Measurement of Gas Velocities in the Presence of Solids in the Riser of a Cold Flow Circulating Fluidized Bed

The local gas velocity and the intensity of the gas turbulence in a gas/solid flow are a required measurement in validating the gas and solids flow structure predicted by computational fluid dynamic (CFD) models in fluid bed and transport reactors. The high concentration and velocities of solids, however, make the use of traditional gas velocity measurement devices such as pitot tubes, hot wire anemometers and other such devices difficult. A method of determining these velocities has been devised at the National Energy Technology Laboratory employing tracer gas. The technique developed measures the time average local axial velocity gas component of a gas/solid flow using an injected tracer gas which induces changes in the heat transfer characteristics of the gas mixture. A small amount of helium is injected upstream a known distance from a self-heated thermistor. The thermistor, protected from the solids by means of a filter, is exposed to gases that are continuously extracted from the flow. Changes in the convective heat transfer characteristics of the gas are indicated by voltage variations across a Wheatstone bridge. When pulsed injections of helium are introduced to the riser flow the change in convective heat transfer coefficient of the gas can be rapidly and accurately determined with this instrument. By knowing the separation distance between the helium injection point and the thermistor extraction location as well as the time delay between injection and detection, the gas velocity can easily be calculated. Variations in the measured gas velocities also allow the turbulence intensity of the gas to be estimated

Enhanced heat transfer to a flat plate from an oscillating annular heated jet.

The purpose of this investigation was to examine one possible method of heat transfer enhancement. A free annular jet was oscillated and impinged on a flat plate to determine the effects of oscillation on the subsequent heat transfer. The nozzle through which the jet was emitted was an axisymmetric bistable fluidic oscillator but was oscillated through mechanical and pneumatic means for this investigation. The frequencies investigated ranged from steady flow to 25 Hz. The investigation used two methods, computational and experimental. The computational fluid dynamics package FLUENT version 4.3 was used to determine instantaneous heat fluxes and determine the flow field. Experimental data were used to corroborate the computational results. Experimental data were obtained using a heated jet, 400{dollar}\\sp\\circ{dollar}F and 500{dollar}\\sp\\circ{dollar}F at the nozzle exit. The jet impinged on an aluminum plate where a constant temperature backwall boundary condition was maintained. Thermocouples were mounted at various points on the front surface of the plate in various configurations. These thermocouples on the front surface of the plate yielded instantaneous air temperatures directly below the surface at distinct radial positions and heat fluxes through the plate. The experiments were conducted at flow rates of 7.5, 10 and 13.4 scfm and spacings of 25 to 75 hydraulic nozzle diameters between the nozzle exit and plate. The computational data indicated increased overall heat flux by nozzle oscillation of 11 to 38 percent over that of a non-oscillating jet under identical nozzle to plate spacings and flow rates. Vortical structures formed by the oscillating nozzle were observed and are believed to be the mechanism for the increased heat transfer. The experimental evidence confirms, to an extent, the conclusions drawn from the computational evidence