Showing papers on "Heat transfer coefficient published in 1988"

Natural convection in enclosures

Abstract: Publisher Summary This chapter discusses the complex nature of the natural convection phenomena in enclosures It discusses the two basic configurations of natural convection— that is, a rectangular cavity and a horizontal circular cylinder In rectangular cavities, consideration is given to the two-dimensional convective motion generated by the buoyancy force on the fluid in a rectangle and to the associated heat transfer The two long sides are vertical boundaries held at different temperatures and the short sides can either be heat conducting or insulated Particular attention is given to the different flow regimes that can occur and the heat transfer across the fluid space between the two plane parallel vertical boundaries Although heat transfer by radiation may not be negligible it is independent of the other types of heat transfer and can be fairly accurately calculated separately To formulate the boundary value problem that describes this phenomena it is assumed that: (a) the motion is two-dimensional and steady, (b) the fluid is incompressible and frictional heating is negligible, and (c) the difference between the hot wall and cold wall temperatures is small relative to the absolute temperatures of the cold wall In horizontal circular cylinder, consideration is given to the large Rayleigh number flow with the Prandtl number large and the Grashof number of unit order of the magnitude

Heat transfer and friction characteristics in rectangular channels with rib turbulators

Abstract: The effect of the channel aspect ratio on the distribution of the local heat transfer coefficient in rectangular channels with two opposite ribbed walls (to simulate turbine airfoil cooling passages) was determined for a Reynolds number range of 10,000 to 60,000. The channel width-to-height ratios (W/H, ribs on side W) were 1/4, 1/2, 1, 2, and 4. The test channels were heated by passing current through thin, stainless steel foils instrumented with thermocouples. The local heat transfer coefficients on the ribbed side wall and on the smooth side wall of each test channel from the channel entrance to the fully developed regions were measured for two rib spacings (P/e = 10 and 20). The rib angle-of-attack was kept at 90 deg. The local data in the fully developed region were averaged and correlated, based on the heat transfer and friction similarity laws developed for ribbed channels, to cover the ranges of channel aspect ratio, rib spacing, rib height, and Reynolds number. The results compare well with the published data for flow in a square channel with two opposite ribbed walls. The correlations can be used in the design of turbine airfoil cooling passages.

Turbulent Transport on the Endwall in the Region Between Adjacent Turbine Blades

Abstract: The complex three-dimensional flow in the endwall region near the base of a turbine blade has an important impact on the local heat transfer. The initial horseshoe vortex, the passage vortex, and resulting corner vortices cause large variations in heat transfer over the entire endwall region. Due to these large surface gradients in heat transfer, conventional measurement techniques generally do not provide in accurate determination of the local heat transfer coefficients. In the present study the heat/mass transfer analogy is used to examine the local transport coefficients for two different endwall boundary layer thicknesses and two free-stream Reynolds numbers. A linear turbine blade cascade is used in conjunction with a removable endwall plate. Napthalene (C{sub 10}H{sub 8}) is cast into a mold on the plate and the rate of naphthalene sublimation is determined at 6,000+ locations on the simulated endwall by employing a computer-aided data acquisition system. This technique allows one to obtain detailed contour plots of the local convection coefficient over the entire endwall. By examining the mass transfer contours, it is possible to infer information on three-dimensional flow in the passage between the blades. Extremely high transport coefficients on the endwall indicate locations of potential overheating and failuremore » in actual turbine.« less

Heat transfer from a flat surface to an oblique impinging jet

Abstract: Experiments are conducted to determine the heat transfer to a jet impinging at different oblique angles to a plane surface The main portion of the test plate contains a composite sheet of temperature-sensitive liquid crystal, which is sandwiched between a thin metallic-foil heater and a specially designed liquid bath The results indicate a displacement of the peak heat transfer from the geometric center of the jet origin, the displacement being a function primarily of impingement angle Contours of constant heat transfer coefficient are obtained and correlated with an empirical equation that permits determination of average Nusselt numbers over areas of interest

Heat transfer in turbulent mixed convection

Abstract: The contents of this book are: Basic Equations of Convective Heat Transfer; Basic Information on the Theory of Turbulent Heat Transfer in Flow Near Walls; Heat Transfer in Laminar Mixed Convection; Turbulent Mixed Convection in Boundary Layers; Turbulent Flow and Heat Transfer in Horizontal Channels; Turbulent Flow and Heat Transfer in Vertical Channels; and Gravitational Effects on Heat Transfer in a Single-Phase Fluid Near the Critical Point.

Stagnation Film Cooling and Heat Transfer, Including Its Effect Within the Hole Pattern

Abstract: Etude du refroidissement par film avec point d'arret ainsi que des transferts de chaleur en tenant compte de l'influence de la distribution des orifices

Wall and particle-shape effects on heat transfer in packed beds

Abstract: The effective radial thermal conductivity and apparent heat transfer coefficient for a packed bed were experimentally determined for beds of spheres, full cylinders and hollow cylinders, for flow rates giving particle Reynolds numbers in the range 100-1000, and for tube to particle diameter ratios of 5-12. Over these ranges the radial Peclet number Per showed significant dependence on solid conductivity, gas flow rate and particle shape, while the wall Biot number Bi showed significant dependence on tube to particle diameter ratio, gas flow rate and particle shape. These dependencies were predicted well by equations incorporating the effects of these variables into individual gas and solid phase parameters, which were then combined to give the effective or lumped parameters

Circulating fluidized bed heat transfer

Abstract: Heat transfer to the walls of a circulating bed is due to conduction from clusters of particles falling along the walls, thermal radiation, and convection to uncovered surface areas. Important hydrodynamic factors are the fraction of the wall covered by particles and the average contact time of particles at the wall. Small active heat transfer probes will give an upper limit to the heat transfer in a circulating bed. Experimental results show lower heat transfer coefficients for larger walls; heat transfer is limited by the temperature change of particle clusters moving down the walls. The contact time for clusters is related to their average displacement before breakup. For displacements of 15 cm or less, the cluster displacement approximates constant gravitational acceleration. The most pressing needs for commercial bed designers are heat transfer and hydrodynamic results valid for large diameter beds.

Subcooled Water Flow Boiling Experiments Under Uniform High Heat Flux Conditions

Abstract: This work involves steady-state high heat flux removal in fusion reactor beam dumps, first walls in compact fusion reactors, and other applications with smooth surfaces and/or irregular coolant channel cross sections. In such applications, the coolant pressure is required to be low (/approx/ 1.0 MPa), and the coolant channels are moderately long (length-to-diameter ratio (L/D) /approx/ 100). The present experiments have resulted in high heat flux data in a region where only sparse data existed. Subcooled flow boiling measurements were performed for the critical heat flux (CHF), local (axial) variations of the coolant channel's heat transfer coefficients, and pressure drop for horizontal, uniformly heated tubes. The tubes had inside diameters of 0.3 cm, a heated L/D ratio of 96.6, and were made of amzirc (zirconium-copper). The coolant was degassed, deionized water. The exit pressure and the inlet water temperature were held approximately constant at 0.77 MPa and 20/sup 0/C, respectively. From experiment to experiment, the inlet temperature varied slightly (+- 1.5/sup 0/C) from 20/sup 0/C. The actual measured inlet temperature was used in reducing the experimental data. Measurements of the above quantities were performed for the mass velocity and exit subcooling, varying from 4.6 to 40.6 Mg/m/sup 2/ . smore » and 30 to 74/sup 0/C, respectively. For these ranges, (a) the subcooled flow boiling CHF varied from 625 to 4158 W/cm/sup 2/, (b) the heat transfer coefficients during fully developed nucleate boiling varied from 30 to 400 kW/m/sup 2/ . K (Nusselt number = 150 to 1700), and (c) the overall pressure drop varied from -- 1.75 to 0.9 times the adiabatic pressure drop. For the flow conditions and geometry parameters given above, least-squares equations for the CHF were developed in terms of both the liquid Reynolds number and the exit subcooling. The average percent deviation of the developed equations from the data was <12.5%.« less

Effects of coolant on temperature distribution in metal machining

Abstract: The effects of coolant on tool temperature in metal turning operations were studied both experimentally and theoretically. Values for the coefficient of heat transfer from the tool to the coolant were obtained. Experimentally, a medium carbon steel was turned at a feed rate of 0·254 mm rev −1, at cutting speeds of 33–61 m min−1, with high speed steel tools and water based coolant. Tool temperatures estimated from structural changes in the tool were compared with values calculated by finite element analysis. Heat transfer coefficients from tool to coolant between 103 and 5 × 103 W m2K−1, depending on coolant flow rate, best match theory and experiment. Further, it is shown by the calculations that tool temperatures are particularly sensitive to changes in heat transfer coefficient between 103 and 104 W m−2K−1. This is the range in which coefficients are to be found in practice, explaining the critical nature of cutting fluid formulation, supply rate, and direction of application in those operations...

Numerical prediction of laminar flow and heat transfer in wavy channels of uniform cross-sectional area

Abstract: Fluid flow and heat transfer in two-dimensional wavy channels of uniform cross-sectional area were analyzed numerically for constant-property laminar flow. The channel walls were maintained at constant temperature. Streamwise periodic geometry caused the flow and heat transfer to repeat periodically far downstream of the channel entrance. Computations were performed for different Reynolds numbers and geometric parameters. Prandtl number was kept constant (0.707). The whole solution domain was divided into five regions, and different coordinates were used for each region. The periodic boundary conditions were implemented by mutual replacements of field values at the two end regions. It was found that both Nusselt number and friction factor were greatly affected by Reynolds number and two geometric parameters.

Package thermal resistance model dependency on equipment design

Abstract: A physical model is presented that describes mechanisms for operating-equipment junction-to-ambient thermal resistance in excess of a typical component manufacturer's data-sheet value by as much as a factor of four under constant cooling conditions. The model accounts for the discrepancy between system thermal performance of a package and data-sheet thermal resistance value which are not accompanied by qualifying data in the form of junction-to-header thermal resistance, board temperature rise over ambient, convection coefficient, mounting sensitivity, and power dissipation. The eight constants used to predict inherent increases in package thermal resistance when going from the data-sheet-specified operating conditions to the excess-value conditions are described. These constants and procedures for obtaining them are given for dual in-line packages (DIPs), pin-grid arrays (PGAs), small-outline transistors (SOTs), and plastic leaded chip carriers (PLCCs). >

An analysis on forced convection in a channel filled with a Brinkman-Darcy porous medium: exact and approximate solutions

Abstract: An analysis was made to investigate non-Darcian fully developed flow and heat transfer in a porous channel bounded by two parallel walls subjected to uniform heat flux. The Brinkmanextended Darcy model was employed to study the effect of the boundary viscous frictional drag on hydrodynamic and heat transfer characteristics. An exact expression has been derived for the Nusselt number under the uniform wall heat flux condition. Approximate results were also obtained by exploiting a momentum integral relation and an auxiliary relation implicit in the Brinkmanextended Darcy model. Excellent agreement was confirmed between the approximate and exact solutions even in details of velocity and temperature profiles.

Heat Transfer With Insulated Combustion Chamber Walls and Its Influence on the Performance of Diesel Engines

Abstract: Recently great expectations were put into the insulation of combustion chamber walls. A considerable reduction in fuel consumption, a marked reduction of the heat flow to the cooling water, and a significant increase of exhaust gas energy were predicted. In the meantime there exists an increasing number of publications reporting on significant increase of fuel consumption with total or partial insulation of the combustion chamber walls. In [1] a physical explanation of this effect is given: Simultaneously with the decrease of the temperature difference between gas and wall as a result of insulation, the heat transfer coefficient between gas and wall increases rapidly due to increasing wall temperature, thus overcompensating for the decrease in temperature difference between gas and wall. Hence a modified equation for calculation of the heat transfer coefficient was presented [1]. In the paper to be presented here, recent experimental results are reported that confirm the effects demonstrated in [1], including the influence of the heat transfer coefficient, which depends on the wall temperature, on the performance of naturally aspirated and turbocharged engines.

Experimental investigation of heat transfer in coiled annular ducts

Abstract: Forced convection heat transfer in coiled annular ducts was investigated experimentally. Average heat transfer coefficients were obtained for both laminar and transition flows. Two coiling diameters and two annulus radius ratios were used in the study. The data were correlated with Dean number and Reynolds number separately and compared with the available studies of coiled circular tubes and straight annular ducts. It was found that coiling augments the heat transfer coefficients above the values for a straight annulus especially in the laminar region. However, the augmentation is less than would be expected for a coiled circular tube. The augmentation decreases as the flow enters the transition region.

Fiber optic coupler

Abstract: A fiber optic coupler and method for forming such coupler is shown in which the substance of the fibers is preselected to have a heat fusing temperature in a range achievable by exterior heating of a shrink sleeve, and the fibers are heat fused together along a limited length region within the shrunken sleeve as a result of heat and inter-fiber pressure applied to the fibers by exterior heating of the shrink sleeve. In preferred embodiments, a filler rod is disposed alongside the optical fibers and pressed thereagainst by the shrink sleeve, the exterior of the rod preferably of fluorocarbon plastic, having an index of refraction lower than the index of refraction of the fibers in the fused region. The interior of the filler rod preferably comprises an elongated heat transfer element having a heat transfer coefficient greatly exceeding the heat transfer coefficient of the fibers. A plastic coupler is also featured in which the cladding layer is completely absent in a fused region, and the cores in that region are deformed and fused together in a core-mode coupling relationship as the result of application of inter-fiber pressure under fusing conditions, preferably heat fusing conditions.