Subcritical Environment Research Articles

Experimental study of Cryogenic jet injection using centrifugal nozzles at supercritical pressure

The new generation of large-thrust rockets commonly employ cryogenic liquids such as liquid hydrogen (LH2), liquid oxygen (LOX), and methane as propellants. Conventional analytical methods established under subcritical (specifically, low-pressure) conditions are inadequate for addressing the atomization issues under trans-critical and supercritical conditions. Therefore, conducting experimental research on atomization of cryogenic fluids under supercritical conditions holds significant practical value. This paper employs liquid nitrogen (LN2) as the working fluid and focuses on conducting injection atomization experiments on LN2 under supercritical pressure conditions. The research involves the design and construction of an experimental system for supercritical cryogenic fluid injection, with the aim of measuring and studying the spray characteristics of centrifugal nozzles under various conditions. Three major categories of experiments were designed for low-temperature LN2 injection: subcritical nitrogen into subcritical environment (BB), subcritical nitrogen into supercritical environment (BP), and supercritical nitrogen into supercritical environment (PP). The experimental results ultimately reveal that for jets in BB, both the spray cone angle and the breakup length increase with higher injection pressure drop. In the case of BP, the spray cone angle decreases while the penetration distance increases with higher pressure drop. As for the jets in PP, the spray cone angle increases and the penetration distance decreases with higher injection pressure drop. Across different geometric parameters of the centrifugal nozzle, it was observed that with an increase in the geometric parameter, the rate of decrease in the spray cone angle with respect to pressure drop amplifies, and the penetration distance exhibits an increasing trend as the geometric parameter grows.

Experimental Study of the Effect of the Initial Droplet Diameter on the Evaporation Characteristics of Unsymmetrical Dimethylhydrazine Droplets in a Subcritical Environment

The evaporation characteristics of unsymmetrical dimethylhydrazine droplets with different initial diameters in a subcritical environment were experimentally investigated with the temperature–pressure separation technique. The evaporation processes of unsymmetrical dimethylhydrazine droplets with different initial diameters in this environment have the same general pattern. All the studied droplets exhibit a short transient heating phase and a steady-state evaporation phase obeying d2. Notably, the expansion of the transient heating phase gradually increases with increasing ambient pressure. The change in diameter squared ∆d2max increases from 1.03% at 1 MPa to 12.48% at 5 MPa. Under subcritical conditions, the evaporation rate decreases linearly with decreasing droplet diameter, and the droplet evaporation lifetime increases linearly. Changes in the initial droplet diameter may still have a large effect on droplets smaller than those studied here. When the ambient pressure is not greater than 3 MPa, the change in the steady-state evaporation time for both medium- and large-diameter droplets accounts for more than 70% of the variation in the droplet evaporation lifetime. As the ambient pressure increases to 4 MPa and 5 MPa, the percentage of the change in the transient heating time contributing to the variation in the droplet evaporation lifetime gradually increases to more than 45%.

Corrosion performance and mechanisms of tantalum coatings on 316 stainless-steel in subcritical aqueous solutions of NaCl and NaBr

The corrosion behavior of 316 stainless-steel with a tantalum (Ta-316) coating deposited using PVD technology was investigated in subcritical environments containing Cl− and Br− at 290 °C and 6 MPa. The corrosion rate of Ta-316 continuously decreased with increasing exposure time, demonstrating its high corrosion resistance. A thin film, mainly composed of Ta2O5, formed on the surface of the tantalum coating, preventing the diffusion of substrate elements. The corrosion process and mechanism of Ta-316 were also examined.

Study on spray characteristics of biodiesel alternative fuels for in-cylinder environment of diesel engine

This paper presents an experimental study of spray characterization of MD-NH (a blend of methyl decanoate and n-heptane in a molar ratio of 1:1 to characterize biodiesel) in an in-cylinder environment of diesel engines to provide a reference for the experimental study of engines in advanced combustion mode. Based on the Peng-Robinson state equation, the thermodynamic model of real fluid was established, and the influence law of ambient temperature and pressure on the diffusion characteristics of fuel flow was analyzed. Based on the constant volume combustion bomb experiments, the variations of spray penetration distance, spray cone angle, spray projection area, spray air entrainment mass, and spray fragmentation forms in different critical environment conditions were studied. The results show that the transport properties of the fuel change abruptly within the critical point neighborhood of the fuel. As the ambient temperature increases, near the critical temperature, the thermal conductivity of biodiesel decreases and then increases; density and kinematic viscosity decrease at an accelerated rate; and surface tension increases to a maximum and then disappears sharply. The diffusion coefficient decreases sharply when the ambient pressure reaches the critical pressure. The spray characteristics of biodiesel in sub/supercritical conditions were different. Compared with the subcritical environment, the biodiesel spray penetration distance decreases, the spray cone angle increases, the spray projection area decreases, and the air entrainment mass increases in the supercritical atmosphere, which induces a gas-liquid interface mixing layer and promotes the mixing of biodiesel with the atmosphere gas. The experimental data in this paper fill the gap in the study of biodiesel spray characteristics in supercritical environments and provide a basis for high-fidelity numerical simulation of the spray model.

Comparative molecular dynamics study of liquid hydrogen annular jets in subcritical and supercritical environments

In this paper, the injection characteristics of a liquid hydrogen propellant jet under subcritical and supercritical operating conditions are comparatively studied at the molecular scale using molecular dynamics methods in the context of spacecraft propulsion systems. To control the temperature and pressure within the environment and nozzle respectively, two sub-computational domains are set up for separate modeling. The sub-computational domain merging process eliminates the broken molecular bonds by using the computational domain reduction in the x-direction. The jet velocity, density and other physical property distributions at different temperatures and pressures were comparatively studied. It is found that the rate of change of the radial density of the jet under supercritical conditions is small. Based on this, the surface area and volume of the liquid phase region of the jet were extracted and calculated. Compared to result of subcritical case, the volume of the liquid hydrogen jet in the supercritical environment is reduced by 1.9%, while the surface area is increase by 6.7%. To explain the variation in the surface area due to molecular diffusion, the molecular stress results revealed that the stress in the hydrogen jet in supercritical environment is only 88.9% of that in subcritical environment.

Chemical regulation in the bonding reconstruction stage of amorphous Zr-Si oxidation and the resultant phase selection

A mechanism based on oxidation induced bonding reconstruction (OBR) was proposed to optimize phase selection during oxidation of amorphous ZrxSi1−x (0.55 ≤ x ≤ 0.75). According to VASP modeling, a self-barrier effect was proposed to regulate the OBR and Zr/Si separation at initial oxidation stage. The depressed Si inner-diffusion leads to the formation of amorphous bilayer structure. A Si-rich Zr-Si layer formed underneath the outer Zr-Si-O amorphous layer, which synergistically impeded oxygen permeation at subcritical environment. As confirmed by EELS and XPS, the alternative of Zr-O and Si-O bonds in Zr-Si-O layer contributed to the enhanced hydro chemical stability.

The effects of sub/supercritical conditions on the spray interface characteristics of alkane fuel

ABSTRACT Alkane fuels are widely used in engines. The spray characteristics in supercritical environments are very different than in subcritical environments, and supercritical environments significantly impact the formation of the gas mixture in the engine. In this paper, the spray images of n-heptane and n-tetradecane in subcritical (500K-2MPa) and supercritical (700K-4MPa, 800K-6MPa) environments were obtained on the constant volume spray test platform. The effects of the supercritical environment and carbon chain length of alkane fuel on spray characteristic parameters were investigated. The results showed that the carbon chain length and ambient pressure are the main factors of penetration. The ambient pressure is the main factor of the spray cone angle and the spray non-liquid layer thickness. Compared with n-heptane, in the 500K-2MPa (subcritical) environment, the cone angle of n-tetradecane was reduced by 2°, the penetration was increased by 10 mm, and the spray non-liquid layer thickness was reduced by about 1 mm. In the 800K-6MPa (supercritical) environment, the cone angle of n-tetradecane was increased by 2°, the penetration was increased by 10 mm, and the spray non-liquid layer thickness was increased by about 1.2 mm. In addition, the calculation model of alkane fuel spray penetration in sub/supercritical environments was modified and achieved 95% prediction accuracy.

Study of fuel spray characteristics using a high-pressure common rail diesel injection system by the method of Schlieren

The supercritical atmosphere created by the increase in temperature and pressure in the diesel engine cylinder is the key factor that causes changes in the fuel atomization process. In this study, the differences in spray morphology of n-heptane under different ambient temperature and pressure conditions were investigated using a constant volume combustion bomb combined with the Schlieren method. Based on the Peng-Robinson equation of state (PR equation), several transport characteristics models are constructed to predict the transport characteristics of n-heptane under different operating conditions. During the experiment, the injection pressure and fuel temperature were kept constant (80 MPa, 300 K), the ambient pressure was varied from 2 to 5 MPa, and the fuel temperature was changed from 400 to 800 K, spanning from subcritical to supercritical state. Three different spray regions have been identified based on the fuel temperature and spray geometry: liquid phase region, gas-liquid interface mixing layer, gas phase region. The results show that in a subcritical environment (400 K, 2 MPa), the fuel injection and atomization process is dominated by evaporation. The degree of heat exchange between the fuel and the medium gas is very low, while a small amount of phase change occurs. When the fuel is in low (600 K, 3 MPa) and medium supercritical (600 K, 4 MPa) ambient atmosphere, turbulence becomes the dominant factor in the fuel atomization process. The mixing layer at the gas-liquid interface at the edge of the liquid jet increases significantly. When the fuel is in a high supercritical (700 K, 4 MPa; 800 K, 5 MPa) atmosphere, the jet takes on a shape similar to that of a gas jet, and the mixing layer at the gas-liquid interface becomes the main part of the spray projection area.

Backwater assessment for the energy conversion through hydrokinetic turbines in subcritical prismatic canals

Hydrokinetic turbines exploit partially the total head variation created in natural and artificial waterways. In subcritical flows, the sum of the energy harvested mechanically and the energy dissipated by fluid-dynamic losses and viscous friction is balanced through a backwater extending far upstream. The latter depends on the turbine operating condition and, thus, on the thrust force generated.Only the downstream momentum is known, whilst upstream momentum and turbine thrust force are undetermined. To solve this problem, a method is developed to update the inflow factor for a single turbine, thus achieving a single physical solution. Froude number and blockage ratio are updated at each iteration and are used as independent variables for solving the turbine model. According to the actual canal geometry, a specific continuity equation allows to infer the backwater depth from the inflow factor. Hence, this method can be integrated with confined BEM and DMS models to extend the capability of the simulation tool.Experimental data of inflow depth are used to validate this model for single rotors in a subcritical environment, whilst its extension allows the assessment of turbine arrays in artificial canals exploited for hydropower purposes.

Selective extraction of lithium from spent LiNixCoyMnzO2 cathode via in-situ conversion of ethylene glycol in subcritical water system

The resources scarcity and explosive growing demand of lithium have made the selective extraction of lithium from spent lithium-ion batteries (LIBs) more and more widely concerned. Yet the current lithium recovery routes still suffer the problems of complex process, low efficiency and unfriendly environment. In this study, a strategy of selective lithium extraction from spent LiNixCoyMnzO2 cathode with ethylene glycol (EG) in subcritical water system is proposed. The in-situ conversion pathway and lithium extraction mechanism in this new system are investigated in detail. Benefit from the reducibility of EG and the dynamic strengthening of the subcritical water system, more than 99% of lithium can be selectively extracted into the solution under the optimal conditions. The leaching kinetics show that the selective leaching process is controlled by chemical reactions and the leaching mechanism suggests that EG plays a dual role of reducing and donating protons in subcritical environment. Furthermore, the lithium-containing leachate can be applid to prepare Li2CO3 products with high purity, and the transition metals (Ni, Co, Mn) in the lithium extraction residue are recovered by acid leaching without adding reducing agent. This process puts forward a innovative idea for preferentially extracting lithium from spent LIBs and even lithium-containing wastes, which has the significant advantages of high efficiency, environmental protection and simplicity.

Molecular dynamics investigation of the vaporization characteristics of n-alkane blended fuels under different ambient conditions

Molecular dynamics (MD) simulation is a powerful tool to reveal the microscopic characteristics of supercritical transitions. However, the accuracy of MD depends strongly on the potential model that describes the interaction forces between atoms. In this study, four commonly used potential models for long-chain n-alkanes in MD simulations are evaluated, and a hybrid model is introduced. The vaporization and phase-transition characteristics of n-alkane blended fuels with different mole fractions are then explored under a wide variety of ambient conditions by using the hybrid model. Compared to the commonly used potentials, the hybrid model shows higher accuracy for predicting the thermodynamic and transport properties. In subcritical environments, vaporization belongs to typical two-phase evaporation with a sharp gas–liquid interface. The preferential evaporation of the light-end component is obvious, and the evaporation rate of the heavy-end component is maximized after the light-end component is consumed. Under supercritical conditions, the interface dissolves rapidly, the evaporation rates for both the light- and heavy-end components increase simultaneously, and both components coexist throughout the evaporation process. Based on the maximum potential energy and evaporation rate, a new criterion for the supercritical transition is proposed. The dimensionless transition time, which reflects the proportion of the sub/supercritical stage within the lifetime, is nearly independent of the ambient temperature and fuel composition; instead, it mainly depends on the ambient pressure. Finally, an empirical formula is obtained by curve-fitting to describe the variation in the dimensionless transition time with ambient pressure.

Transient mixing behavior of a supercritical fluid injected into supercritical and subcritical environments

Transient, near-field mixing of a supercritical fluid injected into supercritical and subcritical environments is investigated experimentally. Fuel (propane) from a 100 μm diameter Bosch diesel injector is injected into a high-pressure test rig with a steady flow of heated, low-velocity nitrogen to control the ambient pressure and temperature. The fuel supply pressure is approximately 190 bar, allowing short injections lasting 3–4 ms to be performed in quick succession to acquire statistically significant datasets to represent the unsteady fuel–air mixing. Two test cases with the same ambient density and injector inlet pressure and temperature were chosen in an attempt to isolate effects of mixture phase behavior. Microscopic rainbow Schlieren deflectometry (RSD) is used for visualization and quantitative data acquisition. In the case of the supercritical ambient, the injector inlet and exit and ambient conditions are all supercritical. The RSD analysis reveals a mixing process similar to the single phase fluid mixing, though condensation is also observed locally, and can be explained by the adiabatic mixing theory presented in the paper. For the subcritical ambient case, the injector inlet is supercritical, but the injector exit and ambient conditions are both subcritical. The results show a rapid expansion in the near-field promoting mixing between propane and nitrogen. At downstream locations, significant condensation is also observed as predicted by the adiabatic mixing theory. The supercritical ambient case exhibited longer and narrower jet penetration compared to the subcritical ambient case.

A molecular dynamics investigation on fuel vaporization and mixing characteristics under sub/supercritical conditions

Molecular dynamics simulation is performed to study the influence of environmental pressure on the mixing process. Based on the OPLS-AA full-atomic potential function, the gas-liquid-gas simulation box model is used to study the evaporation characteristics of n-heptane at different environmental conditions. The results show that compared with the subcritical environment, the nitrogen molecules in the supercritical condition can diffuse into the liquid phase region earlier, and the temperature of the liquid phase rise faster, and then a unified supercritical fluid could be formed. Based on the density profile, a gas-liquid-gas interface thickness is defined and the interface thickness is widened as the ambient pressure increase, resulting in the conventional subcritical evaporation transition turbulent mixing process.

Nano-mechanics based crack growth characterization of concrete under fatigue loading

Developments of scaling laws are crucially important for modeling an engineering phenomenon. Based on the type of physical problems scaling laws have been developed in conjunction with the concept of self-similarity. Scale effect should take into account when the size of an object reduces to extremely small-scale level. Therefore, scaling/power laws and similarity concepts have been considered to be important in nano mechanics in the recent times. In the case of quasi-brittle materials like concrete, crack growth phenomenon can considered as a multi-scale problem comprising of atomistic separation, nano scale level coalesce to form micro and subsequently, major crack. Therefore, it is necessary to have a clear understanding of cracking phenomenon in concrete at different length scales under the action of repetitive loading cycles.In this work, an attempt has been made out to understand the micro-fracture scale effect on the crack growth rate in concrete material under the action of fatigue loading. A theoretical model has been developed based on atomic fracture mechanics theory. Using the concept, the activation energy controls the random movement of the nano-crack front in the subcritical environment of micro-crack growth within the cyclic fracture process zone of quasi-brittle material. Kramer’s formula has been used for the prediction of net frequency of the forward crack front jumps by assuming, fatigue crack propagation rate is governed by thermally activated breakage of interatomic bonds. A multi-scale transition approach has been adopted from micro to the macro using scaling law considering energy dissipation in each cycle to grow a macro-crack is equal to the sum of the energy dissipations associated with the propagation of all the active micro-cracks inside the cyclic fracture process zone.

Comparison of evaporation rate constants of a single fuel droplet entering subcritical and supercritical environments

This work compares the evaporation rate constants of a single fuel droplet under subcritical and supercritical environments by experimental visualizations (in millimeter scale) and molecular dynamics simulations (in nanometer scale). The objection is to understand the differences of fuel droplet evaporation rate constant variations under the subcritical and supercritical environments. Firstly, an experimental apparatus is designed to study the diesel droplet evaporation under subcritical and supercritical environments. The results show that the droplet evaporation obeys the D2 law in subcritical environments, while the D2 law is no longer strictly applicable in supercritical environments and the evaporation rate constant isn’t a constant but a variable that increases with the evaporation time. Secondly, the molecular dynamics simulations are applied to investigate the evaporation of diesel surrogate n-dodecane under subcritical and supercritical environments. The results show that the droplet temperature keeps at the equilibrium temperature in subcritical environments, but keeps increasing in supercritical environments. In supercritical environments, the reason for the evaporation rate constant keeps increasing is that the droplet density keeps decreasing with the increase of droplet temperature. The droplet surface tension will rapidly vanish in supercritical environments, which implies that the formation of fuel–air mixture is no longer dominated by the surface tension, but is the turbulent diffusion in diesel trans/supercritical injection conditions. It can deduce that the evaporation rate constants of a fuel droplet in micrometer scale (that is commonly generated in engines) will follow the same trend and within the same variation scale at the same ambient pressures and temperatures.

A molecular dynamics study of binary-component n-alkane fuel vaporization characteristics at sub/supercritical nitrogen environments

The emphasis of this paper is to explore the gas-liquid interface characteristics of binary-component n-alkane fuels composed of n-heptane and n-dodecane at sub/supercritical nitrogen environments, and examine the vapor-liquid equilibrium (VLE) assumption adopted in the continuum-based vaporization models using the molecular dynamics simulation. In addition, the phase transition characteristics are also revealed. The results indicate that the vaporization of the binary-component fuel follows the classical multi-component vaporization theory in subcritical environment, in which the preferential vaporization of the light-end component is obvious with distinguishing gas-liquid interface. On the contrary, in supercritical environment, the vaporization rates of light-/heavy-end components are simultaneously enhanced even in the early stage. When approaching the critical point of the heavy-end component, the solubility of the ambient gas of nitrogen in the liquid phase is greatly enhanced with the indistinguishable gas-liquid interface. The transition mechanism from VLE to non-VLE for single-component fuels is caused by the remarkable ambient gas solubility in the liquid phase and the non-ideal gas effect at high pressures. However, for binary-component fuels, it is related to the complicated interaction between the light- and heavy-end components with significant differences in the physical properties and critical points. It is also found that ambient temperature and pressure play more dominant roles in controlling the phase transition process compared to the light-end component blended ratio for the binary-component fuels.

Evolution of “Spinodal decomposition”-like structures during the oxidation of Zr(Fe,Nb)2 under subcritical environment

As an important precipitate in Zr-alloy, detailed study on the oxidation behavior of Zr(Fe,Nb)2 in atomic scale will help to develop safer materials for reactors. In this study, Zr(Fe,Nb)2 intermetallic was fabricated and exposed to subcritical water (320 °C, 15.5 MPa). On oxidation frontier, the interplay between element diffusion and structure evolution was analyzed. The oxidation process started with the incorporation of oxygen in local strips, which was perpendicular to the direction of oxygen diffusion. Fe was segregated from the oxygen-rich strips via an uphill diffusion process along {001} lattice plane. The “spinodal decomposition”-like phenomenon was observed and discussed.

Chemical diversity of iron species and structure evolution during the oxidation of C14 Laves phase Zr(Fe,Nb)2 in subcritical environment

C14 Laves phase Zr(Fe,Nb)2 was fabricated and oxidized in a subcritical environment. Electron energy loss spectroscopy was used to study the migration and chemical state evolution of the elements. The oxidation process was accompanied by the formation of multilayered oxide scale, consisting of a crystalline oxide layer, amorphous oxide layer (Fe-poor) and Zr-Nb-Fe-O amorphous layer. The formation of closely bonded, semi-free and metallic iron species as well as the out diffusion of Fe contribute to the multilayered structure growth. The amorphization of the C14 phase occurred with the incorporation of oxygen, which preferentially bonded with Zr and Nb.

Ionisation in turbulent magnetic molecular clouds

Previous studies show that the physical structures and kinematics of a region depend significantly on the ionisation fraction. In this paper, we extend our previous studies of the effect of ionisation fractions on star formation to clouds that include both non-ideal magnetohydrodynamics and turbulence. We aim to quantify the importance of a treatment of the ionisation fraction in turbulent magnetised media and investigate the effect of turbulence on shaping the clouds and filaments before star formation sets in. In particular, we investigate how the structure, mass and width of filamentary structures depend on the amount of turbulence in ionised media and the initial mass-to-flux ratio. We compare the resulting density and mass-to-flux ratio structures both qualitatively and quantitatively via filament and core masses and filament fitting techniques (Gaussian and Plummer profiles.) We find that even with almost no turbulence, filamentary structure still exists. Comparison of simulations show that for turbulent Mach numbers above 2, there is little structural difference between the Step-Like (SL) and Cosmic Ray only (CR-only) ionisation models, while below this threshold the ionisation structure significantly affects the formation of filaments. Analysis of the mass within cores and filaments show decrease in mass as the degree of turbulence increases. Finally, observed filaments within the Taurus L1495/B213 complex are best reproduced by models with supercritical mass-to-flux ratios and/or at least mildly supersonic turbulence, however, our models show that sterile fibres observed within Taurus may occur in highly ionised, subcritical environments. Based on this, we suggest that regions with fertile fibres likely indicate a trans- or supercritical mass-to-flux ratio within the region while sterile fibres are likely subcritical and transient.

Experimental study of elliptical jet from supercritical to subcritical conditions using planar laser induced fluorescence

The study of fluid jet dynamics at supercritical conditions involves strong coupling between fluid dynamic and thermodynamic phenomena. Beyond the critical point, the liquid-vapor coexistence ceases to exist, and the fluid exists as a single phase known as supercritical fluid with its properties that are entirely different from liquids and gases. At the critical point, the liquids do not possess surface tension and latent heat of evaporation. Around the critical point, the fluid undergoes large changes in density and possesses thermodynamic anomaly like enhancement in thermal conductivity and specific heat. In the present work, the transition of the supercritical and near-critical elliptical jet into subcritical as well as supercritical environment is investigated experimentally with nitrogen and helium as the surrounding environment. Under atmospheric condition, a liquid jet injected from the elliptical orifice exhibits axis switching phenomena. As the injection temperature increases, the axis switching length also increases. Beyond the critical temperature, the axis switching is not observed. The investigation also revealed that pressure plays a major role in determining the thermodynamic transition of the elliptical jet only for the case of supercritical jet injected into subcritical chamber conditions. At larger pressures, the supercritical jet undergoes disintegration and formation of droplets in the subcritical environment is observed. However, for supercritical jet injection into supercritical environment, the gas-gas like mixing behavior is observed.