Hamn-Ching Chen

Near-wall turbulence models for complex flows including separation

Results of a computational experiment designed to investigate the performance of different near-wall treatments in a single turbulence model with a common numerical method are reported. The complete fully elliptic, Reynoldsaveraged Navier-Stokes equations have been solved using a low-Reynolds-number model, a new two-layer model, and a two-point wall-function method, in thek-s turbulence model, for the boundary layer and wake of two axisymmetric bodies. These tests enable the evaluation of the performance of the different approaches in flows involving longitudinal and transverse surface curvatures, streamwise and normal pressure gradients, viscous-inviscid interaction, and separation. The two-layer approach has been found to be quite promising for such flows and can be extended to other complex flows.

Flow and Heat Transfer in a Rotating Square Channel With 45 deg Angled Ribs by Reynolds Stress Turbulence Model

Numerical predictions of three-dimensional flow and heat transfer are presented for a rotating square channel with 45 deg angled ribs as tested by Johnson et al. (1994). The rib height-to-hydraulic diameter ratio (e/D h ) is 0.1 and the rib pitch-to-height ratio (P/e) is 10. The cross section of the ribs has rounded edges and corners. The computation results are compared with the experimental data of Johnson et al. (1994) at a Reynolds number (Re) of 25,000, inlet coolant-to-wall density ratio (Δρ/ρ) of 0.13, and three rotation numbers (Ro) of 0.0, 0.12, and 0.24. A multiblock numerical method has been employed with a near-wall second-moment turbulence closure model. In the present method, the convective transport equations for momentum, energy, and turbulence quantities are solved in curvilinear, body-fitted coordinates using the finite-analytic method. Pressure is computed using a hybrid SIMPLER/PISO approach, which satisfies the continuity of mass and momentum simultaneously at every time step. The second-moment solutions show that the secondary flows induced by the angled ribs, rotating buoyancy, and Coriolis forces produced strong nonisotropic turbulent stresses and heat fluxes that significantly affected flow fields and surface heat transfer coefficients. The present near-wall second-moment closure model provided an improved flow and heat transfer prediction.

Prediction of Flow and Heat Transfer in Rotating Two-Pass Rectangular Channels With 45-deg Rib Turbulators

Numerical predictions of three-dimensional flow and heat transfer are presented for a rotating two-pass rectangular channel with 45-deg rib turbulators and channel aspect ratio of 2:1. The rib height-to-hydraulic diameter ratio (e/D h ) is 0.094 and the rib-pitch-to-height ratio (P/e) is 10. Two channel orientations are studied: β=90 deg and 135 deg, corresponding to the mid-portion and the trailing edge regions of a turbine blade, respectively. The focus of this study is twofold; namely, to investigate the effect of the channel aspect ratio and the channel orientation on the nature of the flow and heat transfer enhancement. A multi-block Reynolds-averaged Navier-Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure. In the present method, the convective transport equations for momentum, energy, and turbulence quantities are solved in curvilinear, body-fitted coordinates using the finite-analytic method. The numerical results compare reasonably well with experimental data for both stationary and rotating rectangular channels with rib turbulators at Reynolds number (Re) of 10,000, rotation number (Ro) of 0.11 and inlet coolant-to-wall density ratio (Δρ/ρ) of 0.115.

Computation of heat transfer in rotating two-pass square channels by a second-moment closure model

Abstract A multiblock numerical method has been employed for the calculation of three-dimensional flow and heat transfer in rotating two-pass square channels with smooth walls. The finite-analytic method solves Reynolds-averaged Navier–Stokes equations in conjunction with a near-wall second-order Reynolds stress (second-moment) closure model and a two-layer k–e isotropic eddy viscosity model. Comparison of second-moment and two-layer calculations with experimental data clearly demonstrate that the secondary flows in rotating two-pass channels have been strongly influenced by the Reynolds stress anisotropy resulting from the Coriolis and centrifugal buoyancy forces as well as the 180° wall curvatures. The near-wall second-moment closure model provides accurate heat transfer predictions which agree well with measured data.

Computation of Flow and Heat Transfer in Two-Pass Channels With 60 deg Ribs

Numerical predictions of three-dimensional flow and heat transfer are presented for a two-pass square channel with and without 60 deg angled parallel ribs. Square sectioned ribs were employed along one side surface. The rib height-to-hydraulic diameter ratio (e/D h ) is 0.125 and the rib pitch-to-height ratio (P/e) is 10. The computation results were compared with the experimental data of Ekkad and Han at a Reynolds number (Re) of 30,000. A multi-block numerical method was used with a chimera domain decomposition technique. The finite analytic method solved the Reynolds-Averaged Navier Stokes equation in conjunction with a near-wall second-order Reynolds stress (second-moment) closure model, and a two-layer κ-e isotropic eddy viscosity model

Flow and heat transfer in rotating two-pass rectangular channels (AR=2) by Reynolds stress turbulence model

Abstract Numerical predictions of three-dimensional turbulent flow and heat transfer are presented for a rotating two-pass smooth rectangular channel with channel aspect ratio of 2:1. The focus of this study is to investigate the effect of rotation, channel orientation and the sharp 180° turn on the flow and heat transfer distributions. Two channel orientations are studied: β=90° and β=135°. A multi-block Reynolds-averaged Navier–Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure. The Reynolds number (Re) is fixed at 10,000 while the rotation number (Ro) is varied from 0 to 0.22. Two inlet coolant-to-wall density ratios Δρ/ρ are studied (0.115 and 0.22). The numerical results are compared with the experimental data for both stationary and rotating two-pass rectangular channels.

A Numerical Study of Flow and Heat Transfer in Rotating Rectangular Channels (AR=4) With 45 deg Rib Turbulators by Reynolds Stress Turbulence Model

Computations were performed to study three-dimensional turbulent flow and heat transfer in stationary and rotating 45 deg ribbed rectangular channels for which experimental heat transfer data were available. The channel aspect ratio (AR) is 4:1, the rib height-to-hydraulic diameter ratio (e/D h ) is 0.078 and the rib-pitch-to-height ratio (P/e) is 10. The rotation number and inlet coolant-to-wall density ratios, Δρ/ρ, were varied from 0.0 to 028 and from 0.122 to 0.40, respectively, while the Reynolds number was fixed at 10,000. Also, two channel orientations (β=90 deg and 135 deg from the rotation direction) were investigated with focus on the high rotation and high density ratios effects on the heat transfer characteristics of the 135 deg orientation

Assessment of a Reynolds Stress Closure Model for Appendage-Hull Junction Flows

A multiblock numerical method, for the solution of the Reynolds-Averaged Navier-Stokes equations, has been used in conjunction with a near-wall Reynolds stress closure and a two-layer isotropic eddy viscosity model for the study of turbulent flow around a simple appendage-hull junction. Comparisons of calculations with experimental data clearly demonstrate the superior performance of the present second-order Reynolds stress (second-moment) closure over simpler isotropic eddy viscosity models. The second-moment solutions are shown to capture the most important features of appendage-hull juncture flows, including the formation and evolution of the primary and secondary horseshoe vortices, the complex three-dimensional separations, and interaction among the hull boundary layer, the appendage wake and the root vortex system.

Computation of Flow and Heat Transfer in Rotating Rectangular Channels (AR=4:1) With Pin-Fins by a Reynolds Stress Turbulence Model

Computations with multi-block chimera grids were performed to study the three-dimensional turbulent flow and heat transfer in a rotating rectangular channel with staggered arrays of pin-fins. The channel aspect ratio (AR) is 4:1, the pin length to diameter ratio (HID) is 2.0, and the pin spacing to diameter ratio is 2.0 in both the stream-wise (S 1 /D) and span-wise (S 2 /D) directions. A total of six calculations have been performed with various combinations of rotation number, Reynolds number, and coolant-to-wall density ratio. The rotation number and inlet coolant-to-wall density ratio varied from 0.0 to 0.28 and from 0.122 to 0.20, respectively, while the Reynolds number varied from 10,000 to 100,000. For the rotating cases, the rectangular channel was oriented at 150 deg with respect to the plane of rotation to be consistent with the configuration of the gas turbine blade. A Reynolds-averaged Navier-Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure for detailed predictions of mean velocity, mean temperature, and heat transfer coefficient distributions.

Near-Wall Second-Moment Closure for Rotating Multiple-Pass Cooling Channels

A multiblock numerical method has been employed together with chimera domain decomposition technique to calculate three-dimensional flow and heat transfer in rotating two-pass square channels with smooth walls. The method solves Reynolds-averaged Navier-Stokes equations in conjunction with a near-wall second-order Reynolds stress (second-moment) closure model and a two-layer k-e isotropic eddy viscosity model. The second-moment solutions show that the Coriolis and centrifugal buoyancy forces produced strong nonisotropic turbulent stresses and heat fluxes that significantly affected the friction factors and heat-transfer coefficients in the rotating two-pass square channel, particularly in the 180-deg bend region. The near-wall second-moment closure model provides an improved heat-transfer prediction in comparison with the k-e model