Pressure Undershoot Research Articles

Investigation on the temperature variation of novec-1230 and halon 1301 in a discharge system

The flow inside the aviation engine fire extinguishing system is complicated due to its transient, two-phase and compressible nature. Temperature is one of the important variables of the flow, because it reflects the thermal respect of the physics. However, detailed analysis on the temperature variation during the discharge process is in absence. In this article, factors influencing temperature were analyzed, based on experimental experiments using halon1301 and novec1230. The temperature variation is found to be mainly influenced by the following factors: (1) the pressure gradient in pipe and gas fraction of the flow, (2) the relative magnitude of pressure work rate and convection, (3) pressure undershoot. The negative pressure gradient in the pipe, combined with the raising of gas fraction, leads to a sharp temperature drop of about 50 °C/s in the pipe. This process stops when the magnitude of pressure work is diminished, then heat convection from the pipe wall starts to dominate. Pressure undershoot is observed only in experiments using halon1301. The temperature drops of 6.1–9.8 °C in 0.01 s during the pipe pressure undershoot, and drops 12–14.2 °C in 1 s during bottle pressure undershoot.

Phenomena of Liquid Nitrogen Flashing under Rapid Depressurizations

Flashing Experiment of liquid nitrogen in a pressure vessel was conducted under rapid depressurization rate. Observations of the explosive boiling behavior caused by quick opening of a electro-magnetic valve were undertaken by using a video camera. Pressure and temperature changes in the vessel were measured. Depressurization rates in these experiments fell in the range from 0.01 to 4.0MPa/s. Experimental results showed that the initial temperature distributions and depressurization rates have more intrinsic influence on these flashing phenomena. Relationship between pressure undershoot and depressurization rate was obtained. The rate of pressure recovery was found to increase with the rate of depressurization, which also resulted in the increase of the increment of pressure recovery. The empirical equation proposed by Alamgir and Lienhard was applied to predicted the recovery pressure of the flashing of liquid nitrogen. To obtain more precise prediction of the pressure recovery, another factor such as an orifice diameter that affects the discharge condition of the recovery pressure.

Experimental visualization and characteristics of bubble nucleation during rapid decompression of supercritical and subcooled carbon dioxide

The rapid decompression of sub-cooled and supercritical carbon dioxide was investigated experimentally using schlieren and regular high-speed photography. An optically accessible expansion tube was designed and constructed. A test campaign with initial pressures ranging from 6.45–9.02 MPa and initial temperatures from 297–308 K was performed. High-speed schlieren imagery was used to visualize the decompression waves, evaporation waves, and an oscillatory phase change region present in the blowdown process. The time history of pressures at different locations along the expansion tube were recorded using fast response pressure transducers. Image and pressure signal analyses were performed to extract decompression wave speeds and evaporation wave speeds. Results were compared to analytical models and other experiments reported in the literature. The relationship between pressure undershoot and wave speed to the initial conditions was analysed.

A deterministic approach for design of supervisory control of LPV systems with delay.

Linear Parameter Varying (LPV) systems and their control have gained attraction recently as they approximate nonlinear systems with higher order than ordinary linear systems. On the other hand, time delay is an inherent part of various real-life applications. A supervisory control structure is proposed in this paper for LPV systems subject to time delays. In the proposed control structure, a supervisor selects the most suitable controller from a bank of controllers; which desires to enhance the performance of closed-loop system in contrast with using a single robust controller. The analysis is based on the celebrated Smith predictor for time delay compensation and we provide a sufficient condition to assure the stability of the closed-loop switched system in terms of dwell time. Simulations on blood pressure control of hypertension patients in postoperative scenario are used to exemplify the effectiveness of the utilized technique. The operating region of the system is partitioned into five smaller operating regions to construct corresponding robust controllers and perform hysteresis switching amongst them. Simulation results witnessed that the proposed control scheme demonstrated a pressure undershoot less than the desired value of 10 mmHg while the Mean Arterial Pressure (MAP) remains within ±5 mmHg of the desired value.

Coating Flow Near Channel Exit. A Theoretical Perspective

The planar flow of a steady moving-wall free-surface jet is examined theoretically for moderate inertia and surface tension. The method of matched asymptotic expansion and singular perturbation is used to explore the rich dynamics near the stress singularity. A thin-film approach is also proposed to capture the flow further downstream where the flow becomes of the boundary-layer type. We exploit the similarity character of the flow to circumvent the presence of the singularity. The study is of close relevance to slot and blade coating. The jet is found to always contract near the channel exit, but presents a mild expansion further downstream for a thick coating film. We predict that separation occurs upstream of the exit for slot coating, essentially for any coating thickness near the moving substrate, and for a thin film near the die. For capillary number of order one, the jet profile is not affected by surface tension but the normal stress along the free surface exhibits a maximum that strengthens with surface tension. In contrast to existing numerical findings, we predict the existence of upstream influence as indicated by the nonlinear pressure dependence on upstream distance and the pressure undershoot (overshoot) in blade (slot) coating at the exit.

An asymptotic-numerical comparative study for axisymmetric free surface liquid jet near and far from exit at high Reynolds number

Purpose The purpose of this study is to examine theoretically the axisymmetric flow of a steady free-surface jet emerging from a tube for high inertia flow and moderate surface tension effect. Design/methodology/approach The method of matched asymptotic expansion is used to explore the rich dynamics near the exit where a stress singularity occurs. A boundary layer approach is also proposed to capture the flow further downstream where the free surface layer has grown significantly. Findings The jet is found to always contract near the tube exit. In contrast to existing numerical studies, the author explores the strength of upstream influence and the flow in the wall layer, resulting from jet contraction. This influence becomes particularly evident from the nonlinear pressure dependence on the upstream distance, as well as the pressure undershoot and overshoot at the exit for weak and strong gravity levels, respectively. The approach is validated against existing experimental and numerical data for the jet profile and centerline velocity where good agreement is obtained. Far from the exit, the author shows how the solution in the diffusive region can be matched to the inviscid far solution, providing the desired appropriate initial condition for the inviscid far flow solution. The location, at which the velocity becomes uniform across the jet, depends strongly on the gravity level and exhibits a non-monotonic behavior with respect to gravity and applied pressure gradient. The author finds that under weak gravity, surface tension has little influence on the final jet radius. The work is a crucial supplement to the existing numerical literature. Originality/value Given the presence of the stress singularity at the exit, the work constitutes a superior alternative to a computational approach where the singularity is typically and inaccurately smoothed over. In contrast, in the present study, the singularity is entirely circumvented. Moreover, the flow details are better elucidated, and the various scales involved in different regions are better identified.

Pressure characteristics of a novel double rotor hydraulic transformer

The hydraulic transformer is the core component when it works with the common pressure rail system, which integrates the functions of the pump and the motor, and thus possesses sensitive pressure characteristics. The rotating speed has significant influence on the pressure characteristics of a hydraulic transformer while it has not been considered previously. In this study, aimed at improving the working performance, a novel double rotor hydraulic transformer is proposed and a comprehensive mathematical model considering the dynamic characteristics of the cylinder block is established. At the same time, a prototype is made and the experiment is conducted. The test results show that the robust rotor structure enables a larger pressure range, and the numerical results exhibit a good match with the test results. The parameter sensitivity study shows that the delivery pressure is mainly subject to the valve plate control angle δ and, under the effects of the resistance torques, pressure loss will occur especially under a large control angle and a high rotating speed. The magnitude of the instantaneous angular velocity fluctuation increases sharply when the speed is lower than 400 r/min, which is the main reason for the serious pressure pulsation at a low speed. As a result of the improved low-speed stability and output flow uniformity, the pressure pulsation rate of the double rotor hydraulic transformer is greatly reduced. However, the pulsation rate is still high at an extremely low speed. In addition, when the rotating speed exceeds the capability of the damping grooves, the pressure undershoot becomes serious at the A-T transition region around the control angle of −30°. Consequently, from the perspective of pressure characteristics, the limitation on the rotating speed under small control angles is suggested for the design of the double rotor hydraulic transformer controller.

Mass transfer characteristics of pipeline leak-before-break in a nuclear power station

This paper aims to develop a correlation for pressure undershoot and revises the hypotheses of nucleation inception in the two-phase critical flow model. The theories of rapid depressurization and isothermal bubble growth were used to construct a modified two-phase critical flow model for leak-before-break analysis. The modified model predictions exhibit strong similarities with the experimental values compared with the Moody and Henry–Fauske models, with prediction deviations of less than 8.9% in the inter-granular stress corrosion crack. The effects of the inlet conditions on the nucleation inception point, as well as the distribution characteristics of the mass quality and void fraction along the channel, were investigated to explore the mass transfer process.

A non-equilibrium relaxation model for fast depressurization of pipelines

In this paper, transient depressurization of high pressure pipelines containing initially subcooled liquid is simulated numerically by using thermodynamic non-equilibrium and choking condition model. The numerical method relies on finite volume. The convective terms of cell boundaries are discretized by Advection Upstream Splitting Method (AUSM+ - up) with a proposal of partially implicit approach for source terms. Different void fraction correlations are applied to simulate two phase shock tubes as well as the depressurization process. By comparison between the present results and previous experimental data, the best void fraction correlation is introduced. The results indicate that the propagation of a strong evaporation wave toward the closed end drops the inside pressure of the pipe to a value below the initial saturation pressure. As the wave is reflected from the closed end, a slight pressure undershoot is developed. At the late stage of the depressurization, the pressure along the pipe reduces at a moderate rate. Maximum heat transfer rate and minimum pipe wall temperature occur at the open end.

Surface pressure characteristics of a highly loaded turbine blade at design and off-design conditions; a CFD methodology

The flow field passing through a highly loaded low pressure (LP) turbine cascade is numerically investigated at design and off-design conditions. The Field Operation And Manipulation (OpenFOAM) platform is used as the computational Fluid Dynamics (CFD) tool. In this regard, the influences of grid resolution on the results of k-e, k-ω, and large-eddy simulation (LES) turbulence models are investigated and compared with those of experimental measurements. A numerical pressure undershoot is appeared near the end of blade pressure surface which is sensitive to grid resolution and flow turbulence modeling. The LES model is able to resolve separation on both coarse and fine grid resolutions. In addition, the off-design flow condition is modeled by negative and positive inflow incidence angles. The numerical experiments show that a separation bubble generated on blade pressure side is predicted by LES. The total pressure drop has also been calculated at incidence angles between -20° and +8°. The minimum total pressure drop is obtained by k-ω and LES at design point.

Cycle parameters of an internal combustion spark-ignition gas piston engine with an electronic control system

The purpose of the work is to develop methods of managing the cycle of bifuel and single-fuel vehicleswith engines running both on petrol and gas fuel as well as physically and mathematically reasonable algorithmsfor an electronic control system. On the basis of full-scale tests of an internal combustion spark-ignition pistonengine and a technique developed for designing an ejeсtor with non-stationary physical parameters of workingbodies a method of compensating power losses by ejecting a portion of air with the help of pressure differenceon the valve of a gas electromagnetic injector is proposed. On the basis of the theoretical analysis and experimentaldata an algorithm of determining the fuel injection rate taking into account the subcritical or critical fueldischarge from the injector nozzle, the change in the speed of sound, the gas fuel pressure undershoot in the processof fuel feeding and the power supply voltage on the electromagnetic coil of the gas injector is developed. Amethod of determining the ignition advance angle when switching from petrol to gas fuel is developed for anelectronic control system of an internal combustion spark-ignition engine which serves as the basis for calculatingthe adjusting factors for liquefied petroleum gas and compressed natural gas.

Choking flow modeling with mechanical and thermal non-equilibrium

The mechanistic model, which considers the mechanical and thermal non-equilibrium, is described for two-phase choking flow. The choking mass flux is obtained from the momentum equation with the definition of choking. The key parameter for the mechanical non-equilibrium is a slip ratio. The dependent parameters for the slip ratio are identified. In this research, the slip ratio which is defined in the drift flux model is used to identify the impact parameters on the slip ratio. Because the slip ratio in the drift flux model is related to the distribution parameter and drift velocity, the adequate correlations depending on the flow regime are introduced in this study. For the thermal non-equilibrium, the model is developed with bubble conduction time and Bernoulli choking model. In case of highly subcooled water compared to the inlet pressure, the Bernoulli choking model using the pressure undershoot is used because there is no bubble generation in the test section. When the phase change happens inside the test section, two-phase choking model with relaxation time calculates the choking mass flux. According to the comparison of model prediction with experimental data shows good agreement. The developed model shows good prediction in both low and high pressure ranges.

Low level control of metal belt CVT considering shift dynamics and ratio valve on-off characteristics

In this paper, low level control algorithms of a metal belt CVT are suggested. A feedforward PID control algorithm is adopted for line pressure based on a steady state relationship between the input duty and the line pressure. Experimental results show that feedforward PID control of the line pressure guarantees a fast response while reducing the pressure undershoot which may result in belt slip. For ratio control, a fuzzy logic is suggested by considering the CVT shift dynamics and on-off characteristics of the ratio control valve. It is found from experimental results that a desired speed ratio can be achieved at steady state in spite of the fluctuating primary pressure. It is expected that the low level control algorithms for the line pressure and speed ratio suggested in this study can be implemented in a prototype CVT.

Critical flow in convergent-divergent nozzles with cavity nucleation model

A transient critical flow experiment with convergent-divergent nozzle as the break geometry was conducted in the high-pressure steam-water test loop of Man Jiaotong University. The test parameters were pressure 3.0–16.0 MPa, inlet liquid stagnation subcooling 0–60°C, and corresponding critical mass flow rate (40−120) × 103 kg/(m 2 s). The concept of the incipient flashing, with the choking plane occurring at the throat location, was applied in a wall surface cavity nucleation model. The total pressure difference between the inlet pressure P 0 and the pressure at the throat location P t was divided into two parts, one determined by the inlet fluid properties and the other being the pressure undershoot. Our experimental results show that the pressure undershoot and the liquid superheat at the incipient flashing location reach their maximum values with saturated inlet conditions; with increased inlet subcooling, thermal nonequilibrium decreased. A cavity nucleation model was developed for prediction of the pressure undershoot. The model includes a discharge coefficient, and it has been verified experimentally that this coefficient is a function of only the inlet liquid subcooling. Based on the present theory, the predicted critical mass flow rates are compared with not only our own experimental data but also other experimental data, and good agreement is achieved.

Critical Two-Phase Flow in Pipes for Subcooled Stagnation States With a Cavity Flooding Incipient Flashing Model

Analysis of loss of coolant accident (LOCA) scenarios in nuclear reactor safety evaluation depends on knowledge of many complex phenomena. A primary phenomenon controlling the sequence of events, by determining the residual coolant mass inventory within the primary system, is the critical flow process. Critical flow of a flashing liquid is complicated by marked departure from thermal equilibrium. Several complex models have been proposed to represent the non-equilibrium effects, including six-equation two-fluid models. Amos and Schrock (1983) developed a model based on the premise that the two-phase region is homogeneous and that thermal nonequilibrium is the dominant factor causing the departure from the homogeneous equilibrium idealization. Flashing inception was represented by a modification of the Alamgir-Lienhard (1981) pressure undershoot. Exponential relaxation of the metastable liquid was formulated as suggested by Bauer et al. (1976) and the critical flow criterion used the sound speed formulation of Kroeger (1976). Lee and Schrock (1988) extended the Amos-Schrock work by developing an improved correlation for the pressure undershoot correction factor in terms of Reynolds number and subcooling Jakob number. Improvements were also made in the relaxation constant and in the application of Kroeger’s formulation. In the present paper a new cavity flooding model is used for the evaluation of pressure undershoot at flashing inception. This model is similar to the one developed by Fabic (1964) for the evaluation of liquid superheat required for boiling on a surface subjected to transient heating. The model contains an experimentally deduced factor, which is correlated against stagnation subcooling using the experimental data of Amos and Schrock (1983, 1984), Jeandey et al. (1981), and the Marviken tests (Anon., 1979). The model was then tested against seven additional data sets and shown to be very accurate in predicted mass flux (standard deviation of 10.9 percent for all data). The cavity flooding model is thought to represent the true physics more correctly than does the earlier model, which had its origin in molecular fluctuation theory.

A study of the rapid depressurization of hot water and the dynamics of vapour bubble generation in superheated water

Liquid can be brought to a superheated state by means of very rapid depressurization. Such situations are typical for pipe rupture accidents, which can occur in processing equipment and have to be taken into account, for example, in the design of nuclear power plants with water-cooled reactors. An experimental investigation was carried out using a horizontal pipe attached to a pressure vessel. Rapid depressurization was achieved with the use of a rupture disc assembly. The paper describes the most important test results, a method for calculating the pressure undershoot below the saturation pressure and a simplified model of expansion wave propagation with vapour bubble generation in superheated liquid. On the basis of this model, the density of nucleation sites and the evolution of pressure and void fraction in the two-phase mixture can be estimated.

Performance chararcteristics of convergent-divergent nozzles for subcooled hot water(2nd report, Improvement of nozzle performance).

The purpose of this study is to improve the performance characteristics of a convergent-divergent nozzle for flashing expansion of initially subcooled hot water. Slip between vapor and liquid in the divergent passage and pressure undershoot (that is, thermal nonequilibrium) at the nozzle throat causes the nozzle efficiency to decrease. Initially subcooled hot water at an inlet pressure of 0.47 MPa and inlet temperatures of 409.8 to 421.8 K (inlet subcoolings of 0.9 to 12.9 K) were used in the range of nozzle back pressures of 5 to 101.3 kPa. The results showed that: (1) pressure undershoot is decreased and the thrust coefficient is increased by 10%; (2) a thrust coefficient is independent of the inlet subcooling and exit-stream expansion ratio; (3) a method for determining the exit-stream expansion ratio is proposed; (4) the critical flow rate is lower than that for an ordinary convergent-divergent nozzle.

An Adaptive Controller for Blood Pressure

In clinical cases it is often required to externally control the arterial blood pressure, for which the infusion of the drug sodium nitroprusside (SNP) is usually used to reduce the blood pressure. Because of significant variability among patient's characteristics, the use of a robust adaptive controller is necessary. A self-tuning control algorithm is presented which provides fine tracking of a deSired blood pressure history without any pressure undershoot and produces optimal reduction for stochastic background activity. The adaptive controller is implemented on a microcomputer. Animal experiments in dogs indicated that the behaviour of the adaptive controller was predicted well by theoretical considerations and the system performance was significantly improved.

Vapor generation model for flashing in the initial blowdown phase

Abstract A vapor generation model for flashing in the initial blowdown phase is proposed based on a wall nucleation theory and a bubble transport model. Comparisons are made between the proposed model and the TRAC-PF1 model by using the MINCS code through analyses of three blowdown experiments with different scales. The present model well predicts the pressure undershoot in the vessel, while the TRAC model can not predict this typical thermodynamic nonequilibrium phenomenon.

Spinodal lines and equations of state: A review

The importance of knowing superheated liquid properties, and of locating the liquid spinodal line, is discussed. The measurement and prediction of the spinodal line, and the limits of isentropic pressure undershoot, are reviewed. Means are presented for formulating equations of state and fundamental equations to predict superheated liquid properties and spinodal limits. It is shown how the temperature dependence of surface tension can be used to verify p - v - T equations of state, or how this dependence can be predicted if the equation of state is known.