THE EFFECT OF RADIAL DISTRIBUTION OF VOIDAGE ON THE BURNING RATE OF A CARBON SPHERE IN A FLUIDIZED BED

The local void around a sphere in a packed bed or in the emulsion phase of a fluidized bed has been calculated theoretically; and measured as a function of the radial distance from the centre of the sphere. Effects of this distribution on the burning rate and Sherwood number have been investigated for two types of bed e.g. where inerts are of the same size as the carbon particle and where the inerts are smaller than the carbon particle. Even though the present analysis studies the effect on the diffusive component of the mass transfer alone, the results suggest that the distribution of voidage will yield Sherwood numbers higher than that calculated from the constant voidage assumption. The voidage distribution explains the increase in burning rate of carbon with increasing size of the inerts.     

A Method to Predict the Minimum Fluidization Velocity of Binary Mixtures Based on Particle Packing Properties

An approach was made to predict the minimum fluidization velocity for binary mixtures of spherical particles differing in size and/or density. The spherical multiparticle model proposed by Panigrahi and Murty was employed to describe the relationship between the bed pressure drop and the gas velocity; the voidage at minimum fluidization was estimated by the Westman equation, which was originally used to calculate the packing voidage of mixtures. The predictions agree fairly well with the reported experimental data in the range of Re = 0.12 ? 156, covering both the regions of laminar flow and intermediate flow.     

Effect of elevated temperature and silica sand particle size on minimum fluidization velocity in an atmospheric bubbling fluidized bed

The impact of temperature and particle size on minimum fluidizing velocity was studied and analyzed in a small pilot scale of bubbling fluidized bed reactor. This study was devoted to providing some data about fluidization to the literature under high temperature conditions. The experiments were carried out to evaluate the minimum fluidizing velocity over a vast range of temperature levels from 20 °C to 850 °C using silica sand with a particle size of 300–425 μm, 425–500 μm, 500–600 μm, and 600–710 μm. Furthermore, the variation in the minimum fluidized voidage was determined experimentally at the same conditions. The experimental data revealed that the U_mf directly varied with particle size and inversely with temperature, while ε_mf increases slightly with temperature based on the measurements of height at incipient fluidization. However, for all particle sizes used in this test, temperatures above 700 °C has a marginal effect on U_mf. The results were compared with many empirical equations, and it was found that the experimental result is still in an acceptable range of empirical equations used. In which, our findings are not well predicted by the widely accepted correlations reported in the literature. Therefore, a new predicted equation has been developed that also accounts for the affecting of mean particle size in addition to other parameters. A good mean relative deviation of 5.473% between the experimental data and the predicted values was estimated from the correlation of the effective dimensionless group. Furthermore, the experimental work revealed that the minimum fluidizing velocity was not affected by the height of the bed even at high temperature.     

CFD modeling of solid-liquid fluidized beds of mono and binary particle mixtures

CFD simulation of bed expansion of mono size solid-liquid fluidized beds has been performed in creeping, transition and turbulent flow regimes, where Reynolds number (Re_∞=d_pV_S∞ρ_L/μ_L) has been varied from 0.138 to 1718. It has been observed that the predicted values of bed voidage using the drag law of Joshi [1983. Solid-liquid fluidized beds: some design aspects. Chemical Engineering Research and Design 61, 143-161] and Pandit and Joshi [1998. Pressure drop in packed, expanded and fluidized beds, packed columns and static mixers—a unified approach. Reviews in Chemical Engineering 14, 321-371] (which has been derived from the first principals), exhibited an excellent agreement with the Richardson and Zaki equation. CFD simulations have also been performed for the prediction of segregation and/or intermixing of binary particle systems having the ratio of terminal settling velocity over a range from 3.2 to 1.06. The Reynolds number has also been varied over the range of 0.33 to 2080. It has been observed that the present CFD model explains all the qualitative and quantitative observations reported in the published literature (complete segregation, partial segregation, complete intermixing, etc) and these predictions are in good agreement with the experimental results. The present CFD model also predicts successfully the layer inversion phenomena which occur in the binary particle mixtures of different size as well as density. Further, the critical velocity at which the complete mixing of the two particle species occurs has also been predicted.     

Accurate void fraction calculation for three-dimensional discrete particle model on unstructured mesh

An accurate and efficient analytical method for computing the three-dimensional local void fraction is proposed in the context of discrete particle modeling. It is developed for the general case of unstructured meshes whose use is unavoidable to efficiently simulate modern gas-solid fluidized bed reactors characterized by complex geometries. The method relates the three-dimensional void fraction to several geometrical parameters. This allows the exact voidage evaluation for the frequently occurring case of having particles not wholly contained within one grid cell regardless its shape. Failing to accurately account for these common particle configurations in dense gas-particle systems has proven detrimental to the accuracy of their simulations.     

注水开发油藏合理压力水平研究

合理压力水平是指既能满足油田提高排液量的地层能量的需要 ,又不会造成原油储量损失、降低开发效果的压力水平。本文运用注采平衡原理 ,将油藏驱替能量与注采工艺技术结合起来 ,通过研究不同条件下注采压力系统的合理组合 ,确定油藏在不同条件下的合理压力水平     

Voidage waves in hydraulic conveying through narrow pipes

The nature of voidage waves in hydraulic conveying systems has been investigated computationally in this study. Voidage waves consisting of alternating regions of high and low solid fractions were observed to form spontaneously in hydraulic conveying through vertical pipes. The wavelengths of these voidage waves were found to be inversely related to mean solid fraction but independent of liquid conveying velocities for elastic solid particles. The wave structure in terms of the thickness and shape of the solid fraction profile over each dense phase was dependent on the mean solid fraction used. Pressure distributions along the longitudinal direction of the conveying pipe exhibited an interesting ‘step’ profile due to unequal pressure drops across the dense and dilute regions. With inelastic solid particles, a lower tendency for voidage wave formation was observed and a phase diagram was constructed to show combinations of operating conditions under which voidage waves would form. Finally, the study of voidage waves in hydraulic conveying systems was extended to the case of hydraulic conveying through horizontal channels. In addition to the various system parameters, voidage wave formation in this case was also affected by gravitational settling of the solid particles.     

Dynamic voidage measurements in a gas solid fluidized bed by electrical capacitance tomography

Based on the electrical capacitance tomography technique, a voidage measurement system that can implement a voidage distribution display and voidage measurement of a gas-solid fluidized bed simultaneously is developed. The back projection algorithm is selected as the image reconstruction algorithm to satisfy the requirement of voidage distribution display. Combining the Tikhnov regularization principle with the ART algorithm, a new quantitative hybrid image reconstruction algorithm is presented to implement the voidage measurement of a gas-solid fluidized bed. Experimental results show that the developed measurement system is successful. The speed of image reconstruction of voidage distribution is greater than 20 frames per second. The maximum error of voidage measurement can be less than 5%.     

钙芒硝空隙的CT实验研究

采用μCT225KVFCB高精度显微CT试验系统,利用CT衰减系数,从细观角度研究了自然状态下钙芒硝的空隙情况.研究表明:钙芒硝岩样的空隙度仅为0.59%-0.7%,等效空隙直径仅为1.11μm.这些研究结论对钙芒硝原位水溶开采的实施和控制具有非常重要的意义.