Gradual Pressure Rise Research Articles

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.

Impact of Fuel Injection Distribution on Flame Holding in a Cavity-Stabilized Scramjet

This study investigated the impact of fuel injection distribution on flame stabilization and heat release in a cavity-stabilized scramjet. Experiments were conducted using a hydrogen air heater that provided vitiated air at an average stagnation temperature of 1570 K and stagnation pressure of 12.7 atm, accelerated to a Mach number of 2.2 at the combustor entrance. Ethylene was injected using two fuel injection locations, one upstream of the cavity and another in the cavity. Three different fuel injection distributions were studied for global equivalence ratios of 0.2, 0.3, and 0.4. Measurements included high-speed chemiluminescence and shadowgraph, as well as wall-mounted pressure transducers. It was found that when the cavity fuel flow rate was low, the flame stabilized on the cavity shear layer and resulted in a gradual pressure rise in the combustor. A sufficiently high fuel flow rate in the cavity led to flame stabilization upstream of the cavity, in the fuel jet wake, and a more confined combustion zone. Shadowgraph imaging showed that injection of the fuel from the cavity floor at a sufficient momentum penetrated the shear layer over the cavity and enabled flame propagation upstream of the cavity. It was concluded that the heat release distribution and pressure profile in the combustor could be significantly influenced by the fuel injection distribution through its impact on the flame stabilization mode.

Experimental Investigation on Transient Pressure Characteristics in a Helico-Axial Multiphase Pump

In oil and gas exploitation, the multiphase pump is a vital piece of equipment to guarantee production with high efficiency and stable operation. The complicated pattern of multiphase flow in the multiphase pump affects the pump performance; for this reason, the multiphase performance and the inner flow should be sufficiently investigated. In the present work, a liquid-gas multiphase pump is designed and manufactured, and then tested in a specialized test rig to reveal the transient pressure characteristics of the multiphase pump. Results show that the dominant frequency under water and water-air conditions is the blade passing frequency, which is induced by the rotor stator interaction. In the downstream of the impeller, the pressure fluctuation is obviously weakened, because the splitter-blade design could improve the control ability of flow pattern in the downstream region. In comparison with water condition, the pressure fluctuation of water-air condition greatly increases, and the multiples of impeller rotating frequency are stimulated due to the movement and merging of air bubbles. Finally, the correlation of transient pressure and phase distribution in impeller is revealed by using a high-speed camera. With the gradual pressure rise from impeller inlet to outlet, the relative movement and separation of two phases induce violent pressure fluctuations.

Effect of vacuum deterioration on thermal performance of coaxial evacuated tube solar collector considering single and two phase flow modelling: A numerical study

Boiling and condensation play an important role in heat transfer in evacuated tube solar collectors (ETC), especially for low flow rates. Another factor that influences the efficiency of an ETC is deterioration of vacuum inside the ETC. The present study investigates numerically the effect of deteriorating vacuum during single and two phase flows inside an ETC to analyze its thermal performance. A steady-state one-dimensional model is developed which can simulate the steady-state thermal performance of the Evacuated tube solar collector (ETC) with single and two-phase flows under saturation conditions considering effects of vacuum deterioration to simulate a more real-life situation of solar collectors. The rise in pressure inside the ETC has a significant effect on the suppression of boiling and condensation due to rise in overall heat loss coefficient. The radiative, convective and conductive heat loss coefficients from the collector are studied for gradual pressure rise inside the ETC. The two phase model also indicates that if boiling starts inside the ETC in a particular inlet flow condition, consideration of two phase flow along with the effect of vacuum deterioration shows significant deviation of efficiency and temperature profile from those of single phase flow models and predicts more accurate results. Inducing of two-phase heat transfer in ETC, with all the fluid condensing again within the collector tube, will result in improved efficiency for that range of mass flow rates. It also helps in maintaining uniform temperature and equal heat gain throughout the collector absorber plate. The efficiency of the collector for higher inlet temperatures is seen to be effected more significantly by vacuum deterioration than at lower inlet temperatures in single phase flows. However, in two phase flows for a fixed saturation temperature of the fluid, vacuum deterioration has negligible effect on the efficiency at higher inlet temperatures (Tfi ≥ 85 °C).

Shock wave attenuation by geotextile encapsulated sand barrier systems

This paper presents a laboratory scale experimental technique to study the performance of the encapsulated sand barrier systems in mitigating shock waves. The geotextile encapsulated sand barrier systems are made of cubical wire mesh formwork lined with geotextile and form a thick protective barrier when filled with granular materials. In the present study, dry sand particles of size varying from microns to few millimeters (fine and coarse) are used as infill granular material. Spherical shaped glass beads are also used as the infill material to study the influence of shape of the infill particle on the attenuation behavior. The process of shock wave attenuation by the sand barrier, with and without the geotextile facing formwork is examined. The experiments are performed using a conventional shock tube, where shock waves with incident Mach number in the range of 1.29–1.70 are generated. The experimental results show that the presence of geotextile layer has contributed significantly towards shock wave attenuation. The geotextile also plays an important role as a regulator, which is able to deliver gradual pressure rise at the downstream end of the barrier.

Identification of Baking Expansion Phases of Leavened Dough Using an Experimental Approach

A measurement system was designed to study changes in the volume, pressure, and viscosity of dough leavened by baking powder during model baking. Analysis of the volume changes demonstrated two baking stages, i.e. dough expansion and crumb shrinking. Through the analysis of pressure and viscosity extremes, the expansion stage was divided into five phases: stress relaxation (R) characterised by a mild pressure decline; gluten matrix softening (S), during which the decrease in viscosity is accompanied by a gradual pressure rise contributing to substantial dough expansion (by ∼54 %); starch gelatinisation and protein aggregation (G) characterised by rapidly increasing viscosity; gas bubble opening (O) reflecting a rapid pressure reduction; and boiling of water in dough (B), which ends at initiation of crumb shrinking. The study showed that enrichment of the dough with carob fibre increased the contribution of phases S and O to dough expansion at the cost of phase G. A similar contribution of the expansion phases was reported for the Bombona cultivar, which exhibits the highest gluten content. In contrast, the Finezja and Katoda cultivars, which have a lower gluten level, were characterised by an approximately two-fold higher impact of phase G on the increase in dough expansion. The results indicated that the developed method for identification of baking expansion phases of leavened dough can be useful in baking characteristics of raw materials and bakery additives.

Laser-induced hydrodynamics in water-saturated tissue: III. Optoacoustic effects

Studied in this work are specific features of acoustic vibrations generated at the hot blackened tip of an optical fiber (the so-called hot tip) delivering moderate-power (1–10 W) CW laser radiation in contact with water or a water-saturated biotissue. Generated upon such contact is a wideband acoustic signal whose characteristics largely depend on the object exposed and treatment scheme. Placing the hot tip in an acoustic resonator is demonstrated to cause distinct amplitude modulation of the acoustic noise. The formation of laser channels in an intervertebral disc or the intramedullary cavity of a bovine thighbone gives rise to the emission of a quasiperiodic train of pulses associated with the explosive growth and collapse of steam-gas bubbles in the hot-tip-to-biotissue contact region. The resultant pressure pulses, 20 ± 15 MPa in amplitude, cause damage to the adjacent tissue and facilitate the production of a laser channel at a rate of some 0.4–5 mm s−1. During the course of laser treatment the biotissue gradually gets saturated with steam-gas bubbles, which results in the development of low-frequency pressure oscillations in the range 0.1–10 Hz and a gradual pressure rise to around 200 kPa, leading to reduction of the natural frequencies of the resonance modes of the biotissue. The possible effect of these acoustic vibrations on the biotissue is discussed.

An intraventricular axial flow blood pump integrated with a bearing purge system.

The future development of implantable axial flow blood pumps must address two major issues: mechanically induced hemolysis and shaft seal reliability. The recent revisions to our miniature intraventricular axial flow left ventricular assist device (LVAD) were aimed particularly at addressing these concerns. To improve hemocompatibility, a new impeller has been designed according to the following criteria: 1) gradual pressure rise along the blade chord; 2) minimized local fluid acceleration to prevent cavitation; 3) minimum surface roughness; and 4) radius edges. Subsequent in vitro hemolysis tests conducted with bovine and ovine blood have demonstrated very low hemolysis (normalized index of hemolysis = 0.0051 +/- 0.0047 g/100 L) with this new impeller design. To address the need for a reliable seal, we have developed a purged seal system consisting of a miniature lip seal and ceramic pressure groove journal bearing that also acts as a purge pump. Several spiral grooves formed on the bearing surface provide viscous pumping of the purge fluid, generating more than 3,000 mmHg at 10,000 rpm. This purge flow flushes the lip seal and prevents blood backflow into the bearing. We have found this purge pump to offer several advantages because it is simple, compact, durable, does not require separate actuation, and offers a wide range of flow, depending upon the groove design. In vivo animal tests demonstrated the potential of the purged seal system.

Skeletal muscle arteriolar constriction to ANG II: evaluation of a myogenic component.

This study addressed the hypothesis that an increase in blood pressure contributes to the overall constrictive response of skeletal muscle arterioles to angiotensin II (ANG II). Diameters of second-order arterioles (2A) and third-order arterioles (3A) in the rat cremaster muscle were quantitated after intravenous administration of ANG II. Hindquarter blood pressure was either allowed to increase or was maintained at normal levels. Constriction of 3A to bolus injection of ANG II was the same whether hindquarter pressure increased or not. However, the total vascular constrictive response of the cremaster muscle (based on 2A blood flow) and of the entire hindquarter (based on iliac arterial blood flow) to bolus ANG II was greater when hindquarter pressure was held constant. During slow infusion of ANG II, 3A constriction was unaffected by an abrupt decrease or increase in hindquarter pressure. However, an abrupt reduction of hindquarter pressure caused a significant decline in hindquarter vascular resistance. Thus an increase in blood pressure, whether rapid or gradual, does not influence 3A constriction to ANG II. However, in the entire hindquarter, a rapid rise in blood pressure opposes constriction to ANG II, whereas a gradual pressure rise evokes a mechanism that enhances constrictive response to the peptide.

Outgassing characteristics of an electropolished stainless-steel pipe with an operating extractor ionization gauge

The outgassing of a Bayard–Alpert ionization gauge (BAG) with a tungsten filament was observed to increase with the pressure in an isolation test. This was caused by the temperature rise of the hot filament, resulting from the gradual pressure rise. The outgassing of an extractor ionization gauge with an iridium filament coated with thorium oxide was much lower than that of the BAG. The net outgassing rate of an electropolished (EP)-type 304 stainless-steel (SS304) pipe wall was measured by the isolation method. The outgassing rate of the pipe wall at 3.0×10−6 Pa, after an ‘‘in situ’’ bakeout (150 °C, 20 h), was measured as 1.1×10−11 Pa ℓ/s cm−2 by neglecting the outgassing and pumping effects of the operating EXG.

Mechanics of syneresis I. Theory

Syneresis is spontaneous contraction of a gel, accompanied by expulsion of liquid from the pores. This occurs even if evaporation is prevented and the gel is immersed in liquid. In this paper, the rate of contraction is shown to be dependent on the size of the gel, because the small pores resist flow of liquid out of the gel. An analysis is presented that allows prediction of the influence of size on contraction rate for plates and cylinders of gel. An initial period of slow contraction, which is reported by some experimenters, is shown to be consistent with the gradual rise in pressure in the pore liquid as it is compressed by the viscoelastic gel network. Experimental results on syneresis in silica gels, presented in a companion paper, support the theory.

MHD Pseudoshock and Qualitative Model in the Performance of Supersonic Generators

A preliminary study, based on an extension of Crocco's pseudoshock model, has been carried out to explain and predict qualitatively the rather gradual pressure rise in a supersonic MHD generator, to account for the influence of wall roughness on the core flow. Numerical experiments are presented, which demonstrate indeed the qualitative link between the theory of MHD pseudoshock and the observed pressure ramp caused by rough walls. B b CP o Nomenclature = cross-sectional area of channel, m2 = cross-sectional area of core region, m2 = cross-sectional area of wall layer, m2 = magnetic field strength, T = width of generator duct, m = specific heat capacity, J/kg K = electric field strength, N/C = specific enthalpy, J/kg = electric current density, C/m2 s = loading factor = Eyw/ucB = Eyc/ucB = Eyw/uwB = Mach number = total mass flow rate through generator, kg/s = unit normal vector on interface = pressure, N/m2 = gas constant, J/kg K = MHD interaction parameter = specific entropy, J/kg K = temperature, K — specific internal energy, J/kg = flow velocity, m/s = specific volume, m3 /kg = Cartesian coordinate along the duct axis, in the flow direction, measured from the entrance, m = ratio of Hall field to Faraday field = EX/Ey = Hall-current parameter = ratio of specific heat capacities = cp/cv = ratio of mass flow rate in wall layer to the total mass flow rate through the generator = ratio of cross-section al area of wall layer to that of entire duct = density, kg/m3 = electric conductivity, Q ~; /m3 = angle of inclination of segments = tan ~! aw, rad

Release of pressor substances from renal grafts originating from rats with renal hypertension.

A method is described for grafting kidneys on bilaterally nephrectomized rats without interrupting renal blood flow. Under these conditions, normal kidneys caused a pressor response characterized by a gradual rise in pressure suggesting a continuous endocrine secretion. Kidneys from rats made hypertensive by unilateral clipping of the renal artery, renal encapsulation, or partial infarction were tested during the acute and chronic stages of the experimental disease. Wrapped and infarcted kidneys, as well as the untouched contralateral organs, released little or no pressor substances. On the other hand, clipped kidneys released normal or slightly greater amounts of pressor substances; the eontralateral untouched kidney did not elicit any pressor effect. Morphological examination showed a relationship between presence of nephroselerotic lesions and absence of, or decrease of, reninlike pressor substance released. Both changes are believed to be due to high renal vascular pressure against which the partly clipped kidney is protected. These observations suggest that, if renin is a primary factor in the pathogenesis of renal hypertension, hypertension due to renal artery constriction has a mechanism different from that caused by renal manipulation or infarction.