Showing papers in "Journal of Heat Transfer-transactions of The Asme in 1992"

Direct Numerical Simulation of Passive Scalar Field in a Turbulent Channel Flow

Abstract: A direct numerical simulation (DNS) of the fully developed thermal field in a dimensional turbulent channel flow of air was carried out. The isoflux condition was imposed on the two walls so that the local mean temperature increased linearly in the streamwise direction. With any buoyancy effect neglected, temperature was considered as a passive scalar. The computation was executed on 1,589,248 grid points by using a spectral method. The statistics obtained were root-mean-square temperature fluctuations, turbulent heat fluxes, turbulent Prandtl number, and dissipation time scales. They agreed fairly well with existing experimental and numerical simulation data. Each term in the budget equations of temperature variance, its dissipation rate, and turbulent heat fluxes was also calculated. It was found that the temperature fluctuation [theta][prime] was closely correlated with the streamwise velocity fluctuation u[prime], particularly in the near-wall region. Hence, the distribution of budget terms for the streamwise and wall-normal heat fluxes, [bar u][prime][theta] and [bar v][prime][theta][prime], were very similar to those for the two Reynolds stress components, [bar u][prime]u[prime]and [bar u][prime]v[prime].

Surface Roughness and Its Effects on the Heat Transfer Mechanism in Spray Cooling

Abstract: In the spray cooling of a heated surface, variations in the surface texture influence the flow field, altering the maximum liquid film thickness, the bubble diameter, vapor entrapment, bubble departure characteristics, and the ability to transfer heat. A new method for determining and designating the surface texture is proposed, and the effects of surface roughness on evaporation/nucleation in the spray cooling flow field studied. A one-dimensional Fourier analysis is applied to determine experimentally the surface profile of a surface polished with emery paper covering a spectrum of grit sizes between 0.3 to 22 {mu}m. Heat transfer measurements of liquid flow rates between 1 to 5 l/h and air flow rates between 0.1 to 0.4 l/s are presented. Maximum heat fluxes of 1,200 W/cm{sup 2} for the 0.3 {mu}m surface at very low superheats were obtained.

Turbulent Heat Transfer Augmentation and Friction in Periodic Fully Developed Channel Flows

Abstract: Measurements are presented of the distribution of average friction factors (f) as well as local and average ({ovr Nu}) heat transfer coefficients for fully developed channel flows with two rib-roughened opposite walls. The temperature measurements were made by using both a laser holographic interferometer and thermocouples. In addition, the reattachment length was determined by flow visualization. The Reynolds number (Re) was varied from 5.0 {times} 10{sup 3} to 5.4 {times} 10{sup 4}; the rib pitch-to-height ratios (Pi/H) were 10, 15, 20; and the rib height-to-hydraulic diameter ratios (H/De) were 0.063, 0.081, and 0.106. The detailed results allowed the peaks of heat transfer augmentation and the regions susceptible to hot spots to be located and allowed the relative contribution of the rib surface and the channel wall to the heat transfer augmentation to be determined. Moreover, relative to a smooth duct, the enhancement of both {ovr Nu} and f at various Re, Pi/H, and H/De was documented in detail. Furthermore, compact correlations in terms of Re, Pi/H, and H/De were developed for both {ovr Nu} and f.

Enhancement of Single-Phase Heat Transfer and Critical Heat Flux From an Ultra-High-Flux Simulated Microelectronic Heat Source to a Rectangular Impinging Jet of Dielectric Liquid

Abstract: Jet impingement is encountered in numerous applications demanding high heating or cooling fluxes. Examples include annealing of metal sheets and cooling of turbine blades, x-ray medical devices, laser weapons, and fusion blankets. The attractive heat transfer attributes of jet impingement have also stimulated research efforts on cooling of high-heat-flux microelectronic devices. These devices are fast approaching heat fluxes in excess of 100 W/cm[sup 2], which have to be dissipated using coolants that are both electrically and chemically compatible with electronic components. Unfortunately, fluids satisfying these requirements tend to possess poor transport properties, creating a need for significant enhancement in the heat transfer coefficient by such means as increased coolant flow rate and phase change. The cooling problem is compounded by a need to cool large arrays of heat sources in minimal volume, and to reduce the spacing between adjacent circuit boards. These requirements place severe constraints on the packaging of jet impingement cooling hardware.

Heat Transfer Regimes in Microstructures

Abstract: Submicron dimensions are the hallmark of integrated electronic circuits, photovoltaic cells, sensors, and actuators. The design of these devices requires heat transfer analysis. Often it is not known to the designer whether a given microstructure can be analyzed using macroscale heat transfer theory, i.e., a method not considering the size dependence of a transport property such as thermal conductivity. This study develops regime maps showing the boundary between the macroscale and microscale heat transfer regimes. The maps relate the geometric dimension separating the two regimes to temperature for conduction in solids, to temperature, pressure, and Reynolds number for convection in gases, and to the temperature of the emitting medium for radiative transfer. The material purity and defect structure strongly influence the regime boundaries. Microstructures pertaining to a given technology are marked on these maps to determine whether macroscale heat transfer theory is applicable. By marking regions on the maps for the expected future development of microtechnologies, research needs in microscale heat transfer can be anticipated.

Experimental Heat Transfer Rates of Natural Convection of Molten Gallium Suppressed Under an External Magnetic Field in Either the X, Y, or Z Direction

Abstract: The heat transfer rates of natural convection of molten gallium were measured under various strengths of heating rates and three coordinate directional magnetic fields. Molten gallium (Pr = 0.024) was filled in a cubic enclosure of 30 mm {times} 30 mm {times} 30 mm whose one vertical wall was uniformly heated and an opposing wall was isothermally cooled, with otherwise insulated walls. An external magnetic field was impressed either perpendicular and horizontal to the heated wall (x direction) or in parallel and horizontal to the heated wall (y direction) of the enclosure or in a vertical direction (z direction). For the modified Grashof number, based on the heat flux, less than 4.24 {times} 10{sup 6} and the Hartmann number less than 461, the average Nusselt numbers were measured. These results proved that our previous three-dimensional numerical analyses for an isothermal hot wall boundary were in good qualitative agreement. A much higher suppression effect is given in the x- and z-directional magnetic fields than that in the y-directional one.

Model of the evaporating meniscus in a capillary tube

Abstract: A mathematical model describing the evaporating meniscus in a capillary tube has been formulated incorporating the full three-dimensional Young-Laplace equation, Marangoni convection, London-van der Waals dispersion forces, and nonequilibrium interface conditions. The results showed that varying the dimensionless superheat had no apparent effect on the meniscus profile. However, varying the dispersion number produced a noticeable change in the meniscus profile, but only at the microscopic level near the tube wall. No change in the apparent contact angle was observed with changes in the dimensionless superheat or dispersion number. In all cases, the dimensionless mean curvature was asymptotic to a value equal to that for a hemispherical meniscus. The local interfacial mass flux and total mass transfer rate increased dramatically as the dispersion number was increased, suggesting that surface coatings can play an important role in improving or degrading capillary pumping. The model also predicted that the local capillary pressure remains constant and equal to 2{sigma}/r{sub c} regardless of changes in the dimensionless superheat and dispersion number. It should be noted that the results in this study are theoretical in nature and require experimental verification.

On the Gas Entrapment and Nucleation Site Density During Pool Boiling of Saturated Water

Abstract: A model to describe the effect of wettability on nucleation site density is presented. First, from Helmholtz free energy analysis, a criterion for the entrapment condition in a uniform temperature field is developed. Second, the stability condition of preexisting gas/vapor nuclei during the heating process and the minimum superheat required to initiate nucleation are determined. The prediction of the entrapment condition and the incipient temperature are consistent with the experimental observations made on surfaces having naturally existing cavities. Third, a naturally formed cavity on a heater surface is modeled as a spherical cavity. The cumulative active nucleation site density for a specified contact angle is expressed in terms of the cumulative density of cavities existing on the surface as Na = Pas · Nas where Nas is the heater surface cumulative cavity density with cavity mouth angles less than a specified value and Pas is a function of contact angle and cavity mouth angle. The model successfully predicts active site densities for different contact angles.

Optimization of Multiprobe Cryosurgery

Abstract: This paper describes a general technique for optimising cryosurgical procedures. The method, which is based on the simplex minimisation algorithm, minimises unnecessary freesing by optimising various surgical parameters. The optimisation procedure is illustrated using a simplified model of prostatic cryosurgery. In this illustratine case, the function to be minimised, F, defined as the volume of healthy tissue destroyed during complete freesing of the prostate, is assumed to depend on three parameters: the number of cryoprobes used, the freesing length per cryoprobe, and the cryoprobe diameter

Effect of Nozzle Configuration on Transport in the Stagnation Zone of Axisymmetric, Impinging Free-Surface Liquid Jets: Part 2—Local Heat Transfer

Abstract: This is the second of a two-part study on the flow structure and heat transfer characteristics of turbulent, free-surface liquid jets. Part 2 deals with the effect of selected nozzle configurations on the local heat transfer in the stagnation zone. Infrared techniques have been used to characterize the local heat transfer for the four nozzle configurations whose mean and turbulent flow structure was detailed in Part 1. The results show that for identical jet Reynolds numbers, significant differences exist in the magnitudes of the local Nusselt number for the nozzle types studied. Differences of approximately 40 percent were observed. Local heat transfer results reveal that for already turbulent jets, the mean radial velocity gradient appears to be more influential in determining the heat transfer than incremental changes in the level of turbulence (as measured by the radial component of the fluctuations). An empirical correlation of the experimental data supports this conclusion, and reveals that the stagnation Nusselt number is affected independently by the jet Reynolds number and the dimensionless mean radial velocity gradient. 21 refs., 6 figs.

The Effect of Plate Spacing on Free Convection Between Heated Parallel Plates

Abstract: In the past, considerable attention has been given to free convection between heated vertical parallel plates. This problem is considerable interest to engineers because of its application to electronic equipment cooling and solar energy. Some attempts have been made to optimize the spacing between parallel plates in the past. Bodoia and Osterle analytically derived a criterion for an optimum plate spacing for which the heat dissipation is maximum. The objective of this paper is to predict the optimum plate spacing for single channels by using the governing equations for a continuous system model. No heat transfer coefficient known a priori will be used in these calculations, but will be calculated as part of the solution.

Heat Transfer With Very High Free-Stream Turbulence: Part I—Experimental Data

Abstract: Boundary layer heat transfer with very high freestream turbulence is investigated. The problem is studied experimentally by placing a constant-temperature heat transfer surface at various locations in the margin of a turbulent free jet and measuring both the surface heat transfer rate and the turbulence in the freestream. Freestream turbulent fluctuations 20 to 60 percent relative to the mean velocity augment heat transfer 1.8 to 4 times that which would be predicted locally using accepted correlations for turbulent boundary layers at the same Reynolds number. The correlations of Simonich and Bradshaw (1989), Pedisius et al. (1983), and Blair (1983) each fail to describe the present data. For flows over flat surfaces in air with very high freestream turbulence, greater than 0.2, u-prime determines h. A new heat transfer parameter, St-prime, characterizes turbulent boundary layer heat transfer with freestream turbulence on the domain 0-0.65 to within +/- 15 percent for high Reynolds number flows with uniform thermal boundary conditions.

Comparative Studies on Nonlinear Hyperbolic and Parabolic Heat Conduction for Various Boundary Conditions: Analytic and Numerical Solutions

Abstract: With the advent of lasers with very short pulse durations and their use in materials processing, the effect of thermal wave propagation velocity becomes important. Also, localized heating in laser-aided materials processing causes significant variations in the material properties. To account for these two effects, hyperbolic heat conduction is studied in this paper by considering all the thermophysical properties, except the thermal diffusivity, to be temperature dependent. The resulting nonlinear hyperbolic equations are linearized by using Kirchhoff transformation. Both analytical and numerical solutions are obtained for finite domains. Results are presented and compared with parabolic conduction results.

Numerical Calculation of Convective Heat Transfer Between Rotating Coaxial Cylinders With Periodically Embedded Cavities

Abstract: A numerical study has been performed for the flow and heat transfer in the space between a pair of coaxial cylinders with the outer one fixed and the inner one rotating. Of special interest is the case where either one of the cylinders has an axially grooved surface resulting in twelve circumferentially periodic cavities embedded in it. The ends of the cylinder are bounded by flat impermeable walls that are either fixed to the outer cylinder or rotate with the inner one. Such a geometry is common in electric motors where an improved understanding of thermophysical phenomena is essential for analysis and design. Discretized transport equations are solved for two-dimensional and three-dimensional, steady, constant properly laminar flow using a second-order accurate finite volume scheme within the context of a SIMPLER-based iterative methodology. The two-dimensional calculations reveal a shear-induced recirculating flow in the cavities. For supercritical values of the Reynolds number, the three-dimensional calculations show how the flow in a cavity interacts with Taylor vortices in the annular space to enhance heat transfer. Relative to coaxial cylinders with smooth surfaces, for the conditions of this study the transport of momentum and heat is raised by a factor of 1.2 in themore » case of cavities embedded in the inner cylinder and by a factor of 1.1 in the case of cavities embedded in the outer cylinder.« less

Surface Radiation Exchange for Two-Dimensional Rectangular Enclosures Using the Discrete-Ordinates Method

Abstract: The purpose of this study is to develop a model based on the discrete-ordinates method for computing radiant exchange between surfaces separated by a transparent medium and to formulate the model so that arbitrary arrangements of the surfaces can be accommodated. Heat fluxes from the model are compared to those based on the radiosity/irradiation analysis. Three test geometries that include shadowing and irregular geometries are used to validate the model. Heat fluxes from the model are in good agreement with those from the radiosity/irradiation analysis. Effects of geometries, surface emittances, grid patterns, finite-difference weighting factor, and number of discrete angles are reported.

Prediction of Heat Transfer and Friction for the Louver Fin Geometry

Abstract: This paper is concerned with prediction of the air-side heat transfer coefficient of the louver fin geometry used in automotive radiators. An analytical model was developed to predict the heat transfer coefficient and friction factor of the louver fin geometry. The model is based on boundary layer and channel flow equations, and accounts for the[open quote] flow efficiency[close quotes] in the array, as previously reported by Webb and Trauger. The model has no empirical constants. The model allows independent specifications of all of the geometric parameters of the touver fin. This includes the number of louvers over the flow depth, the louver width and length, and the louver angle. The model was validated by predicting the heat transfer coefficient antifriction factor of 32 louver arrays tested by Davenport, which spanned hydraulic diameter based Reynolds numbers of 300-2800. At the highest Reynolds number, all of the heat transfer coefficients were predicted within a maximum error of [minus]14 /+ 25 percent, and a mean error of +/- percent. The high Reynolds number friction factors were predicted with a maximum error [minus]22 / + 26 percent, with a mean error of +/- 8 percent. The error ratios were slightly higher at the lowestmore » Reynolds numbers. 11 refs., 14 figs., 2 tabs.« less

Effect of uneven wall temperature on local heat transfer in a rotating square channel with smooth walls and radial outward flow

Abstract: The effect of uneven wall temperature on the local heat transfer coefficient in a rotating square channel with smooth walls and radial outward flow is investigated for Reynolds numbers from 2500 to 25,000 and rotation numbers from 0 to 0.352. Three cases of thermal boundary conditions are studied: (1) four walls uniform temperature, (2) four walls uniform heat flux, and (3) leading and trailing walls hot and two side walls cold. It is shown that the heat transfer coefficients on the leading surface are much lower than that of the trailing surface due to rotation. For case 1, the leading surface heat transfer coefficient decreases and then increases with increasing rotation numbers, and the trailing surface heat transfer coefficient increases monotonically with rotation numbers. The trailing surface heat transfer coefficients, as well as those for the side walls, for case 2 are higher than for case 1, and the leading surface heat transfer coefficients for cases 2 and 3 are significantly higher than for case 1.

Measurements of the free surface flow structure under an impinging, free liquid jet

Abstract: The objective of this research was to characterize the flow structure under an impinging liquid jet striking a flat, normally oriented surface The approach was the measurement of the free surface velocities of the jet prior to impingement and the surface velocities of the radially spreading liquid layer A novel laser-Doppler velocimetry technique was used The LDV system was configured such that the measurement volume would span the time-dependent fluctuations of the free surface, with the surface velocity being measured The mean and fluctuating components of a single direction of the velocity vector were measured It was found that the radial liquid layer data collapsed well over the range of jet Reynolds numbers 16,000 < Re < 47,000 if plotted in dimensionless coordinates, where the measured velocity was normalized by the average jet exit velocity and the radial coordinate was normalized by the nozzle diameter Mean liquid layer depths were inferred from the velocity measurements by assuming a velocity profile across the layer, and were reported Pre-impingement jet measurements suggest that the flow development is nearly complete two diameters from the nozzle exit

1990 Max Jakob Memorial Award Lecture: Viscoelastic Fluids: A New Challenge in Heat Transfer

Abstract: A review of the current knowledge on the fluid mechanics and heat transfer behavior of viscoelastic aqueous polymer solutions in channel flow is presented. Both turbulent and laminar flow conditions are considered. Although the major emphasis is on fully established circular pipe flow, some results are also reported for flow in a 2:1 rectangular channel. For fully established turbulent channel flow, it was found that the friction factor, f, and the dimensionless heat transfer factor, jH , were functions of the Reynolds number and a dimensionless elasticity value, the Weissenberg number. For Weissenberg values greater than approximately 10 (the critical value) the friction factor was found to be a function only of the Reynolds number; for values less than 10 the friction factor was a function of both Re and Ws. For the dimensionless heat transfer coefficient jH the corresponding critical Weissenberg value was found to be about 100. The heat transfer reduction is always greater than the friction factor reduction; consequently, the heat transfer per unit pumping power decreases with increasing elasticity. For fully established laminar pipe flow of aqueous polymer solutions, the measured values of the friction factor and dimensionless heat transfer coefficient were in excellent agreement with the values predicted for a power law fluid. For laminar flow in a 2:1 rectangular channel the fully developed friction factor measurements were also in agreement with the power law prediction. In contrast, the measured local heat transfer coefficients for aqueous polymer solutions in laminar flow through the 2:1 rectangular duct were two to three times the values predicted for a purely viscous power law fluid. It is hypothesized that these high heat transfer coefficients are due to secondary motions, which come about as a result of the unequal normal stresses occurring in viscoelastic fluids. The anomalous behavior of one particular aqueous polymer solution—namely, polyacrylic acid (Carbopol)—is described in some detail, raising some interesting questions as to how viscoelastic fluids should be classified. In closing, a number of challenging research opportunities in the study of viscoelastic fluids are presented.

Impingement Cooling of Electronics

Abstract: Experiments were conducted to determine the performance of a system of low-velocity air jets used to cool a simulated electronics package. The test model consisted of a uniform array of rectangular elements mounted to a circuit board. Each element was cooled by a cluster of four jets, and the spent fluid was vented at one end of the channel formed between the circuit board and the plate from which the jets were discharged. Reported are measurements of system pressure drop and convective heat transfer coefficients for elements at various sites within the array. Results indicate that (for the geometry tested) the largest portion of the total pressure drop occurs across the jet orifices. Further, the crossflow of spent air appears to enhance heat transfer for those elements near the exit end of the channel.

Developing Heat Transfer and Friction in a Ribbed Rectangular Duct With Flow Separation at Inlet

Abstract: The local heat transfer and pressure drop characteristics of developing turbulent flows in a rectangular duct with an abrupt-contraction entrance and repeated square-rib pairs on the two opposite walls have been investigated experimentally. Both entrance-region and periodic-fully-developed-region results were obtained. Laser holographic interferometry was employed in the local and average heat transfer measurements. The Reynolds number was varied from 5.0x10[sup 3] to 5.0x10[sup 4]; the rib pitch-to-height ratios were 10, 15, and 20; and the rib height-to-duct height ratio was kept at a value of 0.13. The results allowed the entry length to be determined and the regions susceptible to hot spots to be located. Semi-empirical heat transfer and friction correlations for the periodic fully developed region were developed. Moreover, performance comparisons between the ribbed and smooth ducts were made under two types of constraint, namely equal mass flow rate and equal pumping power. Finally, the effect of thermal entry length on the length mean Nusselt number was also investigated. The results showed that the length mean Nusselt number ratio was a function of only the duct length and independent of PR and Re, and could be further correlated by an equation of the form [bar N]u[sub m]/[bar N]u[sub p]=1more » + 1.844/(X/De).« less

Coupled Structure and Radiation Analysis of Acetylene/Air Flames

Abstract: A coupled radiation-structure analysis of turbulent, non-premixed, strongly radiating acetylene/air flames is described. The analysis extends the laminar flamelet concept to include the effects of local radiative heat loss/gain. A new method for the calculation of the radiative source term is presented. New measurements of mean and fluctuating emission temperatures and radiation intensities, and previous data concerning flame structure are used to evaluate the predictions. Results show good agreement between measurements and predictions of flame structure similar to past uncoupled calculations. The mean emission temperatures and the mean visible radiation intensities are substantially underpredicted by the uncoupled analysis. The coupled calculations provide reasonable estimates of both quantities.

Effect of nozzle configuration on transport in the stagnation zone of axisymmetric, impinging free-surface liquid jets: Part 1-turbulent flow structure

Abstract: This study characterized the mean and fluctuating parts of the radial component of the local velocity in the stagnation region of an impinging, free-surface liquid jet striking a smooth flat plate. Four different nozzle exit conditions were studied, including fully developed pipe flow, a contoured nozzle, and turbulence-damped and undamped sharp-edged orifices. Liquid jet Reynolds numbers in the range 30,000 to 55,000 were investigated. Velocities were measured using laser-Doppler velocimetry. Mean velocities were found to vary nearly linearly with radial location, with the slope of the line being a function of distance from the impingement plate. Dimensionless mean velocity gradients, of relevance to the heat transfer, were found to be a strong function of nozzle type, but roughly independent of jet Reynolds number for a given nozzle type. Turbulence levels were also found to be strongly influenced by the nozzle exit-condition. Local heat transfer data corresponding to the flow structure measurements presented here are reported in Part 2 of this study. 22 refs., 9 figs.

Natural Convection With Radiation in a Cavity With Open Top End

Abstract: The heat transfer by natural convection and radiation in a two-dimensional cavity with one side heated, one side and bottom insulated, and the top open was investigated numerically The first part of the study focused on natural convection alone, and concluded with closed-form correlations for the heat transfer from each of the side walls It was found that the fluid rises along both side walls in the boundary layer regime In the second part of the study, the natural convection correlations were incorporated into a seven-equation model for combined convection and radiation The procedure needed for calculating the floating temperature of the insulated wall was illustrated numerically The individual effects of the dimensions of the cavity, the temperature of the heated wall, and the emissivities of the two side walls were also documented numerically

Free-stream turbulence and concave curvature effects on heated, transitional boundary layers

Abstract: An experimental investigation of the transition process on flat-plate and concave curved-wall boundary layers for various free-stream turbulence levels was performed. Results show that for transition of a flat-plate, the two forms of boundary layer behavior, identified as laminar-like and turbulent-like, cannot be thought of as separate Blasius and fully-turbulent profiles, respectively. Thus, simple transition models in which the desired quantity is assumed to be an average, weighted on intermittency, of the theoretical laminar and fully turbulent values is not expected to be successful. Deviation of the flow identified as laminar-like from theoretical laminar behavior is shown to be due to recovery after the passage of a turbulent spot, while deviation of the flow identified as turbulent-like from the full-turbulent values is thought to be due to incomplete establishment of the fully-turbulent power spectral distribution. Turbulent Prandtl numbers for the transitional flow, computed from measured shear stress, turbulent heat flux and mean velocity and temperature profiles, were less than unity. For the curved-wall case with low free-stream turbulence intensity, the existence of Gortler vortices on the concave wall within both laminar and turbulent flows was established using liquid crystal visualization and spanwise velocity and temperature traverses. Transition was found to occur via a vortex breakdown mode. The vortex wavelength was quite irregular in both the laminar and turbulent flows, but the vortices were stable in time and space. The upwash was found to be more unstable, with higher levels of u' and u'v', and lower skin friction coefficients and shape factors. Turbulent Prandtl numbers, measured using a triple-wire probe, were found to be near unity for all post-transitional profiles, indicating no gross violation of Reynolds analogy. No evidence of streamwise vortices was seen in the high turbulence intensity case.

Finite Element Formulation for Two-Dimensional Inverse Heat Conduction Analysis

Abstract: This paper presents a finite element algorithm for two-dimensional nonlinear inverse heat conduction analysis. The proposed method is capable of handling both unknown surface heat flux and unknown surface temperature of solids using temperature histories measured at a few discrete points. The proposed algorithms were used in the study of the thermofracture behavior of leaking pipelines with experimental verifications.

Unifying the controlling mechanisms for the critical heat flux and quenching : the ability of liquid to contact the hot surface

Abstract: We investigated the hypothesis that the critical heat flux (CHF) occurs when some point on a heated surface reaches a temperature high enough that liquid can no longer maintain contact at that point, resulting in a gradual but continuous increase in the overall surface temperature for most power-controlled systems. This hypothesis unifies the occurrence of the CHF with the quenching of hot surfaces by relating them to the same concept: the ability of a liquid to contact a hot surface, generally defined as some fraction of the liquid's homogeneous nucleation temperature, depending on the contact angle. The proposed hypothesis about the occurrence of the CHF is investigated through a study of the boiling mechanism of the second transition region of nucleate pool boiling of water on copper. An idealized two-dimensional transient conduction heat transfer model was developed to investigate the heat transfer mechanism. The initial macrolayer thickness on the dry portion of the heater, in the second transition region, was found to be bounded between 0 and 11 [mu]m. The radius of the dry patch varied from 15 to 23 mm (60 and 92 percent of the heater radius, respectively) for initial macrolayer thicknesses of 0 and 11 tim, respectively.more » The results indicated that the critical liquid-solid contact temperature at the onset of CHF (the surface temperature at the center of the dry patch) must be lower than the homogeneous nucleation temperature of the liquid for the pool boiling of water on a clean horizontal surface. The liquid-solid contact Temperature was dependent on the initial dry patch liquid macrolayer thickness, varying from 180[degrees]C to 157[degrees]C for initial macrolayer thicknesses of 0 and 11 [mu]m, respectively. Independent assessment of these values shows good agreement with extrapolated contact temperature data at the onset of film boiling.« less