Subcooled Liquid Research Articles

Heat Transfer Coefficient Calculation for Developed Ammonia Boiling in the Evaporator Channel of a Thermal Sink

The subject of this article is the heat transfer of ammonia in the channels of thermal sink evaporators. The objective was to determine a sufficiently simple correlation acceptable for engineering practice, which could be used to calculate the heat transfer in cylindrical channels of a thermal sink, designed for two-phase systems of thermal control systems of uncrewed spacecraft. For this purpose, experiments were performed on two thermal sink having an evaporator channel of ~7 mm diameter made of aluminum alloy and stainless steel, with channel surface roughness Ra 3.33 µm and 0.12 µm. The experiments were carried out with a subcooled liquid or two-phase flow at the channel inlet. The results of the experiments were compared with Kupriyanova's formula obtained under markedly different conditions: with ammonia boiling in a large volume on the external surface of 5...6 mm diameter tubes at –40 °C...+20 °C. It is shown that Kupriyanova's formula can be used on the ground and in microgravity conditions to calculate heat transfer coefficients during the developed boiling of ammonia in the range of flow parameters: saturation temperature +35 °C...+75 °C; mass velocity 27...200 kg/(sec-m²); liquid subcooling to saturation temperature at thermal sink inlet 0 °C...30 °С; mass vapor quality at the inlet 0...0.7. Difference of calculated and experimental values of heat transfer coefficients did not exceed 30 %.

Simulation of vapour bubble condensation using a 3D method

In this work, simulation of a rising vapour bubble condensation in subcooled boiling flow conditions was performed using an interface tracking method, called the InterSection Marker (ISM) method which was previously developed by the authors to track the bubble interface. The rising bubble was condensing under the influence of the buoyancy and the surface tension forces. Immersed boundary method was used as a coupling mechanism between the ISM method and an in-house variable-density and variable-viscosity single-fluid flow solver. To imitate the condensation (i.e., heat and mass transfers from the vapour bubble), the source terms were modelled in the Computational Fluid Dynamics (CFD) governing equations. During the simulation, the condensing bubble properties such as bubble size, shape, and velocity were predicted for various initial bubble sizes and liquid subcooling values. The numerical results compared well against the past works, and the ISM method established itself to be an efficient, reliable, and viable alternative numerical tool for multiphase flow simulations involving heat and mass transfers across the interface.

Dynamic behaviors of in-tank subcooled liquid with depressurization-induced phase change and the impact on primary breakup of flashing jet

Accidental releases of superheated liquids, such as liquefied petroleum or natural gases, are featured by depressurization across the outlet leading to liquid flashing within the upstream container and the downstream jet. In this work, dynamic behaviors of in-tank liquid with phase change throughout depressurized releases and the influence on the primary breakup of flashing jet were studied with an experimental 20 L tank. In-tank parameters (pressure, temperature, and liquid mass) and downstream jet morphology were characterized. A new nondimensional number (ηp), the ratio between the superheat levels of the saturated states corresponding to liquid temperature and pressure, was developed to describe the liquid's thermodynamic state under both release and ambient conditions. A thermodynamics-determined release rate model was established to characterize flow behaviors at the exit. Results showed a strong correlation between the initial ηp0 and key process parameters I (depressurization and release rates): I = aηp0b, where a and b are constants for a particular I. A ηp0-based criterion was derived to characterize in-tank release dynamics and thermodynamics: ηp0 < 0.25 (subcooled), 0.25<ηp0 < 0.4 (transition from subcooled to superheated), and ηp0 > 0.4 (superheated). Mild evaporation and flash evaporation dominated in-tank pressure recovery when ηp0 < 0.25 and ηp0 > 0.25, inhibiting depressurization. The release rate model agreed well with experiments and demonstrated that flashing happens upstream and chokes the induced two-phase flow when ηp0 > 0.25, decreasing the release rate. The evolving upstream conditions changed the breakup regime both in mechanical and thermodynamic aspects. Moreover, the thermodynamic effect increased as the mechanical effect decreased during depressurization.

Experimental investigation of bladeless expander with an incompressible fluid

For small-medium heat pumps and small-scale energy storage (e.g., hydro) and other small-scale applications (up to 10kWe), an economical, reliable, durable, robust, and acceptable performing bladeless expander is an attractive technology. In this article, experiments are performed on a bladeless expander of 1kW design power with water as a working fluid. The complete expansion is in the subcooled liquid phase with an overall pressure drop across the expander in the 2-14 bar range. The water expander is designed as a similitude case study for a butane heat pump, where such a bladeless expander could replace the expansion valve recovering untapped energy from isenthalpic to isentropic expansion. Unlike conventional bladed expanders, the present bladeless expander consists of several co-rotating compact disks, closely spaced and parallelly mounted on the shaft which transmits torque using wall shear forces. The present expander design is an improved version resulting from the detailed loss characterisation done on earlier air expanders.The article begins with the definition of design conditions for water expander starting from the expected butane expansion in the heat pump, fully inside the liquid region. The rotor design of a bladeless expander is outlined using dimensionless parameters that dictate the performance features. The turbine is designed for 1kW of power output and 2kg/s mass flow with an overall pressure drop of 14 bar with a rotational speed of 8000 rpm. The resulting turbine rotor consists of an 80mm disk outer diameter, 120 disks and a 0.1mm gap between them. An experimental test rig employing water as a working fluid is described. Experiments are conducted for overall pressure drop ranging in the 2-14 bar interval, with a maximum rotational speed of 6500 rpm. The performance is recorded with two different stator configurations, having two different throat dimensions for varying mass flow at maximum inlet pressure. Peak total to static efficiency of 30% is obtained with a net power of 670 W at ∼3000 rpm. An experimental ventilation loss (end wall viscous disk friction) is performed with both water and air as working fluids to estimate the power loss. It is found that ventilation loss is the major source of loss in the present turbine prototype with a power loss of 250W@3000 and 1100W@8000 rpm, varying quadratically with rotational speed. It is finally concluded that the expander performance is promising because ventilation losses can be potentially reduced with established strategies used in conventional expanders.

Simulation of heat and mass transfer process in a flat-plate loop heat pipe and experimental comparison

A loop heat pipe is a passive heat transfer device with strong robustness, high efficiency, and good performance, and has been widely utilized in numerous thermal control systems. Herein, an intensive study of a flat-plate evaporator LHP from the perspective of experiment and simulation is presented. First, the vapor leakage problem, which was a common issue for most LHP systems, was solved by filling the assembly clearance with epoxy glue, and a flat-plate LHP system was fabricated to verify its operating performance. Test results indicated that the loop stably performed under various working conditions without temperature oscillation. The maximum heat flux was 11.25 W/cm2 and the minimum thermal resistance was 0.1340 °C/W. The heater surface temperature remained low except during the dry-out state in wick at the maximum heat load condition where a sharp increase was observed, and temperature hysteresis occurred during variable heat load tests owing to the transport lag in long liquid line. Based on the above experiment, an efficient 3-D CFD computational model was built to simulate the heat and mass transfer process in a flat-plate evaporator by the Volume of Fluid method. In addition, the flow regime with two-phase distribution at the scale of the entire evaporator was calculated by considering the structure effect. Boundary conditions were derived from the experiment and the mass flow rate at the evaporator inlet was deduced from the heat leakage effect in liquid line. Calculation results demonstrated that four symmetric vortices developed inside the compensation chamber and generated uneven subcooled liquid infiltration in wick, further leading to a slight offset of high-temperature zone to the evaporator inlet side. Parameter analysis indicated that the ribs conducted 84 % of the heat conduction using thermal paths formed between the heater surface and liquid–vapor interface. The vapor volume percentage in the vapor collector was above 80 %, and increasing the heat load reduced the vapor volume in wick as well as the percentage of heat adsorbed during evaporation. Experimental comparison illustrated that the model exhibited a high accuracy with an error less than 10.29 %.

Investigations of Performance of Mini-Channel Condensers and Evaporators for Propane

This paper provides the results of experimental investigations of the exemplary mini-channel heat exchanger in its application as a condenser and an evaporator in a compressor refrigeration system with propane as a working fluid. The aim of the investigations was to identify the mean heat transfer coefficient of the refrigerant side for the entire operating range of the tested heat exchanger. The experiments covered a mass velocity range from 50 to 160 kg/(m2 × s). The experiments covered a range of liquid subcooling in the condenser from 3 to 15 K and a range of vapour superheating at the outlet of the evaporator from 3 up to 15 K. The experiments on the condenser were conducted at the saturation temperature of 34 °C, and in the case of the evaporator, at the saturation temperature of 8 °C. The average heat transfer coefficients as well as pressure drops in the case of the operation of the tested heat exchanger as an evaporator and condenser were calculated. The heat transfer coefficient was calculated by means of the separated thermal resistance method with the application of the Wilson plot technique. The experiments confirmed the increase in the heat transfer coefficient with the increase in the refrigerant mass flow rate for the tested mini-channel heat exchanger. A dimensionless correlation was proposed for the pressure drop based on the modified Müller-Steinhagen correlation in the case of the operation of the mini-channel heat exchanger as a condenser and as an evaporator.

CFD Investigation on Movement Features of Hydrogen Bubble under Microgravity Environment

A designed cryogenic upper stage adopted liquid hydrogen and liquid oxygen (LH2/LO2) as an aerospace propellant. During a zero-gravity coast period in space, the wall heat leakage into the delivery tube could induce liquid propellant evaporation and two-phase flow phenomenon, so that a bubble discharge operation must be employed prior to engine restart. In this study, a CFD approach was utilized to numerically study the bubble discharge behaviors inside the LH2 delivery tube of the upper stage. The bubble motion properties under two different schemes, including positive acceleration effect and circulation flow operation, were analyzed and discussed. The results showed that the boiled hydrogen bubbles could increase to the size of the tube inner diameter and distribute randomly within the entire tube volume, and that, in order for the bubble to spill upward under the acceleration effect, a higher acceleration level than the needed value of acquiring liquid–vapor separation inside the propellant tank should be provided. When creating an acceleration level of 10−3 g0, most of the bubbles could spill upward within 700 s. Significantly, the bubbles could not be completely expelled in the created acceleration condition since a number of small bubbles always stagnate in the bulk liquid region. In the circulation flow operation, the gas volume reduction was mainly attributed to two mechanisms: the vapor condensation effect; and bubble discharge effect. For the case with a circulation flow rate of 0.2 kg/s, a complete bubble discharge purpose was reached within 820 s, while a large bubble stagnation in the spherical distributor occupied a remarkable proportion of the total time. In addition, both the liquid flow rate and liquid subcooling exert important effects on bubble performance. When applying a high circulation flow, the gas volume reduction is mainly due to the inertial effect of liquid flow, but the bubble stagnation in the spherical distributor still affects the total discharge time. The liquid subcooling influence on the gas volume reduction is more significant in smaller circulation flow cases. Generally, the present study provides valuable conclusions on bubble motions inside a LH2 delivery tube in microgravity, and the results could be beneficial to the sequence design of engine restart for the cryogenic upper stage.

Correlation for CHF during subcooled flow across a single cylinder

A correlation is presented for prediction of CHF (critical heat flux) during upward flow of subcooled liquids across single cylinders. The correlation was verified against all available published test data. It includes five fluids (water, R-12, R-113, isopropanol, FC-72), tube diameter 0.25 – 19.1 mm, reduced pressure 0.0046 – 0.3554, liquid velocities 0.01 – 6.8 m/s, CHF 0.14 – 28.7 MW/m2, and quality −0.001 to −0.34. The new correlation predicts 496 data points from 28 data sets from 12 sources with mean absolute deviation (MAD) of 16.4%. The same data were also compared to other published correlations; their MAD ranged from 47% to 319.0%.

Connecting the Phase State and Volatility of Dicarboxylic Acids at Elevated Temperatures

The partitioning of semivolatile organic molecules between condensed phases and the vapor phase has broad application across a range of scientific disciplines, with significant impacts in atmospheric chemistry for regulating the evolving composition of aerosol particles. Vapor partitioning depends on the molecular interactions and phase state of the condensed material and shows a well-established dependence on temperature. The phase state of solid organic material is not always well-defined, and many examples can be found for the formation of amorphous subcooled liquid states rather than crystalline solids. This can lead to significant changes to vapor equilibrium processes by modifying the thermodynamics and kinetics of evaporation. Here, we explore the influence of phase state on the evaporation dynamics of a series of straight-chain dicarboxylic acids across a range of above-ambient temperatures. These molecules show an odd/even alteration in some of their properties based on the number of carbon atoms that may be connected to their phase state under dry conditions. Using a newly developed linear-quadrupole electrodynamic balance, we levitate single particles containing the sample and expose them to dry conditions across a range of temperatures (ambient to ∼350 K). Using the rate of evaporation measured from the change in the size or relative mass, we derive the vapor pressure and enthalpy of vaporization. Light scattering data allows for unambiguous identification of the phase of the particles (crystal vs amorphous) allowing the vapor equilibrium properties to be attributed to a particular state. This work highlights a new experimental method for characterizing vapor pressures of low volatility substances and extends the temperature range of available data for the vapor pressure of terminal dicarboxylic acids. These measurements show that crystalline and subcooled liquid states persist at elevated temperatures and provide a direct comparison between subcooled and crystal phases under the same experimental conditions.

The Modeling of Bubble Lift-Off Diameter in Vertical Subcooled Boiling Flow

Bubble lift-off diameter is characterized as the size of a bubble rising from a wall, which is vital in the boundary condition of heat transfer model and interfacial area transport equation. In this paper, mechanistic force analysis was conducted to explore a predictive model for bubble lift-off diameter in a vertical channel of subcooled boiling flow. Specifically, the component of growth force normal to the wall and the shear lift force lead to the lift-off of a bubble on the vertical surface. Through force analysis, we found that bubble lift-off diameter is arranged to be related to wall superheat, latent heat, liquid velocity, fluid properties, bulk liquid subcooling, etc. To account for the contribution of the influencing factors, the dimensionless bubble lift-off diameter was correlated with dimensionless parameters, including the Prandtl number, the Reynolds number, the Jacob number, and dimensionless subcooling. The proposed correlation was assessed according to experimental data and the predictions showed good agreement with the data.

Heat transfer correlation for film boiling during quenching of micro-structured surfaces

This work investigates the separate effects of liquid subcooling, substrate material, and surface micro-structure on the film boiling characteristics. A quenching facility was constructed to conduct vertical quenching experiments using rods with various substrate materials and surface morphologies. The surface morphology is characterized using field emission scanning electron microscopy (FESEM). Stainless steel, zirconium, and Inconel-600 rods are used with a diameter of 9.5 mm that simulates the size of fuel rods in commercial nuclear reactors. Other Inconel-600 rods with different porosity percentages are used to study the effect of fouling on the quenching behavior. The surface temperature and wall heat flux at the surface are deducted from the temperatures measured by the thermocouples embedded inside the rods using an inverse heat conduction code, from which the heat transfer coefficient and Nusselt number are calculated. The data are used to develop a generalized heat transfer correlations that includes the effects of liquid subcooling and substrate materials. It predicted the data within ±40% error band. The results also suggest that the heat transfer coefficient increases gradually as the sample cools down and in higher subcooled pools. Moreover, the variation in the substrate material shows a significant effect on the heat transfer characteristics. However, the surface micro-structure impact on the film boiling regime is negligible.

Experimental investigation of pool boiling heat transfer on the radial micro-pillar surfaces

An experimental study on the pool boiling heat transfer with heterogeneous micro-pillar surfaces was performed in this study. Six kinds of surfaces, including single-block radial micro-pillar surfaces (MS-1, MS-2 and MS-3) and four-blocks radial micro-pillar surfaces (BS-1, BS-2 and BS-3), were fabricated by the dry etching technique. FC-72 was used as the working medium under four different liquid subcoolings. A high-resolution camera was used to capture the boiling phenomenon. It could be found that the heat transfer performance of these radial micro-pillar surfaces was remarkably enhanced in comparison with that of a smooth surface, which showed an earlier onset of nucleate boiling (ONB) and an improved heat transfer coefficient (HTC). Moreover, the critical heat flux (CHF) was dependent on the heat transfer area enhancement ratio at the high subcooled temperature. BS-1 surface had the highest CHF 1.57–2.05 times more than the smooth surface. For the MS-1 and BS-2 surfaces, their CHF was almost the same due to a similar heat transfer area enhancement ratio, but the HTC of the MS-1 increased by 20% compared with the BS-2 surface. This could be due to different solid fractions φS (36.78% for MS-1 and 25.81% for BS-2). Under the saturation condition, the overall HTC of micro-pillar surfaces first increased to a critical value, and then decreases with the increase of heat flux. In addition, the maximum heat transfer coefficient enhancement ratio (HTCmax,PF/HTCmax,S) was almost constant when ΔTsub > 15 K, which was somewhat significant for subcooled pool boiling.

Pool boiling heat transfer of dual-scale porous microchannel for high-power electronics cooling

Surface modifications for boiling enhancement are urgently required for cooling high-power electronics. In the study, a dual-scale porous microchannel fabricated by plough-extrusion, wire electrical discharge machining, and ultrasonic machining is developed to meet the pressing needs. Boiling performance and bubble behaviors on the proposed microchannel are investigated, and the effect of the liquid subcooling on heat transfer is analyzed. The proposed microchannel is capable of dissipating heat flux of 2319.7 kW/m2 without reaching CHF, and exhibits a high HTC of 243.3 kW/(m2K) at saturation boiling with water. The complex microchannels with interconnected holes, reentrant cavities and micro-nanopores enhance the heat transfer by enlarging surface area, increasing nucleate sites, strengthening capillary wicking, and inducing macroconvection. The increased subcooling degree of pool liquid inhibits the nucleate boiling at low heat flux, but enhances the heat transfer at high heat flux which is the main region of interest. DPM with all the merits is highly promising for cooling high-power electronics.

Electrospray characteristics and cooling performance of dielectric fluid HFE-7100

Electrospray (ES) cooling is a promising route of efficient heat removal for electronic components with powerful cooling capacity, a small liquid supply, and precise temperature control. In this study, we experimentally investigated the electrohydrodynamic (EHD) disintegration and ES cooling performance of the dielectric fluid HFE-7100. The stainless-steel capillary nozzle was connected to a high voltage direct current (DC) power supply, whereas the hot copper surface was grounded. A high-speed camera was used to capture the spray morphology of the coolant. The results indicated that a small amount of ethanol significantly improved the charging performance of HFE-7100 by increasing the liquid electrical conductivity. An uncharged column liquid jet was stretched into a thin liquid film by the EHD forces, generating numerous ultrafine droplets along the lower edge. The ES cooling heat flux was increased by about 2.2 times compared with the neutral condition. In addition, the influences of the applied voltage, flow rate, spray height, liquid subcooling, and ethanol concentration on the ES cooling capacity were discussed. Finally, correlations of the ES cooling heat transfer based on the Reynolds number, Weber number, Prandtl number, electric Weber number, Jacob number, and normalized surface temperature were established.

DNS and Highly-Resolved LES of Heat and Mass Transfer in Two-Phase Counter-Current Condensing Flow

A comprehensive study of direct-contact condensation heat transfer for turbulent, counter-current, liquid/vapour flow in a nearly horizontal channel at high pressure (i.e. 5 MPa) has been carried out based on Direct Numerical Simulation (DNS) and highly-resolved Large Eddy Simulation (LES) approaches. To simulate the two-phase flow situation, driven in this case by a constant pressure gradient, a single set of Navier–Stokes equations, coupled with an enthalpy conservation equation, have been employed. The interfacial mass transfer, seen in this case to be dominated by condensation, has been calculated directly from the heat flux at the liquid/vapour interface. To investigate the effect of condensation on the turbulence phenomena, and vice versa, cases have been considered involving two friction Reynolds numbers: namely Re_{*}=u_{*}h/nu =178 and Re_{*}=u_{*}h/nu =590 (u_{*}=(hvarDelta P/rho )^{1/2}). At the lower Reynolds number, three levels of water subcooling—0 K, 10 K and 40 K—have been investigated. The use of water subcooling of 0 K has enabled the validation and verification procedures associated with the numerical approach to be compared against experimental and numerical data reported in the literature. The choice of the maximum degree of water subcooling is dictated by the need to justify the periodic boundary conditions applied in this numerical study. In the simulation for the higher Reynolds number, only the case of 10 K subcooling has been included, as a consequence of the very high computation effort involved. A detailed statistical analysis of the DNS and LES data obtained from the application of the well-known wall laws has also been assessed. In the vicinity of the liquid/vapour interface, the characteristics of the turbulent motions appear somewhat diverse, depending on whether the interface is basically flat or wavy in character. For a flat interface, some damping effect of the presence of the interface on the turbulence intensity has been observed, a feature which becomes enhanced as the level of liquid subcooling is increased. In the case of a wavy interface, the damping effect is predicted as considerably less pronounced.

Effects of surface properties on wall superheat at the onset of microbubble emission boiling

Boiling heat transfer (BHT) is a promising technology for cooling the high heat flux emitted from next generation electronic devices. Pool boiling represents an eco-friendly cooling technology because its pumps do not require an external electronic power source. To increase the maximum heat flux of pool boiling beyond the critical heat flux, microbubble emission boiling (MEB) can be implemented with highly subcooled water. However, conventional MEB experiments use a copper heating surface, which can easily be oxidized in air. Thus, copper heating surfaces do not retain constant surface properties, especially when the BHT boiling pool is open to air. Applying a coating to the copper surface can enable further investigations of the MEB phenomenon and improve the reliability of MEB-based cooling devices. However, it is unclear which surfaces are suitable for promoting the onset of MEB. Herein, we experimentally investigated a stable hydrophilic black chrome plating on copper by performing subcooled pool boiling with water at atmospheric pressure. The results confirmed that the black chrome plating did not interrupt the onset of MEB, but rather, it enabled constant wall superheat during repeated experiments spanning the transition from nucleate boiling to MEB. Moreover, the threshold of liquid subcooling for the onset of MEB with the black chrome plating (20 K) was similar to that with a copper heat transfer surface.

Gas-particle partitioning of polycyclic aromatic hydrocarbons from oil combustion involving condensate, diesel and heavy oil

This study focuses on the gas-particle (G-P) partitioning of 16 polycyclic aromatic hydrocarbons (PAHs) from oil combustion, which is one of the important contributors of anthropogenic PAHs but has been rarely studied. The combustions of different types of oils involving ultra-light to heavy oils were investigated, and the PAH partitioning mechanism was determined by the widely used Junge-Pankow adsorption model, Koa absorption model, and dual sorption model, respectively. The results show that the source-specific diagnostic ratios of Ant/(Ant+Phe) are between 0.09 and 0.24, the estimated regression slopes of G-P partition coefficients (KP) of the total PAHs on their sub-cooled liquid vapor pressures (PLO) are in the range of − 0.34 to − 0.25, and the predicted fractions of PAHs in the particle phase (φ) by Koa absorption model are close to the measured values, while the log KPvalues of the LMW PAHs from the combustions of diesel and heavy oil are better represented by the dual sorption model. Our findings indicate that PAHs are derived from mixed sources that include the unburned original oil and combustion products, and the PAH partitioning mechanism is governed by the process of absorption into organic matter because of the unburned oil, but both adsorption and absorption exist simultaneously in the lighter PAHs from the combustions of heavier oils (i.e., diesel and heavy oil). Based on these findings, the understanding of the fate and transport of PAH emissions and the optimization of the emergency responses to accidents such as marine oil spills would be potentially improved.

Surrogate-Assisted Multi-Objective Optimization of a Liquid Oxygen Vacuum Subcooling System Based on Ejector and Liquid Ring Pump

As an important combustion aid for aerospace vehicles, subcooled liquid oxygen of high density can be used to increase loading capacity of a spacecraft. Providing a large amount of cryogenic propellant in a short time with a strict energy consumption limitation is a challenge in the design of the fuel filling system. The authors proposed a vacuumed subcooling system combined with an ejector and liquid ring pump to vacuum a liquid oxygen tank and obtain subcooled liquid oxygen. After the liquid oxygen tank is vacuumed to an intermediate pressure by the ejector, it is further vacuumed to 10 kPa using the liquid ring pump. The infinitesimal method was used to simulate the thermodynamic processes involved. Taking the ejector working fluid mass flow rate, jet pressure, intermediate pressure, initial tank liquid level, and liquid ring pump speed as optimizing variables, optimization was conducted to determine the optimal vacuuming time, remaining liquid level in the tank, pumping speed difference, and nitrogen consumption. The sample set was generated by the optimal Latin sampling algorithm. The surrogate assisted Non-dominated Sorting Genetic Algorithm (NSGA-III) multi-objective algorithm was used to construct a system optimization framework. The non-dominated solutions were added to the sample set to improve the generalization ability of the Gaussian Process Regression (GPR) in the Pareto front. A convergent Pareto solution set was obtained after multiple iterations. The influence of different optimization variables on each optimization objective was analyzed using the Pearson correlation coefficient method. The optimization results show that the trade-off scheme can obtain the subcooled liquid oxygen at 10 kPa and 73 K with a remaining liquid level of 74.84% in a total vacuum time of 19.93 h. The efficiency of the liquid oxygen vacuum subcooling system can be improved significantly.

Geometric and thermodynamic considerations of saturated and slightly subcooled water flow through nozzles

• Deviation from equilibrium is recognized as a manifestation of rapid depressurization events including Rapid Phase Transitions, Boiling Liquid Expanding Vapor Explosions, and flow of saturated and slightly subcooled liquids through converging nozzles and sudden contractions. • Non-equilibrium effects are attributed to nozzle geometry and the pressure driving force for saturated and slightly subcooled liquid flow through nozzles and sudden contractions. • Bubble nucleation theory is used to estimate the deviation from equilibrium and single-phase flow methods are used to estimate the critical mass flux when rapid vaporization occurs at the nozzle throat. It is well known that homogeneous equilibrium methods for calculating the mass flux of initially subcooled or saturated liquids in short nozzles under-predict the measured values and various methods for estimating non-equilibrium effects have been presented in the past. It is shown in this paper that acceleration effects at the entrance of converging nozzles due to changing cross-sectional area and approach to thermodynamic saturation pressure at the point of maximum fluid acceleration can be the most significant causes of non-equilibrium. By properly accounting for non-equilibrium due to the fluid acceleration, single-phase flow methods can be used to estimate the pressure loss and mass flux in nozzles when rapid vaporization occurs at the nozzle throat. For these cases, choking occurs due to the rapid vaporization while the difference between the inlet pressure and choking pressure determines the nozzle mass flux.

분자동역학 해석을 이용한 다양한 응축온도에서 R448A와 R449A의 상변화 및 열물성 평가

Molecular dynamics simulations are performed to evaluate phase transition phenomena, liquid density, shear viscosity, and the isobaric heat capacity of low GWP refrigerants R448A and R449A. Increasing the condensing temperature from 273.15 K to 298.15 K leads to increase in the phase change duration of R448A and R449A. Moreover, the liquid density decreases to 11.24% for R448A, and 12.34% for R449A with the an increase in temperature from 273.15 K to 308.15 K. The particles’ snapshots reveal that vapor molecules undergo a rapid phase transition, to the subcooled liquid at a certain condensing time period. Subcritical clusters of R448A and R449A refrigerants’ molecules are formed in the initial part of the simulations, which then aggregate to produce the critical condensate nucleus over time. The liquid density, shear viscosity, and isobaric heat capacity of the two refrigerants predicted by the present MD simulations agree well with data of REFPROP.