MECHANICAL ENGINEERING SHARIF (SHARIF: MECHANICAL ENGINEERING)
Year:2009 | Volume:24 | Issue:46.2|Papers Count:9

The effect of vortex flow in off-design conditions on the performance of the draft tube in a horizontal Francis turbine has been investigated numerically.DTo validate the numerical approach, having experimental data in the draft tube is the immediate objective of this research. Thus, a three-hole pressure probe has been designed and mounted inside the draft tube for measuring the axial and tangential components of the velocity field. The specific speed of the turbine in the test rig was such that the radial component of the velocity at the inlet of the draft tube could be neglected, which justifies implementation of the applied probe, especially at the inlet of the draft tube.The velocity field is measured by traversing the probe at two sections of the inside of the draft tube. The velocity field data at the inlet of the draft tube are used as the boundary conditions of the 3D numerical analysis.It is well known that a high intensity vortex causes con-sider able degrees of anisotropy in stress and dissipation tensors, leading to a highly anisotropic eddy viscosity. Thus, all conventional eddy-viscosity-based models will no longer be valid under these conditions and Reynolds stresses in Navier-Stokes equations are modeled by using the RSM formulation.In this study, an emphasis is also placed on the influence of the inlet condition. Numerical results are in fairly good agreement with experimental data. Grid independency is carefully checked in the numerical approach.Several operating points, with different flow rates, under the constant head and rotational speed of the turbine, are investigated. For each case, by applying a numerical simulation at a constant mass flow rate, the pressure recovery factors are calculated in two ways, with and without the circumferential velocity component at the inlet of the draft tube. These are necessary tools to find the influence of the vortex on the draft tube efficiency.It is found that the vortex flow has an adverse effect on the performance of the draft tube and may, consequently, decrease turbine production.

Multi-stream plate fin heat exchangers are used for simultaneous heat transfer between more than two streams in a single unit. They are used where high performance and low weight are required. The main applications of this kind of heat exchangers are in cryogenic gas processing as main feed pre-coolers, condensers and liquid chillers, in aerospace industry as oil and fuel coolers, radiators and in HVAC industry as air to air exchangers, condensers and evaporators.High thermal performance and economical advantages make them suitable to be used in heat recovery networks instead of shell & tube heat exchangers. In the current paper, the thermal design of the Multi-stream Plate fin heat exchangers will be mentioned and the economical and technical results of replacing shell & tube heat exchangers by Multi-stream Plate fin heat exchangers in a heat recovery network will be inspected.

A series of experiments are conducted to study the velocity field around a wing-canard body that represents a highly maneuverable aircraft. All experiments were conducted in a subsonic wind tunnel under two different angles of attack, Jmodel=15, 20 degrees, while the canard angle of attack varied between -10 to 10 degrees. The data shows that, at all the angles of attack mentioned before, the canard downwash passes over the main wing surface, which results in a reduction of the pressure over the wing surface. These phenomena delay the model stall angle of attack, hence, increasing the performance of the model. Furthermore, to reduce the experiments, a neural network system was used, which is capable of both prediction and extrapolation. The neural network data shows both canard and main wing vortices at various stations over the wing surface. As the canard angle of attack increases, its corresponding vortices become stronger and cover a layer portion of the wing surface. As a result of this vortex system, the pressure over the wing surface drops considerably.

Generally, in pipe manufacturing industries, roll design, in the cold roll forming process, is performed, based upon experiments and the qualitative choice of the designer. In this paper, a computer program is provided to reduce the number of qualitative decisions. In this research, rolls are designed, based on the Kiuchi method, using a sine shape function. Elasto-plastic analysis is done for determining the 3D shape of strips, strain, stress distribution and the consumed roll power between roll stands. Then, by equalizing the power for all roll stands, the rolls are designed. Finally, considering the technological parameters of the process and the dimensions of the machines, a SCRIPT file for manufacturing the rolls is prepared for the user. Using the results of the point test, based on CAD/CAM principals, it is shown that the results of the provided computer program, for the 3D shape of the strips, are consistent with experimental data.

In the present work, a thermoeconomical optimization approach is applied to a vapor compression refrigeration system. The cooling load and the outdoor and indoor temperatures are considered as constraints in this optimization procedure. Two optimization parameters are considered in this analysis. The first one is the difference between condensation temperature and outdoor temperature, which fixes the condensation pressure. The difference between evaporation temperature and indoor temperature is considered as the second optimization parameter, which, in turn, fixes the evaporation pressure. The objective function is a cost function, which is the sum of the exergy input costs and the capital costs. The analysis shows that optimum cycle parameters may be found by a trade-off between the compression cost and other costs, including those for heat exchangers, fans 4he input power of the fans.

In high-rate anaerobic wastewater treatment reactors such as upflow anaerobic sludge blanket (UASB) system, gas solids separator (GSS) plays a critical role in maintaining stability in reactor performance. Operational and performance problems of the current design include complexity and clogging of gas collection pipes due to sludge entry, excessive number of gas separators, and limitations on upflow velocities due to potential sludge washout. To overcome these problems, a new design is proposed in which the total volume of GSS is filled with gas-supernatant and flow is partially diverted to a side basin adjacent to the main reactor. In this basin, gas is separated from supernatant, the remaining good sludge is settled and returned back to the reaction zone in the main reactor, and undesirable sludge and the associated flow is returned to the influent pump station for recycle back into the bottom of reactor.Advantages of the new design include the possibility of increasing velocity in the blanket zone without changing velocity in the clarification region. The degree of increase is dependent on the extent of flow diversion but an operational velocity of about 2.5 mhr-1 is easily achievable even though much larger velocities are theoretically possible. Inactive sludge accumulated outside the turbulent domain of inlet openings is also made available.Horizontal velocities of at least 20 mhr-1 underneath GSS units due to flow into the side basin is a new phenomenon in the new design which further improves gas separation. Since lesser total overlap area is needed due to larger and fewer number of settlers in the reactor (reduction factor of around 70%), more area for fluid flow is available resulting in lower inter-GSS velocities. Furthermore, larger settlers are easier and more economical to construct and gas-collecting pipes and auxiliary appurtenances to secure GSS structures are eliminated with consequent reduced capital and operating expenses.

In the present work, a free wake unsteady panel method is used to accurately compute vortical wake and aerodynamic characteristics, including the surface pressure distribution and the blade loading for a hovering helicopter rotor. In addition, the effects of blade thickness on aerodynamic characteristics are investigated, by comparing the results of panel and vortex lattice methods. The main advantage of the panel method over the vortex lattice method is that the thickness of blades can be modeled and, therefore, the results of surface pressure are accurately obtained. In the panel method, the blade surface is covered with a source/doublet distribution and the wake is modeled with a doublet distribution.When the blades rotate, the vortices are shed into the wake and freely moved with a velocity induced by the effects of panels located on the blades and in the wake. The wake can freely deform over time to take its natural shape. The aerodynamic calculations, using the free wake panel method, are performed for a NACA0012 wing, to examine the accuracy and validity of the analysis for a three-dimensional geometry. Then, the aerodynamic analysis of a helicopter rotor in hover, for the tip Mach number Mtip=0.44 , is performed and the present calculations are compared with available theoretical and experimental results. The present results, based on the panel method, are compared with those of the vortex lattice method, to investigate the effects of blade thickness on the surface pressure distribution, the blade loading and the structure of the vortical wake. The effects of the number of panels and the computational time step on the aerodynamic results and the wake structure are also studied. Finally, the aerodynamic analysis of the rotor blades with twist is performed and the results of the panel and vortex lattice methods are compared with each other.