Showing papers on "Critical heat flux published in 2005"
TL;DR: In this article, the authors show that the extent of thermal conductivity enhancement sometimes greatly exceeds the predictions of well-established theories, and new theoretical descriptions may be needed to account properly for the unique features of nanofluids, such as high particle mobility and large surface to volume ratio.
824 citations
TL;DR: In this article, the performance of nano-fluids with nano-particles suspended in water is studied using different volume concentrations of alumina nano-partsicles, and the experimental results show that these nano-fluids have poor heat transfer performance compared to pure water in natural convection and nucleate boiling.
655 citations
TL;DR: In this article, the authors present a literature survey for heat transfer to supercritical water flowing in channels, and assess the work that was done and to understand the specifics of heat transfer at these conditions.
289 citations
TL;DR: In this paper, a new version of the flow pattern map presented in Part I of this paper has been used to modify the dry angle in the heat transfer model of Kattan-Thome-Favrat.
286 citations
TL;DR: In this article, the critical heat flux (CHF) limit of a flow boiling system was investigated in the presence of either an upstream compressible volume instability or an excursive instability rather than the conventional dryout mechanism.
Abstract: The critical heat flux (CHF) limit is an important consideration in the design of most flow boiling systems. Before the use of microchannels under saturated flow boiling conditions becomes widely accepted in cooling of high-heat-flux devices, such as electronics and laser diodes, it is essential to have a clear understanding of the CHF mechanism. This must be coupled with an extensive database covering a wide range of fluids, channel configurations, and operating conditions. The experiments required to obtain this information pose unique challenges. Among other issues, flow distribution among parallel channels, conjugate effects, and instrumentation need to be considered. An examination of the limited CHF data indicates that CHF in parallel microchannels seems to be the result of either an upstream compressible volume instability or an excursive instability rather than the conventional dryout mechanism. It is expected that the CHF in parallel microchannels would be higher if the flow is stabilized by an orifice at the entrance of each channel. The nature of CHF in microchannels is thus different than anticipated, but recent advances in microelectronic fabrication may make it possible to realize the higher power levels.
282 citations
TL;DR: In this paper, the authors applied this super-hydrophilic nature to enhancement of boiling and evaporation heat transfer, and found that the critical heat flux (CHF) of TiO2-coated surface is about two times larger than that of noncoated one, and Leidenfrost temperature increases as the contact angle decreases.
239 citations
TL;DR: In this article, the authors visualize the flow pattern and measure heat transfer coefficient during explosive boiling of water in parallel triangular micro-channels, and show that the behavior of long vapor bubbles, occurring in a micro-channel at low Reynolds numbers, was not similar to annular flow with interposed intermitted slugs of liquid between two long vapor trains.
219 citations
TL;DR: In this paper, the authors investigated heat transfer and pressure drop at supercritical conditions using carbon dioxide as a modelling fluid as a cheaper and faster alternative to using SCW and found that the majority of experimental data were obtained in vertical tubes, some data in horizontal tubes and just few in other flow geometry.
179 citations
TL;DR: In this paper, a new silicon micro-channel heat sink, composed of parallel longitudinal microchannels and several transverse micro-channels, which separate the whole flow length into several independent zones, in which the thermal boundary layer is in developing.
175 citations
TL;DR: In this article, the authors investigated the effects of the refrigerant mass flux, imposed heat flux, system pressure (saturated temperature) and mean vapor quality of R-410A on the measured data and proposed an empirical correlation for the R410A condensation heat transfer coefficient in the vertical plate heat exchanger.
167 citations
TL;DR: In this article, heat transfer in silica nanofluids at different acidity and base was measured for various ionic concentrations in a pool boiling experiment, and it was shown that nanosilica suspension increases the critical heat flux 3 times compared to conventional fluids.
Abstract: Heat transfer in silica nanofluids at different acidity and base is measured for various ionic concentrations in a pool boiling experiment. Nanosilica suspension increases the critical heat flux 3 times compared to conventional fluids. The 10-nm particles possess a thicker double diffuse layer compared to 20-nm particles. The catalytic properties of nanofluids decrease in the presence of salts, allowing the particles to cluster and minimize the potential increase in heat transfer. Nanofluids in a strong electrolyte, i.e., in high ionic concentration, allow a higher critical heat flux than in buffer solutions because of the difference in surface area. The formation and surface structure of the deposition affect the thermal properties of the liquid.
TL;DR: In this paper, a discrete double-inclined ribs tube (DDIR-tube) was developed to enhance the heat transfer performance of laminar convection convection in a straight circular tube.
TL;DR: In this article, the onset of nucleate boiling in the flow of water through a microchannel heat sink was investigated and an analytical model was developed to predict the incipient heat flux as well as the bubble size.
TL;DR: In this paper, the authors present both three and two-dimensional numerical analysis of convective heat transfer in microchannels, and compare the measured and computed heat flux and temperature fields.
TL;DR: In this article, the results of a large parametric study of spray cooling using a number of different nozzle patterns were presented, and it was found that the nozzles that use the fluid most efficiently to remove thermal energy were limited by low peak heat fluxes and that the highest peak fluxes were obtained when phase change was avoided.
TL;DR: The performance of single and multiple nozzle sprays for high heat flux electronics cooling using nitrogen-saturated FC-72 was studied in a multi-chip module (MCM) test setup, similar to MCM's used in current high end computer systems.
TL;DR: In this paper, the authors proposed a modified superposition model, where the total heat flux is assumed to be additively composed of a forced convective and a nucleate boiling component.
TL;DR: In this article, the influence of heat flux, system pressure, mass flow rate, vapor fraction, diameter of evaporator channel and tubing distance between evaporator and condenser on the heat transfer coefficient of an advanced two-phase thermosyphon loop is reported.
Abstract: As heat generation from electronic components increase and the limit of air-cooling is reached, the interest for using liquid cooling for high heat flux applications has risen. Thermosyphon cooling is an alternative liquid cooling technique, in which heat is transferred as heat of vaporization from evaporator to condenser with a relatively small temperature difference. The effect of fluid properties, the structure of wall surfaces, and the effect of system pressure was investigated and reported previously by the author. In this paper, the influence of heat flux, system pressure, mass flow rate, vapor fraction, diameter of evaporator channel and tubing distance between evaporator and condenser on the heat transfer coefficient of an advanced two-phase thermosyphon loop is reported. The tested evaporators were made from small blocks of copper with 7, 5, 4, 3 and 2 vertical channels with the diameters of 1.1, 1.5, 1.9, 2.5, and 3.5 mm, respectively and the length of 14.6 mm. Tests were done with isobutane at heat fluxes ranging between 28.3 and 311.5 kW/m 2 .
TL;DR: In this article, time and space resolved temperature and heat flux distributions under nucleating bubbles on a constant heat flux surface were obtained using a 10 × 10 microheater array with 100μm resolution along with high-speed images.
TL;DR: Xu et al. as discussed by the authors analyzed the transient flow patterns for microscale boiling heat transfer and deduced five heat transfer mechanisms from the flow patterns, which are consistent with the heat transfer coefficient measurements with the effects of heat fluxes, mass fluxes and vapor mass qualities.
Abstract: In our previous paper (J L Xu et al 2005 J. Micromech. Microeng. 15 362–76), it is identified that the transient flow patterns for microscale boiling heat transfer are repeated on the timescale of milliseconds. A full cycle could be subdivided into three substages: liquid refilling stage, bubble nucleation, growth and coalescence stage and transient annular flow stage. Five heat transfer mechanisms could be deduced from the transient flow patterns. This paper extends the above work and mainly focuses on the boiling heat transfer behavior, which was performed for 102 runs with the following data ranges: inlet pressures of 1–2 bar, inlet liquid temperatures of 24–45 °C, pressure drops of 10–100 kPa, mass fluxes of 64–600 kg m−2 s−1, heat fluxes of 150–480 kW m−2, exit vapor qualities of 0.07–1.15 and the boiling numbers of 0.69 × 10−3–5.046 × 10−3. The silicon wafer test section consists of ten triangular microchannels with the hydraulic diameter of 155.4 µm. Acetone is selected as the working fluid. The heat transfer coefficients were analyzed with the effects of the heat fluxes, the mass fluxes and the vapor mass qualities. We provide a link between the transient flow patterns and the heat transfer process. The boiling numbers can be used to characterize the microscale boiling heat transfer, which can display three distinct regions by dividing the boiling numbers into three subranges. The transient flow pattern based heat transfer mechanisms are very consistent with the heat transfer coefficient measurements with the effects of the heat fluxes, mass fluxes and vapor mass qualities. The transition boundaries among the three heat transfer regions are given.
TL;DR: In this paper, the authors compared the performance of HFE-7100 dielectric liquid on porous graphite with those of smooth copper (Cu) of the same dimensions, and found that air entrapped in re-entrant type cavities, ranging in size from tens to hundreds of microns, not only enhanced the nucleate boiling heat transfer and the critical heat flux (CHF), but also, the mixing by the released tiny air bubbles from the graphite prior to boiling incipience enhanced the natural convection heat transfer by ∼19%.
TL;DR: In this paper, the boiling flow of deionized water through 227 μm hydraulic diameter microchannels with 7.5 μm wide interconnected reentrant cavities at 47 kPa exit pressure has been investigated.
Abstract: Boiling flow of deionized water through 227 μm hydraulic diameter microchannels with 7.5 μm wide interconnected reentrant cavities at 47 kPa exit pressure has been investigated. Average two-phase heat transfer coefficients have been obtained over effective heat fluxes ranging from 28 to 445 W/cm 2 and mass fluxes from 41 to 302 kg/m 2 s. A map is developed that divides the data into two regions where the heat transfer mechanisms are nucleation or convective boiling dominant. The map is compared to similar atmospheric exit pressure data developed in a previous study. A boiling mechanism transition criterion based on the Reynolds number and the Kandlikar k 1 number is proposed.
TL;DR: In this paper, the authors investigated how the channel size affects the subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct.
TL;DR: In this paper, a set of 27 copper-tubed CEOHPs with internal diameters of 2.03, 1.06 and 0.66mm were used for experimental studies of the heat transfer characteristics at critical state of a closed end oscillating heat pipe (CEOHP).
TL;DR: In this paper, the authors measured the boiling heat transfer coefficients of carbon dioxide (CO 2 ) in mini tubes with inner diameters of 2.0 and 0.98mm and analyzed them as a function of operating parameters.
TL;DR: In this paper, a large number of new experimental data are presented on two-phase pressure drop, bubble size, gas hold-up and heat transfer coefficients for co-current upward gas/liquid flow through beds of different particle sizes under constant wall heat flux.
TL;DR: In this paper, the effects of micro-pin-fins on boiling phenomena and heat transfer from square simulated silicon chips immersed in a pool of FC-72 were investigated, and the enhanced boiling heat transfer mechanisms for the micropin-finned chips were discussed.
Abstract: Experiments were conducted to study the effects of micro-pin-fins on boiling phenomena and heat transfer from square simulated silicon chips immersed in a pool of FC-72. Two kinds of micro-pin-fins having fin thickness of 30 μm and fin heights of 60 and 200 μm, respectively, were fabricated on the silicon chip surface with the dry etching technique. The experiments were conducted at the liquid subcoolings of 3, 25, 35 and 45 K. The effects of dissolved air in FC-72 and chip orientation were also investigated. The boiling curve of the micro-pin-finned chips was characterized by a very small increase in wall superheat with increasing heat flux, and the wall temperatures at the CHF point for all the micro-pin-finned chips were less than the upper limit for the reliable operation of LSI chips (Tw=85°C). Liquid subcooling was very effective in elevating CHF for the micro-pin-finned chips compared to the smooth surface and other treated surfaces. The enhanced boiling heat transfer mechanisms for the micro-pin-finned chips were discussed.
TL;DR: In this paper, the current status of this heat transfer prediction model and flow pattern map is described, covering the following topics: the onset of nucleate boiling, elimination of iterative calculations, evaporation of CO 2 up to pressures as high as 64 bars for tube diameters from 0.9 to 10.0 mm, convective condensation inside horizontal tubes, and the recent experimental verification of the drift flux void fraction model used by the flow pattern maps and heat transfer model.
TL;DR: In this paper, an experimental study of the energy budget, i.e. internal flux, evaporation flux and convective heat flux for monodisperse combusting droplets in linear stream, is presented.
01 Jan 2005
TL;DR: In this paper, the authors conducted pool boiling experiments under saturated conditions (Tsat = 60 °C) using nanofluids as the coolants and found that the performance of the two pure water-based and two pure ZnO-water-based nanoparticles were similar.
Abstract: This investigation conducts pool boiling experiments under saturated conditions (Tsat = 60 °C) using nanofluids as the coolants. Three different nanofluids were tested including zinc oxide (ZnO)-water, aluminum oxide (Al2 O3 )-water and aluminum oxide (Al2 O3 )-water+ethylene glycol (ethylene glycol solution). At saturation (Tsat = 60°C), the pool boiling performance of the two pure water based nanofluids were similar. The maximum CHF enhancement as compared to the predicted Zuber’s [1] CHF evaluated at an equivalent saturation temperature is ∼180% for Al2 O3 -water nanofluids and ∼240% for ZnO-water nanofluids. In both cases, no degradation in the boiling heat transfer rate was observed for lower nanoparticle concentrations. However, higher nanoparticle concentrations demonstrate nucleate boiling heat transfer degradation at high heat fluxes. The dispersion of Al2 O3 nanoparticles in various ethylene glycol solutions is also found to enhance CHF by as much as ∼130%. A significant difference in the diameter of individual grains/particles (27 ± 16.3 nm) and the volume weighted average diameter of particles in solution (155 ± 80 nm) indicates that the Al2 O3 -water nanofluids consist primarily of nanoparticle agglomerates. Gravimetric fractionation of the nanofluid produced nanofluids with particle/particle aggregate average diameters that ranged from 69–346 nm. Over the size range tested, there was no significant CHF dependence on the average particle diameter.Copyright © 2005 by ASME