J.W. Ramsey

Freezing on a finned tube for either conduction-controlled or natural-convection-controlled heat transfer

Experiments were performed to study freezing on a finned vertical tube when either conduction in the solid or natural convection in the liquid controls the heat transfer. Conduction is the controlling mode when the liquid is at its fusion temperature, whereas natural convection controls when the liquid temperature is above the fusion value. The phase change medium was a paraffin, 99% pure n-eicosane, with a fusion temperature of 36.4°C. Auxiliary experiments were also performed with an unfinned tube to obtain comparison data. For conduction control, the enhancement of freezing due to finning is less than the area ratio of the finned and unfinned tubes, whereas for natural-convection control the enhancement is very nearly equal to the area ratio. The liquid-solid interface is a thicket of whisker-like crystals when conduction controls but is straight (i.e. vertical). On the other hand, the interface is smooth but tapered when natural convection controls—yielding bottom-heavy frozen specimens. When conduction controls, freezing continues more or less indefinitely, whereas natural convection severely retards the freezing and ultimately terminates it altogether.

Film cooling with injection through holes - Adiabatic wall temperatures downstream of a circular hole.

Experiments to determine film cooling with air injection through holes into turbulent air boundary layer on flat plate

Heat transfer and pressure drop for a staggered wall-attached array of cylinders with tip clearance

Abstract Experiments have been performed to determine the detailed row-by-row heat-transfer characteristics of a staggered array of circular cylinders situated in a crossflow of air in a flat rectangular duct. The cylinders were attached perpendicular to one of the principle walls of the duct, with a clearance space between the free ends of the cylinders and the other principle wall. The heat-transfer coefficients were obtained by applying the analogy between heat and mass transfer to mass-transfer coefficients measured via the napthalene sublimation technique. Pressure measurements were also made to determine the incremental pressure drop due to the presence of the cylinder array. The row-by-row transfer coefficients were found to vary only in the initial rows and to attain a constant fully developed value for the fourth and all subsequent rows. The fully developed transfer coefficients are quite insensitive to cylinder height (i.e. to the extent of the tip clearance), increasing moderately as the height increases (and the clearance decreases). On the other hand, the array pressure drop increases markedly with increasing cylinder height.

Freezing controlled by natural convection

Experiments were performed for freezing under conditions where the liquid phase is either above or at the fusion temperature (i.e., superheated or nonsuperheated liquid). The liquid was housed in a cylindrical containment vessel whose surface was maintained at a uniform, time-invariant temperature during a data run, and the freezing occurred on a cooled vertical tube positioned along the axis of the vessel. The phase change medium was n-eicosane, a paraffin which freezes at about 36/sup 0/C (97/sup 0/F). In the presence of liquid superheating, the freezing process is drastically slowed and ultimately terminated by the natural convection in the liquid. The terminal size of the frozen layer an the time at which freezing terminates can be controlled by setting the temperature parameters which govern the intensity of the natural convection. The stronger the natural convection, the thinner the frozen layer an the shorter the freezing time. In the absence of liquid superheating, a cylindrical frozen layer grows continuously as predicted by theory, but the growth rate is higher than the predictions because of the presence of whisker-like dendrites on the freezing surface.

Effect of surface roughness on the total hemispherical and specular reflectance of metallic surfaces.

Measurements have been made of the hemispherical and specular reflectance of metallic surfaces of controlled roughness. The surfaces, which were ground nickel rectangles, were irradiated at various angles of incidence by a beam of black-body radiation, the temperature of which was also varied. The instrumentation which was devised to perform the experiments is described. The measurements show that beyond a certain surface roughness, the hemispherical reflectance is virtually independent of further increases in roughness. On the other hand, the specular reflectance decreases steadily with increasing roughness. Additionally, the hemispherical reflectance is found to be quite insensitive to the angle of incidence, while the specular reflectance increases with angle of incidence for the rougher surfaces.

Photographic study of melting about an embedded horizontal heating cylinder

INTRODUCTION THE OBJECTIVE of this note is to provide direct visual information to supplement and extend the results of [l] relevant to the role of natural convection in the melting of solids. In [l], experiments were described in which a horizontal heating cylinder was embedded in a solid at its fusion temperature. By energizing the heater with a constant power input, a steadily growing melt region surrounding the cylinder was created. The heating cylinder was instrumented to provide information about the surface heat-transfer coefficient, and a grid of 92 thermocouples was deployed throughout the phase change medium to detect the passage of the solid-liq~d interface. The experiments were performed with a eutectic mixture of sodium hydroxide and sodium nitrate [fusion temperature u 244°C (471”F)l. The experiments demonstrated that natural convection was the dominant mode of heat transfer. One of the evidences of this dominance was the shapes of the melt region. These shapes were inferred from temperature measurem~ts in the phase change medium. Owing to mechanicat constraints, it was not possible to make detailed temperature measurements below and at the sides of the cylinder, so that the shapes presented in [l] were confined to the region above the cylinder. In the present investigation, photographic evidence of actual meh layer shapes was obtained to corroborate those inferred in [l]. The photographs show the melt Iayer both below and above the cylinder, thereby extending the results of [l]. Another extension was made in connection with the initial state of the solid at the onset of melting. Whereas in [I] all of the experiments were concerned with a solid at its fusion temperature, the present experiments encompassed both that condition and an initially subcooled condition (i.e. initial temperature of the solid lower than the fusion temperature). The general approach used in the investigation was to embed a horizontal heating cylinder in a solid phasechange material and to create a growing melt region about the cylinder by a steady input of heat. At a preselected time, the heating was discontinued and the liquid was extracted from the melt region by means of suction and gravity. The unmetted solid was allowed to cool to room temperature and then was removed from the test apparatus and cut along a vertical plane, thereby revealing the cross section of the melt cavity. To obtain a clear photographic record of the shape of the cavity, it was filled with a dark sand. The experiments were performed using the same eutectic salt as in [l J.

Heat transfer—a review of 1976 literature

INTRODUCTION THIS review surveys results that have been published in various fields of heat transfer during 1976. As in the past, the number ofpapers published during that period was such that only a selection can be included in this review. A more complete listing of papers is available in the heat-transfer bibliographies published periodically in this journal. The Sixteenth National Heat Transfer Conference was held 8-11 August 1976 in St. Louis, Missouri. The recipient of the Max Jakob Memorial Award, R. G. Deissler, presented an invited lecture, ‘“Tornadoes and other atmospheric vortices”. A lecture entitled “An inquiry of selected topics on heat-exchange design” was given by A. C. Mueller, who received the 1975 Donald Q. Kern Award. A third invited lecture on “Water reactor safety research programs” was presented by L. S. Tong. Twenty-eight sessions treated, among other subjects, interfacial phenomena, internal heat generation, process heat transfer, thermonuclear power, and heat transfer in foodstuffs. The papers presented at the conference are available as preprints and many of these will be published in the Journal ofHeat Transfer or in the publication series of the American Institute of Chemical Engineers. The 25th Heat Transfer and Fluid Mechanics Institute was held 21-23 June 1976 at the University of California, Davis, California. An invited lecture by P. Bradshaw discussed progress and problems in the development of turbulence models; another lecture by R. Davis treated numerical methods for interacting boundary layers. Five sessions were concerned with thermal convection, two-phase flow and boiling, compressible and turbulent flow, combustion, and aerospace heat transfer problems. Proceedings are available through Stanford University Press. The Scientific Affairs Division of the North Atlantic Treaty Organization assembled a group of experts to review research needs on thermal energy storage and to make recommendations for future research. One of the panels discussed heat transfer and thermal energy transport. A report of the conference is available through the Scientific Affairs Division, North Atlantic Treaty Organization, Brussels, Belgium. An International Solar Energy Conference was held 15-20 August 1976 at Winnipeg, Canada under the title “sharing the Sun 76”. Many of the papers presented included heat-transfer problems. An advanced course and the 1976 International Seminar were organized by the International Centre for Heat and Mass Transfer 23 August through 4 September 1976 at Dubrovnik, Yugoslavia. The summer course was entitled “Heat Disposal from Power Generation”, and dealt primarily with cooling towers and climatic modifications by energy production. The seminar, “Turbulent buoyant convection”, discussed plumes, stratified fluids, air and smoke movements in buildings, and combustion phenomena. The proceedings of these conferences will be published by Scripta Book Company, Washington, D.C. The American Institute of Chemical Engineers focussed attention on plasma chemical processes in four sessions of its Eighty-Second National Meeting held 29 August through 1 September 1976 at Atlantic City, New Jersey, with heat-transfer information included in some of the papers. Other sessions dealt with transport processes in the oceans and with interfacial phenomena. The 97th ASME Winter Annual Meeting, held 5-10 December 1976 at New York City, included in its program eleven sessions organized by the Heat Transfer Division. Topics ranged from application of computer technology through electric effects, liquid metal fast breeder reactors, thermal energy storage, pipe line heat transfer, heat exchangers, to solar collectors. Reprints of the papers are available from ASME Headquarters and many of them will also be published in the Journal of Heat Transfer. The dinner speaker, P. E. Glaser, talked on ‘The future of power from space”. Heat Transfer Memorial Awards were presented to W. H. Giedt and R. Viskanta. A number of books dealing with heat transfer or including heat-transfer topics have appeared on the market. They are listed in the bibliographic portion of this review. Volume 12 of the series, Advances in Heat Transfer, published by Academic Press, New York, became available in 1976. It contains contributions on dry cooling towers, heat transfer in flows with drag reduction, molecular gas band radiation, and a perceptive on electrochemical transport phenomena. Developments in heat-transfer research during 1976 can be characterized by the following highlights: solidliquid phase change, fins, and contact resistance found special attention in the area of heat conduction. A number of numerical solutions have also been reported. Flow and heat transfer in complex passages and 1097

An experimental determination of the heat and mass transfer coefficients in moist, unsaturated soils

Abstract Ratios of the thermal diffusion coefficient to the moisture diffusion coefficient were experimentally determined for two types of soils. The ratio of the diffusion coefficients was found to increase with increasing moisture content, reach a broad maximum, and thereafter decrease. Except for the drier regions near the warm end, the ratio was found to be in the range 10 −3 -10 −20 C −1 . Analytical predictions for the thermal diffusion coefficients were combined with the experimental results to determine the moisture diffusion coefficients. The thermal diffusion coefficient was predicted to decrease with both decreasing temperature and increasing moisture content. For the soils used in this study, the moisture diffusion coefficient was estimated to be in the range 10 −5 -10 −4 g s −1 cm −1 .

The Transition from Natural-Convection-Controlled Freezing to Conduction-Controlled Freezing

Experiments were performed to study the transition between freezing controlled by natural convection in the liquid adjacent to a freezing interface and freezing controlled by heat conduction in the solidified material. The freezing took place on a cooled vertical tube immersed in an initially superheated liquid contained in an adiabatic-walled vessel. At early and intermediate times, temperature differences throughout the liquid induce a vigorous natural convection motion which retards freezing, but the temperature differences diminish with time and natural convection ebbs. At large times, the freezing rate is fully controlled by heat conduction in the solidified material. The frozen specimens for short and intermediate freezing times are smooth-surfaced and tapered, while those for large times are straight-sided and have surfaces that are overlaid with a thicket of large discrete crystals. These characteristics correspond respectively to those of natural-convection- controlled freezing and conduction-controlled freezing. At early times, the measured mass of the frozen material is identical to that for natural-convection-controlled freezing and conduction-controlled freezing. At early times, the measured mass of the frozen material is identical to that for natural-convection-controlled freezing. At later times, the frozen mass tends to approach that for conduction-controlled freezing, but a residual deficit remains.

Heat transfer—a review of 1975 literature

THIS review surveys results that have been published in various fields of heat transfer during 1975. As in the past, the number ofpapers published during that period was such that only a selection can be included in this review. A more complete listing is available in the heattransfer bibliographies published periodically in this journal. will also be published in the Journul of Heat Transfer. The 1975 International Solar Energy Congress and Exposition was organized by the International Solar Energy Society from 28 July to 1 August at Los Angeles, California. Heat-transfer topics are found interwoven in many of the papers presented at the Conference.