Cryogenic Jet Research Articles

Oxygen atoms and nitrogen molecules as spectroscopic probes for the temperature determination in non-equilibrium cryogenic helium plasma jets

Determination of the gas temperature in the afterglow plasma jet of pre-cooled helium propagating inside a dense helium vapor at 1.4 K is a difficult task. In this work we analyze the possibility of using the emission spectra from oxygen atoms and nitrogen molecules for deducing the local temperature of the gas. Oxygen is always present as a trace impurity (∼1 ppm) in helium gas and strong emission of the atomic 777 nm triplet lines can be a good candidate because the energy gaps of 3.67 cm−1 (T ∼ 5.3 K) and 2.02 cm−1 (T ∼ 2.9 K) between the sub-levels of its upper state O(5P J ), with J = 1, 2 and 3, are comparable to the kinetic energy of atoms in a cryogenic environment. A detailed analysis of the kinetics of atoms in the O(5P J ) state indicates that at temperatures lower than 10 K, the population transfers between sub-levels are not efficient enough for the establishment of a Boltzmann equilibrium within the sublevels before the radiative decay of O(5P J ) atoms. We suggest the presence of small energy barriers in the He-O(5P J ) transient molecule formed during the collision. It was also shown that the temperature of a gas in non-equilibrium cryogenic helium plasmas containing at least 100 ppm of nitrogen can be determined from the rotational spectra of the 2–0 and 3–1 bands of the first positive system and the 8–3 band of the infrared afterglow system of molecular nitrogen. The application of PGOPHER software (1-T method) gives the rotational temperature of N2 molecules averaged over the observation area and is optimal for the gas temperature determination in locally homogeneous plasmas or mapping the temperature in inhomogeneous plasmas with high spatial resolution. A home-made code to simulate rotational spectra with two temperatures (2-T method) allows determination of the temperature span within the observation region of an inhomogeneous plasma.

Simulation study on the cryogenic liquid nitrogen jets: Effects of equations of state and turbulence models

Based on the open source software OpenFOAM, a new computational fluid dynamics (CFD) solver is developed for trans/supercritical jets. Considering the real-fluid effects, the pressure Poisson equation is modified using two real-fluid equations of state (EOSs). Turbulent flows of liquid nitrogen jets are studied by different turbulence models based on Reynolds Average Navier-Stokes (RANS) under trans/supercritical pressure conditions, and the iterative calculation of pressure–velocity-density coupling is implemented by the extended PISO algorithm. The simulation results of different EOSs are compared with the experimental data, and the performance and accuracy of different EOSs coupling with five different turbulence models applied to trans/supercritical jets are analyzed and quantified. The results indicate that the selection of real-fluid EOS is more significant and important than the turbulence model for the numerical performance. Analysis of three cases under different operating conditions shows that none of the conventional RANS models always give satisfactory results, and all need to be extended to adapt to various characteristics under trans/supercritical conditions.

An Empirical Correlation for the Penetration of a Cryogenic Liquid Jet into a Gaseous Crossflow

Empirical correlations have been widely used to map the penetration and trajectory of an injected liquid jet into a gaseous crossflow. Such correlations for the case of cryogenic liquid injection into a gaseous crossflow of the same species are limited. In this study, medium-resolution images of the injection of cryogenic liquid nitrogen into a gaseous nitrogen crossflow with momentum blowing ratios of 1-4 and Weber numbers of 1500-4500 are captured and image analysis is used to identify the jet boundary. Using these data, an empirical correlation for the penetration of liquid nitrogen into a gaseous crossflow as a function of momentum blowing ratio and downstream distance from the point of injection is developed. Minimizing the root mean square error of the correlation parameters over the full range of experimental conditions investigated leads to a power-law correlation with a mean error of 16%.

PDMS material embrittlement and its effect on machinability characteristics by cryogenic abrasive air-jet machining

Abstract The use of cryogenic abrasive jet machining (CAJM) technology for polydimethylsiloxane (PDMS) materials is increasing rapidly due to its high material removal rate and high surface quality. However, how the material properties of PDMS affect the machining properties under cryogenic conditions is still an unsolved problem. Firstly, the thermodynamic and mechanical properties of PDMS materials are studied by dynamic thermo-mechanical analysis and universal material testing. The results show that the PDMS material’s embrittlement temperature is 126 K. Secondly, evaluation of cooling temperature of PDMS during CAJM shows that the machined area has reached embrittled state. Thirdly, the effect of PDMS material’s embrittlement on the machining behaviour is studied. The experimental results show that the PDMS material exhibit increased abrasive erosion at medium-angle under cryogenic conditions and shows a semi-brittle wear mechanism. Furthermore, the erode surface’s roughness is evaluated. The result shows that the roughness decreases with decreasing air pressure at the same impact angle; with increasing impact angle, the roughness shows a trend of increase initially and then decreases. This work shows that the PDMS material’s embrittlement and the erosion response behaviour during CAJM are due to the combined effects of thermo-mechanical properties and material removal mechanisms, providing a better understanding of CAJM technology.

Prediction of erosion volume of PDMS by cryogenic micro-abrasive jet machining based on dimensional analysis method and experimental verification

The polymer polydimethylsiloxane (PDMS) is used widely in microfluidic chips. At room temperature, when PDMS is machined by micro-abrasive jet machining (MAJM), its high elasticity means that it often suffers from processing defects such as abrasive embedding and a surface deterioration layer. Cryogenic micro-abrasive jet machining (CMAJM) based on the glass transition has been proposed, but a model for predicting the volume of material removed is yet to be established. In this study, orthogonal experiments are used to investigate how different processing parameters influence the material removal volume. The results show that the material removal volume is largest when the jet pressure is 0.6 MPa, the erosion angle is 60°, the erosion distance is 3.5 mm, and the scanning speed is 0.2 mm/s. A model is established to predict the material removal volume during the CMAJM of PDMS, and single-factor experiments are used to assess the model qualitatively. The results show that the model offers good predictions of how the material removal volume changes with the scanning speed and jet pressure.

NUMERICAL STUDY ON THERMAL DESTRATIFICATION IN LARGE SCALE HYDROGEN PROPELLANT TANK IN SPACE BY JET INJECTION UNDER ZERO GRAVITY CONDITION1)

Liquid Hydrogen plays a vital role in the future energy system as a space propellant, but it is sensitive to heat leakage from the environment because of its low boiling point and easy evaporation. On the other hand the buoyancy convection in the space microgravity environment is significantly reduced. When there is local heat leakage on the wall of the propellant tank, temperature stratification arises around the heat leakage source causing local overheating. It seriously affects the multiphase heat and mass transfer in the propellant tank which induces the tank pressure rise and jeopardize the structural safety of the system. To prevent the tank pressure from rising above the design of limits, venting or active pressure control techniques must be implemented. The cryogenic jet mixing is an effective means to suppress temperature stratification. The cryogenic fluid is mixed with the fluid inside the tank through a jet nozzle to reduce the local high temperature and achieve uniform temperature. In present paper, the thermal stratification phenomenon caused by the local heat leakage under microgravity condition was numerically simulated by using a fully filled two-dimensional large scale tank model. This paper mainly analyzes the suppression and elimination of local thermal stratification caused by heat leakage near the bottom of the storage tank and the outlet connecting section. The influence of different cryogenic jet mixing conditions on eliminating the temperature stratification effect is analyzed. The results show that the position of jet nozzle has no obvious effect on the elimination of thermal stratification inside the tank when the circular jet nozzle is used and the cryogenic jet condition is the same. When the jet nozzles are located in the same relative position inside the tank and the incident flow rate is the same, the circular jet nozzles have more concentrated flow direction, the flow field in the tank evolves faster, so the thermal stratification elimination effect is better than the hemispherical jet nozzles.

Wear under brittle removal regime of an under-expanded cryogenic nitrogen jet machining of bio-composites

Machining of biocomposites using traditional techniques has shown some limitations due to the multiscale complex cellulosic structure of natural fibrous reinforcement. This paper aims to demonstrate the feasibility of the cryogenic nitrogen jet, which is a novel cutting process that combines sustainable resources and cryogenic temperatures from −175 °C to 150 °C, as a machining process for biocomposites made of unidirectional flax fibers and polylactic-acid polymer (PLA). A high-velocity stream of liquid nitrogen with and without abrasives (400–700 m/s) is hence directed onto the workpiece surface. For the abrasive jet, both conventional garnet abrasives and bio-based abrasives made from walnut shells were used for the nitrogen jet process. The kerf depth, which is representative of the erosion wear mechanisms, was calculated to assess the erosion rate of the biocomposite structure after the nitrogen jet cutting operation. Then, scanning electron microscope observations are performed to characterize the induced damages on the machined biocomposite surfaces. Results show that the pressure and the traverse speed of the nitrogen jet are the relevant process parameters that control the mechanical and thermal stresses viewed by the biocomposite during the machining operation. Each impinging abrasive particle removes a small amount of biocomposite material during the jet cutting process, which depends on the brittleness of the anisotropic work structure and the abrasiveness of the incorporated particles into the jet stream.

Investigation of internal flow of Cryogenic injection

As part of an effort to understand the conditions for the ignition of cryogenic propellants in the low pressure environment, we conducted a research of internal flow of cryogenic jet. In this paper, the experimental investigation was exerted focusing on the qualitative morphology study of the cryogenic flow inside the jet injectors. The test facilities were carefully designed and allow for visualization and characterization of the flow. The results show a strong dependence of mass flow rate on the fluid temperature. The two-phase flow was observed even for a long time chilling down of the injector. The Jacob number is proved to be a good indicator for the flow regimes, and the bubbles are present in the flow every time. The injector geometry has an influence on the flow rate, with the tapered injector demonstrating a higher flow rate than the sharp one.

Dispersion of cryogenic hydrogen through high-aspect ratio nozzles

Liquid hydrogen is increasingly being used as a delivery and storage medium for stations that provide compressed gaseous hydrogen for fuel cell electric vehicles. In efforts to provide scientific justification for separation distances for liquid hydrogen infrastructure in fire codes, the dispersion characteristics of cryogenic hydrogen jets (50–64 K) from high aspect ratio nozzles have been measured at 3 and 5 barabs stagnation pressures. These nozzles are more characteristic of unintended leaks, which would be expected to be cracks, rather than conventional round nozzles. Spontaneous Raman scattering was used to measure the concentration and temperature field along the major and minor axes. Within the field of interrogation, the axis-switching phenomena was not observed, but rather a self-similar Gaussian-profile flow regime similar to room temperature or cryogenic hydrogen releases through round nozzles. The concentration decay rate and half-widths for the planar cryogenic jets were found to be nominally equivalent to that of round nozzle cryogenic hydrogen jets indicating a similar flammable envelope. The results from these experiments will be used to validate models for cryogenic hydrogen dispersion that will be used for simulations of alternative scenarios and quantitative risk assessment.

Development of Cryogenic Electrohydrodynamic Jet Printing for Fabrication of Fine Scaffolds with Extra Filament Surface Topography.

Electrohydrodynamic jet printing (EJP) is a developing additive manufacture technology that enables the fabrication of fine scaffolds directly from polymer solutions or melt. Timely solidification of the polymer jet is the key factor for the success of EJP process. In conventional solution-based EJP methods, it is usually achieved by rapid solvent evaporation and producing a scaffold with smooth filaments. In current study, by combining solution-based EJP with a cryogenic workbench, a cryogenic electrohydrodynamic jet printing (CEJP) system was developed, in which the polymer jet was frozen and solidified quickly at the freezing temperature rather than solvent evaporation. The feasibility and versatility of the CEJP system were verified by successful printing of scaffolds with different hole shapes and pore sizes. Meanwhile, the resulting scaffolds not only had a resolution in the range of 50-80 μm but also possessed oriented "ridges" and "valleys" on surface of the filaments, which was conductive to cell orientation. Therefore, this work provides a novel method to print fine scaffolds with extra surface topography.

Effects of in-process LN2 cooling on the microstructure and mechanical properties of type 316L stainless steel produced by wire arc directed energy deposition

For the first time, in-process cryogenic cooling for Wire Arc Directed Energy Deposition (DED) and its influence on the microstructure and mechanical properties of Type 316L stainless steel is investigated. The in-process cryogenic cooling is applied with a liquid nitrogen cryogenic jet positioned behind the welding torch, targeting the material directly behind the melt pool during deposition. Compared with Wire Arc DED that is conducted with a regulated interpass temperature of 160 °C, the crystallographic grain orientations of the deposit with in-process LN2 cooling were found to be significantly more random, with high numbers of equiaxed grains generated. For the samples produced using in-process cryogenic cooling, the tensile tests resulted in a mean Young’s Modulus of 163 ± 51 GPa. This is significantly higher compared with samples produced using interpass temperature control which resulted in a mean of 72 ± 27 GPa. BS EN 10088-1 guidance for Type 316L specifies a Young’s Modulus of 200 GPa. The stiffness improvement with in-process cooling demonstrated in this research is a significant finding for the additive manufacture of parts by Wire Arc DED for structural applications in the architectural and nuclear industries.

Numerical predictions of cryogenic hydrogen vertical jets

Comparison of Computational Fluid Dynamics (CFD) predictions with measurements is presented for cryo-compressed hydrogen vertical jets. The stagnation conditions of the experiments are characteristic of unintended leaks from pipe systems that connect cryogenic hydrogen storage tanks and could be encountered at a fuel cell refueling station. Jets with pressure up to 5 bar and temperatures just above the saturation liquid temperature were examined. Comparisons are made to the centerline mass fraction and temperature decay rates, the radial profiles of mass fraction and the contours of volume fraction. Two notional nozzle approaches are tested to model the under-expanded jet that was formed in the tests with pressures above 2 bar. In both approaches the mass and momentum balance from the throat to the notional nozzle are solved, while the temperature at the notional nozzle was assumed equal to the nozzle temperature in the first approach and was calculated by an energy balance in the second approach. The two approaches gave identical results. Satisfactory agreement with the measurements was found in terms of centerline mass fraction and temperature. However, for test with 3 and 4 bar release the concentration was overpredicted. Furthermore, a wider radial spread was observed in the predictions possibly revealing higher degree of diffusion using the k-ε turbulence model. An integral model for cryogenic jets was also developed and provided good results. Finally, a test simulation was performed with an ambient temperature jet and compared to the cold jet showing that warm jets decay faster than cold jets.

Modeling of cryogenic hydrogen releases

Hydrogen is likely to be stored and transported as a liquid with high densities to have sufficient hydrogen on-site and for efficient distribution. Thus, cryogenic hydrogen jets and dispersion characteristics need to be studied to quantify the risks of accidental releases. The present study modeled cryogenic hydrogen jets with stagnation temperatures around 50 K released from round nozzles at stagnation pressures of 2–10 bar. The instantaneous flow fields were modeled using the Large Eddy Simulation (LES) turbulence model to accurately and efficiently capture the flow characteristics. The numerical models were validated with experimental data. The concentration and velocity distributions show that the centerline mass fraction and velocity decay at similar rates to the decays for room temperature jets. The radial distributions of the mass fraction, temperature and velocity are Gaussian and self-similar. Additionally, the numerical model can reproduce the shock structures near the nozzle for these underexpanded cryogenic hydrogen jets. The location of the first normal shock was predicted to be closer to the nozzle than for room temperature jets. The present work can be used to develop and revise safety codes and standards for liquid hydrogen facilities.

Numerical characterization of under-expanded cryogenic hydrogen gas jets

High-resolution direct numerical simulations are conducted for under-expanded cryogenic hydrogen gas jets to characterize the nearfield flow physics. The basic flow features and jet dynamics are analyzed in detail, revealing the existence of four stages during early jet development, namely, (a) initial penetration, (b) establishment of near-nozzle expansion, (c) formation of downstream compression, and (d) wave propagation. Complex acoustic waves are formed around the under-expanded jets. The jet expansion can also lead to conditions for local liquefaction from the pressurized cryogenic hydrogen gas release. A series of simulations are conducted with systematically varied nozzle pressure ratios and systematically changed exit diameters. The acoustic waves around the jets are found to waken with the decrease in the nozzle pressure ratio. The increase in the nozzle pressure ratio is found to accelerate hydrogen dispersion and widen the regions with hydrogen liquefaction potential. The increase in the nozzle exit diameter also widens the region with hydrogen liquefaction potential but slows down the evolution of the flow structures.

Optimization of radiochromic film stacks to diagnose high-flux laser-accelerated proton beams.

Here, we extend flatbed scanner calibrations of GafChromic EBT3, MD-V3, and HD-V2 radiochromic films using high-precision x-ray irradiation and monoenergetic proton bombardment. By computing a visibility parameter based on fractional errors, optimal dose ranges and transitions between film types are identified. The visibility analysis is used to design an ideal radiochromic film stack for the proton energy spectrum expected from the interaction of a petawatt laser with a cryogenic hydrogen jet target.

Heat-release dynamics in a doubly-transcritical LO2/LCH4 cryogenic coaxial jet flame subjected to fuel inflow acoustic modulation

Large Eddy Simulations are used to study the heat-release response to fuel inflow acoustic harmonic oscillations, in a coaxial jet flame where both reactants (O2 and CH4) are injected in a dense cryogenic state. The geometry is that of the academic test rig Mascotte, operated by ONERA (France), which high pressure operating conditions are relevant for the characterization of flame dynamics in Liquid Rocket Engines (LREs). The simulations, which are carried out with a real-gas fluid solver using a detailed kinetic scheme for CH4 high-pressure oxycombustion, provide a thorough insight into the flame response for a wide range of forcing frequencies, spanning from approximately 1kHz to 20kHz. Local Flame Transfer Functions (FTF) are computed and analyzed: regions of preferential heat-release response are observed to be highly dependent on the forcing frequency. An analysis based on a flame sheet assumption is conducted to distinguish the main sources of heat release fluctuations: the primary contribution comes from the species diffusivity oscillations, while the density variations have a negligible effect. The second largest contributor is either the mixture fraction gradient or the flame surface area, depending on the forcing frequency. The FTFs are expected to be useful for thermoacoustic Low-Order Models or Helmholtz solvers, and the subsequent analysis has the potential to guide future development of analytical models for flame dynamics in LREs.

Large eddy simulation based multi-environment PDF modelling for mixing processes of transcritical and supercritical cryogenic nitrogen jets

Turbulent cryogenic nitrogen jets at transcritical and supercritical conditions are numerically investigated by LES-based multi-environment probability density function method together with the tabulated thermodynamic properties. To account for the non-ideal thermodynamic effects, the mixture properties are calculated by using Soave-Redlich-Kwong equation of state. The effect of sub-grid scale fluctuations of thermodynamic properties is modelled by the multi-environment PDF. Numerical results indicate that the present multi-environment PDF approach has the prediction capability to capture the essential features of cryogenic nitrogen jets under the supercritical pressure. Comparing with the supercritical jet, the transcritical jet features the longer and denser core which is relatively resilient to core destruction caused by turbulence. It is also found that the stronger pseudo-boiling strength and the larger density difference are directly tied with the longer cold-core length. In terms of the sub-grid scale scalar fluctuation, the transcritical jet shows the larger magnitude than the supercritical jet. In the transcritical nitrogen jet, it is identified that the strong sub-grid fluctuations of density and isobaric specific heat are closely tied with the pseudo boiling process. These numerical results suggest that the high-fidelity numerical modelling for transcritical flows must have the capability to realistically represent the combined effects of turbulence and thermodynamic nonlinearities, sub-grid scale fluctuations, and scalar conditional statistics.

Effect Chain Analysis of Supercritical Fuel Disintegration Processes Using an LES-based Entropy Generation Analysis

ABSTRACT In this work, a large eddy simulation (LES) technique combined with entropy generation analysis (EGA) is employed in order to investigate the complex physics of fuel injection under supercritical conditions. In particular, physical processes that are relevant for the fuel breakup and mixing are identified and their effects on the overall disintegration process are quantified. Thereby, it turned out that (1) a chain of four consecutive physical processes (shearing, separation, pseudo-boiling, and turbulent mixing) drive the overall jet disintegration, (2) entropy is primarily generated due to thermal energy conversion processes including pseudo-boiling rather than hydrodynamics, and (3) LES technique combined with EGA proved to be a promising approach for effect chain analyses, not only for simple flows, but also for those with complex thermodynamic properties like supercritical fuel injection. Based on these findings, a physical model of the disintegration process of a cryogenic jet flow injected at supercritical conditions is proposed.

Disintegration of diminutive liquid helium jets in vacuum

The phenomenon of liquid jets disintegrating into droplets has attracted the attention of researchers for more than 200 years. An overwhelming fraction of these studies considered classical viscous liquid jets issuing into ambient atmospheric gases, such as air. Here, we present an optical shadowgraphy study of the disintegration of a cryogenic liquid helium jet produced with a 5 µm diameter nozzle into vacuum. The physical properties of liquid helium, such as its density, surface tension, and viscosity, change dramatically as the jet flows through the nozzle and evaporatively cools in vacuum, eventually reaching the superfluid state. In this study, we demonstrate that, at different stagnation pressures and temperatures, droplet formation may involve spraying, capillary breakup, jet branching, and/or flashing and cavitation. The average droplet sizes produced in this work range from 3.4 × 1012 to 6.5 × 1012 helium atoms or 6.7-8.3 µm in diameter. This paper also reports on the distributions of sizes and shapes of the resulting droplets.

Understanding Sub and Supercritical Cryogenic Fluid Dynamics in Conditions Relevant to Novel Ultra Low Emission Engines

In this paper we provide insight into the thermophysical properties and the dynamics of cryogenic jets. The motivation of the work is to optimise the use of cryogenic fluids in novel ultra low emission engines. For demonstration, we use conditions relevant to an internal combustion engine currently being developed by Dolphin N2 and the University of Brighton, the CryoPower recuperated split cycle engine (RSCE). The principle of this engine is a split-cycle combustion concept which can use cryogenic injection in the compression cylinder to achieve isothermal compression and thus help maximise the efficiency of the engine. Combined experimental and numerical findings are presented and the effects of atomisation dynamics of the LN 2 are explored at both sub- and supercritical conditions in order to cover different pressure and temperature conditions representative of the engine compression cycle. For subcritical regimes, we observe that the appearance of the jet coincides with the predicted atomisation regimes based on the Weber, Ohnesorge and Reynolds numbers for other common fluids. For the modelling of supercritical jets, a new methodology within OpenFoam which accounts for Real Fluid Thermodynamics has been developed and the jet behaviour under various pressure and temperature conditions has been investigated. To our knowledge this is the first study where a cryogenic spray process evolution is examined for conditions relevant to the ones prevailing in a compression chamber accounting for both sub and supercritical conditions.