Stagnation Pressure Research Articles

Numerical investigation on flow and mixing characteristics of powder fuel under strong shear and shock wave interaction

In this investigation, the mixing process of powder fuel transverse jet and supersonic airflow are discussed. The effects of particle size and induced shock wave on flow and mixing characteristics are also analyzed. The Eulerian-Lagrange method is utilized to simulate the gas-solid two-phase supersonic flow field. This numerical method has been verified using experimental data and simulation data from the open literature. The results reveal that the powder fuel transverse jet interacts violently with the supersonic airflow and forms complex shock wave structures in the flow field. The motion and distribution of particles are affected by their size and the interaction of shock wave/particle. The powder fuel composed of small size particles enhances mixing performance and has less impact on stagnation pressure loss. The existence of induced shock waves leads to part of the particles into the cavity and the stagnation pressure loss is large. With the increase of induced shock wave intensity, the mixing performance of the local region is continuously improved. The case with the inclination angle of 30° owns the best effect with improving the mixing efficiency by 28.33%, but the total pressure loss is increased by 33.88％.

Self-similar characteristics of underexpanded, cryogenic hydrogen and methane jets

Underexpanded, cryogenic hydrogen and methane jets were measured using laser Raman scattering diagnostic. The jets were released from 1 mm to 1.25 mm orifices for the stagnation pressure ranges of 2–6 bar and temperature ranges of 37–46 K (hydrogen) and 112–189 K (methane). Raman signals are inherently small, thus a denoising algorithm was developed to substantially reduce the noise hindering the statistical analysis of the data. The time-averaged concentration and temperature data were plotted to show a hyperbolic decay law along the jet centerline and a Gaussian distribution in the radial direction. The concentration fluctuations of the cryogenic jets are similar to those of warm jets, the centerline RMS mass fraction decays similarly to the mean mass fraction, and the highest radial concentration fluctuations appear in the shear layer. Thus, the self-similar characteristics of the cryogenic jets are comparable with room-temperature jets for the present test conditions.

Detonation simulations in supersonic flow under circumstances of injection and mixing

The unsteady, reactive Navier-Stokes equations with a detailed chemical mechanism of 11 species and 27 steps were employed to simulate the mixing, flame acceleration and deflagration-to-detonation transition (DDT) triggered by transverse jet obstacles. Results show that multiple transverse jet obstacles ejecting into the chamber can be used to activate DDT. But the occurrence of DDT is tremendously difficult in a non-uniform supersonic mixture so that it required several groups of transverse jets with increasing stagnation pressure. The jets introduce flow turbulence and produce oblique and bow shock waves even in an inhomogeneous supersonic mixture. The DDT is enhanced by multiple explosion points that are generated by the intense shock wave focusing of the leading flame front. It is found that the partial detonation front decouples into shock and flame, which is mainly caused by the fuel deficiency, nevertheless the decoupled shock wave is strong enough to reignite the mixture to detonation conditions. The resulting transverse wave leads to further mixing and burning of the downstream non-equilibrium chemical reaction, resulting in a high combustion temperature and intense flow instabilities. Additionally, the longitudinal and transverse gradients of the non-uniform supersonic mixture induce highly dynamic behaviors with sudden propagation speed increase and detonation front instabilities.

Influence of the outflow initial parameters on the transverse dimensions of underxpanded argon jets in presence of condensation

The influence of the outflow initial parameters (stagnation pressure P0 and temperature T0, as well as the residual pressure in the background medium P∞) on the radii in the maximum cross sections of the «traditional» initial jet and the concurrent cluster flow («cluster wake») recently discovered by the authors is considered. The empirical dependencies proposed in the well-known studies of the gas dynamics of supersonic jets are verified, amendment models that take into account the features of the outflow of gases from a supersonic nozzle under condensation conditions are presented, assumptions about the reasons for the similarity of a «traditional» jet and an external «cluster wake» are made, limitations on the conditions for «cluster wake» formation are discussed.

Drug injection and dispersion characteristics of an air-powered needle-free injector

The present study aims to reveal the operational characteristics of the needle-free injector. Effects of operating parameters on the injection performance were experimentally investigated. A visualization experiment was performed to describe the dispersion pattern of water in gel. The results show that the peak stagnation pressure increases continuously with the driving pressure. The injection process comprises two distinct stages, which are characterized by the penetration and the dispersion of the drug, respectively. The nozzle diameter imposes a significant effect on the penetration ability of the needle-free injector. As the nozzle diameter increases, the stagnation pressure decreases nearly linearly and the injection duration is considerably shortened, but the jet power is increased. Among the three nozzle diameters investigated, the nozzle diameter of 0.25mm satisfies the proposed criterion of the injection power. It is evidenced through the visualization experiment that the maximum penetration depth increases with the nozzle diameter. The width of the projection area of the water bulk is insensitive to the nozzle diameter. For a certain nozzle diameter, the projection area of the diffused water bulk increases linearly with increasing liquid volume.

Analytical Study of the Stagnation Point Properties and Impact Forces for Non-Continuum Flow Out of Planar Exit Impinge on Inclined Flat Plate Surface

This paper investigated the stagnation point properties and the impact forces acting on the flat plate exposed to rarefied flow with arbitrary inclination angle issuing from planner nozzle. The gaskinetic method was used to investigate this problem. The analytical solutions for the drag and the shear forces were derived. The parameters that effect on drag, shear forces and their coefficients and the stagnation point properties were studied, focusing on the effect of plate inclination angle. The results were as follows: the drag and shear forces increased with the increasing the plate inclination angle, area of the plate, the temperature of issuing gas, and the temperature of the plate. The drag and the shear forces decreased with increasing the distance between the nozzle and the center of the plate and the exit speed ratio. The stagnation pressure and the heat flux coefficients increased with increasing the plate inclination angle, the exit speed ratio, and by decreasing the distance between the nozzle exit and the center of plate and the gas temperature. The direct simulation Monte Carlo method was used to validate the analytical results of this study. The comparisons between the numerical results that obtained from the direct simulation Monte Carlo method and the analytical solutions were in good agreement.

The impact of the ground on flow structure and aerodynamic characteristics of a double delta wing

The ground effect (GE) on the formation of the vortical flow over a double delta wing having sweep angles of Λ=70°/40° was examined using the Particle Image Velocimetry (PIV) and dye flow visualization methods in this study. Qualitatively and quantitatively measured results were demonstrated for angles of attack, α=15° and 25° cases in the side and cross-flow planes to reveal the detail of the flow structures. The distance, h between surfaces of the ground and the delta wing normalized with the chord length of the double delta wing, c, was selected as h/c=0.1, 0.25 0.55, and the ground-free case (GFC). The results demonstrated that the magnitudes of the strake and wing vortices at α=15° and the coiled-up vortex at α=25° progressively diminish when the delta wing moves from the free-stream region into the boundary layer region close to the ground surface, h/c=0.1. Also, the rate of the inboard movement of leading edge vortices towards the wing central axis increases on the pressure surface of the wing as the altitude of the double delta wing is gradually lowered towards the ground surface. The lift coefficient, CL of the delta wing enhances due to the development of air stagnation and ram pressure between wing and ground surfaces by lowering the level of the double delta wing from the free-stream region into the boundary layer flow region which is strongly influenced by the presence of the ground surface. Also, the drag coefficient, CD increases more than the lift coefficient, CL resulting in a deterioration of the lift to drag ratio, CL/CD under the ground effect (GE). Finally, an increment in CL/CD is much more prominent at lower angles of attack, α regardless of ground effect.

Controlling nozzle and kerf gas dynamics to manage hazardous laser cutting fume

Conventional laser cutting is optimized for cut speed and quality but does not consider process fume. When processing hazardous materials in environments where global extraction is not practical, such as during in situ nuclear decommissioning, consideration of process fume is critically important. Fume generation in laser cutting is governed by forces applied to the molten cut front; dynamic interactions between the assist-gas and the cut front control these forces. This paper investigates the causal links between key gas-dynamic features and fume generation mechanisms in laser cutting. Its aim is to provide the preliminary understanding required to design process setups targeted at reducing the fine particle fraction. During laser cutting, the assist-gas boundary layer separates (BLS) from the molten cut front and, therefore, downstream forces applied on the melt are significantly changed. In this work, the BLS point is analyzed, captured via schlieren imaging, in an idealized environment. This separation point is related to features on the cut wall of laser-cut samples, validating the gas-dynamic interactions. The samples were cut from 3 mm thick 304 stainless steel, using a 1 kW Nd:Yb fiber laser. Control of the surface stagnation pressure and gas flow boundary conditions were used to relocate the point of BLS within the cut front, and the relationship of these parameters was analyzed. The results demonstrate that shifting the point of BLS along the cut front can be used to control fume generation mechanisms. This study successfully provided the preliminary understanding necessary to manage fume generation during the laser cutting process.

Identifying the processes and characterizing the pressure zone of water jet impacting on solid surface

The difficulty in understanding the flow characteristics of water jets impacting on solid surfaces made the water jet technology requiring further investigations. The Arbitrary Lagrange-Euler (ALE) method is used to develop a numerical model of the water jet impacting on a solid surface. It is also utilized in describing the flow characteristics of water and the pressure evolution in water during the impact process, understanding the formation mechanism of the pressure zone, and clarifying the influence factors of the stagnation pressure zone. The results showed that the pressure zone was characterized by the shock wave produced by the water jet impacting the solid surface. When the shock wave was contained in the water, it was the water hammer pressure stage. However, when the shock wave in water was disappeared, the stage was identified as the stagnation pressure. Besides, the water hammer pressure caused a high speed radial jet, and the stagnation pressure resulted in a low speed and stable radial jet. The stagnation pressure zone was the self-buffer zone of the water jet impacting the object, and the geometric characteristics of this zone were affected by the water jet diameter and the impact angle.

Flame–turbulence interactions during flame acceleration using solid and fluid obstacles

A combination of solid and transverse jet obstacles is proposed to trigger flame acceleration and deflagration-to-detonation transition (DDT). A numerical study of this approach is performed by solving the reactive Navier–Stokes equations deploying an adaptive mesh refinement technique. A detailed hydrogen–air reaction mechanism with 12 species and 42 steps is employed. The efficiency and mechanisms of the combined obstacles on the flame acceleration are investigated comprehensively. The effects of multiple jets, jet start time, and jet stagnation pressure on the DDT process are studied. Results show that there is a 22.26% improvement in the DDT run-up time and a 33.36% reduction in the DDT run-up distance for the combined obstacles compared to that having only solid obstacles. The jet acts as an obstruction by producing a suitable blockage ratio and introducing an intense turbulent region due to the Kelvin–Helmholtz instability. This leads to dramatic flame–turbulence interactions, increasing the flame surface area dramatically. The dual jet produces mushroom-like vortices, leading to a significantly stretched flame front and intensive Kelvin–Helmholtz instabilities, and therefore, these features produce a high flame acceleration. As the jet operation time decreases, the jet obstacle almost changes its role from both physical blockage ratio and turbulence and vorticity generator to a physical blockage ratio. There is a moderate jet stagnation pressure that reduces the run-up time to detonation and run-up distance to detonation in the obstacle-laden chamber. While further increasing the jet stagnation pressure, it does not have a positive effect on shortening the detonation transition.

Operational feasibility study of stagnation pressure reaction control for a mid-caliber non-spinning projectile

Controlled, guided munitions can reduce dispersion in the shot, while providing the capability of engaging both stationary and maneuvering targets. The Netherlands Organisation for Applied Scientific Research has developed a fin-less control technology called Stagnation Pressure Reaction Control (SPRC) that takes stagnation pressure air and directs it sideways to control non-spinning projectiles. In a previous study, this technology was demonstrated at Mach 2 wind-tunnel conditions to achieve up to 1.5° controllable angle of incidence for a non-spinning, aerodynamically unstable projectile-like test object. In an operational scenario, the decelerating projectile will experience a decline in control force while the simultaneous forward shift of the center of pressure increases the need for control force. Furthermore, angles of incidence exceeding 1.5° will be experienced under realistic flight conditions, especially against maneuvering targets. This work addresses these challenges and presents an operational feasibility study for a practical application of SPRC in a non-spinning mid-caliber gun-launched projectile, using experiment data on control latency and force of the earlier study. It illustrates the combined effect of the control- and stability dynamics and underlines the potential of an SPRC projectile as a precision-operation ammunition. This research revealed that SPRC technology can stabilize and control the hypothesized projectile in a direct fire scenario against stationary and maneuvering targets.

On the scattering of entropy waves at sudden area expansions

In this work, we investigate both numerically and theoretically the sound generated by entropy waves passing through sudden area expansions. This is a canonical configuration representing internal flows with flow separation and stagnation pressure losses. The numerical approach is based on a triple decomposition of the flow variables into a steady mean, a small-amplitude coherent part, and a stochastic turbulent part. The coherent part contains acoustic, vortical, and entropy waves. The mean flow is obtained as the solution of the Reynolds-Averaged Navier–Stokes (RANS) equations. The equations governing the coherent perturbations are linearised and solved in the frequency domain. To account for the effect of turbulence on the coherent perturbations, a frozen eddy viscosity model is employed. When entropy fluctuations pass through the area expansion, the generated entropy noise behaves as a low-pass filter. The numerical predictions of the noise at low frequencies are compared to the predictions of compact, quasi-one-dimensional, and isentropic theory and large discrepancies are observed. An alternative model for the generated entropy noise tailored for area expansions is then proposed. Such model is based on the conservation of mass, momentum, and energy written in integral form. The model assumes zero frequency and the one-dimensionality of the flow variables far upstream and downstream of the expansion. The predictions of this model agree well with the numerical simulations across a range of finite subsonic Mach numbers including low, intermediate, and high Mach numbers. The contributions of this work are both numerical and theoretical. Numerically, a triple decomposition adapted to high-Mach-number, compressible flows is introduced for the first time in the context of acoustic simulations. From a theoretical point of view, the quasi-steady model proposed here correctly captures the low-frequency entropy noise generated at sudden area expansions, including at high subsonic Mach numbers.

Choked flow behavior of helium-4 at cryogenic temperature

Physical characteristics of choked helium have a significant impact on heat and mass transfer in helium cryogenic systems. Below the liquid hydrogen temperature region, the choked features of helium are numerically calculated, analyzed, and experimentally validated. Stable, one-dimensional, isentropic flows are assumed in the calculations. To analyze cryogenic helium single-phase and two-phase choked states, the developed algorithm with a mass flux criterion includes homogeneous models and different slip models. At stagnation temperatures ranging from 4 to 20 K and larger stagnation pressures (from 0.02 to 2.3 MPa), the choked parameters (pressure, temperature, sound velocity, mass flux, and critical pressure ratio) are calculated. According to the results of the analysis, two-phase choking occurs when the stagnation pressure is less than 300 kPa and the stagnation temperature is less than 6.0 K. In the experiment, the mass flow rates were tested by varying the inlet pressure and temperature of the micro-orifice (34 μm). The trends of mass fluxes calculated using the homogeneous model well match the experimental data. The reason for the difference between experimental and theoretical values is that the computational model does not account for actual fluid losses (structural impedance) and deviation of the helium physical property assumption in the two-phase region. The present study's findings are expected to improve the understanding of a cryogenic helium choked flow behavior and the limitations of theoretically choked flow models currently used in cryogenic systems.

Study on suction pressure loss near suction end and stagnant pressure rise in p-θ diagram of Twin-screw refrigeration compressor for chiller

• A twin-screw refrigeration compressor is tested under different operating conditions. • Two phenomena of efficiency loss are analyzed and causes are founded. • A three-dimensional CFD model is established to further analyze these phenomena. • Some optimization to avoid these efficiency losses is proved to be effective. To improve the efficiency of the twin-screw refrigeration compressor, two pressure characteristics in the p-θ diagram, such as suction pressure loss near the suction end (SESPL) and stagnant pressure rise in the compression process (CPSPR), were investigated in this paper. A three-dimensional CFD model of a twin-screw refrigeration compressor for chillers was established to analyze the generating mechanism of these phenomena. The p-θ diagram and performance of the compressor were tested and compared under variable working conditions, to verify the simulation results. Analysis results showed that the SESPL is caused by the small flow area between the working volume and suction chamber, and the CPSPR is mainly caused by inflow and outflow in the closed cavity of the vapor injection port. On the basis, some improvement directions were put forward to avoid these efficiency losses, i.e. appropriately delaying the suction end angle and blocking the vapor injection port. Experimental results indicated that the volumetric efficiency and total efficiency both increased by about 2% under different condensing temperatures after delaying the suction end angle of the axial suction port by 18° and blocking the vapor injection port of the compressor.

Steady flow of power-law fluids past a slotted circular cylinder at low Reynolds number

Steady laminar flow past a slotted circular cylinder was investigated for non-Newtonian power-law fluids at the low Reynolds number (Re) range (5 ⩽ Re ⩽ 40). Flow simulation was carried out for shear-thinning fluids with their power-law indices (n) varying from 0.2 to 1 (n = 0.2, 0.4, 0.6, 0.8, and 1). The normal (case A) and the slotted (case B) circular cylindrical geometries were considered, where the slit was placed between the front and the base pressure stagnation points. A finite volume method was used to calculate the flow field. The flow characteristics, such as flow separation angles, wake size, coefficients of pressure (Cp), and drag (CD), were studied for different Re and n values. For all n values, the slotted cylinder effectively delayed the flow separation. It showed much better pressure recovery than the normal cylinder due to the interaction between the self-bleed from the slit exit to the cylinder wake. The vorticity of this bleed influenced the wake's vorticity, and an increase of 3%–26.4% in higher maximum surface vorticity was reported for the slotted cylinder. An increase of 0.7%–6.5% in the bubble length was observed for the normal cylinder due to early flow separation. An enhanced pressure recovery across the slotted cylinder resulted in a significant drop in the pressure drag with 0.2%–4.56% reduction in the overall drag coefficient.

Effects of Geometry of Plenum Chamber on Losses in the Bleed System of an Axial Compressor

To study how the geometry of the plenum chamber in the bleed system of an axial compressor influences the losses therein and to establish a theoretical loss model for this system, a 1.5-stage axial compressor with a bleed system is studied numerically by means of computational fluid dynamics (CFD). The results show that losses in the bleed system occur mainly in the axisymmetric bleed slot and plenum chamber, accounting for ca. 85% of the total loss. For a bleed system with a vertical axisymmetric slot, the loss is more sensitive to the radial height than to the axial width of the plenum chamber. A loss model for each part of the bleed system is established via theoretical analysis, and then, a model of the overall bleed system is established by combining these submodels. The predictions of the theoretical loss model agree well with the CFD results: the maximum prediction error for the coefficient of the stagnation pressure loss in the bleed system is −1.38%, and the average prediction error is −0.8%. This loss model can be used when designing the geometry of a bleed system.

Numerical study on unsteady characteristics of high-pressure hydrogen jet ejected from a pinhole

The aim of this study was to delineate the unsteady fluid dynamics of the high-pressure hydrogen jet to clarify the relationship between the forced ignition position and the flame development characteristics in a high-pressure hydrogen jet leaking from a pinhole. The Navier–Stokes equation for a compressible multi-component gas was used to simulate a high-pressure (82 MPa stagnation pressure) unsteady hydrogen jet ejected into the atmosphere through a pinhole (diameter = 0.2 mm). The results indicated that the flapping jet at the base of the jet formed a cloud of highly concentrated hydrogen that flowed downstream. A correlation was observed between the spatio-temporal distribution of hydrogen concentration and velocity was observed. The unsteady high-pressure hydrogen jet obtained by simulation will be used in subsequent studies focusing on flame development under forced ignition.

Flame characteristics of under-expanded, cryogenic hydrogen jet fire

In this work, cryogenic hydrogen fires at various pressures and initial temperatures were investigated experimentally. The flame length, width, thermal radiation fluxes and flame temperatures in the downstream region were measured for the scenarios with 1.6–3 mm jet nozzle, 133 to 273 K and 2–4 barabs. The results show that the flame size is related to not only the jet nozzle diameter but also the stagnation pressure and temperature. Under constant pressure, the flame size, total radiative power and radiation fraction increase with the decrease of temperature, due to lower choked flow velocity and higher density of cryogenic hydrogen. A correlation of normalized flame length Lf/dj is proposed based on the stagnation pressure of hydrogen p0 and the ratio of ambient temperature and hydrogen stagnation temperature Tatm/T0. The results verify that such correlation can be used to a priori prediction of flame size at a wide variety of hydrogen temperatures (133 K ∼ room temperature). Additionally, based on the reciprocal function, a correlation of the radiative fraction χrad is presented as a function of global flame residence time τf.

Research on Flow Field Characteristics in Water Jet Nozzle and Surface Damage Caused by Target Impact

As a new processing method, water jet processing technology has risen rapidly due to its wide range of applications, no pollution, and zero discharge. In this paper, the flow characteristics and failure characteristics of ultra-high-pressure gas-liquid jet in the range of 300 MPa are analyzed by numerical calculation. The research conclusion shows that the jet atomization diffusion is caused by the friction between the liquid medium and the surrounding gas, the mixed flow of broken water droplets and cavitation. The jet diffusion process is essentially the energy exchange process between the jet in the core area and the turbulent flow in the atomization area. The distribution of the turbulent kinetic energy in the atomization area can determine the degree of jet diffusion and the rate of energy decay. The water jet impacted the surface of the target to form a crater-like annular erosion pit. With the increase of the impact pressure, the deformation showed an overall increasing trend, and the increasing trend increased significantly. The central depression of the erosion area is caused by the damage of the material by the stagnation pressure in the core area. The flow characteristics of gas-liquid flow in the process of formation and diffusion in the high-pressure water jet nozzle are explored from the microscopic point of view, and it also provides a theoretical basis for equipment optimization in engineering.

Artificial neural network based shape optimization of supersonic ejectors in the critical flow regime

Supersonic ejectors are widely used in energy, refrigeration, and aerospace propulsion applications. A supersonic ejector is a passive gasdynamic device consisting of a converging–diverging nozzle located in a varying area mixing duct where the secondary fluid is compressed by mixing with high momentum primary fluid. The entrainment ratio (ER, ratio of mass flow rates of the secondary to primary flow) is dependent on the mixing duct geometry, necessitating shape optimization. Complex gasdynamic interactions in the ejector are not fully understood, thus making physics-based or CFD modeling, a challenging exercise. In this paper, experimental data for ejector performance with varying geometries and operating parameters, viz. area ratio, primary flow Mach number, mixing duct length to height ratio (L/D), and stagnation pressure ratio is presented. The mixing length parameter is quantified using data derived from high-speed schlieren images. Improvement in ER to the extent of 20% is achieved by changing L/D to 15 for the AR of 3.33, with further increase in L/D resulting in a reduction in ER. Subsequently, computationally inexpensive model using ANN is developed to predict the ejector performance using the experimental database. The model predicts ER with an accuracy of 7%. Finally, the ANN model is used to optimize the ejector geometry for critical mode operation, which provides a 30% enhancement in entrainment ratio compared to the baseline geometry.