Sharif Digital Repository

The density jump on an inclined surface is analyzed using an integral method by applying mass and momentum conservation equations. The jump occurs in a two-layered fluid flow in which the upper layer is stagnant and very deep. A relation is derived, which gives the conjugate depth ratio as a function of inlet densimetric Froude number, inlet concentration ratio, bed slope and entrainment. A set of experiments are performed to verify the relation. The theory and the measurements are in good agreement. The analysis reveals that increasing the surface inclination results in a decrease in the conjugate depth ratio. This analysis also shows that the densimetric Froude number just after the jump... 

A numerical model based on the boundary element method is proposed for the sloshing of a flowing liquid in a three-dimensional tank. Assuming a mean flow in the tank in addition to a perturbation flow, the nonlinear boundary conditions of the liquid free-surface are linearized. Using the boundary element method along with the modal analysis technique a reduced order model is obtained which is used to calculate the fundamental sloshing frequencies and modes in the tank with an inlet and outlet. The obtained results for a test case are compared with the literature data to validate the proposed model. The results are in a very good agreement with analytical results and show an acceptable... 

A brass vortex tube with changeable parts is used to obtain the optimum nozzle intake numbers and diameter. The effects of inlet pressure and CF (cold fraction) are also investigated. Results illustrate that increasing the number of nozzles causes a temperature drop and the optimum nozzle diameter corresponds to quarter of vortex tube diameter. The distance between cold end orifice and nozzle intakes is investigated in this work and it is found that for a better performance, this distance should be decreased. A series of experiments conducted to investigate the CF effect on VT performance and an optimum amount for this parameter is found. A two-dimensional computational fluid dynamics... 

In this work, we study the gas mixing behavior in a micro T-mixer using the direct simulation Monte Carlo (DSMC) method. The gas mixing process is monitored through a T-mixer, which is fed by two different CO and N2 gases; flowing into the T-mixer through the upper and lower inlets. We investigate the effects of axial and lateral wall temperature gradients on the mixing evolution at different rarefaction levels. The achieved results show that any temperature difference between the channel walls would result in an increase in mixing length for the chosen wall temperature gradient ranges and the studies pressure cases. Our observations show that a positive temperature gradient toward the... 

The pulsating flow in the exhaust gas of a SI engine causes an unsteady flow at the inlet to the turbocharger turbine. In a four cylinder four stroke engine, the pulse frequency varies between 20 and 200 Hz. Three dimensional pulsating flows in a vane-less turbocharger turbine of a 1.7 liters SI engine are simulated numerically and validated experimentally. Simulations are done for 720 degree engine cycle at three engine speeds. The results are shown the inlet pulsating flow has significant effects on several turbine parameters especially the inlet total pressure, the reduced mass flow rate and the efficiency. The results show a very good agreement between the three-dimensional unsteady... 

In this article, the effects of volute cross section shape and centroid profile of a centrifugal compressor volute were investigated. The performance characteristics of a turbocharger compressor were obtained experimentally by measuring rotor speed and flow parameters at the inlet and outlet of the centrifugal compressor. The three dimensional flow field model of the compressor was obtained numerically solving Navier- Stokes equations with SST turbulence model. The compressor characteristic curves were plotted. For model verification, the results were compared with experimental data, showing good agreement. Modification of a volute was performed by introducing a shape factor for volute cross... 

Modern turbine blades are cooled by air flowing through internal cooling passages. Three-Dimensional numerical simulation of these blade cooling passages is too time-consuming because of their complex geometries. These geometrical complexities exist as a result of using various kinds of cooling technologies such as rib turbulators (inline, staggered, or inclined ribs), pin fin, 90 and 180 degree turns (both sharp and gradual turns, with and without turbulators), finned passage, by-pass flow and tip cap impingement. One possible solution to simulate such sophisticated passages is to use the one-dimensional network method, which is presented in the current work. Turbine blade cooling channels... 

Surgical site infection (SSI) is one of the important infectious problems in hospitals. It is well known that appropriate ventilation strategy is most effective way to control bacteria-carrying airborne particles responsible for SSI. With considerable improvement in the computational capacity of computers, computational fluid dynamics (CFD) technique has been widely used to study the indoor airflow and contaminant dispersion over the enclosures. In this research, the effect of inlet air velocity on the ultra-clean ventilation system performance is investigated Eulerian-Lagrangian numerical approach. The results show that surgical wound contamination may be highly affected by inlet velocity.... 

Airlift pumps are devices which are widely used in industrial applications. Parameters such as diameter of the pipe, tapering angle of the upriser pipe, submergence ratio, the gas flow rate, bubble diameter, and inlet gas pressure affect the performance of this type of pumps. In this research, the performances of airlift pumps with a vertical upriser length of 914 mm and initial diameters of 6 and 8 mm and various heights for steps, range 0:2 to 0:9 m, in submergence ratio of 0:6 are investigated numerically. The results show the improvement in the performance of step airlift pump (SALP) in comparison with ordinary type (OALP). Considering the effect of height of steps and secondary pipe... 

In this study, the optimal location of baffle in the header of air-to-air plate heat exchangers and the geometry of the header and the effects of triangular fins arrays on the effectiveness were numerically investigated for different flow rates and Reynolds numbers on different models. The main purpose of inlet baffle and geometry changes is to improve the pattern of flow distribution and assist flow uniformity. The investigations show that the optimal location of the inlet baffle with high Reynolds numbers has an insignificant effect on effectiveness, while with Reynolds numbers 250-3000, the effectiveness is affected significantly by baffle location. On the other hand, the reduction in... 

In primary sedimentation tanks, short-circuiting enlargement of dead zones and high flow mixing problems are caused by circulation regions (dead zones), which can reduce the optimal sedimentation of particles. For proper design of such tanks, the formation of recirculation zones should be avoided. The provision of a baffle as a geometrical modification of a tank may influence the flow field for better sedimentation. Thus, in this study, velocity measurements were performed by a three-dimensional Acoustic Doppler Velocimeter (ADV) to investigate baffle effects on the velocity distribution in a primary rectangular sedimentation tank, quantitatively. Effects of baffle positioning were also... 

We numerically solve the Navier-Stokes equations to study the rarefied gas flow in short micro-and nanoscale channels. The inlet boundary conditions play a critical role in the structure of flow in short channels. Contrary to the classical inlet boundary conditions, which apply uniform velocity and temperature profiles right at the real channel inlet, we apply the same inlet boundary conditions, but at a fictitious position far upstream of the real channel inlet. A constant wall temperature incorporated with suitable temperature jump is applied at the channel walls. Our solutions for both the classical and extended inlet boundary conditions are compared with the results of other available... 

The height of a dense layer underflow is defined as the interface between a dyed saline solution fluid and colorless ambient fluid. In this paper, the density current height or vision height of three-dimensional saline water under pure water is measured empirically, and the relation of this parameter with the location of maximum velocity is investigated. Because of the absence of a clear interface between the dense underflow and pure water, researchers were unable to define a unique parameter for the evaluation of density current height. The parameters used by some researchers include the height corresponding to the location of maximum, half-maximum, and quarter-maximum velocity in the... 

Energy-efficient mixing is vital for chemical and fuel processes. To this end, a flow-focusing configuration is proposed to investigate the effect of a uniform magnetic field on the mixing of a water-based ferrofluid with two streams of deionized water. An external and varying magnetic field is imposed on a straight microchannel, and the mixing between the ferrofluid and deionized waters is qualitatively and quantitatively measured. A commercial code based on the finite-element method is used, and the simulations are validated by two experimental studies in the literature. For a magnetic flux density of 10 mT, a signal frequency of 1 Hz, a duty cycle of 0.3, an inlet velocity of 500 µm/s,... 

In this paper, numerical and experimental investigation of the performance and internal flow field characteristics of the twinentry radial inflow turbine under full and partial admission conditions are presented. The turbine is tested on a turbocharger test facility, which was developed for small and medium size turbochargers. The flow pattern in the volute and impeller of a twin-entry turbine is analyzed using fully three-dimensional viscous program. The computational performance results are compared with the experimental results, and good agreement is found. The flow field at the outlet of the turbine is investigated using a five-hole pressure probe. The tests are performed for both full... 

In this study, the supersonic and choked subsonic flows through micro/nano channels are investigated using direct simulation Monte Carlo (DSMC) method. The supersonic case is simulated at different Knudsen numbers covering slip to transition flow regimes, while the effects of inlet Mach and back pressure are studied in details. The inlet/outlet pressure boundary conditions are suitably implemented benefiting from the basics of characteristics theory. A behavior similar to the one predicted by the Fanno theory is observed here; i.e., the supersonic flow velocity decelerates up to a choking condition where any further increase in Knudsen number is impossible unless strong normal/oblique shocks... 

Compressor fault identification is an important part of gas turbine diagnostic systems. Model based techniques have been used widely in this field. In this paper the performance of two compressors has been simulated using stage stacking method. One of them is the NASA 10 stage constant tip diameter compressor. The other one is the Siemens V94.2 engine's compressor. Then a new and more real compressor fault model is introduced and the effect of different faults on compressor performance has been studied. In this paper an intelligent fault diagnostic system has been developed which is able to detect different faults of the compressor. In all of the cases of faults, degradation in inlet flow... 

In this article, the thermal and hydraulic capacity of a rectangular microchannel heat sink with different trapezoidal porous configuration’s inlet heights (Formula presented.) and outlet heights (Formula presented.) are examined. A three-dimensional model is used for microchannels with miscellaneous trapezoidal porous configuration’s inlet and outlet heights, and the laminar fluid flow and conjugate heat transfer equations are numerically solved. Darcy-Brinkmen-Forchheimer equation is utilized for transport through the porous region. For microchannels with miscellaneous porous distribution’s inlet and outlet heights, the Nusselt number, pressure drop and figure of merit (FOM), a criterion... 

A brass vortextube is used to carry out a series of experiments. The main objective of the present research is to investigate the separation performances of a vortex tube (VT) for a hydrocarbon mixture. Examination is also applied to study the effects of nozzle intakes number and cold fraction on the gas species separation at specific inlet pressure 236.37 kPa in a VT with two gas mixtures (LPG as a hydrocarbon mixture and LPG-N2). A two-dimensional computational fluid dynamic (CFD) model simulation of a VT is presented. CFD code after validation is also applied to investigate the role of cold fraction and nozzle intakes number on the gas species separation. The highly rotating flow field...