Pool Boiling on Wire in Microgravity - What Have We Learned

In the past years, steady pool boiling of degassed R113 on thin platinum wires has been studied systematically in our lab, including experiments in long-term microgravity aboard RS-22, in short-term microgravity in the Drop Tower Beijing / NMLC, and in normal gravity on the ground. Slight enhancement of nucleate boiling heat transfer is observed in microgravity, while dramatic changes of bubble behaviors are much evident. The value of CHF in microgravity is lower than that in normal gravity, but it can be predicted well by the Lienhard-Dhir correlation, although the dimensionless radius in the present case is far beyond its initial application range. The scaling of CHF with gravity is thus much different from the traditional viewpoint. Considering the influence of the Marangoni effects, the different characteristics of bubble behaviors in microgravity have been explained. A new bubble departure model has also been proposed, which can predict the whole observation both in microgravity and in normal gravity.

Three-dimensional fluorescence characteristics of dissolved organic matter produced by Prorocentrum donghaiense Lu

Filtration and cross-flow ultrafiltration techniques were used to separate culture media of Prorocentrum donghaiense at the exponential growth, stationary and decline stages into < 0.45 mu m filtrate, 100 kDa-0.45 mu m, 10-100 kDa and 1-10 kDa retentate and < 1 kDa ultrafiltrate fractions. The fluorescence. properties of different molecular weights of dissolved organic matter (DOM) were measured by excitation-emission matrix spectra. Protein-like and humic-like fluorophores were observed in the DOM produced by P. donghaiense. The central positions of protein-like fluorophores showed a red shift with prolonged growth duration, shifting from tyrosine-like properties at the exponential growth stage to tryptophan-like properties at the stationary and decline stages. The excitation wavelengths of protein-like fluorophores exhibited some change in the exponential growth and stationary stages with increased molecular size, but showed little change in the decline stage. However, the emission wavelengths in the decline stage exhibited a blue shift. Very distinct C type and A type peaks in humic-like fluorophores were observed. With a prolonged culture time, the intensities of both of the peaks became strong and the excitation wavelengths of peak A showed a red shift, while the A:C ratios fell. More than 94% of fluorescent DOM was in the lower than 1 kDa molecular weight fraction.

Interface of chip-based capillary electrophoresis-inductively coupled plasma-atomic emission spectrometry (CE-ICP-AES)

An interface of chip-based capillary electrophoresis (CE)-inductively coupled plasma-atomic emission spectrometry (ICP-AES) that is based on cross-flow nebulization has been developed. A polydimethylsiloxane (PDMS) CE-chip with conventional cross channel layout was used. A stainless steel tube was placed orthogonal to the exit of the CE separation channel for cross flow nebulization. A supplementary flow of buffer solution at the channel exit was used to improve nebulization efficiency. Two capillaries were inserted into the CE chip near the inlet of the separation channel for sample and buffer solution injection. Syringe pumps were used to manipulate the flow rate and flow direction of the sample, buffer, and supplementary buffer solution. Peak broadening due to the shape (bulb and tube-shaped) and size of the spray chambers was studied. The smaller tube-shaped spray chamber was used because of smaller peak broadening effect due to aerosol transport. The nebulization and transport efficiency of the CE-ICP interface was approximately 10%. Ba2+ and Mg2+ ions were eluted from the CE-chip within 30 s. Resolution of the Ba2+ and Mg2+ peaks was 0.7 using the chip-based CE-ICP-AES system.

Effects of Baffle Design on Fluid Flow and Heat Transfer in Ammonothermal Growth System of Nitrides

Efforts have been made in growing bulk single crystals of GaN front supercritical fluids using the ammonothermal method, which utilizes ammonia as fluid rather than water as in the hydrothermal process. Different mineralizers such as amide or azide and temperatures in the range of 200-600degreesC have been used to increase the solubility. The pressure is from 1 to 4 kbar. Modeling of the ammonothermal growth process has been used to identify factors which may affect the temperature distribution, fluid flow and nutrient transport. The GaN charge is considered as a porous media bed and the flow in the charge is simulated using the Darcy-Brinkman-Forchheimer model. The resulting governing equations are solved using the finite volume method. The effects of baffle design and opening on flow pattern and temperature distribution in an autoclave are analyzed. Two cases are considered with baffle openings of 15% and 20% in cross-sectional area, respectively.

The characteristics of BSRs and their derived heat flow on the profile 973 in the northeastern South China Sea

Processing of a recently acquired seismic line in the northeastern South China Sea by Project 973 has been conducted to study the character and the distribution of gas hydrate Bottom-Simulating Reflectors (BSRs) in the Hengchun ridge. Analysis of different-type seismic profiles shows that the distribution of BSRs can be revealed to some extents by single-channel profile in this area, but seismic data processing plays an important role to resolve the full distribution of BSRs in this area. BSR' s in the northeastern South China Sea have the typical characteristics of BSRs on worldwide continental margins: they cross sediment bed reflections, they are generally parallel to the seafloor and the associated reflections have strong amplitude and a negative polarity. The characteristics of BSRs in this area are obvious and the BSRs indicate the occurrence of gas hydrate-bearing sediments in the northeastern South China Sea. The depth of the base of the gas-hydrate stability zone was calculated using the phase stability boundary curve of methane hydrate and gas hydrate with mixture gas composition and compared with the observed BSR depth. If a single gradient geothermal curve is used for the calculation, the base of the stability zone for methane hydrate or gas hydrate with a gas mixture composition does not correspond to the depth of the BSRs observed along the whole seismic profile. The geothermal gradient therefore changes significantly along the profile. The geothermal gradient and heat flow were estimated from the BSR data and the calculations show that the geothermal gradient and heat flow decrease from west to east, with the increase of the distance from the trench and the decrease of the distance to the island arc. The calculated 2 heat flow changes from 28 to 64 mW/m(2), which is basically consistent with the measured heat flow in southwestern offshore Taiwan.

Anisotropic thermopiezoelectric solids with an elliptic inclusion or a hole under uniform heat flow

The two-dimensional problem of a thermopiezoelectric material containing an elliptic inclusion or a hole subjected to a remote uniform heat flow is studied. Based on the extended Lekhnitskii formulation for thermopiezoelectricity, conformal mapping and Laurent series expansion, the explicit and closed-form solutions are obtained both inside and outside the inclusion (or hole). For a hole problem, the exact electric boundary conditions on the hole surface are used. The results show that the electroelastic fields inside the inclusion or the electric field inside the hole are linear functions of the coordinates. When the elliptic hole degenerates into a slit crack, the electroelastic fields and the intensity factors are obtained. The effect of the heat how direction and the dielectric constant of air inside the crack on the thermal electroelastic fields are discussed. Comparison is made with two special cases of which the closed solutions exist and it is shown that our results are valid.

Three-Dimensional Modeling of Heat Transfer and Fluid Flow in Laminar-Plasma Material Re-Melting Processing

Modeling study is performed concerning the heat transfer and fluid flow for a laminar argon plasma jet impinging normally upon a flat workpiece exposed to the ambient air. The diffusion of the air into the plasma jet is handled by using the combined-diffusion-coefficient approach. The heat flux density and jet shear stress distributions at the workpiece surface obtained from the plasma jet modeling are then used to study the re-melting process of a carbon steel workpiece. Besides the heat conduction within the workpiece, the effects of the plasma-jet inlet parameters (temperature and velocity), workpiece moving speed, Marangoni convection, natural convection etc. on the re-melting process are considered. The modeling results demonstrate that the shapes and sizes of the molten pool in the workpiece are influenced appreciably by the plasma-jet inlet parameters, workpiece moving speed and Marangoni convection. The jet shear stress manifests its effect at higher plasma-jet inlet velocities, while the natural convection effect can be ignored. The modeling results of the molten pool sizes agree reasonably with available experimental data.

Conjugate Model for Heat and Mass Transfer of Porous Wall in the High Temperature Gas Flow

Heat and mass transfer of a porous permeable wall in a high temperature gas dynamical flow is considered. Numerical simulation is conducted on the ground of the conjugate mathematical model which includes filtration and heat transfer equations in a porous body and boundary layer equations on its surface. Such an approach enables one to take into account complex interaction between heat and mass transfer in the gasdynamical flow and in the structure subjected to this flow. The main attention is given to the impact of the intraporous heat transfer intensity on the transpiration cooling efficiency.

Numerical Study Of Heat Transfer Mechanism In Turbulent Supercritical Co2 Channel Flow

Direct numerical simulation (DNS) of supercritical CO2 turbulent channel flow has been performed to investigate the heat transfer mechanism of supercritical fluid. In the present DNS, full compressible Navier-Stokes equations and Peng-Robison state equation are solved. Due to effects of the mean density variation in the wall normal direction, mean velocity in the cooling region becomes high compared with that in the heating region. The mean width between high-and low-speed streaks near the wall decreases in the cooling region, which means that turbulence in the cooling region is enhanced and lots of fine scale eddies are created due to the local high Reynolds number effects. From the turbulent kinetic energy budget, it is found that compressibility effects related with pressure fluctuation and dilatation of velocity fluctuation can be ignored even for supercritical condition. However, the effect of density fluctuation on turbulent kinetic energy cannot be ignored. In the cooling region, low kinematic viscosity and high thermal conductivity in the low speed streaks modify fine scale structure and turbulent transport of temperature, which results in high Nusselt number in the cooling condition of the supercritical CO2.

Triple-region structure for turbulent-flow in a square duct - a finite-element approach

The Reynolds-averaged Navier-Stokes equations for describing the turbulent flow in a straight square duct are formulated with two different turbulence models. The governing equations are then expanded as a multi-deck structure in a plane perpendicular to the streamwise direction, with each deck characterized by its dominant physical forces as commonly carried out in analytical work using triple-deck expansion. The resulting equations are numerically integrated using higher polynomial (H-P) finite element technique for each cross-sectional plane to be followed by finite difference representation in the streamwise direction until a fully developed state is reached. The computed results using the two different turbulence models show fair agreement with each other, and concur with the vast body of available experimental data. There is also general agreement between our results and the recent numerical works anisotropic k-epsilon turbulence model.

Computation of turbulent-flow in a square duct - aspects of the secondary flow and its origin

A numerical study of turbulent flow in a straight duct of square cross-section is made. An order-of-magnitude analysis of the 3-D, time-averaged Navier-Stokes equations resulted in a parabolic form of the Navier-Stokes equations. The governing equations, expressed in terms of a new vector-potential formulation, are expanded as a multi-deck structure with each deck characterized by its dominant physical forces. The resulting equations are solved using a finite-element approach with a bicubic element representation on each cross-sectional plane. The numerical integration along the streamwise direction is carried out with finite-difference approximations until a fully-developed state is reached. The computed results agree well with other numerical studies and compare very favorably with the available experimental data. One important outcome of the current investigation is the interpretation analytically that the driving force of the secondary flow in a square duct comes mainly from the second-order terms of the difference in the gradients of the normal and transverse Reynolds stresses in the axial vorticity equation.