Stagnation Pressure Loss Research Articles

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.

Quasi-Two-Dimensional Simulation of a Rotating Detonation Engine Combustor and Injector

A numerical simulation of an annular rotating detonation engine with stoichiometric hydrogen–oxygen is performed. A generic, well-posed, and easily implemented approach using a quasi-two-dimensional method to model the area variations through the rotating detonation engine’s injector and combustor is presented. The detonation–injector interaction is studied for the case with a ratio of four between the combustor and injector’s throat areas. A shock wave is formed in the divergent portion of the injector due to the high backpressure created by the detonation in the combustor. A Favre-averaged steady-state analysis of stream lines and particle paths reveals that the shock causes an irrecoverable loss of stagnation pressure. Stagnation pressure gain in the combustor is insufficient to make up for the loss, and the flow leaves the engine with lower stagnation pressure than in the plenum.

Numerical study on the mixing enhancement induced by the interaction between the hydrogen/air coaxial jets and oblique shock wave at Mach 3 supersonic crossflow

The comprehensive performance of the scramjet is greatly influenced by the supersonic fuel mixing process. In the research, the mixing properties of the combination between the hydrogen/air coaxial jets and oblique shock at Mach 3 supersonic crossflow are investigated. The focus of the investigation is to reveal the mixing enhancement mechanism of coaxial jets and the impact of oblique shock on the mixing process. The numerical simulation is based on three-dimensional compressible RANS equations combined with the SST k-ω turbulence model. The hydrogen spatial distribution, flow features, vortex structure, penetration depth, stagnation pressure loss and mixing efficiency with different working conditions are evaluated and studied. Our computational results demonstrate that the coaxial jets enhance mixing through the intense shear action between the annular fuel jet with the inner and outer air layers, and the oblique shock contributes to the mixing process by strengthening the interaction between jet and airflow, improving fuel distribution and inducing streamwise vorticity. In addition, the hydrogen distribution and mixing properties are affected by the action position of the incident shock. According to the achieved results, the coaxial jets combined with the oblique shock impinging downstream of the jet have the highest mixing efficiency, which is improved by 63.27% compared to the reference case.

Evaluation of the free-windmilling rotational speed of a damaged wide-chord fan rotor

The windmilling imbalance condition occurs after engine shut-down due to fan blade loss, but the engine continues to rotate under aerodynamic forces. This condition can induce undesirable structural excitation throughout the engine and airframe. Especially in the progression of larger high-bypass-ratio fans with fewer blades with larger chords, the excitation is considerably greater and is directly determined by the fan rotational speed. The aim of this study is to explore the windmilling flow through damaged fan rotor numerically according to the zero-torque characteristic, and to develop a prediction method of the windmilling rotational speed of damaged fans. An idealized span-damaged type is introduced to examine the effect of blade-to-blade flow interaction on the rotational speed of fans with various degrees of damage. In full-annulus simulations, whenever the damage of a single blade reaches 35%, the positive torque of the damaged blade reaches its maximum value, and the fan rotational speed instead reaches a new minimum. The results also show that the radial flow profiles at the rotor outlet are independent of airflow and can be determined using only the geometry of the damaged fan. This indicates the self-similar nature of the intra-stage flow. According to numerical results, a speed similarity coefficient of damaged fan is identified, and the dynamic pressure at the damaged fan outlet is the appropriate scale for the stagnation pressure loss across the rotor. Further, the engine is regarded as a finite number of variable cross-section frictional ducts, which facilitates the construction of a total stagnation pressure-loss model, allowing the windmilling airflow for any flight Mach number to be obtained. Finally, this study proposes a mathematical model to predict the windmilling rotational speed using the damaged fan speed similarity coefficient and the one-dimensional windmilling airflow function. The model can be applied to the early stages of high-bypass turbofan development and can be used to evaluate the safety characteristics during engine windmilling operations.

Efficiency of biradial impulse turbines concerning rotor blade angle, guide-vane deflection and blockage

Self-rectifying impulse turbines have two sets of guide vanes symmetrically located on each rotor side. Aerodynamic losses resulting from the inherent misalignment between the rotor outflow and the outlet guide vanes penalize these turbines’ efficiency. This paper presents the aerodynamic design of a radial guide-vane system for self-rectifying biradial impulse turbines and the relation between guide-vane flow deflection and blockage, rotor blade angle and turbine efficiency in design conditions. The system comprises two concentric rows of constant-thickness vanes. The design method solves a multi-objective optimization problem that maximizes deflection while minimizing outflow blockage, returning a Pareto optimal set of guide vanes. A subset of these results is used to configure and assess multiple turbine geometries. Data are obtained numerically with a RANS solver. The new guide-vane system improves flow deflection over reference designs based on aerofoil-section vanes with identical blockage. A turbine efficiency of 68.2% is achieved for a guide-vane deflection of 67.3°, blockage factor of 0.61 and rotor blade angle of 35°. The new design leads to an estimated reduction of 58% on the stagnation pressure losses in the outlet guide vanes.

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％.

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.

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 investigations on the influence of double ramps in a strut based scramjet combustor

This numerical study addresses the performance of double ramps in a strut injection scramjet combustor. Reynolds Averaged Navier Stokes (RANS) equations with the Shear Stress Transport SST k-[Formula: see text] turbulence model are used for numerical analysis. The dual ramps are located symmetrically at the top and bottom wall of a scramjet combustor downstream of the strut injector. The influence of the location of ramps in the scramjet combustor is analyzed based on the key parameters such as shock waves, static pressures, and temperature distribution across the combustor, which are verified with available DLR scramjet experimental data that show similar acceptable values within the range. Simulation is extended for the investigation of other configurations. The performance of the scramjet configurations has been analyzed using the variables such as shock patterns, Mach number contours, wall static pressures, mass fractions of the reactants and products, combustion efficiency, and stagnation pressure loss of combustor. Numerical shadowgraph indicates that numerous shock reflections occur for the dual ramp cases that decelerate the combustor’s flow downstream. In addition, the recirculation regions are created for the dual ramp cases, which help in anchoring the flame. The static pressure distributions show the variation of peaks due to shock patterns and their interactions. The combustion efficiency increases as the ramps are positioned downstream of the strut injector. The mass fractions of reactants and products indicate complete combustion of the hydrogen issued from the combustor in the vicinity of the cavities. The total pressure loss for the DLR strut model is comparatively lower than dual ramp cases due to the multiple shocks from the ramp decelerating the flow downstream of the combustor.

The investigation of inclined AFT wall cavities in a circular scramjet combustor

Achieving good mixing with high combustion efficiency, stable flame holding, and low stagnation pressure losses are still open issues in supersonic reacting flows. Mixing and vorticity in supersonic flows are mainly driven by the baroclinic term that is correlated to the density and pressure gradients. An increase of the baroclinic term has been observed to strongly depend on shock waves arising within the combustion chamber. This work numerically investigated the interaction between the fuel jet and the air stream as a function of the aft wall angle inclination. In fact, this parameter has a primary impact on the shock wave inclination arising from the cavity leading edge and therefore on the baroclinic term. Four different combustor geometries have been investigated: without cavity and with three different aft wall inclinations (90°–15°, 60°–15°, 30°–15°). 3D RANS numerical simulations showed that the primary ramp angle has a critical impact on the fuel-air mixing and consequently on the combustion efficiency. A correlation between the combustor geometry and the shock angle inclination has been proposed. Greater combustion efficiency, homogeneous H2O, and temperature distribution across the combustor are found with the 30–15 aft wall cavity configuration. Lowering the primary aft wall of the cavity from 90° to 30°, resulted in the rising of a stronger bow shock, extended recirculation volume within the cavity, and increased fuel-air mixing.

Precise Fluid-Solid Simulation of Human Left Ventricle along with Aortic Valve during Systole

This paper presents an accurate blood flow model with tissue deformation of the human left ventricle, including the aortic valve. A two-way fluid-solid Interaction (FSI) algorithm is employed to simulate the performance of the human left ventricle during systole. The initial geometry of the left ventricle is extracted from CT scan images of a healthy person. The simulation results produced the systolic anterior motion of the Left Ventricle (LV) identical with the CT scan images at later times during systole. Besides, the numerical results for left ventricular volume change, maximum blood velocity at the aortic valve, and its maximum opening are in good agreement with physiological data. Although no clear image of the aortic valve is apparent in CT images, the FSI simulation predicted the maximum opening of the aortic valve to be 4.38 cm2 which is consistent with physiological observation on a healthy individual. As an application of the above algorithm, a model of Hypertrophic Cardiomyopathy (HCM) or septal wall thickening disease is constructed and studied during systole. This simulation provides an understanding of heart performance under HCM conditions. According to the simulation outcomes, the mitral valve approaches the septal wall under HCM due to the change in pressure gradient and the drag force on the mitral valve. This blockage of the LV blood passage by the mitral valve results in stagnation pressure loss and weaker hearth pumping power. Therefore, the maximum opening of the aortic valve, in this case, is 2.28 cm2, which is much lower than the physiological range, indicating the drastic effect of HCM on the performance of the aortic valve and systolic performance.

Experimentally validated pore-scale numerical analysis for high-temperature (>700°C), high-efficiency (>90%) volumetric solar receivers

• Proposes an innovative, high-temperature semi-transparent packed-bed solar receiver. • Hits SunShot target with one of the highest efficiencies, 92% for T outlet ≥ 700 °C. • Develops a 3-D pore-scale model which couples the optical and thermo-fluid analyses. • Minimises reflection loss via an optical baffle and the pore-scale arrangement. • Reports new experimental validation test data for heat transfer in packed bed. Concentrated solar thermal/power (CST/P) systems are evolving to operate at higher temperatures (i.e., above 700 °C) to drive advanced cycles and thermochemical processes. The development of high temperature, high efficiency solar receivers—as the highest temperature component in a CST/P system—represents a critical technical challenge in this evolution. Commercial solar receivers have been limited to operation below 600 °C due to (1) the historic trade-off between operation temperature and efficiency, and (2) the operating temperature limits of the liquid conventional heat transfer fluids. This paper investigates a gas-phase, packed-bed receiver, which is capable of addressing these limitations. To date, this novel design has shown promise for this application, but it has only been modelled on the macro-scale (i.e., a 1-D model). To obtain a more detailed understanding of this type of receiver, this paper presents the results of a novel 3-D pore-scale analysis of the proposed receiver to characterise the optical-thermo-fluid behaviour. To ensure the validity of this approach, experimental tests were conducted for a complex packed bed of spheres. The 3-D pore-scale analysis enables the development of innovative mechanisms to reduce radiosity losses. It also provides a deeper level of insight regarding the stagnation pressure loss caused by the inlet air-jet flows. The final results demonstrated that the pore-scale modifications of the design can yield an optimised packed-bed receiver with a maximum efficiency of ∼ 92 % (for a hot air outlet temperature well above 700 °C). This optimised receiver design represents a new pathway towards high-temperature solar receivers, which can operate at very high efficiency, to enable CST/P systems to utilise advanced power cycles.

Effects of cavity-induced mixing enhancement under oblique shock wave interference: Numerical study

In this research, the effects of oblique shock on the mixing characteristics in a supersonic combustor equipped with a cavity is numerically investigated. To reveal the flow structure of the supersonic flow field under oblique shock wave interference, three-dimensional steady RANS equations and SST k-ω turbulence model are adopted. The current work focuses on comparing the interaction effects between oblique shock wave and bow shock wave, which are formed by fuel jet on fuel mixing under different conditions. The numerical analysis demonstrates that an optimal angle exists for the mixing efficiency of the ramp. The optimal angle diminishes as the jet-to-crossflow pressure ratio increases. The oblique shock wave in a certain range is conducive to enhance the penetration depth of ethylene. The smaller angle of the ramp does not cause large stagnation pressure losses.

Numerical investigation of cavity-induced enhanced supersonic mixing with inclined injection strategies

In this study, an inclined injection system with a cavity and grooves arranged in series is developed to enhance the supersonic mixing in a scramjet combustor. Three-dimensional steady Reynolds-averaged Navier–Stokes equations, the two-equation shear stress transport k-ω model, and the finite-rate/no turbulence-chemistry interaction model are used to simulate the flow field structures with and without grooves at different inclination angles. The numerical method used in the study has been verified using experimental data from the open literature, and the static wall pressure distribution of the numerical simulation is in good agreement with the experimental data. The numerical results reveal that the grooved configuration has a significant influence on the flow field structure. The downstream streamline of the structure with grooves is more turbulent and produces more streamwise vortices because the geometry of the grooves enhances the shear effect between the vortex and the high-speed incoming flow. In the case of the grooved configuration, the mixing coefficient at the fuel orifices is slightly lower, but the fuel mixing effect in the downstream is enhanced and gradually exceeds the grooveless configuration. Moreover, the grooved configuration can obtain the best mixing coefficient at a jet angle of 30°. The grooved configuration has a greater stagnation pressure loss than the grooveless configuration.

Effect of Valve Opening Manner and Sealing Method on the Steady Injection Characteristic of Gas Fuel Injector

The steady-state injection characteristic of gas fuel injector is one of the key factors that affects the performance of gas fuel engine. The influences of different injection strategies, such as different injection angles and different injection positions, on the mixing performance in gas-fueled engine have been emphasized in previous literatures. However, the research on the injection characteristics of the gas fuel injector itself are insufficient. The three-dimensional steady-state computational fluid dynamics (CFD) models of two kinds of injectors, in different opening manners, and the other two kinds of injectors, in different sealing methods, were established in this paper. The core region speed, stagnation pressure loss and mass flow rate were compared. Additionally, the effective injection pressure (EIP) concept was also used to evaluate the injection efficiency of gas fuel injector. The simulation results show that the jet speed of the pull-open injector is higher than the push-open injector under the same operating conditions. The injection efficiency of the pull-open valve is about 56.0%, while the push-open valve is 50.3%. In general, the steady-flow characteristic of the pull-open injector is better than that of the push-open one. The injection efficiency of the flat sealing injector is 55.2%, slightly lower than the conical sealing method.

Oblique Shock Control with Steady Flexible Panels

Flexible panels deforming under pressure loads have been suggested as a passive form of adaptive oblique shock control. This study investigates oblique shock–boundary-layer interactions on a steady flexible panel in a Mach 2.0 flow. Experiments were performed in the Imperial College supersonic wind tunnel, where shock generators were used to produce an oblique shock followed by a corner expansion. A parametric study was conducted, exploring different shock impingement positions and shock–expansion distances. The steady aerostructural response is studied using schlieren photography, static pressure distributions, photogrammetry measurements, and surface oil flow visualization. Two-dimensional numerical simulations were performed to assess the effects of the flexible panel on downstream total pressure recovery. These were validated against experimental wall pressure distributions and measurements from a Pitot rake. Results show reductions in both separation length (of up to 40%) and stagnation pressure losses (of up to 10%) if the flexible plate is used. These improvements occur for a range of shock positions spanning approximately 50% of the panel length and for all the shock–expansion distances considered. A model that captures the flow physics responsible for these trends is proposed. The results highlight the potential of flexible panels for practical oblique shock control.

Effect of fuel-jet injection angle variation on the overall performance of a SCRAMJET engine

Air-fuel mixing in a SCRAMJET engine is augmented by the interaction of the transverse fuel jet with the incoming supersonic air. The strong bow shock created by this interaction aids in mixing and increases the fuel residence time, but it also leads to loss in performance of the SCRAMJET engine through the loss of stagnation pressure, and rise in entropy in the combustor. One of the ways to address this issue is to weaken the bow shock by changing the angle of injection of fuel into the combustor. In the present study, the effect of variation of the injection angle, measured in the direction of the cross-flow from a line perpendicular to it (and the wall), has been numerically studied and analyzed on a 3-D SCRAMJET combustor of generic design with dual injectors, using Menter's SST model for turbulence on an in-house 3-D unstructured grid RANS solver. The angle for each injector is independently varied between 0° and 45° with an increment of 15°, while the jet positions are kept fixed at locations previously found to be optimum for the chosen flow conditions and zero angle (i.e. transverse) injection. It is observed that in every case that positive non-zero angles of injection, in the direction of the crossflow, increase thermodynamic efficiency, while the negative non-zero angles, opposing the crossflow, augment mixing. As mixing is of paramount importance in the SCRAMJET engine, due to high speeds and low residence times, we conclude that the best option is to have the angle of fuel jet injection in the direction opposing the incoming flow – a recommendation that has not been seen yet in the research literature. The degree to which the injection is slanted towards the incoming flow can be decided on the basis of the desired rate of the simultaneous penetration of the fuel into the recirculating flame-holder, which increases with increasing angle.

Experimental study of a self-rectifying biradial air turbine with fixed guide-vanes arranged into two concentric annular rows

The self-rectifying biradial air turbine was proposed to equip wave energy converters of the oscillating-water-column type. There are two configurations: with fixed guide vanes and a radially more compact one with axially-sliding guide vanes. The added complexity of the movable guide-vane mechanism represents a drawback in offshore environments. The configuration with fixed guide vanes is the object of the present investigation using a novel stator design. The guide-vanes were radially shifted from the rotor and the single row of guide-vanes, located on each side of the rotor, was replaced by two concentric rows of guide-vanes, with wider circumferential spacing for the exit-flow. The work comprised the design, construction and experimental study of a 0.5 m rotor diameter turbine model tested under steady flow. Experimental results show that the new stator design provides significant reduction in the flow stagnation pressure losses at the exit guide vanes, while appropriately turning the flow admitted into the rotor. The measured efficiency reaches a peak of 0.728 and is above 0.6 over a wide range of flow coefficients.

Numerical Study of Mixing Enhancement in 2D Supersonic Ejector

Abstract The present study deals with the numerical study of the mixing process in a 2D supersonic ejector. The performance parameters such as momentum flux, degree of mixing and stagnation pressure loss are used to analyse mixing process. The evolution of the kinetic energy of turbulence is also observed. The investigation is carried out on a conventional 2D supersonic ejector and on a 2D supersonic ejector equipped with wall slot injectors along the mixing chamber. The performance parameters show a significant mixing enhancement when transverse micro-jets are used.

Non-Reacting Examination on Transverse Injection Upstream of Aft Wall Angled Cavities in an Axisymmetric Supersonic Flow

Experiments on an air-fueled non-reacting scramjet test facility with four distinct aft wall angled cavity designs were performed in a Mach 1.8 flow field to explore the effects of transverse fuel injection system on scramjet combustors. Axisymmetric combustor cavity models were featured with two consecutive angles being inclined towards the downstream flow direction at the rear end. The wall mounted injector was located upstream from the cavity at a distance of 10 mm. Mixing performance within the various rear wall angled cavities were evaluated and compared with No-injection case for different injection pressures. Results revealed that the flow within the combustor was more uniform mixing compared to No-cavity configuration and was greatly influenced by injection pressures. Increase in injection pressures were characterized by enhanced mixing, greater stagnation pressure loss values and unstable flow.