Cryogenic Fluids Research Articles

Influence of subsurface temperature on cryogenic fracturing efficacy of granite rocks from Kazakhstan

Liquid nitrogen (LN2) is a new stimulation technology suitable for geothermal energy extraction that increases porosity, permeability, and overall contact area in candidate subsurface formations. It is considered appropriate for hot dry rock (HDR) reservoirs. Due to the significant temperature difference between hot rock and cryogenic fluid, extensive thermally induced cracking is anticipated upon contact, which is helpful in future water-steam cycling operations. To examine the degree of rock integrity failure in granite samples as a function of exposure mode, different procedures were followed for comparison. In particular, three commercial granite samples from three regions of Kazakhstan: Kapal-Arasan, Zhylgyz, and Zheltau, were heated to various elevated temperatures prior to immersion in LN2 with variations in freezing time (FT) and numbers of freezing-thawing cycles (FTC). Samples were examined by various methods to quantify the impact of the cryogenic fracturing process. Compression, acoustic emission (AE), and permeability tests were performed on granite with different starting temperatures ranging from 200 °C to 500 °C. Permeability enhancement was generally in the range of 50-90 %. Scanning electron microscopy (SEM) was also used to document the microscopic characteristics of thermally-induced cracks and indicated most induced fractures to be microscale features. Results showed reductions in Unconfined Compressive Strength of LN2-treated samples by up to 76 %. The experimental outcomes demonstrate that LN2 cooling of hot granite induces mechanical rock failure and permeability augmentation. Furthermore, the degree of thermo-fracturing increases with temperature difference and time of LN2 treatment in both freezing time and freezing-thawing cycle methods. This is the first systematic study of its kind to test three commercially available granites from Kazakhstan with this combination of examination tools to assess alternate cryo-fracturing methods for geothermal energy prospecting and contact area enhancement.

Research on the cavitation characteristics and structural optimisation of spiral grooved thrust bearing lubricated by liquid hydrogen

AbstractIn liquid hydrogen (LH2) delivery, submersible centrifugal pump is an important device. Its reliability and efficiency rely on the stability of the bearings. In a high‐speed range, cavitation occurs due to the hydrodynamic effect induced pressure undulation. Compared with other cryogenic fluids, cavitation in LH2 is prone to happen due to its special properties. In this paper, a cavitation model considering thermal properties of LH2 is proposed based on Rayleigh–Plesset equation. Cavitation characteristics are studied in spiral grooved thrust bearing (SGTB) lubricated by LH2. The effect of cavitation on load capacity and friction torque are investigated. The most severely‐eroded location is predicted by the distribution of mass transfer rate. The influence of bearing structure parameters is studied in respect of cavitation, load capacity and friction torque. The analysis of variance (ANOVA) and range analysis results may provide a reference for optimising LH2 lubricated SGTB.

Numerical simulations of proppant transportation in cryogenic fluids: Implications on liquid helium and liquid nitrogen fracturing for subsurface hydrogen storage

The over dependence on fossil fuels for human energy requirements together with the resultant adverse environmental effects in the form of greenhouse gas discharges has redirected our attention towards renewable energy sources. Researchers have identified hydrogen, a non-carbonaceous energy source holding the capacity to substitute fossil fuels, as a viable fuel option that has the potential to be generated utilizing eco-friendly techniques. Studies have identified underground sites that are operational or have the potential to be used for hydrogen storage. Shale formations, characterized by their low permeability, offer promising opportunities for underground hydrogen storage. These formations require stimulation techniques to generate flow channels. Pumping frac-fluid into these formations at high pressure induces fractures in them and the engineered fractures are supported by the proppants injected along with the fluid. Propped fractures form a vital conduit in unconventional gas reservoirs, which are a potential medium for storage of hydrogen. Cryogenic fracturing is an innovative approach that aims to enhance and broaden the capabilities of conventional hydraulic fracturing processes. The transmission and settlement of proppants are influenced by certain critical aspects including their density, size and volume fraction and fluid velocity, which are investigated in this contribution. This study incorporates water and cryogenic fluids like liquid nitrogen (LN2) and liquid helium (LHe) as fracturing fluids. The primary objective of this effort is to analyze proppant transportation phenomenon and the key influential factors in LN2 and LHe. The proppant volume fraction contour, equilibrium dune level (EqDL) and dune height (EqDH) data are illustrated in this research for better understanding of proppant migration. It was observed that when the fracturing fluid is given a higher velocity, the proppant grains experience increased drag force, causing them to penetrate deeper into the fracture. The desired range of proppant volume fraction for improved grain transmission was determined as 0.45 to 0.50 and the critical proppant density that significantly affected the EqDL as 187.2839 lb/ft3.

Development of constitutive relations for predicting film boiling crisis in bare and inner wall coated cryogenic tubes with a low-thermal conductive layer for heat transfer enhancement

Closure models for transient cryogenic quench flow boiling heat transfer prediction are finding significant attention to support the design of energy-efficient, cost-effective, and reliable cryogenic fluid management systems. In this regard, the characterization of the film boiling crisis, which indicates the beginning of transient wetting of the wall by liquid, is essential due to the significant heat transfer change past this stage. In the current work, a new rewetting database is developed from 112 chill-down experiments conducted using LCH4, LOX, LAr, and LN2 in feed lines having 0.11 m to 11 m length, 0.4 cm to 2.2 cm inner diameter, 0.5 mm to 5 mm wall thickness, and low-thermal conductive coating thickness up to 157 μm, under several pipe orientations. Additionally, the dataset comprises a mass flux from 10.5 kg/m2s to 1805 kg/m2s, pressure from 109.8 kPa to 1153.8 kPa and gravitational acceleration from terrestrial condition to ±0.01 g. Subsequently, 30 existing seminal rewetting correlations are analyzed for their applicability to transient cryogenic rewetting prediction. Moreover, the importance of feed line and flow parameters in this phenomenon is also studied. Accordingly, a new correlation for rewetting temperature is proposed with 4.15% mean absolute percentage error (MAPE). Further, applying a low-thermal conductive coating to the feed line inner wall surface has already been proven to enhance the chill-down efficiency up to 80%. For modeling such feed systems, determining bulk wall rewetting point is necessary. Hence, a new apparent rewetting temperature correlation to characterize the film boiling crisis in cryogenic coated tubes is proposed for the first time with a MAPE of 4%. Finally, the paper presents the predictions from a non-homogeneous two-phase thermal-hydraulic code, employing the previously reported film boiling correlations and the rewetting correlations from the present work. The obtained model performance indicates considerable improvement in prediction while using the proposed closure models. This shows their potential to support optimization tasks to achieve higher chill-down efficiency.

Cryogenic oxygen deficiency hazard assessment at the National Synchrotron Radiation Research Center

The National Synchrotron Radiation Research Center (NSRRC) uses cryogenic fluids to create a low-temperature cooling environment for equipment and to conduct various experiments. However, exposure to these cryogenic fluids can cause frostbite, hypoxic suffocation, behavioral incapacitation, insanity, and even death in severe cases. To evaluate oxygen deficiency hazard (ODH) in the NSRRC, we adopted the Fermilab assessment methodology and conducted ODH assessments in the Cryogenic Compressor Room, Taiwan Light Source (TLS) Tunnel, and TLS15A hutch. The results of the evaluation of the Cryogenic Compressor Room and TLS Tunnel revealed that the ODH class is 0 both when the exhaust fan is operating normally and when the exhaust fan is damaged. The exhaust equipment in the TLS15A hutch is only for emergency use. Without the emergency exhaust fan, the ODH class in the area is 1. If the emergency exhaust fan is always on, the ODH class is 0. Therefore, we recommend that those in TLS15A should undergo safety education and receive hazard notifications. In addition, we strongly recommend installing oxygen detectors in the beamline hutch to ensure safety.

System Resilience of a Liquid Hydrogen Terminal During Loading and Unloading Operations

Hydrogen technologies are playing an increasing role in transportation and industry. Key for taking maximum advantage out of the great gravimetric energy density of hydrogen are feasible and safe storage and transfer concepts. For the successful implementation of a hydrogen-based society with several green industrial initiatives, it is indispensable to develop an infrastructure of liquid hydrogen (LH2) terminals and tanker ships with the capability to bunker LH2. Liquid hydrogen might be used to store and transport large quantities of hydrogen. LH2 is a cryogenic fluid, so double-walled vacuum insulated tanks are required to keep it cold for long periods of time until further distribution or use. Whether transportation sector or industry, to date there is little knowledge on safety related issues available. Main concern when handling hydrogen are accidental releases that can lead to integrity damage on materials and structures, fires, and explosions. Assuming that LH2 infrastructures will be widely deployed and in use all over the globe, accidents are possible to occur during loading and unloading of LH2 bunkering facilities. Therefore, it is necessary to conduct a detailed risk assessment that focuses on system resilience to improve the capabilities of the facility to keep its functionality up when errors occur. This study refers to the LH2 storage tank installed in the LH2 terminal in Kobe, Japan. This stationary storage tank is an essential element of the terminal that was constructed by the Hydrogen Energy Supply Chain Technology Research Association (HySTRA). HySTRA aims at the distribution of hydrogen in liquified form by ship from Australia to Japan. To date, there is only little data available regarding the facility in Kobe. Nevertheless, due to the novelty of the technology the risk for accidents to occur might be higher than in conventional fuel distribution terminals. Accidents might happen due to technical failures, human errors, or external causes such as natural events. The consequences could be catastrophic. Some of these may expose structures and personnel to extreme low temperatures, fires, and explosions which may hinder bunkering operations of the facility in Kobe. This study gives an overview on possible scenarios that lead to loss of containment and provides an insight in the process of evaluating system resilience during such a scenario. This work details with a framework for assessing system resilience applied to LH2 storage facilities. The system resilience calculation involves the examination of a critical hydrogen accident database and provides suitable preventive and mitigative safety barriers.

A new transient hot-wire thermal conductivity apparatus and measurements of helium at temperatures from 20 K to 300 K

In this study, an apparatus based on the transient hot-wire method (THW) was designed to measure the thermal conductivity of cryogenic fluid within the temperature range of 20–300 K and the pressure range of 0–20 MPa, with a relative expanded uncertainty Ur(λ) of 0.0284 (k = 2, 95 %). The experimental data of methane, ethane, and carbon dioxide showed good consistency with the ECS model, validating the reliability of the apparatus. In addition to the linear temperature dependence of hot-wire resistance above 70 K, the nonlinear temperature dependence below 70 K is calibrated, and the feasibility of the THW method for measurements in the temperature range of 20–70 K is validated. Subsequently, the thermal conductivity data for helium are presented from 20 to 300 K. The temperature rises of hot-wire during the measurement agreed well with the simulation calculation results. Furthermore, a thermal conductivity model for helium gas up to 600 K was developed. The presented data in this work, as well as previously published data, demonstrated good agreement with the model calculation results, with an average absolute relative deviation (AARD) of 1.46 %.

Analysis and Simulation on Thermal Behavior of Cryogenic Tank for Different Types of Internal Support Materials

The cryogenic vessel is mainly a double-walled vessel which mainly consists inner vessel, outer vessel, and inner support. The Inner support is the main part designing of the cryogenic vessel for thermal and structural criteria. Inner support material for cryogenic vessels can be made of various materials depending on Net evaporation rate (NER) criteria and structural criteria (i.e. road transport, marine transport, space condition). Material for the Inner support structure has low thermal conductivity and high strength, low weight. To meet the design goal for very low thermal conductivity, high strength, and low weight the next-generation materials like carbon fibber and polymeric-based composites are being developed for cryogenic fuel tank applications. By using different materials like SS304, G-10, Inconel, Invar-36, High strength Carbon fibber, and polymer-based composites same support structure will be analysed in FEA software for Thermal and structural conditions and will compare for different load conditions to check structural integrity. Also will compare the impact of different materials on NER and holding day for cryogenic fluid

Diatreme-hosted fluorite mineralization in S-Namibia – A tale of cryogenic brine formation and fluid mixing below an unconformity in the context of Pangea rifting

The Garub fluorite deposit in south Namibia is hosted by a Neoproterozoic tuffisite diatreme which is cut by a hydrothermal vein with base metal mineralization. Petrographic observations indicate a single hydrothermal event producing a paragenesis of fluorite-baryte with base metal sulfides, quartz and carbonates. Microthermometry data from fluid inclusions show that hydrothermal fluorite and baryte were precipitated from a high-salinity (16.9–22.7 wt% NaCl + CaCl2), moderate-temperature brine (180–210 °C). This brine formed by mixing between a warmer, Ba- and F-rich endmember likely representing a deep-seated basement brine (chemically equilibrated with Namaqua-Natal-Metamorphic Complex basement rocks) and a cooler, SO4- and Ca-rich endmember that likely equilibrated with Nama Group limestone. Chlorine/Br mass ratios between 152 and 612 indicate the mixing of a fluid with a halite dissolution brine signature (endmember 1) and an equally saline brine derived from Nama Group limestone (endmember 2). Rubidium/Cesium (Rb/Cs) ratios between 2.2 and 20.5 provide evidence for significant fluid interaction with clay minerals. Since clay minerals are abundant in the tuffisite of the Garub diatreme, these Rb/Cs ratios indicate that fluids migrated along the diatreme-gneiss contact along a zone of weakness. Late-stage quartz and calcite likely reflect cooling and induced a shutdown of the hydrothermal system. The sulfide phases in this hydrothermal vein are strongly depleted in trace elements compared to other hydrothermal vein districts, which is interpreted as an inherited signature of the fluid source.There is no record of evaporites in the sedimentary rocks of the Nama Group. Based on the fluid chemistry of the mixed fluid, which clearly indicates halite dissolution and water-rock interaction with basement rocks, it is concluded that the basement brine fluid endmember is likely generated through cryogenic brine formation during the large scale Dwyka glaciation event. As a farfield consequence of Pangea breakup fluid pathways were established that acted as host structures for the Garub fluorite deposit. This could be the first evidence for unconformity-related hydrothermal fluorite deposit formation in the context of Dwyka cryogenic brine formation in southern Africa.

Visualization study on double-diffusive convection during a rollover in liquid energy storage tanks

The growing global energy consumption and the transition to the renewable era highlight the urgent need for safe and energy-efficient liquid energy storage tanks. Rollover has been a severe hazard to the efficiency and safety of the storage tank accompanied by significantly enhanced mass and heat transport across the stratified layers, and rapid evaporation and mixing of the layers happen. However there is a lack of experiments sophistically designed for cryogenic storage applications, which leads to a lack of visualized and quantitative experimental data to understand the physics of rollover in energy storage tanks. In this study, a Z-type Schlieren system is established to visually reveal the double-diffusive convection patterns in a cylindrical experimental chamber. The concentrations of salt-water solutions are adjusted to simulate the density difference between the multi-component cryogenic fluids. Cross-correlation algorithm is adopted to analyze the transient velocity distributions in the stratified system. The rollover process is divided into four stages, namely the initial, transition, mixing, and stable stages. Effects of heat flux, concentration difference, and heating location on the rollover characteristics are parametrically investigated. Correlations are proposed with rescaled buoyancy ratio and density gradient, and satisfactory agreement with experimental data is achieved.

Natural convection effects in insulation layers of spherical cryogenic storage tanks

Natural convection effects in the insulation layers of spherical storage tanks are studied for different cryogenic fluids and tank sizes or curvatures. The non-vacuum-based insulation layers of these tanks are made of gas-filled porous materials such as perlite or glass bubbles. The large temperature difference that can exist between the inner cold wall and outer hot wall may create convective patterns with a multi-cellular flow within the insulation-filled annulus and lead to a higher boil-off rate and cryo-pumping effects. A main objective in the design of these systems is the elimination of the convective patterns by a proper choice of insulation properties. This work examines the impact of the insulation permeability or the Rayleigh number (Ra) on the type of convective flows for liquified natural gas (LNG) and liquid hydrogen (LH2) spherical storage tanks. Bifurcation diagrams of possible solutions are generated by numerical solutions of the governing equations at different Ra numbers. It is found that as Ra increases, new convective cells emerge near the south pole of the insulation layer due to unstable density stratification and lead to the distortion of isotherms, bringing the cold boundary towards the external hot boundary. When the temperature difference is large as in the case of LNG and LH2 tanks, there exist regions of multiple solutions in which multi-cell convective solutions may co-exist with a crescent-shaped weakly convective solution. In order to eliminate the possibility of multi-cell convection, inversion of the cold boundary, and reduce the boil-off rate, the Ra value of the insulation system should be below the limit point of the convective branch.

Evaluation of Landweber Coupled Least Square Support Vector Regression Algorithm for Electrical Capacitance Tomography for LN2–VN2 Flow

The electric capacitance tomography (ECT) technique has been widely used in phase distribution reconstruction, while the practical application raised nonideal noise and other errors for cryogenic conditions, requiring a more accurate algorithm. This paper develops a new image reconstruction algorithm for ECT by coupling the traditional Landweber algorithm with the least square support vector regression (LSSVR) for cryogenic fluids. The performance of the algorithm is quantitatively evaluated by comparing the inversion images with the experimental results for both the room temperature working medium with the dielectric constant ratio close to cryogenic fluid and the cryogenic fluid of liquid nitrogen/nitrogen vapor (LN2-VN2). The inversion images based on the conventional LBP and Landweber algorithms are also presented for comparison. The benefits and drawbacks of the developed algorithms are revealed and discussed, according to the results. It is demonstrated that the correlated coefficients of the images based on the developed algorithm reach more than 0.88 and a maximum of 0.975. In addition, the minimum void fraction error of the algorithm is reduced to 0.534%, which indicates the significant optimization of the LSSVR coupled method over the Landweber algorithm.

Experimental investigation of flow orientation effects on cryogenic flow boiling

Cryogenic flow boiling physics is of paramount importance for future cryogenic space applications employing Cryogenic Fluid Management (CFM) technologies. This experimental study investigates the two-phase flow and heat transfer performance of LN2 flow boiling across five distinct flow orientations: vertical upflow, vertical downflow, horizontal flow, 45° inclined upflow, and 45° inclined downflow. The study employed the steady-state heating method within a circular heated tube featuring an 8.5-mm inner diameter and a 680-mm heated length. High-speed video recordings were utilized to capture two-phase flow patterns and interfacial behaviors across various flow orientations. The experiments covered a wide range of operating conditions, including mass velocities ranging from 351.80 to 1572.77 kg/m²s and inlet pressures from 297.14 to 1032.97 kPa, primarily with near-saturated inlet subcooling. Distinct two-phase flow patterns and regime transitions were identified for each flow orientation. Symmetrical flow patterns were evident in vertical orientations, whereas non-vertical orientations exhibited asymmetric flow stratifications, primarily influenced by the buoyancy force in a terrestrial gravity environment. Bubble dynamic parameters were quantified, and bubble collision and dispersion phenomena were visualized. Analyzing heat transfer performance based on local flow boiling curves and variations in heat transfer coefficients (HTCs), it was observed that vertical upflow demonstrated the most enhanced heat transfer performance, while vertical downflow exhibited the lowest. As mass velocity exceeded 830 kg/m²s, the differences in heat transfer among orientations became less distinct, emphasizing the role of flow inertia in mitigating the influence of flow orientation. A direct comparison of microgravity HTC data from a recent study by the authors against the 1-ge HTC data indicated enhanced heat transfer performance in microgravity for flow orientation configurations in terrestrial gravity. However, this enhancement progressively diminished as the heat flux increased. Distinct HTC trends were observed as mass velocity increased for different flow orientations and gravity levels.

Cryogenic distribution system design for cryomodules and cavities test in HIAF project

ABSTRACT High-intensity heavy-ion accelerator facility (HIAF) is an accelerator-driven high-current heavy ion facility, currently under construction in Guangdong, China. The proton linear accelerator of HIAF linac has 96 superconducting cavities and 17 cryomodules cooled by 2 K saturated superfluid helium. Cryogenic tests should be carried out in a test cryogenic system prior to the online operation of each cavity and cryomodule. The cryogenic distribution system (CDS) is the main part of the test cryogenic system, which is used for transporting cryogenic fluid between helium refrigerator and test terminals. A specific CDS is developed due to the long cryogenic test time. Each test terminal can be independently cooled and heated up, saving quite a bit of time without impacting the others. This article provides comprehensive explanations for the process design, process analysis, and special structure design which enhance the system’s maintainability. Furthermore, particular technical designs, such as measuring system, helium guard, and elimination of thermoacoustic oscillations, are described in detail.

An Overview of How the Petrophysical Properties of Rock Influenced After Being Exposed to Cryogenic Fluid

Exposure to cryogenic liquids can significantly impact the petrophysical properties of rock, affecting its density, porosity, permeability, and elastic properties. These effects can have important implications for various applications, including oil and gas production and carbon sequestration. Cryogenic liquid fracturing is a promising alternative to traditional hydraulic fracturing for exploiting unconventional oil and gas resources and geothermal energy. This technology offers several advantages over traditional hydraulic fracturing, including reduced water consumption, reduced formation damage, and a reduced risk of flow-back fluid contamination. In this study, an updated review of recent studies demonstrates how the thermal shock caused by the cryogenic liquid during the fracturing process substantially affects the rock's physical properties. Additionally, changes in permeability, porosity, and pore structure brought about by cryogenic treatments are highlighted. This work aims to draw attention to the studies that deal with the effect of thermal shock on rock petrophysical properties and establish the ideal conditions for employing cryogenic liquids in these contexts. Simulation studies, laboratory trials, and field application cases have been undertaken to assess the efficacy of cryogenic liquid fracturing technology. These investigations have provided important insights into the physical and mechanical impacts of thermal shock on rock and the performance of cryogenic liquid fracturing in real-world situations.

Mixture modeling to simulate helium boiling: Helium gas bubble trapped in high magnetic field

Multi-phase flow hydrodynamics is an essential theoretical foundation for modernizing engineering and is important in guiding and developing modernizing engineering. In the realm of superconducting power technology, it is imperative to submerge and cool high-field superconducting magnets and superconducting cables, constructed from superconducting materials, in cryogenic fluids in order to maintain a superconducting state. Even minor thermal disturbances can induce boiling of the cryogenic fluid, leading to a two-phase flow regime. In the case of high-field superconducting magnets, the presence of liquid helium as the cooling medium gives rise to a diamagnetic effect. Consequently, the formation of a bubble resulting from boiling is confined in close proximity to the central aperture of the magnet owing to the influence of the magnetic field force. This confinement adversely affects the heat transmission properties of the magnet. This work presents a comprehensive analysis of the flow state during the boiling process of liquid helium, using the finite element approach and the mixture model. The heat transfer in a two-phase flow is analyzed by treating the system as a single pseudo-fluid. The model is consistent with empirical formulations of liquid helium boiling in large vessels and is also suitable for modeling the effect of heating surface orientation on the critical heat flux. Consequently, the analysis of the retention of helium bubbles generated by boiling in a high-field magnet has been further examined, using this particular model. The results of numerical hydrodynamic studies show that the width of the helium bubble stagnation zone is larger compared to the results of hydrostatic calculations. This poses a major challenge for superconducting magnets operating at high magnetic fields and high temperatures. The paper further analyses the effect of supercooled liquid helium on helium bubble stagnation, thus providing valuable insight into the construction of superconducting magnets with high magnetic fields.

CFD Modeling of Bidirectional PMDs Inside Cryogenic Propellant Tanks Onboard Parabolic Flights

Future cryogenic propulsion systems will require efficient methods for transferring cryogenic propellants from a depot storage tank to a customer receiver tank to minimize costs and maximize reusability. The Reduced Gravity Cryogenic Transfer Project is currently developing advanced cryogenic fluid management technology and developing and validating new numerical models for three phases of transfer: line chill down, tank chill down, and tank fill. Additionally, multiple liquid nitrogen (LN2) parabolic flight transfer rigs are being designed by universities and NASA to investigate the gravitational sensitivities that exist in these three technologies. To maximize the collection of low-g data during flights, it is required to extract as much LN2 as possible from the supply tank, despite variable gravity levels. The purpose of this study is to present computational fluid dynamics volume of fluid simulations of LN2 behavior in the supply tank onboard parabolic flights to validate the optimal design of a bidirectional propellant management device (PMD) using the commercial software FLOW-3D. A parametric study was conducted on the effects of gravity level, fill level, pore size, open area, thickness, and type of baffle on PMD performance. Based on the results, the designed PMD exceeded the targeted expulsion efficiency.

1G and microgravity tank self-pressurization: Research on cryogenic fluid thermal stratification

A correlation numerical model was constructed, and relevant experiments were simulated to investigate the influence of gravity level on the self-pressurization process of the cryogenic fluid of liquid nitrogen. The calculated results agreed well with the experimental data. On this basis, the influences of a normal gravity environment and a microgravity environment on the self-pressurization process of liquid nitrogen in the normal-temperature fluid Perfluoro-n-Pentane (PnP, or C5F12) tank of National Aeronautics and Space Administration (NASA) were analyzed. According to the calculation results, an explicit horizontal interface is formed between the gas and liquid phases in the tank during the 1G (G = 9.81 m/s2) self-pressurization process. The ullage pressure increases as time progresses, and obvious thermal stratification phenomena are observed in the ullage along the axial and radial directions. However, the thermal stratification of the liquid zone is not obvious. The ullage near the gas‒liquid interface shifts from counterclockwise movement to clockwise movement, while the liquid changes from clockwise movement to counterclockwise movement. In the process of microgravity self-pressurization, the gas‒liquid interface in the tank presents an irregular shape, and the ullage pressure increases continuously before 400 s and then tends to be stable. The axial thermal stratification of fluid in the tank tends to disappear as time progresses, but obvious radial thermal stratification is observed. The ullage in the tank moves up and down as a response to the Marangoni migration effect. The temperature of the liquid zone in the tank during 1G self-pressurization tends to be consistent more quickly than that during microgravity self-pressurization, and the average temperature is higher. However, the temperature reduction rate of the ullage in the tank during microgravity self-pressurization is higher than that during 1G self-pressurization. Numerical simulation results can provide references to further study the on-orbit pressure control technique of cryogenic liquid tanks.

On the unsteady cavitation characteristics of compressible liquid nitrogen flows through a venturi tube

In this paper, the unsteady cavitating flows of liquid nitrogen (LN2) through a three-dimensional (3D) venturi tube in different thermal cavitation modes are modeled based on the mixture multiphase model with the LES method and Sauer-Schnerr cavitation model. A numerical framework considering compressibility and the thermal effects of cryogenic fluids is developed, and the modified temperature-dependent Tait equations of state provide the mathematical closure. The numerical results are compared with those from previous experiments that we have conducted, and it is indicated that the simulations and experiments agree well. Condensation shock wave propagation is observed to be initiated by the impingement of the collapse-induced pressure waves from the previously shedding cloud, and details of the process are presented. The impacts of the thermal effect on the cavitation dynamics, especially the evolution of cavitation patterns and dynamic interactions between cavitation and vortex, are analyzed. The numerical results show that the formulation of condensation shock generated at the rear of the cavity varies with temperature. In addition to the pressure waves caused by shed vapor collapse, the violent mass transfer process at the closure region of the cavity releases a substantial quantity of latent heat, initiating further condensation towards the cavity as liquid temperature decreases. The entropy production theory is derived and implemented, and different entropy production terms are established in unsteady LN2 cavitating flow with different thermal effects to understand the interactions between cavitation and entropy production. The present study provides insight into the interactions of cavitation-condensation shock in the LN2 cavitating flow and contributes to a better understanding of the influences of thermal effects on cryogenic cavitation dynamics.

Cryogenic flow boiling in microgravity: Effects of reduced gravity on two-phase fluid physics and heat transfer

With the growing interest in space exploration, cryogenic technologies involving two-phase flow and heat transfer are in high demand to successfully procure advanced space applications such as fuel depots and nuclear thermal propulsion (NTP) systems for deep space missions. However, the unique and extreme thermal properties of cryogenic fluids introduce distinct flow boiling fluid physics and energy transport phenomena, which differ significantly from those observed with conventional fluids. Understanding the unique two-phase physics in cryogenic flow boiling remains an ongoing challenge. Furthermore, the lack of readily available microgravity cryogenic steady-state heat transfer data hinders the assessment of gravitational effects on cryogenic flow boiling. This study aims to elucidate the gravitational effects on two-phase fluid physics and heat transfer by conducting the first-ever experimental measurement of cryogenic flow boiling performance using a steady-state heated method in a reduced gravity environment. Parabolic flight experiments were performed to acquire both heat transfer measurements and high-speed video of interfacial behaviors, under varying gravity levels (microgravity, hypergravity, Lunar gravity, and Martian gravity). The experiments involved flow boiling of liquid nitrogen (LN2) with a near-saturated inlet along a circular heated tube of dimensions 8.5-mm inner diameter and 680-mm heated length. The operating parameters varied are mass velocity of 398.3 – 1342.8 kg/m2s, inlet quality of -0.08 to -0.01, and inlet pressure of 413.68 – 689.48 kPa. Captured microgravity flow patterns range from bubbly to annular, all having vapor structures that are larger than those under higher gravity levels. Under microgravity, absence of buoyancy yields symmetrical vapor structures without flow stratification, laying a physical foundation for the distinct two-phase heat transfer trends during LN2 flow boiling in microgravity. Transient data collected during the flight parabolas exhibited decreasing heated wall temperature as the aircraft transitioned from hypergravity to microgravity phases. The temperature variation indicated an enhancement in flow boiling heat transfer with decreasing gravity levels and a reduction with increasing gravity levels. The effect of reduced gravity on cryogenic flow boiling heat transfer coefficient (HTC) is discussed based on steady state heat transfer analysis. Seminal HTC correlations are evaluated against the measured microgravity HTC data, of which one is identified for superior accuracy in predicting microgravity data. Finally, a new HTC correlation is proposed to improve accuracy of microgravity predictions, yet there still exists room for further improvement with future terrestrial flow boiling experiments at different flow orientations relative to Earth gravity.