Letters in Heat and Mass Transfer最新文献

Effect of non-linear density-temperature relation on convective stability in a porous medium has been studied using normal mode technique. Computations have been made for both Brinkman and Darcy models. The analysis is an extension of that by Wu and Cheng [5] to Porous media.

The optimal linearization method is used to obtain an approximate analytical solutions for steady state of convective fins with variable thermal conductivity. Furthermore, an approximate analytical solutions for transient state of convective fins with variable thermal conductivity is achieved by using the variational embedding method.

Using the three-zone model proposed by Levich, a new theoretical expression for turbulent heat and mass transport in inelastic non-Newtonian liquids has been proposed. This new model, $\text{j}\text{= 0.075}\text{n}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{(}\text{K}\text{/g9)}{}^{\mathrm{<mtext>-2</mtext><mtext>3</mtext><mtext>n</mtext>}}\text{v}{}_{\mathrm{o}}\text{(4−}\text{n}\text{)}\text{3}\text{n}\text{(}\text{C}{}_{\mathrm{o}}\text{− C}{}_{\mathrm{b}}\text{)}$ which has no adjustable parameters, has been derived under the assumption that the Schmidt number is fairly high as is the case in viscous polymer systems and fermentation broths. The predictive capability of this equation was tested in this work for flow situations for which the friction velocity v_o is known: tubular non-Newtonian heat transfer and turbulent non-Newtonian mass transfer from a spinning disk. The excellent agreement between the available experimental data and the predictions of our model gives confidence to using the above model for that the heat and/or mass transfer rate increases if the pseudoplastic anomaly increases and that this enhancement is more pronounced in turbulent regime than it is under laminar conditions of flow.

This paper presents a numerical model for predicting the performance of liquid-gas mass transfer in a rotating perforated-disc type contactor. The device consists of a cylindrical section situated between two 45-degree conical sections. A liquid flows downward by gravity while a stream of air moves upward by buoyancy thus forming a counter-current flow situation in the contactor. A gas dissolved in the liquid transfers into air bubbles which are sheared to a tiny size as they rise through the perforations on the rotating disc. Both laminar and turbulent flows are treated. Utilizing the velocity distribution [10,11] and bubble trajectory [12] as the basis, the interphase mass transfer performance of carbon dioxide in the water-air system is numerically determined. It is disclosed that in both laminar and turbulent flow cases, the rate of interphase mass transfer increases significantly with a reduction in bubble size. Rotational speed does not affect mass transfer in laminar flow but causes an exponential mass transfer enhancement in higher turbulent flows. There exists an optimum through-flow rate of the liquid for the best mass transfer performance depending on the initial bubble size and disc speed. Test results [9] provide a qualitative confirmation of the theory.

An experimental study of the interaction of a laminar two-dimensional thermal plume with a neigboring vertical surface is carried out. The temperature field in the interaction region, downstream of the source, is studied to determine the nature of the buoyancy-induced flow that arises. Of particular interest was the effect of the buoyant heated flow on the surface temperature level. The flow is found to be strongly deflected towards the surface, for the range of physical variables considered. The dependence of the resulting flow on the variables, such as heat input and distance of the source from the surface, is determined. The underlying physical processes are considered in terms of the observed trends.

A model is proposed for the calculation of effective thermal conductivity of two phase media with a single continuous phase. This model has been shown to satisfy the limiting conditions and also to correlate the experimental data well. Though this model is based on the Ohm's law, it is in very good agreement with rigorously derived results based on the Fourier law.

The purpose of this paper is to test the validity of pulse methods to characterise the thermal properties of a fibrous non random composit material. A tridimensional numerical model shows that the influence of the contact fiber-matrix is slight except when the sample is gaz free and under vacuum; however the longitudinal diffusivity of the fiber is the most important parameter. A “flash” pulse method was used to test the validity of the model. For thick samples, the composit material is similar to an homogeneous material, the thermal properties of which depend on those of the cons tituants and on the geometrical parameters of the composit material.