Wang Buxuan

Effect of variable thermophysical properties on laminar free convection of gas

Abstract The thermal conductivity and dynamic viscosity are assumed to vary with absolute temperature according to a simple power law. The density is taken as inversely proportional to absolute temperature at constant pressure, while the Prandtl number is assumed constant. The governing equations for laminar free convection of gas are changed to dimensionless ordinary equations by similarity transformation. They are solved by a shooting method. The numerical calculation results are given along with a detailed discussion.

Water absorption and measurement of the mass diffusivity in porous media

Abstract The study on water absorption in porous media is briefly reviewed. The mean mass diffusivity for moisture migration in the sense of a constant property is related to the actual mass diffusivity, and its dependence on the initial moisture content is presented. A method is proposed to determine the parameters in the exponential expression of the mass diffusivity according to the mean mass diffusivity measured. The corresponding error transmission is discussed, and experimental results from wet sand are compared with the predicted curves.

Analysis on hysteresis in nucleate pool boiling heat transfer

Abstract Many experiments reported for nucleate pool boiling heat transfer from microelectronic assemblies or porous surfaces in dielectric fluids have shown the existence of hysteresis phenomenon, which is known as a temperature overshoot (TOS) in boiling incipience and constrains engineering applications. Here, theoretical analysis has been carried out. It confirms that hysteresis may occur during both the boiling incipience and the boiling development. Hence, a new type of hysteresis, termed Temperature Deviation (TD), is described. Using thermodynamic nucleation, the physical mechanisms for both TOS and TD are analyzed in detail. Three parameters for quantitatively understanding the hysteresis phenomenon and possible measures for remedying it are proposed and discussed.

Forced convective heat transfer in a vertical annulus filled with porous media

Abstract Experiments on forced convective heat transfer were conducted for water or transformer-oil flowing upward through a vertical annulus filled with different size porous media. The thermal dispersion caused by successive flow disturbance due to the existence of porous media may have significant influence on the Nu value in the thermally fully-developed region, and a new model is proposed to consider the thermal dispersion as related to the geometrical parameters of porous media and the flow characteristics. The temperature distribution near the wall surface, and hence the convective heat transfer coefficient, can be thus predicted, which agrees well with the experimental results.

A method for evaluation of heat and mass transport properties of moist porous media

Abstract A method is proposed for evaluation of the mass diffusivity Dm(w, t), thermo-mass diffusivity D,(w, t), non-linear thermal conductivity ξ(w, t), and mass-thermal conductance α(w, t) of moist porous media at different temperatures and moisture contents based on the measured mass diffusivity D m (w, t ∗ ) and thermo-mass diffusivity D t (w, t ∗ ) at a specified reference temperature t ∗ . It is a valuable and convenient approach for engineering practice.

A semi-empirical theory for forced-flow turbulent film boiling of subcooled liquid along a horizontal plate

Abstract Based on a previous study [1], aphysical model is proposed in this paper to analyze the film boiling heat transfer for turbulent flow of subcooled liquid along a horizontal plate. The corresponding mathematical descriptions are given in detail and an analytical expression has been obtained as equation (28), which is simple in form. The empirical values of coefficient k and exponent m are determined from experiments with subcooled water. As a result, equation (30) will be adaptable for practical use in engineering analysis of high-temperature quenching process for rolled metals.

Natural convection in liquid-saturated porous media between concentric inclined cylinders

Abstract Numerical studies are reported for three-dimensional steady natural convection in liquid-saturated porous media enclosed between concentric inclined cylinders. The inner cylinder is kept at constant heat flux or is isothermal, while the outer cylinder is kept isothermal and the end plates insulated. Numerical studies range for: Rayleigh number, Ra ⩽ 10 3 ; radius ratio, 1 ⩽ R o ⩽ 10; aspect ratio, 1 ⩽ A ⩽ 15; angle of inclination, φ, from 0° to 90°. Numerical results show that, there exists a critical aspect ratio, for which the angle of inclination has no effect on the heat transfer. Good agreements between numerical results and experimental values have been ascertained for mean Nusselt number, Nu m .

Film boiling in laminar boundary-layer flow along a horizontal plate surface

Abstract A mathematical model is presented to cover the effect of a pressure gradient caused by the increasing vapor film thickness along the flow direction. Approximate heat transfer solutions are obtained analytically for laminar film boiling of a liquid deviating greatly from critical state. It is shown that, the heat transfer rate thus calculated may be obviously improved as compared with the results from previous formulae reported in the literature, especially for the region of low flow velocity.

Modeling of heat and mass transfer in unsaturated wet porous media with consideration of capillary hysteresis

Abstract The capillary hysteresis is an important behavior of liquid-unsaturated porous media. The hysteresis effects on the heat and mass transfer are evident in action in many important cases. The present paper introduces the minimum driving gradient concept into the analytical studies on the transport phenomena in unsaturated wet porous media. The individual phase equations and total saturation equation of mass transfer, and the energy equation for temperature profiles are derived. A hysteretic criterion is introduced with discussions. It is believed that the new efforts on the subject will promote the progress in all the relevant respects of theories and applications.

Flow Structures Around Micro-bubbles During Subcooled Nucleate Boiling

The flow structures were investigated around micro bubbles on extremely thin wires during subcooled nucleate boiling. Jet flows emanating from the bubbles were observed visually with the fluid field measurement using high-speed photography and a PIV system. The jet flows induced a strong pumping effect around a bubble. The multi-jet structure was further observed experimentally, indicating the evolution of flow structure around micro bubbles. Numerical simulations explore that the jet flows were induced by a strong Marangoni effect due to high temperature gradients near the wire. The bubble interface with multi-jet structure has abnormal temperature distribution such that the coolest parts were observed at two sides of a bubble extending into the subcooled bulk liquid rather than at the top. Evaporation and condensation on the bubble interface play important roles not only in controlling the intensity of the jet flow, but also in bringing out the multi-jet structure.