Subcooled Conditions Research Articles

Microbubble emission boiling in scbcooled pool boiling and the role of Marangoni convection in its formation

An experimental apparatus was designed and fabricated to visually investigate microbubble emission boiling (MEB) phenomena characterized by its extremely high heat dissipation capacity and potential application in the field of thermal engineering. The heating element was a cone copper block with an upper cylindrical section of 10mm in diameter. A high-speed video camera (Photron: Fastcam SA5) was employed for recording the scenario of MEB. Visualized results show that both the subcooling and noncondensable gas have significant effects on MEB. Remarkable MEB could be observed when the liquid subcooling exceeds 25K, with a maximum heat flux about 9MW/m2 at the subcooling of 60K. In order to explore the mechanism of MEB, numerical simulations were carried out to obtain the velocity field near a single vapor film on the same heating surface to that in the experiments with the software of “FLUENT”. Simulation results indicates that there exists Marangoni convection around the film under subcooled condition, which is influenced significantly by subcooling as well as thermal properties of the liquid. Strong Marangoni convection around the vapor film and the condensation at interface would induce interfacial waves, likely resulting in the collapse of the film in the case of high subcooling. As a result, it is reasonable to conclude that Marangoni convection is probably one of the key factors triggering the occurrence of MEB.

Carbon dioxide gas hydrate crystallization in porous silica gel particles partially saturated with a surfactant solution

This paper reports on investigations into the way carbon dioxide (CO2) hydrate forms in porous silica gel partially saturated with pure water or with a surfactant solution. The experiments, conducted at two different temperatures (278.2 and 279.2K) and under a loading pressure of 3.8MPa, used silica particles of different nominal pore diameters (30 and 100nm), saturated at 80% pore volume with pure water or with a 100ppm solution of either sodium dodecyl sulfate (SDS) or polyoxyethylenesorbitan monoleate (Tween-80). They were run following the “hydrate precursor method” developed in previous works (Duchateau et al., 2009, 2010) to form bulk hydrate under controlled subcooling conditions, and adapted for studying hydrate formation behavior in porous media.The work demonstrated that the successive hydrate formation and decomposition cycles involved in this method do not alter the pore size distribution in the porous media. At the two temperatures investigated, silica gel particles with a nominal pore diameter of 100nm proved better suited to comparing the CO2-hydrate formation behaviors: higher water-to-hydrate conversions (>90mol%) were effectively obtained for all the conditions tested making comparison of the results much easier. Of the two surfactants used, only SDS was found to produce a positive effect on both the hydrate formation kinetics and the amount of hydrate formed. Our visual observations of quiescent bulk systems (without porous silica gel) suggest that when SDS is present, CO2 hydrate forms not only at the w/g interface (where it occurs without SDS too), but also in the bulk water phase. This may explain the beneficial effect observed on the porous medium.

Assessment of CHF characteristics at subcooled conditions for the CANFLEX bundle

Boiling-Length-Average (BLA) Critical Heat Flux (CHF) values for the CANFLEX11CANFLEX® (CANdu FLEXible) is a registered trademark of Atomic Energy of Canada Limited (AECL) and Korea Atomic Energy Research Institute (KAERI). bundle at cross-sectional average subcooled conditions have been evaluated using the ASSERT-PV subchannel code. The predicted BLA CHF values supplement experimental BLA CHF values obtained with full-scale bundle simulators at saturated conditions in developing a BLA CHF correlation applicable over the interested range of cross-sectional average thermodynamic quality in regional overpower protection (ROP) trip and safety analyses. The BLA CHF correlation exhibits similar characteristics to those observed in tubes at subcooled and saturated conditions. Applying this correlation has led to similar prediction accuracy in dryout power to that using the BLA CHF-data-based correlation at saturated conditions. However, it provides improved prediction accuracy in dryout power at dryout conditions near saturation compared to the BLA CHF-data-based correlation (which tends to underpredict the dryout power).

B125 サブクール沸騰における親水・撥水複合伝熱面上の熱伝達および発泡挙動(一般講演(2))

Boiling heat transfer from a hydrophilic/hydrophobic combined surface was studied to examine the effect of the subcooling. The experiment was conducted at the subcooling of 30 K and its results were compared with previous data of the subcooling of 0, 10, 20 K. In subcooled conditions, boiling incipient superheat turns to a negative value and becomes lower with an increase in subcooling. The bubbles stay on the PTFE coating area without departure and maintain the hemispherical shape while oscillating up and down.

Effects of Hydrophobic-Spot Periphery and Subcooling on Nucleate Pool Boiling from a Mixed-Wettability Surface

The effect of subcooling and length of hydrophobic-spot periphery on nucleate pool boiling heat transfer from TiO2-coated surface with and without PTFE (polyetatafluoroethylene) hydrophobic circle spots at intermediate heat flux has been examined. The experiments are performed with liquid subcooling ranging from 0-20 K and heat transfer block used were TiO2-coated copper block with a PTFE hydrophobic circle spot with various diameters with total area of PTFE being constant. Bubble nucleation and behavior were observed by using high-speed camera. The results showed that the heat transfer performance of surfaces with PTFE hydrophobic circle spot is better than superhydrophilic surface in overall condition. Furthermore, the heat transfer performance decreases under subcooled condition for all surfaces. Increase in peripheral length of hydrophobic-spot enhances the heat transfer performance.

Nonlinear Analysis of Chaotic Time Series in a Natural Circulation Boiling Loop

The present study reports chaotic flow oscillations observed in a natural circulation boiling loop. The periodic oscillations of wall temperature (thermal oscillations) initiate at a certain condition of heat flux and inlet subcooling in addition to geysering instability and pressure-drop oscillations. We observed that boiling regime changes from nucleate boiling to transition boiling as a result of decrease in inlet subcooling and increase in heat flux. The thermal oscillations are strongly coupled with pressure-drop oscillations. Nonlinear analysis of the time series of loop flow rate at various heater power and inlet subcooling have been carried out using statistical analysis, fast Fourier transform (FFT), time delay embedding for attractor reconstruction, autocorrelation, and correlation dimension. The analysis confirms that the oscillations are more chaotic at relatively low heater power and high inlet subcooling. The complexity of the oscillations strongly depends on boiling heat transfer regime. Our observations and analysis have been supported by other relevant experiments.

Effectiveness of Alcohol Cosurfactants in Hydrate Antiagglomeration

Hydrates are crystalline inclusion compounds where hydrogen bonded water molecules form cages to trap small hydrocarbon and nonhydrocarbon molecules. Under high subcooling conditions, these crystals grow rapidly into large pieces and may cause enormous problems in transport and deepwater oil capture. One of the most effective methods to address gas hydrate problems is through the formation of small hydrate particles in the nanometer or micrometer range dispersed in the fluid phase. In this approach, special surfactants are required, which are called antiagglomerants (AAs). However, there are major limitations when salt is present in water and when volume ratio of water in fluid (i.e., watercut) is high. In this work, we investigate a wide range of alcohols as cosurfactants along with the rhamnolipid as AA in a multiple screening-tube rocking apparatus by monitoring the temperature of vials and morphology of mixtures. The results show medium-sized alcohols, such as isopropanol (IPA), are effective cosurfac...

B123 中性子ラジオグラフィによる管内沸騰二相流のボイド率定量評価(OS9 熱・流動)

The void fraction in a forced convective flow boiling is very important information for understanding the characteristics of the boiling two-phase flow. Consequently, many experimental investigations have been carried out to obtain the local void fraction so far, but the detail data among the whole of the test-section has not been enough. Especially, the data under subcooled condition are quite limited. In this study, the void fraction distribution in a forced convective boiling was quantitatively measured by using the thermal neutron radiography. These results were compared with several existing void fraction correlations. Although these correlations show a good agreements with experimental results under low heat flux condition, there is no suitable correlation to estimate the void fraction under non-thermal equilibrium condition.

Submerged Jet Impingement Boiling of Water Under Subatmospheric Conditions

An experimental study of jet impingement boiling is presented for water under saturated and subcooled conditions. Unique to this study is the documentation of boiling curves of a submerged water jet under subatmospheric conditions. Data are reported at a fixed nondimensional nozzle-to-surface distance of H/dj = 6 and for a fixed surface-to-nozzle diameter ratio, dsurf/dj, of 23.8. Saturated jet impingement experiments are performed at three subatmospheric pool pressures of 0.176 bar, 0.276 bar, and 0.478 bar with corresponding saturation temperatures of 57.3 °C, 67.2 °C, and 80.2 °C. At each pressure, jet impingement boiling at varying Reynolds numbers are characterized and compared with pool boiling heat transfer. The effect of surface roughness and fluid subcooling is studied at the lowest pressure of 0.176 bar. Boiling curves indicate a strong dependence of heat flux on jet Reynolds number in the partially developed nucleate boiling region but only a weak dependence in the fully developed nucleate boiling region. At a fixed wall superheat, fluid subcooling is found to shift the boiling curve to the left thereby enhancing heat transfer performance. Critical heat flux is found to increase with increases in pressure, surface roughness, and Reynolds number.

Experimental Study on the Lightning Impulse Dielectric Characteristics of Sub-Cooled Liquid Nitrogen for a High Voltage Superconducting Fault Current Limiter

Sub-cooled liquid nitrogen (LN2) condition is a very useful method to increase the current rating capacity of YBCO tapes. Thus, the lightning impulse dielectric characteristics of sub-cooled LN2 condition should be investigated for designing high voltage superconducting machines. The sphere-plane electrode systems were used to simulate the internal structure of high voltage superconducting machines such as superconducting fault current limiters. The lightning impulse dielectric tests on saturated and sub-cooled LN2 condition were carried out by using the sphere-plane electrode systems. In addition, a quasi-uniform electric field distribution between the sphere-plane electrodes was calculated by the finite element method (FEM) simulation tool. The field utilization factor was calculated by the FEM simulation results. The lightning impulse breakdown voltage of various geometrical configurations between the sphere-plane electrodes in sub-cooled LN2 conditions was compared with that of in saturated LN2 condition. Finally, the lightning impulse breakdown voltage with 50% probability and its relations with the utilization factor have been described in this paper.

Experimental Analysis of Unequal Voltage Distribution on a Single YBCO Coated Conductor Affected by Inhomogeneous Critical Currents in Liquid Nitrogen and Sub-Cooled Liquid Nitrogen

To design a large scale superconducting fault current limiter (SFCL) for power grids, a single long-length YBCO coated conductor (CC) wire is preferred. However this single YBCO CC wire can cause a problem of unequal voltage distribution between the parts on the long wire at quench by the faults. Inhomogeneous critical current of the YBCO CCs in the SFCL is a possibility. Furthermore, sub-cooled liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) condition is usually used to increase current capacity of the SFCL. However, this sub-cooled condition can worsen the inhomogeneity between the YBCO CCs. Therefore, it is important to understand the relation between the unequal voltage distribution and the inhomogeneous critical currents in the single long length YBCO CC. In this paper, experiments with the same single YBCO CC wire were conducted in various temperature conditions, LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and sub-cooled LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . In both experiments, the critical currents and voltage distributions generated by faults at short-circuit tests were measured. It was observed that the inhomogeneous condition impacts on the unequal voltage distribution.

Bubble Dynamics and Heat Transfer in Pool Boiling on Wires at Different Gravity

Abstract A series of experimental studies on bubble dynamical behaviors and heat transfer in pool boiling on thin wires in different gravity conditions have been performed in the past years, including experiments in long-term microgravity aboard the 22nd Chinese recoverable satellite RS-22, in short-term microgravity in the drop tower Beijing, and in normal gravity on the ground. Steady pool boiling of degassed R113 on thin platinum wires has been studied using a temperature-controlled heating method. A voltage-controlled heating method has also been used in normal gravity. A slight enhancement of nucleate boiling heat transfer is observed in microgravity, while dramatic changes of bubble behaviors are very evident. Considering the influence of the Marangoni effects, the different characteristics of bubble behaviors in microgravity have been explained. A new bubble departure model including the influence of the Marangoni effects has also been proposed, which can predict the whole observation both in microgravity and in normal gravity. The value of CHF (critical heat flux) in microgravity is lower than that in normal gravity, but it can be predicted well by the Lienhard–Dhir correlation, although the dimensionless radius, or the square root of the Bond number, in the present case is far beyond its initial application range. A further revisit on the scaling of CHF with heater radius in normal gravity, which is focused on the case of a small Bond number, has also been performed in our laboratory using different kinds of working fluids at different subcooling conditions. Interactions between the influences of the subcooling and heater radius will be important for the case of a small Bond number. In addition to the Bond number, there may exist some other parameters, which may be material-dependent, that play important roles in the CHF phenomenon with a small Bond number.

Numerical Prediction of Conjugate Heat Transfer in Fluid Network

An unsteady finite volume procedure for conjugate heat transfer in flow network that takes into account the longitudinal conduction through the solid is presented. It uses a fully coupled approach in which the governing equations for solid and fluid are coupled through solid to fluid heat transfer that is expressed as a function of flow properties and temperature of solid. As an evaluation of the proposed technique, a chilldown problem for a cryogenic transfer line is formulated and solved. Test cases modeling transient flow of liquid hydrogen (LH2) and liquid nitrogen (LN2) under saturated and subcooled liquid conditions are presented. The effects of varying the inlet driving pressure on the chilldown time and flow rates have been evaluated. Increasing the driving pressure decreased the chilldown time and increased the flow rate. Subcooling the inlet cryogen further reduced the chilldown time. Numerical predictions are compared with available experimental data and are found to be in good agreement. The proposed model captures the essential features of conjugate heat transfer and provides an efficient and robust way for predicting chilldown of transfer line at a low computational cost.

Investigating the dynamics of gas hydrates in a gas dominant flow loop

Offshore pipelines transporting hydrocarbon fluids have to be operated with great care to avoid problems related to flow assurance. Of these possible problems, gas hydrate is dreaded as it poses the greatest risk of plugging offshore pipelines and other production systems. As the search for oil and natural gas goes into deeper and colder offshore fields, the strategies for gas hydrate mitigation are evolving to the management of hydrate risks rather than costly complete prevention. CSIRO has been developing technologies that will facilitate the production of Australian deepwater gas reserves. One of its research programs is a recently commissioned investigation into the dynamic behaviour of gas hydrates in gas pipelines using a pilot-scale 1 inch and 40 m long flow loop. This work will provide experimental results conducted in the flow loop, designed to investigate the hydrate formation characteristics in steady state and transient flow. For a given hydrodynamic condition in steady state flow, the formation and subsequent agglomeration and deposition of hydrate particles appear to occur more severely as the subcooling condition is increasing. Transient flow during a shut-in and restart operation represents a more complex scenario for hydrate formation. Although hydrates develop as a thin layer on the surface of water during the shut-in period, most of the water is quickly converted to hydrate upon restart, forming hydrate laden slurry that is transported through the pipeline by the gas flow. These results could provide valuable insights into the present operation of offshore gas pipelines.

Photographic Study on Two-Phase Flow Patterns of Water in a Single-Side Heated Narrow Rectangular Channel

Abstract Visualized experimental observation on flow patterns during flow boiling of water under single-side heated and fluid-inlet subcooled conditions in a vertical narrow rectangular channel with the cross section of 40×3 mm2 have been carried out. Four discernible flow patterns, which names dispersed bubbly, coalesced bubbly, churn flow, and annular flow are observed. Flow visualization in two dimensions of two-phase flow patterns for narrow rectangular channel, which provided clearer evidence to distinguish flow patterns, have been performed. Based on the experimental results, a flow pattern map for single-side heated narrow rectangular channel has been developed and then compared with the exiting maps and flow transition criteria.

Condensation and Collapse of Vapor Bubbles Injected in Subcooled Pool

We focus on condensation and collapse processes of vapor bubble(s) in a subcooled pool. We generate the vapor in the vapor generator and inject it/them to form vapor bubble(s) at a designated temperature into the liquid at a designated degree of subcooling. In order to evaluate the effect of induced flow around the condensing/collapsing vapor bubble, two different boundary conditions are employed; that is, the vapor is injected through the orifice and the tube. We also focus on interaction between/among the condensing/collapsing vapor bubbles laterally injected to the pool. Through this system we try to simulate an interaction between the vapor bubble and the subcooled bulk in a complex boiling phenomenon, especially that known as MEB (microbubble emission boiling) in which a higher heat flux than critical heat flux (CHF) accompanying with emission of micrometer-scale bubbles from the heated surface against the gravity is realized under a rather high subcooled condition.

Pool boiling of perfluorocarbon mixtures on silicon surfaces

The need for higher pool boiling critical heat flux (CHF) in electronic cooling applications has turned attention to the use of binary mixtures of dielectric liquids. The available literature demonstrates that the addition of a liquid with higher saturation temperature, higher molecular weight, higher viscosity and higher surface tension can lead to significant enhancement of CHF, beyond what can be achieved through changes in pressure, liquid subcooling, and the product of surface effusivity and heater thickness. The current study focuses on extending the available data on mixture CHF enhancement, as well as pool boiling, on polished silicon surfaces to FC-72/FC-40 mixture ratios of 10%, 15%, and 20% of FC-40 by weight, a pressure range of between 1 and 3 atm, and fluid temperature from 22 to 45 °C, leading to high subcooling conditions. It is found that peak heat flux can be increased to as high as 56.8 W/cm 2 compared to 25.2 W/cm 2 for pure FC-72 at 3 atm and 22 °C. It is believed that the increase in the mixture latent heat of evaporation and surface tension, accompanying the depletion of the lower boiling point fluid in the wall region plays the major role in enhancing the critical heat flux for binary mixtures.

Rewetting velocity in quenching at reduced gravity

The results of quenching tests in tubes at microgravity and normal gravity conditions are presented. The aim of the experimental campaign was to gather both quantitative data and direct observations regarding phenomena involved in the rewetting of tubes. The rewetting velocity at different refrigerant flow-rate and subcooling conditions is obtained from experimental data. Test sections are made of Pyrex tubes with three different inner diameters: 2.0 mm, 4.0 mm and 6.0 mm. They are coated with Indium-Tin-Oxide (ITO), which is transparent but allowing the direct heating of the Pyrex tube. Tests are performed with vertical test section and downward liquid flow. The working fluid is FC-72, a fluorinert liquid. Measurements included wall temperatures along the flow channel, inlet and outlet temperatures, pressures and mass flow-rate. The 6.0 and 2.0 mm tubes show a significant decrease in the quenching velocity at reduced gravity and a dependence on the coolant mass flow-rate, especially for ground experiments. The results of the 4.0 mm tube show a contradictory behaviour in micro/normal gravity comparison.

Experimental Study on the Electrical Breakdown Characteristics of Sub-Cooled Liquid Nitrogen for Designing a High Voltage Superconducting Machine

The electrical breakdown characteristics of liquid nitrogen should be investigated for developing of a high voltage superconducting machine. This paper deals with the experimental study for the verification of dielectric characteristics of sub-cooled in accordance with utilization factors. AC dielectric experiments were carried out by using sphere-plane electrode systems. The utilization factors of simulated electrode systems were controlled by gap distance between two electrodes and diameter of a sphere electrode. Also, dielectric experiments according to various pressures, 1.25, 1.5 and 2 bar, by gaseous nitrogen were carried out. In this paper, the electrical breakdown criteria of sub-cooled conditions are derived with various sphere diameters. In addition, the sub-cooled condition and saturated condition were compared with AC dielectric characteristics. Research results could be applicable to designing of high voltage superconducting applications such as superconducting fault limiters (SFCLs), cables and large-capacity transformers.

Boiling heat transfer from a horizontal flat plate in a pool of liquid hydrogen

Heat transfer from a flat plate facing upward immersed in a liquid hydrogen pool was measured for the pressures from atmospheric to 1.1 MPa. The flat plate heater used was 10 mm in width, 100 mm in length and 0.1 mm in thickness. Critical heat fluxes (CHFs) in saturated boiling increased with the increase in pressure up to around 0.3 MPa and then decreased with further pressure increase. The CHFs under subcooled condition at each pressure increased with the increase in sub-cooling. Discussions were made on the experimental results by comparing with those of the other cryogenic liquids and also the Kutateladze’s equations under saturated and subcooled conditions. The experimental CHFs were much smaller than the Kutateladze’s equation for higher pressure up to critical. The heater surface temperature was found to reach the critical temperature before the occurrence of hydrodynamic instability and jump to the film boiling regime at the lower heat flux in the higher pressure range. It was suggested that the CHFs are determined not by the heat flux but by the temperature in the higher pressure range.