Tsun Lirng Yang

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.

Heat transfer in a reciprocating duct fitted with transverse ribs

This article describes a detailed experimental investigation of heat transfer in a reciprocating square-sectioned duct fitted with transverse ribs. The parametric conditions involved several nominal Reynolds numbers ranging from 2,700 to 9,000 with five different reciprocating frequencies, namely, 0, 0.415, 0.83, 1.25, and 1.67 Hz. This resulted in a pulsating number, which represented the ratio of reciprocating force to inertial force effect, varying from 0 to 10.5. Inside the reciprocating ribbed duct the typical effects of flow reciprocation on heat transfer were illustrated by examining the periodic spatial-time distributions of Nusselt number. The amplitude of the timewise variable reciprocating Nusselt number was dependent on the axial location and increased with increases of Reynolds number. The time-averaged heat transfer due to flow reciprocation could be accounted for by the pulsating number and, in general, the heat transfer level increased with increases of the pulsating number. At a pulsating...

Heat Transfer in a Rotating Twin-Pass Trapezoidal-Sectioned Passage Roughened by Skewed Ribs on Two Opposite Walls

An experimental study of heat transfer in a radially rotating twin-pass trapezoidal-sectioned duct with two opposite walls roughened by 45° staggered ribs was performed. Two channel orientations of 0° and 45° from the direction of rotation were tested. At each Reynolds number of 5000, 7500, 10000, 12500, and 15000, local Nusselt numbers along the centerlines of two rib-roughened surfaces with five different heating levels were acquired at rotating numbers of 0, 0.1, 0.3, 0.5, 0.7, and 1. A selection of experimental results illustrates the isolated and interactive influences of convective inertial, Coriolis, and rotating buoyancy forces on local and centerline-averaged heat transfers. The isolated Coriolis force-effect improves heat transfer over two unstable surfaces of the rotating twin-pass channel. The rotating buoyancy effect undermines local heat transfer, but its influence is alleviated when the rotating number increases. At rotating number of 0.7 and 1, the rotating buoyancy force acting with counter-flow manner considerably impairs local heat transfer in the end-region of the first passage with radially outward flow. With the rotating numbers in the range of 0.1 to 1, the heat transfer differences between the two channels with orientations of 0° and 45° are in the range of 5–26%. As a strategic aim of the present study, heat transfer correlations are derived to evaluate the centerline-averaged Nusselt numbers over two rib-roughened surfaces that permit the individual and interactive influences of convective inertia, Coriolis force, and rotating buoyancy to be quantified. As the full-field spatial heat transfer variations in the present rotating channel are not measured, the local heat transfer results generated by the present study are limited to the locations measured.

Heat Transfer in Radially Rotating Pin-Fin Channel at High Rotation Numbers

Endwall heat transfer measurements for a radially rotating rectangular pin-fin channel with the width-to-height ratio (aspect ratio) of 8 are performed at the parametric conditions of 5000 ≤ Re ≤ 20,000, 0 ≤ Ro ≤ 1.4, and 0.1 ≤ Δρ/ρ ≤ 0.21. Centerline heat transfer levels along the leading and trailing endwalls of the rotating pin-fin channel are, respectively, raised to 1.77-3.72 and 3.06-5.2 times of the Dittus-Boelter values. No previous attempt has examined the heat transfer performances for the pin-fin channel at such high rotation numbers. A selection of experimental data illustrates the individual and interactive Re, Ro, and buoyancy number (Bu) effects on heat transfer. Spanwise heat transfer variations between two adjoining pin rows are detected with the averaged Nusselt numbers (Nu) determined. A set of empirical equations that calculates Nu values over leading and trailing endwalls in the developed flow region is derived to correlate all the heat transfer data generated by this study and permits the evaluation of interactive and individual effects of Re, Ro, and Bu on Nu. With the aid of the Nu correlations derived, the operating conditions with the worst heat transfer scenarios for this rotating pin-fin channel are identified.

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.

Heat transfer in a swinging rectangular duct with two opposite walls roughened by 45° staggered ribs

Abstract This paper describes an experimental study of heat transfer in a rectangular channel with two opposite walls roughened by 45° staggered ribs swinging about two orthogonal axes under single and compound modes of pitching and rolling oscillations. A selection of heat transfer measurements illustrates the manner by which the swinging oscillations with and without buoyancy interaction modify local heat transfer along the centerline of rib-roughened surface in the range of 0.75–2.25 times of the static channel value. The compound rolling and pitching forces with harmonic and non-harmonic rhythms interacting with buoyancy exhibit synergistic effect to reduce heat transfer. An adverse buoyancy effect that reverses the buoyancy interaction from improving to impeding heat transfer when the relative strength of swinging force increases could develop in the channel that swings with compound mode oscillation. An empirical heat transfer correlation, which is physically consistent, has been developed that permits the individual and interactive effects of single and compound modes of swinging forces with and without buoyancy interaction on forced convection to be evaluated and quantified. This work has been motivated by the need to understand the general effect of swinging oscillation on the performance of the cooling passage in a rib-roughened plate-type heat exchanger under sea-going conditions.

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.