Underexpanded Air Jets Research Articles

Hybrid time-frequency domain dual-probe coherent anti-Stokes Raman scattering for simultaneous temperature and pressure measurements in compressible flows via spectral fitting.

We demonstrate a hybrid time-frequency spectroscopic method for simultaneous temperature/pressure measurements in nonreacting compressible flows with known gas composition. Hybrid femtosecond-picosecond, pure-rotational coherent anti-Stokes Raman scattering (CARS), with two independent, time-delayed probe pulses, is deployed for single-laser-shot measurements of temperature and pressure profiles along an ∼5-mm line. The theory of dual-probe CARS is presented, along with a discussion of the iterative fitting of experimental spectra. Temperature is obtained from spectra acquired with an early, near-collision-free probe time delay (τ 1=0p s) and pressure from spectra obtained at probe delays of τ 2=150-1000p s, where collisions significantly impact the spectral profile. Unique solutions for temperature and pressure are obtained by iteratively fitting the two spectra to account for small collisional effects observed for the near zero probe delay spectrum. A dual-probe pure-rotational CARS system, in a 1D line-imaging configuration, is developed to demonstrate effectively the simultaneous temperature and pressure profiles recorded along the axial centerline of a highly underexpanded jet. The underexpanded air jet permits evaluation of this hybrid time-frequency domain approach for temperature and pressure measurements across a wide range of low-temperature-low-pressure conditions of interest in supersonic ground-test facilities. Single-laser-shot measurement precisions in both quantities and pressure measurement accuracy are systematically evaluated in the quiet zone upstream of the Mach disk. Precise thermometry approaching 1%-2% is observed in regions of high CARS signal-to-noise ratios. Pressure measurements are optimized at probe time delays where the ratio of the late probe delay to the Raman lifetime exceeds four (τ 2/τ R>4). The impact of low-temperature Raman linewidths on CARS pressure measurements is evaluated, and comparisons of CARS pressures obtained with our recent low-temperature pure-rotational Raman linewidth data and extrapolated high-temperature Q-branch linewidths are presented. Considering all measurements with τ 2/τ R≥4.0, measured pressures were on average 7.9% of the computed isentropic values with average shot-to-shot deviations representing a combination of instrument noise and fluid fluctuations of 5.0%.

Analysis of submerged single and twin gas jet injection using URANS-CFD simulations

This paper addresses the analysis and characteristics of submerged underexpanded air jets injected into water by means of single and twin tuyeres (i.e. straight tubes). First, a Computational Fluid Dynamics simulation of a single tuyere is compared with available experimental data found in the literature. In the twin tuyere configuration, spacing is varied and the resultant flow is compared to that obtained with the single tuyere configuration. When two tuyeres are closely placed, compared to a single tuyere injecting the same mass flow, an increase in the potential core of each jet and a combined core are found, along with an increase in mixing levels. With larger tuyere spacing, individual cores, shorter potential core length and a significant increase in mixing levels are found. In addition, a general selection guideline is presented to help the design of a process involving gas being injected into a liquid.

On the mechanism of the influence of medium density on the supersonic core length of an underexpanded jet

Exhaustion of a supersonic underexpanded air jet into air–helium and air–sulfur hexafluoride mixtures is studied experimentally. The air jet escapes from a convergent circular nozzle in the range of moderate Reynolds numbers covering the laminar and transitional regimes of the jet flow. The experiments are performed with “cold” and heated jets in a low-pressure jet setup, which allows the Reynolds number of the jet exhaustion from the nozzle, the jet pressure ratio, and the density of the gas surrounding the jet to be maintained independently. A Pitot tube is used to measure the supersonic core length of the air jet as a function of the density of the ambient medium whereto the jet exhausts. In addition, the spatial distributions of the amplitude–frequency characteristics of acoustic oscillations generated by the jet in the ambient space are measured by a moving sensor of pressure fluctuations. The effect of the ratio of the underexpanded air jet density to the ambient medium density on the supersonic core length is demonstrated. It is shown that the mechanism of influence of the ambient medium density is associated with a change in the conditions for the global instability of the underexpanded air jet and, in particular, the conditions for generation of the screech tone.

Experimental and Numerical Study of Heat Transfer of High-Enthalpy Underexpanded Air Jets with Cylindrical Models

Experimental and Numerical Study of Heat Transfer of High-Enthalpy Underexpanded Air Jets with Cylindrical Models

Turbulent mixing and molecular transport in highly under-expanded hydrogen jets

Highly under-expanded hydrogen jets releasing in quiescent air atmosphere are studied using highly resolved numerical simulations accounting for complex multicomponent molecular transport phenomena. In a first step of the analysis, the main overall features of the hydrogen jet structure are described and compared to those of the classical under-expanded air jet at the same nozzle pressure ratio (NPR). Even if the global flow topology remains quite similar in both cases (i.e., hydrogen and air discharges), the modification of both mean density and mean velocity gradients leads to different relative energy levels for each velocity component. The corresponding change of fluid properties mainly leads to an enhanced mixing at the jet periphery. In comparison to the air case, the turbulence development within the internal part of the under-expanded hydrogen jet surrounding the subsonic core also yields a different structure. While a significantly higher peak of streamwise turbulent stress is observed downstream of the reflected shock, the vorticity dynamics is dampened by viscous diffusion and velocity divergence (i.e., volumetric expansion) contributions. Then, the performance of the simplified Hirschfelder and Curtiss approximation of the multicomponent molecular diffusion phenomena is evaluated with respect to the detailed multicomponent transport representation, as deduced from the EGLIB library. The detailed representation of molecular phenomena is shown to have a significant influence on the estimated local levels of hydrogen mass flux, leading to a non-negligible alteration of the global jet structure.

Flows of Supersonic Underexpanded Jets on the Range of Moderate Reynolds Numbers

The results of an experimental investigation of the gasdynamic structure of supersonic underexpanded air jets flowing out of a sonic nozzle into a low-pressure medium are presented. This setting of the experimentmakes it possible to achieve high values of the nozzle-to-ambient pressure ratio at moderate outflow Reynolds numbers characteristic of underexpanded jets issuing from micronozzles. The data on the supersonic core length, the laminar-turbulent transition location, and the jet flow characteristics are obtained. The results are compared with those obtained in microjets flowing out of sonic nozzles. Emphasis is placed on the earlier discovered effect of inverse transition of a turbulent jet into the laminar flow regime with increase in the Reynolds number.

An experimental study of the structure of supersonic flat underexpanded microjets

We have experimentally studied the structure of supersonic flat underexpanded room-temperature air jets escaping from micro nozzles with characteristic heights from 47 to 175 μm and widths within 2410–3900 μm in a range of Reynolds numbers of 1280–9460. The dimensions of the first shock cell are established. The supersonic core length of supersonic flat underexpanded air jets has been determined for the first time. A flow regime with a large supersonic core length has been observed for air jets escaping from a 47-μm-high nozzle.

Flow structure formed due to interaction of a supersonic jet with a porous obstacle

Results of studying impingement of a supersonic underexpanded air jet onto a finite-thickness porous metal obstacle whose frontal plane is normal to the jet axis and whose side surface is impermeable for the gas flow are reported. The case of a non-porous obstacle of the same diameter is considered for determining the effect of porosity on gas-dynamic characteristics of jet–obstacle interaction.

Results of mathematical simulation of steam jet efflux in atmosphere under various modes affecting noise generation

Mathematical simulation results for various modes of the steam jet efflux into atmosphere are represented. It is shown that, to describe processes in steam jet, in particular during coherent structure formation, there is a need to use a nonstationary model. Mathematical simulation results for the steam jet efflux into the atmosphere under various modes (subsonic, transonic, and supersonic) are described. Features of the formation and emission of noise by the steam jet are shown in dependence of its parameters at the exhaust line section. The obtained results are qualitatively compared with known experimental data on the location of the acoustical center of the steam jet and for instantaneous velocity field of the underexpanded air jet.

Effect of the profile of a convergent nozzle on heat transfer distribution of a flat plate impinged by an under-expanded jet

An experimental investigation is performed to study the effect of the profile of a convergent nozzle on the local heat transfer distribution of a smooth flat surface impinged normally by an under-expanded air jet. Two convergent nozzles of different expansion profiles having same throat diameter of 5.6mm are employed. Measurements are carried out for nozzle pressure ratios (P0/P∞) of 2.36, 3.04, 3.72, 4.4 and 5.08 at different nozzle-to-plate distances (z/d=1, 2, 3, 4, 6, 8, 10, 12 and 15). Flat surface made of thin stainless steel metal foil acts as a heated target plate. Local temperature distribution is measured using thermal infrared camera and the surface pressure distribution is measured on an impingement plate. Local Nusselt numbers and the adiabatic wall temperatures are obtained using linear regression method. The recovery factor distributions for various nozzle-to-plate distances are presented. Nusselt number and the recovery factor distributions are significantly affected by the profile of the convergent nozzle particularly at lower nozzle-to-plate distances in the stagnation point region and not in the wall jet region.

Flow and heat transfer in underexpanded nonequilibrium jets of an induction plasmatron

The results of experimental and numerical investigations of heat transfer to a cylindrical flatended model in underexpanded high-enthalpy air jets, including those flowing out through cylindrical heads of an elongated channel, are presented for the case of the operating parameters of the 100-kW VGU-4 plasma gun.

1417 軸対称不足膨張噴流における凝縮衝撃波の発生(圧力比と湿り度の影響)(熱工学III)

1417 軸対称不足膨張噴流における凝縮衝撃波の発生(圧力比と湿り度の影響)(熱工学III)

ナトリウム-水反応および圧縮性混相流数値解析コードSERAPHIMの高度化 : 高度不足膨張噴流に対する解析手法の適用性検証(<特集>第14回動力・エネルギー技術シンポジウム)

SERAPHIM is a computer program for the simulation of the compressible multiphase flow involving the sodium-water chemical reaction under a tube failure accident in a steam generator of sodium cooled fast reactors. In this study, the numerical analysis of the highly underexpanded air jets into the air or into the water was performed as a part of validation of the SERAPHIM program. The multi-fluid model, the second-order TVD scheme and the HSMAC method considering a compressibility were used in this analysis. Combining these numerical methods makes it possible to calculate the multiphase flow including supersonic gaseous jets. In the case of the air jet into the air, the calculated pressure, the shape of the jet and the location of a Mach disk agreed with the existing experimental results. The effect of the difference scheme and the mesh resolution on the prediction accuracy was clarified through these analyses. The behavior of the air jet into the water was also reproduced successfully by the proposed numerical method.

Hysteretic Phenomenon of Underexpanded Moist Air Jet

Moist air jets, corresponding to the range of moderately underexpanded to strongly underexpanded conditions, have been investigated with the help of computational fluid dynamics methods. The equations for nucleation rate and droplet growth are incorporated into the unsteady, compressible Navier-Stokes equations. A third-order total variation diminishing finite difference scheme with monotonic upstream schemes for conservation lows is employed to discretize the spatial derivatives of the governing equations. The experiment is performed to validate the computational predictions. The initial degree of supersaturation is changed to obtain different values of the relative humidity of moist air. The pressure ratio is changed to investigate the characteristic behavior of the jets generated during the transient processes of startup and shutdown. The results obtained show that underexpanded air jet produced during the startup transient of jet behaves different from the shutdown transient process, leading to a hysteretic phenomenon of underexpanded jet. It is also known that the moist air jet reduces the hysteretic phenomenon compared with the dry air jet, and that nonequilibrium condensation that occurs in the underexpanded moist air jet is responsible for these findings.

Investigation of axisymmetric underexpanded air and helium jets by background oriented schlieren

Investigation of axisymmetric underexpanded air and helium jets by background oriented schlieren

Role of initial conditions on noise from underexpanded pipe jets

An experimental investigation of acoustic radiation from underexpanded air jets of different shear layer thicknesses has been performed. The initial shear layer thickness variation is achieved by allowing the jet to exit through pipes of various lengths. The lengths of the pipes considered for the present study are in the range 1≤L/D≤6. The pressure ratio of the pipe jets is varied from 3 to 7 corresponding to fully expanded jet Mach numbers of 1.35≤Mj≤1.92. Acoustic radiation is characterized in terms of overall sound pressure level, directivity, tonal, and broadband shock associated noise. Increase in initial shear layer thickness in pipe jets results in the decrease of screech tone amplitude (by up to 20 dB), and increase in broadband shock associated noise level. Turbulent mixing noise levels are higher for shorter (L/D) pipe jets compared to longer ones. Longer pipe jets exhibit more number of screech modes while the shorter pipe jets show only one or two screech modes. The screech frequency and the peak frequency of broadband shock associated noise do not show much variation with increase in initial shear layer thickness. The screech frequency of a jet with negligible shear layer thickness (orifice jet) is found to be higher compared to those of finite shear layer thickness from pipe jets.

The effect of nonequilibrium condensation on hysteresis phenomenon of under-expanded jets

Under-expanded jets which are discharged from an orifice or a nozzle have long been subject of researches for aeronautical and mechanical applications. Provided that the jet pressure ratio and nozzle configuration are known, the major features of the steady jet are now well known. However, the jet pressure ratio is often varied even during the process in many practical applications. Many questions remain unanswered with regard to how the supersonic jet responds to the transient process of the pressure ratio and whether the steady jet data for a specific pressure ratio can still bear the same during the transient process of pressure ratio. In the present study, the hysteric phenomenon of under-expanded jets has been investigated with the help of computational fluid dynamics methods. The under-expanded jets of both dry and moist air have been employed to investigate the transient processes of the pressure ratio. The effects of nonequilibrium condensation occurring in the under-expanded moist air jets are explored on the hysteresis phenomenon. It is known that under-expanded air jet produced during the startup transient of jet behaves differently from the shutdown transient process, leading to the hysteric phenomenon of under-expanded jet. It is also known that the moist air jet reduces the hysteric phenomenon, compared with the dry air jet, and that non-equilibrium condensation which occurs in the underexpanded moist air jet is responsible for these findings.

Numerical simulation of flows in a high-frequency induction plasmatron and of heat transfer in underexpanded air jets

Some results of numerical simulation of flows in the discharge channel of the IPG-4 induction plasmatron installed at the Institute for Problems in Mechanics of the Russian Academy of Sciences (IPM RAS) and in underexpanded jets of dissociated and partially ionized air from sonic nozzles are discussed. These results are in good agreement with experimental data on heat fluxes and on the pressure at the stagnation point of a blunt cylindrical body. These experimental data were obtained at the IPM RAS Laboratory for Interaction of Plasma and Radiation with Materials.

A study of under-expanded moist air jet impinging on a flat plate

When a gas expands through a convergent nozzle in which the ratio of the ambient to the stagnation pressures is higher than that of the critical one, the issuing jet from the nozzle is under-expanded. If a flat plate is placed normal to the jet at a certain distance from the nozzle, a detached shock wave is formed at a region between the nozzle exit and the plate. In general, supersonic moist air jet technologies with non-equilibrium condensation are very often applied to industrial manufacturing processes. In spite of the importance in major characteristics of the supersonic moist air jets impinging to a solid body, its qualitative characteristics are not known satisfactorily. In the present study, the effect of the non-equilibrium condensation on the under-expanded air jet impinging on a vertical flat plate is investigated numerically in the case with non-equilibrium condensation, frequency of oscillation for the flow field becomes larger than that without the non-equilibrium condensation, and amplitudes of static pressure become small compared with those of dry air. Furthermore, the numerical results are compared with experimental ones.

406 軸対称不足膨張噴流における噴流境界とマッハディスクの特性(OS4-2 高速流れと衝撃波現象の基礎と応用(噴流),OS4 高速流れと衝撃波現象の基礎と応用,オーガナイズドセッション)

406 軸対称不足膨張噴流における噴流境界とマッハディスクの特性(OS4-2 高速流れと衝撃波現象の基礎と応用(噴流),OS4 高速流れと衝撃波現象の基礎と応用,オーガナイズドセッション)