Jyh- Chen

Numerical computation of heat flow, fluid flow and interface shapes in the float zone of lithium niobate during a melting process

Abstract Computer simulations were performed to study the temperature and velocity distributions and the interface shapes in the floating-zone melt for lithium niobate materials. The input power distribution generated by the heat source was assumed to be Gaussian. The variation of the interface shapes and the transport phenomena in the flow zone relative to the input power was investigated. The results show that the shapes of the gas-melt and melt-solid interfaces and the temperature and velocity fields are modified by the gravity force; the influence of gravity increases as the input power increases. The present results are consistent with the recent experimental observations.

The effects of temperature distribution on the barium titanate crystal growth in an LHPG system

Abstract In this study, we examined the effects of the float-zone length, the growth rate, the heating range and the crystal diameter on surface temperature distribution profiles and temperature gradients for a barium titanate single-crystal rod using an LHPG system. The results indicated that the axial surface temperature gradients were enlarged by decreasing the float-zone length and the width of heating range and increasing the growth rate and the diameter of the crystal. During the growth of a 0.7 mm diameter crystal rod at 0.6 mm/min, a growth rate with a narrow heating range and with a high maximum temperature gradient is high, about 2050°C/mm, screw cracks periodically appearing on the crystal surface.

A general modeling for heat transfer during reflux condensation inside vertical tubes surrounded by isothermal fluid

Abstract An analytical modeling of condensation heat transfer characteristics under reflux cooling mode inside vertical tubes surrounded by isothermal external fluid was conducted in the present work. The dimensionless film thickness, film Reynolds number as well as the heat transfer coefficient in the condensate film were formulated and calculated numerically. The results indicate that the increase in Biot number thickens the film layer, speeds the film Reynolds number, and increases the condensation heat transfer coefficient. As Bi ≫ 1 and Nu = 1, the limiting cases for both isothermal wall and uniform wall heat flux, respectively, were derived for comparison. As compared with the experimental data of reflux condensation from previous studies, the analytical results show good agreement with the observed trend of the condensation heat transfer.

Experimental observation of interface shapes in the float zone of lithium niobate during a CO2 laser melting

Abstract The radiosity from a rod of lithium niobate that was heated uniformly by a CO2 laser with an axisymmetric ring-shaped beam was measured by a two-dimensional infrared imaging radiometer with bandpass filters chosen to be 3–5, 8–12, and 10.6 μm. The radiative characteristics of the volumetric emission and the significant difference of the emitted thermal radiation between the solid or melt and background for wavelengths of 3–5 μm was used to determine the interface shapes of the float zone during the CO2 laser melting process. The trend of the power distribution impinging on the rod surface may be deduced from the reflection of the CO2 laser. The gas bubbles existing in the melt may also be monitored. The surface temperature was determined from the distribution of the radiosity for wavelengths of 8–12 μm. The surface temperature distribution is modified significantly be thermocapillary convection. With an increase of the input power, the gas-melt interface changes from concave towards the melt to convex at the upper zone and concave at the lower zone due to the effect of gravity.

Thermocapillary convection in a rectangular cavity under the influence of surface contamination

Abstract This study examines the influence of insoluble surfactant on the steady thermocapillary flow in a rectangular cavity with an upper, deformable free surface and differentially heated side walls. The numerical solutions are obtained with a finite difference method, together with a boundary-fitted curvilinear coordinate system. The results show that the thermocapillary convection can be stabilized by the addition of an insoluble surfactant. Furthermore, as the concentration of the surfactant increases (higher elasticity number), oscillatory instability caused by that surfactant may occur when the slope of the surface tension becomes negative. For higher Peclet numbers, the concentration boundary is formed near the cold wall, and the clean surface appears in the region near the hot wall. Oscillatory flow may also be induced by a high local surfactant concentration that is created by a concentration boundary.

Thixotropic behavior of semi-solid magnesium alloy

By a high-temperature Couette type viscometer, we studied the thixotropic behavior of the semi-solid AZ91D magnesium alloy slurry. According to different variable conditions, we could measure the change of the shear stress. The results showed that the shear stress of semi-solid AZ91D magnesium alloy slurry increased at starting shearing, and the shear stress fell down at the maintained shear rate shearing. In our experiment data, we found that the semi-solid AZ91D magnesium alloy slurry had the behavior of shear thinning fluid. As the maximum shear rate increased, the measured shear stress increased at a dissimilar level. When the rising time was shorter, the increasing range of shear stress was larger. As the shearing time of the maintained shear rate increased, the falling down degree of the shear stress increased and it can be presented as a function of shearing time, such as: e 0.0159t min 11331 τ = − . At the same shear rate, the area of hysteresis loop was bigger and the thixotropic behavior was more obvious.