A Study of the Relationship Between Free-Stream Turbulence and Stagnation Region Heat Transfer

Abstract

A study has been conducted at the NASA Lewis Research Center to investigate the mechanism that causes free-stream turbulence to increase heat transfer in the stagnation region of turbine vanes and blades. The work was conducted in a wind tunnel at atmospheric conditions to facilitate measurements of turbulence and heat transfer. The model size was scaled up to simulate Reynolds numbers (based on leading edge diameter) that are to be expected on a turbine blade leading edge. Reynolds numbers from 13,000 to 177,000 were run in the present tests. Spanwise averaged heat transfer measurements with high and low turbulence have been made with “rough” and smooth surface stagnation regions. Results of these measurements show that, at the Reynolds numbers tested, the boundary layer remained laminar in character even in the presence of free-stream turbulence. If roughness was added the boundary layer became transitional as evidenced by the heat transfer increase with increasing distance from the stagnation line. Hot-wire measurements near the stagnation region downstream of an array of parallel wires has shown that vorticity in the form of mean velocity gradients is amplified as flow approaches the stagnation region. Finally smoke wire flow visualization and liquid crystal surface heat transfer visualization were combined to show that, in the wake of an array of parallel wires, heat transfer was a minimum in the wire wakes where the fluctuating component of velocity (local turbulence) was the highest. Heat transfer was found to be the highest between pairs of vortices where the induced velocity was toward the cylinder surface.

Oscillatory thermal structures induced by unconfined slot jet impingement

“…Also, at a fixed time, hot and cold spots alternate in the streaks. Such temperature variations are consistent with near-wall flow and surface visualizations reported in the literature [3][4][5]7] and have been attributed to variations in near-wall fluid temperature that is influenced by the presence of near-wall counter-rotating streamwise vortices.…”

“…Other studies have, in addition, spatially locked the near-wall vortical structures by the use of flutes at the exit of the slot nozzle Table 1 Distribution of the temperature fluctuations among the dominant modes during slot jet impingement, scaled with the total fluctuations in the filtered dataset [19]. In a closely related study of cross-flow over a cylinder [3], an array of equally spaced wires upstream of the flow were used to lock the structures spatially. In the present study, the jet is not perturbed either temporally or spatially; hence, slight imperfections in the nozzle geometry or nozzle-to-surface spacing could cause the temporally oscillating thermal structures to be displaced/skewed spatially.…”

“…VanFossen and Simoneau [3] determined the presence of pairs of counter-rotating streamwise vortices at the forward stagnation point of a cylinder when an array of parallel wires was placed perpendicular to the stagnation line upstream of the cylinder. They found that regions of high heat transfer corresponded to locations in between the vortices, where the flow directly impinged on the surface.…”

The role of large-scale vortical structures in transient convective heat transfer augmentation

“…Vanfossen & Simoneau (1987) used wires to create vortices perpendicular to a downstream cylinder and studied the resulting fluid flow and heat transfer near the surface using a combination of smoke visualization and liquid crystals. It was observed that regions where vortices forced fluid towards the surface corresponded to regions of increased heat transfer.…”

Oscillatory thermal structures in a reattaching jet flow

“…The former spacing corresponds to a large gradient in spatial temperature distribution along the x coordinate, while the latter corresponds to an unstable reattachment. Near-wall flow structure and/or the impingement surface temperature have been documented in several studies for jet impingement (VanFossen and Simoneau, 1987;Yokobori et al, 1988;Kataoka et al, 1994). Sakakibara et al (1997) measured, simultaneously, the near-wall vortex structure and fluid temperature distribution in the impingement region.…”