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, 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.

This work evaluates the flow and thermal characteristics of a gas-turbine blade trailing-edge. The study is carried out using large-eddy simulation (LES) for Reynolds number, Re=5900. In view of the realistic conditions, a converging channel with a staggered arrangement of fins with varying aspect ratio is considered for the higher pressure of the coolant and the larger temperature difference between the blade and the coolant. The present results are compared with the parallel channel case to identify the effects of converging flow passage of the channel on the flow and thermal characteristics. The computational setup and methodology are thoroughly validated using reference data from experiments as well as LES available in the literature. The results are analyzed in terms of Q-criterion, streamlines, vorticity and turbulent heat flux contours, and the Nusselt number profiles at several locations in the flow domain. The present results shows, unlike in the parallel channel case, a dominant spanwise component of velocity that helps in the formation of a strong vortex behind the first row itself, and the Nusselt number consistently increases from the first to the last row. The maximum gain and diffusion of turbulent heat flux occur within a recirculation zone and along the free-shear layer, respectively. The heat flux from the end-wall is extracted mainly by the spanwise velocity component, which is also responsible for its advection to the central region. Heat transfer is observed to be higher from fins compared to the end-walls, which underscores the importance of the active participation of fins in the analysis of heat transfer at the trailing-edge.

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.

Several industrial and engineering applications employ ventilated offset-jets such as flow separation control devices, upper surface blowing used in short take-off and landing aircraft, drying processes, and fuel injection systems. The design and efficient operation of these devices relies on controlling the turbulence levels, heat transfer rates, and the locations of flow reattachment points. Therefore, an understanding of the flow and thermal characteristics of ventilated offset-jets helps in the design and operation of these devices. The present study aims to address this by performing large-eddy simulations (LES) of a planar turbulent offset-jet for different velocities of the ventilated-jet. To understand and quantify the effects of ventilation on the offset-jet, a range of velocity-ratios of 0, 0.1, 0.14, and 0.18 are considered for a primary offset-jet Reynolds number of 14,000. First, a grid-sensitivity study is performed to establish the convergence of the solver and the LES index of quality of resolution is obtained to ascertain the quality of the mesh. Thereafter, the mean and second-order statistics of the streamwise component of velocity are compared with reference data from the literature to validate the numerical solver. The effects of ventilation on the offset-jet are studied using the decay of streamwise velocity, jet-spread, the evolution of pressure coefficient, friction coefficient, the mean Nusselt number, and the unsteady characteristics. As the velocity-ratio increases, the suction pressure in the initial region of the domain decreases, which causes the jet to spread away from the bottom wall. Further the peak magnitudes of the mean Nusselt number and coefficient of pressure decrease exponentially with an increase in the velocity-ratio. The peak magnitude of the mean Nusselt number for the unventilated-jet case – the offset-jet without the ventilated-jet – is the largest and its value for the ventilated-jet cases decreases as the velocity-ratio is increased. It is concluded that in the initial region, the ventilated-jet has a profound effect on the offset-jet flow and as the flow develops into a wall-jet the effects of ventilation vanish.

In the present work, compact finite-difference lattice Boltzmann method is extended to simulate thermal flows using double distribution approach. Fourth-order accurate spatial and temporal discretization schemes are utilized in order to accurately simulate the flow fields. The developed solver is validated by simulating natural convection in a square cavity at Rayleigh numbers 106 and 108. Thereafter, natural convection in a circular annulus is studied for a Rayleigh number of 108. Streamlines and isotherms are used in order to visualize the results and the variation of Nusselt number and temperature profiles are studied within the cavity.