مجله مکانیک سیالات و آیرودینامیک
سال چهارم شماره 1 (بهار و تابستان 1394)

The aim of this study is designing the impeller of a centrifugal compressor in order to supply pressure ratio for a 65 kW micro gas turbine. First, the primary thermodynamic analysis has been conducted for the considered cycle using a home made code. Herein, the compressor pressure ratio was considered to be 4. The obtained thermal efficiency was 25.64%. Then, the design of micro compressor in 90,000 rpm rotational speed was conducted and its dimensions were extracted. The designed micro compressor has 9 main blades and 9 splitter blades. The outer diameter of the compressor impeller was 95.5 mm. Then, the three-dimensional geometry of the compressor impeller was extracted. Finally, to validate the designed impeller, CCD software was used and CFD analysis has been conducted on generated geometry, using CFX.

Atomization is a process by which a volume of liquid is converted spray. This process includes the breakup of the liquid jet or the liquid sheet exiting the injector. This paper has experimentally investigated the models and mechanisms for the breakup of the liquid jet and the liquid sheet formed by the jets impingement in a still environment and in an environment with counter flow, coflow, and cross flow. Also, the effects of the impingement angle, injector diameter, and jet length before impingement on the formation of the liquid sheet have been explored. Imaging has been employed to investigate different patterns formed by these interactions. To produce water jets, injectors with 0.4 and 0.9 mm diameters at a velocity range of 1-33 m/s and in accordance with Reynolds numbers between 3,000 and 30,000 have been used. It was found that in the low flow speed range, the water jet breaks up by the Riley's breakup model. While in high speed range, the breakup model is the firstwind - induced breakup model. The measured breakup length has a very good agreement with the result of Sallam’s equation. In investigating the breakup of liquid jets in cross flow, six breakup models were observed, including Rayleigh-like, turbulent, column, bag, multimode and shear breakup models. Also, in paper, an equation has been presented for the penetration of water jet into an air counter flow. Through a qualitative comparison of the images taken from the impingement of two water jets, velocity and Reynolds number ranges of the closed rim, alternating drops, open rim, and fully-developed models were determined. It was observed that, by increasing pre- impingement length of a jet, the instabilities on the liquid sheet increase.

The present paper investigates turbulent mixed convection - radiation flow between two vertical parallel plates kept at different temperatures with variable thermophysical properties. The Reynolds number, based on channel half-width and friction velocity and the Grashof number, based on channel width and wall temperature difference, were assumed to be 150 and 1.6×106, respectively. numerical simulations were carried out by developing a solver in OpenFOAM software, applying finite volume method. Launder-Sharma low-Reynolds RANS based turbulence model was employed. Thermophysical properties approximation (dynamic viscosity and Thermal conductivity coefficient) were based on Sutherland Law and Power Law. The working fluid was assumed to be gray, absorbing, emitting, and scattering. The radiative transfer equation was solved using the discrete ordinates method (DOM). Results show that the variable thermophysical properties condition results in higher Nusselt number on both hot and cold wall sides. Also, the bulk temperature and friction factor increase on hot wall side and decrease on cold wall side. Moreover, the velocity across the channel width and the centerline velocity increase.

In this study, the characteristics of the wake around a rotating circular cylinder have been investigated experimentally. To this end, a cylinder with a circular cross-section of Plexiglas with the length of 400 mm and a diameter of 20 mm have been tested in the wind tunnel. Using existing methods, the characteristics of the wake and the drag coefficients of the model in different cases have been investigated. In order to measure mean velocity profile and turbulence intensity, hot-wire anemometry has been used. The experiments carried out on the rotating cylinder indicates that the rotation of the cylinder causes significant changes in mean velocity profile. The rotation of the cylinder causes increase in the velocity in the vortex and decrease in the velocity of the wake. The variations become more and more by increase in rotational speed. Also, the results show that by increasing the rotation speed, the turbulence intensity in the wake increases. It is found that by increasing the rotational speed, the parameters of drag coefficient and velocity defect reduce.

Performance analysis of jet engines in various environmental conditions is important and is the first step in the design of such engines. In this paper, the performance of a typical turboprop engine was modeled in the commercial software GasTurb10 and the obtained results are validated at three flight regimes take off, military and 100% normal using engine test results. Performance analysis of combustion chamber was essential for estimating its exhaust gas temperature, which has influence on the lifetime and durability of engine hot section. In this study, one-dimensional performance analysis of a turboprop engine combustion chamber was carried out applying semi-experimental correlations. For this, a computer program called CLAD1 was developed in MATLAB. The results obtained for various engine operation regimes were compared with reliable experimental results. showing good agreements. The maximum relative error was 3.92% for military regime. Finally, the gas temperature distribution and emission of pollutants in different operation regimes were calculated and compared. The amount of NOx emission was maximum at take off flight regime, whereas the emission of pollutants CO and UHC have their highest values at 100% normal regime.

Magnetic field effect on unsteady natural convection heat transfer of Cu–Water nanofluid in a square porous cavity was studied numerically in here. At first, initial temprature of the cavity was i T and vertical walls were at temprature c i T T  . Suddenly the right wall's tamprature was changed to hT and the horizontal walls were adiabatic. The effective parameters in this study were Ra , Ha, and  . which appear in the non-dimensionalized equations. Ra is a function of temperature difference between hot and cold walls, Ha is a parameter that depends on the magnetic field, and  is the volume fraction of nano particles. The non-dimensionalized governing equations were obtained based on darcy model. A control volume approach was used for solving these equations. The effect of the variation of parameters, Ra , Ha and , on heat transfer rate, fluid flow, isotherms, and the steady time of solution were investigated. The solutions show that steady time decreases, as Ra increases or  decrease. While, variation of Ha for high Ra has different effects on steady time.