Kameswararao Anupindi

In the present work, the flow and thermal characteristics for flow through a wall-confined array of pin-fins are studied using large-eddy simulation (LES). The geometry considered here resembles the trailing-edge portion of a early-stage gas-turbine blade. The wall-confined array of low-aspect ratio cylindrical fins are arranged in a staggered manner, and the direction of the coolant is perpendicular to the axis of the fins. The analysis is performed for a fixed spacing between the fins and for a Reynolds number of 5900. In view of the actual running conditions, large temperature differences, ranging from 25∘C to 300∘C, are considered between the walls and the coolant. Together with this, the effect of higher pressure, ranging from 5 bar to 10 bar, on the cooling performance is analyzed. The numerical solver used is thoroughly validated with the reference experimental and LES data available in the literature. A detailed investigation of the mechanism for heating of the fluid, transport, and diffusion of the heat flux is presented by analyzing Nusselt number, turbulent heat fluxes and vorticity contours. It is observed that, an increase in Nusselt number is due to encapsulation of small-scale vortices around the pin-fin surfaces and a decrease in Nusselt number downstream is due to the lower frequency of high-energy vortex. The localized Nusselt number is higher over the fin surfaces compared to end-walls, and the base vortex is responsible for higher energy extraction at the junction of the pin-fin and the end-walls and its effect increases downstream. The spanwise component of velocity is responsible for localized mixing than the remaining two components, and the coolant gets heated inside the recirculation zone and diffusion of which occurs mostly at the free shear layer. The analysis of higher pressure and temperature difference shows higher Nusselt number at high pressure, whereas higher heat flux at lower pressure.

In the present work, five different subgrid-scale (SGS) models and implicit large-eddy simulation (LES) are evaluated and compared against the DNS for the simulation of the planar turbulent wall-bounded jet with heat transfer. The SGS models tested are the classical constant coefficient Smagorinsky model and its dynamic version, the wall-adaptive local eddy-viscosity (WALE) model, the turbulent kinetic energy one-equation model, and its dynamic version. The effects of using variable turbulent Prandtl number and the near-wall damping function are also studied in these models. The mean, second-order flow and heat-transfer statistics with the evolution of Nusselt number along the jet downstream are used to assess the different SGS models. The quality of resolution of the present LES are evaluated using the activity parameter and the index of resolution quality. Among the models tested, the constant coefficient Smagorinsky model together with Van-Driest damping predicts the solution accurately in the near-wall region as well as in estimating the thermal parameters. However, the dynamic models performed better in evaluating the Reynolds stress profiles away from the wall in the outer region. Capabilities of the models to predict the turbulent kinetic energy budgets, pressure-velocity gradient correlations and triple velocity correlations are also studied. The implemented variable Prandtl number algorithm is noted to have minimal influence on the evolution of the solution.

In the present work, flow around a heated circular cylinder, at a Reynolds number of Re=3900, is investigated using large-eddy simulation (LES). Large differences in temperature, 25 ∘C, 100 ∘C, 200 ∘C, and 300 ∘C between the cylinder and the oncoming flow are considered, and its effect on the flow and thermal characteristics in the near wake region are studied. The numerical methodology employed is validated for both, the mean and second-order statistics, with the direct numerical simulation (DNS) data available in the literature. The results are analyzed using the mean temperature, velocity, Reynolds stresses, temperature variances, turbulent heat fluxes and energy spectra. The flow and thermal characteristics are studied along the center line in the wake, and in the transverse direction at two locations. The non-isothermal flow characteristics are compared with isothermal flow, to study the effect of temperature on the flow dynamics. Phase-averaging is performed to analyze the regions of turbulence production and convection of heat. It is observed that, the flow characteristics vary non-linearly with the temperature, and the effect is insignificant till a temperature difference of 100 ∘C, however, beyond this significant effect could be noticed. The effect of temperature difference is prominent in the thermal characteristics for all temperature differences, 25 ∘C to 300 ∘C, considered. The transverse component of shear stress fluctuations are observed to be dominant over the stream-wise components at both the locations downstream, thereby enhancing the local mixing of the fluid and hence, the heat transfer.

The trailing edge of a gas turbine blade contains rows of short circular cylinders which extract the heat from blade, with the help of the coolant. Large temperature differences exist between the incoming coolant and the blade, and its effect on the flow and thermal characteristics need to be understood. To this end, in the present work, non-isothermal flow over three side-by-side circular cylinders is investigated for a Reynolds number of 3900 using large-eddy simulation (LES). The numerical solver used is validated for isothermal flow using reference experimental data. Large temperature differences, ranging from 25∘C to 300∘C, between the incoming flow and the surface of the cylinders are considered and their effect on the flow and thermal characteristics in the near wake region of cylinders are analyzed. The cylinder-surface and wake characteristics are analyzed using instantaneous and mean quantities. The surface characteristics of the cylinders are studied using Nusselt number, wall shear stress, and lift and drag coefficients. The wake characteristics are studied using Reynolds stresses, turbulent heat flux, and turbulence anisotropy invariant maps. Phase-averaging is used to analyze the unsteady flow and topology of the wake. It is observed that, compared to the middle cylinder the production of turbulence and diffusion of heat is higher behind the two outer cylinders, and their location remains stationed independent of the time. Convection and diffusion of heat takes place along the free shear layer of the wake of the middle cylinder. The vortex shedding behind the cylinders are noted to have multiple frequencies. Further, the vortices shed by the outer cylinders contain more energy and at higher-frequencies when compared to those shed by the middle cylinder.