Effect of pulse injection on film cooling performance: Experimental and numerical investigation

In this paper, the influence of pulsating air on film cooling of a flat plate at different frequencies and blowing ratios are experimentally and numerically investigated. Square wave pulsed flow is generated at four frequencies of 2, 10, 50, and 100 Hz corresponding to Strouhal numbers of 0.00254, 0.0127, 0.0636, and 0.1271, respectively, and at five blowing ratios of 0.5, 1, 1.5, 2.4, and 3. Reynolds-averaged Navier−Stokes equations are resolved to analyze the coolant film effectiveness based on parameters set in the experiments. The [Formula: see text] model used for turbulent modeling. The obtained results showed that the performance of pulsating cooling decreases with increasing of blowing ratio at the same flow as steady state conditions. The difference between numerical and experimental values for the centerline film effectiveness shows good adaptation at the distances of the injection hole downstream. The lift-off of the local jet increased under pulsation. Increasing the pulse frequency increases the overall efficiency of film cooling. The maximum mean centerline pulsating film cooling effectiveness is obtained at Strouhal number of 0.0636 and a blowing ratio of 0.5, and the minimum value is for Strouhal number of 0.00254 and a blowing ratio of 3. For pulsed flow, the maximum discrepancy of the mean centerline film effectiveness between experimental and numerical results was 17.82%.

Comparative Analysis between Experimental and Numerical Model resultsof a Jetin a Shallow Water Tank

In this work a comparative analysis of a turbulent jet experiment produced by the injection of dyed water into a shallow tank filled with water of the same density and the results of a numerical modelare presented. Dye was injected with the source fluid as a tracer. The concentration of the dye in the shallow turbulent flow was determined using a video imaging technique.The present laboratory experiments were conducted in a tank of small depth, and it is significantly wide to avoid the effect of the side walls. The space between the parallel walls of the tank can be varied during the experiments. The large-scale turbulent flow in the water sheet between the walls of the tank is confined to essentially two-dimensional motion. The shear on the bottom of the tank is a momentum sink to be considered.A comparison of the numerical resultswith the experimental data showed a very good agreement in terms of the position reached by the jet at different times after injection is initialized. These findings are useful for turbulent modeling of the shallow shear flow and for application to the large scale heat and mass exchange processes in lagoons, lakes, the ocean and the atmosphere

Numerical Analysis of Non Contact Transportation System for Wafer Warping

In this work a comparative analysis of a turbulent jet experiment produced by the injection of dyed water into a shallow tank filled with water of the same density and the results of a numerical modelare presented. Dye was injected with the source fluid as a tracer. The concentration of the dye in the shallow turbulent flow was determined using a video imaging technique.The present laboratory experiments were conducted in a tank of small depth, and it is significantly wide to avoid the effect of the side walls. The space between the parallel walls of the tank can be varied during the experiments. The large-scale turbulent flow in the water sheet between the walls of the tank is confined to essentially two-dimensional motion. The shear on the bottom of the tank is a momentum sink to be considered.A comparison of the numerical resultswith the experimental data showed a very good agreement in terms of the position reached by the jet at different times after injection is initialized. These findings are useful for turbulent modeling of the shallow shear flow and for application to the large scale heat and mass exchange processes in lagoons, lakes, the ocean and the atmosphere.

Influence of Dimple Height on Turbulent Heat Transfer of Fin Array with Alternate Convex/Concave Dimples

This study analyzes transient turbulent modeling of three-dimensional multiple dimpled fin array using large eddy simulation (LES). The Navier–Stokes equations as well as the energy equation were constructed by the finite volume method and then discretized to form algebraic equations, which were solved by semi-implicit method for pressure-linked equation (SIMPLE). The solutions of temperature and velocity were obtained by iterating computation until it converged within each step. This simulation places nine fins on the bottom surface of a channel and changes the height of the dimple (0.4, 0.8, and 1.2 mm) with three different levels of Reynolds number (Re) (3500, 5000, and 6500) to investigate the temperature and flow field without gravity in forced convection. The results indicate that the dimpled fin array can generate vortices between the convex/concave dimples and the fin base and increase the influences of the height of the dimple on the flow field around the fin array. The averaged time-mean of the Nusselt number (Nu) for the dimple height of 0.8 mm is higher than that of the no-dimple case up to 14.4%, while the averaged time-mean Nu for the dimple height of 1.2 mm is lower than that of the no-dimple case up to 11.6%.

A Numerical Research on Vortex Street Flow Oscillation in the Double Flapper Nozzle Servo Valve

The oscillating flow field of the double nozzle flapper servo valve pre-stage is numerically analyzed through Large Eddy Simulation (LES) turbulent modeling with the previous grid independence verification. The vortex street flow phenomenon can be observed when the flow passes through the nozzle flapper channel, the vortex alternating in each side produces the periodical flow oscillation. The structural and flow parameter effects on the oscillating flow are emphasized, and it could be determined that the pressure on the flapper is nearly proportional to the flow velocity and inversely proportional to the actual distance between the flapper and the nozzle. On the other hand, the main frequency of oscillation decreases with the velocity and increases with the distance between the nozzle flapper. The main stage movement is further considered with a User Defined Function (UDF), and it could be determined that the influences of the structural and flow parameters on the flow oscillation are rarely changed, but the main frequencies drop, generally.

Experimental investigations of turbulent decaying behaviors in the core-flow region of a propeller wake

This work investigates the turbulent decaying behaviors downstream of a propeller in the core-flow region. Both axial and tangential velocity fluctuations behind a two-bladed propeller were measured using a stationary hot-wire probe. Unexpectedly, the complex near-wake core-flow of the propeller is found to show a similar decay characteristic of homogeneous turbulence, such as grid turbulence. The decay of turbulence intensity is found to be dominated by the level of periodic velocity fluctuations, showing a similar behavior of the homogenous and isotropic turbulence. This turbulent decaying behavior of the core-flow can be adopted for future turbulent modeling techniques.

Comparisons of Numerical Methods for Fluid Flow Simulations in a Microorganism Incubating Chamber

A microorganism incubator has been developed by a prototype performance tests and computational fluid dynamics (CFD) simulations. The microorganism incubator is useful to supply the high quality of fertilizer to agricultural industries. Particularly, a small-sized, movable incubator is desired to be produced for personal agriculturalists. Since the incubator is basically designed as a type of mixing tank for mixing water and powder of microorganism, the decision of an impeller type and size, positions, size of internal parts such as baffles for an efficient mixing process should be considered. To produce the effect of turbulence on the efficient mixing, the location and size of baffles on the chamber wall is particularly significant to the mixing chamber design process. Multiphase CFD simulations are performed to describe the mixing flows inside the tank and the flow physics and patterns are studied in order to find out the optimal conditions for the microorganism incubating. Since water with powder of microorganism is partially filled with the chamber, air-water two phase flows should be considered in the CFD simulation. To simulate such flows, the volume of fluid (VOF) scheme is used. Both steady-state and time-transient simulations are performed and their results between two different time derivative considerations are compared, which enables us to clearly understand the effects of unsteady flow characteristics on the whole flow phenomena in the chamber. Additionally, comparisons of the turbulence modeling for the rotating flows in the chamber will be performed to describe the complex flow phenomena around the rotating impeller and the stationary baffles on the chamber wall. Prior to performing CFD study for the real type of the chamber, the flow simulations for the mixing chamber whose flow characteristics were already studied by experiments are performed with respect to the change of the turbulence modeling and numerical methods. Thus, the proper numerical methods and turbulent modeling are then determined. After validations of the turbulent modeling and numerical schemes, the flow phenomena occurred at the real prototype of the microorganism chamber corresponding to the change of design parameter of the chamber such as the change of the chamber bottom shape, an impeller blade length and the number of baffles will be analyzed.

Numerical Simulations on Flow Separation Within an Axial Turbine at Very Low Reynolds Number

Reynolds number has significant impact on the aerodynamic performance of axial turbines. The internal flow within the stator of a low pressure turbine could be all laminar when the inlet Reynolds number is very low, and large flow separation may occur on the suction surface of the blade. The separated laminar boundary layer and the wake have strong unsteady interactions with the main flow in rotor passages. In order to investigate the laminar flow separation and the interaction, both laminar flow simulation and detached eddy simulation (DES) have been performed on a low speed axial turbine under a very low Reynolds number condition in this paper. For comparison, fully laminar modeling, transitional modeling and fully turbulent modeling are performed, also. The comparison between the computational results and the experimental results shows that both the laminar modeling and transition modeling can capture the laminar separation on the suction side of the stator blade accurately. The separation region locates in a thin zone strengthening from the blade tip to the hub, which is induced by the tip passage vortex. The separation generates a high turbulence intensity zone at the stator outlet. However, this zone in laminar simulation is smaller than that in the experimental due to the absence of turbulence disturbances. Fully turbulent modeling predicts a delayed separation and a smaller separation region. Detached eddy simulation is performed for single stator row, which gives better predictions for both the flow separation and high turbulent zone. The detailed flow structures of the secondary vortices of the stator, the rotor passage vortex and the tip leakage vortex are illustrated. The simulation results show that the laminar separation has obvious three dimensional behaviour. The radial movement of the horse shoe vortex is the main disturbance to the flow separation.