Time-averaged Pressure Research Articles

Natural Gas Conversion and Liquid/Solid Organic Waste Gasification by Ultra-Superheated Steam

The technology of a pulsed detonation gun for gasification of organic waste with ultra-superheated steam has been experimentally demonstrated for the first time. Experiments were performed on natural gas conversion as well as on the gasification of liquid (waste machine oil) and solid (wood sawdust) waste by hot detonation products of natural gas–oxygen mixture at a frequency of detonation pulses f = 1 Hz. Periodic release of detonation products to a 100 L flow reactor provided a time-averaged mean temperature and pressure in the reactor at about 1200 K and 0.1 MPa. It is shown that the technology of a pulsed detonation gun can provide complete (100%) natural gas conversion to syngas containing H2 and CO with a H2/CO ratio of 1.25. During the gasification of liquid and solid wastes, the total volume fraction of combustible gases (H2, CO, and CH4) in the product syngas was 80 and 65% with H2/CO ratios of 0.8 and 0.5, respectively. Comparison of the experiments on natural gas conversion and liquid/solid organic waste gasification under the same conditions at f = 1 Hz showed that the composition of the product syngas in terms of H2 and CO content almost did not depend on the type of used feedstock. The estimated ideal energy gain defined as the ratio of the total energy of product syngas to the energy spent in its production from dry wood sawdust is about 4.6, i.e., the pulsed detonation technology of biomass gasification is economically very attractive.

A numerical study of wave characteristics in axisymmetric gas-liquid annular flow in microchannels

Microreactors have emerged as a viable option to perform mass transfer limited gas-liquid reactions due to the advantage of high gas-liquid interfacial area density. Slug and annular flow regimes are the two most commonly preferred flow regimes to perform reactions in microreactors. In the annular flow regime, characterized by a continuous gas core and an annular liquid film on the wall, waves are observed at the interface between the two phases. The characteristics of these waves depend on the properties of the fluids and their flow rates. In this work, the hydrodynamics of axisymmetric gas-liquid annular flow in millimeter-size channels is investigated computationally employing volume of fluid method in OpenFOAM solver. CFD simulations have been performed for nitrogen-water and nitrogen-water and ethylene glycol mixture over a range of gas and liquid flow rates. Depending on the gas and liquid flow rates and liquid viscosity, coalescing and non-coalescing waves are observed at the interface. The properties of the interfacial waves such as frequency, wave spacing and wave velocity are determined using spatiotemporal analysis and the effect of gas and liquid flow rates on the wave characteristics is investigated. Further, the time-averaged pressure gradient and interfacial shear stress are reported.

Unsteady Aspects of Shock-Wave/Boundary-Layer Interaction Resulting from Control Surface Deflection

A computational investigation of a Mach 6.9 turbulent flow near the juncture (cove) of a wing’s trailing-edge and elevon was carried out. Turbulence was modeled using improved delayed detached eddy simulation (IDDES). The computational data were compared to the results of an experimental study, conducted by Deveikis and Bartlett, which simulated the flow over the windward surface of the NASA space shuttle. Flow visualization, surface variable comparison with experiment, and analysis of unsteady flow statistics were carried out. Reasonable agreement between prediction and measurement was obtained for time-averaged surface pressure and heat flux. Unsteady analysis of these quantities revealed spectra dominated by low-frequency energy content throughout the interaction region. A correlation analysis indicated coupling of the unsteady motions of the shear layer, shock wave, separation vortex, and flow reattachment. The results of the present study suggest that IDDES can be a valuable tool for the analysis of heating and unsteadiness in hypersonic gap flows.

Calculation of the Pressure Field for Turbulent Flow around a Surface-Mounted Cube Using the SIMPLE Algorithm and PIV Data

The calculation of the pressure field on and around solid bodies exposed to external flow is of paramount importance to a number of engineering applications. However, conventional pressure measurement techniques are inherently linked to problems principally caused by their point-wise and/or intrusive nature. In the present paper, we attempt to calculate a time-averaged two-dimensional pressure field by integrating PIV (particle image velocimetry) velocity measurements into a CFD code and modifying them by the respective correction step of the SIMPLE algorithm. Boundary conditions are applied from the PIV data as a three-layer area of constant velocities adjacent to the boundaries. A novel characteristic of the approach is the straightforward inclusion of the Reynolds stresses into the source terms of the momentum equations, calculated directly from the PIV statistics. The methodology is applied to three regions of the symmetry plane parallel to the main boundary layer flow past a surface-mounted cube. In spite of findings of deviations from the planar 2D flow assumption, the derived pressure fields and the adjusted velocity fields are found to be reliable, while the intrinsic turbulent nature of the flow is considered without modelling the Reynolds stresses.

Pressure reconstruction from PIV measurements in the bow region of a fast ship

The paper presents velocity measurements, using particle image velocimetry, as well as a reconstruction of hydrodynamic pressures for the analysis of fast ships. Stereoscopic PIV measurements with a towed underwater PIV system are conducted during towing tank tests to obtain the velocity field in the bow region of a fast ship at speeds up to Fr=0.8. While the model is kept at a fixed trim and sinkage, multi-plane PIV measurements with a total of 68 measurement planes are conducted to reconstruct a volumetric representation of the time-averaged velocity field in the bow region. The obtained velocity field is subsequently used for a volumetric description of the time-averaged hydrodynamic pressure field. In addition to these captive runs, forced oscillation tests are conducted. During these tests, the flow field is recorded in three successive planes to obtain a local phase-averaged description of the velocity and its gradients for the reconstruction of the phase-averaged hydrodynamic pressure field. The postprocessing procedure for the pressure reconstruction, including the solution of the Poisson equation, is implemented into the open-source CFD package OpenFOAM. For the detection of the free surface and the ship hull, an automated procedure is presented. Experimental results are finally compared to results from numerical simulations. Results show that the PIV method is capable of capturing the flow characteristics in the bow region of a fast ship. In addition, it can be used together with the pressure Poisson equation to obtain the hydrodynamic pressure field. However, large out-of-plane velocities require a large dynamic range, which limits the resolution of local effects close to the ship hull.

Effects of dissolved gas on surface pressure of a hydrofoil under cavitation condition

Abstract Dissolved gas in water is known as one of the factors which affect cavitation. The present study focuses on the effect of dissolved gas on cavitation around a hydrofoil, more specifically on the hysteresis of cavitation and surface pressure of the hydrofoil. The pressure measurement on a hydrofoil of Clark Y-11.7% was carried out under various dissolved gas conditions. Pressure in the test section was reduced stepwise from an atmospheric pressure to that in a super-cavitation condition, then was increased stepwise to the atmospheric pressure condition. Amount of dissolved oxygen (DO) was used as a parameter representing the degree of air content, and three DO conditions were set at low DO: under 30%, middle DO: around 50%, and high DO: over 70% of saturation under atmospheric pressure. Also, two angles of attack, 8 and 20 degrees were selected. Although there is no significant difference of the surface pressure and its hysteresis at the angle of attack of 8 degrees, dissolved gas seems to affect the behavior of cavitation. At the angle of attack of 20 degrees, it was found that the appearance of cavity was changed by DO conditions, and it would affect surface pressure of the foil. Under high DO condition, when the cavity oscillated with large amplitude, dissolved gas seemed to enhance the growth of the cavity, and the cavity tended to grow as covering the suction surface of the foil; it would make the time-averaged surface pressure of high DO condition lower than that of the other DO condition.

Numerical investigation of the influence of aerothermoelastic dynamic response on the performance of the three-dimensional hypersonic inlet

The influence of aerothermoelastic dynamic response on the performance of the three-dimensional hypersonic inlet is investigated in this paper. A dynamic aerothermoelastic analysis framework is developed. The reliability of the framework is verified. The effects of aerodynamic heating on the dynamic response, the effects of the aerothermoelastic dynamic response of the inlet on the flow field structure, and the performance parameters are studied. The results indicate that the static deformation of the leading edge structure is large while the vibration amplitude is small. Meanwhile, the vibration amplitude of the trailing edge structure is larger. The aerothermoelastic dynamic response changes the shock wave structure near the exit, strengthens the shock wave intensity, increases the length of the separated region, and changes the flow field of the exit. Simultaneously, the flow field structure also experiences obvious dynamic changes at different moments. The dynamic response increases the time-averaged flow coefficient, reduces the time-averaged total pressure recovery coefficient, and improves the time-averaged reverse pressure ratio. At the same time, the dynamic response leads to the fluctuation of performance parameters, especially the large fluctuation range of the reverse pressure ratio at the exit.

Patterned Interference Radiation Force for Transcranial Neuromodulation.

Compared with the conventional method of transcranial focused ultrasound stimulation using a single transducer or a focused beam, the compression and tensile forces are generated from the high-pressure gradient of a standing wave that can generate increased stimulation. We experimentally verified a neuromodulation system using patterned interference radiation force (PIRF) and propose a method for obtaining the magnitude of the radiation force, which is considered the main factor influencing ultrasound neuromodulation. The radiation forces generated using a single focused transducer and a standing wave created via two focused transducers were compared using simulations. Radiation force was calculated based on the relationship between the acoustic pressure, radiation force and time-averaged second-order pressure obtained using an acoustic streaming simulation. The presence of the radiation force was verified by measuring the time-averaged second-order pressure generated due to the radiation force, by using a glass tube.

Echocardiographic left ventricular hypertrophy and geometry in Chinese chronic hemodialysis patients: the prevalence and determinants

BackgroundTo investigate the prevalence of left ventricular hypertrophy (LVH) and explore left ventricular geometry in maintenance hemodialysis (MHD) patients, and to explore the risk factors of LVH which is an important predictor of cardiovascular events.MethodsThe subjects were patients who are on MHD for more than 3 months in Peking University People's Hospital from March 2015 to February 2017. Demographic and clinical data were retrospectively collected. Left ventricular mass was measured by echocardiography. LVH is defined by Left ventricular mass index (LVMI) > 115 g/m2 for men and > 95 g/m2 in women. LVMI and relative wall thickness were used to determine left ventricular geometry. Logistic regression was used to analyze the risk factors of LVH.ResultsAltogether, 131 patients including 77 males were enrolled. The median age was 60 (47, 69) years, with a median dialysis vintage of 48 (18, 104) months. There were 80 patients with LVH, the prevalence rate was 61.1%, and 66.3% of them were moderate to severe LVH. We found that (1) most of the patients were concentric hypertrophy; (2) one-third of the patients were concentric remodeling; (3) only 4 cases with normal geometry. The pre-dialysis serum sodium level and time average pre-dialysis systolic blood pressure (SBP) were independent risk factors of LVH.ConclusionLVH is prevalent in MHD patients. Concentric hypertrophy and concentric remodeling are the most common geometric patterns. Attention should be paid to long-term pre-dialysis SBP management and pre-dialysis sodium control as they might be potentially modifiable risk factors for LVH.

In Vitro Pressure Measurements Across an Interatrial Shunt for HFpEF Treatment.

Preserved ejection fraction heart failure (HFpEF) can be treated by installing a shunt in the interatrial septum, which relieves excess pressure in the left atrium by allowing blood to flow from left to right. This technique has proven effective in clinical trials, but the details of the flow through the shunted heart are not well understood. The current study aims to collect quantitative data on the relationship between pressure and flow rate in such shunts. An in vitro, shunted double atrium flow phantom was fabricated and used to investigate the relationship between pressure drop and flow across an interatrial shunt. Flow rate was controlled and the resulting pressure drop across the shunt was measured for a variety of flow cases, including steady and pulsatile flow, flow rate waveforms typical of healthy and failing hearts, and low and high heart rates. The results show a positive relationship between shunt flow rate and pressure drop which is more pronounced in steady flow than in pulsatile flow. Increasing heart rate increases the time-averaged pressure drop across the shunt but not the maximum pressure drop. For steady-flow cases, large changes in pressure drop resulting from moderate changes in flow rate suggest a flow regime transition during parts of the cardiac cycle. Comparison of time-averaged pulsatile flow pressure measurements with steady-flow measurements and two analytical plate-orifice models suggests that none approximate pulsatile flow accurately. The flow rate/pressure drop relationship across an in vitro model of an interatrial shunt has been measured for a variety of physiologically relevant cases. Among other things, the results suggest that steady flow approximations to the heart's pulsatile flow should be used with caution and simplified theoretical models do not approximate the flow rate/pressure drop relationship accurately.

The Effect of Inflow Distortion on the Rotordynamic Characteristics of a 1400-MW Reactor Coolant Pump Annular Seal

The annular seal between stator and rotor substantively acts as a bearing that affects the rotordynamic characteristic of the turbomachinery rotor system. The rotor wake turbulence in a canned motor Reactor Coolant Pump (RCP) will lead to inflow pressure distortion at the annular seal entrance, thus further affecting the seal rotordynamic characteristics and threatening the stable operation of RCP. In order to obtain the seal rotordynamic coefficients, a transient numerical method applies the mesh deformation technique to simulate the multiple-frequency elliptical rotor whirling orbit model. The transient solutions were proposed to solve the unsteady reaction forces of annular seals at five excitation frequencies for each case. The inflow pressure distortion patterns were simplified as harmonic functions, including two important influence parameters that are impeller blades number m and pressure fluctuation ratio λ. The numerical results showed that with nonuniform time-averaged pressure distribution at the entrance of the annular seal in Case 2, the inflow distortion significantly affects the seal rotordynamic coefficients, while the rotational spinning speed in Case 3 can weaken the time-averaged nonuniformity and accordingly make a dent in the influence. Increasing impeller blades number m and pressure fluctuation λ both result in a sharp diminution of the negative stiffness Keff, as well as an obvious increase in the effective damping Ceff, which will strengthen rotor misalignment and system stability. In addition, the larger impeller blades number m and higher pressure fluctuation λ will make the effective damping Ceff more independent of the whirling frequency. These results provide theoretical guidance for the operation safety of RCP.

The role of channel height and actuation method on particle manipulation in surface acoustic wave (SAW)-driven microfluidic devices

Surface acoustic wave (SAW) micromanipulation offers modularity, easy integration into microfluidic devices and a high degree of flexibility. A major challenge for acoustic manipulation, however, is the existence of a lower limit on the minimum particle size that can be manipulated. As particle size reduces, the drag force resulting from acoustic streaming dominates over acoustic radiation forces; reducing this threshold is key to manipulating smaller specimens. To address this, we investigate a novel excitation configuration based on diffractive-acoustic SAW (DASAW) actuation and demonstrate a reduction in the critical minimum particle size which can be manipulated. DASAW exploits the inherent diffractive effects arising from a limited transducer area in a microchannel, requiring only a travelling SAW (TSAW) to generate time-averaged pressure gradients. We show that these acoustic fields focus particles at the channel walls, and further compare this excitation mode with more typical standing SAW (SSAW) actuation. Compared to SSAW, DASAW reduces acoustic streaming effects whilst generating a comparable pressure field. The result of these factors is a critical particle size with DASAW (1 upmum) that is significantly smaller than that for SSAW actuation (1.85 upmum), for polystyrene particles and a given lambda _{text {SAW}} = 200 upmum. We further find that streaming magnitude can be tuned in a DASAW system by changing the channel height, noting optimum channel heights for particle collection as a function of the fluid wavelength at which streaming velocities are minimised in both DASAW and SSAW devices.

Plasma selenium and the risk of first stroke in adults with hypertension: a secondary analysis of the China Stroke Primary Prevention Trial

Previous studies have indicated that selenium (Se) may play an important role in cardio-cerebrovascular disease. However, the relation between circulating selenium and risk of first stroke remains inconclusive. We conducted a secondary analysis of the China Stroke Primary Prevention Trial (CSPPT), using a nested case-control design, and aimed to investigate the correlation between Se concentration and first stroke risk in adults with hypertension and examine the potential effect modifiers. In the CSPPT, a total of 20,702 adults with hypertension were randomly assigned to a double-blind daily treatment with either 10 mg enalapril and 0.8 mg folic acid or 10 mg enalapril alone. A total of 618 first stroke cases and 618 controls matched for age, sex, treatment group, and study site were included in this study. During a median follow-up duration of 4.5 y (IQR: 4.2-4.6 y), there was a significant inverse association between plasma Se and the risk of first stroke (per SD increment; adjusted OR: 0.81; 95% CI: 0.68, 0.96) and ischemic stroke (per SD increment; adjusted OR: 0.76; 95% CI: 0.62, 0.93). Furthermore, a stronger inverse association between plasma Se and first stroke was observed in participants with higher folate concentrations at baseline [≥7.7 ng/mL (median), adjusted OR: 0.67; 95% CI: 0.54, 0.85, compared with <7.7 ng/mL, adjusted OR: 0.98; 95% CI: 0.80, 1.21; P-interaction = 0.008] and those with higher time-averaged systolic blood pressure (SBP) over the treatment period (≥140 mm Hg, adjusted OR: 0.71; 95% CI: 0.58, 0.86, compared with <140 mm Hg, adjusted OR: 0.96; 95% CI: 0.77, 1.20; P-interaction = 0.023). There was a significant inverse association between plasma Se and risk of first stroke in Chinese adults with hypertension, especially among those with higher baseline folate concentrations and those with higher time-averaged SBP over the treatment period. This trial was registered at clinicaltrials.gov as NCT00794885.

Effects of viscoelasticity on the onset of vortex shedding and forces applied on a cylinder in unsteady flow regime

The present paper aims to investigate the effect of viscoelasticity on the onset of vortex shedding of a high concentration polymer solution over a cylinder using the finite volume method for the first time. To describe the behavior of the viscoelastic fluid, mathematically, the Phan–Thien–Tanner (PTT) model is employed. The convergence problems are resolved using the rheoFoam solver developed by previous researchers based on the log-conformation method. The exact critical Reynolds number (Recr), which corresponds to the onset of vortex shedding, is estimated by implementing numerous unsteady simulations at each elasticity number (El). The Recr is also calculated at every retardation ratio (β) and elongational viscosity. The results revealed a significant impact of viscoelasticity on Recr so that the flow of a high viscosity ratio PTT becomes unstable at higher Re (at very low El) or lower Re (at higher El), compared to a Newtonian fluid. In addition, Recr decreases linearly with β according to Recr=−34.5β+46.525 and increases with extensional viscosity. It is also found that β plays a vital role in the effect of viscoelasticity on the flow parameters. The averaged drag coefficient (CD¯) and the amplitude of lift coefficient (CLmax) do not have similar behaviors for low and high β. Moreover, viscoelasticity enlarges the vortices and increases the shedding frequency. A comprehensive physical analysis of flow structures is carried out using the distribution of time-averaged stress components and pressure over the cylinder. The numerical results demonstrated the three regimes of drag reduction at El &amp;lt; 0.015, drag enhancement at 0.015 &amp;lt; E1 &amp;lt; 1, and a Newtonian behavior at El &amp;gt; 1 that is an opposite trend compared to a steady regime. The variations of CLmax with El are also similar to CD¯, but at different critical elasticity numbers (El = 0.005 and 2). It is found that the normal stress changes the drag force by the variation of pressure distribution over the cylinder, while the shear stress directly affects the drag and lift forces. In addition, the viscoelasticity decreases the size of the vortices behind the cylinder and increases their vorticity, and changes the position of maximum normal stress, which leads to drag variations. It was also concluded that the higher the elongational viscosities, the lower the shedding frequency.

Iteration-Based Recycling and Reshaping Method for Inflow Turbulence Generation and Its Evaluation

For the analysis of the aerodynamic characteristics of the buildings immersed in the atmospheric boundary layer (ABL), it is necessary to generate a turbulence velocity field with similar temporal and special characteristics to the ABL to obtain a reliable result. In this paper, an improved precursor simulation method called the recycling and reshaping method (RRM) is proposed to generate a turbulent boundary layer in an LES model. The laminar inflow is firstly disturbed by the virtual roughness blocks realized by adding drag force term in the momentum equation, then the inflow velocity profile is reshaped every several steps to adjust the streamwise velocity profile in the downstream target area to meet the requirements. The final turbulence field generated by RRM with virtual roughness blocks is in good agreement with the target velocity conditions. Then, the simulation of the wind-induced pressure on an isolated low-rise building surface is carried out, using the generated turbulence boundary layer as inflow. The comparison between numerical results and TPU aerodynamic database shows that the time-averaged wind-induced surface pressure obtained by LES can be considered in good accordance with the measurements over the whole building surface. However, the non-ignorable deviations for the fluctuating pressure result in the flow separation corners still exist.

Viscoacoustic squeeze-film force on a rigid disk undergoing small axial oscillations

This paper investigates the air flow induced by a rigid circular disk or piston vibrating harmonically along its axis of symmetry in the immediate vicinity of a parallel surface. Previous attempts to characterize these so-called ‘squeeze-film’ systems largely relied on simplifications afforded by neglecting either fluid acceleration or viscous forces inside the thin enclosed gas layer. The present viscoacoustic analysis employs the asymptotic limit of small vibration amplitudes to investigate the flow by systematic reduction of the Navier–Stokes equations in two distinct flow regions, namely, the inner gaseous film where streamlines are nearly parallel to the confining walls and the near-edge region of non-slender flow that features gas exchange with the surrounding stagnant atmosphere. The flow in the gaseous film depends on the relevant Stokes number, defined as the ratio of the characteristic viscous time across the film to the characteristic oscillation time, and on a compressibility parameter, defined as the square of the ratio of the acoustic time for radial pressure equilibration to the oscillation time. A Strouhal number based on the local residence time emerges as an additional governing parameter for the near-edge region, which is incompressible at leading order. The method of matched asymptotic expansions is used to describe the solution in both regions, across which the time-averaged pressure exhibits comparable variations that give opposing contributions to the resulting time-averaged force experienced by the disk or piston. A diagram structured with the Stokes number and compressibility parameter as coordinates reveals that this steady squeeze-film force, typically repulsive for small values of the Stokes number, alternates to attraction across a critical separation contour in the parametric domain that exists for all Strouhal numbers. This analysis provides, for the first time, a unifying viscoacoustic theory of axisymmetric squeeze films, which yields a reduced parametric description for the time-averaged repulsion/attraction force that is potentially useful in applications including non-contact fluid bearings and robot locomotion.

Conformal Mapping-Based Discrete Vortex Method for Simulating 2-D Flows around Arbitrary Cylinders

A novel technique based on conformal mapping and the circle theorem has been developed to tackle the boundary penetration issue, in which vortex blobs leak into structures in two-dimensional discrete vortex simulations, as an alternative to the traditional method in which the blobs crossing the boundary are simply removed from the fluid field or reflected back to their mirror-image positions outside the structure. The present algorithm introduces an identical vortex blob outside the body using the mapping method to avoid circulation loss caused by the vortex blob penetrating the body. This can keep the body surface streamlined and guarantees that the total circulation will be constant at any time step. The model was validated using cases of viscous incompressible flow passing elliptic cylinders with various thickness-to-chord ratios at Reynolds numbers greater than Re = 1 × 105. The force and velocity fields revealed that this boundary scheme converged, and the resultant time-averaged surface pressure distributions were all in excellent agreement with wind tunnel tests. Furthermore, a flow around a symmetrical Joukowski foil at Reynolds number Re = 4.62 × 104, without considering the trailing cusp, was investigated, and a close agreement with the experimental data was obtained.

Research@ZaB: Study of FDS Capabilities to Assess the High-Speed Train Impact on Pressure Pattern Within a\xa0Railway Tunnel

The “wind tunnel” approach is applied to study high-speed train aerodynamics in a railway tunnel using FDS software. The main focus of the research is on the pressure distribution along the tunnel. Proven analytical dependencies based on the experimental observations for air jet centerline velocity and flow entrainment are used to evaluate the model setup. A model verification is carried out based on the pressure drop calculations due to viscous effects where the impact of the surface roughness and the tunnel length are also considered. A sensitivity analysis is performed to evaluate changes in input FDS parameters and to explore interactions between them. It is proposed to use the standard deviation, obtained from the calculated time-averaged pressure values, to specify the appropriate numeric parameter combinations, e.g. DT and PRESSURE_TOLERANCE, considering the desired results consistency and the computational time consumed. The simulated cases with and without a train inside a tunnel provide data on the aerodynamic characteristics of the models. The obtained volumetric and cross-sectional profiles for pressure and airflow velocity distribution form the basis for an informed decision regarding the tunnel design or safety solutions, for example, defining areas under maximal and minimal pressure loads. The analysis displays the necessity to carefully manage each investigated case considering the FDS features and limitations that largely affect a model setup and calculations.

Data-based estimation and simulation of compressible pulsating flow with reverse-flow through an orifice

Highly compressible pulsating flows are often encountered in devices where knowledge of the flow rate is required but elimination of pulsations is not an option. The current work is a continuation of a previous investigation that characterized the orifice discharge coefficient Cd as a function of dimensionless groups based on pulsation characteristics. The experimental apparatus has been rebuilt in the current work to mitigate temperature and vibration problems, allowing pressure and ΔP measurements to be made very close to the test section with 159-mm of nylon tubing. Data was acquired for 77 operating conditions spanning a range of pulsation frequencies, mass flow rates and system pressures. They confirm previously reported low Cd's in 0.20 range (calculated from time-averaged pressures) at some high-pressure low-flow operating conditions. Computational Fluid Dynamics (CFD) simulations of 12 of these data points suggest that the low Cd's result from reverse flow. Flow direction changed several times during each pulsation cycle closely tracking the orifice ΔP. A ‘core-and-sheath’ phenomena was observed for reverse-flow operating conditions: a positive core flow surrounded by a sheath of negative flow transitioned to a negative core and positive sheath several times during each pulsation cycle. The simulations also suggested that velocity profiles at the orifice stay stable and similar to steady-state profiles except for periods of rapid transitions. Based on these results a data-based quasi-steady method of estimating pulsating flow has been proposed. A pair of forward and reverse flow Cd's chosen by the data are used to predict instantaneous forward and reverse flows using the steady-state orifice discharge equation for compressible flow. The instantaneous values are then summed up over the pulsation cycle to estimate average mass flow rate. Average prediction errors were within 6%. A previously proposed method where regression was used to model Cd as a function of dimensionless groupings was shown to produce similar results. Both methods are designed to extract information from experimental data in order to overcome theoretical limitations and experimental error. The data is available upon request for further understanding of the flow physics.

Experimental Investigation of Secondary Flows and Length Reduction for a Low-Pressure Compressor Transition Duct

Abstract The need to reduce fuel-burn and emissions is pushing turbofan engines toward geared architectures with higher bypass ratios and small ultrahigh-pressure ratio cores. However, this increases the radial offset between compressor spools leading to a more challenging design for compressor transition ducts. For the duct connecting the fan to the engine core, this is further complicated by poor-quality flow generated at the fan hub, which is characterized by low total pressure and large rotating secondary flow structures. This paper presents an experimental evaluation of a new rotor designed to produce these larger flow structures and examines their effect on the performance of an engine sector stators (ESS) and compressor transition duct. Aerodynamic data were collected via five-hole probes, for time-averaged pressures and velocities and phase-locked hot-wire anemometry to capture the rotating secondary flows. The data showed that larger structures promoted mixing through the ESS increasing momentum exchange between the core and boundary layer flows. Measurements within the duct showed a continued reduction in the hub boundary layer, suggesting the duct had moved further from separation. Consequently, an aggressive duct with 12.5% length reduction was designed and tested and measurements confirmed the duct remained fully attached. Total pressure loss was slightly increased over the ESS, but this was offset by reduced loss in the duct due to improved flow quality. Overall, this length reduction represents a significant cumulative effect in reduced fuel-burn and emissions over the life of an engine.