Bubble growth rates in nucleate boiling of water at subatmospheric pressures

The growth rate of vapour bubbles has been investigated experimentally up to departure in water boiling at pressures varying from 26·7 to 2·0 kPa (the corresponding Jakob number increasing from 108 to 2689).Comparison of the data with existing theory shows the substantial influence of liquid inertia during initial growth, in agreement with previous results of Stewart and Cole [1]on water boiling at 4·9 kPa, the Jakob number varying from 955 to 1112. As an extreme case, at a pressure of 2·0 kPa, large “Rayleigh” bubbles are observed during the entire adherence time. During advanced growth, bubble behaviour is gradually governed by heat diffusion, especially at relatively high (subatmospheric) pressures.Experimental bubble growth in the investigated pressure range is in quantitative agreement with the van Stralen, Sohal, Cole and Sluyter theory [10]. This model combines the Rayleigh solution with a diffusion-type solution, which accounts for the contributions to bubble growth due to both the relaxation microlayer (around the bubble dome) and the evaporation microlayer (beneath the bubble).Finally, a curious bubble cycle is observed at the lowest investigated pressures, which is attributed to the combined action of a high-velocity liquid jet (originating in the wake following a large primary bubble) and a succeeding secondary vapour column (generated at the adjacent dry spot at the heating wall beneath the primary bubble).     

Bubble behavior and heat transfer during pool boiling of binary mixtures

Heat transfer coefficients were measured during pool boiling of binary mixtures on a heated wire hung horizontally and bubble behavior was simultaneously captured with a high‐speed video camera. The experiment was carried out at a pressure of 0.4 and 0.7 MPa for the whole range of mass fractions in a binary mixture of R22/R11. We clarified the change in bubble behavior and heat transfer by measuring the bubble departure diameter, frequency and growth rate on the basis of the video images. Furthermore, we discussed the relationship between the bubble behavior and the boiling heat transfer coefficient in the binary mixtures. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 449–459, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20087     

基于多效蒸发技术的油田污水淡化系统及分析

针对油田污水污染物成分复杂、污染性强不适合膜法脱盐的特点,提出用多效蒸发(multiple-effect evaporation,MEE)技术对油田污水进行集中脱盐处理的技术方案.建立了基于MEE的油田污水集中脱盐系统的工艺流程设计计算模型,系统分析了蒸汽加热温度、油田污水温度、浓缩液含盐质量分数及系统效数的影响.结果...     

Periodic heat transfer in sunlit moist ground: Evaluation of rates of moisture evaporation

This paper present an analysis of the periodic heat transfer in moist ground exposed to periodic solar radiation and atmospheric temperature. This analysis yields a novel method for the evaluation of daily/hourly moisture evaporation from the ground. It is found that in harsh climates a significant amount of moisture ( 4 lb/ft²/day) is evaporated from sunlit ground near the sea and in windy regions. The rate of moisture evaporation decreases with relative humidity and increases with wind speed.     

Performance of a solar still: Predicted effect of enhanced evaporation area on yield and evaporation temperature

A solar still with enlarged evaporation area was simulated on the computer in order to explore the quantitative relationship between evaporation area and the distillation yield. The results showed a gain of 19.6% in the yield when the evaporation area was quadrupled, and an asymptotic (infinite area) gain of around 30.2%. A thermodynamic analysis of the two separate energy conversion processes occurring within the still led to “availability” based definitions of efficiencies for the collector and the evaporator-condenser. This analysis showed that all of the asymptotic improvement in yield is attributable to the more efficient evaporation-condensation process in the still with enlarged evaporation area. The analysis also suggested that stills with enlarged evaporation area could be operated on cheap low temperature thermal energy such as from solar ponds. Finally, the exergy based efficiency defined for the evaporator-condenser was shown to be useful in describing the performance of multiple-effect solar stills as well.     

Nucleate Pool Boiling Heat Transfer of Pure Refrigerants and Binary Mixtures

Heat transfer coefficients in nucleate boiling on a smooth flat surface were measured for pure fluids of R-134a, propane, isobutane and their binary mixtures at different pressure from 0.1 to 0.6 MPa. Series of experiments with different heat flux and mixture concentrations were carried out. The influences of pressure and heat flux on the heat transfer coefficient for different pure fluids were studied. Isobutane and propane were used to make up binary mixtures. Compared to the pure components, binary mixtures show lower heat transfer coefficients. This reduction was more pronounced as the heat flux increasing. Several heat transfer correlations are obtained for different pure refrigerants and their binary mixtures.     

Bubble growth in the presence of a magnetic field

The problem of the growth of a bubble in the presence of a magnetic field is examined under the assumption that the process is heat transfer controlled. The work reveals the existence of a new nondimensional number which physically represents the ratio of the ponderomotive forces over pressure forces computed on the basis of length-scale and time related to the coefficient of thermal diffusion. It is found that for a spherical magnetic field, growth of the bubble remains parabolic in time but the rate of growth is slower. Heat-transfer estimates are also made in the fashion of Forster and Zuber indicating that heat transfer in nucleate boiling is reduced in the presence of magnetic fields. In a numerical example, this reduction is more substantial for potassium than it is for mercury.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：part II—Analytical Model for Boiling Heat Transfer of Binary Mixtures on Smooth Tubes and Comparison of Analytical Models for both Pure Liqu

A combined physical model of bubble growth is proposed along with a corresponding bubble growth model for binary mixtures on smooth tubes. Using the general model of Wang et al.^[1] and the bubble growth model for binary mixtures, an analytical model for nucleate pool boiling heat transfer of binary mixtures on smooth tubes is developed. In addition, nucleate pool boiling heat transfer of pure liquids and binary mixtures on a horizontal smooth tube was studied experimentally. The pure liquids and binary mixtures included water, methanol, ethanol, and their binary mixtures. The analytical models for both pure liquids and binary mixtures are in good agreement with the experimental data.     

Experimental investigation of burning rates of pure ethanol and ethanol blended fuels

A fundamental experimental study to determine the burning rates of ethanol and ethanol-blended fossil fuels is presented. Pure liquid ethanol or its blends with liquid fossil fuels such as gasoline or diesel, has been transpired to the surface a porous sphere using an infusion pump. Burning of the fuel takes place on the surface of the porous sphere, which is placed in an air stream blowing upwards with a uniform velocity at atmospheric pressure and temperature under normal gravity conditions. At low air velocities, when ignited, a flame envelopes the sphere. For each sphere size, air stream velocity and fuel type, the fuel feed rate will vary and the same is recorded as the burning rate for that configuration. The flame stand-off distances from the sphere surface are measured by post-processing the digital image of the flame photograph using suitable imaging software. The transition velocity at which the flame moves and establishes itself at the wake region of the sphere has been determined for different diameters and fuel types. Correlations of these parameters are also presented.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures ：Part I—Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

A mechanism is proposed for nucleate pool boiling heat transfer along with a general model for both pure liquids and binary mixtures. A combined physical model of bubble growth is also proposed along with a corresponding bubble growth model for pure liquids on smooth tubes. Using the general model and the bubble growth model for pure liquids, an analytical model for nucleate pool boiling heat transfer of pure liquids on smooth tubes is developed.     

Bubble growth and transient heat transfer at the onset of boiling

An experimental study was conducted to investigate transient local heat transfer around a bubble at onset of boiling on a thin glass heating plate immersed in saturated n-hexane at low pressure. Eight rapid response Cu-Ni thermocouples consisting of a vacuum deposited thin film were used to measure the temperature change of the heating surface. Simultaneous high-speed video photographs were also obtained. The surface temperatures near a nucleation site decreased rapidly owing to the evaporation of a thin layer (microlayer) of liquid formed beneath the bubble in the early period and the rate of bubble growth increased with increasing incipient boiling superheat (ΔT_IB). The thickness of the microlayer decreased markedly with increasing ΔT_IB. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(7): 484–492, 1997     

Performance investigation of flat-plate CLPHP with pure and binary working fluids for PEMFC cooling

While proton exchange membrane fuel cell (PEMFC) generates electricity, about half of the energy is converted into heat. According to structural characteristics and heat dissipation requirements of PEMFC, a flat-plate micro closed-loop pulsating heat pipe (CLPHP) cooling method is designed. The flat-plate CLPHP is an aluminum alloy plate with a thickness of 2.4 mm, and the inside is a 2.3 mm × 1.4 mm rectangular flow channel, which transfers heat mainly through the internal working fluid's vapor-liquid phase change and forced convection. The experiment tested the heat transfer performance and the internal pressure of pure working fluids methanol, ethanol, isopropanol, deionized water, and methanol-deionized water with different mass ratios. By comparison, it is found that the binary working medium methanol-deionized water with a mass ratio of 5:1 has the best startup performance, lower internal pressure, and less temperature fluctuation, which has great potential in the application of PEMFC. Through the dimensionless number correlation analysis of the internal working fluid's thermophysical parameters, a CLPHP heat flux prediction equation with an average deviation of 15.0% is fitted.     

Onset of oscillatory thermocapillary convection in acetone liquid bridges: The effect of evaporation

Experiments have been carried out for half-zones of acetone (Pr = 4.3) to investigate the effects of evaporative cooling on the flow structures and temperature fields during transition from steady to oscillatory convection. The unstable flow phenomena have been measured using a variety of diagnostic techniques to determine the effects of evaporative cooling on Marangoni convection in liquid bridges of intermediate Prandtl number. The results show that Marangoni convection in acetone liquid bridges with and without strong evaporation becomes unstable due to the same mechanism but the evaporation has a strong stabilizing effect on the onset of oscillatory Marangoni convection.     

Filmwise condensation of a pure fluid and a binary mixture in a bundle of enhanced surface tubes

An experimental study has been carried out to investigate the characteristics of condensation in a bundle of horizontal tubes. These tubes have different types of external surfaces: smooth surface (1D), low trapezoidal fins with several fin pitches (2D) and specific fins (3D, C+ tube). The used fluids are either pure refrigerant (HFC 134a) or binary mixture of refrigerants (HFC 134a/HFC23).For the pure fluid and a single tube, the influence of fin spacing has been studied (11, 19, 26, 32 and 40 fins/inch) and a comparison has been made with the Gewa C+ tube. The results were analysed with the Beatty and Katz theory and compared to a specific model, taking into account both gravity and surface tension effects, developed for the Gewa C+ tube.For the bundle and for a pure fluid, the inundation of the lowest tubes has a strong effect on the Gewa C+ tubes performances contrary to the finned tubes. For the mixture the heat transfer coefficient decreases dramatically especially for Gewa C+ tube.     

Bubble characteristics in time periodic evaporation flow of R-134a in a narrow annular pipe due to heat flux oscillation

In this work, bubble characteristics of periodic evaporation flow with refrigerant R-134a in a horizontal narrow annular pipe were examined experimentally in details. Attention is focused on the time periodic evaporation flow characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The photos of the R-134a time periodic evaporating flow taken from the duct side are presented to show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. The results show that at the low vapor quality, the bubbles get smaller with time and become less crowded in the duct in the first half of the cycle in which the R-134a heat flux decreases. The changes of the bubble characteristics with the instantaneous heat flux become more pronounced for an increase in the amplitude of the heat flux oscillation. At the very high mean vapor quality the bubble nucleation can be barely seen in the entire periodic cycle since the liquid film covering the heating surface is very thin. In addition, the duct flow is dominated by the annular two-phase flow at all time.     

Evaporation rates of pure hydrocarbon liquids under the influences of natural convection and diffusion

Evaporation of liquid hydrocarbons was studied in order to better understand the relative influences of diffusion and buoyancy-induced convection of the vapors on the evaporation rate. Evaporation rates were measured using a simple gravimetric technique and the behavior of the vapor layer that quickly forms above the film was observed using schlieren imaging. Even for conditions for which the influence of buoyancy is strong, the evaporation rates are well correlated by a one-dimensional diffusion model if an effective vapor layer thickness is used.     

Analysis of variability and correlation in long-term economic growth rates

Quantifying the uncertainty in future climate change is an important input into policy decisions. Two important sources of uncertainty are economic growth and technological change, which in turn contribute to uncertainty in future emissions. In this paper, we focus on uncertainty in one type of technical change: productivity growth. Estimates of uncertainty in future growth must necessarily include expert judgment, since the future will not necessarily look like the past. But previous uncertainty studies have taken expert judgments based on annual national growth rates, and applied them to models with regional aggregations and multi-year time steps, and often have made crude assumptions about the correlation between regions. This paper analyzes data on the variability and covariability of historical economic productivity growth rates, and investigates the effect of spatial and temporal aggregation on variance. The results are intended to inform participants in expert elicitation exercises on future economic growth uncertainty.     

Exergetic evaluation and comparison of quintuple effect evaporation units in Indian sugar industries

Present work analyzes and compares two quintuple effect evaporation units with and without heat recovery devices being employed. It is based on actual operational data of sugar plant. This study is primarily based on exergy analysis. Case A is a quintuple effect evaporation unit without heat recovery devices, while case B is with heat recovery devices. The average exergy efficiency is found to be 70.53% for case A, while it is 86.71% for case B. Highest exergy destruction for case A is in second effect with a value of 1562.20 kW, and for case B, it is for the first effect with a value of 1871.68 kW. Steam economy for case A is 1.99, while for case B, it is 3.46. This is due to high evaporation rates and heat recovery devices being employed for case B. The fifth effect evaporator in case A and first effect evaporator in case B are found to be the least efficient components from exergy point of view. As energy economy is concerned in terms of exhaust steam demand, case B is more attractive than case A. However, in terms of exergy, case B is less sustainable than case A. A parametric study indicated that increase in the exhaust/inlet steam temperature is highly disadvantageous in terms of exergy and quality of the end product. The authors expect that the exergy analyses results would facilitate the designers and professional practioners in the field of sugar engineering in furthering the goal of improving energy systems. Copyright © 2012 John Wiley & Sons, Ltd.     

An analytical solution for the stationary behaviour of binary mixtures and pure fluids in a horizontal annular cavity

This study aims at analysing the characteristics of the two-dimensional flow established by binary mixtures and pure fluids in a horizontal annular cavity isothermally heated inside and cooled outside. Some complications arise considering binary mixtures, as a consequence of thermogravitational diffusion due to the coupling of convection with thermodiffusion.An analytical model has been defined assuming a two-dimensional laminar flow of a visco-elastic fluid, under validity of Boussinesq’s assumption and negligible viscous dissipation. For the case of binary mixtures, the buoyancy determined by concentration variations has been ignored, so that momentum equation is independent on concentration and is only coupled with the energy balance equation.The analytical model has been validated, for both cases of binary mixtures and pure fluids, through comparison with numerical results in a wide range of the non-dimensional parameters that govern the problem. In particular, the effect of the geometry and of the boundary conditions on the separation of the mixture compounds due to thermogravitational diffusion has been exploited.