Stagnation Pressure Research Articles

Experimental Study on Cryogenic Compressed Hydrogen Jet Flames

Cryogenic compressed hydrogen (CcH2) technology combines the advantages of high pressure and low temperature to achieve high hydrogen storage density without liquefying the hydrogen, which has broad application prospects. However, the safety concerns related to cryogenic hydrogen need to be carefully addressed beforehand. In the present work, cryogenic hydrogen jet flames are experimentally investigated for various release pressures and initial temperatures. The flame length and thermal radiation flux were measured for horizontally releasing with nozzle diameters of 0.5–2 mm, temperatures ranging from 93 to 298 K, and initial pressures of 2–10 MPa. The results show that the flame length is dependent on the nozzle diameter, stagnation pressure and temperature. At a given pressure, the flame length, size and total radiant power increase with decreasing temperature, which is attributed to the lower jet flow velocity and higher density of low-temperature hydrogen. The normalized flame length Lf/D is correlated with the pressure ratio and temperature ratio. The correlation can be used to predict the flame length at various hydrogen pressures and temperatures. The normalized flame length of the cryogenic hydrogen jet flame is greater than that of the room-temperature hydrogen jet flame. The radiative heat flux of the flame can be predicted by the mass flow rate of the jet flow.

An efficient dimensionality reduction approach for modelling cryogenic hydrogen release

This study develops a precise and cost-effective method for reducing the complexity of cryogenic hydrogen unintended release. For this purpose, the dominant modes of a well-characterised cryogenic hydrogen jet (Sandia National Laboratories), with stagnation pressure and temperature fixed at 5 bar and 50 K, are extracted using the proper orthogonal decomposition (POD) method. Building on validating the large eddy simulation (LES) against experimental measurements, the velocity field within the LES results is analysed near the release point. The mass fraction half-width of cryogenic hydrogen jets grows linearly with a slope of 0.1115 against the downstream distances, resembling room-temperature jets. The first 10 extracted modes from 500 LES snapshots demonstrate dominance over the remaining modes. In addition, this study evaluates the flammable gas dispersion using LES and 10-mode POD, showing POD effectively identifies high-risk areas within hydrogen's flammability range. Minor deviations (2.1–3.3%) confirm POD's reliability for hazard assessment compared with LES. Furthermore, an in-depth analysis is performed on the radial distributions of hydrogen mass fraction and centreline hydrogen mole fraction.

Laser ablation ignition modes in a cavity-based supersonic combustor

A numerical and experimental study was conducted to investigate the Laser Ablation (LA) ignition mode in an ethylene-fueled supersonic combustor with a cavity flameholder. The experiments were operated under a Mach number 2.92 supersonic inflow, with stagnation pressure of 2.4 MPa and stagnation temperature of 1600 K. Reynolds-averaged Navier-Stokes simulations were conducted to characterize the mixing process and flow field structure. This study identified four distinct LA ignition modes. Under the specified condition, laser ablation in zero and negative defocusing states manifested two distinct ignition modes termed Laser Ablation Direct Ignition (LADI) mode and Laser Ablation Re-Ignition (LARI) mode, correspondingly. LA ignition in a local small cavity, created by depressing the flow field regulator, could facilitate the ignition mode transforming from LARI mode to Laser Ablation Transition Ignition (LATI) mode. On the other hand, the elevation of the flow field regulator effectively inhibited the forward propagation of the initial flame kernel and reduced the dissipation of LA plasma, further enhancing the LADI mode. Based on these characteristics, the LADI mode was subdivided into strong (LADI-S) and weak (LADI-W) modes. Facilitating the transition of ignition modes through alterations in the local flow field could contribute to attaining a more effective and stable LA ignition.

Particle impact in high-pressure homogenizer valves – a step towards understanding wear and cell breakup in food and beverage processing

In many liquid food processing applications using high-pressure homogenizers (HPHs), particles impact with the solid surfaces of the homogenization device. This may lead to costly wear. For some applications, impact is also postulated to control the desired cell disruption. This contribution uses computational fluid dynamics to study impact of particles on solid surfaces in HPHs, as a step towards design optimization. Effects of particle diameter, density, homogenizing pressure, and impact distance are studied. Results show impacts both on the forcer and on the impingement ring. Few particles hit the forcer, at low velocities and with low angles (‘bracing impacts’). More particles hit the impact ring. These impacts are with higher velocities and typically occur head-on. The effect of both homogenizing pressure and impact ring distance scales according to a previously suggested stagnation pressure model. Results are discussed in the light of wear and cell disruption observations.

Optimization of Low-Pressure Turbine Blade by Means of Fine Inspection of Loss Production Mechanisms

Abstract In this study, Proper Orthogonal Decomposition (POD) was applied to Large Eddy Simulations (LES) of two high-loaded low-pressure turbine cascades under unsteady inflow conditions to investigate entropy production within different parts of the blade passage. The terms for Turbulent Kinetic Energy (TKE) production, diffusion, and dissipation from the stagnation pressure transport equation were integrated across the computational domain. The POD-based method enables the decomposition of contributions from various coherent flow dynamics to TKE production and dissipation, such as the migration, bowing, tilting, and reorientation of incoming wake filaments, as well as the breakdown of streaky structures in the blade boundary layer and the formation of Von Karman vortices in the blade wake. This approach helps designers identify the dominant POD modes (i.e., turbulent flow structures) responsible for loss generation, understand their dynamics and locate where they primarily act. Using this optimization strategy, a new low-pressure turbine profile was designed. LES calculations on the optimized geometry showed that it is possible to design a higher-loaded profile with a lower global loss coefficient. The new loading distribution, characterized by stronger acceleration in the early part of the blade passage, results in reduced upstream wake migration losses and lower TKE production in the trailing edge wake zone due to early suction side boundary layer transition.

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.

Accelerated pump tube concept for hypersonic ground testing

This paper examines a new hypersonic wind tunnel concept which is referred to here as an ‘accelerated pump tube’ (APT). The facility can be considered a hybrid between a gun tunnel and an expansion tube. Like a gun tunnel, the test gas is isentropically compressed with a piston to raise its pressure and temperature; like an expansion tube, the compressed test gas is subject to an unsteady expansion to further increase its enthalpy and stagnation pressure. State-to-state analytical calculations incorporating high temperature equilibrium chemistry are presented which demonstrate that the APT is theoretically able to simulate a wide hypersonic terrestrial flight envelope from Mach 5–14, with significant capacity for pressure-length scaling. Potential practical benefits of this concept include relatively simple internal flow processes, removal of the need for a film secondary diaphragm and helium as a driver gas, and a reduction in experimental uncertainty through better defined internal boundary conditions and relatively low mechanical complexity. To assess the feasibility of the APT concept in a practical context, the paper considers how an APT based around a 25 m long, 600 mm diameter pump tube should be configured for a Mach 10 test flow. One-dimensional Lagrangian CFD analysis is used to compute piston dynamics for three different pump tube compression ratios and the associated test flows are characterised. The CFD results suggest that the concept should be viable in practice, and also validate the Mach 10 state-to-state calculations.

Experimental study on flame behaviors and dynamic parameters induced by hydrogen jet flame in an enclosed compartment

Hydrogen is considered as one of the most superior future energy carriers due to its clean, low-carbon and extensive sources. However, a wide flammable concentration range and low ignition energy of hydrogen can cause potential safety hazards, including hydrogen jet flames. Currently, a series of hydrogen jet flame experiments were performed in an enclosed compartment with internal dimensions of 1.82 m (L) × 1.99 m (W) × 1.85 m (H), using different nozzle diameters and stagnation pressures. The results indicate that the combustion process can be divided into four stages, including growth stage, relatively stable stage, stretching stage, and self-extinguishing stage. The oxygen concentration in an enclosed compartment decreases rapidly with increasing stagnation pressure. The self-extinguishing time is positively correlated with the compartment volume and negatively correlated with the mass flow rate. Based on thermal equilibrium analysis, the correlations for average gas temperature rise and oxygen concentration were proposed.

Submerged surface texturing of AISI 304L using the pulsating water jet method

Submerged jets have a variety of practical applications due to their versatility in providing efficient and environmentally friendly options for treatment in various industries. The physical background is based on the continuous water jet (CWJ) application powered via stagnation pressure. However, it is known that impact pressure is much more effective than static pressure. When the impact pressure is repeated with a high frequency per time unit, the erosive effects of water can be used even at pressures below 100 MPa, which is attractive from the point of view of the low demands of the hydraulic system. Surface modification utilising impact pressure can be achieved by employing the pulsed water jet (PWJ) method. The combination of parameters such as the traverse speed and trajectory pattern can control the number of water clusters impacting the material surface. So far, the field of application of PWJ for surface treatment has mostly been investigated water atmospheric conditions. This article focuses on the possibility of the surface modification of AISI 304L stainless steel using the PWJ method under submerged conditions. The results are compared to those obtained under atmospheric conditions. The reference samples were treated by the same technological conditions using a continuous water jet (CWJ). The affected surfaces were characterised using areal surface roughness parameters Sa, Sz, Sp, and Sv, and the surface topography and mechanism of erosion wear were evaluated by scanning electron microscopy. A significant increase in all roughness parameters was confirmed using the PWJ compared to the CWJ method (both in atmospheric and submerged conditions), which confirms the importance of using impact pressure. The surface treatment by PWJ under submerged conditions resulted in a decrease of the surface roughness parameter Sa by approximately 97% compared to atmospheric conditions at a traverse speed of 2 mm/s for perpendicular interleaved trajectory, nevertheless, the homogeneity of treatment over a larger area was improved.

Experimental study of jet and cavity coupling under vertical motion of underwater vehicle

After the underwater vehicle detaches from the launch tube and starts its engine, the interaction between the tail cavity and the high-speed jet significantly impacts its motion stability and engine efficiency. Therefore, it is crucial to study the evolution mechanism of cavities in vertical motion. However, in traditional water tunnel experiments, the evolution of the cavity differs from the actual situation of vertical launch due to the influence of buoyancy. This study investigates the dynamics and motion trajectories of tail cavities and high-speed gas jets in the vertical movement of vehicles under different Froude numbers and nozzle stagnation pressure ratios through experimentation. The experimental results show two modes of tail cavity evolution: intact cavity (IC) and foam-conical cavity (FC-CC). Increasing the Froude numbers and stagnation pressure ratios facilitates the transition of the cavity from the IC mode to the FC-CC mode, effectively suppressing the formation and intensity of re-entrant jets, thereby reducing its impact on the bottom of the vehicle and enhancing motion stability. By deriving the formula for the length of the jet core area, it was found that the jet length is linearly related to the nozzle stagnation pressure, further revealing the mechanism of cavity evolution. These findings offer a new perspective for a deeper understanding and prediction of the dynamic behavior of underwater vehicles.

Transient Plasma Wind Tunnel Testing Under Thermomechanical Loads for Reentry Break-Up Analysis

This paper introduces a novel approach for transient plasma wind tunnel testing in an arcjet facility, enabling the experimental simulation of the aerothermal loads along reentry trajectory segments. Such experiments newly allow for the investigation of the causes of break-up events. The trajectory segment is simulated by duplicating the time-resolved profiles of the characteristic flow parameters, namely, heat flux and stagnation pressure as well as the mechanical load, which represents the aerodynamic forces. In arcjet facilities, the parameters that govern the plasma condition are split into variable and constant parameters, which define the available altitude range that can be replicated. The variable conditions arc current and ambient pressure were determined with steady-state trajectory points and traces determined between these conditions resulting in a full trajectory segment. The test time then relates directly to an altitude on the trajectory segment, which allows for an experimental determination of the main break-up altitude. The methodology is applied to a trajectory segment of the transport vehicle CYGNUS OA6, covering an altitude range from 85 to 70 km. A transient break-up experiment on an aluminum bar yielded a failure at an altitude corresponding to 77.6 km, showing a thermomechanical characteristic. These results align well with the observed break-up altitude of CYGNUS OA6, showing the potential of the proposed method.

Neural Network-Based Hypersonic Crossflow Transition Model

A model is proposed for predicting crossflow transition in hypersonic flows. The model is based on transport equations for the crossflow amplification factor and a modified intermittency. The instability onset is estimated with a correlation function. The form of the transport equations is identical to that of typical transport equations for Reynolds-averaged Navier–Stokes (RANS) turbulence models, thus facilitating an easy integration into existing RANS codes. The crossflow amplification factor is estimated from local flow quantities using a neural network. The neural network is trained with a comprehensive database of linear stability theory data. The sensitivity of the amplification factor growth rate prediction to different dimensionless input parameters is evaluated. The input parameters that provide the most-accurate growth rate predictions are selected for the final form of the model. Based on the volume of the training data set, the model is expected to perform reliably for Mach numbers between 5 and 11 and within given upper bounds of the stagnation temperature and pressure. The model is validated for the HiFiRE-5 geometry, a circular cone at angle of attack, and a blunt swept flat plate. The amplification factor distributions obtained from the model are in good agreement with experimental data and linear stability analysis results.

Optimization of Thrust of a Generic X-51 Hypersonic Vehicle

A prune-and-search optimization algorithm was combined with a reduced-order model of the scramjet engine within a generic X-51 vehicle. The goal was to maximize the thrust in order to achieve thrust-to-drag ratios that equal or exceed 1, even for the difficult cases of flight at high altitudes and high Mach numbers up to 10. For the inlet to the engine, the lengths and inclination angles of three wall panels were varied. For the combustor, three parameters were varied: the diameter and number of fuel injector ports and the wall divergence angle. These variables affect both the combustion efficiency and the finite-rate chemistry of the surrogate JP-7 fuel. The optimization algorithm showed that at higher flight Mach numbers an inlet that maximizes static pressure ratio is preferred over one that maximizes the stagnation pressure ratio, because good combustion efficiency requires a sufficient static pressure in the combustor. As the flight Mach number is increased, thrust decreases while the drag increases. Thus, it becomes difficult to achieve thrust that exceeds drag. A solution is shown to be increasing the engine inlet area above a critical value that was computed. Advantages of a reduced-order model over a high-fidelity CFD approach are discussed when thousands of computations of all components within an entire engine are required for optimization.

Angle-of-attack characteristics of opposing jet for improving drag and heat reduction

While the opposing jet technique has the potential to achieve efficient drag and heat reduction, it can be severely affected by the incoming angle of attack. To analyze the angle-of-attack characteristics of opposing jet for improving drag and heat reduction, a three-dimensional blunt model was studied under various jet stagnation pressure ratios and angles of attack using the verified numerical method. The results showed that the enhanced reattachment shock on the windward side resulted in a higher pressure and temperature rise, which led to the deterioration of drag and heat reduction. Under the influence of the incoming angle of attack, the recirculation vortex transformed into a longitudinal vortex, resulting in a slanted U-shaped distribution of the surface pressure coefficient and Stanton number. Increasing the jet stagnation pressure ratio widened the coverage of the recirculation vortex on both the windward and leeward sides, which brought an improvement in drag and heat reduction. The interaction between the incoming angle of attack and the opposing jet caused a double-peak distribution of Stanton number due to the recirculation vortex reattachment and the compression of the incoming flow. The inclined opposing jet could reduce the peak values of pressure coefficient and Stanton number when subjected to the incoming flow with an angle of attack by spreading the recirculation vortex along the windward side. There should exist an optimal inclination angle that can effectively reduce the peak caused by the compression of the incoming flow without generating an excessive peak due to the recirculation vortex reattachment.

A numerical study on the effect of rib size and location on thermo-fluid performance of synthetic jet

A two-dimensional, axisymmetric numerical model is used to study the effect of synthetic jet flow structure and thermal performance, validated with experimental data. The Reynolds number is set at 5473, and orifice-to-surface spacing (z/d) varies (2, 6, 10). Three square cross-section ribs at different radial positions are analyzed. Results show that ribs reduce heat transfer at the stagnation point, with a maximum decrease of 74.37% at z/d=10 for a 5mm×5mm rib at r/d=0.5 and a minimum of 47.70% at z/d=6 for the same rib. Heat transfer increases near the rib's leading edge but drops before and after the rib. No area-averaged heat transfer improvement is observed, with a maximum reduction of 69.24% at z/d=10 for a rib at r/d=2 and a minimum of 2.43% at z/d=6 for a rib at r/d=1. Stagnation pressure increases by 24.42% at z/d=2 and decreases by 15.66% at z/d=10 due to a rib at r/d=2.

Optimization of Radial Jet drilling for hard formations present in deep geothermal wells

Creating controllable fractures in geothermal wells with deep crystal structures is challenging due to rock strength. Radial Jet Drilling (RJD), which creates micro holes through rock, could stimulate these wells. However, it is challenging to implement RJD when drilling hard magmatic formations that are difficult to jet. Several factors influence RJD's effectiveness, including the jetting bit design, fluid properties, circulation rate, injection pressure, stress state in the formation, borehole pressure, and temperature. These factors determine RJD's performance parameters, such as jet stagnation pressure, jet hydraulic power, and maximum lateral hole length. This study aims to develop a generalized model for optimizing RJD's application in geothermal wells with hard formations. A unique feature of the study is the coupling of a hydraulic model with jetting criteria (threshold conditions) developed for evaluating jet-rock interactions under high-pressure conditions.This study introduces an advanced hydraulic model for RJD application, distinct from classical models that rely on field measurements and typically analyze only horizontally oriented nozzles. Our model encompasses a broader scope of operational and design parameters, facilitating the optimization of RJD in challenging geothermal environments without necessitating direct field data. Key performance metrics such as jet stagnation pressure, jet hydraulic power, and maximum lateral hole length are evaluated, emphasizing the critical role of each in overcoming the inherent challenges of hard rock formations, which require stagnation pressure of more than 100 MPa and jet hydraulic power of more than 5.2 KW. The findings reveal that achieving these conditions requires carefully balancing injection pressure, fluid flow rate, and equipment capabilities. Notably, the model underscores the necessity for robust equipment capable of high-pressure operations, including flexible tubing, compact downhole pressure intensifiers, and more advanced inclination and azimuth detection systems for lateral hole trajectory measurement and control.According to the new model, the RJD technique can be optimized by varying the densities of flexible tubes, the inclination angles of the lateral hole, and the number of back-jetting (BJ) nozzles. Results demonstrate that using an aluminum tube or adjusting the lateral hole to an inclination of 74° can enhance the maximum lateral hole length (MHL) by up to four times for the baseline case. Furthermore, increasing BJ nozzles from four to six can lead to an eightfold improvement in the MHL for the baseline scenario. Overall, the study enhances our understanding of RJD's potential in geothermal well stimulation. It offers a comprehensive framework for effectively implementing it in hard rock settings, creating a scenario for more efficient and economically viable geothermal energy production.

The Role of Turbulence Transport in Mechanical Energy Budgets

Abstract In this article, we study the role of turbulence transport on loss prediction using high fidelity scale-resolving simulations. For this purpose, we use eight high-fidelity simulation datasets and compute flux of entropy and stagnation pressure transport equation budgets for all the cases. We find that under certain unsteady inflow conditions, the stagnation pressure coefficient is not a reliable loss metric even at low Mach number conditions. This is due to the turbulence transport terms. The impact of these terms, typically assumed to be negligible, made stagnation pressure loss coefficient underpredict loss from 0% to over 40% when compared to entropy loss coefficient for cases considered here. This effect was most pronounced for the cases with highly unsteady inflow conditions and at low Reynolds numbers.

Multi-fidelity CFD for obtaining the runner blade profile parameters of a Francis turbine for optimum hydrodynamic performance

The high fidelity Navier Stokes equations based CFD codes precisely take care of the turbulent and viscous effects of complex flows passing through the hydraulic turbines. However, for preliminary design synthesis and analysis purposes, this full 3D flow can be assumed as a combination of two 2D flows (meridional axi-symmetric flow and blade-to-blade or cascade flow) which is the basis of low fidelity potential flow theory for hydraulic turbines. This paper presents the utilities of high as well as low fidelity CFD analyses for obtaining a runner blade design which yields an optimum hydrodynamic performance of a prototype Francis Turbine. The complete flow domain of the Francis turbine has been considered for the high-fidelity analysis which is carried out by using ANSYS CFX 16.0 for different operating conditions. Whereas, for the low fidelity investigations, full three-dimensional flow domains of different blade rows (i.e., stay vanes, guide vanes and runner) of the Francis turbine is divided into two 2D flow domains (i.e., an axisymmetric annulus of the turbine space for meridional flow analysis and cascades at different sections from hub to shroud for blade-to-blade analysis). By using the low fidelity analysis, runner blade design parameters have been adequately varied in order to obtain a population of different promising runner blade designs. Performances of these promising designs are evaluated, and the obtained results are thoroughly analysed through the response surface analysis. An optimized hydrodynamic performance design of the runner is obtained for the target values of responses (here, swirl velocity at trailing edge of the runner (Cu2) and stagnation pressure at trailing edge of the runner (P02/ρ)) at different sections from hub to shroud by determining the design parameters of blade profiles at different sections. The adopted multi-fidelity CFD approach is found very effective and has the potential to yield reasonably accurate hydraulic designs of the Francis turbine in considerably lesser time and cost.

CFD Investigation of a Co-Flow Nozzle for Cold Spray Additive Manufacturing Applications

This current work evaluates the efficacy of a co-flow nozzle for cold spray applications with the aim of mitigating nozzle clogging issues, which can occur during long-duration operations, by replacing the solid wall of a divergent nozzle section with an annular co-flow fluid boundary. Simulations were conducted on high-pressure nitrogen flowing through convergent–divergent (C–D) axisymmetric nozzles, with a stagnation pressure of 6 MPa and a stagnation temperature of 1273 K. In these simulations, Inconel 718 particles of varying sizes (15 µm to 35 µm) were modeled using a 2-way Lagrangian technique, and the model’s accuracy was confirmed through validation against experimental results. An annular co-flow nozzle with a circular cross section and straight passage covering the primary C–D nozzle has been designed and modeled for cold spray application. Co-flow was introduced to the reduced nozzle length to compensate for particle velocity loss at higher operating conditions. It was found that co-flow facilitates momentum preservation for primary flow by providing an annular gas boundary, resulting in increased particle speed for a longer axial distance beyond the nozzle exit of the reduced divergent length nozzle. The particle acceleration performance of the reduced divergent section nozzle, when combined with co-flow, is comparable to the original length nozzle.

Investigation of Unsteady Combustion Regimes in a Kerosene-Fueled Scramjet with Air Throttling

This paper describes an experimental study investigating unsteady combustion regimes in a kerosene-fueled scramjet. The results are obtained under inflow conditions of a 2.9 MPa stagnation pressure, 1900 K stagnation temperature, and a Mach number of 3.0. The air throttling position is 240 mm downstream of the combustor entrance, with an air throttling flow rate (ratio of air throttling mass flux to inflow mass flux) of 38% and a fuel equivalence ratio of 0.37. Combustion is relatively stable when air throttling is applied and is dominated by auto-ignition. When air throttling is turned off, the combustion becomes more unsteady and is dominated by flame propagation. At the same time, the combustion mode changes, and the frequency of the combustion mode transition is 286 Hz. Schlieren images and one-dimension analysis show that the effect of air throttling is the coupling of cold throat (aerodynamic throat) and hot throat (thermal throat). The proper orthogonal decomposition and dynamic mode decomposition analysis present that when air throttling is applied or removed, the frequencies of injector–flame feedback are almost the same, while the frequencies of shock–flame feedback exhibit considerable variation, which is caused by the location of the precombustion shock affected by air throttling.