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361.
A theoretical estimation of the pre-breakdown-heating time in the underwater discharge acoustic source 下载免费PDF全文
One of the common characteristics of the electrothermal breakdown in an underwater discharge acoustic source(UDAS) is the existence of a pre-breakdown-heating phase.In our experiment,two phenomena were observed:(1) the breakdown time that takes on high randomicity and obeys a "double-peak" stochastic distribution;(2) the higher salt concentration that reduces the residual voltage and causes 100% non-breakdown.The mechanism of electrothermal breakdown is analysed.To specify the end of the pre-breakdown-heating phase,a "border boiling" assumption is proposed,in which the breakdown time is assumed to be the time needed to heat the border water around the initial arc to 773 K.Based on this ’border boiling’ assumption,the numerical simulation is performed to evaluate the effects of two heating mechanisms:the Joule heating from the ionic current,and the radiation heating from the initial arc.The simulation results verify the theoretical explanations to these two experiment phenomena:(1) the stochastic distribution of the radius of the initial arc results in the randomicity of the breakdown time;(2) the difference in efficiency between the radiation heating and the Joule heating determines that,in the case of higher salt concentration,more energy will be consumed in the pre-breakdown-heating phase. 相似文献
362.
With the long-term objective of Critical Heat Flux (CHF) prediction, bubble dynamics in convective nucleate boiling flows has been studied using a Direct Numerical Simulation (DNS). A sharp-interface phase change model which was originally developed for pool boiling flows is extended to convective boiling flows. For physical scales smaller than the smallest flow scales (smaller than the grid size), a micro-scale model was used. After a grid dependency study and a parametric study for the contact angle, four cases of simulation were carried out with different wall superheat and degree of subcooling. The flow structures around the growing bubble were investigated together with the accompanying physics. The relation between the heat flux evolution and the bubble growth was studied, along with investigations of bubble diameter and bubble base diameter evolutions across the four cases. As a validation, the evolutions of bubble diameter and bubble base diameter were compared to experimental observations. The bubble departure period and the bubble shapes show good agreement between the experiment and the simulation, although the Reynolds number of the simulation cases is relatively low. 相似文献
363.
364.
Boiling flow simulations are conducted on adaptive octree grids. A phase change model consistent with the mixture formulation, in conjunction with the Volume-of-Fluid (VOF) model, is used to track the liquid–vapor interface. Test cases including Rayleigh Taylor instability and bubble growth in a uniform superheat are conducted to validate the phase change model on adaptive grids. The validated model is then used to conduct film boiling simulations on both two-dimensional and three-dimensional adaptive grids. The average wall Nusselt number agrees well with the widely accepted correlations of Berenson (1961) and Klimenko (1981) and Klimenko and Shelepen (1982) for film boiling on a horizontal surface. For the test cases presented, the efficiency of the adaptive technique, as measured by the adaptive mesh refinement (AMR) efficiency, is mostly in the range of 50–80%. Although this efficiency is a function of the nature and dimensionality of the problem, this range of efficiency is comparable to those obtained in the simulations of primary jet atomization conducted by Fuster et al. (2009). This work opens the prospect of conducting more realistic (three-dimensional) multi-modal boiling flow simulations, and problems of similar complexity, in an efficient manner. 相似文献
365.
Researches on two-phase flow and pool boiling heat transfer in microgravity, which included ground-based tests, flight experiments, and theoretical analyses, were conducted in the National Microgravity Laboratory/CAS. A semi-theoretical Weber number model was proposed to predict the slug-to-annular flow transition of two-phase gas–liquid flows in microgravity, while the influence of the initial bubble size on the bubble-to-slug flow transition was investigated numerically using the Monte Carlo method. Two-phase flow pattern maps in microgravity were obtained in the experiments both aboard the Russian space station Mir and aboard IL-76 reduced gravity airplane. Mini-scale modeling was also used to simulate the behavior of microgravity two-phase flow on the ground. Pressure drops of two-phase flow in microgravity were also measured experimentally and correlated successfully based on its characteristics. Two space experiments on pool boiling phenomena in microgravity were performed aboard the Chinese recoverable satellites. Steady pool boiling of R113 on a thin wire with a temperature-controlled heating method was studied aboard RS-22, while quasi-steady pool boiling of FC-72 on a plate was studied aboard SJ-8. Ground-based experiments were also performed both in normal gravity and in short-term microgravity in the drop tower Beijing. Only slight enhancement of heat transfer was observed in the wire case, while enhancement in low heat flux and deterioration in high heat flux were observed in the plate case. Lateral motions of vapor bubbles were observed before their departure in microgravity. The relationship between bubble behavior and heat transfer on plate was analyzed. A semi-theoretical model was also proposed for predicting the bubble departure diameter during pool boiling on wires. The results obtained here are intended to become a powerful aid for further investigation in the present discipline and development of two-phase systems for space applications. 相似文献
366.
Nucleate boiling heat transfer in aqueous solutions with carbon nanotubes up to critical heat fluxes
In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHFs) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60 °C. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001%, 0.001%, 0.01%, and 0.05%. For the dispersion of CNTs, polyvinyl pyrrolidone polymer is used in distilled water. Pool boiling HTCs are taken from 10 kW/m2 to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the aqueous solutions with CNTs are lower than those of pure water in the entire nucleate boiling regime. On the other hand, critical heat flux of the aqueous solution is enhanced greatly showing up to 200% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle on the test surface and extends the nucleate boiling regime to very high heat fluxes and reduces the formation of large vapor canopy at near CHF. Thus, a significant increase in CHF results in. 相似文献
367.
Mamoru Ozawa Takeyuki AmiIsao Ishihara Hisashi UmekawaRyosuke Matsumoto Yasuhiko TanakaTaku Yamamoto Yuya Ueda 《International Journal of Multiphase Flow》2009
Increasing attention has been focused on carbon dioxide (CO2) heat pump system where the temperature level is rather low, while the operating pressure is rather high. In this system, the density difference between vapor and liquid becomes rather small, which significantly affects flow patterns. Low surface tension and latent heat also have significant influence on two-phase flow patterns and heat transfer. This paper describes experimental and numerical investigation on flow patterns and heat transfer characteristics of boiling flow CO2 at high pressure in horizontal small-bore tubes ranging from 1.0 mm to 3.0 mm I.D. Even though the density difference is rather small at high pressure, phase stratification takes place, which leads to the intermittent dryout at the upper wall. So far developed discrete bubble model by the authors for vertical flows is modified so as to include horizontal flow mechanisms. The predicted flow patterns with this new model agree on the whole with the experimental observation. 相似文献
368.
Flow boiling heat transfer in a single circular micro-channel of 0.19 mm ID has been experimentally investigated with R123 and R134a for various experimental conditions: heat fluxes (10, 15, 20 kW/m2), mass velocities (314, 392, 470 kg/m2 s), vapor qualities (0.2–0.85) and different saturation pressures (158, 208 kPa for R123; 900, 1100 kPa for R134a). The heat transfer trends between R123 and R134a are clearly distinguished. Whether nucleate boiling is suppressed at low vapor quality or not determines the heat transfer trend and mechanism in the flow boiling of micro-channels. High convective heat transfer coefficients in the two-phase flow of micro-channels enables nucleate boiling to be suppressed even at low vapor quality, depending on the wall superheat requirement for nucleate boiling. In the case of early suppression of nucleate boiling, specifically R123, heat transfer is dominated by evaporation of thin liquid films around elongated bubbles. In the contrary case, namely R134a, nucleate boiling is dominant heat transfer mechanism until its suppression at high vapor quality and then two-phase forced convection heat transfer becomes dominant. It is similar to the heat transfer characteristic of macro-channels except the enhancement of nucleate boiling and the short forced convection region. 相似文献
369.
Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several
studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters
such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models
are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study
the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically
with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and
is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments
in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the
distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies
range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to
condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor
layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found. 相似文献
370.