Single bubble growth in saturated pool boiling of binary mixtures

Nucleate pool boiling experiments for binary mixtures, which are consisted of R11 and R113, were performed with constant wall temperature condition. Results for binary mixtures were also compared with pure fluids. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant temperature of the heating surface and to obtain heat flow rate measurements with high temporal and spatial resolutions. Bubble growth images were captured using a high-speed CCD camera synchronized with the heat flow rate measurements.The departure time for binary mixtures was longer than that for pure fluids, and binary mixtures had a higher onset of nucleate boiling (ONB) temperature than pure fluids. In the asymptotic growth region, the bubble growth rate was proportional to a value between t^1/6 and t^1/4. The bubble growth behavior was analyzed to permit comparisons with binary mixtures and pure fluids at the same scale using dimensionless parameters. There was no discernible difference in the bubble growth behavior between binary mixtures and pure fluids for a given ONB temperature. And the departure radius and time were well predicted within a ±30% error.The minimum heat transfer coefficient of binary mixtures occurred near the maximum |y−x| value, and the average required heat flux during bubble growth did not depend on the mass fraction of R11 as more volatile component in binary mixtures. Finally, the results showed that for binary mixtures, a higher ONB temperature had the greatest effect on reducing the heat transfer coefficient.     

A review of pool and forced convective boiling of binary mixtures

Boiling of binary mixtures is characterized by a close linking between heat and mass transfer processes, with the evaporation rate usually being limited by the mass transfer process. This is significantly different from single-component systems where interfacial mass transfer rates are normally very high. Information on pool boiling of binary mixtures is widely available in the literature, whereas research on forced convective boiling of mixtures has become significant only over the last few years. This paper presents a brief review of experimental results obtained in pool and forced convective boiling of binary mixtures and upgrades the empirical or theoretical predictive tools for both situations.     

Characteristics of heat transfer coefficient during nucleate pool boiling of binary mixtures

Experimental studies were conducted on heat transfer on a horizontal platinum wire during nucleate pool boiling in nonazeotropic binary mixtures of R12+R113, R134a+R113, R22+R113 and R22+R11, at pressures of 0.25 to 0.7 MPa and at heat fluxes up to critical heat flux. The substances employed were chosen such that the components of a given mixture had a large difference in saturation temperatures. The boiling features of the mixtures and the pure substances were observed by photography. The relationship between the boiling features and the reduction in heat transfer coefficient in binary mixtures is discussed in order to propose a correlation useful for predicting the experimental data measured over a wide range of low and high heat fluxes. It is shown that the correlation is applicable also to alcoholic mixtures. The physical role of k, which was introduced to evaluate the effect of heat flux on the reduction in heat transfer coefficient, is clarified based on the measured nucleate pool boiling heat transfer data and the visual observations of the boiling features. Received on 13 May 1997     

Saturated nucleate boiling to binary and ternary mixtures on horizontal cylinder

In this investigation, a large number of experiments have been performed to determine saturated nucleate pool boiling heat transfer coefficients of MEA/water and DEA/water binary mixtures and that of water/MEA/DEA ternary mixtures. These heat transfer coefficients have been measured at atmospheric pressure and over a wide range of heat fluxes and solution concentrations. The heat flux has been varied in 14 different levels from 7 to about 230 kW/m² and amines concentration has been changed in 10 different levels from zero to 84 wt%. Results show that strong reduction of heat transfer coefficient occurs as a result of mass transfer interference in this phenomenon. Furthermore, in this study, all the correlations proposed during the last years for the prediction of nucleate boiling heat transfer coefficient of mixtures have been categorized in three groups. Some experimental results have been compared with the most accurate representatives of these three groups and the corresponding RMS errors have been calculated. Also, impacts of important existing parameters in these correlations like ideal heat transfer coefficient (h_id.) on the prediction have been discussed.     

Semi-empirical modeling of pool boiling heat transfer in binary mixtures

Pool boiling heat transfer has been investigated for various binary mixtures, including acetone/isopropanol, water/acetone, water/methanol, water/ethanol, water/isopropanol, water/monoethanolamine, water/diethanolamine and water/triethyleneglycol as test solutions. Many correlations have been developed to predict the pool boiling heat transfer coefficient in mixtures in the past few decades, however the predicted values are not confirming. In addition, the application of many existing correlations requires some individual adjusting parameters that may be not available for every system. In this investigation, a new set of experimental data are presented. These data have been compared to major existing correlations. It is observed that the pool boiling heat transfer coefficients in mixtures are less than the ideal boiling heat transfer coefficient. A new semi-empirical model has been proposed based on the mass transfer resistance to predict the boiling heat transfer coefficient with satisfactory accuracy. The new model does not include any tuning parameter and is applicable to any given binary system. The performance of the proposed model is superior to most existing correlations.     

Mass transport effect on the heat transfer coefficient during boiling of multicomponent mixture

In this work a simplified calculation method taking into account the effect of mass transport on the heat transfer coefficient (HTC) during boiling of multicomponent mixture has been elaborated. The calculation results were compared with own experimental data for ternary system methanol–isopropanol–water and Grigoriev data [1] (acetone–methanol–water). The experiments were performed in different hydrodynamic conditions such as: pool boiling and liquid evaporation at the free surface of the falling film. The experimental data covered wide range of heat fluxes from 6 to 30 kW/m² in the case of liquid evaporation from the falling film and from 30 to 240 kW/m² for pool boiling. The analysis of the results indicates that the mass transfer resistance in the liquid phase caused a significant reduction of experimental value HTC in comparison to so-called ideal HTC.     

Radial oscillations of vapor bubbles in solutions

The dynamics and heat and mass transfer of vapor bubbles in binary systems are investigated. An anomalous effect of the component composition on the bubble dynamics in boiling nonideal solutions is established. It is shown that in some binary systems the value of the logarithmic decay rate for small free radial oscillations does not lie within the limiting values calculated for the pure components, which is associated with the cardinal importance of the effect of diffusion in the liquid phase on the intensity of the phase transitions.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 108–113, May–June, 1989.     

Nucleate boiling heat transfer: possibilities and limitations of theoretical analysis

The paper considers the different ways of nucleate boiling investigation, and in particular, presents the analysis of the latest numerical studies of the nucleate boiling primary processes (isolated bubble growth, thin liquid film flow and evaporation near the nucleation site). Many features of the process that were only the hypothesis became established facts after the direct numerical simulation. However, in spite of their undoubted usefulness these investigations cannot be applied for practical calculations. The high complexity of the phenomena comprising nucleate boiling excludes practically the possibility of strict theoretical analysis of the process. Under this situation development of an approximate theory of nucleate boiling retains its validity at present. Such a theory has been developed by the author in 1988. It reveals the main regularities of the nucleate boiling and leads to the predicting equation for heat transfer, which includes two empirical numerical factors. On the basis of the model developed the method of calculation of heat transfer in boiling of binary mixtures is proposed. To improve the existing predicting equations for boiling heat transfer it seems to be especially important to ground theoretically the nucleation sites density dependence on the wall superheat and liquid properties.     

Dynamik des Blasenwachstums und Wärmeübergangs beim Sieden binärer Gemische von Äthylalkohol—Wasser

Measurements of bubble growth rate, detachment diameter and heat-transfer are reported for nucleate boiling of binary mixtures in various concentrations on a horizontal heating surface. The growth rate, characterized by the Jakob-number exhibits maximum at 30% (mass) ethylalcohol in the liquid phase. At this concentration the smallest heat transfer coefficient was measured. For growth rate this proves an influence of mass transfer across the phase boundary besides hydrodynamic and thermal effects.     

Periodic boiling in parallel micro-channels at low vapor quality

Based on experimental investigations the present study evaluates instability and heat transfer phenomenon under condition of periodic flow boiling of water and ethanol in parallel triangular micro-channels. Tests were performed in the range of hydraulic diameter 100–220 μm, mass flux 32–200 kg/m² s, heat flux 120–270 kW/m², vapor quality x = 0.01–0.08. The period between successive events depends on the boiling number and decreases with an increase in the boiling number. The initial film thickness decreases with increasing heat flux. When the liquid film reached the minimum initial film thickness CHF regime occurred. Temporal variations of pressure drop, fluid and heater temperatures were periodic. Oscillation frequency is the same for the pressure drop, for the fluid temperature at the outlet manifold, and for the mean and maximum heater temperature fluctuations. All these fluctuations are in phase. The CHF phenomenon is different from that observed in a single channel of conventional size. A key difference between micro-channel heat sink and single conventional channel is amplification of parallel-channel instability prior to CHF. The dimensionless experimental values of the heat transfer coefficient are presented as the Nusselt number dependence on the Eotvos number and the boiling number.     

Enhancement of nucleate pool boiling heat transfer to dilute binary mixtures using endothermic chemical reactions around the smoothed horizontal cylinder

Experimental studies on enhancing the pool boiling heat transfer coefficient of binary dilute mixtures of water/glycerol, water/MEG (Mono-ethylene glycol) and water/DEG (di-ethylene glycol) have been carried out. Some particular endothermic chemical reactions related to ammonium salts were used to enhance the pool boiling heat transfer coefficient, simultaneously with occurrence of pool boiling heat transfer. Accordingly, 100?g of Ammonium nitrate, ammonium perborate and Ammonium sulfate were selected to dissolve into mixtures. High and extreme solution enthalpies of each of these ammonium salt powders are employed to reduce the surface temperature around the horizontal cylinder locally. Results demonstrated that presence of ammonium salts into the mixtures deteriorates the surface temperature of cylinder and as the result, higher pool boiling heat transfer coefficient is reported for tested solutions. Results are also reported and compared for different ammonium salts to find the influence of inducing different enthalpies of solution on pool boiling heat transfer coefficient. Obtained results also indicated that presence of endothermic reaction besides the pool boiling heat transfer enhances the heat transfer coefficients in comparison with nucleate pool boiling phenomenon solely.     

Study of two-phase bubbly flow dynamics in binary mixtures

The results of an investigation of the bubble dynamics and wave phenomena in two-component vapor-liquid mixtures are presented. These mixtures are widely used in industrial systems as heat transfer media. The effects of various additives on the wave dynamics of vapor-liquid mixtures are of particular interest. A single-velocity two-pressure model was used which takes into account both the liquid radial inertia due to medium volume changes, and the temperature distribution around the bubbles. The claim that mixture composition may have a peculiar effect on the bubble dynamics of a boiling non-ideal solution is substantiated. It is noted that the small free radial oscillation damping ratio for some binary systems lies outside the domain defined by the damping ratio of the constituents as a result of phase change diffusion effects. A criterion is proposed to identify cases of diffusion resistance responsible for the anomalous effect of component concentration on bubble behavior. The phase change delay due to diffusion results in observably higher mixture wave velocities and a smaller damping ratio than for respective single-component systems.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1992 and the AMS fonts, developed by the American Mathematical Society.     

Heat transfer in pool boiling of ammonia/water mixture

The nonazeotropic binary mixtures such as, methanol/water, ethanol/water and ammonia/water, have variable boiling and dew points, depending on the combination of substance and those mass fraction. It is expected to have a higher performance as a result of decreasing the thermodynamically irreversible loss, when there is a relevant mass fraction. Therefore, ammonia/water mixture is expected to use as working fluid in small temperature difference power generation cycles and absorption refrigeration cycles. However, few experiments were carried out for measuring heat transfer coefficient for ammonia/water mixture in the world. An experimental study has been carried out to measure boiling heat transfer coefficient of an ammonia/water mixture on a horizontal heated surface at low pressure of 0.2, 0.4 and 0.7 MPa and at low mass fraction of 0 < C < 0.27 and at high pressure 0.7, 1.0 and 1.5 MPa and at mass fraction of 0.5 < C < 1.0 and at heat flux under critical heat flux the heat transfer coefficient are compared with existing correlations prediction and a revised correlation can be proposed to predict them well.     

Evaporation of binary mixtures in upward annular flow

A theoretical study is presented of the evaporation of binary mixtures in upward annular flow. Heat and mass balances are written and the resulting equations solved to give axial and radial variations of concentrations, temperatures and flowrates of ethanol-water mixtures. Mass and heat transfer within the film are calculated by an extension of the Dukler-Hewitt method for heat transfer in single-component films. It is concluded from the worked examples that, for the mixture considered, the film flowrates and wall temperatures are not significantly controlled by mass transfer in either phase and can be calculated by flash vaporization methods.     

Nucleate pool boiling of aqueous solution of citric acid on a smoothed horizontal cylinder

Nucleate pool boiling heat transfer coefficients were measured during pool boiling of the mixtures of Citric acid/water on a horizontal heated Cylinder. The experiment was done at atmospheric pressure and heat fluxes up to 113 kW m⁻² and mass fraction range 0.1496–0.613 over all ranges of mass fraction, the heat transfer coefficients of the mixtures are markedly less than those in single component substances and, in particular, are dramatically deteriorated in the vicinity of both single component substances. An applicability of existing correlations to the present experimental data is discussed. As a result, it is difficult for any existing correlation to predict the true values of pool boiling heat transfer coefficients over all ranges of mass fraction in mixtures of citric acid/water. Available correlation results were not exactly adapted to experimental data and for the best estimation, a new modified model based on Stephan-Kroner has been achieved with reasonable accuracy. Also the status of bubble generation showed that nucleation site density is strictly functioning of heat flux.     

Study of transient heat transfer and synchronized flow visualizations during sub-cooled flow boiling in a small aspect ratio microchannel

The challenges that microchannel flow boiling technology faces are the lack of understanding of underlying mechanisms of heat transfer during various flow boiling regimes and a dearth of analytical models that can predict heat transfer. This paper aims to understand flow boiling heat transfer mechanisms by analyzing results obtained by synchronously captured high-speed flow visualizations with local, transient temperature data. Using Inverse Heat Conduction Problem (IHCP) solution methodology, the transient wetted surface heat flux and temperature as well as heat transfer coefficient are calculated. These are then correlated with the visual data. Experiments are performed on a single microchannel embedded with fast response temperature sensors located (630 µm) below the wetted surface. The height, width and length of the microchannel are 0.42 mm, 2.54 mm and 25.4 mm respectively. De-ionized, de-gassed water is used as the working fluid. Two heat fluxes are tested at each of the mass fluxes of 182 kg/(m²s) and 380 kg/(m²s). Because of vapor confinement, slug flow is observed for the tested conditions. The present study provides detailed insights into the effect of various events such as passage of vapor slug, 3-phase contact line, partial-dry-out and liquid slug on transient heat transfer coefficient. Transient heat transfer coefficient peaks when thin film evaporation mechanism is prevalent. The peak value is influenced by the distance of bubble incipience as well as downstream events obstructing the flow. Heat transfer coefficient during the passage of liquid slug and 3-phase contact line were relatively lower for the tested experimental conditions.     

Numerical simulation of flow boiling for organic fluid with high saturation temperature in vertical porous coated tube

A semi-analytical model is developed for the prediction of flow boiling heat transfer inside vertical porous coated tubes. The model assumes that the forced convection and nucleate boiling coexist together in the annular flow regime. Conservations of mass, momentum, and energy are used to solve for the liquid film thickness and temperature. The heat flux due to nucleate boiling consists of those inside and outside micro-tunnels. To close the equations, a detailed analysis of various forces acting on the bubble is presented to predict its mean departure diameter. The active nucleation site density of porous layer is determined from the pool boiling correlation by introducing suppression factor. The flow boiling heat transfer coefficients of organic fluid (cumene) with high saturation temperature in a vertical flame-spraying porous coated tube are studied numerically. It is shown that the present model can predict most of the experimental values within ±20%. The numerical results also indicate that the nucleate boiling contribution to the overall heat transfer coefficient decreases from 50% to 15% with vapor quality increasing from 0.1 to 0.5.     

Study on laminar film condensation of saturated steam on a vertical flat plate for consideration of various physical factors including variable thermophysical properties

The extended theory of the steady state laminar film condensation process of pure saturated vapour at atmospheric pressure on an isothermal vertical flat plate is established. Its equations provide a complete account of the physical process for consideration of various physical factors including variable thermophysical properties, except for surface tension at the liquid-vapour film interface. First, similarity considerations are proposed to transform the governing system of partial differential equations and its boundary conditions into the corresponding dimensionless system. Then, the dimensionless new system is computed numerically in two steps: First neglecting shear force at the interface, so that the initial values of the boundary conditionsW _{xl, s} andW _{yl, s} are obtained. Then, the calculations of a problem of the three-point boundary-value for coupling the equations of liquid film with those of vapour film are carried out. Furthermore, the correlations for heat transfer coefficient and mass flow rate are proposed by analysis of heat and mass transfer and it is found that the heat transfer coefficient is function of dimensionless temperature gradient $\dot L$ , and that the condensate mass flow rate is function of the mass flow rate parameter (η _lδ W _{xl, s} ? 4W _{yl, s} )of liquid. In addition, the corresponding heat and mass transfer correlations expressed by subcooled temperature Δt are developed. According to Nusselt's theory four different assumptions are set up for an investigation of the effects of the film condensation of saturated vapour, so that the validity of Nusselt's theory can be further clarified. Quantitative comparisons from the results of the heat transfer coefficient and mass flow rate of the condensate indicate that the effect of variable thermophysical properties on the heat and mass transfer is appreciable. The effect of thermal convection in the condensate film is obviously larger than those of shear force at liquid-vapour interface, and the effect of the inertia in the condensate film is very small. Finally, it is also shown that Nusselt's theory, in using Drew reference temperature, will decrease the heat transfer coefficient by at most 5.11%, and will increase the mass flow rate of the condensate by at most 2.45%, provided that the effect of the surface tension is not taken into account.     

Flow boiling behaviors in hydrophilic and hydrophobic microchannels

Surface wettability is a critical parameter in small scale phenomena, especially two-phase flow, since the surface force becomes dominant as size decreases. In present study, experiments of water flow boiling in hydrophilic and hydrophobic rectangular microchannels were conducted to investigate the wettability effect on flow boiling in rectangular microchannels. The rectangular microchannels were fabricated with a photosensitive glass to visualize flow pattern. The hydrophilic bare photosensitive glass microchannel was chemically treated to obtain a hydrophobic microchannel. And, visualization of flow patterns was carried out. And boiling heat transfer and two-phase pressure drop was analyzed with visualization results. The boiling heat transfer coefficient in the hydrophobic rectangular microchannel was higher than that in the hydrophilic rectangular microchannel, which was highly related with nucleation site density and liquid film motion. And the pressure drop in the hydrophobic rectangular microchannel was higher than that in the hydrophilic rectangular microchannel, which was highly related with unstable motions of bubble and liquid film. Finally, we find out the wettability is important parameter on the flow pattern, which were highly related with two-phase heat and mass transfer.