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1.
Experimental Investigation of Subcooled Vertical Upward Flow Boiling in a Narrow Rectangular Channel
Accurate models for the onset of nucleate boiling, density of active nucleation sites (Na), bubble departure size (Dd), and departure frequency (fd) are essential to the success of computational fluid dynamics analysis of two-phase thermal-hydraulics involving subcooled flow boiling in nuclear reactor systems. This work presents an experimental study of subcooled flow boiling in a vertical upward narrow rectangular channel that mimics the flow passage in the plate fuel assembly of boiling water reactors. The experiments are conducted over a range of mass flux (G = 122–657 kg/m2s), inlet subcooling (ΔTsub = 4.7–33.3?C), and heat flux (q″ = 1.7–28.9 W/cm2). Based on the experimental data, empirical correlations are developed for the prediction of onset of nucleate boiling, Na, Dd, and fd for given flow conditions. These correlations are valid in the nucleate boiling regime when the wall superheat is less than 12°C and can be incorporated in the computational fluid dynamics codes to enable more precise simulation of subcooled flow boiling heat transfer and two-phase flow in nuclear energy applications. 相似文献
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The present work is an experimental investigation of the incipient boiling of R134a inside a circular glass minichannel mounted horizontally and equipped with a series of transparent indium tin oxide heaters. The effects of heat flux input levels and refrigerant mass fluxes on the onset nucleate boiling process and on the saturated boiling heat transfer rate are quantitatively explored. The flow pattern visualizations, carried on by means of a high-speed camera, show that the nucleation process is oddly non-uniform: the first vapor bubbles are always generated on the upper side of the tube and lead to a first wall temperature drop. A further increase in the heat flux values results in an increased wall superheat until bubble nucleation also originates on the lower side of the tube, causing a second wall temperature drop. Finally, at higher heat input levels, the boiling process becomes uniformly distributed on the inner tube surface. This phenomenon occurred also after a 180° rotation of the glass tube, and, after a critical analysis of the potential origins, it remains presently unexplained. An evaluation of heat transfer coefficients for low vapor quality regimes is finally presented. 相似文献
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Mohamed El-Genk 《实验传热》2013,26(4):281-301
The effects of orientation and subcooling on pool boiling of the HFE-7100 dielectric liquid near atmospheric pressure (0.085 MPa) from a 10 × 10 mm smooth copper surface are investigated experimentally. Results are obtained for inclination angles θ = 0° (upward-facing), 30°, 60°, 90°, 120°, 150°, and 180° (downward-facing) and liquid subcoolings ΔTsub = 0, 10, 20, and 30 K. Increasing θ decreases the saturation nucleate boiling heat flux at high surface superheats (ΔTsat > 20 K), but increases it only slightly at lower surface superheats. The critical heat flux (CHF) decreases slowly with increasing θ from 0° to 90°, and then deceases faster with increasing θ to 180°. CHF increases linearly with increased subcooling, but the rate increases from 0.016 K?1 at 0° to 0.048 K?1 at 180°. At θ = 0° and ΔTsub = 30 K, CHF is ~ 36 W/cm2 and 24.45 W/cm2 for saturation boiling, while at θ = 180° CHF = 10.85 W/cm2 at ΔTsub = 30 K and only 4.30 W/cm2 at saturation. The developed correlation for CHF of HFE-7100, as a function of θ and ΔTsub, is within ±10% of the present data. The recorded still photographs of the boiling surface in the experiments illustrate the effects of liquid subcooling and surface orientation at different nucleate boiling heat fluxes and surface superheats on vapor bubble accumulation and/or induced mixing at the surface. 相似文献
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A Study on Nucleate Boiling Heat Transfer Characteristics of Acetone on Smooth and Indented Surfaces
This article presents the nucleate boiling heat transfer characteristics of acetone at one bar on smooth and enhanced circular stainless steel surfaces (SS 316) of 20 mm diameter for heat flux between 1 and 4 W cm? 2, which mimic the operating condition of a typical immersion electronic cooling system. The experimental heat transfer coefficient from the smooth surface is validated against Borishanski correlation [1] within acceptable limits of ± 5%. The steel smooth surface is enhanced by providing 100 equally spaced indents of 0.5 mm diameter and 0.05 mm depth. The experimental results indicate that the enhanced surface shows a good shift in the boiling curve and thus, enhancing the nucleate boiling heat transfer at a lesser wall super heat when compared to the smooth surface by around 35% for tested condition. The effect of subcooling on nucleate boiling in enhanced surface reveal that the heat transfer coefficient degrade by 40 to 55% for a sub cooling of 5 to 10 K. The influence of material is studied by a similar enhanced surface made of brass and compared for the same working condition. The brass enhanced surface showed an improved of around 50% against the steel-enhanced surface. Also, the influence of fluid is studied by comparing acetone and n-pentane, which showed that the latter an enhancement in heat transfer coefficient of 50% over the former. 相似文献
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Abstract A pool boiling experiment with either water or Freon-113 was conducted to investigate nucleate boiling from a heated wall with either a spherical or a cylindrical attachment. The result revealed that nucleate boiling can be enhanced by applying a horizontal cylindrical attachment to a vertical heated wall, owing to the favorable thermal environment characterized by a small-gradient liquid temperature profile within the restricted regions between the attachment and the heated wait. Nucleate boiling is enhanced in terms of a lower wall superheat required for incipient boiling and more bubbles generated than from an open heated wall. As a result of the enhanced nucleate boiling, heal transfer of the vertical heated wall above the attachment was improved due to excessive bubbles moving upward along the heated wall, causing removal of the thermal layer near the wall and evaporation of the thin liquid film between the bubbles and the wall. The boiling curve hysteresis with Freon-113 was significantly reduced as a result. The effects of diameter, length and surface roughness of the cylindrical attachment were also investigated. 相似文献
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The problems of simulation of heterogeneous nucleate pool boiling on a horizontal surface on the ascending branch of the boiling curve from the formation of a steam lens (SL) to the boiling crisis are considered. The proposed hypothesis provides in a number of cases a logically consistent interpretation of experiments and outlines the organizational principle of transferring the wall–liquid–steam system into the regime of nonwettable “dry spot” formation. The model includes the following types of nucleate boiling: (a) cyclic boiling with the contact line reverse to the bubble bottom center and bubble departure from the surface (at low heat flux q and the contact angle θ < 90°); (b) single steam bubble conversion into a steam lens, i.e., local film boiling with the possibility of spreading of a single “dry spot” at the variation of the contact angle θ ≥ 90°, and substantial growth of the departure diameter Dd and SL lifetime τd; (c) formation of a single steam cluster of four SLs at a given pressure, the liquid underheating, and the average wall overheating. 相似文献
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This article deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. Electrohydrodynamic (EHD) effects on heat transfer coefficients for dielectric liquids are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n-pentane, R-113, and R-123 are used as working fluids and the boiling phenomenon takes place on a horizontal plane copper surface. The experimental results have shown: (1) a threefold increase of nucleate pool boiling heat transfer coefficients, (2) a threefold increase of the critical heat flux (CHF), and (3) the disappearance of the hysteresis phenomenon. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed. The results obtained by the proposed EHD model are in good agreement with the experimental results. 相似文献
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Presented are results of an experimental study of local heat transfer characteristics in boiling of the dielectric liquid perfluorohexane under forced convection in a horizontal microchannel heat exchanger. The experiments with a copper microchannel heat exchanger comprising 21 channels with sections of 335 × 930 μm were conducted with a mass velocity of 250 to 1000 kg/m2s and a heat flux through the outer wall of the heat exchanger of 3 to 60 W/cm2. The dependence of the local heat transfer coefficient on the heat flux density on the inner wall of the microchannels was established, as well as the critical heat flux. The experimental data are compared with calculations based on known models of heat transfer. 相似文献
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The influence of nucleation on the flow boiling heat transfer coefficient of R-134a/R-290/R-600a refrigerant mixture is experimentally
studied in a smooth horizontal tube of 12.7 mm diameter. The heat transfer coefficients are experimentally measured for stratified
flow patterns under a varied heat flux condition; a condition found in the evaporator of refrigerators and deep freezers.
The experiments are conducted in a counter-current heat exchanger test section. By regulating the flow rate and inlet temperature
of acetone, which is the heating fluid flowing in the outer tube, a varied heat flux is provided to the refrigerant flowing
in the inner tube. The refrigerant mass flow rate is fixed between 3 and 5 g s−1 and its inlet temperature between −8.59 and 5.33°C, which corresponds to a pressure of 3.2 to 5 bar. The significance of
nucleate boiling prevailing in the above-mentioned evaporators is highlighted. The experimental heat transfer coefficients
are also compared with well known heat transfer correlations. 相似文献
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Visualization experiments in upward forced convective nucleate boiling flows were carried out. The bubble growth and collapse have been measured using high-speed photography technique with distilled water under atmospheric pressure. The experiments show that the bubbles depart from nucleating sites shortly after nucleation and slide along the heater surface. The bubbles grow while sliding, attain a maximum size, then lift from the surface sometime during condensation, and quickly vanish in the bulk liquid. Parametric studies show that bubble diameter and departure frequency increases with an increase in heat flux, a decrease in subcooling, and a decrease in mass flux. 相似文献
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An experimental study was performed to investigate the effect of surface coating on the critical heat flux for downward facing boiling on the outer surface of a hemispherical vessel. Steady-state boiling experiments were conducted in the subscale boundary layer boiling (SBLB) facility using test vessels with metallic microporous coatings to obtain the local boiling curves and the local critical heat flux (CHF) limits. Similar heat transfer performance was observed for microporous aluminum and microporous copper coatings. When compared to the corresponding data without coatings, the boiling curves for the coated vessels were found to shift upward and to the right. This meant that the CHF limit was higher with surface coating and that the minimum film boiling temperatures were located at higher wall superheats. In particular, the microporous coatings were found to enhance the local CHF values appreciably at all angular locations explored in the experiments. Results of the present study showed that the microporous aluminum coating was very durable. Even after many cycles of steady state boiling, the vessel coating remained rather intact, with no apparent changes in color or structure. Although similar heat transfer performance was observed for microporous copper coatings, the latter were found to be much less durable and tended to degrade after several cycles of boiling. 相似文献
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Sarit K. Das G. Prakash Narayan Anoop K. Baby 《Journal of nanoparticle research》2008,10(7):1099-1108
Pool boiling heat transfer using nanofluids (which are suspensions of nano-sized particles in a base fluid) has been a subject
of many investigations and incoherent results have been reported in literature regarding the same. In the past, experiments
were conducted in nucleate pool boiling with varying parameters such as particle size, concentration, surface roughness etc.
and all sort of results ranging from heat transfer enhancement, deterioration and no effect were reported. This work tries
to segregate a survey on pool boiling of nanofluids with respect to particle concentration. This is due to the fact that a
major drift in heat transfer behavior is observed at higher and lower particle concentration. But upon deep perusal it has
been found that deterioration in heat transfer coefficient are mainly observed at higher particle concentrations (4–16% by
weight) and enhancements mainly at lower particle concentrations (0.32–1.25% by weight). Moreover, the relative size of the
particle with respect to the surface roughness of the heating surface seems to play an important role in understanding the
boiling behaviour. Also, recent works have reported that change in ‘surface wetting’ of the heating surface due to nanofluids
and the formation of a porous layer modifiying nucleation site density can be of importance in predicting nucleate pool boiling
characteristics of nanofluids. In the present paper, attempts are made to make systematic analysis of results in literature
and try to bring out a common understanding of the results in literature. 相似文献
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In this paper, a fractal model for nucleate pool boiling heat transfer of nanofluids is developed based on the fractal distribution of nanoparticles and nucleation sites on boiling surfaces. The model shows the dependences of the heat flux on nanoparticle size and the nanoparticle volume fraction of the suspension, the fractal dimension of the nanoparticle and nucleation site, temperature of nanofluids and properties of fluids. The fractal model predictions show that the natural convection stage continues r... 相似文献
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Experimental investigation into the pool boiling heat transfer of aqueous based γ-alumina nanofluids
This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations over the past few years. The work is motivated by the controversial results reported in the literature and the potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to formulate stable aqueous based nanofluids containing γ-alumina nanoparticles (primary particle size 10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼
∼40% at a particle loading of 1.25% by weight. Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids under the nucleate pool boiling conditions be associated with the properties and behaviour of the nanofluids and boiling surface, as well as their interactions. 相似文献