High Stagnation Pressure Research Articles

Analytical Model of Critical Ventilation Flow Rate for Accidental Hydrogen Leakage in a Confined Space

The determination of the critical ventilation flow rate is significant for risk control and standard development during accidental hydrogen leakage in a confined space with hydrogen-related equipment. This paper presents an analytical model for calculating the critical ventilation flow rate through the quantification and constraint solution of the ventilation effect and ventilation cost. The experimental method was used to investigate the effects of nozzle diameter and stagnation pressure on the diffusion and ventilation of horizontal hydrogen leakage in a cuboid chamber. Ventilations from 30 to 180 m3/h were carried out through the rectangular vent. It was shown that the peak concentration of the measuring point was positively correlated with the stagnation pressure and the nozzle diameter. The experimental data were used to verify the analytical model by calculating the effective ventilation time. This study demonstrates that the critical ventilation flow rate can be increased significantly at higher stagnation pressures and larger nozzle diameters. Furthermore, the discrepancy of critical ventilation flow rates under different nozzle diameters will be enhanced with the increase of stagnation pressure. For a stagnation pressure of 0.4 MPa, the critical ventilation flow rate under a 4 mm nozzle even increased by 52% relative to the 2 mm nozzle.

High pitch sweep rates in supersonic wind tunnel tests

A typical test in a high-Reynolds-number blowdown wind tunnel, such as the VTI T-38 facility in Belgrade, involves a model sweep in a range of angles of attack. Increasing pitch sweep rates can bring two-fold benefit. The angle-of-attack range can be extended in tests at high stagnation pressures with limited run time, or test time can be shortened for the desired angle-of-attack range in other tests, in both cases bringing significant energy savings. The potential effect of the increased pitch rates on measurement data in the VTI T-38 wind tunnel was investigated in a recent supersonic wind tunnel test campaign of a hypervelocity standard model. The Mach 2 tests were performed at pitch rates of 2, 3, 4, 8 and 12° per second. While the wind tunnel balance measurement data were not seemed to be affected, the base pressure data revealed a high dependency on changing pitch sweep rates. It was found that the high lag in pressure piping affected the measured base pressure and, consequently, the forebody axial force, which is an important fact to know when dealing with ‘base drag corrected’ supersonic data. The conclusions derived from the Mach 2 tests have been verified in the additional Mach 4 tests. The verification tests confirmed that, for the currently employed base pressure measurement system, reliable measurement results can be obtained in wind tunnel tests by applying pitch rates up to 3° per second. The recommendations for modifications of the base pressure measurement system have been given.

Low-surface-temperature jump behavior of C/SiC composites prepared via precursor impregnation and pyrolysis in high-enthalpy plasma flows

The response of C/SiC composites prepared via precursor impregnation and pyrolysis was investigated in a 1 MW plasma wind tunnel. Under a considerable aero heating of up to 26.2 MJ/kg of specific total enthalpy, the samples were exposed to heat fluxes exceeding 5.7 MW/m2 and low pressures of 4.5–6.6 kPa. The samples were able to withstand low heat fluxes and low stagnation pressures, and their carbon-rich nature improved the thermal conductivity, presenting a low steadystate surface temperature. However, a spontaneous jump in the surface temperature at around 1700 °C was observed at high heat fluxes and high stagnation pressures. The jump temperature was lower compared with that reported in previous studies, and was found to increase rapidly to temperatures above 2000 °C. This low-temperature jump phenomenon was associated with the evolution of microstructure during testing, and the underlying mechanism was revealed through the use of thermodynamics analysis.

Characterization of a radio-frequency inductively coupled electrothermal plasma thruster

A radio-frequency (RF) inductively coupled electrothermal plasma thruster operating with argon is experimentally characterized for different mass flow rates, RF powers, and propellant injection configurations. Depending on the propellant mass flow rate, significant neutral gas heating is observed with effective stagnation temperatures around 2000 K (giving a maximum estimated thrust and specific impulse of about 100 mN and 125 s, respectively) for absorbed powers between 300 and 500 W. A self-consistent theoretical discharge model is developed and used to study the basic physics and operation of RF electrothermal thrusters, and predictions of the gas temperature are in good agreement with experimental measurements. The model identifies primary power inefficiencies as electron-neutral excitation losses and neutral gas heat losses to the thruster walls. Both experimental and theoretical results indicate that a relatively high stagnation pressure (of the order of 100 Torr or higher) is critical for high performance. For pressures significantly below this the electron-neutral collisional power transfer is too low to effectively heat the neutral gas.

고온유체의 벤츄리노즐 공동현상 수치해석

Understanding cavitation dynamics considering thermodynamic effect is critical to handle specific cavitation phenomena such as high temperature fluid flow or cryogenic fluid flow. In this study, a numerical analysis has been performed to investigate a the Venturi nozzle cavitation characteristic at 5 different points; 4 0℃, 50℃, 65℃, 80℃ and 95℃. A compressible homogeneous mixture model is applied, which is based on a dual-time preconditioning method. The results show that the evolution of cavitation in the Venturi nozzle and its cyclic motion can be described by re-entrant jet behavior, which is formed at a moment when high stagnation pressure occurs at the end of the cavity. After the re-entrant jet cuts off the head of the leading part, cloud shedding cavitation detaches from the main body of the cavity and it starts a vortex motion. In addition, increasing fluid temperature from 50℃ to 80℃ makes thermal effects dominant, which suppress and interrupt further development of cavitation due to the decrease of the local vapor pressure as cavitation occurs. Nonetheless, the 95℃ case, which is almost close to the boiling point of water, induces a thick and long cavity due to its higher saturated vapor pressure value when compared to others.

Flow Characterization of Bluff Bodies: A Two-Dimensional Transformation From Square to Triangular Cylinder

Abstract In the present study, two-dimensional unsteady, incompressible flow around a square body that is being transformed into a vertex oriented towards the flow configuration of a triangular body is numerically investigated at Re =100 using ANSYS FLUENT 19.0 software. The purpose is to explore the effect of this transformation on the wake characteristics of a square body with l/d = 1 to a triangular body with l/d = 0; where l is the length of lateral and front surface, and d is the body height. The effect on the flow behavior caused by the leading-edge transformation from the prospect of wake width, recirculation length and stagnation pressure difference is discussed. It is seen that as the l/d ratio decreases, the vortex strength increases which is attributed to the higher stagnation pressure difference value resulting in more intense rolling of the shedding vortex and a smaller wake width. For lower l/d, the fluid traverses a longer distance along the lateral surfaces resulting in greater loss of momentum and hence the lower vortex formation length. The mean drag coefficient is found to be minimum for l/d = 0.75 with stagnation pressure difference and recirculation length being the more dominating factor on this variation. The flow in all the cases separates at the rear surface and the general trend of decrease in drag coefficient with decrease in wake width is not followed. However, such modification leads to better aerodynamic outcome by weakening the periodic drag and lift forces.

Impaction of thin films from polar solvent Si/SiOx nanoparticle inks

Impaction techniques have so far been efficiently used for formation of films from nanoparticles when coupled to synthetic process. However, this approach limits the choice of shapes and compositions of particles, so techniques of impaction of ready-made particles are desirable. To this end, the use of nanoparticle inks as a source of solid material is first described in this paper. The films are cast from dispersions of polar solvent-susceptible Si/SiOx nanoparticles. Water, ethanol, and acetonitrile are studied for solvents, and the latter two are shown to be suitable, as they possess lower surface tension and lower boiling points. Microscopically uniform films of nanoparticles are formed when aerosol and substrate are heated, with density of films approaching 40% of that of the bulk material. Higher stagnation pressure is beneficial for deposition, while further heating of the aerosol beyond the solvent evaporation can cause oligomerization of the solvent. Deposition of dense films in subsonic part of the jet appears possible with the technique.

The interaction between the supersonic molecular beam and electron beam for the optimization of an electron ionization ion source

The interaction between the supersonic molecular beam (SMB) and electron beam is a key issue in the design of electron ionization mass spectrometry with SMB (EI-MS with SMB). In this paper, one EI ion source prototype was designed based on an electron gun, whose filament was far away from the interaction range and the influence of the filament field could be neglected. The interaction was studied by one high-speed camera and one Faraday cup. The ion flux as well as the variation with stagnation pressure and axial distance was studied. The experimental results were consistent with the simple simulated results about the ionization and photon emission process in the interaction. It was found that higher stagnation pressure was necessary for higher electron energy to acquire the maximum ion flux. The optimized parameters for the EI ion source including the axial position, stagnation pressure, and electron energy were reported. These results were meaningful for the design and optimization of the ion source.

Effects of component geometries and inflow conditions on ejector operational mode

The ejector system is a useful device for creating high altitude conditions for ground tests of supersonic engines. The ejector performance can be immediately changed by varying the ejector operational mode. The ejector nozzle pressure ratio is well known to have a significant effect of the operational mode of an ejector. However, the effects of the mixing duct length and other geometric design parameters on the ejector mode change have not been clearly determined. In this study, the effects of ejector component geometries and inflow conditions on ejector operational mode are investigated by numerical analysis. By changing the inflow conditions and geometric parameters, twelve test cases are studied. Using the numerical test results, the flow pattern and suction pressure performance of the ejector with a fixed secondary mass flow rate are compared. In the numerical test results, a high primary nozzle stagnation pressure induces a highly underexpanded flow, resulting in the critical operational mode. For the critical operational mode, the mixing duct must be sufficiently long to accommodate the shock train and the choking zone. Primary nozzles with wide angles also induce widely expanded nozzle flows and result in the critical operational mode.

Use of Dimensional Analysis in the X-, Y-, and Z-Axis to Predict Occurrence of Injury in Human Stampede.

Human stampedes (HS) may result in mass casualty incidents (MCI) that arise due to complex interactions between individuals, collective crowd, and space, which have yet to be described from a physics perspective. HS events were analyzed using basic physics principles to better understand the dynamic kinetic variables that give rise to HS. A literature review was performed of medical and nonmedical sourced databases, Library of Congress databases, and online sources for the term human stampedes resulting in 25,123 references. Filters were applied to exclude nonhuman events. Retrieved references were reviewed for a predefined list of physics terms. Data collection involved recording frequency of each phrase and physics principle to give the final proportions of each predefined principle used a single-entry method for each of the 105 event reports analyzed. Data analysis was performed using the R statistics packages "tidyverse", "psych", "lubridate", and "Hmisc" with descriptive statistics used to describe the frequency of each observed variable. Of the 105 reports of HS resulting in injury or death reviewed, the following frequency of terms were found: density change in a limited capacity, 45%; XY-axis motion failure, 100%; loss of proxemics, 100%; deceleration with average velocity of zero, 90%; Z-axis displacement pathology (falls), 92%; associated structure with nozzle effect, 93%; and matched fluid dynamic of high pressure stagnation of mass gathering, 100%. Description or reference to principles of physics was seen in differing frequency in 105 reports. These include XY-axis motion failure of deceleration that leads to loss of human to human proxemics, and high stagnation pressure resulting in the Z-axis displacement effect (falls) causing injury and death. Real-time video-analysis monitoring of high capacity events or those with known nozzle effects for loss of proxemics and Z-axis displacement pathology offers the opportunity to prevent mortality from human stampedes.

Experimental Investigations of Film Cooling in a Conical Nozzle Under Rocket-Engine-Like Flow Conditions

The current work presents an experimental parametric study on tangential, supersonic film cooling in the expansion part of a conical nozzle. For simulating rocket-engine-like hot gas conditions, the nozzle is attached to a detonation tube providing hydrogen-oxygen combustion gas at high stagnation pressures and temperatures. The Mach number of the hot gas at the point of injection is about , whereas the coolant is injected at Mach numbers of . Within the presented study the coolant injection conditions, the hot gas conditions, and the coolant gases are varied to determine the influence of different parameters on film cooling behavior. The results indicate that film cooling effectiveness massively depends on the blowing ratio and slot height. Film cooling efficiency is also found to depend on the coolant gas heat capacity, molar mass, and the density and velocity ratio of coolant and hot gas at the point of injection. To compare the experimental data the film cooling efficiencies are calculated using the measured wall heat fluxes and a newly developed, simple approach for the calculation of heat transfer coefficients. For a theoretical investigation of the experimental results the common Goldstein model is extended for axisymmetric flows. Finally, the gained experimental data are correlated by the extended Goldstein model.

Response of Silicon Nitride Ceramics under High-enthalpy Plasma Flows

Silicon nitride ceramics were prepared by means of a hot-press sintering method. The ablation behavior was studied by plasma wind tunnel tests. The data suggested that, under low heat flux and stagnation pressure conditions, the ablation process was controlled by the atomic oxidation of Si3N4, leading to the elimination of Si3N4. By contrast, the erosion was mainly produced by the decomposition of Si3N4 under high heat flux and stagnation pressure conditions. Under these conditions, a fraction of Si phase, formed upon decomposition of Si3N4, was volatilized. The remained Si had melted due to high temperatures and scoured away from stagnation point area, generating mushroom-shaped samples.

Theoretical and Numerical Study of a Preheated Ludwieg Tube with Adiabatic Compression

This paper describes a novel hypersonic facility to reproduce the high stagnation pressures and temperatures necessary for the accurate simulation of hypersonic flows in the range Mach 5–7, while providing test times sufficiently long to study unsteady flow effects. The facility uses a preheated Ludwieg tube together with a piston-compression stage to generate reservoir temperatures difficult to achieve with electrical heating alone, but also avoiding the introduction of vitiation contaminants. The absence of shocks in the generating flow is expected to lead to increased flow quality while also providing longer test times compared with shock-driven facilities of comparable size. The operational concept is described and a condition for optimal operation is defined based on minimizing the dimensions of the heated section for a given set of desired flow conditions. A simple theoretical treatment shows that this condition constrains the ratio of the nozzle exit and Ludwieg tube diameters, assuming accurate simulation of flight conditions. A method-of-characteristics solver is used to determine the constrained facility parameters more accurately. Modeling of the unsteady adiabatic compression cycle is performed with quasi-one-dimensional finite volume computations, and various configurations are explored to minimize the pressure oscillations generated during compression.

Effect of Condensation on the Length of Strongly Underexpanded Jets Exhausting Into a Rarefied Submerged Space

Exhaustion of supersonic argon and nitrogen jets through sonic and supersonic nozzles into a rarefied submerged space at high stagnation pressures is studied experimentally. The shapes and lengths of the jets are visualized by means of detecting radiation excited in the considered flow by an electron beam. Dependences of the geometric parameters of the jets on exhaustion and clusterization conditions at low Reynolds numbers based on the reference length of the jet are obtained. It is found that the coefficient of proportionality between the length of the first “barrel” of the supersonic jet and the degree of jet expansion increases with an increase in the stagnation pressure. Empirical dependences of the proportionality coefficient on the size of clusters formed in supersonic flows are derived for the first time.

Influence of condensation on heat flux and pressure measurements in a detonation-based short-duration facility

Detonation-based short-duration facilities provide hot gas with very high stagnation pressures and temperatures. Due to the short testing time, complex and expensive cooling techniques of the facility walls are not needed. Therefore, they are attractive for economical experimental investigations of high-enthalpy flows such as the flow in a rocket engine. However, cold walls can provoke condensation of the hot combustion gas at the walls. This has already been observed in detonation tubes close behind the detonation wave, resulting in a loss of tube performance. A potential influence of condensation at the wall on the experimental results, like wall heat fluxes and static pressures, has not been considered so far. Therefore, in this study the occurrence of condensation and its influence on local heat flux and pressure measurements has been investigated in the nozzle test section of a short-duration rocket-engine simulation facility. This facility provides hot water vapor with stagnation pressures up to 150 bar and stagnation temperatures up to 3800 K. A simple method has been developed to detect liquid water at the wall without direct optical access to the flow. It is shown experimentally and theoretically that condensation has a remarkable influence on local measurement values. The experimental results indicate that for the elimination of these influences the nozzle wall has to be heated to a certain temperature level, which exclusively depends on the local static pressure.

Development of an innovative validation strategy of gas–surface interaction modelling for re-entry applications

This paper summarises the final synthesis of an ESA technology research programme entitled “Development of an Innovative Validation Strategy of Gas Surface Interaction Modelling for Re-entry Applications”. The focus of the project was to demonstrate the correct pressure dependency of catalytic surface recombination, with an emphasis on Low Earth Orbit (LEO) re-entry conditions and thermal protection system materials. A physics-based model describing the prevalent recombination mechanisms was proposed for implementation into two CFD codes, TINA and TAU. A dedicated experimental campaign was performed to calibrate and validate the CFD model on TPS materials pertinent to the EXPERT space vehicle at a wide range of temperatures and pressures relevant to LEO. A new set of catalytic recombination data was produced that was able to improve the chosen model calibration for CVD-SiC and provide the first model calibration for the Nickel–Chromium super-alloy PM1000. The experimentally observed pressure dependency of catalytic recombination can only be reproduced by the Langmuir–Hinshelwood recombination mechanism. Due to decreasing degrees of (enthalpy and hence) dissociation with facility stagnation pressure, it was not possible to obtain catalytic recombination coefficients from the measurements at high experimental stagnation pressures. Therefore, the CFD model calibration has been improved by this activity based on the low pressure results. The results of the model calibration were applied to the existing EXPERT mission profile to examine the impact of the experimentally calibrated model at flight relevant conditions. The heat flux overshoot at the CVD-SiC/PM1000 junction on EXPERT is confirmed to produce radiative equilibrium temperatures in close proximity to the PM1000 melt temperature.This was anticipated within the margins of the vehicle design; however, due to the measurements made here for the first time at relevant temperatures for the junction, an increased confidence in this finding is placed on the computations.

Ablation behavior of C/SiC-HfC composites in the plasma wind tunnel

In order to satisfy the requirements for the thermal protection system to serve at ultrahigh temperatures, carbon fiber reinforced silicon carbide-hafnium carbide (C/SiC-HfC) composites were fabricated by reactive melt infiltration method. The ablation behaviors of C/SiC-HfC composites were examined in plasma wind tunnel. The results indicated that ablation was controlled by the oxidation of HfC-SiC under dissociated oxygen conditions at low heat flux and stagnation pressure. However, a sudden temperature jump caused by the change in the catalytic surface from silica to hafnia was observed at high heat flux and stagnation pressure during ablation. The increase in surface temperature was further accelerated by the burning of carbon fiber and recombination of carbon and nitrogen on it.

Ablation behavior of C/SiC composites in plasma wind tunnel

The ablation behavior is crucial to the application of carbon fiber reinforced silicon carbide (C/SiC) composites in thermal protection systems of spacecraft to enter the atmosphere or to cruise hypersonically in near space. Plasma wind tunnel is a suitable tool to simulate the service conditions of space vehicles. In this study, we systematically studied the ablation behaviors of C/SiC composites in plasma wind tunnel. The oxidation behavior of C/SiC composites in plasma is discussed. The results indicated that the oxidation under atomic oxygen condition dominated at low heat flux and stagnation pressure; however, rapid recession mechanism by sublimation and decomposition of SiC was observed at high heat flux and stagnation pressure. The temperature jump phenomenon during the ablation performed under high heat flux and stagnation pressure was attributed to the exposure of the carbon fibers to the plasma flow after consumption of the SiC coatings.

Quantification of interparticle forces by energy controlled fragmentation analysis

A molecular beam apparatus (MBA) with an implanted time of flight (tof) measurement cell was used for energy controlled fragmentation experiments on Pt agglomerates. Due to high vacuum conditions, drag forces and stagnation pressure in front of the impaction plate slowing down agglomerates were negligible. Thus, this set-up permitted a precise and convenient detection of the impact velocity of agglomerates. Results on the velocity measurement and the related degree of fragmentation are presented. The current results on fragmentation were compared to the work of Seipenbusch, Toneva, Peukert, and Weber (2007). This was motivated by the question whether the exceedance of the theoretical van der Waals binding energy found in the above-quoted work was caused by calculating the impact velocity with the commonly employed approach of Marple (vimpact=0.85·vgas). The theoretical van der Waals binding energy was found to exceed the experimentally gained binding energy by the factor of 55.4, which was in good agreement with the comparable published work.

Self-excited oscillations of two opposing planar air jets

This paper investigates the self-excited oscillations of two co-planar air jets impinging on each other, referred to, hereafter, by opposing planar jets. The study is performed for a single jet thickness of h = 2 mm and a span-wise aspect ratio of 50. The effects of flow velocity of the jets and the distance between them are examined. The opposing planar jets are found to produce strong acoustic tones over a wide range of jet Mach number and separation distance. Phase-locked averaged particle image velocity measurements showed that the tone is generated by out-of-phase flapping oscillations of the planar jets. These oscillations are sustained by two effects. The first is the high stagnation pressure at the impingement region, which deflects the jets laterally in opposite directions away from their common centerline, while the second effect is the cross stream oscillating velocity in the entrainment regions around the jet exits, which forces the jets back towards their common centerline.