Effects of initial bubble size on geometric and motion characteristics of bubble released in water

To seek and describe the influence of bubble size on geometric and motion characteristics of the bubble,six nozzles with different outlet diameters were selected to inject air into water and to produce different bubble sizes.High-speed photography in conjunction with an in-house bubble image processing code was used.During the evolution of the bubble,bubble shape,traveling trajectory and the variation of bubble velocity were obtained.Bubble sizes acquired varied from0.25to8.69mm.The results show that after the bubble is separated from the nozzle,bubble shape sequentially experiences ellipsoidal shape,hat shape,mushroom shape and eventually the stable ellipsoidal shape.As the bubble size increases,the oscillation of the bubble surface is intensified.At the stabilization stage of bubble motion,bubble trajectories conform approximately to the sinusoidal function.Meanwhile,with the increase in bubble size,the bubble trajectory tends to be straightened and the influence of the horizontal bubble velocity component on the bubble trajectory attenuates.The present results explain the phenomena related to relatively large bubble size,which extends the existing relationship between the bubble terminal velocity and the equivalent bubble diameter.

Investigation of the compressibility and sinterabilty of AZ91 powder production and particle production by gas atomisation method

This study intends to determine the pressability and sinterability of AZ91 powder production by gas atomisation method and that of the produced powder for partial production.Therefore,first,a gas atomisation unit has been designed and manufactured in the laboratories of the Karabiik University,Department of Manufacturing Engineering.Atomised powder production has been achieved at a temperature of 795℃,with nozzle diameters of 2 and 4 mm and four different gas pressures(5,15,25,35 bars).Argon gas has been used for atomisation and as a protective gas atmosphere.Scanning electron microscope(SEM)is used to determine the shape of the produced AZ91 powder,and a laser particle size analyzer is used to analyze the powder size.Additionally,a microhardness(HV0.025)measurement has been conducted to determine the hardness of the produced powders.To achieve a homogeneous distribution,the produced powders are mixed in a three-dimensional moving turbulator for 30 min.Mixed powders have been pressed at 300,400,500 and 600 MPa and have been sintered at 500℃,550℃and 600℃.Additionally,the density values have been determined before and after sintering of the materials.SEM images have been obtained from the fractured surfaces of the samples before and after sintering.XRD and EDX analyses have been performed to determine the chemical composition.Further,microhardness(HV0.5)is obtained from the pressure surfaces of the samples to determine the effects of the pressing pressure and the sintering temperature on the hardness.As a result of the experimental studies,it has been observed that the powder size decreases with the increase in gas pressure and that the powder shape generally changes from ligament and complex shape to droplet and spherical shape.From the XRD,XRF and EDX results,it has been determined that the structure comprises an a phase(Mg main matrix)and Mg17Al12 interphase,which isβphase,and very small amounts of MgO have been observed.The hardness of the produced powders increased based on the increase in gas pressure.The densities of the samples increased with both increasing pressing pressure and sintering temperature.It has been observed from the fractured surface SEM images that the number of pores formed in the samples decrease with an increase in the pressing pressure.It has been determined that the post-sintering structure exhibitsαtypical dendritic structure.In addition to theα-Mg matrix phase,β(Mg17Al12)intermetallic andα+βeutectic were formed in the structure.The microhardness values of the samples decreased depending on the sintering temperature;the highest hardness value was measured as 64,02 HV0.5 at a pressing pressure of 300 MPa and a sintering temperature of 500℃,whereas the lowest hardness value was measured as 54,86 HV0.5 at a pressing pressure of 600 MPa and a sintering temperature of 600℃.

Numerical analysis of plasma arc behaviors

Completely understanding the physical mechanisms of the plasma arc is critical to its application in welding of medium thickness plates. In this study, a mathematical model is developed to analyze the temperature, fluid flow, electromagnetic fields and pressure distribution in plasma arc welding. The correlations between the torch structure （ nozzle diameter） and the plasma are properties are analyzed qualitatively. The influence of the plasma gas flow rate on the plasma arc behavior is also simulated numerically. The temperature distribution and current density of the plasma are change greatly with a little variation of the nozzle diameter and^or the plasma gas flow rate. Compared to the tungsten-inert-gas arc with almost same conditions, the heat intensity, fluid velocity and pressure at the anode suoCace rise by one order of magnitude for a plasma arc. The analysis results lay solid foundation for effective usage ofplnsma arc welding.

Bubbling to Jetting Transition during Argon Injection in Molten Steel

Bubbling to Jetting Transition is of the outmost importance in metallurgical processes given that the flow regime influences the refining rates, the refractory erosion, and the blockage of injection nozzles. Bubbling to jetting transition during subsonic bottom injection of argon in molten steel is studied here. The effect of the molten steel height, the injection velocity, the nozzle diameter, and the molten steel viscosity on the jet height and the bubbling to jetting transition is numerically analyzed using Computational Fluid Dynamics. Five subsonic argon injection velocities are considered: 5, 25, 50, 100 and 150 m/s. Three values of the metal height are taken into account, namely 1.5 m, 2 m and 2.5 m. Besides, three values of the nozzle diameters are considered: 0.001 m, 0.005 m and 0.01 m. Finally, three values of the molten steel viscosity are supposed: 0.0067, 0.1 and 1 kg/(m·s). It is observed that for the argon-molten steel system, the bubbling to jetting transition occurs for an injection velocity less than 25 m/s and that for the range of viscosities considered, the molten steel viscosity does not exert significant influence on the jet height and the bubbling to jetting transition. Due to the jet instability at subsonic velocities, a second transition, namely jetting to bubbling, is appreciated.

Visualization study on atomization characteristics and heat transfer performance of R1336mzz flash spray cooling

Visualization experiments are carried out to investigate the atomization characteristics of R1336mzz flash spray cooling.The influences of superheat,spray distance,and nozzle orifice diameter on spray cooling performance are analyzed experimentally.As the superheat increases,finer droplets and thinner liquid film are observed;this is helpful to improve the two-phase heat transfer efficiency.Enlarging atomization angle under high superheat is also observed for flash spray cooling,and it benefits for reducing the spray distance.It can be found that when the inlet superheat is 19.8℃ and the spray distance is 6 mm,the critical heat flux(CHF)reaches 251 W/cm^(2) and the maximum heat transfer coefficient(HTC)reaches 37.4 kW/(m^(2)℃),which are 55%and 11.6%higher than those when the inlet subcooling is 6.9℃ and the spray distance is 12 mm,respectively.Using flash spray reduces the spray distance,which benefits for designing compact spray cooling device.In addition,the nozzle orifice diameter has great influence on the cooling performance of flash spray,and the choice of the nozzle depends on the superheat.This study provides a physical insight into the heat transfer enhancement in flash spray cooling.