Heat Transfer in Channels in Parallel-Mode Rotation at High Rotation Numbers

Abstract

This study attempts to understand one of the most fundamental and challenging problems in fluid flow and heat transfer for rotating machines. The study focuses on electric generators for high energy density applications, which employ rotating cooling channels so that materials do not fail under high temperature and high stress environment. Prediction of fluid flow and heat transfer inside internal cooling channels that rotate at high rotation number and high wall heat flux is the main focus of this study. Rotation, buoyancy, and boundary conditions affect the flow inside these channels. A fully computational approach is employed in this study. Reynolds stress turbulence model with enhanced near-wall treatment is validated against available experimental data (which are primarily at low rotation and buoyancy numbers). The model was then used for cases with high rotation number (as much as 0.35) and high wall heat flux. Particular attention is given to how turbulence intensity, Reynolds stresses, and transport are affected by Coriolis and buoyancy/centrifugal forces caused by high levels of rotation number and wall heat flux. Variations of flow total pressure along the rotating channel are also predicted. The results obtained are explained in view of physical interpretation of Coriolis and centrifugal forces.conductivity of coolant Nu = local Nusselt number, hD h =K Nu o = Nusselt number in fully-developed turbulent nonrotating tube flow Pr = Prandtl number R = radius from axis of rotation Re = Reynolds number, W o D h = Ro = rotation number, D h =W o S = distance in streamwise direction T = local coolant temperature T o = coolant temperature at inlet T w = wall temperature W o = inlet velocity = = density ratio, T w T o =T w , same as DR = dimensionless temperature, T T o =T w T o = dynamic viscosity of coolant = density of air = rotational speed

Developing convective flow in a square channel partially filled with a high porosity metal foam and rotating in a parallel-mode

“…Overall, the performance is improved significantly with increasing Reynolds number unlike the flow case through a clear channel S=1, where it clearly deteriorates. To confirm this outcome, a comparison was done between the data obtained by Sleiti and Kapat [7] for turbulent air flow at Re=20,000 in a clear square channel rotating in parallelmode and the current data of laminar air flow at Re=2000 through a channel occupied with metal foam. The same dimensions, wall heat flux, and rotation rates were used, where L/D h =64, H/D h =32, q w =6.45 KW/m 2 , and Re Ω =0, 1260, 2800, and 7000.…”

“…It was noticed that rotational effects in the case of constant heat flux are more important than those in the isothermal case, where secondary flow at the fully developed region retains its vortices in the iso-flux channels unlike the isothermal ducts where they almost vanish. Recently, a numerical study of developing turbulent flow and heat transfer in a square channel rotating in parallel mode was conducted by Sleiti and Kapat [7]. The problem was examined for high levels of both rotation and applied heat flux.…”

“…For example, the hollow ratio (0≤S≤1) as shown in Figs. (3,4,5,6,7,10,12,13,14,15), medium porosity has been varied from 0.89 to 0.97 as shown in Figs. (6,9,10,12,13,14), pore density (5PPI≤ω≤40PPI) as shown in Figs.…”

“…(6,9,10,12,13,14), pore density (5PPI≤ω≤40PPI) as shown in Figs. (6,7,8,9,10), solid to fluid-phase thermal conductivity ratio (250≤κ≤4000) as shown in Fig. (14), rotation number (0≤Ro≤1.0) as shown in Figs.…”

High-Porosity Metal Foams: Potentials, Applications, and Formulations

“…Similarly, they have been proposed as an effective way to enhance the heat dissipated from heavy-duty electrical generators through filling their rotating cooling passages either fully or partially with open-cell metal foams [23][24][25]. The value of the this proposal was inspected by introducing an enhancement factor as the ratio of heat transported to the pumping power required, that is, Nu Re p in Àp e ðÞ =ρu 2 in , and comparing it with the corresponding values from a previous work regarding turbulent flow in a rotating clear channel [26], as shown in Figure 4 [25], where it was confirmed that the proposed enhancement is practically justified and efficient.…”