Showing papers on "Heat transfer coefficient published in 1969"

Hyperbolic Heat-Conduction Equation—A Solution for the Semi-Infinite Body Problem

Abstract: Heat finite propagation velocity effects on temperature distribution and heat flux for step temperature change at semiinfinite body surface

Evaluation of internal heat-transfer coefficients for impingement-cooled turbine airfoils.

Abstract: Although internal impingement cooling of the leading edge of gas-turbine airfoils has been shown to be effective, previously available heat-transfer data are not generally applicable to present-day turbine designs because of the unique geometry requirements. An experimental system which allows the ready acquisition of heat-transfer data necessary for thermal design of turbine airfoils is described. A cold-flow model is developed, and the measurement of local heat-transfer coefficients on a full-size model is accomplished by considering Joulean dissipation in very thin platinum strips bonded to the model. Heattransfer results are given which show the dependence of Nusselt number on Reynolds number, geometry, and chordwise location on the inside leading-edge region of the airfoil. Dimensionless correlations are presented which allow the designer to predict heat transfer for impingement cooling in these geometries for the range of parameters tested.

Deterioration in Heat Transfer to Fluids at Supercritical Pressure and High Heat Fluxes

Abstract: At slightly supercritical pressure and in the neighborhood of the pseudo-critical temperature (defined as the temperature corresponding to the peak in specific heat at the operating pressure), the heat transfer coefficient between fluid and tube wall is strongly dependent on the heat flux. For large heat fluxes, a marked deterioration takes place in the heat transfer coefficient in the region where the bulk fluid temperature is below and the wall temperature above the pseudo-critical temperature. An analysis has been developed, based on the integration of the transport equations, to predict the deterioration in heat transfer at high heat fluxes, and the results have been compared with the previously available experimental results for steam. Experiments have been performed with carbon dioxide for additional comparison. Limits of safe operation in terms of the allowable heat flux for a particular flow rate have been determined both theoretically and experimentally. Experiments with twisted tape inserted in the test section to generate swirl have shown that the heat transfer rates can be improved by this method. Qualitative visual observations have been made of the flow under varying conditions of heat flux and flow rate.

An Experimental Study of Turbulent Natural Convection Boundary Layers

Abstract: An experimental investigation on turbulent natural convection boundary layers has been conducted with water on a vertical plate of constant heat flux. Local heat transfer data are presented for laminar, transition, and turbulent natural convection, with the emphasis on the turbulent regime. The data extend to a modified Rayleigh number of 1016 for a threefold range in Prandtl number. The results indicate that natural transition occurs in the range 1012 < Ra* < 1014 ; i.e., fully developed turbulent flow occurs by Ra* = 104 . This latter value can be as low as 2 × 1013 with the use of a trip rod. The physical structure of the turbulent boundary-layer flow was studied using the combined time-streak marker hydrogen bubble method. Temperature data and temperature corrected velocity data obtained by hot-film sensors are presented for Ra* values between 8.7 × 1013 and 7.1 × 1014 . For the range of variables investigated, the major conclusions are (a) the local heat transfer coefficient exhibits a slight decrease with length, (b) confirmation that the vortex street layer in the transition region decays into a longitudinal-vortex-type structure, and (c) the outer portion of the thermal and velocity fields can be approximated by power profiles that fit almost all the data available to date.

Real Gas Effects in Carbon Dioxide Cycles

Abstract: The potential performance of carbon dioxide as working fluid is recognized to be similar to that of steam, which justifies thorough thermodynamic analysis of possible cycles.The substantially better results achievable with CO2 with respect to other gases are due to the real gas behaviour in the vicinity of the Andrews curve. Simple cycles benefit from the reduced compression work, but their efficiency is compromised by significant losses caused by irreversible heat transfer. Their economy, however, is appreciably better than that of perfect gas cycles. More complex cycle arrangements, six of which are proposed and analyzed in detail, reduce heat transfer losses while maintaining the advantage of low compression work and raise cycle efficiency to values attained only by the best steam practice.Some of the cycles presented were conceived to give a good efficiency at moderate pressure which is of particular value in direct-cycle nuclear applications.The favourable influence on heat transfer coefficients of the combined variation with pressure of mechanical, thermal and transport properties, due to real gas effects, is illustrated. Technical aspects as turbo-machines dimensions and heat transfer surfaces needed for regeneration are also considered.Cooling water requirements are found to be not much more stringent than in steam stations.Copyright © 1969 by ASME

Effects of ultrasonic vibrations on heat transfer to liquids by natural convection and by boiling

Abstract: The effects of ultrasonic vibrations on heat transfer to water and methanol by natural convection and by boiling were measured at three ultrasonic energy levels with frequency ranging from 20.6 to 306 kcycles/sec., using electrically heated platinum wires of diameters 0.007 and 0.010 in. Up to an eight-fold increase in heat transfer coefficient was obtained in natural convection, but the effects diminished with increased temperature difference and became negligible in the well-developed nucleate boiling region. High-speed photographs showed that the increase was due to the motion of cavitation bubbles on the wire surface. The heat transfer results were correlated by local cavitation activity values measured by a technique developed for this work.

Buoyancy Effects in Forced-Convection Flow and Heat Transfer

Abstract: An analysis is made for the effects of buoyancy forces in laminar forced-convection on a vertical flat plate. In general, since it is difficult to obtain an exact solution, an approximate similarity solution is given by introducing the assumption that a role of Gr/Re2 in stream function, [numerical formula] is negligibly small as compared with η. The validity of this assumption is examined by comparing with the experimental data of Kliegel. Numerical calculations are performed for both the constant fluid properties and the variable fluid properties, and the results are compared with Szewczyk's solution previously obtained by perturbation analysis. The velocity and temperature distribution, the friction factor and heat transfer coefficient are given for various values of the parameter Gr/Re2.

Convective Heat Transfer in a Gas-Fired Pulsating Combustor

Abstract: The effect of longitudinal combustion-driven oscillations on convective heat transfer rates was studied using a tubular, propane-fired combustor which resonated as a quarter-wavelength organ pipe at a frequency of 100 cps. The combustor was provided with damping tubes of variable length which enabled the amplitude of the oscillations to be varied, to the extent of damping them out completely. Heat transfer coefficients were measured with and without the presence of combustion-driven oscillations. It was found that heat transfer coefficients were highest at a position of maximum velocity amplitude, where improvements of over 100 percent were obtained. Satisfactory prediction of the effects of the oscillations was obtained by using the quasisteady-state theory.

Mass and heat transfer relations in evaporation through porous membranes

Abstract: This study concerns rates of evaporation and mass transfer of water vapor from a heated salt solution through a water repellent porous membrane to a cooled water condensate. This transfer is a result of temperature differences and corresponding vapor pressure differences across the membrane. Three groups of experiments were carried out which indicate that the major factor influencing the rates of transfer is diffusion through a stagnant gas in the membrane pores. However, an equation considering film heat transfer coefficients, membrane thermal conductivity, and an empiricial correction based on temperature driving force appears to be necessary for representing all the data. The empirical correction appears to be related to internal condensation and possibly diffusion along surfaces.

Non-Boiling and Film Boiling Heat Transfer to a Saturated Bath of Liquid Helium

Abstract: It is well known that the liquid phase of the normal isotope 4He exhibits some unique physical properties and thus, heat transfer characteristics [1–5]. In discussing previous investigations of heat transfer from solid surfaces to the superfluid, helium-II, it is convenient to distinguish three regions: (1) nonfilm boiling; (2) transition to film boiling; and (3) film boiling.

Turbulent Flow and Heat Transfer in the Entrance Region of a Parallel Wall Passage

Abstract: Measurements of boundary layer development and heat transfer were made in the entrance region of a parallel passage and compared with a computer solution based on the law of the wall. Little difference was found between the heat transfer, both measured and predicted, with a developing flow and that predicted with a fully developed flow. The experiments also show that boundary layer parameters, such as momentum thickness, do not approach their fully developed values asymptotically.

Effect of Vibration on Heat Transfer From Spheres

Abstract: The effect of vibration on free and forced convection heat transfer from spheres was investigated. Test spheres made of copper were subjected to sinusoidal vibration in the vertical plane, this being perpendicular to the direction of airstream in the case of forced convection studies. In free convection studies the amplitude of vibration was varied from 4 mm to 25.5 mm and the frequency of vibration from 150 cpm to 930 cpm. It was found that the effect of vibration on Nusselt number was negligible for values of vibrational Reynolds number less than 200. For values of vibrational Reynolds number greater than 200, the vibration increased the heat transfer coefficient considerably and values of heat transfer coefficients as high as seven times the free convection values without vibration were obtained. The following correlations were obtained for heat transfer from spheres to air: free convection without vibration, NNu = 2 + 0.401 (NGr)0.25 for 4 × 103 < NGr < 6 × 104 and free convection with vibration: hvho = 0.83 (NRe)v0.5(a/D)0.1(NGr)0.251.28 In the case of forced convection studies with vibration, the amplitude of vibrations varied between 4 mm and 12.4 mm, and the frequency of vibration from 200 cpm to 1600 cpm. The flow velocity was varied from 24.5 ft/sec to 84 ft/sec. The results in the absence of vibration could be represented by: NNu = 0.304 (NRe )0.56 or NNu = 2 + 0.222 (NRe )0.587 in the range 6 × 103 < NRe < 3.3 × 104 . Nusselt numbers were not found to be affected by the imposition of vibrational velocity even as high as 19.6 percent of the flow velocity.