Effect Of Pressure Oscillations Research Articles

Effects of pressure oscillations on impinging-jet atomization and spray combustion in liquid rocket engines

Effects of pressure oscillations on impinging-jet atomization and spray combustion in liquid rocket engines

Flame-acoustic interactions in high-pressure H2/air diffusion flames

Understanding flame-acoustics interaction (FAI) that can potentially trigger thermoacoustic instabilities is of critical importance to mitigate the serious side effects of pressure oscillations. While FAI has been studied extensively in premixed flames, comparatively less analysis has been conducted to explore FAI mechanisms in non-premixed flames, especially under high-pressure conditions. This study aims to numerically investigate FAI in high-pressure hydrogen/air counterflow diffusion flames, with special attention on the impact of pressure on FAI. To this end, a fully compressible unsteady counterflow solver combined with Navier-Stokes Characteristic Boundary Conditions (NSCBC) is employed to capture the reflection of the acoustic waves at the boundaries. The results show that for all ambient pressures ( 1 ≤ P a ≤ 60 bar) considered here, real-gas effects on flame structures are negligible and that increasing P a would lead to a power-law dependence of density gradient on P a with an exponent of 1.5. Furthermore, all tested cases show acoustic growth with time under fully reflecting boundary conditions, which is found to be pressure dependent. The flames become less resilient to FAI when the pressure (or density gradient) is increased. The results from the spectral analysis show that for all ambient pressures, there is only a single dominant frequency at the low-density fuel side and two dominant frequencies at the high-density oxidiser side. This indicates that the pressure oscillation is more sinusoidal at the H 2 side than that at the air side. Moreover, the phase space portraits of pressure signals indicate that the dynamics of the system are becoming periodic and approaching a limit cycle. It is found that the integrated heat release rate and the pressure fluctuations are partially in phase, which is responsible for the growth of high-frequency acoustic waves at both fuel and oxidiser sides. On the other hand, the significant density gradient contributes to the growth of low-frequency acoustic waves at the oxidiser side due to the high degree of acoustic reflectivity at the density boundary.

Acoustic pressure oscillation effects on mean burning rates of plateau propellants

• Influence of acoustic pressure oscillations on mean burning rates of plateau propellants. • Non-aluminized and aluminized propellants exhibit plateau burning trends are examined with corresponding matrixes. • Magnitudes of imposed acoustic amplitudes are varied from 0.1% to 1.4% with respect to mean chamber pressure conditions. • Both non - aluminized propellants and aluminized propellants show significant enhancement in the mean burning rate due to the fluctuations imposed by acoustic pressure amplitudes and frequencies on the propellant combustion. • Maximum burning rate augmentation factors of non-aluminized and aluminized are 1.27 and 1.47 respectively. Combustion instabilities constitute a well pronounced problem in all large rocket motors due to the inherent acoustic pressure oscillations established based on the port geometries. Suppression of the combustion instabilities in the solid rocket motor is required for controlling the mean burning rate variation which arises due to the interaction of acoustic pressure wave with propellant combustion. An experimental study has been carried out to investigate the effects of acoustic pressure oscillations on mean burning rates of non-aluminized and aluminized propellants which exhibit low pressure exponent index (n) in the burning rate trends. Steady and unsteady mean burning rates are determined from combustion photography method using a window bomb test facility over the pressure range of 17 MPa. A rotary valve is coupled with the window bomb setup to generate acoustic pressure oscillations inside the test chamber (cylindrical pressure vessel), which imposes the required frequencies of 140, 180 and 240 Hz respectively. The acoustic pressure amplitudes are varied from 0.04% to 1.4% of the mean chamber pressure. Both non-aluminized propellants and aluminized propellants have shown significant enhancement in the mean burning rate due to the fluctuations imposed by acoustic pressure amplitudes and frequencies on the propellant combustion. The enhancement in the mean burning rate also depends upon dynamic response of the flame to the excited frequencies and acoustic pressure amplitudes. The plateau burning behaviour of the non-aluminized propellant is completely distorted whereas it is retained in aluminized compositions. Conversely, it also shifts the mean pressure range of plateau burning rate trend. The maximum burning rate augmentation factors resulted from imposed acoustic pressure wave on non-aluminized and aluminized propellants are observed as 1.27 and 1.47 respectively.

Effects of pressure oscillation on aerodynamic characteristics in an aero-engine combustor

The effects of pressure oscillation on aerodynamic characteristics in an aero-engine combustor are investigated. A combustor test rig is designed to simulate the pressure drop characteristics of a practical annular combustor. The pressure drop characteristics are firstly measured under atmosphere condition with non-reacting flow (or cold flow), and the air mass flow proportion of each component (dome/liner) are obtained; these properties are base lines for comparison with combustion state. The combustion tests are then carried out under conditions of inlet temperature 340–450 K, fuel air ratio 0.010–0.028. The stability map and the oscillation frequencies are obtained in the tests, the results show that pressure oscillation amplitude increases with the increase of fuel air ratio. Phase trajectory reconstruction is applied to classify the pressure oscillation motion; there are three motions captured in the tests including: “disk”, “ring” and “cluster”. The pressure drops across the dome under strong pressure oscillation are distinctly divergent from the cold flow, and the changes of pressure drops are mainly affected by pressure oscillation amplitude, but is less influenced by pressure oscillation motion nor oscillation frequencies. Based on the mass flow conservation, the reduction of effective flow area of combustor under strong pressure oscillation is demonstrated. Liner wall temperatures are analyzed through Multiple Linear Regression (MLR) method to estimate the reduction of the air mass flow proportion of the liner cooling under strong pressure oscillation. Finally, the air mass flow proportions of each component under strong pressure oscillation are estimated, the results show that the pressure oscillation motion also has influence on air mass flow proportion.

The effect of pressure-oscillation on the suspension flowing through a sudden contraction-sudden expansion channel

The effect of pressure-oscillation on the suspension flowing through a sudden contraction-sudden expansion channel

Effect of pressure-oscillation on gas absorption rate from a bubble in viscous fluid

Effect of pressure-oscillation on gas absorption rate from a bubble in viscous fluid

Effect of Pressure Oscillations on Flashback Characteristics in a Turbulent Channel Flow

A flashback phenomenon is reproduced by a direct numerical simulation (DNS) of a premixed hydrogen–air flame in a turbulent channel flow, and effects of the pressure oscillation, is caused by a virtual combustion instability on the flashback characteristics, are investigated. In addition, the interaction between the flashback flame structure and the turbulent flow structure is examined in detail. For the hydrogen–air reaction, a chemical reaction model, which considers 9 chemical species and 20 reactions, is employed. The results show that the pressure oscillation strongly affects the flashback characteristics, such as the flashback speed and heat flux on the wall, and is at risk for increasing the flashback speed, because the acceleration of the flashback speed by the pressure oscillation is greater than the deceleration of it. Independent of the pressure oscillation, the turbulent eddies passing through the flame tend to be suppressed and vanish by the acceleration of the streamwise flow caused by therm...

Analysis of pressure oscillations and safety relief valve vibrations in the main steam system of a Boiling Water Reactor

Steam flow inside the main steam lines of a Boiling Water Reactor can generate high-amplitude pressure oscillations due to coupling between the separated shear layer at the mouth of the safety relief valves (SRVs) and the acoustic modes of the side branches where the SRVs are mounted. It is known that certain combinations of flow velocities and main steam line geometries are capable of generating self-excited pressure oscillations with very high amplitudes, which can endanger the structural integrity of main steam system components, such as safety valves, or reactor internals such as steam dryers. However, main steam systems may also experience lower amplitude pressure oscillations due, for example, to coupling of higher order hydrodynamic modes with acoustic cavity modes, or to incipient resonances where the free stream velocity is slightly lower than the critical flow velocity required to develop a stable locked-on acoustic resonance. The amplitude of these pressure oscillations is typically insufficient to cause readily observable structural damage to main steam system components, but may still have subtle effects on safety relief valves. The investigation presented in this article focuses on the characterization of the response of SRVs under the effects of pressure oscillations associated with acoustic excitations that are insufficient to cause structural damage to the valves or associated equipment. It is shown that valve internals exhibit non-periodic impulsive vibratory behavior when submitted to a continuous sinusoidal pressure oscillation caused by acoustic resonances inside the SRV standpipes. These acceleration peaks are caused by small impacts between the valve disc and nozzle surface.

Effect of pressure oscillations on in-cylinder heat transfer – through large eddy simulation

As fuel consumption is a key issue for next-generation internal combustion engines, the heat release rate is increased and the duration shortened towards partially premixed combustion and in extreme cases towards homogeneous charge compression ignition to increase thermal efficiency. However, a steep rise in the heat release rate may trigger pressure oscillations in the combustion chamber, which have shown to increase the heat transfer, lowering efficiency and increasing fuel consumption. The aim of this research is to find the physical mechanisms that cause the increased in-cylinder heat transfer in the presence of pressure oscillations. According to the author’s knowledge, the physical mechanisms responsible for the increased heat transfer have yet not been well understood for this application. Several of the hypotheses for this work are therefore based on the research performed for pulsating turbulent pipe flow. A numerical study has been performed using the large eddy simulation approach, where the pressure oscillations in the combustion chamber have been triggered by an artificially imposed heat source. The results show an increase in heat transfer in relation to pressure amplitude, in accordance with previous experimental studies. The mechanism found is a rapid transport of high-temperature fluid from the heat source towards the wall due to large-scale velocity fluctuations emerged from the pressure oscillations resulting in increased heat transfer.

Meteotsunami in the Great Lakes and on the Atlantic coast of the United States generated by the “derecho” of June 29–30, 2012

Tsunami-like intense sea-level oscillations, associated with atmospheric activity (meteorological tsunamis), are common in the Great Lakes and on the East Coast of the United States. They are generated by various types of atmospheric disturbances including hurricanes, frontal passages, tornados, trains of atmospheric gravity waves, and derechos. “Derecho” is a rapidly moving line of convectively induced intense thunder storm fronts producing widespread damaging winds and squalls. The derecho of June 29–30, 2012 devastatingly propagated from western Iowa to the Atlantic coast, passing more than 1,000 km and producing wind gusts up to 35 m/s. This derecho induced pronounced seiche oscillations in Lake Michigan, Chesapeake Bay, and along the US Atlantic coast. Sea-level records from the updated National Oceanic and Atmospheric Administration (NOAA) tide gauge network, together with the NOAA and automated surface-observing system air pressure and wind records, enabled us to examine physical properties and temporal/spatial variations of the generated waves. Our findings indicate that the generation mechanisms of extreme seiches in the basins under study are significantly different: energetic winds play the main role in seiche formation in Chesapeake Bay; atmospheric pressure disturbances are most important for the Atlantic coast; and the combined effect of pressure oscillations and wind is responsible for pronounced events in the Great Lakes. The “generation coefficient,” which is the ratio of the maximum observed sea-level height and the height of air pressure disturbance, was used to map the sea-level response and to identify “hot spots” for this particular event, i.e., harbors and bays with amplified seiche oscillations. The Froude number, Fr = U/c, where U is the speed of the atmospheric disturbance and c is the long-wave speed, is the key parameter influencing the water response to specific atmospheric disturbances; the maximum response was found for those regions and disturbance parameters for which Fr ~1.0.

Simulation and Study of the Effect of Pressure Oscillations on Linear Combustion Instability in a Double Base Solid Rocket Motor

The combustion instability remained as a problem during solid rocket motors progress. In this study, combustion instability of a double base solid rocket motor was estimated wherein acoustical and erosive burning was considered. The ballistic parameters of the solid rocket motor were presented and the results were compared with simulations, which show a good agreement. The result shows that specific impulse of the solid propellant rocket motor was obtained 211 s and the maximum pressure of the combustion chamber reaches 13.8 MPa. Combustion instability analysis indicates that the motor became unstable after 0.5 s.

Effects of Pressure Oscillations on Drainage in an Elastic Porous Medium

The effects of seismic stimulation on the flow of two immiscible fluids in an elastic synthetic porous medium is experimentally investigated. A wetting fluid is slowly evacuated from the medium, while a pressure oscillation is applied on the injected non- wetting fluid. The amplitude and frequency of the pressure oscillations as well as the evacuation speed are kept constant throughout an experiment. The resulting morphology of the invading structure is found to be strongly dependent on the interplay between the amplitude and the frequency of the applied pressure oscillations and the elasticity of the porous medium. Different combinations of these properties yield morphologically similar structures, allowing a classification of structures that is found to depend on a proposed dimensionless number.

Effect of Pressure Oscillation in Fluids on Structure by MPS Method

For unreal pressure oscillation in fluid particles due to MPS method, two algorithms which corrected the source term in the Poisson equation are compared with original MPS method. In addition, wave boundary condition is improved to generate solitary wave by a wave maker with piston type. The incident wave heights are compared to experimental data. In dynamic analysis of fluid-structure interaction problems, the influence of pressure oscillation on the structure deformation and the fluid force is investigated in detail. As a result, although it was indicated that the two algorithms restrain the pressure oscillation, their coefficients suited to numerical conditions will be required. Further, we made clear that the oscillation of pressure strongly affects the structure deformation because the fluid force does not accurately act on the elastic body.

Gold Nanoparticle Deposition Using CO2 Expanded Liquids: Effect of Pressure Oscillation and Surface−Particle Interactions

Postsynthesis processing of nanoparticles to obtain mesoscale hierarchal nanostructures is the key for the development of nanotechnology and smart composites/coatings from these materials. We have utilized gas-expanded liquid deposition of alkyl-coated gold nanoparticles to study the effects of variable process flowrates, variable flow oscillation and variable interaction potential of the substrate on nanoparticle array quality. Array quality is measured here as completeness of area surface coverage of approximately a monolayer of nanoparticles. Quantitative values for surface coverage are averages obtained from multiple TEM photomicrographs using Image J digital analysis. The process was modified using higher CO2 addition rate outside of the pressure range necessary for deposition, and this modified process produced an excellent film quality while reducing overall processing time by 45%. The effects of pressure oscillation during deposition appeared to anneal the film at the lower flow rates, 0.5 and 1.0 mL/min, but a reduction in area coverage was observed with pressure oscillation at 3.0 mL/min. Pressure oscillation has emerged as a useful tool for researchers to tune the film uniformity and therefore the surface roughness. Calculations based on Hamaker theories for surface-particle interactions on various substrates were performed, and better surface coverage was predicted for C-based surfaces compared to Si3N4 and SiO2 surfaces. Indeed, experimental studies verified this general ordering, indicating that if surface interactions with the particles are strong deposition directly on the surface rather than on pre-existing nanoparticle islands may govern uniform deposition.

모형 가스터빈 연소기의 입구 형상변화에 따른 연소 불안정성에 관한 LES 연구

The effects of combustion instability on flow structure and flame dynamics with the inlet configurations in a model gas turbine combustor were investigated using large eddy simulation (LES). A G-equation flamelet model was employed to simulate the unsteady flame behaviors. As a result of mean flow field, the change of divergent half angle(α ) at combustor inlet results in variations in the size and shape of the central toroidal recirculation (CTRZ) as well as the flame length by changing corner recirculation zone (CRZ). The case of α = 45° show smaller size and upstream location of CTRZ than those of 90° and 30° by the development of higher swirl velocity. The flame length in the case of α = 45° is shorter than other cases, while the case of α = 30° yields the longest flame length due to the decrease of effective reactive area with the absence of CRZ. Through the analysis of pressure fluctuation, it was identified that the case of α = 45° shows the largest damping effect of pressure oscillation in all configurations and brings in the noise reduction of 2.97㏈, compared to that of α = 30° having the largest pressure oscillation. These reasons were discussed in detail through the analysis of unsteady phenomena related to recirculation zone and flame surface. Finally the effects of flame-acoustic interaction were evaluated using local Rayleigh parameter.

Semi-Analytical Solutions for the Effect of Well Shut-Down on Rock Stability

Abstract Poorly consolidated rock around the wellbore may become unstable after the process of well shut-down and restart. Possible reasons for instability include pressure recovery and water hammer effects. These are defined and analyzed by two different fluid mechanics models. The pressure recovery model results in a quasi-static solution defined in the context of Darcy theory. The water hammer model is based on inertial effects inside the wellbore, and seeks to specify the magnitude of short-term stress variation from the generation of a compressional shockwave due to sudden shut-down of a flowing well. To quantify the effect of well shut-down on rock stability, a fully coupled geomechanics model accounting for the changes of fluid pressure is developed. The redistribution of fluid pressure in a reservoir are analytically solved and coupled with the stress model, while the water hammer equations provide a boundary condition for the bottom-hole pressure. This approach allows a direct solution for the relationships among fluid properties, rock properties, and production parameters, within the context of rock stability, and defined as an effective stress-dependent strength or yield threshold. Model calculations demonstrate that the fluctuations of effective stresses and shear stresses may reach several hundred kPa because of the pressure wave created by a water hammer inside the wellbore. The model can be applied to evaluate the risks of triggering rock instability and select the wells that may start sanding if they are shut-down or started up abruptly. Furthermore, the paper provides a method to quantify the effect of pressure fluctuation on rock stability. Introduction Dynamic pressure fluctuations near a well in a reservoir can, in principle, lead to rock instability; however, there is little field documentation available. Santarelli et al. reported the injectivity decline of some water injectors in the Norwegian Sea after well shut-in(1). Dusseault et al. developed a new workover method to clean up wellbore damage based on a strong dynamic pressure pulse method, clearly demonstrating that dynamic pressure perturbation can re-initiate sand influx in heavy oil wells that use sand production as a means of recovery(2). Santos has indicated that during the well drilling process, pressure oscillation at bottom hole not only can destabilize sand but also shale(3). The lack of more extensive documentation of dynamic pressure pulse induced instability is, we believe, due to several reasons: first, monitoring rapid downhole pressure fluctuations has traditionally been difficult; second, the presence of gas in the oil dampens the effect of pressure oscillations; and third, the impact of dynamic effects on sand instability has not been widely understood. We note also that the solution to this problem is, in principle, relatively simple: smooth production rate or injection rate reductions can be easily implemented to avoid abrupt pressure changes. However, it is valuable to evaluate rock stability before a production strategy is chosen, and a thorough understanding of mechanisms and quantitative analysis of their potential effects should be pursued. When production (or injection) is adjusted at the wellhead, or when the drilling string is moved up or down during drilling, pressure fluctuations will appear within the wellbore.

Pressure Oscillation Effects on the Saffman-Taylor Instability

Pressure Oscillation Effects on the Saffman-Taylor Instability

Effects of periodic air pressure oscillation on growth of Rhizobium leguminosarum in a solid state fermenter

Charcoal impregnated with a medium was used to culture Rhizobium leguminosarum in a periodic pressure oscillating, solid-state fermenter. Effects of pressure oscillation amplitude, the span of high pressure, and the duration of low pressure on the growth of R. leguminosarum were investigated in detail. At 30 °C and 3 atm, R. leguminosarum was fermented and subsequently reached 4.9×10 10 cfu g −1 substrate dry matter in about 36 h, while only 1.6×10 10 cfu g −1 substrate dry matter was obtained in a conventional static tray fermenter.

SYNOPTIC: Drag of Rectangular Cavities in Supersonic and Transonic Flow Including the Effects of Cavity Resonance,

The drag of relatively short rectangular cavities (length-to-depth ratios of 0.50-3.0) with turbulent shear layers has been measured at transonic and supersonic Mach numbers (0.33.0). Of particular importance is the effect of pressure oscillations within the cavity (commonly referred to as cavity resonance) which is shown to increase the drag as much as 250%. Cavity resonance is found to occur over the entire Mach number range investigated and other work has shown it to occur at both lower and higher Mach numbers. The frequency of the pressure oscillations is best predicted by the vortex-wave interaction model presented by Rossiler. The effects of external reinforcement of resonance by reflection of radiated pressure waves are examined both experimentally and analytically. Existing methods for predicting cavity drag are inadequate to cope with this phenomena. A new model for predicting the lower bound of cavity drag (nonresonating cavity) is presented and agrees well with experimental data. Analytical considerations are utilized to indicate the qualitative effect of resonance on cavity drag.

Response of a burning fuel plate to sound vibrations.

Combustion instability is considered from the viewpoint of the time-dependent response of a flat-plate boundary layer with mass transfer to standing or traveling acoustic vibrations in the gas. The acoustic admittance is denned in terms of the response of the boundary layer at i] = co and is employed to obtain necessary conditions for wave amplification. Results for the gasification rate at the wall, the wall shear stress, and the acoustic admittance are obtained as expansions in the parameter ux/U; co = oscillation frequency, x = distance from the upstream edge of the plate, and 17 = velocity of gas relative to the plate. It is shown that the zero-order (quasisteady) response rarely leads to amplification of a standing wave. Effects of pressure oscillations alone seldom produce wave amplification. The velocity oscillations lead to the greatest acoustical energy emission and absorption, and the presence of a flame in the boundary layer greatly enhances the influence of velocity oscillations. Results to first and second orders in the parameter ux/U are presented. The opposite limiting case, in which (i)X/Ui& large compared with unity, is also considered.