Interfacial mass transfer around a vapor bubble during nucleate boiling

Interfacial mass transfer from vapor bubbles affects markedly the heat transfer efficiency of nucleate boiling. The position of the interfacial zone that exhibits zero net mass flux, namely, the zero-flux zone, represents an essential parameter in detailed modeling works on nucleate boiling. Assuming a linear temperature profile in the superheated liquid adjacent to the heating wall, our previous work (Li et al. [10]) demonstrated the zero-flux angle as a function of wall superheat, solid-liquid- vapor contact angle, and bubble growth rate. To make a more realistic framework, we refined in this paper the proposed mass flux model by taking into account the role of thermocapillary flow that is induced by the temperature gradient around the vapor bubble, and that of non-condensable gas presented in the boiling liquid. The Hertz-Kundsen-Schrage equation describes the interfacial mass flux distribution along the vapor bubble surface. Owing to the flattened temperature distribution produced by thermocapillary flow, which significantly reduces the interfacial area to evaporation, the zero-flux zone shifts to the bubble base with most of the cap regime to condense vapor at the interface and to produce the thermal jet. This occurrence also weakens the dependence of bubble growth rate and of the contact angle on the location of zero-flux zone, and yields early occurrence of the non-condensation limit at which the entire bubble surface is subjected to evaporation. Sensitivity analysis demonstrated the significance of process parameters on the evaluation of zero-flux angle using the HKS equation.     

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.     

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.     

Experimental studies on the effect of tube inclination on nucleate pool boiling

Experiments were conducted to study the effect of tube inclination on nucleate pool boiling heat transfer for different tube diameters and surface roughness values. The results show that as the tube is tilted from the vertical to the horizontal, the temperatures at the top and bottom (with respect to circumference) increase and decrease, respectively. The increase and decrease is such that they almost compensate for each other, resulting in very little variation of the average heat transfer coefficient with tube inclination. The increase in bubble sliding length at the bottom wall and decrease in bubble sliding length at the top wall are thought to be the reasons for this behaviour. Experiments have been conducted with water, ethanol and acetone at atmospheric pressure, to confirm similar effects of inclination irrespective of fluid property.     

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.     

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.     

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.     

Review of nucleate pool boiling bubble heat transfer mechanisms

Enhanced convection, transient conduction, microlayer evaporation, and contact line heat transfer have all been proposed as mechanisms by which bubbles transfer energy during boiling. Models based on these mechanisms contain fitting parameters that are used to fit them to the data, resulting a proliferation of “validated” models. A review of the recent experimental, analytical, and numerical work into single bubble heat transfer is presented to determine the contribution of each of the above mechanisms to the overall heat transfer. Transient conduction and microconvection are found to be the dominant heat transfer mechanisms. Heat transfer through the microlayer and at the three-phase contact line do not contribute more than about 25% of the overall heat transfer.     

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.     

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.     

Experimental study on nucleate boiling of water in vertical upflow and downflow

An experimental study on nucleate boiling of water was carried out using an annular vertical channel both in upflow and downflow. Heat transfer data are given in different conditions of subcooling and fluid velocity. Photographs show different behaviour of heat transfer mechanism.     

Transport-mechanisms in natural nucleate boiling in absence of external forces

In literature it is generally supposed that under terrestrial conditions the driving force in natural, nucleate boiling heat transfer is namely buoyancy caused by earth gravity, which is expressed in the empirical correlations for technical applications. However, experiments in microgravity performed during the past three decades demonstrate unanimously that up to a medium level heat flux the overall heat transfer in pool boiling is nearly independent from gravity. We refer and discuss in this paper on results of experiments performed with various liquids and liquid states and also using various heater geometries on mission platforms which provide low gravity for short and long periods. Beside the measurements of the experimental parameters to determine the heat transfer, we observed the macroscopic boiling process itself with movie films and videos in order to study the bubble dynamics. From these records we learned about the mechanisms of heat and vapour bubble transport, about the interaction between solid heater, superheated liquid, and vapour without gravity or other external force only generated by the bubbles themselves, and we observed significant details about the boiling process not recognized so far. These findings are essential for a better understanding of the complex physical process; and therefore they are important for the formulation of empirical correlations, and in future for numerical simulations to predict properly boiling heat transfer for technical applications.     

Modes of bubble growth in the slow-formation regime of nucleate pool boiling

The shape and size of a bubble formed slowly on a sharp- or round-edged orifice are derived with the help of a new analytical solution for the bubble profile. Two modes of formation are distinguished, depending on the natural contact angle, ?₀: bubble confined to the orifice (?₀ small); bubble spreading beyond the orifice (?₀ large: Fritz mode). The limits of the slow-formation regime in mucleate pool boiling are estimated, involving an assessment of the influences of liquid inertia, viscosity and surface-tension gradients.“Slow” formation is predicted for large cavities or high pressures and this is borne out by data for water. The Fritz mode of growth, however, is seen to be suppressed.     

Experimental and numerical study on nucleate bubble deformation in subcooled flow boiling

Experimental and numerical study was conducted to investigate the bubble behaviors in subcooled flow nucleate boiling. The bubble behaviors in subcooled flow boiling in an upward annular channel were investigated in the range of subcooling degree 5–30 K by visualization with high spatial and temporal resolutions using a high speed video camera and Cassegrain tele-microscope. Obvious deformation on the upstream side surface of the bubble during its growth process was frequently observed. This deformation phenomenon was caused by the condensation occurring at the upstream side bottom of the bubble, which results from the Marangoni flow along the bubble surface from the bubble bottom to the top. Since the Marangoni flow cannot be directly observed by the current experiments because it occurs in a very thin interface along the bubble surface, the numerical simulations of bubble growth and departure behaviors in subcooled flow boiling were carried out. As a result, it was confirmed that the bubble deformation was caused by the Marangoni flow along the bubble surface. Moreover, the phenomenon of wave propagation on the bubble surface during the condensation process was observed, and it can enhance the heat transfer between the bubble and the surrounding subcooled liquid.     

Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid

Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid was investigated experimentally. Three types of surfactants including Sodium Dodecyl Sulfate (SDS), Cetyltrimethyl Ammonium Bromide (CTAB) and Sorbitan Monooleate (Span-80) were used in the experiments. The refrigerant-based nanofluid was formed from Cu nanoparticles and refrigerant R113. The test surface is horizontal with the average roughness of 1.6 μm. Test conditions include a saturation pressure of 101.3 kPa, heat fluxes from 10 to 80 kW m⁻², surfactant concentrations from 0 to 5000 ppm (parts per million by weight), and nanoparticle concentrations from 0 to 1.0 wt.%. The experimental results indicate that the presence of surfactant enhances the nucleate pool boiling heat transfer of refrigerant-based nanofluid on most conditions, but deteriorates the nucleate pool boiling heat transfer at high surfactant concentrations. The ratio of nucleate pool boiling heat transfer coefficient of refrigerant-based nanofluid with surfactant to that without surfactant (defined as surfactant enhancement ratio, SER) are in the ranges of 1.12-1.67, 0.94-1.39, and 0.85-1.29 for SDS, CTAB and Span-80, respectively, and the values of SER are in the order of SDS > CTAB > Span-80, which is opposite to the order of surfactant density values. The SER increases with the increase of surfactant concentration and then decreases, presenting the maximum values at 2000, 500 and 1000 ppm for SDS, CTAB and Span-80, respectively. At a fixed surfactant concentration, the SER increases with the decrease of nanoparticle concentration. A nucleate pool boiling heat transfer correlation for refrigerant-based nanofluid with surfactant is proposed, and it agrees with 92% of the experimental data within a deviation of ±25%.     

The degeneration of nucleate boiling conditions under conditions of free convection

An experimental investigation was made of the effect of the degeneration of nucleate boiling conditions under conditions of natural convection for alkali metals (potassium, cesium) in working sections with diameters of 14 and 11 mm in a pressure range of 0.04–1.7 bar, and for nonmetallic liquids (ethanol, benzene, and carbon tetrachloride) in working sections with diameters from 0.1 to 10 mm in a pressure range of 0.05–2.5 bar. The regions were determined for thermal flows, with which the appearance of a vapor phase after single-phase natural convection leads to the formation of a stable vapor film. The results of high-speed movingpicture photography of this phenomenon are also given.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 69–73, January–February, 1972.     

Maximum size of bubbles during nucleate boiling in an electric field

By taking account of the electric field effects on the shape of bubbles. Fritz's analysis on maximum bubble volume during boiling was extended to the boiling process in the presence of a uniform electric field. It was found that the maximum bubble volume decreases with increase in electric field strength and the dielectric constant of the boiling liquid. The decrease of bubble departure size with increase in electric field strength was confirmed by experimental observations.     

Shape of the vapor bubble upon explosive boiling

A relation for the shape of a vapor bubble forming during propagation of a vaporization front is proposed. Kutateladze Institute of Thermal Physics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 2, pp. 122–123, March–April, 2000.