Liquid Flux Distribution Research Articles

Droplet characteristics in the multi-staged high speed disperser with single inlet

Abstract Droplet characteristics in the cavity zone of a multi-staged high speed disperser with single inlet were studied in this paper. The influences of both the operating and structural parameters on the mean droplet diameter, size distribution and liquid flux distribution were quantitatively analyzed. The result showed that the mean droplet diameter decreased with the increase of rotational speed and the number of rotors; whilst there is little influence on the inlet flow rate. In the experimental range, the minimum value of mean droplet diameter is 0.57 mm, 0.48 mm, 0.41 mm in the two-staged, three-staged and four-staged rotors, respectively. The Rosin–Rammler (R–R) distribution could describe the droplet size distribution appropriately, and it became uniform with the increase of rotational speed and the number of rotor, while the inlet flow rate had little effect on the droplet size distribution. The liquid flux distribution curves were always unimodal. With the increase of rotational speed, the location of maximum liquid flux ratio moved from zone 3 to zone 4 and this value decreased from 22.1% to 18.1%. Using Coefficient of Variation (CV) to indicate the uniformity of liquid flux distribution, it was found that the CV decreases from 47.5% to 22.7% when the number of rotor increased from 2 to 4.

Effects of Liquid-Oxygen Post Biasing on SSME Injector Wall Compatibility

An experimental investigation has been carried out to examine the effects of liquid-oxygen (LOX) post biasing of a shear coaxial injector on the behavior of the spray near a chamber wall. The experimental work was performed with inert propellant simulants in a high-pressure chamber. Injector flow rates and chamber pressure were designed to match the space-shuttle-main-engine (SSME) injector gas-to-liquid density and velocity ratio at the point of propellant injection. Measurements of liquid mass-flux, gas-phase velocity, and droplet size were made using mechanical patternation and phase Doppler interferometry techniques. The measurements revealed that the liquid mass-flux distribution shifts away from the wall with increasing LOX postbias away from the wall. The shift in the liquid flux distribution was much greater than that caused by the angling of the LOX post alone. Gas velocity near the wall simultaneously increased with increasing LOX post bias away from the wall. The increase in wall-side gas velocity was caused by the higher fraction of gas injected on the wall side of the injector as a result of the eccentricity at the injector exit. The net result was a decrease in mixture ratio near the wall. Estimates of heat transfer and engine performance relative to the unbiased case are presented.

Effect of Polydispersity of Droplets in the Prediction of Flux Distribution in a Venturi Scrubber

The effect of polydispersity of droplets on the liquid flux distribution in a Venturi scrubber is investigated using a 2-D model along the length of a Venturi scrubber. Two average drop-sizes (the Sauter mean diameter and the mass median diameter) and two drop-size distribution functions (the upper-limit distribution function and the universal root-normal distribution function) are used in the model to predict liquid flux distribution. It is found that the flux distribution is a strong function of the polydispersity of droplets. It is also found that the throat gas velocity has a greater effect on the polydispersity of droplets than L/G ratio. At low throat gas velocity, flux distributions, predicted by the model using drop-size distributions, match with experimental data for a wide range of L/G ratios. At high throat gas velocities, flux distributions, predicted using average drop-sizes, follow experimental results closely. An analysis of overall collection efficiency reveals that it is nearly independent of polydispersity of droplets at low liquid-to-gas ratios and high gas velocities.

Effect of Nozzle Arrangement on Venturi Scrubber Performance

The effect of nozzle arrangement on flux distribution is studied in a rectangular, pilot-scale, Pease−Anthony-type Venturi scrubber. The annular, two-phase, heterogeneous, three-dimensional gas−liquid flow inside the scrubber is modeled using a commercial computational fluid dynamic (CFD) package, FLUENT. The comparison of predicted liquid drop concentration shows good agreement with experimental data. The model predicts the fraction of liquid flowing as film on the walls reasonably well. Visualization of flux patterns studied using four typical nozzle configurations indicate that the nonuniformity in flux distribution increases when the nozzle-to-nozzle distance is greater than 10% of the width of the side on which the nozzles are placed. An analysis of the effect of multiple jet penetration lengths on liquid flux distribution yielded a comparable distribution at 10−45% less liquid than uniform penetration for a particular nozzle configuration. This would lead to significant improvements in scrubber perf...

Heat and mass transfer during low intensity convection drying

A diffusion model of heat and mass transfer in moist porous media is obtained for low intensity convection drying found typically in food products and vegetable dehydration. The effective diffusivity was analyzed based on the coupled effects of moisture content and temperature on mass transfer in capillary flow, evaporation–condensation, and transition regions. The temperature effect was found to be significant in the transition and evaporation–condensation regions. The model was solved numerically using the finite difference method. Physical properties of banana were determined. The results show that the most intensive heat and mass transfer occur in the transition region where both capillary flow and vapor diffusion play a significant role. The length of the transition region is determined clearly by analyzing the liquid flux distribution in the material. Its value depends very much on the permeability of the material. The model proves that the assumption of evaporation front is valid for drying of very permeable materials. For the case of food and vegetable dehydration with the permeability below 10 -19 m 2, more comprehensive analysis is necessary.

Predicting the Liquid Flux Distribution and Collection Efficiency in Cylindrical Venturi Scrubbers

A simplified two-dimensional model to predict liquid flux distribution and collection efficiency in cylindrical Venturi scrubbers with Pease−Anthony mode operation is evaluated with experimental data from a pilot-scale unit. Prediction of the liquid flux distribution near the point of injection appears to be far from agreement with experimental values while the accuracy of prediction has been found to improve with distance from the injection point. The initial location of the liquid source immediately after atomization has been found to affect the liquid flux distribution significantly. Although the overall liquid distribution pattern is found to be in good agreement with the experimental data, the prediction of liquid distribution appears to be a strong function of jet penetration length and turbulence caused by operating conditions. Concentric injection at high liquid rates could result in collision between jets from nozzles as they converge at the center of the scrubber. Turbulence caused by interaction between the jets is accounted for by using varying Peclet numbers. A dimensionless group, Venturi number, developed from jet penetration correlation has been found to predict conditions that give rise to uniform flux distribution and maximum collection efficiency. Venturi numbers between 1 × 10-3 and 1.5 × 10-3 appear to predict conditions for cylindrical Venturi scrubbers (with radial injection into the throat using nozzles) for optimal liquid utilization and maximum collection efficiency.

Estimating maximum removal efficiency in Venturi scrubbers

AbstractA simplified 2‐D model to predict liquid flux distribution and collection efficiency in a Venturi scrubber was tested successfully with experimental data from pilot‐ and industrial‐scale units. Establishing nonuniformity of flux distribution is the key to estimating collection efficiencies accurately. Liquid jet penetration and liquid injection velocity are critical in defining flux distribution for the entire range of conditions simulated. Increase in gas velocity makes flux distribution more uniform and enhances collection efficiencies. This effect becomes insignificant beyond a gas velocity as finer droplets get accelerated quickly to the throat gas velocity giving the collector very little interaction time. Increase or decrease in the liquid to gas (L/G) ratio beyond an optimum value made flux distribution more nonuniform and decreases collection efficiencies. Design parameters of the Venturi scrubber (aspect ratio and nozzle diameter) have an optimum that offers the most uniform flux distribution and the maximum collection efficiency. A new dimensionless group, the Venturi number, derived from the jet penetration correlation, could define both nonuniformity influx distribution and collection efficiency for changes in the L/G ratio, aspect ratio, nozzle diameter, and number of nozzles. Simulation results indicate that a maximum efficiency exists for the Venturi number between 1.0 and 1.5 × 10−3. The useful length of the scrubber throat could be determined from the L/G ratio defined by the Venturi number concept and appropriate throat gas velocity.

Structure of nonevaporating sprays. I - Initial conditions and mean properties

Structure measurements were completed within the dilute portion of axisymmetric nonevaporating sprays. Measurements included: mean velocities, velocity fluctuations, and Reynolds stress of the gas phase; and mean velocities, fluctuating velocities, mass flux, and diameter distributions of the drop phase. The measurements were used to evaluate three typical methods of analyzing sprays: (1) a locally homogeneous flow (LHF) analysis, where slip between the phases is neglected; (2) a deterministic separated flow (DSF) analysis, where slip is considered but effects of drop interactions with turbulence are ignored; and (3) a stochastic separated flow (SSF) analysis, where effects of both slip and turbulence are considered using random-walk computations for drop motion. Measurements of initial conditions of both phases near the injector, mean gas-phase properties, and liquid flux distributions are described herein. Best agreement between predictions and measurements was obtained with the SSF model, which provided a reasonable representation of turbulent dispersion of drops. A companion paper presents additional measurements of drop continuous phase properties.

Measurements and Predictions of the Structure of Evaporating Sprays

An experimental and theoretical study of turbulent evaporating sprays is described. Experiments considered round, Freon-11 sprays (Sauter mean diameters of 31 and 58 μm) produced by an air-atomizing injector directed vertically downward in still air. The following structure measurements were made: mean and fluctuating gas velocities, total concentration of Freon-11, drop size and velocity distributions, mean gas temperature, and liquid flux distributions. Three spray models were evaluated using the new measurements: (a) a locally homogeneous flow (LHF) model where interphase transport rates are assumed to be infinitely fast; (b) a deterministic separated flow (DSF) model where finite interphase transport rates are considered but drop-turbulence interactions are ignored; and (c) a stochastic separated flow (SSF) model where effects of finite interphase transport rates and drop-turbulence interactions are considered using random-walk computations for drop motion and transport. The LHF and DSF models performed poorly, since both finite interphase transport rates and drop-turbulence interactions were important for present test conditions. The SSF model gave best agreement between predictions and measurements and appears to be an encouraging approach for treating practical sprays.