Radial Void Fraction Distribution Research Articles

Radial void fraction distributions in randomly packed fixed beds of uniformly sized spheres in cylindrical containers

The radial void fraction distribution in randomly packed fixed beds of uniformly sized spheres in cylindrical containers is analyzed by a nondestructive method in which the center position coordinates of specially prepared spheres are experimentally obtained using X-ray radiography and the radial void fraction distribution is geometrically determined from these center coordinates. The geometrical solution for the radial void fraction distribution is derived by dividing the fixed packed bed of spheres into a large number of radial annular layers of equal thickness and the local void fraction in each layer is expressed in terms of the solid volume segments coming from each sphere. The radial void fraction distribution is presented for four different diameter aspect ratios, D/d, ranging from 2.02 to 7.99. The height aspect ratio, H/d, of each cylindrical container is approximately 7.84. Using the new data and existing data for the radial void fraction distribution, an empirical correlation with one principal equation is determined which accurately predicts the radial void fraction distribution for fixed packed beds of uniformly sized spheres in cylindrical containers. The correlation can be used for fixed packed beds with D/d > /2.02. The correlation can be also used in analytical and numerical transport models for fixed packed beds of uniformly sized spheres to closely approximate the radial void fraction in the near-wall region.

Experimental and analytical study of countercurrent flow limitation in vertical gas/liquid flows

AbstractAn experimental and analytical study of adiabatic countercurrent flow limitation (flooding) in single vertical ducts is reported. The experiments were carried out in a rectangular channel using saturated liquid and vapour of Refrigerant 12 (CCl2F2). The steady‐state liquid delivery (flooding) curves as well as local pressure drop and void fraction distributions in the countercurrent flow were measured in a range of system pressures from p/pcrit = 0.16 to p/pcrit = 0.31, and for various total liquid injection rates and locations. The measured flooding curves j1 = f(jg) as well as pressure drop and void fraction during partial liquid delivery (j1 < j1in) were not affected either by the axial liquid feed location or by the excess liquid rate carried upwards by the vapour. Moreover, for given flow conditions during flooding pressure drop and void fraction were essentially the same at different axial positions. Radial void fraction distributions evaluated from optical fibre probe data indicate an annular‐type flow pattern. Based on this experimental evidence, a mechanistic core/film flow model was developed for the calculation of flooding. The analytical results are compared with the present high pressure and with comparable atmospheric pressure experimental data, showing reasonable overall predictions not only of the flooding curves, but also of the pressure drop in countercurrent flow.

On the various forms of the conservation equations in two-phase flow

The conservation equations for one-dimensional two-phase flow are derived from first principles. The effects of the radial distributions of velocities, enthalpies, and void fraction are taken into account through the use of correlation coefficients. Several simplified separated-flow model formulations that have appeared in the literature are derived from these equations by specializing the values of the correlation coefficients. The equivalence of these formulations under certain assumptions is demonstrated. Finally, new Lagrangian forms of the conservation equations, written in terms of the velocities of the center of mass, momentum, and energy are presented.

Two-Phase Flow in Spirally Grooved Tubes

An experimental investigation into vertical upward air-water two-phase flow was performed using a smooth tube and three spirally grooved tubes. All the test tubes are about 13 mm in inner diameter and 8m in length. The experimental data on each test section include pressure drop, radial distribution of void fraction, cross-sectional mean void fraction, and mean film thickness in annular flow regime. The summary of the results is as folows : (1) The transition boundaries of flow patterns are very similar to those of smooth tube. (2) The pressure gradients do not always depend on the magnitude of relative roughness of test tube, but a ribbed tube, having the greatest relative roughness, shows the greatest values in pressure gradient. (3) The variation of grooves produces remarkable difference in radial distribution of void fractions. (4) Mean film thickness for grooved tubes become definitely greater than for smooth tube.

531. Desorption of hydrogen from platinum catalyst: Y Kubokawaet al, J Phys Chem, 68, May 1964, 1244–1245

Flashing of initially sub-cooled water in a vertical circular converging-diverging nozzle is simulated with two-fluid model incorporating drag and non-drag forces. Phase change is assumed to be induced only by interphase heat transfer while pressure jump across the interface is ignored. The reliability of CFD simulations is confirmed by comparison with experimental data and one-dimensional results basing on a drift-flux model. Satisfactory prediction of mass flow rate and cross-section averaged parameters is achieved. The predicted radial distribution of void fraction is however much more flat than the measured one. The discrepancy is found to be caused by the effect of lift force and the reversal of relative velocity. Cases characterised with large pressure-undershoot exhibit significant pressure non-equilibrium effect in form of a pressure recovery process. Sensitivity tests on bubble number density, heat transfer coefficient as well as nucleation are performed.