Time-averaged Static Pressure Research Articles

Performance Characteristics of a Rotating Detonation Combustor Exiting Into a Pressurized Plenum to Simulate Gas Turbine Inlet

Abstract The present study aims to experimentally characterize the performance of a rotating detonation combustion (RDC) system integrated with a pressurized downstream plenum to simulate the high-pressure inlet conditions of power-generating gas turbines. A thorough understanding of the operational behavior including wave mode behavior, static pressure profile along the combustor length, and dynamic features of pressure fluctuations is crucial for successful integration of RDC with the turbine. In this study, two RDC configurations are investigated, RDC with a constant area annulus and RDC with a converging nozzle. In both cases, the RDC flow exited into a plenum chamber kept at pressures varying from 155 kPa to 330 kPa. RDC was operated on methane and oxygen-enriched air to represent reactants used in land-based power generation. Experiments were conducted for the two RDCs configurations operated at three reactant mass flow rates (0.23, 0.32, and 0.46 kg/s). The RDC performance is characterized by time-averaged static pressure measurements, and wave velocity determined by ionization probes. In addition, dynamic pressure measurements were recorded both inside and near the exit of RDC channel to investigate wave interactions between RDC and downstream plenum. Results show that the RDC with the converging nozzle achieved superior performance while minimizing detrimental interactions with the reflected shock and/or acoustic waves from the downstream plenum.

Impact of mixture mass flux on hydrodynamic blockage ratio and Mach number of rotating detonation combustor

To analyze non-ideal phenomena, such as burned gas backflow and non-detonation combustion, which affect the rotating detonation wave Mach number, simultaneous self-luminous visualization, time-averaged static pressure, fluctuating pressure, and thrust measurements with gaseous ethylene and oxygen were performed. Consequently, by doubling the number density of the fuel injectors, the hydrodynamic blockage ratio at the oxidizer inlet increased approximately 1.7-fold under the same oxidizer inlet area conditions. This may be attributed to the increase in the detonation propagation Mach number owing to the enhanced mixing of fuel and oxidizer. The relationship between the parasitic combustion fraction in front of the rotating detonation wave and the Mach number was also investigated by using a distributed heat release model. Consequently, it was suggested that experimental Mach number decreased from approximately 4.1 to 2.8 with increase in a mixture mass flux, and the theoretical detonation wave propagation Mach number was 7.3.

DEVELOPMENT OF A LIFETIME PRESSURE SENSITIVE PAINT PROCEDURE FOR HIGH-PRESSURE VANE TESTING

Abstract Optical measurements based on fast response Pressure Sensitive Paint (PSP) provide enhanced spatial resolution of the pressure field. This paper presents lifetime PSP at 20 kHz, with precise calibrations, and results from a demonstration in an annular vane cascade. The laser lifetime PSP methodology is first evaluated in a linear wind tunnel with a converging-diverging nozzle followed by a wavy surface. This test section is fully optically accessible with maximum modularity. A data reduction procedure is proposed for the PSP calibration, and optimal pixel binning is selected to reduce the uncertainty. In the annular test section, laser lifetime PSP was used to measure the time-averaged static pressure field on a section of the suction surface of a high-pressure turbine vane. Tests were performed at engine representative conditions in the Purdue Big Rig for Annular Stationary Turbine Analysis module at the Purdue Experimental Turbine Aerothermal Lab. The 2-D pressure results showed a gradual increase of pressure across the spanwise and flow directions, corroborated with local static pressure taps and computational results. The variation in PSP thickness was measured as a contribution to the uncertainty. The discrete Fourier transform of the unsteady pressure signal showed increased frequency content in wind-on compared to wind-off conditions at the mid-span and 30% span. Compared to the mid-span, the hub end wall region had an increase in frequencies and pressure amplitude. This result was anticipated given the expected presence of secondary flow structures at the near hub region.

Investigation of combustion modes and pressure of reflective shuttling detonation combustor

Detonation combustors are considered promising alternatives to conventional combustors because they offer high thermal efficiency and fast combustion. However, especially for the rotating detonation combustor, the theoretical propulsive performance has not been confirmed in experimental studies because the highly unsteady flow field hinders the measurements process. To understand the involved phenomena in more detail, a reflective shuttling detonation combustor (RSDC) with a rectangular combustion chamber was developed. The interior of the chamber can easily be visualized owing to its two-dimensional quality. Utilizing the RSDC, several combustion tests with gaseous ethylene and oxygen were conducted for different values of mass flow rates and equivalence ratios. Combustion modes from the tests were classified into four types based on the fast Fourier transform (FFT) analysis of the luminous intensity of the CH* self-luminescence images captured by a high-speed camera and a band pass filter. Simultaneously, the theoretical total pressure of a conventional isobaric combustor was compared to the static pressure measured at the bottom of the RSDC chamber. For the detonation modes, the ratio between experimentally measured static pressure and the theoretical pressure varied depending on the location in the chamber owing to the distribution of the time-averaged static pressure. Furthermore, the pressure ratio of the detonation modes was up to 18% lower than that of the deflagration mode potentially owing to the flow velocity induced by the detonation waves.

Investigation on the Stall Inception Circumferential Position and Stall Process Behavior in a Centrifugal Compressor With Volute

The present paper numerically and experimentally investigates the stall inception mechanisms in a centrifugal compressor with volute. Current studies about stall inception pay more attention on the axial compressors than the centrifugal compressors; especially, the circumferential position of stall inception onset and the stall process in the centrifugal compressor with asymmetric volute structure have not been studied sufficiently yet. In this work, the compressor performance experiment was conducted and the casing wall static pressure distributions were obtained by 72 static pressure sensors first. Then, the full annular unsteady simulations were carried out at different stable operating points, and the time-averaged static pressure distributions were compared with the experimental results. Finally, the stall process of the compressor was investigated by unsteady simulations in detail. Results show that the stall inception onset is determined by the impeller leading edge (LE) spillage flow, and the occurrence time of trailing edge (TE) backflow is prior to the LE spillage. The nonuniform static pressure circumferential distribution at impeller outlet induced by volute tongue causes the two stall inception regions locating at certain circumferential positions, which are 120 deg and 300 deg circumferential positions at impeller LE, corresponding to the circumferential static pressure peak (PP) and bulge regions at impeller outlet, respectively. In detail, at rotor revolution 2.86, a small disturbance that the incoming/tip clearance flow interface is perpendicular to axial direction occurs at 120 deg position, but this disturbance did not cause the compressor stall. Then at revolution 7, the first stall inception zone (spillage region) occurs at 120 deg position, causing the compressor stall with positive pressure ratio performance. At approximately revolution 23, the second stall inception zone occurs at about 300 deg position; however, both the intensity and size of this stall inception zone are smaller than those of the first stall inception zone. These two stall inception zones are not moving along circumferential direction because the stall inception circumferential position is dominated by the impeller outlet static pressure distribution. Even then, the obvious low frequency signals appear after the spillage crossing two blade LEs, because at this moment, the spillage vortex caused by the tip leakage flow begins to shed. However, due to the asymmetric structure limitation, this vortex cannot move across full annular. Furthermore, the spillage vortexes cause the local low static pressure zone ahead of blade LE in the centrifugal compressor with volute, suggesting that the spillage can be predicted by the steady casing wall static pressure measuring. The development of blockage zones at impeller LE is also investigated quantitatively by analyzing the stall blockage effect.

Three-dimensional numerical thrust performance analysis of hydrogen fuel mixture rotating detonation engine with aerospike nozzle

The propulsive performance for an H2/O2 and H2/Air rotating detonation engine (RDE) with conic aerospike nozzle has been estimated using three-dimensional numerical simulation with detailed chemical reaction model. The present paper provides the evaluation of the specific impulse (Isp), pressure gain and the thrust coefficient for different micro-nozzle stagnation pressures and for two configurations of conic aerospike nozzle, open and choked aerospike. The simulations show that regardless of the nozzle, increase the micro-nozzles stagnation pressure increases the mass flow rate, the pre-detonation gases pressure and consequently the post-detonation pressure. This gain of pressure in the combustion chamber leads to a higher pressure thrust through the nozzle, improving the Isp. It was also found that the choked nozzle increases the chamber time-averaged static pressure by 50–60% compared with the open nozzle, inducing higher performance for the same reason explained before.

Modeling Analysis of an Actively Cooled Scramjet Combustor Under Different Kerosene/Air Ratios

Improved delayed detached Eddy simulation (IDDES) modeling based on a developed skeletal combustion mechanism of kerosene/air is conducted for a full-scale actively cooled scramjet combustor under two different global equivalence ratios. A reformulated partially stirred reactor (PaSR) is proposed to adapt it for high mesh resolution. A skeletal mechanism consisting of only 28 species and 92 reactions is reduced for the kerosene combustion modeling. A one-dimensional solid-gas-liquid coupling method is developed to simulate the active cooling effect. The time-averaged static pressure and wall heat flux profiles are well predicted for both fuel-lean and stoichiometric cases. The supersonic flow, mixing, and combustion characteristics under the two fuel/air ratios are quantitatively compared based on the efficiency indices, Takeno Flame Index, and the correlation statistics between heat release rate and mixture fraction. The vorticity and its evolution are analyzed through the five source terms for the two cases. The turbulence-chemistry interaction is then analyzed by the aid of numerous quantitative indices, such as Damköhler number, PaSR coefficient, and scalar dissipation rate, as well as their correlation statistics to identify the main combustion modes in the scramjet combustor.

Effect of a volute on the unsteady flow in the vane diffuser of a centrifugal compressor

A volute is the only circumferential asymmetric component in a centrifugal compressor, and thus, it should account for the circumferential asymmetry of the flow in a vane diffuser. This study performs a transient numerical analysis to investigate the effect of a volute on the flow in the vane diffuser of a centrifugal compressor under three operating conditions (near-stall, middle, and high mass flow). We compare numerical and experimental performance of the compressor, including polytropic efficiency, total pressure ratio, and unsteady pressure on a diffuser vane. The numerical scheme is proven valid owing to the fact that the numerical and experimental results considerably agree well with each other. Under middle and high mass flow conditions, the time-averaged static pressure recovery and the total pressure loss coefficients for all the diffuser passages indicate that the performance of the passages near and upstream of the volute tongue is affected negatively by the volute, whereas that of the passages downstream of the volute tongue is less affected. Under near-stall condition, the performance of all the passages is disturbed, and the diffuser passage marked as DP 3 demonstrates the worst performance. Investigation on the time-averaged aerodynamic forces, loading, and pressure on the vanes yields results that are consistent with those of the investigation on the performance of the passages. The harmonics with 0.5 fb and fb, which are included in the unsteady loading and pressure on the pressure and suction sides of the vanes, are dominant, where fb is the impeller main and splitter blades passing frequency. Their amplitude values increase as mass flow deviates from the middle mass flow condition. Under middle and high mass flow conditions, the harmonic with 0.5 fb is affected more negatively because of the larger amplitude on the vanes near and upstream of the volute tongue than those downstream, whereas the harmonic with fb is less affected by the volute. Under the near-stall condition, the transient vorticity fields along with the harmonics of 0.5 fb and fb are investigated to evaluate the performance of the diffuser passages. DP 3, which is located at approximately 90° downstream of the volute tongue, suffers the strongest flow deterioration and is inferred to stall first. Further researches for designing more matching diffuser/volute combination will be performed by referring this study.

Main Gas Ingestion in a Turbine Stage for Three Rim Cavity Configurations

Experiments were carried out in a model air turbine stage to study the influence of rotor-stator rim cavity configuration on the ingestion of mainstream gas into the cavity. The three rim cavity configurations differed in their aspect ratio (height/width); the rim seal geometry remained the same. The aspect ratio was changed from the baseline ratio by installing an inner shell on the stator at an appropriate radius; this effectively introduced an axial-gap seal between the rim cavity and the cavity radially inboard. The initial step in each experiment was the measurement of time-average static pressure distribution in the turbine stage to ascertain that proper flow condition had been established. Subsequently, tracer gas concentration and particle image velocimetry techniques were employed to measure the time-average but spatially local main gas ingestion and the instantaneous velocity field in the rim cavity. At low purge air flow, regions of ingestion and egress could be identified by inspecting the instantaneous radial velocity distribution near the rim seal obtained from cavity gas velocity maps close to the stator. While the tangential velocity tended to be slightly larger for the so determined ingested gas, a more clear-cut indicator of ingestion was the strong inward gas radial velocity. Information provided by ensemble-average velocity maps was not sufficient for identifying ingestion because the averaging smeared out flow details, which varied from instant to instant. Velocity fields obtained from three-dimensional, time-dependent numerical simulation of a rim seal-cavity sector with similar dimensions qualitatively showed similar characteristics in the outer part of the cavity and provided insight into the complex flow in the seal region.

Measurement of pressure field around a NACA0018 airfoil from PIV velocity data

In solving these equations numerically, Eqs. (1) and (2) are transformed into general coordinates and they are solved by SOR scheme to obtain the time-averaged static pressure field around the airfoil. Note that the numerical grids are given by O type in the present study and the grid spacing is controlled to be fine near the airfoil and outer boundary, which is set at 5 times of the chord length C of the airfoil from the airfoil center. The number of grids is 600 x 300 in circumferential and radial direction, respectively. In solving these equations, all velocities on the right hand side of Eq. (1) are evaluated from the experimental velocity data measured by PIV. 3. Experiment The velocity field around a NACA0018 airfoil is measured using a standard PIV system, which consists of Nd:YAG laser (532 nm, 30 mJ/pulse), CCD camera with frame straddling function (1280 x 1024 pixels with 12 bits in gray level) and the pulse controller. The experiment is carried out in an { } ( ) j i j u u

WALL PRESSURE FLUCTUATIONS OF TURBULENT FLOW OVER BACKWARD-FACING STEP WITH AND WITHOUT ENTRAINMENT: MICROPHONE ARRAY MEASUREMENT *

Wall pressure fluctuations in turbulent boundary layer flow over backward-facing step with and without entrainment were investigated. Digital array pressure sensors and multi-arrayed microphones were employed to acquire the time-averaged static pressure and fluctuating pressure, respectively. The differences of two flows were scrutinized in terms of static pressure characteristics, pressure fluctuations, cross-correlation and coherence of wall pressure. Introduction of the entrainment increased scale of large-scale vortical structure and reduced its convection velocity. However, shedding frequency of large-scale vortical structures was found to be the same for both flows.

Experiment on Gas Ingestion Through Axial-Flow Turbine Rim Seals

It has been suggested by researchers that ingestion, through rim seals, of mainstream gas into axial-flow turbine disk cavities is a consequence of the prevailing unsteady three-dimensional flow field. The cause–effect relationship is complex—to help understand it, experiments were performed in a model single-stage turbine rig using two different vane-blade configurations. Selected measurements from one of the configurations were reported earlier (1999–2001). The second configuration is new, featuring smaller numbers of vanes and blades and a larger vane turning angle. Selected measurements are presented and compared to those from the first configuration. The measurements include unsteady and rotor revolution time-average static pressure spatial distributions, and spatial distribution, in the rotor-stator cavity, of time-average ingestion. The parameters in the experiments were the main airflow rate, the purge/seal airflow rate, and the rotor speed. Unsteady three-dimensional CFD simulation may be helpful in identifying the roles of the many intertwined phenomena in the ingestion process.

Vortex shedding in a two-dimensional diffuser: theory and simulation of separation control by periodic mass injection

We develop a reduced-order model for large-scale unsteadiness (vortex shedding) in a two-dimensional diffuser and use the model to show how periodic mass injection near the separation point reduces stagnation pressure loss. The model estimates the characteristic frequency of vortex shedding and stagnation pressure loss by accounting for the accumulated circulation due to the vorticity flux into the separated region. The stagnation pressure loss consists of two parts: a steady part associated with the time-averaged static pressure distribution on the wall, and an unsteady part caused by vortex shedding. To validate the model, we perform numerical simulations of compressible unsteady laminar diffuser flows in two dimensions. The model and simulation show good agreement as we vary the Mach number and the area ratio of the diffuser. To investigate the effects of periodic mass injection near the separation point, we also perform simulations over a range of the injection frequencies. Periodic mass injection causes vortices to be pinched off with a smaller size as observed in experiments. Consequently, their convective velocity is increased, absorption of circulation from the wall is enhanced, and the reattached point is shifted upstream. Thus, in accordance with the model, the stagnation pressure loss, particularly the unsteady part, is substantially reduced even though the separation point is nearly unchanged. This study helps explain experimental results of separation control using unsteady mass injection in diffusers and on airfoils.

Probe systems for static pressure and cross-stream turbulence intensity

A recent study of total-pressure probes for use in highly turbulent streams is extended herein by developing probe systems that measure time-averaged static or ambient pressure and turbulence intensity. Arrangements of tubular probes of circular and elliptical cross section are described that measure the pressure at orifices on the sides of the probes to obtain different responses to the cross-stream velocity fluctuations. When the measured data are combined to remove the effect of the presence of the probes on the local pressure, the time-averaged static pressure and the cross-stream components of turbulence intensity can be determined. If a system of total pressure tubes, as described in an accompanying paper, is added to the static pressure group to form a single cluster, redundant measurements are obtained that permit accuracy and consistency checks.