Gocha Chochua

A COMPUTATIONAL AND EXPERIMENTAL INVESTIGATION OF TURBULENT JET AND CROSSFLOW INTERACTION

The flowfield induced by a single circular jet exhausting perpendicularly from a flat plate into a crossflow has been investigated numerically. The flow regime investigated corresponds to that encountered in a modern gas-turbine combustor. Reynolds-averaged solutions were obtained using a pressure-based Navier-Stokes solver. The standard k -epsilon turbulence model with and without nonequilibrium modification was employed. Two different momentum flux ratios, J, between the jet and the free stream are investigated, namely, J = 34.2 and J = 42.2. To aid the evaluation of the computational capability, experimental information also has been obtained, including mean and root-mean-square (RMS) velocity distributions downstream of the jet, and the detailed velocity profile at the jet exit. An evaluation of the different convection schemes reveals that the second-order upwind scheme does a noticeably better job than the first-order scheme to predict the velocity profile at the jet exit while predicting less mixin...The flowfield induced by a single circular jet exhausting perpendicularly from a flat plate into a crossflow has been investigated numerically. The flow regime investigated corresponds to that encountered in a modern gas-turbine combustor. Reynolds-averaged solutions were obtained using a pressure-based Navier-Stokes solver. The standard k -epsilon turbulence model with and without nonequilibrium modification was employed. Two different momentum flux ratios, J, between the jet and the free stream are investigated, namely, J = 34.2 and J = 42.2. To aid the evaluation of the computational capability, experimental information also has been obtained, including mean and root-mean-square (RMS) velocity distributions downstream of the jet, and the detailed velocity profile at the jet exit. An evaluation of the different convection schemes reveals that the second-order upwind scheme does a noticeably better job than the first-order scheme to predict the velocity profile at the jet exit while predicting less mixing than the experimental measurement during the jet and free stream interaction. It appears that turbulence modeling primarily is responsible for the deficiency the accounting for the physics of the jet and free stream interaction.

Numerical Modeling of Rotordynamic Coefficients for Deliberately Roughened Stator Gas Annular Seals

A method is proposed for computations of rotordynamic coefficients of deliberately roughened stator gas annular seals using computational fluid dynamics. The method is based on a transient analysis with deforming mesh. Frequency-dependent direct and cross-coupled rotordynamic coefficients are determined as a response to an assigned rotor surface periodic motion. The obtained numerical results are found to be in good agreement with the available test data and one-dimensional tool predictions. The method can be used as a research tool or as a virtual annular seal test rig for seal design and optimization.

Computational modeling for honeycomb-stator gas annular seal

Abstract Gas annular seals are commonly adopted for leakage control in turbomachinery applications. Honeycomb seals are attractive from the viewpoints of leakage control as well as rotordynamic stability. To improve our understanding of thermo-fluid-physics in such seals, a computational capability is developed for low Mach number, compressible, turbulent flows. The emphases of the present study include (i) development of an original numerical scheme with periodic boundary conditions for flows around repeated geometries, (ii) evaluation of a low Reynolds number version of the k – e turbulence model suitable for the operating conditions of honeycomb seals, and (iii) 3-D computations to assess the implications of the numerical predictions for practical configurations including velocity, pressure, temperature characteristics and loss mechanisms.

Computational Modeling of Turbulent Flows Over Rough Surfaces

Turbulent flows over rough surfaces are often encountered in nature and engineering practices and are often difficult to analyze. In this study, combined modeling and computational techniques is involved to investigate such flows over a surface covered with a large-scale roughness pattern. A simplified empirical engineering model is validated by taking area average of the flow field data over the surface. The approach can interpret fluid physics based on the empirical correlation. The area-averaged mean momentum transport resulting from the wall-normal time-averaged velocity component is found to be a significant contributing term into the near-boundary shear stress balance. This makes its behavior different from the flow over a smooth surface. Comparing alternative approaches for estimating the roughness coefficients, it is found that the mass-flow-rate-deficit approach produces superior results. Flow in a channel with one wall covered with an array of cylindrical cavities and the other smooth is used as an example. The extended wall functions, based on the k-e closure and the simplified engineering model, can be applied for a large-scale roughness pattern. The approach can significantly reduce required computational cost. On the other hand, the small domain periodic computations are needed to produce roughness lengths for a particular surface geometry. This model can develop a general correlation relating the roughness lengths to a surface geometry to aid engineering design.Copyright

Modelling of compressible periodic flows with application to turbomachinery seals

Gas annular seals are commonly adopted for leakage control in turbomachinery applications. Due to repeated geometric patterns of the seals, it is desirable to develop suitable strategies to treat a small portion of the flow domain and extrapolate the information to the entire seal. This paper presents such a technique. The following issues are addressed: validation of results obtained using periodic boundary conditions accounting for turbulent and compressible flows; effect of the rotor rotation and inlet swirl ratio on flow development in the seal; extrapolation of the full-length results from limited periodic simulations; and distribution of the friction factor on contributing pressure and shear stress components. The computational techniques presented can help address the design-related issues more economically, and with better insight into the fluid physics.

Computational and Analytical Study of Multiphase Rotary Separator Turbine Liquid Outlet

Gas-liquid separation is typically performed in settling tanks using gravitational force, or in cyclones generating higher centrifugal forces by swirl generators. However, in some applications such as offshore platforms or subsea compression stations, further compactness of the separating units is important. The Rotary Separator Turbine, a special type of cyclonic separator, generates even higher centrifugal forces, which allows the design of very compact separators. Due to the complexity of the flow fields in this type of machine, Computational Fluid Dynamic (CFD) analyses were used in the recent design of such a rotary separator. Reynolds Averaged Navier-Stokes equations are solved for multiphase turbulent flows in multiple frames of reference. This paper describes basic functionality of the RST and concentrates on one particular analysis of the lip discharge geometry. In addition to the numerical study, an analytical model is presented for this particular flow.Copyright

Analytical and Computational Study of Radial Loads in Volutes and Collectors

The physical mechanism of the radial load generation in discharge volutes and collectors of a centrifugal compressor is analytically studied. A new equation is presented enabling prediction of radial load magnitude and direction based on compressor geometry and operational conditions. The analytical approach is compared with computational fluid dynamics (CFD) results utilizing different rotor-stator interface models and test data measurements. The effect of a low solidity vaned diffuser on radial load attenuation is presented.Copyright