NUMERICAL SIMULATION OF SUCCINONITRITE DENDRITIC GROWTH IN A FORCED FLOW

Numerical simulation based on phase field method is performed to describe solidification process of pure material in a free or forced flow. The evolution of the interface is showed, and the effects of mesh grid and flow velocity on succinonitrite shape are studied. These results indicate that crystal grows into an equiaxial dendrite in a free flow and into an asymmetrical dendritic in a forced flow. With increasing flow velocity, the upstream dendritic arm tip grows faster and the downstream arm grows slower. However, the evolution of the perpendicular tip has no significant change. In addition, mesh grid has no influence on dendritic growth shape when mesh grid is above 300×300.

Distinctions of dendritic behavior influenced by constant pressure and periodic pressure

The distinctions of dendritic morphology and sidebranching behavior when solidified under atmosphere pressure,constant pressure which is higher than atmosphere pressure (hereinafter referred to as constant pressure) and periodic pressure were investigated using 3-D phase field method.When growing at atmosphere pressure,side branches (secondary dendritic arms) are irregular.When solidified under constant pressure with a relatively high value,side branches are much more luxuriant,with more developed high-order side branches.When applied with periodic pressure,resonant sidebranching happens,leading to many more regular side branches and the smallest secondary dendritic arm spacing (SDAS) in the three cases.The significant difference in dendritic morphology is associated with tip velocity modulated by total undercooling including pressure and temperature undercooling.In the case of constant pressure,tip velocity increases linearly with total undercooling,and it varies periodically in periodic pressure case.The different variation trend in tip velocity is the reason for the distinct dendrite growth behavior in different cases.Unlike the phenomenon in constant pressure case where the dendrite grows faster with higher pressure,the dendrite grows slower under periodic pressure with higher amplitude,resulting in less developed primary dendrite and side branches.This is influenced by tip remelting due to low undercooling or even negative undercooling.It is revealed that the accelerated velocity of tip remelting increases with the decline of undercooling.The greater the amplitude of periodic pressure,the faster the tip remelting velocity during one period.This is the reason why the average tip velocity decreases with the rise of amplitude of periodic pressure.

Experimental study of flow field in interference area between impeller and guide vane of axial flow pump

Axial flow pump is a kind of typical pumps with rotor-stator interaction, thus the measurement of the flow field between impeller and guide vane would facilitate the study of the internal rotor-stator interaction mechanism. Through a structural modification of a traditional axial flow pump, the requirements of particle image velocimetry（PIV） measurement are met. Under the condition of opt.0.8Q, the axial vortex is identified between impeller hub and guide vane hub, which is developed into the main flow and to affect the movement when the relative positions of impeller and guide vane at different flow rates are the same. Besides, the development and the dissipation of the tip leakage and the passage vortex in impeller passages are mainly responsible for the difference of the flow field close to the outer rim. As the flow rate decreases, the distribution of the meridional velocities at the impeller outlet becomes more non-uniform and the radial velocity component keeps increasing. The PIV measurement results under the condition of opt.1.0Q indicate that the flow separation and the trailing vortex at the trailing edge of a blade are likely to result in a velocity sudden change in this area, which would dramatically destroy the continuity of the flow field. Moreover, the radial direction of the flow between impeller and guide vane on the measurement plane does not always point from hub to rim. For a certain position, the direction is just from rim to hub, as is affected by the location of the intersection line of the shooting section and the impeller blade on the impeller as well as the angle between the intersection line and the rotating shaft.