Influence of surface roughness and waviness upon thermal contact resistance

This work deals with the phenomenon of thermal resistance between contacting solids. Attention is directed towards contiguous solids possessing both surface roughness and waviness. When two such surfaces are brought together under load, they actually touch at isolated microcontacts, and the resulting real area is the sum of these microcontacts. Because of the waviness the microcontacts are confined to a region called the contour area which may occupy some fraction of the total available area. The non-uniform pressure distribution over the contour area results in microcontacts which vary in size and density. In the absence of an interstitial fluid and negligible radiation heat transfer, all the heat crossing the interface must flow through the microcontacts. A thermal analysis, based on size and spatial distribution, results in a thermal resistance equation which differs from previously developed theories. The equation is verified by liquid analog tests which show that the size and spatial distribution are very significant.(cont.) A surface deformation analysis considers the influence of surface roughness upon the elastic deformation of a rough hemisphere. An equation is developed which shows the extent of the contour area as a function of the surface geometry, the material properties, and the applied load. The equation is compared with existing theories and qualitatively checked against experimental results. Experimental heat transfer data were obtained to verify the thermal and deformation theories. The agreement between theory and test is quite good over a large range of surface geometry and applied loads.Sponsored by the National Aeronautics and Space Administration DS

Forced-convection surface-boiling heat transfer and burnout in tubes of small diameters

A basic heat-transfer apparatus was designed and constructed for the study of forced-convection boiling in small channels. The various regions of forced-convection surface boiling were studied experimentally and analytically. In the region of low wall superheat, the heat flux can be predicted by available correlations for forced convection. Data indicate, however, that these correlations do not properly account for the radial variation of properties for water at high temperature difference. The conventional Dittus and Boelter-McAdams relationship is recommended for design purposes on the basis of its simplicity and conservative predictions. An analysis for the prediction of the inception of first significant boiling was developed. Experimental results are in good agreement with analytical predictions. The analysis provides information necessary for the prediction of the complete forced-convection surface-boiling curve. Data for a small-diameter tube indicate that the bubbles formed at incipient boiling can trip the laminar or transition boundary layer to a fully-developed turbulent boundary layer. The region of vigorous boiling coincides approximately with the extrapolation of the pool-boiling curve in one set of experiments. In other experiments, pool-boiling data were strongly influenced by fluid and surface conditions, as well as by bubble-induced convection in the pool. Due to the complexities in these pool-boiling data, it is impossible to make a conclusive comparison with forced-convection-boiling data. The heat flux obtained by a superposition of pool boiling and forced convection is close to the apparent asymptote for fully-developed boiling. For design purposes, it is concluded that fully-developed -forced-convection boiling can be related to pool boiling by either direct extrapolation or superposition of forced convection.(cont.) The burnout heat flux under conditions of forced convection and surface boiling is shown to be a complicated function of subcooling at low values of subcooling. This appears to be due to the velocity increase caused by the relatively large volume fraction of vapor. Burnout flux is shown to increase with decreasing tube diameter. This effect can be attributed to an increase in void fraction with decreasing tube diameter. Entrance effects are significant in forced-convection surface boiling as shown by the decrease of burnout flux with increasing length. Flow oscillations caused by system compressibility can greatly reduce the burnout heat flux in the subcooled region. This instability is particularly difficult to avoid with tubes of very small diameter.Air Force Office of Scientific Research D.S.R. Projec

Film boiling of saturated liquid flowing upward through a heated tube : high vapor quality range

Film boiling of saturated liquid flowing upward through a uniformly heated tube has been studied for the case in which pure saturated liquid enters the tube and nearly saturated vapor is discharged. Since a previous study at the M.I.T. Heat Transfer Laboratory covered the case in which only a small percentage of the total mass flow is vaporized, this investigation has been concentrated on film boiling in the region where the vapor quality is greater than 10 percent. Visual studies of film boiling of liquid nitrogen flowing through an electrically conducting pyrex tube have been made to determine the characteristics of the two-phase flow regimes which occur as a result of the film boiling process. It was found that the annular flow regime with liquid in the core and vapor between the liquid and the tube wall, which exists at very low qualities, is broken up at higher qualities to form a dispersed flow of droplets and filaments of liquid carried along in a vapor matrix. A stainless steel test section having a .319 inch ID and heated electrically, has been used to obtain experimental data of wall temperature distributions along the tube and local heat transfer coefficients for different heat fluxes and flow rates with liquid nitrogen as the teit fluid. Heat flux has been varied from 3500 2 to 30000 BTU/hr-ft and mass velocity from 70000 to 210000 lbm/hr-ft2. From these tests, values of wall superheat, (Tw -Ts ), from 200 to w S 975[degree]F and heat transfer coefficients from 11.1 to 65.5 BTU/hr-ft2-[degree] F have been obtained. A theoretical derivation using the Dittus-Boelter equation as an i,7mptote for the heat transfer to pure vapor has demonstrated that a significant amount of vapor superheat is present throughout the film boiling process. The mechanism of the heat transfer process in the dispersed flow region has been described by a two step theory in which 1) all ofthe heat from the wall is transferred to the vapor and 2) heat iSponsored by the National Science Foundatio

Mechanism and behavior of nucleate boiling heat transfer to the alkalai liquid metals

A model of boiling heat transfer to the alkali liquid metals is postulated from an examination of the events and phases of the nucleate boiling cycle. The model includes the important effect of microlayer evaporation which causes a wave of temperature depression to penetrate into the heating solids; calculated results predict the periodic boiling behavior in the heating solid as a function of the heat flux, the system pressure, the cavity size, and the thermophysical properties of the liquid and the solid. An experimental program was designed to examine the microscale boiling behavior of sodium and to verify the calculated predictions of the boiling model. Artificial cylindrical cavities are used in most of the test sections; a thermocouple is placed close to the boiling surface and adjacent to the cavity wall, and microscale temperature measurements were obtained for stable boiling of sodium from artificial cavities and also from a natural cavity. Horizontal heating surfaces were made from nickel "A", stainless steel 316, and molybdenum-1/2%-.titanium; the range of saturation pressure is from 20 to 780 mm Hg. Favorable comparisons with the predictions of the boiling model are obtained with data for the bubble period and for the amplitude of temperature oscillation at a thermocouple close to the boiling surface; these results indicate the importance of the microlayer in a model for boiling sodium.(cont.) The effect of pressure-temperature history on incipient superheats for boiling was examined; the presence of inert gas in a cavity can lower the incipient superheat. Also, an analysis of unstable boiling indicates that outgassing from the heating solid at high temperatures can cause erratic temperature fluctuations by sporadically triggering nucleation at a previously inactive site.Sponsored by United States Atomic Energy Commissio

The influence of return bends on the downstream pressure drop and condensation heat transfer in tubes

The influence of return bends on the downstream pressure drop and heat transfer coefficient of condensing refrigerant R-12 was studied experimentally. Flow patterns in glass return bends of 1/2 to 1 in. radius and 0.315 in. I. D. were examined visually and photographically using a high frequency xenon light source. Local pressure drop and heat transfer measurements were made along a horizontal 14 1/2 ft. test section immediately following the return bend. The refrigerant mass flux ranged from 1.32 X 105 to 4.58 X 105 lbm/hr-ft 2, saturation temperature from 90 to 107*F, and return bend quality from 0.24 to 1.0. The pressure drop and heat transfer data were compared to previous data for condensation without return bends. Effects on the downstream pressure drop and heat transfer were found to be small, if not negligible.Sponsored by Technical Committee 1.3, American Society of Heating, Refrigeration, and Air Conditioning Engineers DSR Projec

Thermal non-equilibrium in dispersed flow film boiling in a vertical tube

The departure from thermal equilibrium between a dispersed liquid phase and its vapor at high quality during film boiling is investigated, The departure from equilibruim is manifested by the high resistance to heat transfer between the dispersed and continuous phases, which result in much higher vapor temperatures and a defect in the amount of vapor generated. The effect on the overall heat transfer is to raise the tube wall temperature, and incomplete evaporation occurs within the tubes. Film boiling tests with liquid nitrogen (70,0 90 SG 190,000 lbm/hr/ft2 and 5000sq/A!25,000 Btu/hr/ft ) were made with 0.228, 0,323, and 0.462 inch ID tubes, 4 and 8 foot long. Visual observations showed that complete evaporation occurs at heat inputs much greater than the required heat of evaporation based on thermal equilibruim (A Hinput >Hfg); in terms of quality, the heat inp t was as large as 300% quality for G = 70,000 lbm/hr/ft . The departure from equilibruim is principally a function of the total mass velocity, being less at higher mass velocities. The non-equilibruim quality was measured experimentally by a helium tracer ggs technique; reliable quality data at G = 70,000 lbm/hr/ft was found to be in agreement with the departure from equilibruim calculated by applying a modified single phase heat transfer coefficient to the film boiling data. A kinematic-heat transfer analysis of the core flow, which takes into account the acceleration, evaporation and breakup of a droplet, confirmed the trends in the departure from equilibrium. A Weber number criterion (Wec = 7.5) was found to adequately describe the breakup of droplets over a partial range of test conditions. Film boiling pressure drop is also reported.Sponsored by the National Science Foundation Contract D.S.R

Heat transfer and pressure drop data for high heat flux densities to water at high subcritical pressures

Local surface ooeffioients of heat t-ansfer, overall pressure drop data and mean friction factor are presented for heat flamms up to 3.52106 BtuAr ft2 for water flowing in a nickel tabe isder the following conditions: mass rates of flow up to 5.6x 106 lb.m/hr ft2 (or inlet velocities up to 30 ft/sec), absolute pressures up to 2000 pula, and liquid suboooling between 50 F and 250 T. The test section dmnsins were 0.160 inoh I.D. and 9.4 inches long.Office of Naval Researc

Film boiling on the inside of vertical tubes with upward flow of the fluid at low qualities

Flow regimes, local heat transfer coefficients, and temperature distributions along the wall have been studied for film boiling inside a vertical tube with upward flow of a saturated liquid. The area of interest has been limited to cases of constant heat flux from the tube wall, small inlet liquid velocities, and film boiling which completely covers the entire heated portion of the tube. The last restriction means that there is no large region of nucleate boiling prior to the film boiling section. A visual test section made of electrically conducting glass tubing has been used for flow visualization studies at low qualities. Visual observations with this test section have indicated that the flow regime is annular with liquid in the center and vapor along the walls of the tube. Based on interpretations of temperature distribution data, it has been concluded that the annular flow regime changes at higher qualities to one of dispersed flow--where the liquid is dispersed in the form of drops through a predominately vapor flow. Two different diameter test sections made of stainless steel and heated electrically have been used to obtain experimental data of temperature distributions along the tube wall and local Nusselt numbers for different heat fluxes and flow rates. The fluid used in all the experimental tests has been freon 113. For the larger tube, 0.408" I.D., heat fluxes have been varied from 14,400 to 25,600 Btu/hr-ft2 , and mass velocities have been varied from 482,000 to 818,000 lbm/hr-ft2 . For these conditions, values of heat transfer coefficients from 24.0 to 41.4 Btu/hr-ft 2 -*F and values of Tw-Ts from 407 to 697*F have been obtained. These conditions have resulted in exit qualities up to 10 per cent. For the smaller tube, 0.180" I.D., heat fluxes have been varied from 22,500 to 41,800 Btu/hr-ft2 , and mass velocities have been varied from 332,000 to 398,000 lbm/hr-ft". For these conditions, values of heat(cont.) transfer coefficients from 27.0 to 87.5 Btu/hr-ft2 -*F and values of Tw-T from 261 to 883*F have been obtained. These conditions have resulted in exit qualities up to 50 per cent. A theoretical derivation based on an annular flow model with turbulent flow in the vapor film has given good agreement with the experimental data at low qualities. A dispersed flow theory using a modified form of the Dittus and Boelter-McAdams equation seems to be an asymptote which the experimental data approaches with increasing qualities.Sponsored by the National Science Foundatio

Thermal contact resistance

This work deals with phenomena of thermal resistance for metallic surfaces in contact. The main concern of the work is to develop reliable and practical methods for prediction of the thermal contact resistance for various types of surface characteristics under different conditions. In particular, consideration is restricted to the following cases: (i) rough nominally flat surfaces in a vacuum environment; (ii) rough nominally flat surfaces in a fluid environment; (iii) smooth wavy surfaces in a vacuum environment (with either of the following three types of waviness involved; spherical waviness, cylindrical waviness in one direction and cylindrical waviness in two perpendicular directions) and (iv) rough wavy surfaces in a vacuum environment. The problem is divided into three parts: thermal analysis, surface analysis and deformation analysis. The thermal analysis, based upon the proposed models, investigates the analytical solutions for the thermal contact conductance under steady state conditions. It was found convenient, due to the extensive analytical work connected with various models and different methods used here, to present all details of the thermal analysis separately in the appendices. The surface analysis, treating the surfaces as random processes with Gaussian distribution of height, relates the interface geometry to the actual contact area. The method suggested in this analysis has been checked against some autoradiographical experimental data. The deformation analysis, in its two parts, gives dependence between the load supported by the interface and (i) the actual contact area and (ii) the contact spots distribution for rough spherically wavy surfaces, respectively. The result of the first part of(cont.) the analysis is based on the plastic deformation of the surface asperities. The second part considers, through the model of the equivalent contour area, the combined effect of spherical waviness and roughness on the problem of contact spots spreading at the interface. Limitations and possible deviations of the proposed models are discussed. Prediction of the thermal contact conductance is compared with experimental data obtained in this work (in a vacuum environment) together with some data obtained by other investigators (for which necessary surface parameters were available). Agreement between the measured and predicted values was good in the whole tested range of system variables.Sponsored by the National Aeronautics and Space Administratio

Heat transfer during film condensation of a liquid metal vapor

The object of this investigation is to resolve the discrepancy between theory and experiment for the case of heat transfer durirnfilm condensation of liquid metal vapors. Experiments by previous investigators have yielded data which are extremely scattered and markedly below the predictions of both the classical IUusselt theory and more recent modifications to it. All theoretical treatments so far have taken account only of the thermal resistance presented by the condensed film. However, calculations from kinetic theory show that with liquid metals a significant thermal resistance can exist at the liquid-vapor interface. This resistance increases with decreasing vapor pressure and is dependent on the value of the "condensation coefficient." Experimental work to back up this hypothesis of a liquid-vapor interfacial resistance is presented. The working fluid for the experiments is mercury condensing at low pressures in the absence of non-condensable gases on a vertical nickel surface. Data of previous investigators are analyzed, and possible reasons for being unable to interpret these results meaningfully are cited.Sponsored by the U. S. Atomic Energy Commissio