Li Duan

The convection during NaClO3 crystal growth observed by the phase shift interferometer

Abstract An optical diagnostic system consisting of the Mach–Zehnder interferometer with the phase shift device and an image processor has been developed for the study of the kinetics of the crystal growing process. The dissolution and crystallization process of NaClO 3 crystal has been investigated. The concentration distributions around a growing and dissolving crystal have been obtained by using phase-shift of four-steps theory for the interpretation of the interferograms. The convection (a plume flow) has been visualized and analyzed in the process of the crystal growth. The experiment demonstrates that the buoyancy convection dominates the growth rate of the crystal growing face on the ground-based experiment.

Mass transfer process during the NaClO3 crystal growth process

Abstract Particle image velocimetry (PIV) technology was used directly to measure the convection pattern in sodium chlorate (NaClO 3 ) crystal growth. The two-dimensional velocity distributions in the solution of NaClO 3 have been obtained from experiments, and the velocity fields were compared with concentration fields that have been obtained by a phase-shift interferometer. The structures of both results of the velocity fields and concentration fields agree well. The coupling action of velocity and concentration was analyzed.

Epitaxial lateral overgrowth of cubic GaN by metalorganic chemical vapor deposition

The epitaxial lateral overgrowth (ELO) of cubic GaN by metalorganic chemical vapor deposition has been performed on SiO2-patterned GaN laver. The mechanism of lateral overgrowth is studied It was found that the morphology of ELO GaN stripes strongly depended on the direction of stripe window openings, which was discussed based on the different growth rates of (1 1 1)A and (1 1 1)B. Under the optimized growth condition, single-phase cubic GaN was deposited successfully. The peak position of near-band emission in ELO GaN has a redshift of 13 meV compared with the conventionally grown sample, which may be due to the partial release of stress during the ELO process

Experimental study of surface deformation and flow pattern on buoyant-thermocapillary convection

Free surface deformation is one of the most important physical phenomena in fluids with free surface. In the present paper, convection and surface deformation caused by thermocapillary effect in a rectangular cavity were investigated. In ground experiments, the convection was also affected by gravity. The cavity has a horizontal cross section of 52mm×42mm and the thikkness of the liquid layer is 4mm. Temperature difference between two sides of the liquid layer was increased gradually, and the flow in liquid layer will develop from steady to unstable convection. An optical diagnostic system consisting of a revised Michelson interferometer with image processor was developed to study fluid surface deformation in convection, and the displacements of free surface oscillation were determined. PIV technique was adopted to observe the evolution of flow pattern, and the velocity fields were obtained quantitatively. The present experiments demonstrate that surface deformation is quite distinct in buoyant-thermocapillary convection. in order to understand the mechanism of buoyant-thermocapillary convection, not only the hydrothermal wave instability but also the surface wave instability should be discussed.

Scanning electron microscope studies of cubic AlxGa1-xN films grown on GaAs(1 0 0) by metal organic vapor phase epitaxy (MOVPE)

Cubic AlGaN films were grown on GaAs(100) substrates by MOVPE. Scanning electron microscope and photoluminescence were used to analyze the surface morphology and the crystalline quality of the epitaxial layers. We found that both NH, and TEGa fluxes have a strong effect on the surface morphology of AlGaN films. A model for the lateral growth mechanism is presented to qualitatively explain this effect. The content of hexagonal AlGaN in the cubic AlGaN films was also related to the NH3 flux

Instabilities of thermocapillary–buoyancy convection in open rectangular liquid layers

This article presents the experimental investigation on instabilities of thermocapillary- buoyancy convection in the transition process in an open rectangular liquid layer subject to a horizontal temperature gradient. In the experimental run, an infrared thermal imaging system was constructed to observe and record the surface wave of the rectangular liquid layer. It was found that there are distinct convection longitudinal rolls in the flow field in the thermocapillary- buoyancy convection transition process. There are different wave characterizations for liquid layers with different thicknesses. For sufficiently thin layers, oblique hydrothermal waves are observed, which was predicted by the linear-stability analysis of Smith & Davis in 1983. For thicker layers, the surface flow is distinct and intensified, which is because the buoyancy convection plays a dominant role and bulk fluid flow from hot wall to cold wall in the free surface of liquid layers. In addition, the spatiotemporal evolution analysis has been carried out to conclude the rule of the temperature field destabilization in the transition process.

Oscillatory and Chaotic Buoyant-Thermocapillary Convection in the Large-Scale Liquid Bridge*

To cooperate with Chinese TG-2 space experiment project, the transition process from steady to regular oscillatory flow, and finally to chaos is experimentally studied in buoyant-thermocapillary convection. The onset of oscillation and further transitional convective behavior are detected by measuring the temperature in large-scale liquid bridge of 2cSt silicone oil. To identify the various dynamical regimes, the Fourier transform and fractal theory are used to reveal the frequency and amplitude characteristics of the flow motion. The experimental results indicate the co-existence of quasi-periodic and the Feigenbaum bifurcation in chaos.

Wavenumber Selection by Bénard–Marangoni Convection at High Supercritical Number

Marangoni-Benard convection which is mainly driven by the thermocapillary (Marangoni) effect occurs in a thin liquid layer heated uniformly from the bottom. The wavenumber of supercritical convection is studied experimentally in a 160 x 160-mm(2) cavity that is heated from the bottom block. The convection pattern is visualized by an infrared thermography camera. It is shown that the onset of the Benard cell is consistent with theoretical analysis. The wavenumber decreases obviously with increasing temperature except for a slight increase near the onset. The wavenumber gradually approaches the minimum when the supercritical number.. is larger than 10. Finally a formula is devised to describe the wavenumber selection in supercritical convection.

Transition in high Prandtl number buoyant-thermocapillary convection

Combined buoyant-thermocapillary convection in a rectangular vessel is experimentally studied for the 100cSt silicone oil with a large temperature-dependent Prandtl number (Pr=909 at 25°C). Utilizing the PIV and IRT techniques, the two-dimensional velocity fields in the mid-plane and the free surface temperature field are investigated. We have established the relationship between the total kinetic energy in the mid-plane and the imposed temperature difference for the first time through experiments. The well known return flow has been achieved and the first transition is observed from unicellular- to bicellular- flow at a much higher critical temperature difference relying on the layer depth. The transition conditions have also been transformed into a series of nondimensional parameters and mapped in the Ma-Bod and Ra-Bod spaces.

Preliminary Investigation of Fluid Flow in Meso-Scale Rectangular Tube

The fluid flow associated with micro and meso scale devices is currently of interest. Experiments were performed to study the fluid flow in meso-scale channels. A straight flow tube was fabricated with 1.0×4.0mm2 in rectangular cross section and 200mm in length, which was made of quartz for flow visualization and PIV measurements. Reynolds numbers were ranged from 311 to over 3105. The corresponding pressure drop was from 0.65KPa to over 16.58KPa between the inlet and outlet of the tube. The micro PIV was developed to measure the velocity distribution in the tube. A set of microscope object lens was mounted ahead of CCD camera to obtain optimzed optical magnification on the CCD chip. The velocity distributions near the outlet of the tube were measured to obtain full-developed flow. A CW laser beam was focused directly on the test section by a cylinder lens to form a small light sheet. Thus, high power density of light was formed on the view region. It is very important to the experiment while the velocity of the flow reaches to a few meters per second within millimeter scale. In this case, it is necessary to reduce exposure time to microseconds for PIV measurements. In the present paper, the experimental results are compared with the classical theories.