Akira Murata

Comparison between laminar and turbulent heat transfer in a stationary square duct with transverse or angled rib turbulators

Abstract Heat transfer in a rib-roughened duct was numerically simulated by using the second-order finite difference method in coordinates fitted to transverse or angled ribs. Turbulent and laminar cases of which Reynolds numbers were 350 and 50, respectively, were computed for rib angles of 60° and 90°. The present turbulent results clearly showed the locally high heat transfer spots at several locations. The comparison between the laminar and turbulent results showed clear differences in heat transfer distribution because the higher momentum fluid of the turbulent case was more disturbed by the ribs as compared to the laminar case.

Large eddy simulation with a dynamic subgrid-scale model of turbulent heat transfer in an orthogonally rotating rectangular duct with transverse rib turbulators

Abstract In order to investigate the effects of transverse ribs, the Coriolis force, and cross-sectional aspect ratios on turbulence, the large eddy simulation was performed changing the rotation number and the aspect ratio. The results reproduced the experimentally observed high heat transfer areas: in front of the rib on the rib-roughened walls and around the rib on the smooth side walls. In the rotating case, an interesting dissimilarity between the velocity and temperature fields was seen. The heat transfer enhancement caused by the rotation was larger for the higher aspect ratios because of the intensified Coriolis induced secondary flow.

Effect of cross-sectional aspect ratio on turbulent heat transfer in an orthogonally rotating rectangular smooth duct

Abstract In order to investigate the effects of the Coriolis force and the duct cross-sectional aspect ratio on turbulent heat transfer, the large eddy simulation was performed changing the aspect ratio and the rotation number in the range, 0.25–4.0 and 0–5.0, respectively, while the turbulent Reynolds number was 350. In the results, both direct and indirect influences of the Coriolis force on the turbulence were seen. The relative intensity of these influences depended on the aspect ratio. The Colburn’s j factor showed a higher value for a larger aspect ratio because of larger heat transfer enhancement on the pressure and side surfaces.

Effects of Rib Arrangements on Heat Transfer and Flow Behavior in a Rectangular Rib-Roughened Passage: Application to Cooling of Gas Turbine Blade Trailing Edge

Experimentation was conducted to examine the heat transfer and pressure drop characteristics in a rib-roughened rectangular passage with aspect ratio 2:1 for four rib configurations: 90 deg, 75 deg, 60 deg and 45 deg oblique ribs. The ribs were attached to two opposing long side walls instead of short side walls. In this study the oblique ribs were intended to function as secondary flow inducers as well as turbulators to improve the heat transfer of the bottom wall (one of the short side walls)

Effect of centrifugal buoyancy on turbulent heat transfer in an orthogonally rotating square duct with transverse or angled rib turbulators

Abstract The effect of the Coriolis and buoyancy forces on the turbulent heat transfer disturbed by rib turbulators was numerically investigated by using the large eddy simulation (LES). The computational condition was as follows: the turbulent Reynolds number of 350, the rotation number of 1.0, the rib angles of 90° and 60°, and the Rayleigh number from −5×10 4 to +5×10 4 . The heat transfer was enhanced and suppressed by the buoyancy-induced aiding and opposing flows, respectively. The friction factor was dominated by the form drag, and the results of 90° and 60° rib showed different behavior for the Rayleigh number change.

Effect of cross-sectional aspect ratio on turbulent heat transfer in an orthogonally rotating rectangular duct with angled rib turbulators

Abstract Heat transfer in a rotating rib-roughened rectangular duct was numerically simulated by using the large eddy simulation with a Lagrangian dynamic subgrid-scale model. The rotation number and the duct cross-sectional aspect ratio (0.25, 0.5, 1.0, 2.0, and 4.0) were varied for a friction Reynolds number of 350 and rib angle of 60°. The present results clearly showed the locally high heat transfer at several locations and how it was changed by the duct aspect ratio. The dissimilarity between the streamwise velocity and temperature was observed for all the aspect ratio cases in the rotating case.

Local heat transfer measurements of an orthogonally rotating square duct with angled rib turbulators

Abstract The effect of angled rib turbulators on the heat transfer of an orthogonally rotating square duct were experimentally investigated. The Reynolds and rotation numbers were varied in 10,000–20,000 and 0–0.08, respectively. The leading and trailing walls were rib-roughened with angles of 90° and 60°. The results showed that the rotation induced a peripheral variation of the heat transfer and the higher heat transfer was seen using the 60° angled rib; judging from the local heat transfer variation, the skewed secondary flow induced by the angled rib seemed to result in additional heat transfer enhancement.

Large Eddy Simulation of Turbulent Heat Transfer in an Orthogonally Rotating Square Duct with Angled Rib Turbulators

Heat transfer in a rotating rib-roughened duct was simulated with a Lagrangian dynamic subgrid-scale model. The angled 60 deg rib induced a pair of strong vortices in the stationary condition, and the heat transfer and the friction factor were larger than the 90 deg rib case. In the rotating condition, the high heat transfer areas at the midpoint between ribs and in front of the rib were observed only on the trailing wall. The friction factor and overall Nusselt number were decreased by the duct rotation for 60 deg rib case, although they were increased for 90 deg rib case

Detailed Measurement of Convective Heat Transfer in Rotating Two-Pass Rib-Roughened Coolant Channels

The objective of the study is to investigate, through experiments, heat transfer characteristics in a rotating two-pass rib-roughened coolant channel with a sharp 180-deg turn. Detailed measurements of local surface temperatures of the coolant channel are conducted using a newly designed and constructed test section which has as many as 458 thermocouples installed on the rotating channel walls. Description of this unique experimental rig is given. The cross section of the channel is rectangular with 15 mm in width and 30 mm in height. In addition to a rib-free (smooth) surface, two types of rib configurations are employed: 90° and 45° parallel ribs. The measured temperature distribution for the entire test section are shown and the effects of rib configuration and rotation on the cooling performance are examined. It was also found that the secondary flow created in the bend has substantial influence on the heat transfer in the channel after the turning section.Copyright

Effect of the parameters of traveling waves created by blowing and suction on the relaminarization phenomena in fully developed turbulent channel flow

A series of direct numerical simulations of a fully developed turbulent channel flow controlled by traveling waves induced by blowing and suction is performed. Relaminarization, i.e., the transition from turbulent flow to laminar flow, is observed for some sets of parameter when the wave is traveling in the downstream direction. Since the downstream traveling wave produces the drag, the drag of the flow is slightly larger than the corresponding laminar flow. A parametric study is performed, and reveals that the range of control parameters that produce relaminarization are the wave speed and amplitude of the wave which scale with the mean bulk flow rate corresponding to laminar flow and the wavelength which scales with the viscous scale. When relaminarization occurs, the amplitude of the wave, wavelength, and wave speed are in the range of a/u¯ lam >0.1, 200 1.5, respectively. These ranges are organized by displacement thickness and are between 3 and 10 wall units when the relamin...