Effect of Uneven Wall Heating Conditions Under Different Buoyancy Numbers for a One Side Rib-Roughened Rotating Channel

This paper investigates the impacts of uneven wall heating conditions under different buoyancy numbers on flow field and heat transfer performance of a rotating channel with one side smooth and one side roughened by 45 deg inclined ribs. Parametric Reynolds-averaged Navier–Stokes (RANS) simulations were conducted under two different wall heating conditions: only ribbed wall heated, as in experiment setup, and all walls heated, under three different buoyancy numbers. Results are compared, when available, with experimental results. Numerical results show that uneven wall heating has only a minor impact on nonrotating cases and very low buoyancy rotating cases. However, it has a significant influence, on both, the heat transfer behavior and the flow field, when the buoyancy number is large. In the ribbed trailing rotating tests, the all walls heated cases show significantly higher heat transfer rate than only the ribbed wall heated cases. The discrepancy is enlarged as buoyancy number increases. The heat transfer in the all walls heated case increases monotonically with the buoyancy number, whereas in the ribbed wall, heated case is slight reduced. In the ribbed leading rotating tests, the heat transfer sensitivity to the heating conditions is not conspicuous, and for both cases, the heat transfer level slightly reduced as the buoyancy number increased. The flow field investigation shows that there is a significant displacement of main flow in the all walls heated cases than only the ribbed wall heated cases under high buoyancy numbers. This displacement is due to the buoyancy effect and responsible for the heat transfer differences in uneven heating problems. According to the results obtained in the paper, we conclude that when buoyancy effects are relevant, the heating settings can play a significant role in the heat transfer mechanisms and therefore in the experimental and numerical results.

Effect of Uneven Wall Heating Conditions Under Different Buoyancy Numbers for a One Side Rib-Roughened Rotating Channel

This paper investigates the impacts of uneven wall heating conditions under different Buoyancy numbers on flow field and heat transfer performance of a rotating channel with one side smooth and one side roughened by 45 degree inclined ribs. Parametric RANS simulations were conducted under two different wall heating conditions: only ribbed wall heated, as in experiment setup, and all walls heated, under three different Buoyancy numbers. Results are compared, when available, with experimental results. Numerical results show that uneven wall heating has only a minor impact on non-rotating cases and very low buoyancy rotating cases. However, it has a significant influence, on both, the heat transfer behaviour and the flow field, when the Buoyancy number is large. In the ribbed trailing rotating tests, the all walls heated cases show significantly higher heat transfer rate than only the ribbed wall heated cases. The discrepancy is enlarged as Buoyancy number increases. The heat transfer in the all walls heated case increases monotonically with the Buoyancy number whereas in the ribbed wall heated case is slight reduced. In the ribbed leading rotating tests, the heat transfer sensitivity to the heating conditions is not conspicuous, and for both cases, the heat transfer level slightly reduced as Buoyancy number increases. The flow field investigation shows that, there is a significant displacement of main flow in the all walls heated cases than only the ribbed wall heated cases under high Buoyancy numbers. This displacement is due to the buoyancy effect and responsible for the heat transfer differences in uneven heating problems. According to the results obtained in the paper, we conclude that when buoyancy effects are relevant, the heating settings can play a significant role in the heat transfer mechanisms and therefore in the experimental and numerical results.

Convective Heat Transfer of Turbulent Gas Flow in Micro-Tubes With Constant Wall Temperature (Cooled Case)

Numerical computations were performed to obtain for heat transfer characteristics of turbulent gas flow in micro-tubes with constant wall temperature whose temperature is lower than the inlet temperature (cooled case). The numerical methodology was based on Arbitrary-Lagrangian-Eulerinan (ALE) method to solve compressible momentum and energy equations. The Lam-Bremhorst Low-Reynolds number turbulence model was employed to evaluate eddy viscosity coefficient and turbulence energy. The tube diameter ranges from 100 μm to 400 μm and the aspect ratio of the tube diameter and the length is fixed at 200. The stagnation temperature was fixed at 300 K and the computations were done for wall temperature, which ranged from 250 K to 295 K. The stagnation pressure was chosen in such a way that the flow is in turbulent flow regime. The results in wide range of Reynolds number and Mach number were obtained. The bulk temperature based on the static temperature and the total temperature of the cooled case are compared with those of heated case and also with temperatures of the incompressible flow. The result shows that different heat transfer characteristics are obtained for each cooled and heated case. A correlation for the prediction of the heat transfer rate of the turbulent gas flow in a micro-tube is proposed.

Turbulent Thermal Diffusion Over a Locally-Heated Two-Dimensional Hill

We measure flow and thermal fields over a locally heated two-dimensional hill. The heated sections on the wall are divided into upstream and downstream portions of the hill model. These sections are heated independently, yielding various thermal boundary conditions in contrast to the uniformly heated case. In the separated region formed behind the hill, it is found that the mean temperature profiles in the uniformly heated case are well decomposed into the separately heated cases. This is because the velocity fluctuation produced by the shear layer formed behind the hill is large, so the superposition of a passive scalar in the thermal field can be successfully realized. The rapid increase in the mean temperature near the uniformly heated wall should be due to the heat transfer near the leeward slope of the hill. On the other hand, the mean temperature distributions away from the wall are strongly affected by the turbulent thermal diffusion on the windward side of the hill.

1998 Heat Transfer Committee Best Paper Award: Complementary Velocity and Heat Transfer Measurements in a Rotating Cooling Passage With Smooth Walls

An experimental investigation was conducted on the internal flowfield of a simulated smooth-wall turbine blade cooling passage. The square cross-sectioned passage was manufactured from quartz for optical accessibility. Velocity measurements were taken using Particle Image Velocimetry for both heated and non-heated cases. Thin film resistive heaters on all four exterior walls of the passage allowed heat to be added to the coolant flow without obstructing laser access. Under the same conditions, an infrared detector with associated optics collected wall temperature data for use in calculating local Nusselt number. The test section was operated with radial outward flow and at values of Reynolds number and Rotation number typical of a small turbine blade. The density ratio was 0.27. Velocity data for the non-heated case document the evolution of the Coriolis-induced double vortex. The vortex has the effect of disproportionately increasing the leading side boundary layer thickness. Also, the streamwise component of the Coriolis acceleration creates a considerably thinned side wall boundary layer. Additionally, these data reveal a highly unsteady, turbulent flowfield in the cooling passage. Velocity data for the heated case show a strongly distorted streamwise profile indicative of a buoyancy effect on the leading side. The Coriolis vortex is the mechanism for the accumulation of stagnant flow on the leading side of the passage. Heat transfer data show a maximum factor of two difference in the Nusselt number from trailing side to leading side. A first-order estimate of this heat transfer disparity based on the measured boundary layer edge velocity yields approximately the same factor of two. A momentum integral model was developed for data interpretation, which accounts for coriolis and buoyancy effects. Calculated streamwise profiles and secondary flows match the experimental data well. The model, the velocity data, and the heat transfer data combine to strongly suggest the presence of separated flow on the leading wall starting at about five hydraulic diameters from the channel inlet for the conditions studied.

Complementary Velocity and Heat Transfer Measurements in a Rotating Cooling Passage With Smooth Walls

An experimental investigation was conducted on the internal flowfield of a simulated smooth-wall turbine blade cooling passage. The square cross-sectioned passage was manufactured from quartz for optical accessibility. Velocity measurements were taken using Particle Image Velocimetry for both heated and non-heated cases. Thin film resistive heaters on all four exterior walls of the passage allowed heat to be added to the coolant flow without obstructing laser access. Under the same conditions, an infrared detector with associated optics collected wall temperature data for use in calculating local Nusselt number. The test section was operated with radial outward flow and at values of Reynolds number and Rotation number typical of a small turbine blade. The density ratio was 0.27. Velocity data for the non-heated case document the evolution of the coriolis-induced double vortex. The vortex has the effect of disproportionately increasing the leading side boundary layer thickness. Also, the streamwise component of the coriolis acceleration creates a considerably thinned side wall boundary layer. Additionally, these data reveal a highly unsteady, turbulent flowfield in the cooling passage. Velocity data for the heated case show a strongly distorted streamwise profile indicative of a buoyancy effect on the leading side. The coriolis vortex is the mechanism for the accumulation of stagnant flow on the leading side of the passage. Heat transfer data show a maximum factor of two difference in the Nusselt number from trailing side to leading side. A first-order estimate of this heat transfer disparity based on the measured boundary layer edge velocity yields approximately the same factor of two. A momentum integral model was developed for data interpretation which accounts for coriolis and buoyancy effects. Calculated streamwise profiles and secondary flows match the experimental data well. The model, the velocity data, and the heat transfer data combine to strongly suggest the presence of separated flow on the leading wall starting at about five hydraulic diameters from the channel inlet for the conditions studied.

Oscillatory convection in a porous medium heated from below

The stability of natural convective flow in a porous medium heated both uniformly and non-uniformly from below is studied in order to determine the possibility of oscillatory and other unsteady flows, and to explore the conditions under which they may occur. The results of the numerical work are directly comparable with experiments using a Hele Shaw cell and also, in the uniformly heated case, with the results of Combarnous & Le Fur (1969) and Caltagirone, Cloupeau & Combarnous (1971). It is shown that for the uniformly heated problem there exist, in certain cases, two distinct possible modes of flow, one of which is fluctuating, the other being steady. However in the non-uniformly heated case the boundary conditions force the solution into a unique mode of flow which is regularly oscillatory when there is considerable non-uniformity in the heat input at the lower boundary, provided that the Rayleigh number is sufficiently high.