Shyr Fuu Chiou

Heat Transfer in High-Speed Rotating Trapezoidal Duct With Rib-Roughened Surfaces and Air Bleeds From the Wall on the Apical Side

An experimental study of heat transfer in a radially rotating trapezoidal duct with two opposite walls roughened by 45 deg staggered ribs and bleed from the apical side wall is performed. Centerline heat transfer variations on two rib-roughened surfaces are measured for radially outward flows with and without bleeds at test conditions of Reynolds number (Re), rotation number (Ro), and density ratio (Δρ/ρ) in the ranges of 15,000-30,000, 0-0.8, and 0.04-0.31, respectively. Geometrical configurations and rotation numbers tested have considerably extended the previous experiences that offer practical applications to the trailing edge cooling of a gas turbine rotor blade. A selection of experimental data illustrates the individual and interactive influences of Re, Ro, and buoyancy number (Bu) on local heat transfer with and without bleeds. Local heat transfer results are generated with the influences of bleeds on the apical side examined to establish heat transfer correlations with Re, Ro, and Bu as the controlling flow parameters for design applications. The rotation of present trapezoidal duct with rib-roughened surfaces and air bleeds on the apical side worsens the impairing heat transfer impacts due to bleeds. Within the Ro range of 0.1-0.8, bleeds on the apical side of the rotating channel respectively produce 25-50% and 25-40% of heat transfer reductions from the rotational no-bleed references along the leading and trailing centerlines. Such heat transfer reductions due to the combined bleeds and Ro-Bu impacts need design precautions for turbine rotor blades.

Enhanced heat transfer of shaker-bored piston cooling channel with twisted tape insert

This experimental study investigates the heat transfer augmentation in a reciprocating anti-gravity open thermosyphon using a twisted tape insert with relevance to the “shaker-bored” piston cooling system for marine propulsive diesel engine. A selection of experimental data illustrates the interactive effects of inertial, reciprocating, and buoyancy forces on heat transfer in the anti-gravity open thermosyphon with and without a twisted tape insert for subcooled and superheated conditions. The impacts of gravitational buoyancy on heat transfer in the static plain thermosyphon tube are reversed from impairing to improving heat transfer when the flow condition yields from subcooled to superheated condition. In the static thermosyphon tube fitted with twisted tape insert and in the reciprocating thermosyphon tubes with and without twisted tape insert, the buoyancy interactions enhance heat transfer coefficients. Due to the isolated reciprocating force effect, heat transfer coefficients are initially impaired from the static levels at low pulsating numbers but recovered to be enhanced at high pulsating numbers in the reciprocating plain thermosyphon tube. For the reciprocating thermosyphon tube fitted with a twisted tape insert, the isolated reciprocating force effect consistently improves heat transfer. The impacts of isolated reciprocating force and buoyancy interaction on heat transfer are Reynolds number-dependent. Heat transfer coefficients in the reciprocating thermosyphon tube fitted with the twisted tape insert could be augmented to the range of 1.2–6 times of plain tube levels. A set of empirical heat transfer correlations that considers the synergistic effects of inertial force, reciprocating force, and buoyancy interaction in the reciprocating anti-gravity open thermosyphon tube fitted with a twisted tape insert is developed to assist the design activity of the piston-cooling system.

High Rotation Number Heat Transfer of Rotating Trapezoidal Duct With 45-deg Staggered Ribs and Bleeds From Apical Side Wall

An experimental study of heat transfer in a radially rotating trapezoidal duct with two opposite walls roughened by 45° staggered ribs and mid-rib bleeds from the apical side wall is performed. Centerline heat transfer variations on two rib-roughened surfaces are measured for radially outward flows with and without bleeds at test conditions of Reynolds number (Re), rotation number (Ro) and density ratio (Δρ/ρ) in the ranges of 15000–30000, 0–0.8 and 0.04–0.31, respectively. Geometrical configurations and rotation numbers tested have considerably extended the previous experiences that offer practical applications to the trail edge cooling of a gas turbine rotor blade. A selection of experimental data illustrates the individual and interactive influences of Re, Ro and buoyancy number (Bu) on local heat transfer with and without bleeds. Local heat transfer results are generated with the influences of sidewall bleeds examined to establish heat transfer correlations with Re, Ro and Bu as the controlling flow parameters for design applications.Copyright

Heat Transfer in a Reciprocating Thermosyphon Fitted with Staggered Transverse Ribs

This paper describes an experimental investigation of heat transfer in a reciprocating antigravity open thermosyphon. The thermosyphon tube is square in cross section, and two opposite walls are roughened with staggered transverse ribs. This flow/heat-transfer system has relevance, as a fundamental study, to the shaker-cooling system for the pistons of reciprocating engines. A series of experiments are undertaken to demonstrate the complex interaction that exists between inertial, reciprocating, and buoyancy forces on the performance of this form of cooling system. It is shown that reciprocation of the system significantly improves heat transfer in comparison with the stationary antigravity thermosyphon performance. The buoyancy effects from gravity and reciprocation improve heat transfer. Reductions in the effectiveness of buoyancy forces are produced when the relative strengths of inertial or/and pulsating forces increase. In the extrapolated case of zero buoyancy, it was found that the local heat transfer along the thermosyphon tube was reduced in comparison to the static case at relatively low levels of reciprocation frequency. As the reciprocation frequency was increased, this effect was reversed and heat transfer improved in comparison with the stationary case.

Free Convective Heat Transfer in Tilted Longitudinal Open Cavity

Heat transfer experiments were performed to investigate the effects of inclination and channel height-to-gap ratio on free convection in a simulated fin-passage with a strategic aim of devising a criterion for selecting the optimal fin length that could provide the maximum free convective capability. The ranges of parameters investigated include the Grashof number, up to 500,000; channel height-to-gap ratios of 1, 2, and 3; and tilt angles of 0°, 30°, 60°, 90°, 120°, 150°, and 180°. Selections of local and spatially averaged Nusselt number results demonstrate the manner by which the Grashof number, tilt angle, and channel height-to-gap ratio interactively affect the heat transfer. In conformity with the experimentally revealed heat transfer physics, the correlation of a spatially-averaged Nusselt number over two parallel walls and the bottom surface of an open-ended channel is derived that permits the individual and interactive effects of the Grashof number, tilt angle, and channel height-to-gap ratio on heat transfers to be evaluated. A criterion for selecting the optimal height-to-gap ratio of the fin channel is subsequently formulated as a design tool for maximizing the convective capability of a free convective fin assembly.

Numerical Heat Transfer and Flow in Narrow Channel with Attached and Detached Pin Fin

This numerical study comparatively examines the flow and heat transfer characteristics hi two rectangular channels with attached and detached pin-fin arrays at Reynolds numbers (Re) of 10000. 15000, 20000, 25000 and 30000 using the commercial code Star CD. Clearances (C) between pin-tips and endwall for the present detached pin-tin array are fixed at 1/4 pin-diameter (d). The presence of pin-to-endwall clearances modifies the endwall flow structures by way of generating the accelerating flows over the pin-tips and results in the large-scale three dimensional vortical flows along two side walls of the detached pin-tin channel that leads to regional heat transfer enhancements. The lack of vortices and flow separations tripped by the attached pin-fins considerably reduces the inlet-to-exit pressure drops hi the detached pm-fin channel. But the absence of horseshoe vortices and the downstream wakes as well as the shear-layer separations over the endwall in the detached pin-fin channel moderates the gradients at endwall Nusselt number (No) distributions and reduces the area averaged endwall Nu (average)(Nu). As the endwall Nit with detached pin-tins is somewhat less than that with attached pin-fins but the channel pressure drop is much lower than that of the later, the thermal performance factors for the detached pin-fin channel ire improved from those of the attached pin-fin channel.

Heat Transfer of Forced Convective Fin Flow with Cooling Applications to Electronic Chipset

The detailed heat transfer measurements over three smooth-walled rectangular channels with open boundaries on their two side profiles and a sealed-bottom end, simulating a cooling passage in the fan-fin type heat sink of CPU cooling unit performed. The interactive effects of side-profile leakage flow and streamwise coolant stream are functionally related with Reynolds number and channel length-to-gap ratio, which demonstrate considerable influences on local and spatially averaged heat transfers over the fin surfaces Heat transfer modifications caused by varying Reynolds number (Re) and length-to-gap ratio of channel (L/B) over the ranges of 500≤Re≤3000 and 5.89≤L/B≤21.33 are disclosed by examining the detailed heat transfer distributions over the fin surfaces. In conformity with the experimentally revealed heat transfer physics, a regression-type analysis is performed to develop the correlation of spatially-averaged Nusselt number over fin surface, which permits the individual and interactive effect of Re and L/B on heat transfers to be evaluated. A criterion for selecting the optimal length-to-gap ratio of a confined fin channel that provides the maximum convective heat flux from fin surface is formulated.

An Experimental Study of Multiple-jets Impingement Heat Transfer

This study experimentally examined the detailed heat transfer distributions over a free surface of an array of orthogonal impinging air jets. Detailed heat transfer measurements were made at jet Reynolds numbers of 830, 1110, 1660, 2220 and 2770 for five different heating levels with separation distances keeping at 0.1, 0.25, 0.75, 1, 1.5. 2, 2.5, 3, 4 and 6 jet diameters. The graphic representations of the local Nusselt number contours evaluated from the infrared thermal images of the impinging surface under different flow conditions and the dependence of Nusselt number values on Reynolds number, heating level and separation distance were presented. It was found that four regions of impeded closed-loop heat transfer contours located between jets developed in the jet array assembly. A geostrophic heat transfer phenomenon among the central jet region was observed when the separation distance became very small (S/D(subscript j)≤1) which phenomenon could weaken the heat transfer identities of central jet and was ini.tiated at the lower value of heat flux if the jet Reynolds number reduced. Due to the complicate flow phenomena and the geostrophic flow behavior developed over the central jet region, the variations of heat transfer values did not correspond monotonically with the increase of separation distance.