ordinary differential equations are numerically solved for a range of non-dimensional parameters. Results show that the presence of a magnetic field would significantly affects the boundary layer profiles. An increase in magnetic parameter would decrease the reduced Nusselt and Sherwood numbers.

In this paper, an integration of a symbolic power series method - Padé approximation technique (PS - Padé), was utilized to solve a system of nonlinear differential equations arising from the similarity solution of laminar thermal boundary layer over a flat plate subjected to a convective surface boundary condition. As both boundary conditions tended to infinity, the combination of series solutions with the Padé approximants was used for handling boundary conditions on the semi-infinite domain of solution. The combination of power series and Padé proposed an alternative approach of solution which did not require small parameters and avoided linearization and physically unrealistic assumptions. The results of the present approach were compared with numerical results as well as those of previous works reported in the literature. The obtained results represented remarkable accuracy in comparison with the numerical ones. Finally, reduced Nusselt number, as an important parameter in heat transfer, was calculated by the obtained analytical solution. The present power series-Padé technique was very simple and effective, which could develop a simple analytic solution for flow and heat transfer over the flat plate. The results of the present study could be easily used in practical applications.

In the present paper, the flow and heat transfer of two types of nanofluids, namely, silver-water and silicon dioxide-water, were theoretically analyzed over an isothermal continues stretching sheet. To this purpose, the governing partial differential equations were converted to a set of nonlinear differential equations using similarity transforms and were then analytically solved. It was found that the magnitude of velocity profiles in the case of SiO2-water nanofluid was higher than that of Ag-water nanofluid. The results showed that the increase of nanoparticle volume fraction increased the non-dimensional temperature and thickness of thermal boundary layer. In both cases of silver and silicon dioxide, increase of nanoparticle volume fraction increased the reduced Nusselt number and shear stress. It was also demonstrated that the increase of the reduced Nusselt number was higher for silicon dioxide nanoparticles than silver nanoparticles. However, the thermal conductivity of silver was much higher than that of silicon dioxide.