Showing papers on "Heat transfer coefficient published in 1994"

Finite volume method for radiation heat transfer

Abstract: This chapter presents a finite-volume (FV) method for computing radiation heat transfer processes The main ingredients of the calculation procedure were presented by Chai et al [1] The resulting method has been tested, refined and extended to account for various geometrical and physical complexities

Basic Heat and Mass Transfer

Abstract: 1. Introduction and Elementary Heat Transfer. 2. Steady One-Dimensional Heat Conduction. 3. Multidimensional and Unsteady Conduction. 4. Convection Fundamentals and Correlations. 5. Convection Analysis. 6. Thermal Radiation. 7. Condensation, Evaporation, and Boiling. 8. Heat Exchangers. 9. Mass Transfer. A. Property Data. B. Unit, Conversion Factors, and Mathematics. C. Charts. Bibliography. Nomenclature. Index.

Free convection on a vertical stretching surface with suction and blowing

Abstract: The natural convective heat transfer from a vertical stretching sheet with surface mass transfer is analyzed. A three dimensional similarity solution to the governing momentum and energy equations is presented. Numerical data for the friction factor and Nusselt number has been tabulated for a range of surface mass transfer rates and Prandtl numbers. Surface mass transfer has a considerable influence on the heat transfer mechanism.

Experimental heat transfer and friction in channels roughened with angled, v-shaped and discrete ribs on two opposite walls

Abstract: Experimental investigations have shown that the enhancement in heat transfer coefficients for air flow in a channel roughened with angled ribs is on the average higher than that roughened with 90 deg ribs of the same geometry. Secondary flows generated by the angled ribs are believed to be responsible for these higher heat transfer coefficients. In an effort basically to double the area of high heat transfer coefficients, the angled rib is broken at the center to form a V-shaped rib, and tests are conducted to investigate the resulting heat transfer coefficients and friction factors. Three different square rib geometries, corresponding to blockage ratios of 0.083, 0.125, and 0.167, with a fixed pitch-to-height ratio of 10, mounted on two opposite walls of a square channel in a staggered configuration, are tested in a stationary channel for 5,000 < Re < 30,000. Heat transfer coefficients, friction factors, and thermal performances are compared with those of 90 deg, 45 deg, and discrete angled ribs. The V-shaped ribs are tested for both pointing upstream and downstream of the main flow. Test results show that: (a) 90 deg ribs represent the lowest thermal performance, based on the same pumping power, and is essentially themore » same for the 2:1 change in blockage ratio, (b) low-blockage-ratio (E/D{sub h} = 0.083) V-shaped ribs pointing downstream produced the highest heat transfer enhancement and friction factors. Among all other geometries with blockage ratios of 0.125 and 0.167, 45 deg ribs showed the highest heat transfer enhancements with friction factors less than those of V-shaped ribs, (c) thermal performance of 45 deg ribs and the lowest blockage discrete ribs are among the highest of the geometries tested in this investigation, and (d) discrete angled ribs, although inferior to 45 deg and V-shaped ribs, produce much higher heat transfer coefficients and lower friction factors compared to 90 deg ribs.« less

Heat transfer characteristics of water flowing through microchannels

Abstract: The forced-flow convection of water through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and aspect ratios of H/W = 0.333-1 was investigated experimentally. The flow friction was measured to analyze the heat transfer regimes and to explore the physical aspects of the connective heat transfer. The experimental measurements indicated that the upper bound of the laminar heat transfer regime occurred at a Reynolds number of 200-700, and fully turbulent connective heat transfer was reached at Reynolds numbers of 400-1,500. The transition Reynolds number diminished with the reduction of the microchannel dimension, and the transition range was observed to become smaller in magnitude. For the laminar heat transfer regime, the Nusselt number was found to be proportional to Re0.62while the turbulent heat transfer case exhibited a typical relationship between Nu and Re, but with a different empirical coefficient, CH,t. The geometric parameters were found to be important variables that could...

Film Cooling With Compound Angle Holes: Heat Transfer

Abstract: Heat transfer coefficients have been measured for film cooling injection from a single row of holes laterally directed with a compound angle of 60 deg. Two hole configurations were tested, round holes and holes with a diffusing expansion at the exit. Streamwise-directed round holes were also tested as a basis for comparison. All the holes were inclined at 35 deg with respect to the surface. The density ratio was 1.0, momentum flux ratios ranged from I = 0.16 to 3.9, and mass flux ratios ranged from M = 0.4 to 2.0. Results are presented in terms of h{sub f}/h{sub 0}, the ratio of film cooling heat transfer coefficient to the heat transfer coefficient for the undisturbed turbulent boundary layer at the same location. Results indicate that for the streamwise directed holes, the heat transfer rates are close to the levels that exist without injection. Similarly, at low momentum flux ratio, holes with a large compound angle had little effect on heat transfer rates. However, at high momentum flux ratios, holes with a large compound angle had significantly increased heat transfer levels. The results were combined with adiabatic effectiveness results to evaluate the overall performance of the three geometries. It ismore » shown that for evaluation of film cooling performance with compound angle injection, especially at high momentum flux ratios, it is critical to know the heat transfer coefficient, as the adiabatic effectiveness alone does not determine the performance. Compound angle injection at high momentum flux ratios gives higher effectiveness values than streamwise-directed holes, but the higher heat transfer levels result in poorer overall performance.« less

A new model for nucleate boiling heat transfer

Abstract: A new model to calculate heat transfer coefficients in nucleate boiling is presented. Heat transfer and fluid flow around a single bubble are investigated taking into account the influence of meniscus curvature, adhesion forces and interfacial thermal resistance on the thermodynamic equilibrium at the gas-liquid interface. The model requires only bubble site densities and departure diameters. Further quantities except the thermophysical properties are not needed. From the results bubble growth rates can be derived. As an example nucleate boiling heat transfer coefficients of R-114 were calculated. They agree with experimental values within the experimental accuracy.

Comparison of Wing-Type Vortex Generators for Heat Transfer Enhancement in Channel Flows

Abstract: Longitudinal vortices can be generated in a channel flow by punching or mounting small triangular or rectangular pieces on the channel wall. Depending on their forms, these vortex generators (VG) are called delta wing, rectangular wing, pair of delta winglets, and pair of rectangular winglets. The heat transfer enhancement and the flow losses incurred by these four basic forms of VGs have been measured and compared in the Reynolds number range of 2000 to 9000 and for angles of attack between 30 and 90 deg. Local heat transfer coefficients on the wall have been measured by liquid crystal thermography. Results show that winglets perform better than wings and a pair of delta winglets can enhance heat transfer by 46 percent at Re=2000 to 120 percent at Re=8000 over the heat transfer on a plate.

Heat Transfer and Friction in Rectangular Channels With Ribbed or Ribbed-Grooved Walls

Abstract: The effect of compound turbulators on friction factors and heat transfer coefficients in rectangular channels with two opposite ribbed-grooved walls was determined for a Reynolds number range of 10,000 to 50,000. The channel width-to-height ratio was 10. The fully developed heat transfer coefficients and friction factors on the ribbed-grooved and smooth side walls of each test channel were measured for six rib-groove spacings (p/e=8, 10, 15, 20, 25, and 30). The fully developed friction and heat transfer in similar aspect ratio rectangular channels with two opposite ribbed walls with two rib spacings (p/e=8.5 and 11.5) was also measured for comparison

Analysis of forced convection enhancement in a channel using porous blocks

Abstract: This work presents a detailed investigation of forced convection enhancement in a channel using multiple emplaced porous blocks. The Brinkman-Forchheimer-extended Darcy model is used to characterize the flowfield inside the porous regions in order to account for the inertia effects as well as the viscous effects. Solution of the coupled governing equations for the porous/fluid composite system is obtained using a stream function-vorticity approach. Important fundamental and practical results have been presented and discussed. These results thoroughly document the dependence of the streamline, isotherm, and local Nusselt number distributions on the governing parameters defining the problem, such as the Reynolds number, Darcy number, Prandtl number, inertial parameter, and two pertinent geometric parameters. An in-depth discussion of the formation and variation of the recirculation caused by the porous medium is presented, and the existence of an optimum porous matrix is demonstrated. It is shown that altering some parametric values can have significant and interesting effects on both the flow pattern as well as heat transfer characteristics.

Heat Transfer Measurement and Analysis for Sintered Porous Channels

Abstract: This paper presents the results of heat transfer measurement and analysis for two 5×5×1 cm porous channels. The channels were made of sintered bronze beads with two different mean diameters, d p =0.72 and 1.59 mm. The local wall temperature distribution, inlet and outlet pressure and temperatures, and heat transfer coefficients were measured for heat flux of 0.8, 1.6, 2.4, and 3.2 W/cm 2 with air velocity ranging from 0.16 to 5 m/s and inlet air pressure of 1∼3 atm

Similarity solutions for flow and heat transfer of a viscoelastic fluid over a stretching sheet

Abstract: This paper presents a study of the flow and heat transfer of an incompressible second-order fluid past a stretching sheet using Dandapat and Gupta's boundary layer solution. Two cases are studied, namely, (i) the sheet with constant surface temperature (CST case) and (ii) the sheet with prescribed surface temperature (PST case). The solutions for the temperature and the heat transfer characteristics for large Prandtl number (σ) are obtained using a Runge-Kutta method of fourth order with step size Δz = 0.01.

Use of the Kelvin-Clapeyron Equation to Model an Evaporating Curved Microfilm

Abstract: A Kelvin-Clapeyron change-of-phase heat transfer model is used to evaluate experimental data for an evaporating meniscus. The details of the evaporating process near the contact line are obtained. The heat flux and the heat transfer coefficient are a function of the film thickness profile, which is a measure of both the intermolecular stress field in the contact line region and the resistance to conduction. The results indicate that a stationary meniscus with a high evaporative flux is possible. At equilibrium, the augmented Young-Laplace equation accurately predicts the meniscus slope. The interfacial slope is a function of the heat flux

Effects of turbulator profile and spacing on heat transfer and friction in a channel

Abstract: Turbulators in cooling passages of small turbine blades often have geometric constraints placed upon them due to casting limitations associated with small dimensions. Problems such as core die wear, cavity fill imperfections, and casting tolerance variations change turbulator profiles in their final form. This change of turbulator geometry affects, often in the wrong direction, the heat transfer coefficient and friction factor in the cooling cavity. Liquid crystals are used in this experimental investigation to study the effects of turbulator profile and spacing on heat transfer coefficient. Friction factors are also measured, and both heat transfer and friction factor results for 15 turbulator geometries are compared. For all test configurations, the turbulators are positioned on two opposite walls of a rectangular test section in a staggered arrangement with an angle of attack to the mainstream flow, a = 90 deg. A range of turbulator blockage ratios e/Dh, pitch-to-height ratios S/e, and Reynolds numbers are tested. It is concluded that while turbulators with aspect ratios (ARf = turbulator height/ turbulator width) greater than unity produce higher heat transfer coefficients at the expense of higher pressure losses, trapezoidal-shaped turbulators, spaced properly, are very effective in heat removal with moderate pressure losses. Low aspect ratio (ARt < 1) turbulators, especially with round corners, produce lower heat transfer coefficients. Furthermore, an optimum pitch-to-height ratio for 90-deg square turbulators is found to be around 8.

Am Experimental Investigation of the Rib Surface-Averaged Heat Transfer Coefficient in a Rib-Roughened Square Passage

Abstract: Turbine blade cooling, a common practice in modern aircraft engines, is accomplished, among other methods, by passing the cooling air through an often serpentine passage in the core of the blade. Furthermore, to enhance the heat transfer coefficient, these passages are roughened with rib-shaped turbulence promoters (turbulators). Considerable data are available on the heat transfer coefficient on the passage surface between the ribs. However, the heat transfer coefficients on the surface of the ribs themselves have not been investigated to the same extent. In small aircraft engines with small cooling passages and relatively large ribs, the rib surfaces comprise a large portion of the passage heat transfer area. Therefore, an accurate account of the heat transfer coefficient on the rib surfaces is critical in the overall design of the blade cooling system.The objective of this experimental investigation was to conduct a series of thirteen tests to measure the rib surface-averaged heat transfer coefficient, in a square duct roughened with staggered 90° ribs. To investigate the effects that blockage ratio, e/Dh, and pitch-to-height ratio, S/e, have on hrib and passage friction factor, three rib geometries corresponding to blockage ratios of 0.133. 0.167 and 0.25 were tested for pitch-to-height ratios of 5, 7, 8.5 and 10. Comparisons were made between the rib average heat transfer coefficient and that on the wall surface between two ribs, hflor, reported previously. Heat transfer coefficients of the upstream-most rib and that of a typical rib located in the middle of the rib-roughened region of the passage wall were also compared.It is concluded that:1) the rib average heat transfer coefficient is much higher than that for the area between the ribs,2) similar to the heat transfer coefficient on the surface between the ribs, the average rib heat transfer coefficient increases with the blockage ratio,3) a pitch-to-height ratios of 8.5 consistently produced the highest rib average heat transfer coefficients amongst all tested,4) under otherwise identical conditions, ribs in upstream-most position produced lower heat transfer coefficients than the mid-channel positions,5) the upstream-most rib average heat transfer coefficients decreased with the blockage ratio, and6) thermal performance decreased with increased blockage ratio. While a pitch-to-height ratio of 8.5 and 10 had the highest thermal performance for the smallest rib geometry, thermal performance of high blockage ribs did not change significantly with the pitch-to-height ratio.Copyright © 1994 by ASME

Detailed Measurements of Local Heat Transfer Coefficient and Adiabatic Wall Temperature Beneath an Array of Impinging Jets

Abstract: A transient method of measuring the local heat transfer under an array of impinging jets has been developed. The use of a temperature-sensitive coating consisting of three encapsulated thermochromic liquid crystal materials has allowed the calculation of both the local adiabatic wall temperature and the local heat transfer coefficient over the complete surface of the target plate. The influence of the temperature of the plate through which the impingement gas flows on the target plate heat transfer has been quantified. Results are presented for a single in-line array configuration over a range of jet Reynolds numbers

Application of a Complex Nusselt Number to Heat Transfer During Compression and Expansion

Abstract: Heat transfer during compression and expansion can be out of phase with bulk gas-wall temperature difference. An ordinary convective heat transfer model is incapable of predicting this phenomenon. Expressions for compression/expansion heat transfer developed from simple conduction models use a complex heat transfer coefficient. Thus, heat flux consists of one part proportional to temperature difference plus a second part proportional to rate of change of temperature. Surface-averaged heat flux was calculated from experimental pressure-volume data for piston-cylinder gas springs over a range of speeds, pressures, gases, and geometries. The complex Nusselt number model proved capable of correlating both magnitude and phase of the measured heat transfer as functions of an oscillation Peclet number.

Member having internal cooling passage

Abstract: The present invention effectively cools members with a small amount of cooling air. Turbulence promotor ribs are formed so that cooling fluid along a wall flows from a center of the wall to both end portions of the wall. A highly enhanced thermal conducting effect, namely high cooling heat transfer coefficient, can be obtained, and it is possible to cool members effectively with the small amount of cooling air.