Dynamic Pressure Transducers Research Articles

Development of Fibre Bragg grating (FBG) dynamic pressure transducer with diminutive voltage inconsistency

In this work, a diaphragm-type Fibre Bragg grating (FBG) dynamic pressure transducer was designed and developed. Pressure transducer in this study consists of elastic thin metal diaphragm which acts as a primary sensing element; and integrated with an FBG sensor for the secondary sensing element. In the common match filter interrogation system, converting optical signal to voltage is the challenging due to voltage reading inconsistency, which would cause variation in pressure reading. New alternative arrangements of matched filter interrogation system are used to address the issue. For automated pressure measurement, the optical signal from FBG was converted into voltage by using the proposed arrangement of matched filter interrogation system. Additional FBG was added as reference sensor which installed in the system for detection the change of broadband light source. Reduction of voltage inconsistency was achieved by normalizing the voltage reading from sensor with the voltage reading from reference FBG. The result shows that the FBG pressure transducer is proven to be suitable for pressure measurement of gas or liquid with an average error of 5.348%. Furthermore, the FBG sensor has a good linearity with a linear correlation coefficient of 97.29% in pressure measurement.

Customized Software and Hardware applied to Assesment of Outwardly Spherical Flames Using the Pressure Trace: a Thermodynamic Approach to Improve Accuracy of Laminar Flame Speed Measurements

Outwardly propagating spherical flames are well suitable for measurements of Laminar Flame Speed which is a very important thermochemical property of flammable mixtures. There are several facilities, approaches and methods applied to measure Laminar Flame Speed. Due this variety of measurements, consistency and accuracy of the results are mandatory for useful comparisons. Confined premixed flames were assessed using a customized spherical stainless steel vessel, equiped with a Schlieren setup and a dynamic pressure transducer, allowing the comparison between the syncronized results obtained using both approaches. The thermodynamic method presented in this paper uses experimental pressure traces as input to a FORTRAN code that calculates temporal profiles of flame radius, flame surface area, laminar flame speed and some additional quantities. The proposed algorithm processes the raw pressure trace, controls flow of data to an external code to calculate chemical equilibrium and thermodynamic properties and calculates the rate of reactants burned by the flame propagation. The strategy allows to obtain the laminar flame speed without the need to model heat transfer from the flame, diminishing uncertainties in the calculations. The calculations of the proposed FORTRAN code enables to obtain the Laminar Flame Speed at initial datum and the flame behavior for higher pressures and temperatures, maximizing the results for each experiment. Measurements were carried out for air mixtures with methane, iso-octane, n-heptane, Jet A and Jet A-1. The laminar flame speed is the highest for the Jet fuels at Ti = 408 K and lowest for iso-octane at Ti = 300 K, about 50% higher for stoichiometric mixtures. The faster flames for Jet fuels presented the highest exergy destruction, about 30%. The joint analyses of the results of flame radius, Laminar Flame Speed, the exergetic assessment of the flame propagation and the calculated equilibrium composition of the burned gases showed the consistency and good accuracy of the thermodynamic approach here presented.

Experimental Verification of Buffet Calculation Procedure Using Unsteady Pressure-Sensitive Paint

Typically, a limited number of dynamic pressure sensors is employed to determine the unsteady aerodynamic forces on large, slender aerospace structures. This paper describes a robust calculation procedure based on frequency-specific correlation lengths and validation from an experiment conducted on a flat panel coated with fast-response pressure-sensitive paint. The first part of the paper describes the procedure used to analyze the pressure-sensitive paint images and a calibration method using dynamic pressure transducers. Excellent comparison in spectra, coherence, and phase, measured via pressure-sensitive paint and dynamic pressure sensors, validates the pressure-sensitive paint data. The second part of the paper describes the buffet validation process, the first step of which was to use pressure histories from all pixels to determine the true force fluctuations. In the next step, only a selected number of pixels was chosen as virtual sensors, and a correlation-length-based buffet calculation procedure was applied to determine modeled force fluctuations. By progressively decreasing the number of virtual sensors, it was observed that the present calculation procedure was able to make a close estimate of the true unsteady forces only from eight sensors. It is believed that the present work provides the first validation of a buffet calculation procedure.

An investigation on characterization of dynamic pressure transducers using material impact test machine

The need for reliable dynamic measurements and characterization of pressure transducers are important, especially in the fields of aerospace, defense and machinery manufacturing. Such measurements are performed on systems in which input pressure varies over time and must accurately capture the dynamic variations in pressure over short time intervals. This study presents a research work and measurements on the dynamic calibration of pressure transducers using a drop mass system and a high-speed impact tester. The impact test machine also uses the drop mass principle similar to some secondary level dynamic measurement instruments. The pulses which are produced by the impact test machine can be used as a dynamic pressure generator for the test and characterization of pressure transducers. For this purpose, a new experimental calibration setup was constructed on the impact test machine. The dynamic behaviors of pressure transducers were investigated using pulses obtained from varying dropping speeds and energies of the drop mass in this new setup. Experimental results were compared with the results obtained from a manual drop mass pulse generator. Results show that while there is some variance about 15% at 200 MPa as the variance becomes less than 5% at 300, 400, and 500 MPa. As a result, it is concluded that an impact test machine can be used as a reliable dynamic pulse generator.

Structural instability and water hammer signatures from shock-initiated implosions in confining environments

An experimental study is conducted to investigate the dynamic response and instability of cylindrical structures subjected to hydrostatic pressure in conjunction with explosive loading. Full-field displacements/velocities, and localized pressures, of imploding aluminum structures within a confining environment are captured during the experiments. Also, polyurea coatings of 1:1 volume ratios are evaluated as a possible energy mitigation technique. Two high-speed cameras are used to capture the imploding structures while seven dynamic pressure transducers measure the emitted pressure pulses. The specimens are confined inside a thick-walled cylindrical structure that had one end open to the hydrostatic pressure and the other end closed. This confinement configuration generates a water hammer at the closed end of the confinement. The results show that the pressure profiles of the hydrostatic and explosive initiated implosions are about the same after collapse. Moreover, the energy from the implosion's high-pressure pulses, present at the confinement's closed end, was greater than the energy of the explosive itself due to the water hammer effect. The polyurea coatings used in this study caused a sufficient phase shift in the implosion pressures such that the hammer and implosion high-pressure pulses were not superimposed; thus, the maximum pressures and energy after the implosion was reduced. However, the polyurea coatings did not significantly mitigate neither the hammer nor the implosion pressures individually. Finally, a non-linear Riks model from ABAQUS was used to show that the energy input requirement for dynamic-initiated implosions decreases rapidly as initial hydrostatic pressure increases.

Experimental Determination of Mechanical Stress Induced by Rotating Stall in Unshrouded Impellers of Centrifugal Compressors

Unshrouded centrifugal compressor impellers typically operate at high rotational speeds and volume flow rates. The resulting high mean stress levels leave little margin for dynamic excitations that can cause high-cycle fatigue. In addition to the well-established high-frequency impeller blade excitations of centrifugal compressors caused by the stationary parts, such as vaned diffusers or inlet guide vanes (IGVs), the presented study addresses an unsteady rotating flow feature (rotating stall) which should be taken into account when addressing the high-cycle fatigue during the design phase. The unsteady fluid–structure interaction between rotating stall and unshrouded impellers was experimentally described and quantified during two different measurement campaigns with two full-size compression units operating under real conditions. In both campaigns, dynamic strain gauges and pressure transducers were mounted at various locations on the impeller of the first compression stage. The casing was also equipped with a set of dynamic pressure transducers to complement the study. Rotating pressure fluctuations were found to form an additional impeller excitation at a frequency that is not a multiple of the shaft speed. The measurements show that the excitation amplitude and frequency caused by the rotating pressure fluctuations depend on the operating conditions and are therefore challenging to predict and consider during the design phase. Furthermore, the excitation mechanism presented was found to cause resonant impeller blade response under specific operating conditions. For the experimentally investigated impeller geometries, a rotating pressure fluctuation caused approximately 1.5 MPa of additional dynamic stress in the structure per 1 mbar of dynamic pressure amplitude when exciting the first bending mode of the impeller. The induced dynamic mechanical stresses due to rotating stall are in the order of 10% of the endurance limit of the material for the tested impeller geometries; therefore, they are not critical and confirm a robust and reliable design.

Comparison of model measured runner blade pressure fluctuations with unsteady flow analysis predictions

An accurate prediction of pressure fluctuations in Francis turbines has become more and more important over the last years, due to the continuously increasing requirements of wide operating range capability. Depending on the machine operator, Francis turbines are operated at full load, part load, deep part load and speed-no-load. Each of these operating conditions is associated with different flow phenomena and pressure fluctuation levels. The better understanding of the pressure fluctuation phenomena and the more accurate prediction of their amplitude along the hydraulic surfaces can significantly contribute to improve the hydraulic and mechanical design of Francis turbines, their hydraulic stability and their reliability.With the objective to acquire a deeper knowledge about the pressure fluctuation characteristics in Francis turbines and to improve the accuracy of numerical simulation methods used for the prediction of the dynamic fluid flow through the turbine, pressure fluctuations were experimentally measured in a mid specific speed model machine. The turbine runner of a model machine with specific speed around nq,opt = 60 min-1, was instrumented with dynamic pressure transducers at the runner blades. The model machine shaft was equipped with a telemetry system able to transmit the measured pressure values to the data acquisition system. The transient pressure signal was measured at multiple locations on the blade and at several operating conditions. The stored time signal was also evaluated in terms of characteristic amplitude and dominating frequency.The dynamic fluid flow through the hydraulic turbine was numerically simulated with computational fluid dynamics (CFD) for selected operating points. Among others, operating points at full load, part load and deep part load were calculated. For the fluid flow numerical simulations more advanced turbulence models were used, such as the detached eddy simulation (DES) and scale adaptive simulation (SAS). At the different operating conditions, distinct flow phenomena were available for evaluating the accuracy of the simulation method, e.g. rotor- stator interaction, rotating vortex rope and runner channel vortex. The experimentally obtained pressure time signals at the runner, characteristic amplitude and frequency at several operating points offered the possibility to assess the precision in the prediction of the pressure fluctuations in the Francis turbine using the numerical simulation methods.

Measurement of unsteady transition on a pitching airfoil using dynamic pressure sensors

Unsteady boundary layer transition on a pitching OA209 airfoil in a wind tunnel was detected by using pressure fluctuation measurement at different oscillation frequency. Thirty Kulite dynamic pressure transducers flush-mounted on the airfoil surface recorded pressure signatures, and root mean square of pressure signatures were calculated. Results indicated that the criterion of transition for static airfoil defined as the peak of root mean square of pressure fluctuation was still suitable for detection of transition on a pitching airfoil. Fixed transition experiment for pitching airfoil was performed to validate the conclusion. Effect of oscillation frequency on transition was investigated. For small reduced frequency, the hysteresis loop is larger near leading edge. With increasing in the oscillation frequencies, the transition was promoted and relaminarization was enhanced.

Mitigation of pressure pulses from implosion of hollow composite cylinders

An experimental study on the underwater collapse of composite tubes with polymeric coatings is conducted in an attempt to mitigate the implosion pressure pulse released. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure during collapse. Filament-wound carbon-fiber/epoxy tubes are studied with polyurea coatings of different thicknesses on the interior and exterior of the tube to explore the effects of these configurations on implosion pulse mitigation. 3-D Digital Image Correlation (DIC) technique is used to capture the full-field deformation and velocities during the implosion event. Local pressure fields generated by the implosion event are measured using dynamic pressure transducers to evaluate the strength of the emitted pressure pulses. Local pressure data and DIC results are used to obtain a measure of normalized energy released during implosion. Results show that thick interior coatings significantly reduce the energy released in the pressure pulse by slowing the collapse and softening the initial wall-to-wall contact. In contrast, thick exterior coatings increase this energy by suppressing damage, thereby reducing the energy absorption capacity of the structure.

Experimental Investigation on Underwater Buckling of Thin-Walled Composite and Metallic Structures

An experimental study on the underwater buckling of composite and metallic tubes is conducted to evaluate and compare their collapse mechanics. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure through the collapse. Filament-wound carbon-fiber/epoxy, glass/polyester (PE) tubes, and aluminum tubes are studied to explore the effect of material type on the structural failure. Three-dimensional digital image correlation (DIC) technique is used to capture the full-field deformation and velocities during the implosion event. Local pressure fields generated by the implosion event are measured using dynamic pressure transducers to evaluate the strength of the emitted pressure pulse. The results show that glass/PE tubes release the weakest pressure pulse and carbon/epoxy tubes release the strongest upon collapse. In each case, the dominating mechanisms of failure control the amount of flow energy released.

Amplitude characteristics of pressure oscillation caused by steam jet condensation in a confined channel

Steam jet injector is a prospective device with advantages of impact volume, strong heat transfer, without moving part, which can be applied in various industrial fields such as nuclear/conventional power plant, heat-supply system and heat recovery system, etc. However, as the steam jet injector is driven by high speed steam jet condensation in subcooled water flow in the mix chamber, the instability of steam-water direct contact condensation will lead to loud noise and severe vibration. Thus, loud noise will evolve into environmental pollution, and vibration will endanger the equipment, which prevent the promotion of this device in induatrial applications. Moreover, the published works generally involved in the pressure oscillation caused by steam jet condensation in large space water pool, where the water was considered to be stagnant. However, the case becomes more complex when direct contact condensation occurs in a confined channel like in a steam jet injector, where the effect of turbulence on water side and wall can not be ignored. Thus, the reported results can not be applied in the present condition. So it is necessary to study the mechanism of the instability of steam-water direct contact condensation in a confined channel. In this paper, experimental investigation was performed on the amplitude characteristics of pressure oscillation caused by direct contact condensation in a confined channel, aiming at revealing the mechanism of noise and vibration and providing some guidance for the design and operation of the steam jet injector. In order to better observe the behavior of the steam-water interface, a special visualized test section was designed and a high speed camera was applied. The dynamic pressure data was obtained by a dynamic pressure transducer arranged on the wall of channel. Three main inlet parameters, including steam mass flux, water mass flux and water temperatures were tested in the ranges of 150–600 kg/m2s, 6×103–18×103 kg/m2s and 15–40°C, respectively. The results indicated that when direct contact condensation occurred in a confined channel, flow patterns could be divided into two categories-unstable and stable flow patterns with respect to the interface behavior. The effect of the inlet parameters on amplitude characteristics of pressure oscillation was discussed. With the increase of water mass flux, the amplitude altered slightly in stable flow pattern region, but increased suddenly and dramatically in unstable region. With the increase of steam mass flux, the amplitude increased a little in unstable region, but decreased suddenly in stable region and then increased again. And the increase of water temperature leaded to the increasing of amplitude in all flow pattern regions. In a word, the equipment would bear stronger impact load when operating in unstable flow pattern region.

Experimental study of pressure fluctuation in a gas-solid cyclone separator

In order to get further insight into the dynamic property of the vortex, pressure signals at various axial, radial and circumferential positions in a gas-solid cyclone were measured by multi-input dynamic pressure transducer. By adopting probability density function, standard deviation, and power spectral density methods, the characteristics of the pressure signals were analyzed and discussed. Experimental results show that there are two dominant frequencies (about 71Hz and 179Hz) of the pressure fluctuation in the gas flow: the former (about 71Hz) is related to the quasi-forced vortex rotation; the latter (about 179Hz) is related to the swing of the quasi-forced vortex core. The particle strands motion in the gas-solid flow reduces the dispersion of pressure fluctuation, mitigates the off-axis phenomenon of the core to some extent, and adds a dominant low frequency (0.31Hz–0.86Hz) which gets higher with the increase of inlet solids loading. The experimental results can provide insight into the pressure fluctuation in a gas-solid cyclone separator.

Instrumentation Development and Validation for an Experimental Two-Phase Blowdown Facility

An experimental facility was designed and built to study the loading on steam generator tubes during a blowdown. The facility used refrigerant R-134a and measurements were taken for static and dynamic pressures as well as tube loading and temperatures. Commissioning experiments indicated that the off-the-shelf dynamic pressure transducers and load cells could not take the mechanical and thermal shock loading caused by the blowdown and produced spurious results of no value. This paper presents the instrumentation problems found, explains why they occurred, describes the remedial procedures employed, and outlines the instrumentation validation methodologies developed. The success of the instrumentation development is demonstrated in a series of experiments designed to assess the rapid transient measurement system.

Experimental Investigation of Flow Instability in a Turbocharger Ported Shroud Compressor

Turbocharger centrifugal compressors are equipped with a “ported shroud” to reduce flow instabilities at low mass flow rates. This passive stability control device using flow recirculation has been demonstrated to extend the surge margin of a compressor without substantially sacrificing performance. However, the actual working mechanisms of the system are not well understood. In this paper, the relationship between inlet flow recirculation and instability control is studied using stereoscopic particle image velocimetry (PIV) in conjunction with dynamic pressure transducers at the inlet of the turbocharger compressor with and without ported shroud. Both stable and unstable operational points are analyzed using phase-locked PIV measurements during surge. Detailed description of unstable flow in the centrifugal compressor is presented by reconstructing the complex flow structure evolution in the compressor inlet during surge. Rather than one-dimensional, the surge flow is characterized by a three-dimensional structure of both entering and exiting swirling flows, alternating in magnitude during a self-excited pressure cycle. The correlation between pressure and velocity measurements shows that the development of compressor unsteadiness is concurrent with swirling reversed flow at the impeller tip. The impact of the ported shroud on the inlet velocity flowfield is evidenced by the presence of localized flow recirculation. Stability improvement due to the ported shroud is thus a result of removing swirling backflow from the impeller inducer tip and recirculating it into the impeller inlet to increase the near shroud inlet blade loading and the incidence angle.

Laser Ignition and Flame Speed Measurements in Oxy-Methane Mixtures Diluted With CO2

Ignition and flame propagation in methane/O2 mixtures diluted with CO2 are studied. A laser ignition system and dynamic pressure transducer are utilized to ignite the mixture and to record the combustion pressure, respectively. The laminar burning velocities (LBVs) are obtained at room temperature and atmospheric pressure in a spherical combustion chamber. Flame initiation and propagation are recorded by using a high-speed camera in select experiments to visualize the effect of CO2 proportionality on the combustion behavior. The LBV is studied for a range of equivalence ratios (ϕ = 0.8–1.3, in steps of 0.1) and oxygen ratios, D = O2/(O2 + CO2) (26–38% by volume). It was found that the LBV decreases by increasing the CO2 proportionality. It was observed that the flame propagates toward the laser at a faster rate as the CO2 proportionality increases, where it was not possible to obtain LBV due to the deviation from spherical flame shape. Current LBV data are in very good agreement with existing literature data. The premixed flame model from chemkin pro (Reaction Design, 2011, CHEMKIN-PRO 15112, Reaction Design, San Diego, CA) software and two mechanisms (GRI-Mech 3.0 (Smith et al., 1999, “The GRI 3.0 Chemical Kinetic Mechanism,” http://www.me.berkeley.edu/gri_mech/) and ARAMCO Mech 1.3 (Metcalfe et al., 2013, “A Hierarchical and Comparative Kinetic Modeling Study of C1–C2 Hydrocarbon and Oxygenated Fuels,” Int. J. Chem. Kinetics, 45(10), pp. 638–675)) are used to simulate the current data. In general, simulations are in reasonable agreement with current data. Additionally, sensitivity analysis is carried out to understand the important reactions that influence the predicted flame speeds. Improvements to the GRI predictions are suggested after incorporating latest reaction rates from literature for key reactions.

Combustion instability of pilot flame in a pilot bluff body stabilized combustor

Combustion instability of pilot flame has been investigated in a model pilot bluff body stabilized combustor by running the pilot flame only. The primary objectives are to investigate the pilot flame dynamics and to provide bases for the study of the interaction mechanisms between the pilot flame and the main flame. Dynamic pressures are measured by dynamic pressure transducers. A high speed camera with CH∗ bandpass filter is used to capture the pilot flame dynamics. The proper orthogonal decomposition (POD) is used to further analyze the high speed images. With the increase of the pilot fuel mass flow rate, the pilot flame changes from stable to unstable state gradually. The combustion instability frequency is 136Hz when the pilot flame is unstable. Numerical simulation results show that the equivalence ratios in both the shear layer and the recirculation zone increase as the pilot fuel mass flow rate increases. The mechanism of the instability of the pilot flame can be attributed to the coupling between the second order acoustic mode and the unsteady heat release due to symmetric vortex shedding. These results illustrate that the pilot fuel mass flow rate has significant influences on the dynamic stability of the pilot flame.

Hydrostatic Implosion of GFRP Composite Tubes Studied by Digital Image Correlation

The mechanisms and pressure fields associated with the hydrostatic implosion of glass fiber reinforced polymer (GFRP) tubes with varying reinforcement are investigated using high-speed photography. Experiments are conducted in a large pressure vessel, designed to provide constant hydrostatic pressure throughout collapse. Three-dimensional (3D) digital image correlation (DIC) is used to capture full-field displacements, and dynamic pressure transducers measure the pressure pulse generated by the collapse. Results show that braided GFRP tubes release pressure waves with significantly greater impulse upon collapse as compared to filament-wound tubes, increasing their damage potential.

Nonlinear acoustic response of stand-off tubes used in acoustic pressure measurements—Part II: Analysis

Rocket engine combustion instabilities, which lead to rapid engine failure through enhanced heat transfer rates and high-cycle fatigue, continue to be the most serious concern facing engine designers. Experimental testing and pressure measurements remains the best approach to determine the susceptibility of an engine design to acoustically coupled combustion instabilities. But, the harsh, high-temperature environment requires remotely mounting dynamic pressure transducers to the engine’s main chamber using “sense-tubes.” Experiments reveal that the acoustic response of the sense-tube is highly nonlinear due to the area-contraction at the connection point between the engine and sense-tube. This nonlinearity leads to large discrepancies between the combustor's actual and the remotely measured acoustic amplitudes. Our aim was to develop an accurate, nonlinear sense-tube acoustic response model including steady flow effects. Therefore, the governing equation for the area-contraction pressure drop was approximated using a Fourier based technique to develop expressions for the steady and acoustic pressure drop across the area change. The acoustic pressure drop model was then incorporated into a response model for the tube. Measurements of the acoustic response of the area-contraction without mean flow agree well with predictions of the developed model.

Achieving a High-Speed and Momentum Synthetic Jet Actuator

A detailed analysis of a finite-span synthetic jet in a quiescent fluid is presented, with the goal of achieving a high speed and momentum synthetic jet, with a peak velocity exceeding 200 m/s. A total of two scales of actuator apparatuses having either 40- or 80-mm-diameter piezoelectric discs were used. A temperature-compensated hot wire, laser displacement sensor, and dynamic pressure transducers were used to quantify the performance of a given actuator. The synthetic jet generation process was divided into its main components, diaphragm displacement, cavity pressure, and orifice velocity, which were analyzed in detail in both their peak values and time responses. The phase shift between the disc displacement and velocity signals were found to be mainly attributed to compressibility effects. Doubling the diameter and thickness of the piezoelectric disc was found to double the output jet velocity and shift the operation frequency range of the system. In addition, a dual disc configuration was used, which yielded an approximately 40% higher jet velocity than its single disc configuration. Using the knowledge acquired from these experiments resulted in a synthetic jet having a peak velocity of 211 m/s out of a 12×1-mm rectangular orifice at 700 Hz, which is significantly stronger than any similar actuator reported in the literature.

Shock-Initiated Buckling of Carbon/Epoxy Composite Tubes at Sub-Critical Pressures

A comprehensive investigation on the implosion of composite cylinders subjected to a nearby explosion is performed. Experiments are conducted in a large pressure vessel, designed to provide constant hydrostatic pressure throughout the event. Carbon fiber/epoxy filament-wound tubes are studied with constant hydrostatic pressure and varying charge standoff distances to determine the effect of the explosive loading on the mechanisms of collapse. 3-D Digital Image Correlation (DIC) is used to capture the full-field displacements and velocities during the implosion event, and to characterize the initial dynamic response of the tube. Dynamic pressure transducers measure the shock waves generated by the explosive and also the pressure pulse generated by the collapse. Results show that different magnitudes of explosive loading produce drastic differences in the way implosions are initiated, and in the extent of damage to the structure. Experiments with strong explosive loading show immediate collapse of the tube upon the arrival of shock wave. Relatively smaller explosive loading result in collapses due to the additional bubble pulse loading, or after accumulating damage for extended periods of time.