Showing papers on "Critical heat flux published in 1996"
TL;DR: In this paper, the effect of volumetric flux distribution on critical heat flux (CHF) on spray cooling of a hot surface was investigated, and it was determined that CHF can be maximized when the spray is configured such that the spray impact area just inscribes the square surface of the heater.
Abstract: Spray cooling of a hot surface was investigated to ascertain the effect of nozzle-to-surface distance on critical heat flux (CHF). Full cone sprays of Fluorinert FC-72 and FC-87 were used to cool a 12.7 X 12.7 mm 2 surface. A theoretical model was constructed that accurately predicts the spray's volumetric flux (liquid volume per unit area per unit time) distribution across the heater surface. Several experimental spray sampling techniques were devised to validate this model. The impact of volumetric flux distribution on CHF was investigated experimentally. By measuring CHFfor the same nozzle flow rate at different nozzle-to-surface distances, it was determined CHF can be maximized when the spray is configured such that the spray impact area just inscribes the square surface of the heater. Using this optimum configuration, CHF data were measured over broad ranges of flow rate and subcooling, resulting in a new correlation for spray cooling of small surfaces.
267 citations
TL;DR: In this paper, the effects of surface orientation on pool boiling characteristics of a highly wetting fluid from a flush-mounted micro-porous-enhanced square heater were investigated by applying copper and aluminum particle coatings to the heater surfaces.
Abstract: Experiments are performed to understand the effects of surface orientation on the pool boiling characteristics of a highly wetting fluid from a flush-mounted, micro-porous-enhanced square heater Micro-porous enhancement was achieved by applying copper and aluminum particle coatings to the heater surfaces Effects of heater orientation on CHF and nucleate boiling heat transfer for uncoated and coated surfaces are compared A correlation is developed to predict the heater orientation effect on CHF for those surfaces
169 citations
TL;DR: In this paper, the impact of water droplets on heated surfaces with different roughness was studied using still and high speed photographic techniques and heat transfer measurements and the study encompassed droplet Weber numbers of 20, 60 and 220 and surface temperatures of 100-280°C.
168 citations
TL;DR: In this article, the authors focus on the fundamental issues that influence boiling heat transfer to a free-surface, planar jet of water and present local boiling curves at several streamwise distances from the stagnation line, while streamwise distributions of the surface temperature and convection coefficient.
126 citations
TL;DR: In this paper, a model based on an additive mechanism of heat transfer is proposed for pool boiling of single component systems, and the authors estimate the nucleation site density from a literature correlation that includes the boiling surface characteristics.
110 citations
TL;DR: The Marangoni effect on the critical heat flux (CHF) condition in pool boiling of binary mixtures has been identified and its effect has been quantitatively estimated with a modified model derived from hydrodynamics.
Abstract: The Marangoni effect on the critical heat flux (CHF) condition in pool boiling of binary mixtures has been identified and its effect has been quantitatively estimated with a modified model derived from hydrodynamics. The physical process of CHF in binary mixtures, and models used to describe it, are examined in the light of recent experimental evidence, accurate mixture properties, and phase equilibrium revealing a correlation to surface tension gradients and volatility. A correlation is developed from a heuristic model including the additional liquid restoring force caused by surface tension gradients. The CHF condition was determined experimentally for saturated methanol/water, 2-propanol/water, and ethylene glycol/water mixtures, over the full range of concentrations, and compared to the model. The evidence in this study demonstrates that in a mixture with large differences in surface tension, there is an additional hydrodynamic restoring force affecting the CHF condition.
90 citations
TL;DR: In this article, an analytical investigation of the heat transfer characteristics for evaporating thin liquid films in V-shaped microgrooves with nonuniform input heat flux was conducted, where the combined heat transfer mechanisms of both liquid conduction and interfacial vaporization were used to describe the local interfacial mass flux in the interline region.
Abstract: An analytical investigation of the heat transfer characteristics for evaporating thin liquid films in V-shaped microgrooves with nonuniform input heat flux was conducted. This investigation assumed that the capillary pressure difference caused by the receding of the meniscus is responsible for the axial liquid flow along the groove, and that the disjoining pressure difference along the groove side wall provided the driving force for the flow up the groove wall. The combined heat transfer mechanisms of both liquid conduction and interfacial vaporization were used to describe the local interfacial mass flux in the interline region. Based on this approach, a local heat transfer coefficient was defined. The local and average heat transfer coefficients were both found to be sensitive to the characteristic thermal resistance ratio. In addition, when the film superheat was constant, the primary factor affecting the length of the evaporating interline region was found to be the heat flux supplied to the bottom plate, and for high heat flux conditions, the highest heat transfer coefficient did not necessarily exist at the axial dryout point. The expression developed for the evaporating film profile was shown to assume an exponential form if the heat flux distributed on the active interline region was assumed to be uniform.
77 citations
TL;DR: In this paper, the authors have performed heat transfer experiments on smooth circular and swirl tubes in the regions from non-boiling to high sub-cooled partial nucleate boiling.
75 citations
TL;DR: In this article, the local heat transfer coefficient distribution over all four walls of a large-scale model of a gas turbine cooling passage has been measured in great detail and the contribution of the rib, at 5 percent blockage, to the overall roughened heat transfer coefficients was found to be considerable.
Abstract: The local heat transfer coefficient distribution over all four walls of a large-scale model of a gas turbine cooling passage have been measured in great detail. A new method of determining the heat transfer coefficient to the rib surface has been developed and the contribution of the rib, at 5 percent blockage, to the overall roughened heat transfer coefficient was found to be considerable. The vortex-dominated flow field was interpreted from the detailed form of the measured local heat transfer contours. Computational Fluid Dynamics calculations support this model of the flow and yield friction factors that agree with measured values. Advances in the heat transfer measuring technique and data analysis procedure that confirm the accuracy of the transient method are described in full.
58 citations
TL;DR: In this article, a theory on both the heat consumed in pervaporation and the heat transfer in the liquid feed boundary layer is used to experimentally evaluate the heat consumption, as well as the heat-transfer resistance of the boundary layer.
57 citations
TL;DR: In this article, 11 correlations and models for critical heat flux (CHF) of subcooled flow boiling in water were evaluated and the Celata model was the best with respect to accuracy.
TL;DR: In this paper, the authors investigated the subcooled flow boiling heat transfer characteristics of binary mixtures in microchannel plates and found that the heat transfer coefficient at the onset of flow boiling and in the partial nucleate boiling region was greatly influenced by liquid concentration, microchannel and plate configuration, flow velocity and amount of subcooling.
TL;DR: In this paper, the effects of surface treatments and gassy-subcooling on pool boiling heat transfer are quantified by testing both smooth and treated surfaces at gassy subcooling levels from 0°C to 40°C (1 atm) and 40°c to 85°C(3 atm).
Abstract: The effects of surface treatments and gassy-subcooling on pool boiling heat transfer are quantified by testing both smooth and treated surfaces at gassy-subcooling levels from 0°C to 40°C (1 atm) and 40°C to 85°C (3 atm). Incipient and nucleate boiling wall superheats decrease over this range of gassy-subcooling. At gassy-subcooling levels greater than 20°C, the boiling curves for the smooth surface indicate two distinct regions governed by different heat transfer mechanisms, one in which the boiling process is influenced by the presence of dissolved gas, the other by boiling of the pure liquid. The critical heat flux (CHF) for each surface continually increases with increased levels of gassy-subcooling and the CHF sensitivity to gassy-subcooling is higher for the treated surface. The CHF increase due to combined surface treatment and gassy-subcooling (85°C) is ∼400 percent (78 W/cm 2 ).
TL;DR: In this article, the effects of gap sizes, channel heights, and heat fluxes on heat transfer and critical heat flux (CHF) were studied systematically, and the results based on observation and measurement illustrate that smaller the gap size, the smaller the incipient heat flux.
TL;DR: In this article, the laminar thermal and hydraulic boundary layers were divided into five regions of flow and general expressions of heat transfer coefficients were obtained in all the four regions of stagnation and wall jet zones before the hydraulic jump.
Abstract: An analytical research was conducted to study heat transfer from horizontal surfaces to normally impinging circular jets under arbitrary-heat-flux conditions. The laminar thermal and hydraulic boundary layers were divided into five regions of flow. General expressions of heat transfer coefficients were obtained in all the four regions of stagnation and wall jet zones before the hydraulic jump.
TL;DR: In this article, the heat transfer characteristics of a low temperature latent heat storage system have been determined for circular finned and unfinned tubes using sodium acetate tribydrate as a phase change material (PCM).
TL;DR: In this paper, the divertor plate of fusion experimental machines is developed for removal of high heat loads with heating on one side, which is essential for removing high heat load with one side.
Abstract: Development of high-heat-flux components such as the divertor plate of fusion experimental machines is essential for removal of high heat loads with heating on one side. For this purpose, the autho...
TL;DR: In this article, a model for heat transfer without condensation is derived, resulting in a set of classical differential equations, and the experiments performed are reported and the results compared with the models presented.
TL;DR: In this paper, a numerical study of upward fluid flow and the corresponding convective heat transfer in a vertical porous annulus was presented, which investigated the effects of the inertia term, thermal dispersion, variable porosity, variable properties, buoyancy, particle diameter, and fluid pressure on the flow and heat transfer.
Abstract: This article presents a numerical study of upward fluid flow and the corresponding convective heat transfer in a vertical porous annulus. The study investigated the effects of the inertia term, thermal dispersion, variable porosity, variable properties, buoyancy, particle diameter, and fluid pressure on the flow and heat transfer. The heat transfer augmentation produced by the porous matrix was also analyzed. It was found that for the conditions studied, the effect of thermal dispersion and variable porosity had to be considered. For Ree ≥ 5, the flow inertia had nonnegligible influence on the flow field and local heat transfer coefficient near the inlet (x / L ≤ 0.02). When Ree ≥ 400, the flow inertia significantly influenced the overall friction factor in the vertical porous annulus. The porous medium greatly enhanced the heat transfer coefficient and also sharply increased the friction resistance. At supercritical pressures (25 Mpa), variable properties significantly influenced the heat transfer.
TL;DR: In this article, a unique method to determine the vapor volumetric flow rate above a heated wire utilizing a single photograph and laser-Doppler anemometry is developed and discussed.
Abstract: A unique method to determine the vapor volumetric flow rate above a heated wire utilizing a single photograph and laser-Doppler anemometry is developed and discussed. The volumetric flow rate is combined with additional analyses to determine the overall contributions to the total heat flux from four nucleate boiling heat transfer mechanisms (latent heat, natural convection, Marangoni flow, and microconvection). This method is applied to a 75-μm wire immersed in a saturated, highly wetting liquid (FC-72). Latent heat is identified as the dominant mechanism in the fully developed nucleate boiling regime.
31 Dec 1996
TL;DR: In this paper, the authors show that the heat transfer coefficient is a function of the flow quality, the mass flux, nd of course, the heat flux and the related surface superheat.
Abstract: Micro-channel surfaces have been shown to be effective in thermal management of electronic components. Such channels are often 20 to 200 {micro}m in width and depth. The large number of channels per unit width of the surface offer a significantly higher area of heat transfer. However, a large number of variables control the two-phase flow heat transfer coefficient. Specifically, the phenomenon surrounding the bubble generation plays a very important role in two-phase flow heat transport. The bubble generation, the diameter, and the frequency are functions of the channel geometry, the channel surface, and its shape. In addition, the pressure, the surface heat flux, and the mass flux affect the heat transport significantly. Experiments were conducted on a setup that has been specially built for testing micro-channel heat exchangers. The range of parameters considered in the study are: power input: 20 to 400 W, mass flux: 35 to 300 ml/min, quality: 0 to 0.9, inlet subcooling: 5 C. The results indicate that the heat-transfer-coefficient is a function of the flow quality, the mass flux, nd of course, the heat flux and the related surface superheat. The heat transfer coefficient decreases with wall superheat from a value of 12,000 W/m{sup 2}-K atmore » 10 C to 9,000 W/m{sup 2}-K at 80 C. The coefficient decreases by 30% when the quality is increased from 0.01 to 0.65. Also, the pressure drop increases with increasing heat flux.« less
TL;DR: In this paper, an experimental study on pool boiling heat transfer from finned copper surfaces immersed in a saturated dielectric liquid (Galden HT-55) is presented.
TL;DR: In this paper, it is shown that the application of the heat balance method to such correlations, which leads to expressing the CHF margins in terms of the critical power ratio, may be more appropriate.
TL;DR: In this article, the authors presented a software to determine heat transfer through terra-cotta bricks full of large vertical cavities, based on Karman-Polhausen's method for convection and on the radiosity method for radiative heat transfer.
TL;DR: In this article, the electrohydrodynamically enhanced heat transfer in pool boiling in the nucleate regime was studied using R-123 as the working fluid, and the results indicated that the heat transfer coefficient at a heat flux of 1.6 kW/m{sup 2} and a voltage of 10 kV was 4.6 times higher than the heat-transfer coefficient without the electric field presence.
Abstract: The electrohydrodynamically enhanced heat transfer in pool boiling in the nucleate regime was studied using R-123 as the working fluid. An experimental apparatus was designed and built which allowed accurate measurements. The evaporator consisted of an electrically heated single horizontal smooth tube. Several different electrode designs were investigated. This study included higher heat fluxes than most of those previously reported in the literature. A summary of the previous work is provided. The results indicated that the heat transfer coefficient at a heat flux of 1.6 kW/m{sup 2} and a voltage of 10 kV was 4.6 times higher than the heat transfer coefficient without the electric field presence. However, the heat transfer coefficient at 52 kW/m{sup 2} and 10 kV was improved only by 38%. These enhancements are significant even at large heat flux levels. The power consumption for establishing the electric fields was on the order of 0.1% of the heat transfer power in the evaporator. When R-123 fluid was contaminated with a few percent ethanol, the boiling heat transfer at 3.3 kW/m{sup 2} was increased by a factor of 12.6 at 15 kV compared to zero kV. Finally, the presence of the electric fields nearly eliminated the hysteresis effect.
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TL;DR: In this paper, a mathematical model to evaluate the critical heat flux required to ignite thermoplastics in radiative ignition experiments is described, where the concept of a critical mass flux from the solid phase into the gas phase is used as the criticality condition.
Abstract: We describe a mathematical model to evaluate the critical heat flux required to ignite thermoplastics in radiative ignition experiments. The concept of a critical mass flux from the solid phase into the gas phase is used as the criticality condition. Using this criterion, the solid temperature corresponding to the critical heat flux is related to the 'characteristic temperature' determined in Thermogravimetric experiments (TG). We investigate the dependence of the critical heat flux and critical surface temperature on the value taken for the critical mass flux. The specific application we have in mind is piloted ignition in the cone calorimeter.
TL;DR: In this paper, the current definition of critical heat flux (CHF) margins and their differences are reviewed and discussed. But the definition of CHF margins is not defined in this paper.