Jet Expansion Research Articles

High-Resolution Spectroscopy and Structure of Heavy Carbon Subchalcogenides: Tricarbon Selenide, C3Se.

Linear tricarbon selenide, C3Se, has been studied spectroscopically for the first time using a combination of high-resolution infrared and microwave techniques. Probing laser ablation products from carbon-selenium targets in a free jet expansion with He, initial spectroscopic detection was accomplished in the infrared at a wavelength of 5 μm in search of the ν1 vibrational fundamental. Along with the band of the most abundant isotopic species C380Se found centered at about 2039 cm-1, the corresponding bands of the C382Se, C378Se, C376Se, and C377Se isotopologues were also detected. Pure rotational spectra of the five C3Se isotopologues in the 8-18 GHz frequency range were observed in a supersonic jet expansion using chirped-pulse microwave spectroscopy of the discharge products of selenophene, c-C4H4Se. Spectroscopic analyses were guided by results from high-level quantum-chemical calculations carried out at the coupled-cluster level of theory using large correlation-consistent basis sets. Using the experimentally derived ground-state rotational constants of five isotopologues and calculated zero-point vibrational corrections, an accurate semiexperimental equilibrium carbon-selenium bond length is derived.

Fluorescence imaging of plume-surface interaction in large-scale reduced pressure environments

Planar laser-induced fluorescence (PLIF) was applied for the first time in the 20-ft vacuum chamber at the Marshall Space Flight Center to visualize the plume-surface interaction (PSI) of a nitrogen jet seeded with nitric oxide (NO). A Mach 5.3 nozzle was used to simulate the exhaust of a landing spacecraft for two different jet stagnation pressures and one jet stagnation temperature. A flat plate was used to simulate the landing surface, and two different dimensionless altitudes were investigated. The chamber pressure was reduced such that both lunar-relevant environments at 0.01–28 Pa and Martian-relevant environments at ∼600 Pa were investigated. PLIF flow visualization was performed using a pulsed, tunable, ultraviolet laser, which entered the vacuum chamber through a window, and was directed to the test article using remote-controlled mirrors. Fluorescence at ultraviolet wavelengths was imaged using an intensified camera, which was placed inside a pressurized enclosure located inside the vacuum chamber. For the Martian-relevant condition, a Mach disk and stagnation bubble were observed at h/De = 10, whereas a pair of oblique stagnation shocks were observed at h/De = 3. Significantly complex flows, such as different stagnation shock behaviors, were observed for the lunar-relevant conditions based on the h/De and Reynolds number. The results presented here are the first NO-PLIF measurements of the PSI flowfield within rarefied environments. The unique information on jet expansion and plume structure will be useful to aid researchers in validating complex computational simulations and to inform engineering designs of extraterrestrial landing systems.

Analysis of the unsteady flow characteristics of underwater supersonic gaseous jets

To examine the unsteady flow characteristics of underwater supersonic gaseous jets under different jet expansion conditions, a sophisticated numerical model is created. This model accurately predicts the intricate multiphase flow by considering the compressibility of the jet gas and energy exchange, which is then rigorously validated against experimental data. The development process of underwater supersonic gaseous jets displays notably unsteady features in terms of jet morphology, flow structure, and various flow field parameters when compared to atmospheric conditions. The unsteady phenomena, such as necking, breaking, bulging, and back-attack, are observed alongside significant pressure pulsations. These unsteady phenomena occur at a considerable distance from the nozzle exit under under-expanded conditions, while pressure pulsations do not impact the internal gas flow within the nozzle. However, under full-expanded and over-expanded conditions, unsteady phenomena near the nozzle exit lead to oscillatory pressure, causing shock waves to propagate inside the nozzle. This results in a notable increase in internal pressure pulsation and mass flow rate within the nozzle, ultimately affecting engine performance significantly.

The Effect of Hyperconjugation and Hydrogen Bonding on the Conformers of Methylated Monosaccharides.

The conformational landscapes of four 1-O-methylated monosaccharides-methyl a-glucose, methyl b-glucose, methyl a-galactose, and methyl b-galactose-were characterized using jet-cooled broadband rotational spectroscopy, supported by density functional theory calculations. A newly designed, simple pulsed nozzle assembly was used to introduced the sugar samples into a jet expansion without thermal degradation, eliminating the need for a complex and expensive laser ablation system. Ten conformers were experimentally identified by assigning their rotational spectra, and the intricate methyl internal rotation splittings were analysed. Notably, methylation alters the directionality of intramolecular hydrogen bonding of a-galactose highlighting its impact on structural preference. Natural bond orbital, intrinsic bond strength, and non-covalent interaction analyses were conducted to explore the interplay between hydrogen bonding and hyperconjugation. A set of σ to σ* neutral hyperconjugative interactions were found to override a strong hydrogen bond, driving a preference for the gauche conformers.

On the enhancement of acceptable blockage of open jet wind tunnel by employing modifications based on concepts of boundary layer and jet flow for testing of aero-rotors

The conventional closed and open jet tunnels suffer from various adverse blockage effects that alter the flow conditions generating erroneous results when larger than allowable blockage models are tested. The maximum acceptable blockage is limited to 25 % for an open jet tunnel. The physics behind the inferior blockage capacity and their redressal is investigated. Hitherto, numerical models are developed to rectify the blockage troubles that introduce an additional complex case-based mathematical modeling to reckoning the results. In the current paper novel approach to redesigning the tunnel to solve blockage troubles is investigated that facilitates evading of mathematical techniques and securing accurate results straightforwardly. A hypothetical design incorporating the remedies is proposed, capable of conditioning the flow in a manner that debilitates the adverse effects of solid blockage, wake blockage, jet expansion, etc. The hypothesis is theoretically expounded by the boundary layer and jet flow phenomenon. The modified blower-type open jet wind tunnel constitutes a centrifugal blower, long inlet duct, plenum chamber around the test section, and collector. The modified wind tunnel is tested through experiments executed on four different-sized Savonius rotors creating 10 %, 30 %, 50 %, and 60 % blockage. The static pressure along the contours of the blades of rotors is obtained. The experimental results are juxtaposed with CFD results carried out at free-wind boundary conditions and standard classic experimental results performed in conventional wind tunnel at 11 % blockage. A blockage enhancement of 25 % is achieved by the approach of modification in the design of a conventional open jet wind tunnel.

Laser induced fluorescence spectroscopy of the transition of jet cooled 14NO3 and 15NO3 II: the dispersed fluorescence spectrum from the 3rd E level of the ν4 mode (approximately the 2ν4 (e'), l = 0, level).

14NO3 and 15NO3 isotopomers were generated in a supersonic free jet expansion, and laser induced fluorescence (LIF) of the electronic transition was observed. Dispersed LIF spectra from the vibronic level at ∼770 cm-1 above the vibrationless level have been measured for each isotopomer. One remarkable characteristic of the dispersed fluorescence (DF) spectrum is that the ν2 fundamental is clearly observed, even though the transition 201 is forbidden in the electronic transition. Another is that two sets of vibrational structure are observed in the DF spectrum: one of the two sets is the structure with the origin at the excitation energy, and the other is that with the origin at the ν2 fundamental. These observations suggest that the fluorescent level has contribution from the ν2 mode. In part I (M. Fukushima, J. Mol. Spectrosc., 2022, 387, 111646), it has been reported that the ν4 progressions are one of the typical characteristics of the vibrational structure of the DF spectrum from the vibrationless level; the progressions have intensity gradually decreasing with increasing ν4 quantum number (named the "regular" distribution), and have members forbidden for the Δl = 0 selection rule. The intensity distribution of the ν4 progression with the origin at the ν2 fundamental is similar to that from the vibrationless level, while that with the origin at the excitation energy is drastically different from that from the vibrationless level. The Jahn-Teller (J-T) and Renner-Teller (R-T) interactions in the B̃2E' state enable us to interpret the intensity distributions of the ν4 progressions, in which relatively weak interactions are enough to reproduce the observed distributions. The spectral intensity distribution analyses adopting the two vibronic couplings suggest that the fluorescent level at ∼770 cm-1 above the vibrationless level is the 3rd E eigenstate of the ν4 mode. The major component of the ν4 3rd E state is the |Λ = ±1; v4 = 2, l4 = 0〉 level of the B̃2E' state, which is a vibrationally and a vibronically E' level, and it is therefore concluded that the major components of the fluorescent level are both of the ν2 = 1 level and ν4 = 2 level .

New Infrared Spectra of Acetylene-Water Dimers: First Determination of the A Rotational Constant and Another K-Dependent Anomaly.

Infrared spectra of acetylene-water complexes are studied in the regions of the H2O ν2 bend (1600 cm-1) and the D2O ν1/ν3 stretches (2670-2808 cm-1), using tunable infrared sources to probe a pulsed supersonic slit jet expansion. In the H2O bend region, there is a puzzling absence of Ka = 0 transitions for C2H2-H2O, while both Ka = 0 and 1 are observed for C2D2-H2O. This continues a pattern of "missing" Ka states noted in previous infrared studies of acetylene-water. Noticeable line broadening gives estimates of upper state predissociation lifetimes of about 1.6 ns for C2D2-H2O with Ka' = 1, 0.8 ns for C2H2-H2O with Ka' = 1, and 0.8 ns for C2D2-H2O with Ka' = 0. The lifetime for C2H2-H2O Ka' = 0 must be much shorter (<0.05 ns) since rapid predissociation seems to be the only explanation for its absence in the spectrum. In the D2O ν3 region, clear sub-bands with Ka = 0 ← 1, 1 ← 0, and 2 ← 1 are observed which enable a good determination of the A rotational constant of C2H2-D2O (7.313 ± 0.007 cm-1), a first for acetylene-water dimer and larger than previously expected.

Regularities and Anomalies in Neon Matrix Shifts of Hydrogen-Bonded O-H Stretching Fundamentals.

O-H bond stretching vibrations in hydrogen-bonded complexes embedded into cryogenic neon matrices are subtly downshifted from cold gas phase reference wavenumbers. To the extent that this shift is systematic, it enables neon matrices as more universally applicable spectroscopic benchmark environments for quantum chemical predictions. Outliers are indicative of either an assignment problem in one of the two cryogenic experiments or they reveal interesting dynamics or structural effects on the complexes as a function of the environment. We compile 6 literature-known pairs of experimental data in jet and neon matrix expansions and realize a 6-fold expansion of that number through targeted matrix isolation and/or slit jet expansion spectroscopy presented in this work. In many cases, the neon matrix shift is less than the uncertainty of the currently best-performing blind quantum chemical predictions for the gas phase, but in specific cases, it may exceed the currently achievable theoretical accuracy. Some evidence for a positive correlation of the matrix shift with the hydrogen bond shift is found, similar to observations for helium nanodroplets. Outliers in particular for water acting as a donor are discussed, and in a few cases they call for a future reinvestigation. Substantial improvement in the correlation of the matrix shift with the hydrogen bond shift is achieved for ketone monohydrates by removing a vibrational resonance. New insights into nitrile hydration isomerism are obtained, and the linear OH stretching spectrum of the jet-cooled ammonia-water complex is presented for the first time. Vibrational spectroscopy in weakly perturbing solid rare gas quantum matrices for the benchmarking of gas phase theory and future explicit theoretical treatments of the quantum matrix environment to better understand the outliers are both encouraged.

Influence of Double-Ducted Serpentine Nozzle Configurations on the Interaction Characteristics between the External and Nozzle Flow of Aircraft

To clarify the influence of the serpentine nozzle configurations on the flow characteristics and aerodynamic performance of aircraft, the flow features and aerodynamic performances of the double-ducted serpentine nozzles with different aspect ratios (AR), length–diameter ratios (LDR) and shielding ratios (SR) are numerically investigated. The results show that the asymmetric nozzle flow occurs due to the curved profile of serpentine nozzles, and a local accelerating effect exists at the S-bend, causing the increase in wall shear stress. The unilateral unsymmetrical expansion of the tail jet in the upward direction interacts with the separated external flow of the afterbody, forming an obvious cross-shock wave and shear layer structure. The surface pressure of the afterbody increases along the external flow direction, and decreases sharply in the separation point of the boundary layer. With the increase in AR and LDR, the local accelerating effect of the nozzle flow weakens, while with the increase in SR, the accelerating effect increases. The total pressure recovery coefficient, flow coefficient and axial thrust coefficient all decrease with the increase in AR, LDR, and SR. The thrust vector angle decreases with the increase in AR but is less affected by LDR and SR.

Electric nuclear quadrupole coupling reveals dissociation of HCl with a few water molecules

Investigating the dissociation of acids in the presence of a limited number of water molecules is crucial for understanding various elementary chemical processes. In our study, focusing on HCl(H 2 O) n clusters (where HCl is hydrogen chloride and H 2 O is water) formed in a cold and isolated jet expansion, we used the nuclear quadrupole coupling tensor obtained through rotational spectroscopy to decipher the nature of the hydrogen-chlorine (H-Cl) chemical bond in a microaqueous environment. For n = 1 to 4, the H-Cl bond is covalent. At n = 5 and 7, the contact ion pair of H 3 O + Cl − is spontaneously formed within the hydrogen bond networks of book and cube acid-water clusters, respectively.

Competition between O-H and S-H Intermolecular Interactions in Conformationally Complex Systems: The 2-Phenylethanethiol and 2-Phenylethanol Dimers.

Noncovalent interactions involving sulfur centers play a relevant role in biological and chemical environments. Yet, detailed molecular descriptions are scarce and limited to very simple model systems. Here we explore the formation of the elusive S-H···S hydrogen bond and the competition between S-H···O and O-H···S interactions in pure and mixed dimers of the conformationally flexible molecules 2-phenylethanethiol (PET) and 2-phenylethanol (PEAL), using the isolated and size-controlled environment of a jet expansion. The structure of both PET-PET and PET-PEAL dimers was unraveled through a comprehensive methodology that combined rotationally resolved microwave spectroscopy, mass-resolved isomer-specific infrared laser spectroscopy, and quantum chemical calculations. This synergic experimental-computational approach offered unique insights into the potential energy surface, conformational equilibria, molecular structure, and intermolecular interactions of the dimers. The results show a preferential order for establishing hydrogen bonds following the sequence S-H···S < S-H···O ≲ O-H···S < O-H···O, despite the hydrogen bond only accounting for a fraction of the total interaction energy.

Mechanism of Evolution of Shock Wave of Muzzle Jet under Initial Interference and Its Simplified Model

Large-caliber and long-barrel weapons are important experimental devices for exploring the impact resistance and reliability of warheads. The force of impact of the muzzle jet has a significant influence on the overload resistance of the warhead and surrounding devices. The mechanism of motion of the body inside the tube cannot be ignored owing to the high kinetic energy at the muzzle. In this study, we designed the relevant experiment and a simulation model to analyze the structural characteristics and mechanism of evolution of the shock wave and the vortex structure in a muzzle jet. The aim was to examine the evolution of the shock wave with initial jet-induced interference. And we established three other simulation models to compare the similarities and differences between the results of the models. The results showed that, in the original complex model, the initial jet restricted the free expansion of the muzzle jet, and this led to many shock–shock collisions that retarded the development of the shock waves. Multiple reflected shock waves were thus formed under a high local pressure that distorted the shock structure, while the structure of the shock wave in the simplified models was clear and simple. The parameters of motion of the body changed by a little when the initial jet-induced interference was ignored. The difference in values of the strongest impact force measured at monitoring points far from the muzzle was small, with an error of about 2%, such that the simplified model without the initial jet could be used in place of the original complex model. The other simplified models yielded significant differences. Our results provide an important theoretical basis for further research on the muzzle jet and its applications in engineering.

Gamma-Ray Burst Pulses and Lateral Jet Motion

We propose that gamma-ray burst (GRB) pulses are produced when highly relativistic jets sweep across an observer’s line of sight. We hypothesize that axisymmetric jet profiles, coupled with special relativistic effects, produce the time-reversed properties of GRB pulses. Curvature resulting from rapid jet expansion is responsible for much of the observed pulse asymmetry and hard-to-soft evolution. The relative obliqueness with which the jet crosses the line of sight explains the known GRB pulse morphological types. We explore two scenarios: one in which a rigid/semirigid jet moves laterally and another in which a ballistic jet sprays material from a laterally moving nozzle. The ballistic jet model is favored based upon its consistency with standard emission mechanisms.

Numerical investigation in effects of refrigerants’ thermodynamic properties on performance of supersonic ejector

ABSTRACT Using zeotropic mixtures with unique temperature glide characteristics as the working medium in heat-driven ejector refrigeration system will potentially reduce the irreversible losses in heat exchange process. However, the intrinsic link between the pure/mixture refrigerants’ thermophysical properties and the entrainment performance of supersonic ejector has not yet been elucidated. In this study, a novel ejector CFD model is established by coupling the real-gas thermodynamic equation and species transport equation, then the relationships between thermophysical properties of pure/mixture refrigerants and ejector’s dimensionless operational parameters were thoroughly investigated. The results indicated that the ejector expansion ratio rises as primary flow inlet temperature or refrigerant’s normal boiling point increases, while the dimensionless parameters of primary nozzle are relatively insensitive to the variation of primary flow inlet temperature and refrigerant’s boiling point, resulted in changes of supersonic jet expansion behaviour. Moreover, the specific volume and adiabatic index change characteristics resulted in minor fluctuations of primary nozzle’s dimensionless parameters. When the bubble point temperature is fixed at 30℃ in the condenser, the saturated vapor/liquid phase pressure glide characteristics of R600a/R1234ze mixture may lead to an increasement of ejector’s compression ratio.

On the M87 jet structure near the central engine

ABSTRACT At present, there is no doubt that relativistic jets observed in active galactic nuclei pass from highly magnetized to weakly magnetized stage, which is observed as a break in the dependence on their width $d_{\rm jet}(z)$ on the distance z to the central engine. In this paper, we discuss the possibility of observing another break, which should be located at shorter distances. The position of this break can be associated with the region of formation of the dense central core near the jet axis which was predicted both analytically and numerically more than a decade ago, but has not yet received sufficient attention. In this case, the observed width should be determined by the dense core, and not by the total transverse size of the jet. The calculations carried out in this paper, which took into account both the transverse electromagnetic structure of the jet and the change in the spectrum of emitting particles along its axis, indeed showed such behaviour. We also found the evidence of the predicted break in the jet expansion profile using stacked 15 GHz Very Large Baseline Array (VLBA) image of M87 radio jet and constrain the light cylinder radius.

Rotational Spectroscopy and Conformational Flexibility of 2-Phenylethanethiol: The Dominant S-H⋅⋅⋅π Intramolecular Hydrogen Bond.

We present a rotational-computational investigation of the aromatic mercaptan 2-phenylethanethiol, addressing its potential energy surface, conformational equilibrium, internal dynamics and intramolecular interactions. The experiment used broadband chirped-pulse Fourier transform microwave spectroscopy in a supersonic jet expansion, recording the rotational spectrum in the 2-8 GHz frequency region. Two different conformers were detected in the spectrum. The most intense transitions correspond to a skew (gauche-gauche) conformation, identified as the global minimum. The spectra of ten different isotopologues were assigned for this species, leading to accurate effective and substitution structures. The weaker spectrum presents small tunnelling doublings caused by the torsional motion of the thiol group, which are only compatible with an antiperiplanar skeleton and a gauche thiol. The larger stability of the global minimum is attributed to an intramolecular S-H⋅⋅⋅π weak hydrogen bond. A comparison of the intramolecular interactions in the title molecule and 2-phenylethanol, similarly stabilized by a O-H⋅⋅⋅π hydrogen bond, shows the different strength of these interactions. Density functional (B3LYP-D3, B2PLYP-D3) and ab initio (MP2) calculations were conducted for the molecule.

Flow characteristics in a semi-confined circular-pipe impinging jet

The impinging jet is a complex heat and mass transfer technique that involves several process variables, such as the jet Reynolds number, impingement distance, and jet configuration. In this study, the flow characteristics of a semi-confined circular-pipe impinging jet over different Reynolds numbers and impingement distances were experimentally investigated using a two-dimensional particle image velocimetry technique. The confinement was achieved by positioning a plate parallel to the impinging plate at the nozzle exit. The time-averaged velocity field exhibited a recirculation structure that gradually shifted downstream with increasing Reynolds numbers or impingement distances. Notably, at H/d = 2, this downstream shift of the structure was accompanied by an increase in the vortex intensity. Moreover, the confined plate induced alterations in the overall flow pattern within the confined region, significantly reducing the wall jet decay rate compared with both unconfined and confined radial wall jets for H/d ≥ 3. Conversely, the confinement did not affect the expansion of the wall jet. Unlike the free (unconfined) impinging jets, the semi-confined circular-pipe impinging jet did not exhibit self-similar behavior in the conventional outer-scaled coordinates, particularly concerning the turbulence intensity and Reynolds shear stress. Finally, self-similarity in the time-averaged velocity and various turbulence parameters was achieved using the parameter scale proposed in this study, thereby obtaining the corresponding scaling laws in the wall jet region. Our study results can deepen the current understanding of the flow characteristics of semi-confined circular-pipe impinging jets and are significant for optimizing the performance and efficiency of compact electronic packaging equipment.

Binary conformers of a flexible, long-chain fluoroalcohol: dispersion controlled selectivity and relative abundances in a jet.

The complex conformational panorama of binary 4,4,4-trifluoro-1-butanol (TFB) aggregates was investigated using chirped-pulse Fourier transform microwave spectroscopy, aided by conformational searches using CREST (Conformer-Rotamer Ensemble Sampling Tool) and quantum chemistry calculations. From nearly 1500 initial dimer geometries, 16 most stable binary candidates were obtained within a relative energy window of ∼4 kJ mol-1. Rotational spectra of five binary conformers were experimentally observed in supersonic expansion and assigned. Interestingly, three out of the five observed binary conformers are composed solely of monomer conformers, which were not observed in their isolated gas phase forms in jet expansion. In addition, an observed dimer that is made exclusively of the most stable TFB monomer subunits does not correspond to the global minimum. The intricate kinetically and thermodynamically controlled dimer formation mechanisms are discussed, and a modified kinetic-thermodynamic model was developed, providing conformational abundances that are in good agreement with the experiment. Subsequent non-covalent interaction analyses reveal that the observed conformers are held together by one primary O-H⋯O hydrogen bond and secondary intermolecular C-H⋯O, C-H⋯F, and/or O-H⋯F interactions, as well as C-H⋯H-C London dispersion interactions between the methylene groups. Further symmetry-adapted perturbation theory analyses of the TFB dimer conformers and related alcohol dimers reveal a considerable rise in dispersion contributions with increasing n-alkyl carbon chain length and highlight the role of dispersion interactions in preferentially stabilizing the global minimum of the TFB dimer.

Radio outburst from a massive (proto)star

Context. Growing observational evidence indicates that the accretion process leading to star formation may occur in an episodic way, through accretion outbursts revealed in various tracers. This phenomenon has also now been detected in association with a few young massive (proto)stars (&gt;8 M⊙), where an increase in the emission has been observed from the IR to the centimetre domain. In particular, the recent outburst at radio wavelengths of S255IR NIRS 3 has been interpreted as due to the expansion of a thermal jet, fed by part of the infalling material, a fraction of which has been converted into an outflow. Aims. We wish to follow up on our previous study of the centimetre and millimetre continuum emission from the outbursting massive (proto)star S255IR NIRS 3 and confirm our interpretation of the radio outburst, based on an expanding thermal jet. Methods. The source was monitored for more than 1 yr in six bands from 1.5 GHz to 45.5 GHz with the Karl G. Jansky Very Large Array, and, after an interval of ~1.5 yr, it was imaged with the Atacama Large Millimeter/submillimeter Array at two epochs, which made it possible to detect the proper motions of the jet lobes. Results. The prediction of our previous study is confirmed by the new results. The radio jet is found to expand, while the flux, after an initial exponential increase, appears to stabilise and eventually decline, albeit very slowly. The radio flux measured during our monitoring is attributed to a single lobe, expanding towards the NE. However, starting from 2019, a second lobe has been emerging in the opposite direction, probably powered by the same accretion outburst as the NE lobe, although with a delay of at least a couple of years. Flux densities measured at frequencies higher than 6 GHz were satisfactorily fitted with a jet model, whereas those below 6 GHz are clearly underestimated by the model. This indicates that non-thermal emission becomes dominant at long wavelengths. Conclusions. Our results suggest that thermal jets can be a direct consequence of accretion events, when yearly flux variations are detected. The formation of a jet lobe and its early expansion appear to have been triggered by the accretion event that started in 2015. The end of the accretion outburst is also mirrored in the radio jet. In fact, ~1 yr after the onset of the radio outburst, the inner radius of the jet began to increase, at the same time the jet mass stopped growing, as expected if the powering mechanism of the jet is quenched. We conclude that our findings strongly support a tight connection between accretion and ejection in massive stars, consistent with a formation process involving a disk-jet system similar to that of low-mass stars.

Fuel Injector Requirements to Achieve Supercritical Flow at the Exit

Advanced engine designs and alternative fuels introduce the possibility of supercritical fuel injection in aviation gas turbines and diesel engines, as is already the case for many rocket engines. Previous studies have focused mainly on fuel–air mixing in the supercritical regime after injection. However, injector requirements to achieve supercritical flow at the exit have not been investigated systematically. In this study, supercritical flow in an injector is analyzed using computational fluid dynamics with a real gas model and fluid properties derived from Helmholtz equations of state. Three operational challenges are illustrated depending upon the fuel: 1) large decreases in pressure and temperature within the injector, 2) injector choking, and 3) supersonic expansion of the supercritical jet. These challenges are addressed by developing and validating a one-dimensional, nonisentropic model of supercritical flow in the injector. This reduced-order model can guide injector designs for different fuels and applications and help decouple the injector supercritical flow from that in the downstream chamber to significantly reduce the computational effort for fuel–air mixing simulations. Results show that larger-diameter injectors are generally required to achieve supercritical injection with a fuel energy injection rate per unit area matching that of a typical diesel injector.