共查询到20条相似文献,搜索用时 15 毫秒
1.
Milada Pezo Vladimir Stevanovic 《International Journal of Heat and Mass Transfer》2011,54(15-16):3296-3303
Three-dimensional numerical simulations of the atmospheric saturated pool boiling are performed with the aim of predicting the critical heat flux. The two-phase mixture in pool boiling is described with the transient two-fluid model. The transient heat conduction in the horizontal heated wall is also solved. Dynamics of vapor generation on the heated wall is modeled through the density of nucleation sites and the bubble residence time on the wall. The heater’s surface is divided into zones, which number per unit area equals the density of nucleation sites, while the location of nucleation site within each zone is determined by a random function. The results show a replenishment of the heater’s surface with water and surface wetting for lower heat fluxes, while heater’s surface dry-out is predicted at critical heat flux values. Also, it is shown that the decrease of nucleation site density leads to the reduction of critical heat flux values. Obtained results of critical heat flux are in good agreement with available measured data. The presented approach is original regarding both the application of the two-fluid two-phase model for the prediction of boiling crisis in pool boiling and the defined boundary conditions at the heated wall surface. 相似文献
2.
In Cheol Bang Soon Heung Chang Won-Pil Baek 《International Journal of Heat and Mass Transfer》2005,48(25-26):5371-5385
A new experimental work was made to discover a principle mechanism of the burnout in pool boiling. Here, we directly observed a liquid layer structure under a massive vapor clot and the liquid layer-related burnout phenomenon. Based on the present observations, we have made a visual model for the formation and dryout of a liquid film under its vapor environment. At the formation process, liquid is trapped in interleaved space between growing bubbles and surface and the liquid trapping continues between coalesced bubbles and surface. In the dryout process, we especially observed vapor “holes” made by spontaneous breakup of discrete nucleating bubbles inside a vapor clot. The burnout can be triggered by the evaporation of the liquid film region expanded from rims of the holes. 相似文献
3.
This paper deals with a study of enhanced critical heat flux (CHF) and burnout heat flux (BHF) in pool boiling of water with suspended silica nanoparticles using Nichrome wires and ribbons. Previously the current authors and other researchers have reported three-digit percentage increase in critical heat flux in silica nanofluids. This study investigates the effect of various heater surface dimensions, cross-sectional shapes as well as surface modifications on pool boiling heat transfer characteristics of water and water-based nanofluids. Our data suggest that the CHF and BHF decrease as heater surface area increases. For concentrations from 0.1 vol% to 2 vol%, the deposition of the particles on the wire allows high heat transfer through inter-agglomerate pores, resulting in a nearly 3-fold increase in burnout heat flux at very low concentrations. The nanoparticle deposition plays a major role through variation in porosity. The CHF enhancement is non-monotonic with respect to concentration. As the concentration is increased, the CHF and BHF decrease prior to increasing again at higher concentrations. Results show a maximum of 270% CHF enhancement for ribbon-type heaters. The surface morphology of the heater was investigated using SEM and EDS analyses, and it was inferred that the 2 vol% concentration deposition coating had higher porosity and rate of deposition compared with 0.2 vol% case. 相似文献
4.
《International Journal of Heat and Mass Transfer》2007,50(5-6):997-1009
An experimental investigation into the effects of pressure and subcooling on the pool boiling critical heat flux from a bare silicon chip-like heater and from a silicon heater coated with microporous layers, is reported. The dual inline heater package was immersed in FC-72, a dielectric fluid, and the experiments were performed in the horizontal orientation, with subcooling varying between 0 K and 72 K, and the pressure between 101.3 kPa and 303.9 kPa. The maximum CHF values on the diamond-base microporous-coated silicon heater were found to reach 47 W/cm2, at 3 atm and nearly 50 K of subcooling, and to provide an average enhancement of approximately 60% over the values attained with un-treated silicon surfaces. An available CHF correlation, with a reported standard deviation of 12.5% for un-treated surfaces over a large range of pressures, subcoolings, and surface conditions, was shown to predict the pressure and subcooling effects on CHF from the surface-enhanced chip with a standard deviation of 12%. 相似文献
5.
Bao Truong Lin-wen Hu Jacopo Buongiorno Tom McKrell 《International Journal of Heat and Mass Transfer》2010,53(1-3):85-94
Nanofluids are colloidal dispersions of nanoparticles in homogenous base fluids. Previous studies have shown that nanofluids can increase pool boiling critical heat flux (CHF) by forming a porous deposition on the heated surface. However, questions remain whether nanoparticles can further enhance the CHF on a passively engineered heat transfer surface, such as a sandblasted metal plate. In this study, three water-based nanofluids (diamond, zinc oxide and alumina) were used to modify sandblasted stainless steel 316 plate heaters via boiling induced deposition. The pool boiling CHF of these pre-coated heaters increased by up to 35% with respect to that of the bare, sandblasted heaters. The enhancements are highest for alumina and zinc oxide nanofluids. Detailed surface characterization of these pre-coated heaters showed different surface morphology depending on the type of nanofluids used. Additionally, the deposited nanoparticles layers were found to alter the wettability of the heaters. Contact angle measurement provided quantitative data to determine possible CHF enhancement based on existing correlations. 相似文献
6.
The pool boiling characteristics of dilute dispersions of alumina nanoparticles in water were studied. Consistent with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). During experimentation and subsequent inspection, formation of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly changes surface texture of the heater wire surface which could be the reason for improvement in the CHF value. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20301 相似文献
7.
8.
《International Journal of Heat and Mass Transfer》2007,50(19-20):4105-4116
The pool boiling characteristics of dilute dispersions of alumina, zirconia and silica nanoparticles in water were studied. Consistently with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). Buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improves the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. A review of the prevalent CHF theories has established the nexus between CHF enhancement and surface wettability changes caused by nanoparticle deposition. This represents a first important step towards identification of a plausible mechanism for boiling CHF enhancement in nanofluids. 相似文献
9.
《International Journal of Heat and Mass Transfer》2007,50(19-20):3780-3787
This article suggests a method of the determination of main parameters and dynamic characteristics of heat transfer crisis on a surface of fast heated wall. The new physical models describing process of transition from nucleate to film boiling are presented. The results of transient critical heat flux modeling are compared with the experimental data for saturated water pool boiling under atmospheric pressure. 相似文献
10.
A.M. Bhat J.S. Saini R. Prakash 《International Journal of Heat and Mass Transfer》1986,29(12):1953-1961
Analytical expressions for macrolayer thickness and the rate of heat transfer through a macrolayer in a high heat flux region near the critical value were reported in previous papers by the authors. The results of an experimental investigation into the liquid macrolayer formation are being reported in this paper. The initial thickness of this liquid layer formed between the heated surface and the vapour mass and the frequency of the vapour mass as a function of impressed heat flux have been measured. Using these data, the contribution of macrolayer evaporation to the heat flow from heated surface to bulk has been estimated. Experimental results of macrolayer thickness and frequency of vapour mass have been found to compare well with analytically predicted values. Contribution of heat conduction through the macrolayer has also been found to account for a considerable portion of wall heat flux. 相似文献
11.
Ricardo Diaz 《Numerical Heat Transfer, Part A: Applications》2017,72(12):891-903
Molecular dynamics simulations were employed to investigate the effects of wettability (contact angle) and pitch on nanoscale evaporation and pool boiling heat transfer of a liquid argon thin film on a horizontal copper substrate topped with cubic nano-pillars. The liquid–solid potential was incrementally altered to vary the contact angle between hydrophilic (~0°) and hydrophobic (~127°), and the pitch (distance between nano-pillars) was varied between 21.7 and 106.6?Å to observe the resultant effect on boiling heat transfer enhancement. For each contact angle, the superheat was gradually increased to initiate nucleate boiling and eventually pass the critical heat flux (CHF) into the film boiling regime. The CHF increases with pitch, and tends to decrease substantially with increasing contact angle. A maximum overall heat flux of 1.59?×?108?W/m2 occurs at the largest pitch investigated (106.6?Å), and as the contact angle increases the superheat required to reach the CHF condition also increases. Finally, in certain cases of small pitch and large contact angle, the liquid film was seen to transition to a Cassie–Baxter state, which greatly hindered heat transfer. 相似文献
12.
《International Journal of Heat and Mass Transfer》2006,49(1-2):259-268
Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length at different reduced pressures ranging from 0.24 to 0.99 with R-134a as the working fluid. The onset of CHF was determined by the sudden rise in the wall temperature of the electrically heated tube. Experiments were performed over a wide range of parameters: mass flux values from 200 to 2000 kg/m2 s, pressure from 10 to 39.7 bars and heat flux from 2 to 80 kW/m2 and exit quality from 0.17 to 0.94. The results show considerably lower critical heat flux at high pressures. Well known CHF prediction methods, such as the look-up table and correlations of earlier workers show poor agreement at high pressures. A new correlation has been proposed to estimate the CHF in uniformly heated vertical tubes up to the critical pressure and over a wide range of parameters. 相似文献
13.
《International Journal of Heat and Mass Transfer》2006,49(5-6):1058-1072
In view of practical significance of a correlation of critical heat flux (CHF) in the aspects of engineering design and prediction, this study is aiming at evaluation of existing CHF correlations for flow boiling of water with available databases taken from small-diameter tubes, and then development of a new, simple CHF correlation. Available CHF databases in the literature for flow boiling of water in small-diameter tubes (0.33 < Dh < 6.22 mm) are collected, covering wide parametric ranges. Three correlations by Bowring, Katto and Shah are evaluated with the CHF data for saturated flow boiling, and three correlations by Inasaka–Nariai, Celata et al. and Hall–Mudawar evaluated with the CHF data for subcooled flow boiling. The Hall–Mudawar correlation and the Shah correlation seem to be the most reliable tools for CHF prediction in the subcooled and saturated flow boiling regions, respectively. In order to avoid the defect of predictive discontinuities often encountered when applying previous correlations, a simple, nondimensional, inlet conditions dependent CHF correlation for saturated flow boiling has been formulated. Its functional form is determined by the application of the artificial neural network and parametric trend analyses to the collected database. Superiority of this correlation has been verified by the database. The new correlation has a mean deviation of 16.8% for this collected databank, smallest among all tested correlations. Compared to many inordinately complex correlations, this new correlation consists only of a single equation. 相似文献
14.
Ho Seon Ahn Chan Lee Joonwon Kim Moo Hwan Kim 《International Journal of Heat and Mass Transfer》2012,55(1-3):89-92
It is well known that nanoparticles deposited on a heating surface during nanofluids boiling can change the characteristics of the heating surface and increase the critical heat flux (CHF) dramatically. We considered a new approach to investigate the nanoparticle surface effect on CHF enhancement using surfaces modified with artificial micro, nano, and micro/nano structures similar to deposited nanoparticle structures through the anodic oxidation on the zirconium alloy heater. We examined the effect of the capillary wicking action ability on the CHF enhancement due to the micro, nano, and micro/nano structured surfaces. The results demonstrated that the CHF enhancement on the modified surfaces was a consequence of the capillary wicking action ability of the artificial micro/nano structures through the high-speed visualization of the capillary wicking action. 相似文献
15.
《International Journal of Hydrogen Energy》2020,45(11):7141-7150
Critical heat flux (CHF) of liquid hydrogen (LH2) flow boiling under microgravity is vital for designing space cryogenic propellant conveying pipe since the excursion of wall temperature may cause system failure. In this study, a two-dimensional axisymmetric model based on the wall heat flux partition (WHFP) model was proposed to predict the CHF condition under microgravity including the wall temperature and the CHF location. The proposed numerical model was validated to demonstrate a good agreement between the simulated and experimentally reported results. Then, the wall temperature distribution and the CHF location under different gravity conditions were compared. In addition, the WHFP and vapor-liquid distribution along the wall under microgravity were predicted and its difference with terrestrial gravity condition was also analysed and reported. Finally, the effects of flow velocity and inlet sub-cooling on the wall temperature distributions were analysed under microgravity and terrestrial gravity conditions, respectively. The results indicate that the CHF location moves upstream about 5.25 m from 1g to 10−4g since the void fraction near the wall reaches the breakpoint of CHF condition much earlier under the microgravity condition. Furthermore, the increase of the velocity and decrease of the sub-cooling have smaller effects on the CHF location during LH2 flow boiling under microgravity. 相似文献
16.
Observations of boiling behaviors and measurements of critical heat flux (CHF) were carried out for saturated water boiling on a horizontal, upward-facing plate at pressures from atmospheric to 7 MPa. The primary bubbles diminish in size almost in inverse proportion to pressure and commence to coalesce in the very low heat flux region. The diameter of detached coalesced bubbles increases with increases in the heat flux and reaches about 10 mm even at a pressure of 5 MPa. Detachment frequency of the coalesced bubbles was unaffected by the heat flux and pressure. The CHF predicted based on the macrolayer dryout model agrees well with the measured data. 相似文献
17.
《International Communications in Heat and Mass Transfer》1999,26(1):23-32
We study the effects of externally applied electric fields on the critical heat flux in pool boiling for the case of dielectrophoretic forces over the bubbles pointing towards the heater. Experimental tests have been performed for cylindrical heaters with an axial wire as electrode. The results show substantial reductions in the value of the critical heat flux when the adverse dielectrophoretic forces over the bubbles are on the order of magnitude of the buoyancy forces. After that point, an increase in the strength of the applied field does not have an impact in the critical heat flux. In addition, the effects are smaller at saturation temperature than in subcooled conditions. 相似文献
18.
Cristiano Bigonha Tibiriçá Gherhardt Ribatski John Richard Thome 《International Journal of Heat and Mass Transfer》2012,55(25-26):7873-7883
This paper presents experimental results for flow boiling heat transfer coefficient and critical heat flux (CHF) in small flattened tubes. The tested flattened tubes have the same equivalent internal diameter of 2.2 mm, but different aspect height/width ratios (H/W) of ¼, ½, 2 and 4. The experimental data were compared against results for circular tubes using R134a and R245fa as working fluids at a nominal saturation temperature of 31 °C. For mass velocities higher than 200 kg/m2s, the flattened and circular tubes presented similar heat transfer coefficients. Such a behavior is related to the fact that stratification effects are negligible under conditions of higher mass velocities. Heat transfer correlations from the literature, usually developed using only circular-channel experimental data, predicted the flattened tube results for mass velocities higher than 200 kg/m2s with mean absolute error lower than 20% using the equivalent diameter to account for the geometry effect. Similarly, the critical heat flux results were found to be independent of the tube aspect ratio when the same equivalent length was kept. Equivalent length is a new parameter which takes into account the channel heat transfer area. The CHF correlations for round tubes predicted the flattened tube data relatively well when using the equivalent diameter and length. Furthermore, a new proposed CHF correlation predicted the present flattened tube data with a mean absolute error of 5%. 相似文献
19.
In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998 相似文献
20.
K.J.L. Geisler A. Bar-Cohen 《International Journal of Heat and Mass Transfer》2009,52(11-12):2427-2436
Pool boiling and CHF experiments were performed for vertical, rectangular parallel-plate channels immersed in the dielectric liquid FC-72 at atmospheric pressure to elucidate the effects of geometrical confinement in immersion cooled electronics applications. Heat transfer enhancement in the low flux region of the nucleate boiling curve was observed for channel spacings near and below expected bubble departure diameters, but was widely different for two different heater materials. Relative degradation of CHF with decreasing channel spacing was found to be a strong function of channel aspect ratio and independent of surface material and finish. 相似文献