Subcooled Liquid Research Articles

Influence of coolant and material properties on cooling high-temperature steel spheres in subcooled ethanol-water mixtures

The paper presents the new experimental results on cooling high-temperature steel spheres in subcooled ethanol-water mixtures at mass fractions of ethanol in the range of 10–80% under atmospheric pressure. Three test samples of 39 mm in diameter each have been prepared for the experiments: a carbon steel sphere with the oxidized surface and two stainless steel spheres. In the latter samples, one sphere had a polished surface, while the other one had a carbon coating. Usage of binary liquid mixtures as cooling liquids allows changing their thermophysical properties in a wide range. In general, when decreasing ethanol mass fraction boiling heat transfer intensifies, and it leads to an earlier (at higher surface temperature) transition to the intensive heat transfer regime. The experimental results are qualitatively consistent with a model that was proposed by authors earlier. This model describes the conditions of transition from a stable film boiling to an intensive heat transfer regime (so-called “microbubble boiling”). The main idea of the model is that this regime can arise due to the local contacts between the crests of the wavy liquid flow at the liquid/vapor interface and the protrusions of the surface roughness. The model considers the properties of a coolant (latent heat of evaporation, surface tension and kinematic viscosity) and thermophysical properties of a cooled surface (thermal effusivity). The experimentally obtained values of heat transfer coefficients in stable film boiling regimes are in agreement with the calculation results of the semi-empirical model of heat transfer in stable film boiling of subcooled liquids developed by authors earlier.

Explosive Vaporization of Ethanol on Microheater during Pulse Heating

The article is devoted to the research of transient heat transfer, explosive vaporization and condensation in subcooled ethanol on microheater during pulsed heating. Experiments were performed using high-speed infrared thermography and visualization for various initial liquid temperatures 30–60 °C and heat fluxes in the range of 3.3–7.8 MW/m2. The evolution of temperature field of the microheater and liquid during transient heating and the nucleation temperatures for different heat fluxes and liquid subcooling were determined. It was shown that the nucleation temperature was almost independent on heating power and amounted to 194 °C, which is close to the calculations based on homogeneous nucleation theory. New experimental data were obtained on the times of convection and boiling onset, evolution of vapor bubbles, including the dependence of their maximum sizes on the heat flux and initial liquid temperature. It was shown that the maximum bubble size decreases with an increase in the heat flux and liquid subcooling, varying in the same range as for water. It was demonstrated that at low heat fluxes, during condensation, a liquid funnel was formed at the bubble top, which resembled the formation of a liquid jet during the collapse of cavitation bubbles.

Boiling in Forced Convection of Subcooled Liquid as a Method for Removing High Heat Fluxes (Review): Part 2. Critical Heat Fluxes and Heat-Transfer Enhancement

Boiling in Forced Convection of Subcooled Liquid as a Method for Removing High Heat Fluxes (Review): Part 2. Critical Heat Fluxes and Heat-Transfer Enhancement

The gas phase retention volume behavior of organophosphate esters on polyurethane foam

In this study, the gas-phase retention volume behavior of four highly volatile organophosphate esters (OPEs) on polyurethane foam (PUF) was studied using a frontal chromatogram. The breakthrough volumes (VB) of trimethyl phosphate (TMP), triethyl phosphate (TEP), tripropyl phosphate (TPrP), and tri-n-butyl phosphate (TnBP) were 0.33, 0.59, 10.5, and 32.4 m3, respectively. A significant negative correlation was found between the logarithm value of VB and the subcooled liquid vapor pressure (PL) with an r2 of 0.905, indicating that the vapor penetration depends on both the volatility and total air volume. The enthalpy of desorption and vaporization (ΔHS−ΔHV) showed a significant difference between the tested OPEs and polycyclic aromatic hydrocarbons (PAHs), revealing that the strength of the sorption interaction of the tested OPEs vapor on PUF should be different from that of PAHs. The PUF-air partition coefficients (KPUF-air) of the tested OPEs ranged from 4.3 to 8.1, which were lower than those of three-to four-ring PAHs (7.02–10.2) and organochlorines (8.01–9.72), revealing that the partition of highly volatile OPEs on PUF is lower than that of low-volatile compounds. The PUF/XAD cartridge improved the absorption efficiency of the tested OPEs; however, its sorptive capacity was still limited.

Boiling in Forced Convection of Subcooled Liquid as a Method for Removing High Heat Fluxes (Review): Part 1. Characteristics, Mechanism, and Model of the Process, Heat Transfer, and Hydraulic Resistance

Boiling in Forced Convection of Subcooled Liquid as a Method for Removing High Heat Fluxes (Review): Part 1. Characteristics, Mechanism, and Model of the Process, Heat Transfer, and Hydraulic Resistance

Refueling of LH2 Aircraft—Assessment of Turnaround Procedures and Aircraft Design Implication

Green liquid hydrogen (LH2) could play an essential role as a zero-carbon aircraft fuel to reach long-term sustainable aviation. Excluding challenges such as electrolysis, transportation and use of renewable energy in setting up hydrogen (H2) fuel infrastructure, this paper investigates the interface between refueling systems and aircraft, and the impacts on fuel distribution at the airport. Furthermore, it provides an overview of key technology design decisions for LH2 refueling procedures and their effects on the turnaround times as well as on aircraft design. Based on a comparison to Jet A-1 refueling, new LH2 refueling procedures are described and evaluated. Process steps under consideration are connecting/disconnecting, purging, chill-down, and refueling. The actual refueling flow of LH2 is limited to a simplified Reynolds term of v · d = 2.35 m2/s. A mass flow rate of 20 kg/s is reached with an inner hose diameter of 152.4 mm. The previous and subsequent processes (without refueling) require 9 min with purging and 6 min without purging. For the assessment of impacts on LH2 aircraft operation, process changes on the level of ground support equipment are compared to current procedures with Jet A-1. The technical challenges at the airport for refueling trucks as well as pipeline systems and dispensers are presented. In addition to the technological solutions, explosion protection as applicable safety regulations are analyzed, and the overall refueling process is validated. The thermodynamic properties of LH2 as a real, compressible fluid are considered to derive implications for airport-side infrastructure. The advantages and disadvantages of a subcooled liquid are evaluated, and cost impacts are elaborated. Behind the airport storage tank, LH2 must be cooled to at least 19K to prevent two-phase phenomena and a mass flow reduction during distribution. Implications on LH2 aircraft design are investigated by understanding the thermodynamic properties, including calculation methods for the aircraft tank volume, and problems such as cavitation and two-phase flows. In conclusion, the work presented shows that LH2 refueling procedure is feasible, compliant with the applicable explosion protection standards and hence does not impact the turnaround procedure. A turnaround time comparison shows that refueling with LH2 in most cases takes less time than with Jet A-1. The turnaround at the airport can be performed by a fuel truck or a pipeline dispenser system without generating direct losses, i.e., venting to the atmosphere.

Enhanced pool boiling on micro-nano composited surfaces with nanostructures on micro-pin-fins

In the present study, the effect of the distribution of nanostructures on the pool boiling heat transfer performance of micro-nano composited surfaces in FC-72 was investigated. For this purpose, a micro-pin-fined surface (MPF), a micro-nano composited surface with nanoforests covering the top and bottom sides of micro-pin-fins (MPF-NF), a micro-pin-fined surface with nanostructures etched at the top corners of micro-pin-fins (MPF-TC), and a micro-pin-fined surface with nanostructures etched at the top edges of micro-pin-fins (MPF-TE) were fabricated. The pool boiling heat transfer performance was compared under the liquid subcooling (ΔTsub) of 0, 15, 25, and 35 K. It is found that MPF-NF has the lowest wall superheat at the onset of the nucleate boiling point due to its highest nucleation site density, while MPF-TE has the highest maximum heat transfer coefficient because its critical heat flux (CHF) is significantly higher than that of MPF-NF. The bubbles of MPF-TC and MPF-TE are mainly generated on the tops of micro-pin-fins, thereby alleviating the blockage of the capillary wicking channels by the bubbles in the high heat flux region. Furthermore, the condensation effect of the subcooled liquid on the bubbles generated on the tops micro-pin-fins, with a thinner thermal boundary layer, is stronger compared with that generated in the microchannels. On the contrary, the bubbles of MPF are mainly generated in the microchannels, leading to a blockage of the capillary wicking channels at high heat fluxes. Therefore, the CHF of MPF-TE at ΔTsub = 35 K is increased by 25% compared to MPF. However, the wake effect dominates the liquid supply and numerous lateral bubble coalescence on MPF-TE weakens the wake effect on the liquid supply, resulting in a lower CHF than that of MPF at ΔTsub = 0 K.

Performance Investigation of Power Inverter Components Submersed in Subcooled Liquid Nitrogen for Electric Aircraft

Investigating the performance of power electronics devices and thus power inverters at cryogenic temperatures for electric aircraft systems are of great interest. Accordingly, the purpose of this study is to examine the inverter circuit technologies used in cryogenically-cooled electric aircraft applications from three perspectives: inverter topologies, power capabilities, and electromagnetic interference (EMI) that may occur. At a cryogenic temperature, the characteristics of five power semiconductor switches with different technologies (Si MOS, SiC MOS, and GaN HEMT) used in cryogenically-cooled electric aircraft inverters were tested and the results were presented. Furthermore, the low-temperature performance of three types of capacitors commonly used in power electronics inverters was investigated. The research findings provide crucial considerations for the research and development of power inverters cooled by sub-cooled liquid nitrogen for modern electric aircraft.

Correlation between pressure loss and heat transfer coefficient in boiling flows in printed circuit heat exchangers with semicircular and circular mini-channels

• Boiling flow heat transfer coefficient and pressure drop were evaluated. • Printed circuit heat exchangers with semicircular and circular channels were employed. • Nucleate boiling in corners affected the heat transfer and flow resistance. • Heat transfer coefficient in semicircular channel was twice as high at low quality. • Increase in pressure drop was suppressed by effect of corners at a low quality. In this study, we investigated the correlation between the pressure loss and heat transfer coefficient for boiling flows in printed circuit heat exchangers. The effect of channel cross-sectional shapes—semicircle and circle—was evaluated. The hydraulic diameters of the semicircular and circular channels were 1.04 mm and 1.0 mm, respectively. A subcooled liquid with a subcooling temperature of 10 K was heated with hot water, and the saturation temperature was 30 °C. The refrigerant mass flux was set in a range of 100–400 kg/m 2 ·s. Consequently, despite the lower pressure drop, for low outlet quality, the heat transfer coefficient was higher for the semicircular channel than for the circular channel. The heat transfer was enhanced by the promotion of bubble nucleation in the thick liquid film at the corners, and the flow resistance was suppressed by the decrease in the vapor shear stress. However, for high outlet quality, the difference in the heat transfer coefficient decreased with an increase in the outlet quality. The experimental results were compared with the results obtained by correlation equations. The lowest mean absolute error of heat transfer coefficient and pressure drop for the semicircular mini-channel were 15.1 % and 26.7 %, respectively.

Thermodynamic performance on the pressurized discharge process from a cryogenic fuel storage tank

Due to excellent performance, cryogenic propellants, such as liquid hydrogen and liquid oxygen, are widely used in aerospace engineering. However, low storage temperature and low kinetic viscosity bring a lot of technique issues for high efficient thermal management on cryogen. An actual cryogenic fuel storage tank is selected as the research object, and a two-dimensional axial symmetrical computational model is established to study the pressurized discharge process, by adopting the volume of fluid (VOF) model. Both external environment heat leakage and the heat exchange occurring between the liquid and vapor are considered. Compared to the experimental results, the relative error is limited in 20.0%. Based on the developed numerical model, the temperature variation and heat flux through the insulation and tank wall, the pressurized discharge performance and the fluid temperature distribution are analyzed. The results show that during the pressurized discharge process, the lowest temperature appears in the inner side of the foam, and the external heat invasion does not absolutely penetrate into the tank. The vapor mass experiences fluctuating variations, and the vapor is always in condensation. In the first 200s, the temperature of the outflow fluid keeps constant, and then increases gradually. Under the present initial setting, the violent boiling phenomenon does not form during the whole process. The present study is significant to the depth understanding on the pressurized discharge of cryogenic fuels. • The lowest temperature appears in the inner surface of the foam with high-temperature gas occupied. • The external heat invasion does not penetrate into the tank during pressurized discharge. • The vapor is condensed by the subcooled liquid, and its mass experiences fluctuating variations. • The outflow pressure is higher than the saturated pressure, and the violent boiling does not form.

Does Explicit Polarizability Improve Molecular Dynamics Predictions of Glass Transition Temperatures of Ionic Liquids?

Molecular dynamics simulations are used for predictions of the glass transition temperatures for a test set of five aprotic ionic liquids. Glass transitions are localized with the trend-shift method, analyzing volumetric and transport properties of bulk amorphous phases. A classical nonpolarizable all-atom OPLS force-field model developed by Canongia Lopes and Pádua (CL&P) is employed as a starting level of theory for all calculations. Alternative approaches of charge scaling and the Drude oscillator model, accounting for atomic polarizability either implicitly or explicitly, respectively, are used to investigate the sensitivity of the glass transition temperatures to induction effects. The former nonpolarizable model overestimates the glass transition temperature by tens of Kelvins (37 K on average). The charge-scaling technique yields a significant improvement, and the best estimations were achieved using polarizable simulations with the Drude model, which yielded an average deviation of 11 K. Although the volumetric data usually exhibit a lesser trend shift upon vitrification, their lower statistical uncertainty enables to predict the glass transition temperature with lower uncertainty than the ionic self-diffusivities, the temperature dependence of which is usually more scattered. Additional analyses of the simulated data were also performed, revealing that the Drude model predicts lower densities for most subcooled liquids but higher densities for the glasses than the original CL&P, and that the Drude model also invokes some longer-range organization of the subcooled liquid, greatly impacting the temperature trend of ionic self-diffusivities in the low-temperature region.

Experimental and unified mathematical frameworks of water-ice phase change for cold thermal energy storage

Cold thermal energy storage (CTES) is a process that supplies cold thermal energy to a medium for storage and extracts it whenever is needed. The storage medium is phase change material (PCM), which makes great use of the large quantity of latent heat released during solidification or melting. However, some key fundamental and applied issues have not yet been resolved: how do we overcome the thermal interference in PCMs from unstable microscopic crystallization and external mechanical forces? Are there any unified and robust models to predict the whole time evolution of phase change accurately and rapidly? To fulfill these research gaps, we firstly establish a novel, well-controlled experimental system for PCMs that mitigates the thermal disturbance over a medium to large volume during solidification, capable of measuring transient temperature data and characterizing freezing stages at both macro- and micro-scale. We also develop in detail a unified semi-analytical mathematical framework to model the solidification of PCMs, consisting of five subsequent stages: liquid supercooling, nucleation, recalescence, equilibrium freezing, and solid subcooling. The modeling results yield a good agreement with our experimental data in several scenarios, particularly the nucleation temperature and time as well as the total freezing time are accurately predicted. Lastly, we extend our study to investigate the thermal effects of various radial positions, geometries, initial temperatures, heat transfer fluid temperature, and heat transfer coefficients in the context of CTES system.

Experimental Study of Horizontal Flow Boiling Heat Transfer Coefficient and Pressure Drop of R134a from Subcooled Liquid Region to Superheated Vapor Region

For the past few years, research in the field of flow boiling heat transfer has gained immense popularity for unravelling the dominant mechanism responsible for controlling heat transfer and identifying a parametric trend for understanding the characteristics of flow boiling heat transfer. This has led to several assumptions and models for predicting heat transfer during flow boiling without any known generalized mechanism. This study therefore seeks to experimentally study the characteristics of heat transfer during flow boiling over a wide range but small increase in vapor quality from a single-phase subcooled region through to a two-phase superheated vapor region. The study was performed with an R134a refrigerant in a single horizontal circular stainless-steel smooth tube that had an internal diameter of 5 mm. In this experimental study, local heat transfer coefficients and frictional pressure drop were measured for low heat fluxes of 4.6–8.5 kW/m2, mass fluxes of 200–300 kg/(m2s), vapor quality from −0.1 to 1.2 and a low constant saturation pressure of 460 kPa. Flow patterns observed during the study were recorded with a high-speed camera at 2000 fps. In covering a wide range of vapor quality, a peak of heat transfer coefficient near a vapor quality of zero and a local minimum observed in the low vapor quality region were observed, and both were sensitive to heat flux and mildly sensitive to mass flux. Generally, at low vapor quality, the heat transfer coefficient deteriorated with vapor quality and this was sensitive to heat flux but insensitive to mass flux and vapor quality, indicating nucleate boiling dominance in low vapor quality regions. In high vapor quality regions, the heat transfer coefficient was sensitive to mass flux and insensitive to heat flux. This indicates the dominance of convective boiling. In the low vapor quality regions, the flow patterns observed were slug and intermittent, while in the high vapor quality region, annular and mist flow patterns were observed. Generally, frictional pressure drop increased with increasing mass flux and vapor quality in the two-phase region.

A novel total-flow geothermal power generator using Turgo turbine: Design and field tests

This paper presents a unique design of a total-flow geothermal power generator that uses Turgo turbine and two-phase supersonic nozzles. The high-speed flashing jets are formed by high-pressure subcooled liquid from the well through the nozzles and impinge turbine blades obliquely to drive the Turgo turbine. This new generator converts geothermal power directly from the geothermal fluid without implementing any phase separator or heat exchanger, which results in a simple and easy-to-maintain system. A prototype of the total-flow geothermal power generator was built and field-tested in Yilan, Taiwan. Results show that compared to traditional Organic Rankine Cycle generators and double flash geothermal power plants, the new design has a competitive geothermal efficiency operating at moderate reservoir enthalpies.

Formation Mechanism of a Cumulative Jet Resulting from the Collapse of a Laser-Induced Vapor Bubble in a Subcooled Liquid

Formation Mechanism of a Cumulative Jet Resulting from the Collapse of a Laser-Induced Vapor Bubble in a Subcooled Liquid

Experimental study on flow boiling characteristics of hybrid micro-channels with gradient distribution pillars and bypass

Aimed at efficient cooling of high power micro-electric devices, the flow boiling characteristics of a novel hybrid micro-channel with gradient distribution pin fins and bypass is experimentally investigated using HFE7000. The onset of flow boiling, flow pattern, pressure drop, heat transfer coefficient and temperature uniformity are elucidated with the heat flux from 3.6 to 45 W/cm2 and mass velocity from 567 to 1290 kg/m2•s. The results show that the upstream denser and smaller pin fins reduce pin-fin recirculation zone and put off the bubble nucleation to a higher wall superheat. At a given heat flux of 30 W/cm2, the pressure drop of the gradient pin fin-bypass is reduced by 43.2% compared to that in the uniform pin fin. A large exit vapor quality of the latter induces a longer taper two-phase flow length and a larger contribution of vapor acceleration loss. Notably, the flow boiling stability is improved at a higher heat flux by the gradient pin fin-bypass with inhibiting vapor backflow and promoting subcooled liquid supply. As a result, the addressed heat flux increases by 10 W/cm2. Whereas temperature uniformity is better for uniform pin fin due to a larger taper two-phase flow region.

Hydraulic similitude assessment for cryogenic cavitation in propellant lines: The case of thick orifice

Cryogenic cavitation can be easily encountered in rocket liquid propulsion systems at the very initial phase of the launch due to the propellant passing through orifices and valves. This leads to flow instability, performance degradation, and even damages to the structures. Hence, the design of the propulsion systems cannot overlook cavitation phenomena. Despite the relevance of the topic, their accurate prediction is still hampered by the difficulty in modeling the high non-equilibrium phenomena involved. From an engineering point of view, the main interest is to design such restrictions present in propellant systems and, hence, to predict the maximum flow rate they can reach. This work questions the possibility to design orifices for cryogenic applications based on purely isothermal testing. Therefore, we performed cavitation experiments with both water and liquid nitrogen by considering two orifices hydraulically similar. This implies a geometric similitude in terms of the orifice characteristic ratio (β) and its dimensionless thickness (th). The results show that the hydraulic similitude is not sufficient to predict the orifice behavior for a cryogenic flow correctly. In particular, the level of liquid subcooling (ΔTsub) upstream of the orifice plays an important role since it impacts the minimal value of pressure of the flow downstream the orifice and thus its thermodynamic state.

Effects of liquid subcooling on droplet-wall collision heat transfer in film boiling

The effects of subcooling on single droplet-wall collision heat transfer characteristics during film boiling were experimentally investigated. Water droplet subcooling was varied from 0 to 60 K under atmospheric conditions, while the collision velocity and substrate temperature remained constant at 0.7 m/s and 500 °C. The instantaneous and localized heat transfer properties associated with dynamic collision of a water droplet were measured by integrated high-speed shadowgraph and infrared thermometry. Liquid subcooling greatly affected heat transfer. The residence time, effective heat transfer area, and local heat flux were measured. Only the local heat flux correlated strongly with liquid subcooling. When heat flux partitioning at the liquid–vapor interface was considered, i.e., conduction heat transfer into the subcooled droplet and the heat required for evaporation, heat transfer effectiveness was accurately predicted under various subcooling conditions.

Experimental and computational investigation of flow boiling in microgravity

This study explores use of Computational Fluid Dynamics (CFD) to predict near-saturated flow boiling of FC-72 in microgravity. The computational method employs transient analysis to predict detailed interfacial behavior and heat transfer characteristics along a rectangular channel heated along two opposite walls. Predicted results are validated against experimental temperature measurements and high-speed video images captured during a series of parabolic aircraft maneuvers for three sets of operating conditions which include variations of both mass velocity and wall heat flux. The computational method is based on the multi-phase volume of fluid (VOF) model, which is combined with appropriate phase change and turbulence models, and accounts for both shear-lift force on bubbles and conjugate heat transfer along the heating walls. A key advantage of the CFD method is ability to capture details that are very difficult to measure experimentally, including detailed spatial variations of bubble shape, void fraction, mixture fluid temperature, liquid velocity, and vapor velocity, results for which are presented for each of the three test cases. Different flow regimes predicted along the heated length exhibit a number of dominant mechanisms including bubble nucleation, bubble growth, coalescence, vapor blankets, interfacial waviness, and residual liquid sub-layer, all of which agree well with experiment. Vapor velocity is shown to increase appreciably along the heated length because of increased void fraction, while liquid velocity experiences large fluctuations. Non-equilibrium effects are accentuated with increasing mass velocity, contributing minor deviations of fluid temperature from simulations compared to those predicted by the analytical method. Predicted wall temperature is fairly uniform in the middle of the heated length but increases in the entrance region, due to sensible heat transfer in the subcooled liquid, and decreases toward the exit, mostly because of flow acceleration resulting from increased void fraction.

Quenching experiments of vertical Inconel and Zircaloy tubes in internal water flow

In this study, quenching experiments of vertical Inconel and Zircaloy tubes in internal water flow were conducted. The effects of the initial tube wall temperature, liquid subcooling, and liquid injection velocity on the quench temperature, the reaching time to quench temperature, and the quench front velocity were examined. For the experimental conditions, the initial tube wall temperatures were 700 and 1000 °C, the liquid subcoolings were 10, 50, and 75 °C, and the liquid injection velocities were 0.05 and 0.1 m/s. An increase in the initial tube wall temperature increased the quench temperature and prolonged the reaching time to quench temperature with the decrease of the quench front velocity. An increase in liquid subcooling shortened the reaching time to quench temperature and increased the quench temperature and quench front velocity. In addition, an increase in the liquid injection velocity reduced the reaching time to quench temperature, and increased the quench temperature and quench front velocity. For all cases, the Inconel tube exhibited a longer reaching time to quench temperature with lower quench temperature and quench front velocity than those of the Zircaloy tube. Based on the present experimental data for the quench temperatures of Inconel and Zircaloy tubes, the previous correlations were compared and assessed. The correlation of Kim and Lee (1979) predicted the quench temperature data with reasonable accuracy.