Sharp Pressure Rise Research Articles

Combustion Instability due to Combustion Mode Transition in a Cavity-Stabilized Scramjet

This study experimentally investigates flame dynamics during a combustion-driven mode transition in a cavity-stabilized scramjet model with an inlet Mach number of 2.2 using a split fuel injection scheme. Fueling into the cavity was gradually increased while keeping a constant global equivalence ratio of 0.3. A cavity-fueling percentage below 15% resulted in low-intensity combustion that corresponded to operation in scram mode. Redirecting 29% of the fuel from the upstream injection port to the cavity yielded flame spreading into the main flowpath, inducing intense combustion. A sharp pressure rise and an intense combustion region in the diverging section suggest dual-mode combustion, with thermal choking and a pseudoshock formation downstream of the cavity. An intermediate cavity-fueling fraction of 21% led to a low-frequency instability () with intermittent transitions between the combustion modes, indicating operation at the thermal choking limit. Spectral and modal analyses of chemiluminescence images and static-pressure measurement indicate peak response at 10–20 Hz and , originating from oscillations in the air heater and a fuel jet instability, respectively. Thermally chocked conditions significantly amplify oscillations at these frequencies. At the thermal choking limit, there was high-amplitude response at , preventing stable dual-mode operation.

Autoignition of Reacting Hydrocarbon Mixture With Negative Temperature Coefficient Due to the Cold-Spot and Cold Chamber Wall

AbstractAutoignition of an n-heptane/air mixture was simulated in nonuniform temperature environments of a rapid compression machine (RCM) and shock-tube (ST) with and without the presence of a cold-spot. The simulations were performed to investigate how the presence of a cold-spot and the cold boundary layer of the chamber wall may affect the ignition delay of the hydrocarbon mixture with negative temperature coefficient (NTC) behavior. The simulations were performed using three models: (1) three-dimensional (3D) computational fluid dynamics (CFD) model, (2) zero-dimensional (0D) homogenous batch reactor model by including the heat transfer model, and (3) 0D adiabatic homogenous batch reactor model. A detailed n-heptane mechanism was reduced in this work and used for 3D combustion modeling. A cold-spot critical radius of 7 mm was determined, which affects the ignition delay by more than 9%. In addition, two combustion modes were observed in the combustion chamber with a nonuniform temperature environment. With the first combustion mode, combustion starts at the high gas temperature region of the combustion chamber and quickly propagates toward the periphery of the chamber. In this combustion mode, the location of the maximum concentration of hydroxyl radical and the maximum temperature are the same. With the second combustion mode, the combustion starts at the periphery of the chamber, where the temperature is lower than the center of the chamber due to heat transfer to the cold chamber wall. The location of maximum concentration of the hydroxyl radical and maximum temperature is different with this combustion mode. The two observed combustion modes are due to the NTC behavior of the n-heptane mixture. The 0D homogenous batch reactor model (with and without heat transfer models) failed to mimic the ignition delay accurately when the second combustion mode was present. In addition, a propagating combustion has been observed in the simulation which is in agreement with some of the optical autoignition diagnostics of these hydrocarbons. This propagating combustion leads to a gradual pressure rise during autoignition, rather than a sharp pressure rise. The results of this work show that 0D homogenous batch reactor models are unable to simulate autoignition of mixtures with NTC behavior.

Spike-type disturbances due to inlet distortion in a centrifugal pump

An inlet elbow installed upstream of centrifugal pump produces distorted suction flow. In this paper, numerical simulation is conducted to investigate interactions between impeller and inlet distortion, with emphasis on the consequent upstream disturbances. At the design point, velocity, pressure and swirling strength at the upstream section are extracted successively to describe the initial upstream disturbance, i.e., a spanwise vortex bubble (IVB1) on inner wall. Additionally, a detailed analysis allow identifications of flow separation and vortex: high incidence due to IVB1 generates two focus-type separations, then separations evolve into two concentrated separation vortices (CSV1 and 2) spanning from the suction surface to the upstream casing. Vortical flow field is further discussed to reveal CSV’s effect on the suction flow: casing-side end of CSV1 moves upstream to form a new vortex bubble (IVB2), which approaches to IVB1 and blocks impeller inlet; vortex-trail of CSV2 with an associated blockage induces another vortex bubble (IVB3) between IVB2 and CSV2. Therefore, new IVBs enhance upstream disturbances and exert a new pressure trace on inner wall, where a vortex bubble produces a rapid pressure drop followed by a sharp pressure rise due to a partial blockage. The resultant down-up waveform demonstrates enhanced disturbances are spike-type.

Tribological Optimization of Thrust Bearings Operated With Lubricants of Spatially Varying Viscosity

In the present work, a computational optimization study of thrust bearings lubricated with spatially varying viscosity lubricants is presented, with the main goal of minimizing friction coefficient. In practice, spatial variation of viscosity could be achieved by utilizing electrorheological or magnetorheological fluids. The bearings are modeled as two-dimensional (2D) channels, consisting of a smooth moving wall (rotor) and a parallel or inclined stationary wall (stator), which can be (i) smooth, (ii) partially textured with rectangular dimples, and (iii) smooth and partially hydrophobic. The bearings are considered to be operated with an ideal lubricant that exhibits different values of viscosity in two distinct regions of the fluid domain: a high viscosity area is considered at the channel inflow, with the viscosity acquiring a reference (low) value farther downstream. The flow field is calculated from the numerical solution of the Navier–Stokes equations for 2D incompressible isothermal flow. The bearing geometry is defined parametrically. Three optimization problems are formulated, corresponding to: (I) a conventional smooth converging slider, (II) a parallel slider with artificial surface texturing at part of the stator surface, and (III) a parallel or converging slider with hydrophobic properties at part of the stator surface. Here, the geometry parameters, as well as the increased viscosity value and the corresponding application regime, form the problem design variables. Bearings are optimized for maximum load capacity and minimum friction coefficient. Optimal solutions are compared against corresponding ones for operation with constant viscosity. It is demonstrated that, by using spatially varying viscosity, a substantial reduction of friction coefficient can be achieved, for all optimization problems considered. This decrease is shown to be a consequence of a sharp pressure rise in the high viscosity regime, resulting in a corresponding rise in load capacity, accompanied by a less pronounced increase in wall shear stress, and thus in total friction force.

337 High-Speed Video Analysis of Highamplitude Bubble-Continuous Positive Airway Pressure Demonstrates That “Water Hammer” Effects Cause Large Oscillations in Airway Pressure

Background and aims: We have shown, in lavaged rabbits, that High-Amplitude Bubble-Continuous Positive Airway Pressure (HAB-CPAP, see figure) provides greater gas exchange and lower work of breathing than conventional bubble-CPAP (PedsResearch, June,2010). The mechanisms generating the high amplitude pressure waves are the focus of this study. Methods: A high-speed camera (PhantomV9.1;900frames/s) was used to visualize, simultaneously, the bubbling action in the HAB-CPAP device and airway pressures (Paw) measured in the patient circuit. Signals were generated with/without an infant lung model attached to the patient circuit. Measured changes in Paw were compared with calculated values obtained using a force balance. Results: Videos revealed sharp increases in Paw concomitantly with the release of bubbles. Water appears to rush down the inclined tube and transfers momentum to the air it contacts, producing a water hammer effect. The rapid rise in pressure was followed by rarefaction as the wave propagates through the air system. The waveform without the lung model, see figure, is similar to an under-damped mechanical oscillator. Adding the lung model resulted in an over-damped system and appeared to act as a RCI filter removing the highfrequency components. A typical sharp pressure rise was 19.8 cmH2O with a calculated value of 17.1 cmH2O. Conclusions: The “water hammer” effect can account for the sudden oscillations in Paw. Lung mechanics dampen these pressure oscillations suggesting that the stiffer the lung, the greater the oscillations in Paw.

Ignition and Oxidation of 50/50 Butane Isomer Blends

One of the alkanes found within gaseous fuel blends of interest to gas turbine applications is butane. There are two structural isomers of butane, normal butane and isobutane, and the combustion characteristics of either isomer are not well known. Of particular interest to this work are mixtures of n-butane and isobutane. A shock-tube experiment was performed to produce important ignition-delay-time data for these binary butane isomer mixtures, which are not currently well studied, with emphasis on 50-50 blends of the two isomers. These data represent the most extensive shock-tube results to date for mixtures of n-butane and isobutane. Ignition within the shock tube was determined from the sharp pressure rise measured at the end wall, which is characteristic of such exothermic reactions. Both experimental and kinetics modeling results are presented for a wide range of stoichiometries (ϕ=0.3−2.0), temperatures (1056–1598 K), and pressures (1–21 atm). The results of this work serve as a validation for the current chemical kinetics model. Correlations in the form of Arrhenius-type expressions are presented, which agree well with both the experimental results and the kinetics modeling. The results of an ignition-delay-time sensitivity analysis are provided, and key reactions are identified. The data from this study are compared with the modeling results of 100% normal butane and 100% isobutane. The 50/50 mixture of n-butane and isobutane was shown to be more readily ignitable than 100% isobutane but reacts slower than 100% n-butane only for the richer mixtures. There was little difference in ignition time between the lean mixtures.

Mesoscale Surface Pressure and Temperature Features Associated with Bow Echoes

Abstract This study examines observed mesoscale surface pressure, temperature, and wind features of bow echoes. Bow-echo events in the area of the Oklahoma Mesonet are selected for study to take advantage of high-resolution surface data. Thirty-six cases are identified using 2-km-resolution radar reflectivity data over a 4-yr period (2002–05); their surface features are interrogated using the mesonet data. Distinct surface features usually associated with squall lines, the mesohigh and cold pool, are found to also accompany bow echoes. A common surface pattern preceding bowing is identified. Prior to new bowing development, the mesohigh surges ahead of the convective line while the cold pool remains centered behind it. Surface winds shift to a ground-relative outflow pattern upon arrival of the mesohigh surge. Approximately 30 min later, a new bowing segment forms with its apex slightly to the left (with respect to the direction of system motion) of the mesohigh surge. The cold pool follows the convective line as it bows. This process is termed the “pressure surge–new bowing” cycle, and a conceptual model is presented. In one representative case, the surface signature of a gravity wave, identified through spatial and temporal filtering, is tracked. It is presumed to be generated by deep heating within the convective line. The wave moved at nearly 35 m s−1 and has heretofore been undetected in mesoanalysis studies. Two other distinct features, a sharp pressure rise and temperature drop, were also observed at all mesonet stations affected by the system. Possible explanations for these features in terms of a gravity current, gravity wave, or atmospheric bore are explored.

High Thermal Performance CFC Divertor

AbstractThe paper presents the conceptual study of an innovative divertor plate for the physical phase of the ITER/NET reactor. The main distinguishing feature of the new concept is the use of a single material, a Carbon Fibre reinforced Carbon (CFC) composite with ultra-high-thermal conductivity carbon fibres, for the whole structure, i.e. for the protective armour, heat sink and cooling channels. The main potential advantages of such a solution are perceived to be: elimination of the severe joint-interface problems inherent to other multimaterial solutions; weak interaction with runaway electrons; low-activation properties; reduction of mechanical stresses induced by electromagnetic transient. Moreover, the use of helium as a coolant leads to the following additional advantages: avoidance of the risk of burn-out; flexibility towards different operating scenarios; ease of baking at high temperature; lower tritium inventory in the CFC material; avoidance of the sharp pressure rise in cooling circuit becau...

A New Look at Fractures Tip Screenout Behavior

Summary Increasing pressure during a fracture treatment has been attributed to theformation of a "tip screenout," which occurs as a result ofinsufficient pad fluid volume or excessive leakoff. Results from a numericalsimulator of fracture propagation that incorporates the effects of proppantconcentration on fluid rheology, fracture transmissibility, and proppanttransportability suggest that the observed pressure rise can proppanttransportability suggest that the observed pressure rise can result from theformation of a low-mobility proppant bank at or near the perforations. Theeffect is related to proppant scheduling and transport perforations. The effectis related to proppant scheduling and transport characteristics, not toexcessive leak-off or insufficient pad volume. Introduction During a hydraulic fracturing treatment, the observed wellbore pressurenormally behaves in a stable fashion throughout most of the job. While the padfluid is pumped, a stable pressure trend is often established. The trend mayshow nearly constant, slightly increasing, or steadily decreasing pressure, depending on the growth mode of the fracture and the degree of heightcontainment established by surrounding rock layers. Nolte and Smith discussedthese pressure trends and their relative fracture geometries. The treatingpressure observed during a fracture job frequently will begin to increasesharply after proppant is added to the fracture. This pressure proppant isadded to the fracture. This pressure rise usually occurs late in the job. Nolteand Smith have attributed the pressure rise to the occurrence of a tipscreenout in the fracture. A tip screenout generally is assumed to occurbecause of excessive fluid loss, which results in dehydration of the proppantslurry or consumption of the pad fluid ahead of the proppant bank. The proppantbank then bridges off at or near the tip of the fracture, thus stopping furtherlateral growth. Injected fluid is stored in the existing fracture volume whilethe fracture "inflates" and pressure rises. In some treatments, a sharppressure rise has been noted almost coincident with the introduction ofproppant to the fracture. This proppant-induced pressure increase (PIPI) earlyin the job is not pressure increase (PIPI) early in the job is not consistentwith the previous explanation of tip screenout behavior. Conway et al. and Palmer and Veatch hypothesized that early-time PIPI is caused by offsets in thecreated fracture, multiple fracture strands, or related tortuosity in thefracture channel. These effects combine to create severe flow restrictions orproppant bridges near the wellbore. Conway et al. and Palmer and Veatch statethat the observed phenomenon is related almost exclusively to fracturing incarbonate reservoirs where natural fracture frequency is high. The phenomenonhas also been observed in formations of other mineralogy, including friable andwell-consolidated sandstones, conglomerates, and carbonates. Because fracturefaces are by nature rough and tortuous, some restriction to fluid and proppanttransport must be expected above that proppant transport must be expected abovethat predicted by equations governing fluid flow predicted by equationsgoverning fluid flow between parallel plates or in elliptical channels. Concentrations of suspended particles also influence the apparent viscosity, particles also influence the apparent viscosity, or flow resistance, of thecarrier fluid. Furthermore, the high concentrations of proppant particles canbe expected to create a resistance particles can be expected to create aresistance to movement of the particles similar to that represented byhindered-proppant-settling correlations. To account for these effects, a morecomplete model of proppant transport during fracturing operations wasimplemented in a fully 3D fracture-growth simulator, the Grid-Oriented Hydraulic Fracture Extension Replicator (GOHFER). The model solves thediffusivity equation for particle concentration, including horizontal andvertical flow, hindered settling, and the effects of proppant concentration onflow resistance and proppant concentration on flow resistance and particletransport. These features are particle transport. These features are requiredto account for the complex interactions that occur during proppant transport. GOHFER Formulation GOHFER provides the capabilities to model the complex problem of proppanttransport. The mathematical formulation of the model is described in detail in Ref. 6. The validity of the model and its accuracy at representing fracturegeometry development were shown by comparing the model's results to analyticalsolutions for fracture width and published results for other 3D simulators.published results for other 3D simulators. The model uses a 2Dfinite-difference formulation to solve for fluid pressures and flow velocitieswithin the fracture. The fluidflow solution is based on the Navier-Stokesequation for viscous flow between parallel plates. The distribution of fluidpressures plates. The distribution of fluid pressures within the fracture thatresults from the fluid-flow solution is used to calculate the fracture-widthdistribution with the Boussinesq solution for the displacement of the surfaceof a semi-infinite half-space caused by a distributed load. A tensile-strength criterion is used to determine fracture extension by comparing thetensile-stress distribution around the fracture perimeter with an assumed maximum allowable perimeter with an assumed maximum allowable tensilestress. JPT P. 138

Prediction of pressure surge in a one-component two-phase bubbly flow

Shock phenomena caused by a rapid valve closure in a one-component, two-phase bubbly flow were investigated experimentally using Refrigerant R-113 as the working fluid, and a theoretical analysis was conducted. The pressure transient in a one-component, two-phase bubbly flow is characterized by the existence of an exponential pressure decay just after the initial sharp pressure rise. This phenomenon is due to the mass transfer between the vapor and the liquid, that is, condensation. The profile of the pressure transient at each location along the channel was calculated numerically by solving the fundamental equations. By this analysis, the effects of heat transfer and mass transfer between the two phases were clarified. Simplified estimation methods for the potential surge and the equilibrium pressure rise are presented.

Effects of a sharp pressure rise on a compressible laminar boundary layer, when the Prandtl number is σ = 0.72

SynopsisFollowing an earlier paper (Curle 1978) we consider a compressible laminar boundary layer with uniform pressure when the distance x along the wall satisfies x < x0 and a prescribed large adverse pressure gradient when x > x0. The viscosity and absolute temperature are again taken to be proportional, but the Prandtl number is no longer assumed to be unity. After applying the Illingworth-Stewartson transformation, the transformed external velocity u1(x) is chosen so thatis large and constant, where Ts is the stagnation temperature, Tw is the (constant) Wall temperature and u0 is the upstream value of u1(x).The flow reacts to this sharp pressure rise mainly in a thin inner sublayer, so inner and outer asymptotic expansions are derived and matched for functions F and S which determine the stream function and the temperature.The skin friction, heat transfer, displacement thickness and momentum thickness are determined as functions of , andinvolve two parameters B1, B2, which depend upon the Mach number and the walltemperature. Detailed numerical calculations are presented here for σ = 0.72. In particular, it is seen that the heat transfer rate varies roughly like σ⅓ except near to separation, where it varies like σ¼.

Influence of Connecting Passage on Diesel Knock in Divided Chamber Type Engines

Combustion noise of the Diesel engine of a pre-combustion chamber type will be originated by some mechanical cause involved with sharp pressure rise in the main chamber. This pressure rise is determined by the rate of heat release in it and the quantity of gas issued from the auxiliary chamber. In order to clarify the latter effect, which will be closely related with the pressure rise in the auxiliary chamber and the area of the connecting passage, an air model was proposed. The pressure rise in the main chamber caused by a sudden pressure jump in the auxiliary chamber was measured in various areas of the connecting passage, and was compared with the theoretical calculation in which gas inertia was neglected. As the result, it was found that the rate of pressure rise in the main chamber is proportional to the area ratio of the passage and inversely proportional to the volume ratio of the main chamber. Application of the result to the actual engines shows that transmission of rapid pressure rise in the pre-chamber to the main chamber is strongly suppressed, resulting in a smooth pressure rise with the pre-combustion chamber type engine, while it cannot be with the swirl chamber type.

Analysis of Pressure Propagation in the Combustion Chamber of Diesel Engine by Means of Water Model

It was suggested in the previous paper that the area of the connecting passage in a divided chamber type Diesel engine determines wheither the sharp pressure rise in the auxiliary chamber transmits to the main chamber. No attention, however, was paid to the pressure wave propagation in the chamber that would possibly be of importance. In order to make up this, the water analogue method was applied to simulate such a two-dimensional unsteady problem. Test results show that, with a wide connecting passage, a sharp pressure pulse is formed and spreads in the main chamber, followed by a rapid pressure rise and pressure vibration of relatively low pitch. Such pulse and vibration decline with the decrease of the passage area and with smoothing the pressure rise in the auxiliary chamber. Owing to this pulse, the maximum rate of pressure rise in the main chamber is exceedingly larger than the theoretical one predicted in the previous paper, while the average rate coincides virtually with it, if fine waves are ignored.

Reflex release of an oxytocic hormone by stimulation of genital organs in male and female sheep studied by a cross-circulation technique.

SUMMARY Cross-circulation experiments were carried out using either two lactating ewes (six experiments) or one sexually mature ram and one lactating ewe (seven experiments). Under local anaesthesia an anastomosis was established by means of two plastic tubes between a jugular vein of one animal (A) and a jugular vein of the second animal (B) in each pair. The other jugular veins were clamped. Vaginal distension performed in ewe A by means of an inflated balloon often caused a sharp rise of pressure in the udder cisterns of ewe B after a minimum latency of 30 sec. Many pressure curves could be obtained successively in the same preparation by repeating the vaginal distension stimulus. Massage of the seminal vesicles and ampullae of the ram often caused a sharp pressure rise in the udder of the ewe after a minimum lag period of 30 sec. This effect could be obtained several times with the same preparation, but considerable individual variation was observed between the different rams used. Massage of the seminal vesicles and ampullae induced emission in many cases but not always. Intravenous injection of a synthetic oxytocin, Syntocinon (500–1000 mu.), in the ram or distension of the vagina in the ewe failed to elicit emission. Pressure responses were induced by injection of suitable doses of Syntocinon into the plastic tube leading to the ewe in which the cistern pressure was recorded, after a latency of approx. 30 sec. It may be concluded that vaginal distension in the ewe causes the discharge of a hormone from the head. A similar phenomenon occurs in the ram after massage of the seminal vesicles and ampullae. These effects are not abolished by atropine. It is postulated that this hormone is oxytocin in both sexes. The amount of hormone released after one stimulus was usually equivalent to 40–100 mu. oxytocin in the ram and 20–50 mu. oxytocin in the ewe.