C.W. Leung

Heat transfer from an impinging premixed butane/air slot flame jet

Abstract Experiments were performed to study the heat transfer characteristics of a premixed butane/air slot flame jet impinging normally on a horizontal rectangular plate. The effects of Reynolds number and the nozzle-to-plate distance on heat transfer were examined. The Reynolds number varied from 800 to 1700, while the nozzle-to-plate distance ranged from 2 d e to 12 d e . Comparisons were made between the heat transfer characteristics of slot jets and circular jets under the same experimental conditions. It was found that the slot flame jet produces more uniform heat flux profile and larger averaged heat fluxes than the circular flame jet.

Heat transfer of a row of three butane/air flame jets impinging on a flat plate

Experiments were performed to investigate the heat transfer characteristics of a row of three premixed, laminar, butane/air flame jets impinging on a water-cooled flat plate. The between-jet interference was found to reduce the heat transfer rate in the jet-to-jet interacting zone due to the depressed combustion. The interference became stronger when the jet-to-jet spacing and/or the nozzle-to-plate distance were/was small. The positive pressure existed in the between-jet interacting zone caused the asymmetric flame and heat transfer distribution of the side jet. The meeting point of the spreading wall jets of the central and the side jets did not occur at the midpoint of the neighboring jets, but at a location shifted slightly outwards. The maximum local heat flux and the maximum area-averaged heat flux occurred at a moderate nozzle-to-plate distance of 5d with a moderate jet-to-jet spacing of 5d. The lowest area-averaged heat flux was produced when both the jet-to-jet spacing and the nozzle-to-plate distance were small. Comparing with a single jet under the same experimental conditions, the heat transfer rates in both the stagnation point and the maximum heat transfer point were shown to be enhanced in a row of three-jet-impingement system. The present study provided detailed information on the heat transfer characteristics of a row of three in-line impinging flame jets, which had rarely been reported in previous study.

Thermal tuning of phononic bandstructure in ferroelectric ceramic/epoxy phononic crystal

Thermal tuning of phononic bandgaps in megahertz range was demonstrated in ferroelectric ceramic-based phononic crystal structure. Temperature variation across ferroelectric phase transition, accompanied by substantial changes in acoustic velocities, leads to a shift in the phononic bandstructure of a two-dimensional (Ba,Sr)TiO3/epoxy composite sample over a range of 10 °C. Experimental results are supported by modelings based on plane-wave expansion calculations. The high tunability of phononic bandstructure is advantageous for active control of ultrasound transmissions.

NUMERICAL SIMULATION OF LAMINAR FORCED CONVECTION IN AN AIR-COOLED HORIZONTAL PRINTED CIRCUIT BOARD ASSEMBLY

A numerical solution of the steady-state forced convection for air flowing through a horizontally oriented simulated printed circuit board (PCB) assembly under laminar flow condition has been developed. The considered assembly consists of a channel formed by two parallel plates. The upper plate is thermally insulated, whereas the bottom plate is attached with uniformly spaced identical electrically heated square ribs perpendicular to the mean air flow. The bottom plate is used to simulate the PCB, and the ribs with heat generation are used to simulate the electronic components. A second-order upwind scheme is adopted in the calculation and a very fine mesh density is arranged near the obstacle and the channel surface to achieve higher calculation accuracy. Four Nusselt numbers (Nu) are of particular interest in this analysis: local distribution along the ribs surfaces, mean value for individual surfaces of the rib, overall obstacle mean value, and overall PCB mean value between the central lines of two o...A numerical solution of the steady-state forced convection for air flowing through a horizontally oriented simulated printed circuit board (PCB) assembly under laminar flow condition has been developed. The considered assembly consists of a channel formed by two parallel plates. The upper plate is thermally insulated, whereas the bottom plate is attached with uniformly spaced identical electrically heated square ribs perpendicular to the mean air flow. The bottom plate is used to simulate the PCB, and the ribs with heat generation are used to simulate the electronic components. A second-order upwind scheme is adopted in the calculation and a very fine mesh density is arranged near the obstacle and the channel surface to achieve higher calculation accuracy. Four Nusselt numbers (Nu) are of particular interest in this analysis: local distribution along the ribs surfaces, mean value for individual surfaces of the rib, overall obstacle mean value, and overall PCB mean value between the central lines of two obstacles. The effect of the obstacle size and the separation between two obstacles is discussed systematically.

Simulation of Turbulent Flow and Forced Convection in a Triangular Duct with Internal Ribbed Surfaces

ABSTRACT A three-dimensional problem of fully developed turbulent flow through an equilateral triangular duct with internal ribbed surfaces has been simulated numerically through two two-dimensional approaches: turbulent forced convection between two parallel plates with ribbed bottom surface and that in an equilateral triangular duct with smooth internal surfaces. Effects of the complicated geometry on the turbulent forced convection, as well as the formation of the secondary flows around the ribs and in the triangular ducts corners, are analyzed in detail. Comparisons between the calculated and experimental results have been carried out and it has been found that the geometry effect from the attached uniformly spaced ribs on the triangular ducts thermal performance is more significant than that from the ducts corners.

Numerical prediction of laminar forced convection in triangular ducts with unstructured triangular grid method

The flow and heat transfer characteristics of smooth triangular ducts with different apex angles of 15, 30, 60, and 90 under the fully developed laminar flow condition were predicted numerically using a finite volume method. The SIMPLE-like algorithm was employed together with an unstructured triangular grid method, where the grid was generated by a Delaunay method. The triangular grid was adopted instead of the traditional rectangular grid to fit better into the triangular cross section of the duct. Two kinds of boundary condition (uniform wall temperature and uniform wall heat flux) were considered. Comparison of the predictions with previous computational results indicated a very good agreement. Both the friction factor and Nusselt number (Nu) showed a strong dependence on apex angle of the triangular duct. When the apex angle was 60, the duct provided the highest steady-state forced convection from its inner surface to the airflow under the laminar flow condition.The flow and heat transfer characteristics of smooth triangular ducts with different apex angles of 15, 30, 60, and 90 under the fully developed laminar flow condition were predicted numerically using a finite volume method. The SIMPLE-like algorithm was employed together with an unstructured triangular grid method, where the grid was generated by a Delaunay method. The triangular grid was adopted instead of the traditional rectangular grid to fit better into the triangular cross section of the duct. Two kinds of boundary condition (uniform wall temperature and uniform wall heat flux) were considered. Comparison of the predictions with previous computational results indicated a very good agreement. Both the friction factor and Nusselt number (Nu) showed a strong dependence on apex angle of the triangular duct. When the apex angle was 60, the duct provided the highest steady-state forced convection from its inner surface to the airflow under the laminar flow condition.

Forced-convective turbulent-flows through horizontal ducts with isosceles-triangular internal cross-sections

Three electrically heated triangular ducts were used to simulate the thermal behaviours of turbulent air-flows through triangular passages in compact heat-exchangers (Fig. 1). Three sharp-cornered isosceles-triangular duralumin ducts were fabricated, each of the same length of 2.4 m and hydraulic diameter of 0.44 m, but with three different apex-angles, namely 40°, 60° and 90°. The experimental investigations were performed over a wide range of Reynolds number (Re), namely 5000

Enhanced forced-convection from ribbed or machine-roughened inner surfaces within triangular ducts

An experimental investigation has been conducted to study the steady-state forced-convection heat-transfer characteristics of the hydrodynamic fully-developed turbulent flow in air-cooled horizontal equilateral triangular ducts (i.e. of 60° apex angle), which were each fabricated with the same length of 2.4 m and hydraulic diameter of 0.44 m. The inner surfaces of the triangular ducts were roughened by a milling process, shaping process or fixing uniformly-spaced parallel square ribs orthogonal to the mean air flow. The average surface roughness of the inner surfaces, which were produced by milling and shaping processes, were 3.0 and 11.5 [mu]m, respectively. The square-sectioned ribs, adopted to produce the roughened surface, had different protrusions of 6.35, 9.525 and 12.7 mm, and the uniform separation between the centre lines of two successive ribs was kept constant at 57.15 mm. Both the triangular ducts and the square ribs were fabricated out of duralumin. The experiments were performed with the hydraulic-diameter based Reynolds numbers ranging from 4000 to 15000. The entire inner wall of the duct was heated uniformly, while its outer surfaces were thermally well insulated. By comparing the heat-transfer performances with those of a smooth triangular duct (i.e. average inner-surface roughness of less than 1.0 [mu]m) having the same geometry, it was found that forced convection was enhanced by the roughened surfaces. In addition, a much enhanced forced convection was obtained by fixing uniformly-spaced parallel square ribs, rather than by fabricating random roughness on its inner surfaces by machining. However, the heat-transfer enhancement was not proportional to the rib size; the maximum forced convection heat-transfer augmentation was obtained using the smallest (i.e. 6.35 mm) ribs of those tested. Non-dimensional expressions for the determination of the steady-state heat-transfer coefficient of the equilateral triangular ducts, which were fabricated with the various kinds of artificial inner-surface roughness, were also developed.

Forced convection from a horizontal ribbed rectangular base-plate penetrated by arrays of holes

A method has been employed to augment the forced convection to ambient air from a horizontal rectangular plate with its upper surface mounted with transverse (to the mean air-flow) rectangular ribs, which are uniformly spaced and heated. The enhancement has been achieved by drilling uniformly-spaced holes in the base plate between consecutive ribs. The steady-state effects of varying the penetration ratio, [beta], (i.e. of the total cross-sectional areas of the holes to the whole base-plate area between consecutive ribs), diameter of the hole, d, ratio of channels height to ribs common height, H/B, and Reynolds number, Rec, of the air-flow on the rate of forced convection from the rectangular plate were investigated experimentally for both laminar and turbulent flows over the ribs. Heat-transfer measurements were obtained for the ranges: 500[less-than-or-equals, slant]Rec[less-than-or-equals, slant]19,000; 4[less-than-or-equals, slant]H/B[less-than-or-equals, slant]10 and 0.05[less-than-or-equals, slant][beta][less-than-or-equals, slant]0.20. The following correlation applied for the prediction of the average heat-transfer coefficient for this heat-transfer system (i.e. as contained in the wind-tunnel): To achieve the maximum heat-transfer performance with this type of heat exchanger--for a specified application in which the degree of inhibition experienced by the air in flowing through the holes is known--optimised choices of the location, diameter and number of holes, as well as for the materials of the ribs and base, have to be made.

Forced convection and pressure drop in a horizontal triangular-sectional duct with V-grooved (i.e. orthogonal to the mean flow) inner surfaces

An experimental investigation has been performed to study the forced convection and pressure-drop characteristics fully-developed steady turbulent flows in air-cooled, horizontal, equilateral-triangular ducts. These ducts were constructed of duralumin, each of the same axial length and hydraulic diameter. The inner surfaces of each duct were either plane or machined with uniformly-spaced parallel, identical V-grooves. For each tested duct, these grooves had a depth of 1 mm, but were all of the same apex angle, [theta], where 0[less-than-or-equals, slant][theta]