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EDITORIAL

J. Heat Transfer. 1994;116(4):797-798. doi:10.1115/1.2911449.
FREE TO VIEW
Abstract
Topics: Heat transfer
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Invited Review Paper

J. Heat Transfer. 1994;116(4):799-807. doi:10.1115/1.2911450.

This work addresses challenges in the emerging field of microlength scale radiative and conductive heat transfer in solids and recommends specific directions of future research. Microlength scale heat transfer involves thermal energy transport processes in which heat carrier characteristic lengths become comparable to each other or the characteristic device dimension. Identification of these characteristic lengths leads to the definition of different microscale heat transfer regimes. A review of the theoretical bases describing heat transfer in each regime is followed by a discussion of the obstacles confronted in current research. Engineering challenges are illustrated with the applications of microscale heat transfer in cryogenic systems, material processing, and electronic, optical, and optoelectronic devices. The experimental difficulties discussed have hampered the development of microscale heat transfer research and deserve great efforts to overcome them.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Heat Diffusion

J. Heat Transfer. 1994;116(4):808-814. doi:10.1115/1.2911451.

This paper addresses the question in the design of experiments of where to place sensors for optimal sensitivity and the post-experiment determination of which sensors yield relevant data. The authors in their previous works have described the spatial dependence of the response sensitivities and the importance of conducting a sensitivity analysis for a better understanding of the system response. This paper describes the formulation of the method for a transient analysis and its application to thermal problems. The results have been verified using the Monte Carlo sampling technique to simulate the variations in the parameters. The results show that there are not only optimal locations to maximize the sensitivities of the responses, but also optimal times of measurement. Sample test cases are used to demonstrate the effects of time of measurement and placement of sensors on the accuracy of the measured temperatures.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):815-822. doi:10.1115/1.2911452.

A direct simulation of phonon-mediated heat transfer is described and preliminary results are reported. The method is derived from past work in simulating gas-dynamic flow and uses a linear array of cells for modeling a one-dimensional heat transfer problem. Central to the development of the technique is the Debye model for heat capacity of a crystal. The energy equation for this type of solid is presented and a phonon frequency distribution is obtained leading to a simulation technique that naturally takes into account changes in heat capacity. Using the linear array of cells, two fundamental problems are investigated. The first deals with the time evolution of the temperature profile in an array of 40 cells where the initial temperature distribution is 300 K, and at time zero the temperature of the first cell is raised to 500 K and maintained at this value. The second problem involves determining the steady-state heat transfer through an array of 20 cells where the two boundary cells are held at 500 K and 300 K. In this latter problem, the phonon mean free path is varied for each run and the results compared to both a continuum and radiation model for the heat transfer. Considering the simplistic approach used in modeling the phonon collisions, the results from both the time evolution problem and the steady energy transfer one are encouragingly close to predictions made with analytical solutions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):823-828. doi:10.1115/1.2911454.

Experimental interface pressure distributions and thermal conductance data are presented for a bolted joint. The variables considered included the bolt torque and associated axial load, the upper and lower plate thicknesses, and the mean interface temperature within the bolt radius. For 7.62-cm-dia Aluminum 6061-T6 plates, axial loads of 6.69 to 13.425 kN (1500 to 3000 lb), three heat fluxes, and mean junction temperatures of up to 310 K were considered. Pressure distribution data obtained with a pressure-sensitive film compared favorably with both theoretical predictions and published experimental data. Thermal conductance data obtained at three radial locations for the bare interface compared favorably with published data. These data also were compared with a previously published correlation for heat transfer in bolted joints. Thermal conductance data for high-conductivity elastomeric gasket materials were obtained to ascertain their suitability for thermal enhancement. The results of this investigation will be useful in the thermal analysis of bolted and riveted joints.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):829-837. doi:10.1115/1.2911455.

Combined conduction and radiation heat transfer in packed beds of spherical particles was investigated. Three different packing materials (alumina, aluminum, and glass) of various particle diameters (2.5 to 13.5 mm) were tested. Internal bed temperature profiles and corresponding effective thermal conductivities were measured under steady-state conditions for a temperature range between 350 K and 1300 K. The effects of particle diameter and local bed temperature were examined. It was found that higher effective thermal conductivities were obtained with larger particles and higher thermal conductivity packing materials. The measured values for the effective thermal conductivity were compared against the predictions of two commonly used models, the Kunii–Smith and the Zehner–Bauer–Schlünder models. Both models performed well at high temperatures but were found to overpredict the effective thermal conductivity at low temperatures. An attempt was made to quantify the relative contributions of conduction and radiation. Applying the diffusion approximation, the radiative conductivity was formulated, normalized, and compared with the findings of other investigators.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Forced Convection

J. Heat Transfer. 1994;116(4):838-843. doi:10.1115/1.2911456.

A new, simple and approximate analytical method based on linearization of the energy equation is proposed to develop solutions for forced convection heat transfer from isothermal spheres. Furthermore, heat transfer correlations from spheres are proposed in the range of Reynolds number, 0 ≤ ReD ≤ 2 × 104 , and all Prandtl numbers. This technique is performed as follows. The first step is to approximate the energy equation to the form of a transient heat conduction equation that has an existing solution. The second step is to evaluate the effective velocity through scaling analysis in the limit of Pr → ∞ and Pr → 0 and then resubstitute the effective velocity into the solution of the energy equation. Finally, a “blending method” is used to provide a general model for all Prandtl numbers. Comparison of the heat transfer correlations for NuD versus ReD from the present study with the available correlations in the literature reveals very good agreement.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):844-854. doi:10.1115/1.2911457.

Near-wall turbulence models for the velocity and temperature fields based on the transport equations for the Reynolds stresses, the dissipation rate of turbulent kinetic energy, and the temperature variance and its dissipation rate are formulated for flows with widely different Prandtl numbers. Conventional high-Reynolds-number models are used to close these equations and modifications are proposed to render them asymptotically correct near a wall compared to the behavior of the corresponding exact equations. Thus formulated, two additional constants are introduced into the definition of the eddy conductivity. These constants are found to be parametric in the Prandtl number. The near-wall models are used to calculate flows with different wall thermal boundary conditions covering a wide range of Reynolds numbers and Prandtl numbers. The calculated Nusselt number variations with Prandtl number are in good agreement with established formulae at two different Reynolds numbers. Furthermore, the mean profiles, turbulence statistics, heat flux, temperature variance, and the dissipation rates of turbulent kinetic energy and temperature variance are compared with measurements and direct numerical simulation data. These comparisons show that correct near-wall asymptotic behavior is recovered for the calculated turbulence statistics and the calculations are in good agreement with measurements over the range of Prandtl numbers investigated.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):855-863. doi:10.1115/1.2911459.

At present, it is not clear how the fluctuating temperature at the wall can be properly specified for near-wall turbulent heat-flux models. The conventional approach is to assume zero fluctuating temperature or zero gradient for the temperature variance at the wall. These are idealized specifications and the latter condition could lead to an ill-posed problem for fully developed pipe and channel flows. In this paper, the validity and extent of the zero fluctuating wall temperature condition for heat transfer calculations are examined. The approach taken is to assume Taylor series expansions in the wall normal coordinate for the fluctuating quantities that are general enough to account for both zero and nonzero temperature fluctuations at the wall and to develop a near-wall turbulence model allowing finite values of the wall temperature variance. As for the wall temperature variance boundary condition, it is estimated by solving the coupled heat transfer problem between the fluid and the solid wall. The eddy thermal conductivity is calculated from the temperature variance and its dissipation rate. Heat transfer calculations assuming both zero and nonzero fluctuating wall temperature reveal that the zero fluctuating wall temperature assumption is quite valid for the mean field and the associated integral heat transfer properties. The effects of nonzero fluctuating wall temperature on the fluctuating field are limited only to a small region near the wall for most fluid/solid combinations considered.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):864-870. doi:10.1115/1.2911460.

Convective heat transfer data are presented for coaxial jet mixing in a constant-diameter tube. The inner jet diameter was approximately twice the annular gap dimension. Water, with a nominal inlet Prandtl number of 6, was used as the working fluid. For the inner jet, Reynolds numbers of 30,000 and 100,000 were examined and the swirl number was varied from zero to one. Annular flow rates were characterized by a ratio of annular-to-inner jet axial momentum, which was varied from 0 to 8.3. In all cases the annular jet was unswirled. Plots of local Nusselt numbers show minima and maxima corresponding to the separation and reattachment associated with wall-bounded recirculation. As inner jet swirl strength increased from zero to its maximum value, the location of peak Nusselt number shifted upstream. Local Nusselt numbers achieved magnitudes as high as 9.7 times fully developed values for cases with high swirl and low annular flow rate. As the annular jet’s flow rate was increased, the heat transfer enhancement decreased while the near-wall recirculation zones were stretched and shifted downstream, until at sufficiently high values of the momentum flux ratio, the zones were no longer in evidence from the heat transfer data.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):871-879. doi:10.1115/1.2911461.

Heat transfer and fluid mechanics data were obtained for a turbulent boundary layer with arrays of embedded streamwise vortices containing both counterrotating and corotating vortex pairs. The data show that these arrays can cause both large local variations in the heat transfer rate and significant net heat transfer augmentation over large areas. Close proximity of other vortices strongly affects the development of the vortex arrays by modifying the trajectory that they follow. The vortices in turn produce strong distortion of the normal two-dimensional boundary layer structure, which is due to their secondary flow. When one vortex convects another toward the wall, a strong boundary layer distortion occurs. The heat transfer is elevated where the secondary flow is directed toward the wall and reduced where the secondary flow is directed away from the wall. When adjacent vortices lift their neighbor away from the wall, minimal modification of the heat transfer results. The primary influence of grouping multiple vortex pairs into arrays is the development of stable patterns of vortices. These stable vortex patterns produce vortices that interact with the boundary layer and strongly modify the heat transfer far downstream, even where the vortices have decayed in strength.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):880-885. doi:10.1115/1.2911462.

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.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):886-895. doi:10.1115/1.2911463.

An experimental investigation has been performed to study the effect of flow pulsations on local, time-averaged convective heat transfer to an impinging water jet. Sinusoidal and square-pulse waveforms were considered. For the square waveform, the flow was completely halted for a portion of the pulsation cycle. Hot-film anemometry was used to characterize both the steady and the pulsating flows with regard to turbulence level and the spatial uniformity in the velocity profile across the nozzle width in order to assess separately the influence of flow pulsation on convective heat transfer. Pulse magnitude, which was defined as the ratio of the mean-to-peak velocity change to the mean flow velocity, was varied from 0.5 to 100 percent. Pulse frequencies ranged from 5 to 280 Hz, which corresponded to Strouhal numbers based on jet width and velocity of 0.014 to 0.964. Observed effects on convective heat transfer are explained in terms of nonlinear dynamic responses of the hydrodynamic and thermal boundary layers, boundary layer renewal, and bulges in the jet free surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):896-903. doi:10.1115/1.2911464.

The influence of mainstream turbulence on surface heat transfer coefficients of a gas turbine blade was studied. A five-blade linear cascade in a low-speed wind tunnel facility was used in the experiments. The mainstream Reynolds numbers were 100,000, 200,000, and 300,000 based on the cascade inlet velocity and blade chord length. The grid-generated turbulence intensities at the cascade inlet were varied between 2.8 and 17 percent. A hot-wire anemometer system measured turbulence intensities, mean and time-dependent velocities at the cascade inlet, outlet, and several locations in the middle of the flow passage. A thin-foil thermocouple instrumented blade determined the surface heat transfer coefficients. The results show that the mainstream turbulence promotes earlier and broader boundary layer transition, causes higher heat transfer coefficients on the suction surface, and significantly enhances the heat transfer coefficient on the pressure surface. The onset of transition on the suction surface boundary layer moves forward with increased mainstream turbulence intensity and Reynolds number. The heat transfer coefficient augmentations and peak values on the suction and pressure surfaces are affected by the mainstream turbulence and Reynolds number.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):904-911. doi:10.1115/1.2911465.

The mass transfer from an array of naphthalene-coated parallel rectangular cylinders, through which air passes in a slitlike flow, has been measured. The local Sherwood numbers indicate that the flow pattern is asymmetric in spite of using an array of two-dimensional, equally spaced identical cylinders. Smoke-wire flow visualization verifies this asymmetry, showing alternate short and long wakes around the cylinders, due probably to the instability of vortex shedding. On the side surfaces of the cylinders with the short wakes, the airflow deflects and reattaches, resulting in a high mass transfer. Also, a strong impinging effect is observed on the leeward (back) surface of these cylinders at high Reynolds numbers. Reattachment is not observed on the side surface for cylinders with the long wakes. On these, however, the mass transfer on the leeward surface is higher than on the short wake cylinders. This may be due to the relatively low naphthalene vapor concentration in the long wakes. The distribution of the short wakes (and the long wakes) is periodic and relatively stable. However, their position can be changed from one cylinder to the adjacent one by a disturbance. Measurements were taken over a moderate Reynolds number range of 300 to 3000 (based on the cylinder-to-cylinder pitch and approaching velocity). The laminar, transition, and turbulent nature in the wake flows can be inferred from the results.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):912-920. doi:10.1115/1.2911466.

Turbulent heat transfer and friction in a rectangular channel with perforated ribs arranged on one of the principal walls are investigated experimentally. The effects of rib open-area ratio, rib pitch-to-height ratio, rib height-to-channel hydraulic diameter ratio, and flow Reynolds number are examined. To facilitate comparison, measurements for conventional solid-type ribs are also conducted. Laser holographic interferometry is employed to determine the rib permeability and measure the heat transfer coefficients of the ribbed wall. Results show that ribs with appropriately high open-area ratio at high Reynolds number range are permeable, and the critical Reynolds number of initiation of flow permeability decreases with increasing rib open-area ratio. By examining the local heat transfer coefficient distributions, it is found that permeable ribbed geometry has an advantage of obviating the possibility of hot spots. In addition, the permeable ribbed geometry provides a higher thermal performance than the solid-type ribbed one, and the best thermal performance occurs when the rib open-area ratio is 0.44. Compact heat transfer and friction correlations are also developed for channels with permeable ribs.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):921-928. doi:10.1115/1.2911467.

The effect of unsteady wake and film injection on heat transfer coefficients and film effectiveness from a gas turbine blade was found experimentally. A spoked wheel type wake generator produced the unsteady flow. Experiments were done with a five airfoil linear cascades in a low-speed wind tunnel at a chord Reynolds number of 3 × 105 , two wake Strouhal numbers of 0.1 and 0.3, and a no-wake case. A model turbine blade injected air or CO2 through one row of film holes each on the pressure and suction surfaces. The results show that the large-density injectant (CO2 ) causes higher heat transfer coefficients on the suction surface and lower heat transfer coefficients on the pressure surface. At the higher blowing ratios of 1.0 and 1.5, the film effectiveness increases with increasing injectant-to-mainstream density ratio at a given Strouhal number. However, the density ratio effect on film effectiveness is reversed at the lowest blowing ratio of 0.5. Higher wake Strouhal numbers enhance the heat transfer coefficients but reduce film effectiveness for both density ratio injectants at all three blowing ratios. The effect of the wake Strouhal number on the heat transfer coefficients on the suction surface is greater than that on the pressure surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):929-937. doi:10.1115/1.2911468.

Local instantaneous heat transfer between a submerged horizontal cylinder and a gas-fluidized bed operating in the bubble-flow regime was measured and the resulting signals analyzed. Unique to this investigation is the division of particle convective heat transfer into transient and steady-state contact dynamics through analysis of instantaneous heat transfer signals. Transient particle convection results from stationary particles in contact with the heat transfer surface and yields a heat transfer rate that decays exponentially in time. Steady-state particle convection results from active particle mixing at the heat transfer surface and results in a relatively constant heat transfer rate during emulsion phase contact. The average time of contact for each phase is assessed in this study. Signals were acquired using a constant-temperature platinum film heat flux sensor. Instantaneous heat transfer signals were obtained for various particle sizes by varying the angular position of the heat transfer probe and the fluidization velocity. Individual occurrences of emulsion phase heat transfer that are steady-state in nature are characterized by contact times significantly higher than both the mean transient and mean emulsion phase contact times under the same operating conditions. Transient and steady-state contact times are found to vary with angular position, particle size, and fluidizing velocity. Due to the extremely short transient contact times observed under these fluidization conditions, mean transient heat transfer coefficients are approximately equal to the mean steady-state heat transfer coefficients.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):938-945. doi:10.1115/1.2911469.

A new physical model for the spreading dynamics of fluids with an apparent finite contact angle on solid substrates is presented. The model is based on the premise that both interfacial intermolecular forces and temperature control change-of-phase heat transfer and (therefore) motion in the moving contact line region. Classical change-of-phase kinetics and interfacial concepts like the Kelvin–Clapeyron, Young–Dupre, and augmented Young–Laplace equations are used to compare the effects of stress (change in apparent dynamic contact angle) and temperature (superheat). Explicit equations are obtained for the velocity, heat flux, and superheat in the contact line region as a function of the change in the apparent contact angle. Comparisons with experimental data demonstrate that the resulting interfacial model of evaporation/condensation not only describes the “apparently isothermal” contact line movement in these systems at 20°C but also describes the substrate superheat at the critical heat flux.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):946-953. doi:10.1115/1.2911470.

A numerical model has been developed to simulate the transport of heat and moisture in thin beds of microporous silica gel, assuming surface and pore volume diffusion of moisture and conduction of heat as the dominant intraparticle transport mechanisms in the desiccant. Predictions from the model have been compared with the results of single-blow tests for packed beds of silica gel from the literature and for a matrix of silica gel-coated parallel passages from this study. The model has been used to explore the significance of intraparticle diffusion resistance to heat and moisture transport in packed beds and parallel passage matrices of silica gel for different desiccant sizes and air velocities.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):954-959. doi:10.1115/1.2911471.

Small laminar diffusion flames (flame height ≃2–3 mm) established by a fuel jet issuing into a quiescent medium are investigated. It was found that for these flames buoyancy effects disappeared as the flame size decreased (Fr≫1), and diffusive transport of the fuel was comparable to the convective transport of the fuel. The effect of buoyancy on these flames was studied by examining the flame shape for horizontally oriented burners. A phenomenological model was developed (based on experimentally determined flame shapes) to compare diffusion and convection transport effects. Finally, the flame shapes were theoretically determined by solving the conservation equations using similarity methods. It was seen that when the axial diffusion (in momentum and species equations) terms are included in the conservation equations, the calculated flame shape is in better agreement (as compared to without the axial diffusion term) with the experimentally measured flame shape.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Mixed Convection

J. Heat Transfer. 1994;116(4):960-970. doi:10.1115/1.2911472.

A numerical study of the combined-forced and natural convective cooling of heat-dissipating electronic components, located in a rectangular enclosure, and cooled by an external throughflow of air is carried out. A conjugate problem is solved, describing the flow and thermal fields in air, as well as the thermal field within the walls of the enclosure and the electronic components themselves. The interaction between the components is of interest here, depending on their relative placement in the enclosure, and different configurations are considered. For Re = 100 laminar, steady flow is predicted for up to Gr/Re2 = 10, but a single-frequency oscillatory behavior is observed for most of the configurations studied, at Gr/Re2 = 50. Heat transfer results are presented for both the laminar and the oscillatory domains. The mixed convection regime, where the buoyancy effects are comparable to the forced flow, occurs at values of Gr/Re2 between 0.01 and 10. The results are of value in the search for a suitable placement of electronic components in an enclosed region for an effective heat removal.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Radiative Transfer

J. Heat Transfer. 1994;116(4):971-979. doi:10.1115/1.2911473.

The structure and optical properties of soot were studied in the fuel-rich (underfire) region of buoyant laminar diffusion flames of ethylene and acetylene burning in coflowing air. The objective was to evaluate scattering predictions based on the Rayleigh–Debye–Gans (RDG) approximation for polydisperse fractal aggregates of spherical primary soot particles having constant diameters, for conditions where the Guinier (small angle) regime, and the transition between the Guinier and the power-law (large-angle) regimes, were dominant, in order to supplement earlier work for conditions where the power-law regime was dominant. Soot structure was measured using thermophoretic sampling and analysis by transmission electron microscopy (TEM) to yield primary particle diameters, distributions of the number of primary particles per aggregate, and the aggregate mass fractal dimensions. Soot optical property measurements included vv, hh, hv, and vh differential scattering cross sections, total scattering cross sections, and the albedo at 514.5 nm, as well as several soot structure parameters inferred from these measurements using the approximate theory. The approximate RDG theory generally provided an acceptable basis to treat the optical properties of the present soot aggregates over a range of conditions spanning the Guinier and power-law regimes. Other scattering approximations were less satisfactory with performance progressively becoming less satisfactory in the order: RDG polydisperse fractal aggregate scattering using a single mean squared radius of gyration (from the Guinier regime), Mie scattering for an equivalent sphere, and Rayleigh scattering—the last underestimating differential scattering levels by a factor of roughly 100 for the present test conditions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):980-985. doi:10.1115/1.2911474.

The K-distribution technique is presented for predicting nongray gas radiation in the presence of particle scattering. This technique transforms the otherwise formidable spectral integration problem from the frequency domain to the K or gas absorption coefficient domain. This transformation is made possible by the negligible variation of blackbody intensity and particle radiative properties with frequency within a gas band. Application of the K-distribution technique to an isothermal emission problem is made. Results are compared with those from the Monte-Carlo method, These results show that, with the K-distribution technique, the detailed absorption line structure within a gas band as well as the scattering effect due to particles can be simulated with great numerical accuracy and computational efficiency. Essentially, line-by-line accuracy is achieved without line-by-line spectral integration.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):986-992. doi:10.1115/1.2911475.

A new space radiator concept has been proposed (Kim et al., 1991, 1992a, b, 1993) in which a thin film of hot liquid, flowing along the inside of a closed membrane, rejects waste heat by radiation to the surroundings. In previous versions, the radiator rotates, supplying most of the driving force for the liquid flow. In the present design, the cylinder is stationary, and the liquid flows circumferentially under its initial momentum. Moderately large Reynolds numbers are required to overcome viscous drag, and prevent excessive thickening of the film. The major design consideration involves the application of an internal electrostatic field to pull the liquid away from the site of a membrane puncture due to micrometeorite impact. Calculations are presented that show that leaks can be stopped with a safety factor of two or more, while the surface wave thus produced is washed harmlessly out of the system. Some preliminary heat transfer performance characteristics are presented. The advantages of this concept include the absence of moving parts and the ease of deployment, compared to rotating units, and a factor of at least three for the reduction of the weight per unit surface area compared to heat pipes.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):993-998. doi:10.1115/1.2911476.

This work consists of a detailed thermal modeling of two different radiometers operated at cryogenic temperatures. Both employ a temperature sensor and an electrical-substitution technique to determine the absolute radiant power entering the aperture of a receiver. Their sensing elements are different: One is a germanium resistance thermometer, and the other is a superconducting kinetic-inductance thermometer. The finite element method is used to predict the transient and steady-state temperature distribution in the receiver. The nonequivalence between the radiant power and the electrical power due to the temperature gradient in the receiver is shown to be small and is minimized by placing the thermometer near the thermal impedance. In the radiometer with a germanium resistance thermometer, the random noise dominates the uncertainty for small incident powers and limits the ultimate sensitivity. At high power levels, the measurement accuracy is limited by the uncertainty of the absorptance of the cavity. Recommendations are given based on the modeling for future improvement of the dynamic response of both radiometers.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Evaporation

J. Heat Transfer. 1994;116(4):999-1006. doi:10.1115/1.2911477.

The evaporation of a single droplet of liquid nitrogen, levitated during film boiling above a solid, impervious surface, was studied experimentally. The droplet initial diameter (1.9 mm), surface temperature (~20°C), ambient temperature (~20°C), and ambient pressure (~0.1 MPa) were held constant. The principal parameters varied were the surface material (copper or glass), and roughness (0.35 to 50 μm). Measurements were made of the droplet diameter evolution and the surface temperature variation during droplet impact. Predictions from existing models of droplets in Leidenfrost evaporation agree well with measurements of the droplet evaporation rate. The droplet lifetime was found to be slightly longer on the glass surface than it was on the copper surface, corresponding to the greater cooling of the glass surface during droplet impact. The droplet evaporation rate was unchanged by small increases in surface roughness. However, ridges on the surface with a height of the same magnitude as the thickness of the vapor film under the drop caused vapor bubble nucleation in the droplet, and significantly reduced the droplet evaporation time.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):1007-1015. doi:10.1115/1.2911436.

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.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Solid/Liquid Phase-Change Heat Transfer

J. Heat Transfer. 1994;116(4):1016-1027. doi:10.1115/1.2911437.

The transient freezing of water impinging vertically on a subzero disk through a circular jet is studied experimentally to determine the interaction of the fluid flow and the solidification process. Experiments are performed over a range of the jet Reynolds number (1600 < Rei < 3500) based on the average velocity and radius of the falling jet at the impingement point. For this range of Reynolds numbers, that corresponds to tube Reynolds numbers less than 1100, and in the absence of solidification, the thin liquid film is characterized by a smooth circular hydraulic jump whose diameter is measured and correlated with the jet Reynolds number. The solidification process is initiated away from the jet (i.e., outside of the hydraulic jump) and moves inward toward the jet. The formation and growth of ice on the cold surface affect the flow field over the surface. This effect manifests itself in the form of a rapid reduction of the hydraulic jump diameter accompanied by instability in its position until its complete disappearance. The effect of fluid flow on the solidification process is found to be a small reduction in the nucleation temperature. The ice layer profiles at different times for different values of jet Reynolds number, and Stefan numbers of the surface and jet are also measured and reported. An approximate model is developed for the calculation of the transient crust growth by neglecting the interaction between the flow and solidification. The predicted solid crust profiles are compared with the measured ones, and the extent of the flow-freezing interactions is discussed. The approximate model is also used for a parametric study of the problem for a constant temperature surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):1028-1033. doi:10.1115/1.2911438.

Melting of a solid plug in the gap between two coaxial pipes by inserting a moving heat source in the inner pipe is investigated. Using a scale analysis, closed-form solutions for temperatures of liquid in the inner pipe, solid plug and liquid in the annular gap, and the surrounding medium around the outer pipe are determined. It is shown that eight independent dimensionless parameters are required to specify the entire process. The effects of independent parameters on the shapes of the molten region in the gap are found. The analysis and results provided are useful for the design of oil pipes.

Commentary by Dr. Valentin Fuster

ERRATA

TECHNICAL BRIEFS

J. Heat Transfer. 1994;116(4):1034-1038. doi:10.1115/1.2911439.
Abstract
Topics: Temperature
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):1041-1044. doi:10.1115/1.2911441.
Abstract
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):1049-1053. doi:10.1115/1.2911444.
Abstract
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):1053-1055. doi:10.1115/1.2911445.
Abstract
Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1994;116(4):1055-1058. doi:10.1115/1.2911446.
Abstract
Topics: Design
Commentary by Dr. Valentin Fuster

DISCUSSIONS

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