مجله مهندسی مکانیک امیرکبیر
سال چهل و هفتم شماره 1 (تابستان 1394)

In this research, the performances of different turbulence models for simulating flow field in turbine stator have been numerically investigated. To this end, incompressible fluid flow around a high-turning stator blade in Reynolds 2.23×105 has been simulated using FLUENT CFD software. Navier-Stokes equation is discretized on a Hybrid computational grid based on the finite volume approach. Considered turbulence models in this research are “Spalart-Allmaras”, “Standard k-ε”, “Realizable k-ε”, “RNG k-ε” , “SST k-ω” and “five-equation Reynolds-Stress Model (RSM)”. The performances of these models are evaluated by comparing the pressure coefficients obtained from numerical simulations with corresponding experimental data at four different stator regions. It has been observed that the ability of a turbulence model to predict flow field is not uniform throughout the stator blade. Moreover, all models show relatively poor performance in flow field regions with intense velocity gradients. Comparing the overall accuracy of different models, SST k-ω and RSM turbulence models show the best agreement with the experimental data.

Utilizing an airfoil Parameterization method is one of the essential requirements for airfoils optimization. The selection of this method plays an important role, as using an unsuitable method yields the weak results. In addition, it will impose delay on convergence of the solution. Hence, in this work, an improved Harmony Search meta-heuristic optimization algorithm has been developed for investigating three common airfoil parameterization methods (Bezier curves, Parces method and 4-digit-NACA formula) using an inverse optimization design and a non-aerodynamics objective function. The obtained results show that the Bezier curves and Parces method are more efficient than 4-digit-NACA formula. Finally, because of having few control parameters, the Parces method has been used along with an improved Harmony Search algorithm for the shape optimization of an airfoil under a viscose and turbulent flow, with the objective of maximizing lift to drag ratio. To do this, 2-dimentional compressible Navier-Stokes equations with Spalart-Allmaras turbulent method have been solved around the airfoil. The results reveal that the improved optimization algorithm is highly capable of evaluating the airfoil parameterization methods and aerodynamics optimization.

Modification of frictional drag reduction due to the presence of small bubbles and axial flow is investigated experimentally using a Couette-Taylor system. Flow condition between concentric cylinders is fully turbulent and Taylor vortices are appeared into flow when rotational Reynolds number is changed from 5000 to 70000. Torque acting on rotating inner cylinder and bubble behavior are measured while air bubbles and axial flow are injected constantly from the bottom of the system into annulus gap. Pure water is used to avoid the uncertain interfacial property of bubbles and to produce nearly mono-sized bubble distributions. Bubble diameter is measured by image processing method. The result showed that in the absence of small bubbles, axial flow reduces the friction drag. Moreover, it is observed that axial flow improves positive effect of bubbles on drag reduction. In this case, a drag reduction of 28% is obtained which is decreased by increasing the rotational Reynolds number.

The design of air-conditioning systems for energy saver buildings relies on fast and accurate methods able to predict the details of the indoor environment. In this paper, it is shown that the air zonal method is able to quickly assess the indoor environment quality. The zonal method is based on energy and mass balance equations in macroscopic volumes. Zonal models use a coarse grid and balance equations, state equations, hydrostatic pressure drop equations and power law equations. The aim of this paper is to study the simulation of temperature and humidity distributions through a large horizontal opening in a room with mixed ventilation by means of air zonal method and the results are compared to Computational Fluid Dynamics (CFD) calculations and experimental data. It has been shown that for the simple rectangular geometries, the air zonal method gives reasonably an accurate air temperature and humidity results in engineering applications even for the whole year.

Stratum ventilation method is the most promising option for reducing energy consumption of ventilation systems in near future. In this method, which is currently implemented in some modern countries, the ventilation of only a stratum of indoor space in which occupants’ head and chest are located, is pursued. In the current study, 17 stratum ventilated cases with a manikin sited behind a desk and with different manikin’s sitting locations, outlet positions and contaminant source locations are numerically modeled. To investigate the effects of sitting location, outlet position, and contaminant source location on thermal comfort and inhaled air quality, a parametric study with four different evaluation indexes is performed. The results of this study can help improve the design and performance of stratum ventilation systems

In the current study, the problem of heat transfer in non-Newtonian fluid flow over a cylinder has been simulated using the Immersed Boundary – thermal lattice Boltzmann method and direct forcing algorithm. The sharp interface scheme isused to transfer the values of velocity and temperature between the fluid Eulerian and boundary Lagrangian nodes. In order to consider the effects of both discrete grid and boundary forces (thermal forces), the split-forcing lattice Boltzmann method is developed for non-Newtonian power-law fluids. A simple technique for calculating the Nusselt number based on the sharp immersed boundary method is extracted. Heat transfer of different fluid regimes consist of steady and unsteady flow in wide ranges of Reynolds numbers (20

In this paper, numerical investigation of Joule heating effects on the electroosmotic flow through a microchannel with the triangular cross section and constant wall temperature have been presented. The energy equation for the temperature distribution, Navier–Stokes equation for the velocity distribution and a Poisson equation for the electric potential distribution have been solved by using the finite-volume method in a system curvilinear coordinates. Thermophysical properties such as the dynamic viscosity and electric conductivity vary with temperature. The results show that by increasing the Joule number, the temperature, velocity and mass flow rate increase with constant EDL number. With constant Joule number, the increments of EDL number causes the mass flow rate to increase. Mean temperature and velocity reduced by increasing the angle between sides and base of the cross-section in the particular Joule number.

Experimentally and numerically investigation of triangular ducts heat transfer is very important for many heating and cooling systems because of their very low pressure drop. But/ Nevertheless , this non-circular duct has very bad heat transfer performance. In the present study, the heat transfer performance of triangular ducts using nanofluid as heat transfer media is experimentally investigated. Nanofluid (which is the stable suspension of nanoparticles inside base fluid) is a new kind of heat transfer fluid and has very good potential for the heat transfer enhancement. Two kinds of nanofluid (Al2O3/water and CuO/water) produced and injected to the experimental set up, and the Nusselt number and heat transfer coefficients of these nanofluids are determined experimentally and compared with each other. The result expressed that Cuo/water presents better heat transfer performance compared to Al2O3/water nanofluid.

One of the major challenges in high-speed flights is aerodynamic heating. This is why thermal protection system (TPS) is being used. One of the main components of TPS is ablative insulation. In present study, one-dimensional theoretical and experimental analysis of ablative insulations have been done. Phenolic resins with maximum thermal destruction efficiency are being used in charring ablative insulations. When an ablative insulation is exposed to heat flux, its surface gets warmer and as the destruction begins, it produced gases to go out and do cooling. Governing equations of these phenomena have been discretized by the finite difference method and have been solved transient and implicitly. Thermophysical properties have been evaluated by nominal curves and Pyrolysis constants have been obtained by / through the thermochemical reactions. Validation of numerical solution has been done by oxy-acetylene test. By increasing the time, the difference between numerical and experimental results increases. One reason for difference between results could be 1D-modeling, where all of the actual 3D energy is accumulated in one dimension in the numerical solution. Nonetheless, there is good agreement between numerical and experimental results and the average of absolute errors is 7.54%.