Nucleate boiling of FC-87/FC-72 zeotropic mixtures on a horizontal copper disc

This paper presents nucleate boiling experimental results, at atmospheric pressure, for heat fluxes q ≤ 40 kW/m², for FC-87/FC-72 binary mixtures in molar fractions of 0/100, 25/75, 50/50, 75/25, 85/15 and 100/0, at saturation temperatures for pure fluids and bubble points for mixtures. The test section was an upward facing copper disc of 12 mm diameter and 1 mm thickness. The experimental heat transfer coefficient was compared with the correlations of Rohsenow (1952), as reported by Rohsenow et al. (Handbook of heat transfer, McGraw-Hill, New York, 1998), Stephan and Abdelsalam (Int J Heat Mass Transfer 23;73–78, 1978) and Cooper (Int Chem Eng Symp Ser 86:785–792, 1984) for pure fluids and the semi-empirical models of Stephan and Körner (Chem Ing Tech Jahrg 7:409–484, 1969), Thome (J Heat Transfer 104:474–478, 1982), Fujita et al. (1996), as reported by Rohsenow et al. (Handbook of heat transfer, McGraw-Hill, New York, 1998), Fujita and Tsutsui (Int J Heat Mass Transfer 37(1):291–302, 1994) and Calus and Leonidopoulos (Int J Heat Mass Transfer 17:249–256, 1973) for mixtures.     

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     

Flow boiling heat transfer of R134a,R236fa and R245fa in a horizontal 1.030 mm circular channel

This research focuses on acquiring accurate flow boiling heat transfer data and flow pattern visualization for three refrigerants, R134a, R236fa and R245fa in a 1.030 mm channel. We investigate trends in the data, and their possible mechanisms, for mass fluxes from 200 to 1600 kg/m²s, heat fluxes from 2.3 kW/m² to 250 kW/m² at T_sat = 31 °C and ΔT_sub from 2 to 9 K. The local saturated flow boiling heat transfer coefficients display a heat flux and a mass flux dependency but no residual subcooling influence. The changes in heat transfer trends correspond well with flow regime transitions. These were segregated into the isolated bubble (IB) regime, the coalescing bubble (CB) regime, and the annular (A) regime for the three fluids. The importance of nucleate boiling and forced convection in these small channels is still relatively unclear and requires further research.     

Photographic study of bubble dynamics for pool boiling of refrigerant R11

A digital photographic study of pool boiling with R11 was performed on a horizontal transparent heater at pressure at 0.1 MPa. A high-speed digital camera was applied to record the bubble behaviors of boiling process. The departure diameter, departure time, and nucleation site density at different heat flux were obtained. From the video images, it can be concluded that evaporation of microlayer is very important to the growth of bubble. It was also observed that there is not any liquid replenished into the microlayer below the bubble. In addition, bubble growth curve and dry-out area growth curve could be determined by analyzing the images. Based on the experimental result, boiling curve for R11 was predicted by using the dynamic microlayer model. As a result, the agreement between the predictions and the experiment data is good at high heat flux.     

Subcooled flow boiling of R-134a in vertical channels of small diameter

Subcooled flow boiling heat transfer for refrigerant R-134a in vertical cylindrical tubes with 0.83, 1.22 and 1.70 mm internal diameter was experimentally investigated. The effects of the heat flux, q″ = 1–26 kW/m², mass flux, G = 300–700 kg/m² s, inlet subcooling, ΔT_sub,i = 5–15 °C, system pressure, P = 7.70–10.17 bar, and channel diameter, D, on the subcooled boiling heat transfer were explored in detail. The results are presented in the form of boiling curves and heat transfer coefficients. The boiling curves evidenced the existence of hysteresis when increasing the heat flux until the onset of nucleate boiling, ONB. The wall superheat at ONB was found to be essentially higher than that predicted with correlations for larger tubes. An increase of the mass flux leads, for early subcooled boiling, to an increase in the heat transfer coefficient. However, for fully developed subcooled boiling, increases of the mass flux only result in a slight improvement of the heat transfer. Higher inlet subcooling, higher system pressure and smaller channel diameter lead to better boiling heat transfer. Experimental heat transfer coefficients are compared to predictions from classical correlations available in the literature. None of them predicts the experimental data for all tested conditions.     

Dissolved gas effects on thermocapillary convection during boiling in reduced gravity environments

The mechanisms by which thermocapillary convection arises during boiling of nominally pure fluids in low-g environments are currently not known. It has recently been suggested that small amounts of dissolved gas within the bulk liquid can accumulate within the vapor bubble, forming localized concentration gradients that results in a temperature gradient to form along the liquid–vapor interface that drives thermocapillary convection. This hypothesis was tested by boiling > 99.3% pure n-perfluorohexane with and without noncondensible gas in a low-g environment using a 7.0 × 7.0 mm² microheater array to measure time and space resolved heat transfer at various wall superheats. The thermocapillary convection around the primary bubble that formed in the gassy fluid was found to be much weaker than in the degassed fluid, and the primary bubble diameter was much larger in the gassy fluid due to the accumulation of noncondensible gas within the bubble. The results suggest that the accumulation of noncondensible gas in the bubble can result in temperature variations along the interface but due to the increased vapor/gas bubble size, the driving thermocapillary temperature gradient along the interface is significantly reduced and result in much weaker thermocapillary flow. The highest CHF values in a reduced gravity environment (19 W/cm²) occurred when the fluid was highly subcooled and degassed.     

Experimental study on bubble departure characteristics in subcooled nucleate pool boiling

The boiling models use departure diameter and frequency in closure relations for the calculation of nucleate boiling heat flux. These parameters are normally derived from empirical correlations which depend heavily on experiments. While these parameters are studied mostly for saturated conditions, there is not sufficient data for the values of departure diameter and frequency in subcooled boiling. In this work, the bubble departure characteristics, i.e. the departure diameters and frequency have been measured using high speed visualization experiments with subcooled demineralized water at atmospheric pressure for nucleate pool boiling conditions. The water pool dimensions were 300 mm × 135 mm × 250 mm with four different heating elements to carry out the parametric studies of bubble departure behavior. The considered parameters were heater surface roughness, heater geometry and heater inclination along with the experimental conditions like degree of subcooling (ΔT_sub = 5−20 K), superheat (ΔT_sat = 1−10 K) and the heat flux. The departure diameters and frequencies were directly measured from the images captured. It was intended to generate the subcooled nucleate pool boiling data under a wide range of conditions which are not present in the literature. The departure diameter was found to increase with the wall superheat, heater size and the inclination angle while the liquid subcooling and surface roughness produced a damping effect on the diameter. The departure frequency was found to increase with the wall superheat and the inclination angle, but decreases with an increase in the heater size. The frequency increases with the degree of subcooling except very close to the saturation, and is unaffected by the surface roughness beyond a certain superheat value.     

Experimental study of pool temperature effects on nucleate pool boiling

Nucleate pool boiling experiments with constant wall temperature were performed using pure R113 for subcooled, saturated, and superheated pool conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant wall temperature and to measure the instantaneous heat flow rate accurately with high temporal and spatial resolutions. Images of bubble growth were taken at 5000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble geometry was obtained from the captured bubble images. The effect of the pool conditions on the bubble growth behavior was analyzed using dimensionless parameters for the initial and thermal growth regions. The effect of the pool conditions on the heat flow rate behavior was also examined. The bubble growth behaviors during subcooled, saturated, and superheated pool boiling were analyzed using a modified Jakob number that we newly defined. Dimensionless time and bubble radius parameters with the modified Jakob number characterized the bubble growth behavior well. These phenomena require further analysis for various pool temperature conditions, and this study will provide good experimental data with precise constant wall temperature boundary condition for such works.     

Bubble nucleation characteristics in pool boiling of a wetting liquid on smooth and rough surfaces

Quantitative measurements are obtained from high-speed visualizations of pool boiling at atmospheric pressure from smooth and roughened surfaces, using a perfluorinated hydrocarbon (FC-77) as the working fluid. The boiling surfaces are fabricated from aluminum and prepared by mechanical polishing in the case of the smooth surface, and by electrical discharge machining (EDM) in the case of the roughened surface. The roughness values (R_a) are 0.03 and 5.89 μm for the polished and roughened surfaces, respectively. The bubble diameter at departure, bubble departure frequency, active nucleation site density, and bubble terminal velocity are measured from the monochrome movies, which have been recorded at 8000 frames per second with a digital CCD camera and magnifying lens. Results are compared to predictions from existing models of bubble nucleation behavior in the literature. Wall superheat, heat flux, and heat transfer coefficient are also reported.     

Prediction of film boiling heat transfer coefficients for binary mixtures

Film boiling of binary liquid mixtures may be significantly different from that of single-component liquids due to the mass diffusion effect. A theoretical analysis is performed to outline the effects of mass diffusion phenomena on film boiling heat transfer process from a horizontal cylinder heating surface to the binary liquid mixtures of ethylene oxide/water and ethanol/benzene over whole range of compositions. These two binary systems are chosen for illustrating the strong and weak mass diffusion effects, respectively, on film boiling. Furthermore, a simple correlation for predicting heat transfer coefficient is proposed to demonstrate the idea that the dimensionless F factor can satisfactorily account for the mass diffusion effect on film boiling heat transfer of binary mixtures.     

On the departure behaviors of bubble at nucleate pool boiling

Dimensionless scales of radius and time, proposed by the authors in a previous study, were used to quantitatively analyze the bubble departure radius and time during nucleate pool boiling. The results obtained from dimensional analysis were compared with experimental data reported in many studies. These experimental data are including partial nucleate pool boiling data with constant heat flux and temperature conditions acquired over the past 40 years at atmospheric and sub-atmospheric pressures, as well as data obtained at subcooled, saturated, and superheated pool temperature conditions.It was shown that the departure radius and time could be well correlated with respect to Jakob number as proposed by the previous studies. And the bubble departure behaviors well categorized between atmospheric and sub-atmospheric pressure, which is occurred from the different growth rate near the departure time partial nucleate pool boiling.For almost all obtained under atmospheric pressure, the dimensionless departure radius and time scales were about 25 and 60, respectively. For higher Jakob number, the square root of Bond number was proportional to the power of 0.7 of Jakob number, little different from the previous correlations. The dimensional departure radius and time estimated from the relationships proposed in this study were compared with measured departure scales and the results obtained with the previous correlations. And it was shown that the relationships could well predict and describe the departure behaviors of bubble during nucleate pool boiling.     

Experimental verification of heat transfer coefficient for nucleate boiling at sub-atmospheric pressure and small heat fluxes

In this paper we study the influence of sub-atmospheric pressure on nucleate boiling. Sixteen correlations for pool boiling available in literature are gathered and evaluated. Analysis is performed in the pressure range 1–10 kPa and for heat flux densities 10–45 kW/m². Superheats are set between 6.2 and 28.7 K. The results of calculations were compared with experimental values for the same parameters. The experiments were conducted using isolated glass cylinder and water boiling above the copper plate. Results show that low pressure adjust the character of boiling curve—the curve flattened and the natural convection region of boiling is shifted towards higher wall temperature superheats due to the influence of low pressure on the bubble creation and process of its departure. In result, 8 of 16 analyzed correlations were determined as completely invalid in subatmospheric conditions and the remaining set of equations was compared to experimental results. Experimentally obtained values of heat transfer coefficients are between 1 and 2 kW/m²K. With mean absolute deviation (MAD) we have found that the most accurate approximation of heat transfer coefficient is obtained using Mostinski reduced pressure correlation (0.13–0.35 MAD) and Labuntsov correlation (0.12–0.89 MAD).     

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.     

Macro-to-microchannel transition in two-phase flow: Part 2 - Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].     

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.     

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.     

Single bubble growth in saturated pool boiling on a constant wall temperature surface

Nucleate pool boiling experiments with constant wall temperatures were performed using R11 and R113 for saturated pool boiling conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain a constant wall temperature condition and to obtain measurements with high temporal and spatial resolution. Accurate heat flow rate data were obtained from microscale heater array by controlling the surface conditions at a high temporal resolution. Images of the bubble growth were captured using a high-speed CCD camera synchronized with the heat flow rate measurements. The geometry of the bubble was obtained from the images. In the asymptotic growth region, the bubble showed a growth rate that was proportional to t^1/5, which was slower than the growth rate proposed in previous analytical analyses. The bubble growth behavior was analyzed using a new dimensionless parameter to permit comparisons with previous results at the same scale. The comparisons showed good agreement in the asymptotic growth region. A non-dimensional correlation for the bubble radius that can predict the bubble growth and the heat flow rate simultaneously, was suggested. The required heat flow rate for the volume change of the observed bubble was estimated to be larger than the instantaneous heat flow rate measured from the wall. Heat, other than the instantaneous heat supplied from the wall, is estimated to be transferred through the interface between bubble and liquid, even with saturated pool conditions. This phenomenon under a saturated pool condition needs to be analyzed and the data from this study can supply the good experimental data with the precise boundary condition (constant wall temperature).     

2D lattice Boltzmann investigation of saturated pool boiling using a tunable surface tension model: Prandtl number effects on film boiling

In this paper, the saturated pool boiling is investigated using lattice Boltzmann method. The written FORTRAN code is validated in two aspects: For flow, the thermodynamic consistency test and Laplace law are applied and for heat transfer, the space- and time- averaged Nusselt number is compared with Berenson analytical solution in film boiling regime. In addition, the results of bubble generation and departure are compared with some well-known analytical solutions to show the accuracy of the code. It is confirmed that bubble departure diameter and the departure frequency are related to the gravity acceleration with powers of ? 0.505 and 0.709, respectively, which is in a very good agreement with the existing analytical expressions. The present model has the ability to tune different surface tensions independent of liquid/vapor density ratio, which was unreachable using other existing numerical models of boiling. Thus, the sole effects of surface tension on boiling can also be taken into consideration using the present model. It is also shown that the departure diameter is related to the surface tension with a power of 0.485, which is in good agreement with the analytical expressions. Temperature contours are shown together with flow lines to have a better viewpoint for studying the bubble’s behavior. An intensive temperature gradient is observed in the necking area at the departure time. All the four boiling regimes in the boiling curve are simulated under constant temperature boundary condition. The Prandtl number effects on vapor bubble dynamics in the film boiling regime are investigated using the improved Shan and Chen model for the first time. Results revealed that bubbles are more resistant to depart from the vapor blanket with increasing the Prandtl number.     

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.