Predictions of breakup length of a liquid sheet emanating from a pressure-swirl (simplex) fuel atomizer have been carried out by computationally modeling the two-phase flow in the atomizer coupled with a nonlinear analysis of instability of the liquid sheet. The volume-of-fluid (VOF) method has been employed to study the flow field inside the pressure-swirl atomizer. A nonlinear instability model has been developed using a perturbation expansion technique with the initial amplitude of the disturbance as the perturbation parameter to determine the sheet instability and breakup. The results for sheet thickness and velocities from the internal flow solutions are used as input in the nonlinear instability model. Computational results for internal flow are validated by comparing film thickness at exit, spray angle, and discharge coefficient with available experimental data. The predictions of breakup length show a good agreement with semiempirical correlations and available experimental measurements. The effect of elevated ambient pressure on the atomizer internal flow field and sheet breakup is investigated. A decrease in air core diameter is obtained at higher ambient pressure due to increased liquid-air momentum transport. Shorter breakup lengths are obtained at elevated air pressure. The coupled internal flow simulation and sheet instability analysis provides a comprehensive approach to modeling sheet breakup from a pressure-swirl atomizer.

The pressure swirl atomizer, or simplex atomizer, is widely used in liquid fuel combustion devices in the aerospace and power generation industries. A computational, experimental, and theoretical study was conducted to predict its performance. The Arbitrary-Lagrangian-Eulerian method with a finite-volume scheme is employed in the CFD model. Internal flow characteristics of the simplex atomizer, as well as its performance parameters such as discharge coefficient, spray angle and film thickness, are predicted. A temporal linear stability analysis is performed for cylindrical liquid sheets under three-dimensional disturbances. The model incorporates the swirling velocity component, finite film thickness and radius that are essential features of conical liquid sheets emanating from simplex atomizers. It is observed that the relative velocity between the liquid and gas phases, density ratio and surface curvature enhance the interfacial aerodynamic instability. The combination of axial and swirling velocity components is more effective than only the axial component for disintegration of liquid sheet. For both large and small-scale fuel nozzles, mean droplet sizes are predicted based on the linear stability analysis and the proposed breakup model. The predictions agree well with experimental data at both large and small scale.

Both experimental and three-dimensional CFD investigations are carried out in a scale model of an industrial gas turbine exhaust system to better understand its complex flow field and to validate CFD prediction capabilities for improved design applications. The model consists of an annular diffuser passage with struts, followed by turning vanes and a rectangular plenum with side exhaust. Precise measurements of total/static pressure and flow velocity distributions at the model inlet, strut outlet and model outlet are made using aerodynamic probes and locally a Laser Doppler Velocimeter (LDV). Numerical analyses of the model internal flow field are performed utilizing a three-dimensional Navier-Stokes (N-S) calculation method with the industry standard k-ε turbulence model. Both the experiments and computations are carried out for three load conditions: full speed no load (FSNL), full speed mid load (FSML, 57 percent load), and full speed full load (FSFL). Based on the overall comparison between the measurements and CFD predictions, this study concludes that the applied N-S method is capable of predicting complicated gas turbine exhaust system flows for design applications.

This work addresses primary atomization modeling, multidimensional spray prediction, and flow characteristics of compound nozzle gasoline injectors. Compound nozzles are designed to improve the gasoline spray quality by increasing turbulence at the injector exit. Under the typical operating conditions of 270-1015 kPa, spray atomization in the compound nozzle gasoline injectors is mainly due to primary atomization where the flow turbulence and the surface tension are the dominant factors. A primary atomization model has been developed to predict the mean droplet size far downstream by taking into account the effect of turbulent intensity at the injector exit. Two multidimensional spray codes, KIVA-2 and STAR-CD, originally developed for high-pressure diesel injection, are employed for the lower-pressure gasoline injection. A separate CFD analysis was performed on the complex internal flows of the compound nozzles to obtain the initial and boundary conditions for the spray codes. The TAB breakup model used in KIVA-2 adequately facilitates the atomization process in the gasoline injection.

Fuel atomization with prefilming airblast nozzles has been investigated. The present analysis is directed toward a detailed investigation of the atomization processes and the clarification of the fundamental phenomena. Two-dimensional models were utilized. High-speed films, showing the deterioration of the liquid film close to the atomizing edge, reveal the dynamics of the liquid’s deterioration and show the motion of the film during the drop formation. The liquid separation is shown to be a periodic process with the drop formation caused by momentum transfer. The frequency spectrum of the liquid separation is determined by means of an optical technique. It is seen that the main frequencies depend only on the air velocity. They are always lower than the corresponding wave frequencies. The droplet size measurements obtained by a light scattering technique emphasize the dominant role of the air velocity at the atomizing edge. A decrease in the surface tension provides an improvement in atomization quality. Other parameters such as liquid flow rate, liquid viscosity, gap height, and length of the prefilming surface within the nozzle were found not to affect directly the droplet size distribution produced, if the air velocity in each of the two ducts of the nozzle is kept constant. The pressure drop of the air, however, rises. It is shown that the droplet size distribution can be easily determined, if the arithmetic mean value of the air velocity in both ducts is known, e.g., from a calculation of the internal flow. Due to the high liquid mass flow rates of airblast nozzles, the wavy film is partly atomized within the nozzle before the liquid separates at the atomizing edge. The measurements show that the portion of the liquid mass flow atomized remains relatively small and that the droplet sizes are equivalent to those produced at the atomizing edge.

Predicted and measured surface velocity and pressure distributions in the internal flow channels of a centrifugal fan impeller are presented for volume flow rates between 80 and 125 percent of design flow rate. Predictions are based on a fully three-dimensional, finite element analysis of the inviscid, incompressible blade channel flow. Additional predictions using a conventional quasi-three-dimensional analysis are presented for comparison. Experimental results were developed using extensive blade and sidewall surface pressure taps installed in a scale model of an airfoil-bladed centrifugal fan impeller designed for heavy industrial and power generation applications. The results illustrate the ability of both flow analyses to predict the dominant features of the impeller flow field, including peak blade surface velocities and adverse gradients at flows far from the design point. In addition, the experimental results provide valuable insight into the limiting channel diffusion values for typical centrifugal cascade performance, and the influence of viscous effects as seen in deviations from the ideal flow predictions.

A new computational geometry for the blades and flow passages of centrifugal compressors is described and examples of its use in the design of industrial compressors are given. The method makes use of Bernstein-Bezier polynomial patches to define the geometrical shape of the flow channels. This has the following main advantages: the surfaces are defined by analytic functions which allow systematic and controlled variation of the shape and give continuous derivatives up to any required order: and the parametric form of the equations allows the blade and channel coordinates to be very simply obtained at any number of points and in any suitable distribution for use in subsequent aerodynamic and stress calculations and for manufacture. The method is particularly suitable for incorporation into a computer-aided design procedure.

The flow field development within a centrifugal compressor stage was analyzed using an advanced laser velocimetry [4]. A splitter blade impeller coupled with a vaned and vaneless diffuser has been found to have similar internal flow patterns for both the vaneless and vaned diffuser design. Different velocity profiles have been analyzed for adjacent channels behind the splitter blade leading edge. A considerable wake flow was observed near the impeller exit. Detailed optical measurements within the vaned diffuser entrance region gave evidence of a periodically fluctuating, highly distorted diffuser inlet flow. Unsteady flow angle deviations of 13 degrees have been discovered within the diffuser throat. Maximum flow angle differences up to 27 degrees occurred from hub to shroud.

Recent developments in inviscid prediction methods of internal flow fields of centrifugal impellers and related flows are critically reviewed. The overall picture which emerges provides the reader with a state-of-the-art perspective on the subject. Restricting simplifying assumptions of the various methods are identified to stimulate future research. Topics included in this review are: two-dimensional subsonic and transonic inviscid flows as well as three-dimensional inviscid flows.

An analytical technique is presented which permits calculating the off-design performance of radial, or mixed, flow centrifugal compressers and pumps. The method is based on the variation of blade loading diagrams as they are affected by the operating conditions of the machine. The influence of the changes of blade loading is expressed in terms of changes in the boundary layer momentum thickness in the internal flow passages. In turn the boundary layer thicknesses are expressed as a loss and as a flow acceleration in the critical passages such as the throat and the exit. The method makes use of no external arbitrary inputs except one relating to the development of the boundary layer thickness with the amount of fluid deceleration. The method was computerized and checked against five impellers and three diffusers. Acceptable agreement is found in the calculation of the shape of the characteristics and the location of the surge and choke points.