Frequency Pressure Measurements Research Articles

High temperature fuel impacts on combustion characteristics of a swirl-stabilized combustor

Future aircraft will require advanced thermal management systems to manage the increased heat loads resulting from the avionics, turbomachinery and other subsystems of higher performance, and high speed flight systems. Fuel is the primary coolant to alleviate these thermal management challenges. Heated fuel entering the combustor is likely to affect combustion performance due to dramatic changes in properties that impact fuel atomization, distribution, vaporization and fuel/air mixing. In this study, the combustion performance, emissions, and flame characteristics of a single-nozzle, swirl-stabilized combustor operating with high temperature fuels were investigated. Four fuels representing a wide range of jet fuel physical and chemical properties were studied at injection temperatures of 32, 121 and 260 °C. The test fuels included a conventional Jet A (A-2), a low cetane number alcohol-to-jet (ATJ) alternative fuel (C-1), n-dodecane (n-C12) and a high viscosity jet fuel blend (C-3A). Tests were conducted at a combustor pressure of 2 atm and inlet air temperature of 121 °C at global equivalence ratios (ϕ) of 0.10–0.24 for most fuel temperatures. Particulate matter (PM) emissions (concentrations and size distributions), and primary gaseous emissions were measured. High frequency pressure measurements and high-speed video were collected to assess fuel temperature impacts on combustion stability and flame characteristics, respectively. Results show that the heated fuel impacts on combustion were highly dependent on fuel type, temperature and ϕ. Qualitative results show that as fuel temperature increased, the flame in the primary zone appeared more compact, and less thermally radiant (lower soot). PM emissions were significantly impacted with reductions exceeding 95 % in particle number density for all fuels, and between 20 and 55 % reductions in mean particle diameters for aromatic-containing fuels. Gaseous emissions show up to ∼ 90 % reduction in unburned hydrocarbons (UHC) and up to 60 % reduction in carbon monoxide (CO), and corresponding moderate increases in NOx and CO2 with the heated fuel. The combustor acoustic response was slightly suppressed in frequencies of interest as fuel temperature increased. In general, the impacts of heated fuel on combustion were mostly beneficial and are primarily attributed to improved fuel atomization and reduced fuel evaporation timescales of the heated fuel. Specific impacts of fuel physical and chemical properties, and potential implications of heated fuel combustion are discussed.

Mechanism and detection of oscillatory flow in a supersonic cascade

The throttling process in a supersonic cascade is investigated in a free-jet wind tunnel at Mach 2.4. High-speed schlieren imaging and high-resonance frequency pressure measurements are used to capture the flow features. A flap is placed at the channel exit to choke the flow linearly. In the throttling process before the onset of oscillatory flow, the shock train leading shock leads to large area of flow separation near the suction surface and high total pressure loss. The oscillatory flow is triggered by the expansion of the flow separation induced by the impingement of the shock train leading shock onto the suction surface, which belongs to the Dailey criterion. The alarm is triggered during oscillatory flow detection, based on the pressure data at the optimal S5 transducer, either before the large-amplitude pressure fluctuation begins or after it ends. Comparing with other algorithms, the derivative-based algorithm has no data to be stored and no expensive calculation is the best technique for detection. To adjust the alarm margin, three detection points are set upstream of the S5 transducer and detection is performed based on the schlieren images. All algorithms detect the sudden drop in light intensity caused by the arrival of the shock train leading shock at the detection location. The alarm margin is the largest at point C, which is farthest from the suction surface tip, and smallest at point A, which is closest to the suction surface tip.

Singular spectrum analysis as a tool for early detection of centrifugal compressor flow instability

Centrifugal compressor machinery is subject to a potentially damaging phenomenon called surge at low mass flow rates. This effect may be preceded by a phenomena known as inlet recirculation — a flow reversal upstream of the impeller. A methodology to isolate inlet recirculation as a characteristic feature for monitoring of centrifugal compressor instability is presented in this study. The methodology is based on a nonparametric time series analysis technique called as singular spectrum analysis (SSA). SSA decomposes a signal into a number of Reconstructive Components (RCs), from which data trends and oscillatory components may be extracted. The frequency spectra of each RC and their relative contributions to the reconstruction of the original signal were examined and comparisons were made with spectral maps in existing literature. Individual and independent RCs were chosen to construct a compressor’s instability monitoring system. Additionally, the performance of SSA was determined by the Window length parameter. The effect of modification of this parameter was also studied, and the various viable choices of component for the basis of inlet recirculation diagnosis were considered. The methodology was implemented in pressure dynamical signals measured in an experimental centrifugal compressor rig. High frequency pressure measurements were taken at a number of flow conditions and locations within the compressor. The results demonstrated the potential of a methodology based on SSA to identify and extract oscillatory components with information about the local effect of inlet recirculation and eventually successfully monitor centrifugal compressor’s instability.

Propagation of shock-wave/boundary-layer interaction unsteadiness in attached and separated flows

The origin and propagation of low-frequency shock oscillation unsteadiness in the attached and separated flows are investigated. Wind tunnel experiments are performed in an isolator at Mach 1.85 and 2.7 with three types of upstream wedges, generating weak and strong background waves. High-speed schlieren imaging and high-resonance frequency pressure measurements are used to capture the flow features. In the attached flow with weak background waves, the impingement of the reflected shocks along the flow strengthens the original instability waves from the shock oscillation, resulting in the correlation drop and time-delay rise with the original instability waves. In the attached flow with strong background waves, two-point correlation analyses show that the shock oscillations propagate along the shock structure and convection of the boundary layer structures, which enhances the turbulence pulsation in the boundary layer. The correlation and coherence results for pressure indicate that the incident points of two independent background waves move in opposite directions, while the incident points of two merged background waves move in the same direction. Using downstream throttling, the shock train in the separated flow is introduced. Based on the phase analysis of schlieren images, the feedback mechanism of the shock train oscillation is described, which is related to the acoustic wave propagation and the duct volume effect. Power spectra of the pressure in the upstream attached and downstream separated flows of the shock train show that the perturbation pathways in the attached and separated flows do not affect each other.

Statistical Short Time Analysis for Proton Exchange Membrane Fuel Cell Diagnostic‐Application to Water Management

AbstractHigh frequency concomitant voltage and pressure measurements on a single proton exchange membrane fuel cell are presented in this paper. Voltage, anodic and cathodic pressure measurements were shown to be highly sensitive to gas relative humidity and temperature of the cell. Fast voltage dips corresponding to pressure spikes at the anode were observed and indicate a flooding occurrence due to the blockage of anode inlet by liquid water. In a generalize flooding of the cell, pressure drop fluctuations are easily recognizable and result in voltage fluctuations and drift. A methodology based on short‐time analysis on both, voltage and pressure signals, is used to quantify the effects of flooding and drying within proton exchange membrane fuel cell (PEMFC) systems. Intermittent behavior is highlighted on the standard deviation of the voltage for flooding and drying cases. Simultaneous analysis of the standard deviation short‐time for voltage and pressure signals allows for the differentiation between flooding and drying cases. Concomitant pressure and voltage fluctuations can be easily detected and monitored for a reliable diagnostic of proton exchange membrane fuel cell systems.

A heterogeneous benchmark dataset for data analytics: Multiphase flow facility case study

Improvements in sensing, connectivity and computing technologies mean that industrial processes now generate data from a variety of disparate sources. Data may take a number of forms, from time-domain signals, sampled at various rates using a variety of sensors, to alarm and event logs. Novel techniques need to be developed to tackle the challenges of heterogeneous data. Testing such algorithms requires benchmark datasets that allow direct comparison of the performance of the methods. This work presents the PRONTO heterogeneous benchmark dataset. Experiments were conducted on a multiphase flow facility under various operational conditions with and without induced faults. Data were collected from heterogeneous sources, including process measurements, alarm records, high frequency ultrasonic flow and pressure measurements. The presented dataset is suitable for developing and validating algorithms for fault detection and diagnosis and data fusion concepts. Three algorithms are tested using the dataset, illustrating the applicability of the dataset.

Investigation of internal flow pattern of a multiphase axial pump

Due to recent advances in computational techniques, single phase performances of pumps can be easily estimated using CFD. However, when the working fluid is a mixture of liquid and gas (multi-phase flow) computations may struggle to capture the more complex physical phenomena that occurs, and the precision of the prediction worsen as the gas volume fraction at pump inlet is increased. To provide reliable benchmark for the improvement of simulations, in this study we experimentally investigated the internal flow pattern of a multiphase pump using high speed camera and high frequency pressure measurements, so as to obtain the data of pressure distribution inside of impeller, or to obtain blade loading, for several gas concentration conditions. Unsteady phenomena were identified at low flow rate conditions, and an explanation of their nature has been attempted.

Oscillation of the shock train in an isolator with incident shocks

The oscillation characteristics of the shock train in an isolator have been investigated in a direct-connect wind tunnel at Mach 2.7. High-speed schlieren imaging and high-resonance frequency pressure measurements were used to capture the flow features during the shock train movement. The oscillation features without the effects of incident shocks were analyzed first. As the shock train moved upstream, the low-frequency part of the oscillation was found to develop. The analysis was then extended to complex situations with incident shocks. It was revealed that the shock wave-boundary layer interactions considerably influence the shock train behavior. The interactions were classified into three patterns: (I) single interaction, (II) multi-interactions on the same side, and (III) multi-interactions on different sides. Experimental results indicated that the oscillation could be affected in temporal scale by pattern II and enhanced in spatial scale by pattern III. The data also showed that the pressure rise induced upstream propagates to the exit, causing phase offsets in the wall pressure histories and making the pressure distributions diverge from their stable state. This phenomenon suggested a possible physical mechanism for the oscillation during shock train movement, which was verified by additional tests with large backpressure rising rate. It was found that there exists a critical frequency which is related to the pressure ramping rate during the oscillation. If the dominant frequency of the backpressure varies beyond this critical frequency, the pressure distribution could be forced into a steady state before the oscillation was induced. Otherwise the oscillation could not be suppressed.

Experimental investigation into the impact of density wave oscillations on flow boiling system dynamic behavior and stability

In order to better understand and quantify the effect of instabilities in systems utilizing flow boiling heat transfer, the present study explores dynamic results for pressure drop, mass velocity, thermodynamic equilibrium quality, and heated wall temperature to ascertain and analyze the dominant modes in which they oscillate. Flow boiling experiments are conducted for a range of mass velocities with both subcooled and saturated inlet conditions in vertical upflow, vertical downflow, and horizontal flow orientations. High frequency pressure measurements are used to investigate the influence of individual flow loop components (flow boiling module, pump, pre-heater, condenser, etc.) on dynamic behavior of the fluid, with fast Fourier transforms of the same used to provide critical frequency domain information. Conclusions from this analysis are used to isolate instabilities present within the system due to physical interplay between thermodynamic and hydrodynamic effects. Parametric analysis is undertaken to better understand the conditions under which these instabilities form and their impact on system performance. Several prior stability maps are presented, with new stability maps provided to better address contextual trends discovered in the present study.

Field Measurements of Terrestrial and Martian Dust Devils

Surface-based measurements of terrestrial and martian dust devils/convective vortices provided from mobile and stationary platforms are discussed. Imaging of terrestrial dust devils has quantified their rotational and vertical wind speeds, translation speeds, dimensions, dust load, and frequency of occurrence. Imaging of martian dust devils has provided translation speeds and constraints on dimensions, but only limited constraints on vertical motion within a vortex. The longer mission durations on Mars afforded by long operating robotic landers and rovers have provided statistical quantification of vortex occurrence (time-of-sol, and recently seasonal) that has until recently not been a primary outcome of more temporally limited terrestrial dust devil measurement campaigns. Terrestrial measurement campaigns have included a more extensive range of measured vortex parameters (pressure, wind, morphology, etc.) than have martian opportunities, with electric field and direct measure of dust abundance not yet obtained on Mars. No martian robotic mission has yet provided contemporaneous high frequency wind and pressure measurements. Comparison of measured terrestrial and martian dust devil characteristics suggests that martian dust devils are larger and possess faster maximum rotational wind speeds, that the absolute magnitude of the pressure deficit within a terrestrial dust devil is an order of magnitude greater than a martian dust devil, and that the time-of-day variation in vortex frequency is similar. Recent terrestrial investigations have demonstrated the presence of diagnostic dust devil signals within seismic and infrasound measurements; an upcoming Mars robotic mission will obtain similar measurement types.

Unstart phenomena induced by mass addition and heat release in a model scramjet

The unstart phenomena in a model scramjet with a free stream Mach number of 4.5 were investigated at an arc-heated hypersonic wind tunnel. High-speed schlieren imaging and high resonance frequency pressure measurements were used to capture the flow features during the unstart process. Three unstart conditions were tested: (i) a low-enthalpy free stream with mass loading, (ii) a high-enthalpy free stream with mass loading and (iii) a high-enthalpy free stream with mass loading and heat release. It was revealed that the unstart threshold and the time from the onset to the completion of unstart depended strongly on the mass loading rate and the heat exchange. The negative heat addition (cooling) significantly increased the threshold of mass flow rate triggering unstart. The decrement of the mass flow rate threshold for unstart was observed in the presence of heat release by combustion. The observed transient and quasi-steady behaviours of the unstart shockwave system and the jet motion were similar in all of the test conditions. On the other hand, at the lip of inlet model, the unstart shockwave under the cold free stream condition exhibited a relatively steady behaviour while severe oscillatory flow motions of the jet and the unstart shockwave were observed in the combustion-driven unstart process. The different unstarted flow behaviours between the three flow conditions were explained using a simplified one-dimensional flow choking analysis and use of the Korkegi criterion.

Hypersonic Wind-Tunnel Measurements of Boundary-Layer Transition on a Slender Cone

Boundary-layer transition was studied on a sharp 7 deg cone in two hypersonic wind tunnels at Mach numbers of 5, 6, 8, and 14 over a range of freestream Reynolds numbers between 3.3 and . High-speed schlieren measurements visualized the intermittent formation of instabilities and turbulent spots within the transitional boundary layer. Surface pressure and heat-transfer measurements revealed how the intermittent behavior of the boundary layer produces the mean character of these quantities. Transition at Mach 5 appeared to be initiated by a combination of first- and second-mode instabilities. These disturbances were isolated and surrounded by an otherwise smooth boundary layer. At higher Mach numbers, the boundary layer was dominated by second-mode instabilities, which covered most of the model before breakdown into turbulent spots. The spots remain surrounded by second-mode waves throughout the transitional region. These differences alter the pressure fluctuations and heat transfer profiles during transition. Higher frequency pressure measurements peaked upstream of the onset of transition because of the growth of second-mode instabilities. Lower frequency pressure fluctuations and the surface heat transfer did not rise significantly until further downstream where turbulent regions developed.

Nanosecond pulsed sliding dielectric barrier discharge plasma actuator for airflow control: Electrical, optical, and mechanical characteristics

Plasma actuators used for active flow control are widely studied because they could replace mechanical actuators. Industrial applications of these plasma actuators sometimes require a large surface plasma sheet in view of increasing the interaction region between the discharge and the incoming flow. Instead of using a typical two-electrode nanosecond pulsed dielectric barrier discharge for which the interaction region is limited to about 20 mm, this study proposes to characterize a nanosecond sliding discharge based on a three-electrode geometry in order to increase the extension length up to the electrode gap. This sliding discharge is compared to the typical nanosecond dielectric barrier discharge by means of electrical, optical, and mechanical diagnostics. Electrical characterization reveals that the deposited energy can be widely increased. Time-resolved Intensified Charge Coupled Device (iCCD) images of the discharge development over the dielectric surface highlight that the intensity and the propagation velocity of streamers are strongly affected by the DC voltage applied at the third electrode. Finally, qualitative and quantitative characterizations of the pressure wave due to the surrounding gas heating are proposed by means of Schlieren visualizations and high frequency pressure measurements, respectively.

Experimental Research on the Propagation Characteristics of Continuous Rotating Detonation Wave Near the Operating Boundary

Propagation characteristics of continuous rotating detonation wave near the operating boundary are presented. A series of tests have been carried out, and an operating domain is obtained, beyond which the continuous rotating detonation wave cannot be maintained. Based on the high frequency pressure measurements, flow-field structure in a continuous rotating detonation engine (CRDE) is constructed. The features include an upstream detonation wave and a downstream oblique shock wave. Propagation velocity deficits of the detonation wave have been calculated, and a critical value of 15–16.5% is obtained. When the velocity deficit is less than the critical value, the reactants are mainly consumed by the upstream detonation wave. Otherwise, further heat release may occur when the unburned mixture encounters the downstream shock wave.

Transient behavior of shock train with and without controlling

The present paper is a study of the transient behavior of shock train in a constant area isolator with and without controlling method. Two kinds of vortex generators were installed and tested. Planar laser scattering technique was performed to study the three dimensional structure of the shock train and its temporal evolution. High frequency pressure measurements were conducted to reveal the shock train movement and its frequency-domain feature. Three states of the shock train during its formation and movement were revealed based on the wavelet decomposition. The shock train location and its movements were detected by method of integral of pressure derivative and wavelet decomposition. And based on the comparison of the flow structures, shock train movements and the pressure recovery effect, a recommendation of applying passive control method for the isolator was proposed.

Experimental Study of Gas Turbine Combustion With Elevated Fuel Temperatures

The effects of elevated fuel temperatures on the performance and emissions of a Rolls-Royce 501K combustor with a modified fuel injector designed to handle two-phase flow is investigated in this study. A series of tests were run using Jet-A fuel at ambient, 250°F, 450°F, and 600°F fuel temperatures. The influence of the new fuel injector design was compared with previous collected test data using a legacy 501K fuel injector. In order to mitigate autoxidative coking, nitrogen sparging was employed to remove dissolved oxygen from the fuel during high temperature tests. Oxygen levels in the fuel prior to heating were on average less than 0.2% of fully saturated values. At the highest fuel temperatures, test conditions with combustion pressures below the vaporization pressure of the fuel led to lower combustion efficiencies most likely due to flashing in the injector. During all test conditions, high frequency pressure measurements and emissions samples including unburned hydrocarbons, carbon monoxide, and NOx were recorded in addition to conventional pressure and temperature measurements. The trends in combustion efficiency and levels of emissions due to increasing fuel temperatures are discussed in this paper.

Semi-empirical Method for Estimation of Energy Losses in a Large- scale Pipeline

The high frequency (0.1kHz) pressure and velocity measurements of two-phase unsteady flow are used to determine the hydraulic-grade-line (HGL) along the pipeline with a complex layout. The semi-empirical method is applied to estimate the pressure drop for the outflow, and thus determine the pipe downstream-end conditions for the modeling purposes. A Control- Volume (CV) model coefficient β, representing residual motion due to the pressurized water-column stratification is specified. Pressurized two-phase unsteady flow phases in a 250mm diameter and 275 meters long PVC pipeline with steel inlet and outlet counterparts are considered for the evaluation of this method.

Simultaneous time-resolved fluctuating temperature and acoustic pressure field measurements in a premixed swirl flame

A micro-thermocouple, microphones and microphone probes were employed to provide detailed information on key ongoing processes in a premixed swirl flame. High frequency pressure and temperature measurements were carried out to identify temperature field and acoustic characteristics of a swirl stabilized premixed flame. The local distributions of fluctuating pressure and temperature were measured in different regions, in and around the flame. Noise sources were investigated by using microphone probes and microphone arrays. Temperature and pressure fluctuations were examined both inside and outside the flame boundary. Sensor network used here is essential for the development of advanced combustion systems for it can provide detailed temporal and spatial information that can allow for better control to achieve higher efficiency and performance.

Vector sound intensity measurements with a tetrahedral arrangement of microphones in a spherical probe

The three dimensional sound intensity vector may be measured using a number of microphone configurations, an example of which is an orthogonal arrangement of three matched pairs of microphones. This setup is prone to unwanted reflections, however, and uses more microphones than theoretically required. Results are therefore presented on the calculation of the sound intensity vector using atetrahedral arrangement of microphones housed within a solid sphere. The acoustic presence of the sphere is quantified; specifically, frequency dependent phase and pressure measurements due to diffraction are obtained for various incidence angles and compared with theoretical values. The proposed intensity probe is further evaluated against traditional arrangements for the calculation of the acoustic intensity vector and advantages of adopting the spherically mounted, tetrahedral arrangement are highlighted.

Acoustic characterisation of a rectangular rocket combustor with liquid oxygen and hydrogen propellants

High frequency combustion instabilities have plagued the development of liquid propellant rocket engines since their invention. Continuing research efforts aim to understand the mechanisms by which the oscillating combustion chamber pressure of self-sustaining combustion instabilities is driven. To this end, a rectangular combustor with acoustic forcing, designated ‘BKH’, was developed to study flame-acoustic interaction under conditions which are representative of real rocket engines. This article describes the acoustic characterisation of the BKH combustor using high frequency dynamic pressure measurements from the first hot-fire tests using liquid oxygen and ambient temperature or cryogenic hydrogen injectants. Analysis of the resulting high frequency pressure measurements shows excellent agreement with the predicted acoustic properties of the system which were calculated using finite element methods.