فصلنامه مکانیک هوافضا
سال دهم شماره 2 (پیاپی 36، تابستان 1393)

This paper presents a numerical simulation of liquid fuel droplets combustion suspended in air in porous media. Solving the equations of conservation of energy and chemical components numerically، combustion in a porous medium is modeled. Fuel droplets vaporize and mix thoroughly in the preheating zone filled with air and then enter the combustion zone. Singlestage and five-stage combustion mechanisms were used for calculating the rate of change in the concentration of chemical constituents. For modeling of radiation heat flux of porous medium corresponding differential equations are used. Thermodynamic properties are obtained from the curves suggested in references. In this numerical model the effects of the absorption coefficient on temperature distribution، radiant flux and mass fraction of numerical components، droplet size، and the amount of pollutant are shown. The results indicate that in the porous medium with low absorption coefficient، the radiation flux is increased and thus can be used for larger diameter of the fuel droplets. As a consequence of using porous media with low absorption coefficient، the small size fuel droplets are produced more easily.

In this paper، an inverse analysis is performed for estimation of temperature-dependent absorption coefficient in a twodimensional absorbing، emitting and isotropically scattering medium. The finite volume method has been adopted as the numerical technique. The S method is employed to solve the radiative transfer equation. The inverse problem is solved by using a combined method of genetic algorithm and conjugate gradient. The accuracy of the presented method for solving the inverse radiation problem is checked by using four examples. The effect of the measurement errors on the accuracy of the inverse analysis is also investigated. The results show that the algorithm can estimate the unknown absorption coefficient when the measurement errors are neglected. Also it is found that increasing the measurement error decreases the accuracy of estimation of temperature-dependent absorption coefficient.

This paper investigates the effects of swirling air stream on a viscous hollow cone spray atomization with sinuous instabilities. The dimensionless dispersion equation of wave growth rate was derived with linear stability analysis. The dispersion equation is solved by numerical method and swirling air stream effect on maximum growth rate and its corresponding unstable wave number is investigated. The results show that the maximum growth rate of a swirling liquid sheet subjected to purely outer axial air is more pronounced compared to its counterpart subjected to purely inner axial air. In swirling air stream، swirling of outer air has more effect compared to inner air on maximum growth rate and improves the atomization. The combination of inner and outer air stream swirling has the highest effect on wave growth rate. The growth rate can be related to the breakup length of the liquid sheet and when the growth rate increases، breakup length is shorter. The drop diameter is dependent to the wave number and decreases with the increase of the wave number that can afford improvement of combustion and decrease the specific fuel consumption.

Conjugated natural convection and conduction in a square cavity containing a porous medium with a solid body subjected to a uniform energy generation per unit volume is studied numerically in this paper. The vertical walls of the cavity are assumed to have constant temperatures (h T and c T) and the horizontal walls to be adiabatic. The effective parameters in this case are Ra and sb λ which appear in the nondimensionalized equations. Ra is a function of temperature difference between hot and cold walls، and sb λ is a parameter that depends on the energy generation in the porous medium. Nondimensionalized governing equations in both cases are obtained based on the darcy model; a control volume approach is used for solving these equations. The effect of the variation of two parameters، Ra and sb λ، on the heat transfer rate، fluid flow and isotherms are investigated. The results show that the maximum temperature decreases as Ra increases or sb λ decreases.

In this article thermal stress of graphite nose of aerospace projectiles is investigated. The governing equations، boundary conditions، rate of surface recession، ablation model of graphite، and also moving mesh generation are described. In order to calculate thermal stress، at first unsteady heat transfer equation in an unstructured moving mesh is solved using finite volume method for reentry period، then the results of the thermal analysis are transferred to a structure analysis software. Aerodynamic pressure is applied as a variable with time and position and thermal stress is calculated for the reentry flight. There is a good agreement between the current results and those of the other researchers.

This numerical study is focused on variation of mixed convection heat transfer and fluid flow of nanofluid with effective viscosity and thermal conductivity which are dependent on temperature and nanoparticles size and concentration insida double lid-driven cavity. The Navier–Stokes and energy equations are obtained numerically using a FORTRAN code. New Xu’s and Jang’s models are used for calculating the effective thermal conductivity and the effective dynamic viscosity، respectively. The impact of increase in buoyancy force while the shear force was constant and effects of increase in shear force when the buoyancy force was kept constant were investigated. In the current work، we examine the effects of temperature، non-uniform diameter of suspended nanoparticles in base fluid، volume fraction of nanoparticle، Grashof and Reynolds number on hydrodynamic and thermal characteristics. The obtained results show that the heat transfer increases with decrease in diameter of nanoparticle for a particular Re; it also increases with increase in Grashof and Reynolds number for a particular volume fraction and diameter size.

In this paper hyperbolic heat conduction equation in spherical media with its outer surface subjected to time dependent laser heat source and combined convective-radiative cooling is solved using Laplace transform method. In order to perform this method، the nonlinear boundary condition is linearized by Taylor’s series expansion. The Riemann-sum approximation method is used to obtain the inverse of Laplace transform. The temperature distribution is studied for different values of thermal relaxation time، pulse duration، convection-conduction and radiation-conduction parameters and the solutions of hyperbolic and Fourier equations are also compared. The results of the present method is verified by being compared with available analytical solution results under a simpler boundary condition.

The need for use of outside fresh air according to standard ASHRAE 62-1989 (1999) Ventilation for acceptable inside air quality، increases fresh air usage and heat load، thus system performance and equipment costs increase. Energy recovery studies such as run-around heat recovery is useful for optimization of energy consumption in HVAC systems. Several theoretical analyses on run-around heat recovery systems have been done by researchers، but generally heat losses from pipework circuit was ignored. The main purpose of this paper is to investigate the effect of pipework circuit length on runaround heat recovery systems performance by a precise model based on system hydraulic and thermodynamic relations. Simulation results showed that heat transfer coefficient is linearly decreased by increasing distance of hot and cold streams and pump power is increased too. If pipework circuit pipelines are insulated properly، effect of pipework circuit length on run-around heat recovery systems performance will be negligible.