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.     

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     

Heat transfer in pool boiling of binary and ternary non-azeotropic mixtures

Heat transfer coefficients in nucleate pool boiling of binary and ternary non-azeotropic hydrocarbon mixtures were obtained experimentally using a vertical electrically heated cylindrical carbon steel surface at atmospheric pressure with several surface roughness. The fluids used were Methanol/1-Pentanol and Methanol/1-Pentanol/1,2-Propandiol at constant 1,2-Propandiol mole fraction of 30%. Heat fluxes were varied in the range 25–235 kW/m². The cylindrical heater surface was polished to an average surface roughness of 0.2 μm, and sandblasted yielding surface roughness of 2.98 and 4.35 μm, respectively. The experimental results were compared to available prediction correlations, indicating that the correlations based on the boiling range are in better qualitative agreement than correlations based on the phase envelope. Increasing surface roughness resulted in an increase in the heat transfer coefficient, and the effect was observed to be dependent on the heat flux and fluid composition.     

FC72 and FC87 nucleate boiling inside a narrow horizontal space

This paper presents new experimental results for saturated nucleate boiling of FC72 and FC87 on a horizontal copper disc, at atmospheric pressure, for different degrees of confinement, s, in the range of 0.1-13 mm, and with two kinds of confining element, for low and moderated heat fluxes (?40 kW/m²), on both a downward and an upward facing heating surface. For low heat flux a decrease of the confinement gap causes an enhancement of the boiling and a decrease in the dryout heat flux. A visualization of the boiling phenomenon shows the effect of confinement and heat flux on the liquid-vapor configuration.     

Circumferential temperature distribution during nucleate pool boiling outside smooth and modified horizontal tubes

In the work an approach to avoid a circumferential temperature distribution existing during nucleate pool boiling on a horizontal cylinder within low heat flux densities is presented. The idea of the approach is local heat transfer enhancement by a porous layer application on a part of the heating surface. An experiment on nucleate pool boiling heat transfer from horizontal cylinders to saturated R141b and water under atmospheric pressure is reported. Experiments have been conducted using stainless steel tubes with the outside diameter between 8 mm and 23 mm with the active length of 250 mm. The outside surface of the tubes was smooth or partially coated with a porous metallic layer. In particular, measurements of inside circumferential temperature distribution have been performed.     

Nucleate pool boiling heat transfer in binary mixtures

Heat and Mass Transfer - Experimental studies have been made for heat transfer during nucleate pool boiling on a horizontal platinum wire in nonazeotropic binary mixtures of R12+R113, R134a+R113,...     

Enhanced nucleate boiling on copper micro-porous surfaces

Saturation boiling of PF-5060 dielectric liquid on Cu micro-porous surface layers (95, 139, 171, 197 and 220-μm thick) is investigated. These layers are deposited on 10 × 10 mm Cu substrates using two-stage electrochemical process. The basic micro-structure, obtained in the first stage using current density of 3 A/cm² for 15–44 s, depending on thickness, is strengthened by continuing electrochemical deposition using much lower current density for 10’s of minutes. For conditioned surface layers, after a few successive boiling tests, the pool boiling curves are reproducible and the temperature excursion prior to boiling incipience is either eliminated or reduced <7 K. Present nucleate boiling results are markedly better than those reported for dielectric liquids on micro- and macro-structured surfaces. Present values of CHF (22.7–27.8 W/cm²) and h_MNB (2.05–13.5 W/cm² K) are ∼40–70% higher than and >17 times those reported on plane surfaces (<16 W/cm² and ∼0.8 W/cm² K). Best results are those of the 171-μm thick layer: CHF of 27.8 W/cm² occurs at ΔT_sat of only 2.1 K and h_MNB of 13.5 W/cm² K occurs at ΔT_sat = 2.0 K.     

Heat transfer in nucleate boiling of R134a/R152a mixtures

Heat transfer coefficients were measured on a horizontal platinum wire and converted to data on horizontal copper tubes. The measurements spanned a large region of pressures p* = p/p_crit = 0.05–0.50 and heat fluxes of q = 10³–1.5 × 10⁵ W/m². The preparation of the test equipment is described. The effects of pressure and concentration on the heat transfer coefficients are shown. The mixture behaves very much like an azeotropic mixture; concentration has only a small effect, the heat transfer coefficients can be obtained from the heat transfer coefficients of the pure components according to their molar fractions. The conversion steps from wire- to tube-data are presented. A comparison of wire-data with correlations given in literature is shown. It renders good agreement.     

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.     

Pressure drop during forced convection boiling of binary refrigerant mixtures

An experimental investigation was carried out in an electrically heated horizontal tube to measure pressure drop for various flow rates and heat fluxes during forced convection boiling of pure refrigerant 12 and four compositions of refrigerants 13 and 12 mixtures. The Martinelli-Nelson correlation, using the properties of the flowing refrigerant mixture, could not predict satisfactorily the pressure drop data. Total, as well as frictional pressure drops were found to be function of concentration. Two separate models each for total, as well as frictional pressure drop were developed for predicting the corresponding pressure drop. In each case, the maximum per cent deviation between predicted and measured pressure drop was within $\text{± 30\%}$.     

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.     

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.     

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.     

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.     

Observations on braided thin wire nucleate boiling in microgravity

A microgravity experiment was conducted on the Space Shuttle Endeavor (STS-108) to observe sustained nucleate boiling of water. Subcooled water was boiled with a single strand and a braid of three 0.16 mm diameter and 80 mm long Nichrome resistive wires. A CCD video camera recorded the experiment while six thermistors recorded the temperature of the fluid at various distances from the heating element. This paper reports experimental results in observations, measurements, and data analysis. Bubble explosions were found to take place shortly after the onset of boiling for both the single and braid of wires. The explosion may produce a high heat transfer rate, as it generates a cloud of microbubbles. The number, size, and departure rate of the bubbles from the heater wire were measured and compared with theoretical models as a function of time. The temperature measurements revealed a complex temperature distribution in the fluid chamber due to bubbles ejected from the wire that carried thermal energy close to the temperature sensors. Drag forces on departing bubbles were calculated based on bubble movement and used to predict bubble propagation. Results from this experiment provided further understanding of nucleate boiling dynamics in microgravity for the eventual design and implementation of two-phase heat transfer systems in space applications.     

Numerical simulation of nucleate pool boiling on the horizontal surface for ferrofluid under the effect of non-uniform magnetic field

Nucleate pool boiling heat transfer of ferrofluid on a horizontal plate; has been explored numerically. Extra necessary equations have been used in this model to simulate mass transfer and effect of magnetic field that had not considered in previous researches using mixture model. Also effect of negative and positive gradient of magnetic field on the heat transfer rate and bubble shape has been investigated. Results are in good agreement with experimental data.     

The influence of oil on nucleate pool boiling heat transfer

The influence of various oil contents in R134a is investigated for nucleate pool boiling on copper tubes either sandblasted or with enhanced heating surfaces (GEWA-B tube). Polyolester oils (POE) (Reniso Triton) with medium viscosity 55 cSt (SE55) and high viscosity 170 cSt (SE170) were used. Heat transfer coefficients were obtained for boiling temperatures between −28.6 and +20.1°C. The oil content varied from 0 to 5% mass fraction. For the sandblasted tube and the SE55 oil the heat transfer coefficients for the refrigerant/oil-mixture can be higher or lower than those for the pure refrigerant, depending on oil mass fraction, boiling temperature and heat flux. In some cases the highest heat transfer coefficients were obtained at a mass fraction of 3%. For the 170 cSt oil there is a clear decrease in heat transfer for all variations except for a heat flux 4,000 W/m² and −10.1°C at 0.5% oil content. The heat transfer coefficients are compared to those in the literature for a smooth stainless steel tube and a platinum wire. For the enhanced tube and 55 cSt oil the heat transfer coefficients are clearly below those for pure refrigerant in all cases. The experimental results for the sandblasted tube are compared with the correlation by Jensen and Jackman. The calculated values are within +20 and −40% for the medium viscosity oil and between +50% and −40% for the high viscosity oil. A correlation for predicting oil-degradation effects on enhanced surfaces does not exist.     

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.     

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.