mohammad vajdi

The cylinder-piston reciprocating mechanism has been one of the most dominant designs since the invention of internal combustion engines. By reducing friction between the cylinder and piston of the engine, the reciprocating movement of the piston can be made possible, easily. The pressure inside the cylinder during the compression and expansion strokes is much higher than in the crankcase. Due to the movement of the piston inside the cylinder and its temperature change during the engine operation, a clearance needs to be between them. The clearance creates some crevices between the cylinder chamber and the crankcase. By placing the open-mouthed elastic rings in their special grooves on the piston, some serious flowing obstacles are created. However, in addition to the loss of work, the friction between the rings and cylinder can cause the relative motion of the rings in their grooves during the reciprocating movement of the piston and change the blowby geometry, so the ring friction forces are important. In the present work, an accurate experimental method is proposed and used to measure the friction of a ring with a cylinder applying a mass-pulley-string set, equipped with a displacement measurement sensor. After the experimental-analytical estimation of pulley-resistant torques, to extract the friction force of the ring, the friction force was measured and calculated in two states piston without ring and piston with ring. The results showed that the friction force of a compression ring was 2.87 N and although the piston under vertical movement had no noticeable lateral load, it was observed that its friction force was about 77% of the compression ring friction.

Convective heat transfer from a flat plate, which is enhanced by an oscillating blade, was numerically investigated at the present study. It was assumed that the blade is made of a rigid and thin plate and it is vertically oriented at the top of the target plate. Numerical analysis was performed using commercial software ANSYS Fluent 6.3 and the periodic oscillation of the blade was modeled by the moving mesh method. Conservation equations of mass, momentum and energy was solved in 2-D and transient form for the laminar airflow with constant physical properties. Constant temperature was considered for the plate and the details of both the flow and thermal fields were determined. The distribution of convective heat transfer coefficient was then calculated for the target plate. The effect of various parameters including the amplitude and frequency of the blade oscillation as well as the geometrical parameters was investigated on the convective heat transfer from the target plate. The results indicated that a wider area of the plate was affected by increasing the oscillation amplitude of the blade. Convective heat transfer was also enhanced over the entire target plate as the rotational Reynolds number was increased.

In this research, combined cycle of heat, cold and power production has been simulated and function of the combined cycle has been evaluated in terms of energy and exergy. Moreover, the effect of different parameters including the generator temperature, Pinch point temperature difference and superheated temperature difference on the efficiency of energy, exergy, output work, irreversilbilities and the coefficient of performance of ejector refrigeration cycle has been studied. The combined system proposed in this study includes organic Rankine cycles including pump, steam generator, compressor, turbine, heat exchanger, separator, mixer and ejector refrigeration cycle, which in order to generate power and meet the thermal and refrigeration needs, including heating, Cooling as well as spa is designed for consumption. The results imply that by increasing the input heat to the generator, thermal efficiency of the cycle mainly increases. Also, under the same experimental conditions, with an increase in the temperature of the condenser and the evaporator, thermal efficiency of the whole cycle, studied in this research, has an increasing trend. Highest exergy dissipation is in the heater and the generator. The results show that the generator and the turbine are the most effective equipment in the function of the whole system.

Heat transfer efficiency has always been at the center of attractions for many researchers and industries and demand for higher efficiency methods and materials are increased in the last decades. One of the effective methods to enhance the thermal performance of instruments is using nanofluids as a solid-liquid suspension.  In the present paper, the thermal conductivity, viscosity, and heat transfer process of the nanofluids are discussed. The effect of volume fraction, temperature, particle size, shape, base fluid type, and other factors on thermal conductivity, viscosity, and convective heat transfer coefficient of nanofluids are presented. Also, the preparation methods of nanofluids are exhibited and the stability and the effective factors on the stability of nanofluids are brought in the manuscript. Besides, a summarized number of experimental and mathematical studies about the thermal conductivity, viscosity, and stability of nanofluids are listed, compared, and analyzed. The works about the Nusselt number in fluids and nanofluids are presented in detail to determine the future challenges of nanofluids.

Investigation and determination of residual frictional properties of rock joints is important for designing structures on or within the rock masses. Tribometer is a device to measure frictional properties of various types of contacting surfaces. In this research, several direct shear experiments conducted on two types of artificial limestone joints named Onyx marble and Travertine. Two types of surfaces geometry such as rough tensile joint surfaces with standard dimensions (based on ISRM suggested methods) and grinded planar small surfaces with dimensions ranged from 1 cm2 to 25 cm2 were prepared. Direct shear experiments by constant shearing rate and under different normal stresses conducted in CNL boundary condition. Results obtained from the residual state of the shear were gathered and investigate and it found that the residual friction coefficient of limestone rock joints, under almost similar normal stress and shearing rate conditions, remains approximately constant with differing the contact size of the specimens. In addition, under approximately similar stress concentrations in contact regions, the residual shear behavior of rough surfaces with standard dimensions is very similar to that of small planar ground surfaces in limestone joints. Finally, based on findings in this research and some other past researches, it is proposed to apply tribometers for measuring the residual shear strength of rock joints. In this paper, a conceptual design of a tribometer is proposed to develop for measuring residual frictional properties of a point contacts of rock surfaces during 5 to 20 mm shear displacements.

In this research, convective heat transfer in a flow over rotating elliptic cylinders with various axis ratios and rotation rates is investigated numerically. The governing equations including continuity, momentum and energy equations are solved by numerical finite element method. The rotating cylinder was assumed at a constant temperature and heat dissipation to the flow was defined. Numerical analysis was performed by means of COMSOL Multiphysics software. Two domains were defined in the geometry, the rotating one with constant velocity and the second one was assumed to be stationary and the domains are paired to each other. The heat transfer rate and streamlines are obtained for all cases. The unsteady flow is studied when the cylinder starts to rotate suddenly. The results indicate an increase in heat transfer by increasing the rotational ratio. In the higher rotational ratios, the vortices in rotational path of the solid wall are removed and immediately transferred to the edge of the cylinder. In terms of the ratio of heat transfer to drag force, rotating cylinder with axis ratio of unity is the best one.

Venous valves are a type of one-way valves which conduct blood flow toward the heart and prevent its backflow. Any malfunction of these organs may cause serious problems in the circulatory system. Numerical simulation can give us detailed information and point to point data such as velocity, wall shear stress, and von Mises stress from veins with small diameters, as obtaining such data is almost impossible using current medical devices. Having detailed information about fluid flow and valves' function can help the treatment of the related diseases.

In the present work, the blood flow through a venous valve considering the flexibility of the vein wall and valve leaflets is investigated numerically. The governing equations of fluid flow and solid domain are discretized and solved by the Galerkin finite element method.

The obtained results showed that the blood velocity increases from inlet to the leaflets and then decreases passing behind the valve. A pair of vortices and the trapped region was observed just behind the valves. These regions have low shear stresses and are capable of sediment formation.

The von Mises stress which is a criterion for the breakdown of solid materials was obtained. It was also observed that a maximum value occurred at the bottom of the leaflets.

Centrifugal pumps as a heart of the system which are used to move fluids are used widely in most of the industries and have considerable contribution in the amount of energy consumption, so improving of their performance has been attended for researchers .In this paper the aim of studying is the effect of double splitter blades on pump’s performance numerically and experimentally. Three type impellers have been made as experimental investigation. Pump with this impellers is tested and extracted the performance curve. Also, for investigation of the flow pump has been simulated numerically by ANSYS-CFX commercial code. Numerical method of finite volume with k-ω SST turbulence model for numerical analysis. Numerical and experimental results show reasonable agreement that increasing of head and variation of NPSHR due to adding of double splitter blades. The maximum head increased was obtained related to third type of Impeller about 6.33 percent. Furthermore, third type is selected as best impeller. Also, it is observed that around point of designing of pump the effect of double splitter blades on pump’s performance is more significant and deviation from this point will decrease the effect of it.

As pumps are used frequently in industrial plants, their performance improvement is important. In this study, performance improvement of centrifugal pumps by application of splitter blades have been investigated both numerically and experimentally. Radial impellers with different length of splitter blades have been manufactured and tested to obtain performance charts. On the other hand, the flow in impeller and volute has been investigated numerically by ANSYS-CFX commercial code. Numerical study has been done using Finite volume method and k-ωSST turbulence model. Rotating and stationary frames have been used to analyze flow in impeller and volute respectively and the results have been coupled by Frozen Rotor. Three impellers with the lengths of splitter blades equal to 0, 33% and 66% of original blades were tested. Results show head increase when the splitter blades added while the amount of increase depends on the splitter blades length. At BEP (Best Efficiency Point) the maximum head increase was reported for impeller type three (the length of splitters equal to 66% of original blades) about 10.5 percent. It should be noted that as the capacity tends to BEP, the effect of splitter blades is more significant.