Transient Pressure Variations Research Articles

Study on the influence of intermediate airshafts on the aerodynamic effect of urban rapid rail transit

This study explores how intermediate airshafts affect the aerodynamics of urban rapid transit trains in tunnels. It employs a three-dimensional turbulence model to simulate train-tunnel interactions, specifically examining airshafts’ impact on transient pressure variations along surfaces. The model’s accuracy and reliability are confirmed by comparing results with full-scale experimental data. The results show that airshafts effectively mitigate peak tunnel wall pressures, with the lowest peak overpressure observed at the airshaft location. With the airshaft-reflected expansion wave arriving ahead of the train, the pressure between the train front and tunnel wall decreases compared to scenarios without airshafts. Similar transient pressure changes at the tunnel entrance and rear half suggest the emergence of a “secondary compression wave” due to the train’s transit.

Suppression mechanism of transient pressure during the turbine runaway of an ultra-high head pump-turbine

Abstract Pressure fluctuation (PF) is a serious issue in transitions of the ultra-high head (UHH) pump-turbines (PTs). To explore the mechanism and control method of PF, the turbine runaway process (TRP) of an UHH PT was simulated using the one- and three-dimensional coupled method. The study suggested that the local backflow (BF) near the high-pressure (HP) inlet of the impeller blocked the impeller channel and induced water hammer. The local BF near the HP inlet of the impeller were closely related to the ratio of rotational speed to PT head. To suppress the BF this study proposed an improved impeller channel with 5 degree spread angle in meridian plane. The study found that the increase of rotational speed and transient pressure variations induced by water hammer could be suppressed successfully by using the improved impeller. The study provided important reference for the design and optimization of the UHH PTs.

Numerical solution to binary quasi-linear partial differential equations for drilling surge pressures based on the characteristic-lines method

Accurately predicting surge pressures caused by tripping drill string or running casing string is fundamental to implementing Managed Pressure Drilling in areas with narrow safety mud density margin. In this work, the region of interest is divided into 3 coupled regions, i.e., inside moving pipe, annulus, and that below the moving pipe. Mass conservation equations and momentum conservation equations depicting the 3 regions are developed considering mud compressibility, pipe elasticity and wellbore wall elasticity. Applying the characteristic-line theory for partial differential equations, a series of discretized equations have been developed for the 3 regions, including internal nodes within uniform size pipe conduit, coupling nodes between two adjacent different size conduits, surface and bottom-hole nodes, and the conjunction node of the 3 regions. Based on the discretized equations, a computational flowchart is programmed to get numerical transient solutions. Both the transient surge pressure variation with time at the given interested point and the surge pressure variation with depth along the whole well axis can be depicted using this method. Two case studies show that computational values are in good consistent with the measurements with a relative error less than 10%.

Understanding the influence of unsteady and convection terms in heat transfer model on the predicted heat flux in engines operated under different combustion modes

The aim of current research is to develop an enhanced heat transfer model to accurately predict the heat flux through the cylinder walls during the working process of internal combustion engines operated under different loads and combustion modes. Two enhanced models, i.e., the unsteady model and the steady convective contribution (SCC) model were respectively constructed based on the thermal wall function. The effects of the transient ambient gas temperature and pressure variations, as well as the chemical reaction source term, were included in the unsteady model. Then the contributions of the unsteady terms, such as the temporal derivative of gas temperature, the pressure work, and the chemical heat source in the energy equation were analyzed in detail. The results indicate that the mechanism of the unsteady heat transfer process is different in the engine operated under motored conditions and different combustion modes, including conventional diesel combustion (CDC), homogeneous charge compression ignition (HCCI), and reactivity controlled compression ignition (RCCI). Afterward, the influence of the gas velocity and the pressure difference perpendicular to the walls were included in the SCC model to consider their contributions to the heat fluxes near the chamber walls. It is found that the heat fluxes predicted by the SCC model are in better agreement with the experimental data, especially in the CDC mode under different swirl ratios and loads. The peak of the heat flux predicted by the SCC model shows superiority to that of the previous steady heat transfer models and the unsteady heat transfer model.

Instantaneous acceleration-based modeling of pumping systems response under transient events

Water hammer can cause pipeline eruptions after uncontrolled pump trip. In order to protect the pipeline from water hammer effects and monitor the transient flow, it is imperative to accurately predict pressure oscillation and surge energy dissipation for longer simulation periods. This study investigates the effectiveness of the water hammer control strategy by adjusting the rotational inertia of a pumping system alongside valve closing operations. In addition, it also explores the complex nature of the transient energy dissipation by using the one-dimensional instantaneous acceleration-based (IAB) unsteady friction models. To this end, the IAB unsteady friction (UF) models were improved to calculate the water hammer pressure by investigating the elastic pipe's response under transient events that are caused by pump trip and valve closure. Various boundary equations were derived and combined with water hammer governing equations to form the final system of equations to predict the transient pressure variation with time. After that, the numerical results of two IAB friction models (i.e., one and two-decay coefficient models, respectively) were compared with previous experimental data to verify the proposed transient solver. This solver was based on Method of Characteristics (MOC) and UF models. The results indicate that one-coefficient and two-coefficients UF models yield almost similar results that match with the experimental outcomes; however, the latter model is comparatively more complex in terms of formulation. Furthermore, a parametric study shows that increasing the moment of inertia of the pump and integrating it with the optimal operation of the valve helps in significantly controlling the transient pressure. This helps in reducing the check-valve slam and controls the water hammer with no need for additional protection devices. Results of the integral energy equation describe that the role of UF on the total dissipation of transient energy diminishes with an increase in the valve's closing time and inertia of the pumping system. Finally, the IAB-UF model acts as a valuable numerical tool that provides comprehensive information about the physical parameters required to protect high-head water supply systems from transient incidents.

Experimental study on transient pressure induced by high-speed train passing through an underground station with adjoining tunnels

Transient pressure variations on train and platform screen door (PSD) surfaces when a high-speed train passed through an underground station and adjoining tunnel were studied using a moving model test device based on the eight-car formation train model. The propagation characteristics of the pressure wave that was induced when the train passed through the station and tunnel at a high speed were discussed, and the effects of the train speed and station ventilation shaft position on the surface pressure distribution of the train and PSDs were analyzed and compared. The results showed that the pressure fluctuation law is different for the train and PSD surfaces, and the peak pressure increases significantly with an increase in the train speed. Ventilation shafts changed the pressure waveform on the surface of the train and PSDs and greatly reduced the peak pressure. A single shaft at the rear end of the platform and a double shaft at the station had the most significant effect on relieving transient pressure on the surface of the train and PSDs, respectively. Compared with the case with no shaft, these two shafts reduced the maximum amplitude pressure variation of the train and PSD surfaces by 46.3% and 67.4%, respectively.

Transient Analysis of Flow Unsteadiness and Noise Characteristics in a Centrifugal Compressor with a Novel Vaned Diffuser

The demands to apply transonic centrifugal compressor have increased in the advanced gas turbine engines. Various techniques are used to increase the aerodynamic performance of the centrifugal compressor. The effects of the inclined leading edges in diffuser vanes of a transonic centrifugal compressor on the flow-field unsteadiness and noise generation are investigated by solving the compressible, three-dimensional, transient Navier–Stokes equations. Diffuser vanes with various inclination angles of the leading edge from shroud-to-hub and hub-to-shroud are numerically modeled. The results show that the hub-to-shroud inclined leading edge improves the compressor performance (2.6%), and the proper inclination angle is effective to increase the stall margin (3.88%). In addition, in this study, the transient pressure variations and radiated noise prediction at the design operating point of the compressor are emphasized. The influences of the inclined leading edges on the pressure waves were captured in time/space domain with different convective velocities. The pressure fluctuation spectra are calculated to investigate the tonal blade passing frequency (BPF) noise, and it is shown that the applied inclination angles in the diffuser blades are effective, not only to improve the aerodynamic performance and stall margin, but also to reduce the BPF noise (7.6 dB sound pressure level reduction). Moreover, it is found that the diffuser vanes with inclination angles could suppress the separation regions and eddy structures inside the passages of the diffuser, which results in reduction of the overall sound pressure level and the broadband noise radiated from the compressor.

Efficacy of Volume of Fluid Method in Computational Simulation of Sloshing Phenomenon

The present study deals with the analysis of stochastic transient pressure variations and wall impacts for scaled down models of LNG carriers by using the VOF technique for interface characterization. Numerical simulation of sloshing under sway using multiphase liquid-gas condition has been carried out. Use of VOF method for multiphase numerical simulation is compared with the results of previously performed SPH study and established sloshing experiments. The efficacies of the VOF interface capturing technique is established by comparison with high-resolution images taken during sloshing experiments performed under the same external excitation frequency, amplitude and fill level. The pressure data from experiments when compared with respective simulations involving SPH and VOF methods helps to capture the fluid flow behaviour and intricacies they can handle.

Erratum to: Experimental investigations of transient pressure variations in a high head model Francis turbine during start-up and shutdown

In the published article [1] there is an error in the listing of the last author. “Dahlhaug G. Ole” should be “Ole Gunnar Dahlhaug” as listed in this erratum.

Numerical Investigation on the Mechanism of Double-Volute Balancing Radial Hydraulic Force on the Centrifugal Pump

Double-volute is an effective technique to reduce radial hydraulic force on the centrifugal pump and thereby mitigate the pump-casing vibration induced by unsteady flow characteristics. The mechanism of the double-volute structure balancing radial force on the impeller and volute was investigated on the basis of volute cross-sections by using Computational Fluid Dynamics (CFD) method. The tested performances and simulated inner-flow characteristics of two pumps with single-volute and double-volute were compared in this paper. The performance-test results verify the veracity of CFD method and illustrate that double-volute pump has some losses in terms of pump head and operation efficiency. The numerical simulations reveal that double-volute pump has smaller radial-force magnitude than single-volute pump on the abnormal conditions. Steady pressure field and transient pressure variations of pumps were explored to account for radial-force characteristics of double-volute pump. Compared with the single-volute structure, obvious pressure increases were found in the upper chamber (single part) of the double-volute, while the static pressure decreased in the lower chamber (double chambers). This situation reduces the pressure difference between two volute cross-sections in the collinear radial direction, resulting in smaller radial hydraulic force. Moreover, the transient simulations present the same phenomenon. The radial-forces distribute more uniformly in the double-volute pump, which can alleviate some vibrations.

A new method for early gas kick detection based on the consistencies and differences of bottomhole pressures at two measured points

Early gas kick detection is an important measure to ensure well control safety. In accordance with the fluctuation of the bottomhole pressure during early gas kick, the gas kick can be detected rapidly and early with downhole measurement tool. At present, erroneous diagnosis often occurred when gas kick detection was performed using pressure fluctuations of a single measured point. In this paper, firstly, considering the water hammer effect, a wellbore pressure fluctuation model at the initial stage of gas kick was established to accurately describe the transient annular pressure variations. The model was solved using the composite difference scheme and was verified using a laboratory gas kick experiment. Secondly, a dual-measured-points (DMP) method for early gas kick detection was presented based on the consistencies and discrepancies of bottomhole pressures at two measured points. The distance between two measured points was optimized. Simulation results indicated that the annular pressure at the measured point increased and periodically changed with time in the initial stage of gas kick, and then declined approximately linearly after the gas-liquid interface reached the measured point. The distance between the two measured points was mainly dominated by the borehole diameter, and increased with the decrease of borehole diameter. The gas kick confirmation time (GKCT) obtained using DMP method was 2–7 min and the corresponding gas kick confirmation volume (GKCV) was 0.3–0.5 m3. The GKCT diminished with the increase of the formation permeability and bottomhole negative differential pressure, and with the decrease of the borehole diameter and well depth.

Flow transportation inside shale samples during low temperature combustion: The effect of desorption and pyrolysis

Flow transportation inside shale formations is a complex process mainly including slip flow, transition flow, and free molecular diffusion. Recently, using combustion to promote gas transportation inside shale formations for gas extraction has been studied. In this study, the shale samples were combusted at 280 °C and 400 °C respectively, to obtain transient pressure variation behaviors inside the shale samples. The effects of organic matter pyrolysis and adsorbed gas desorption on internal flow transportation were investigated. Moreover, a mathematic scaling power law was used to predict the pressure rising and declining processes. It was found that the pressure exhibited similar trend under 280 °C and 400 °C. Shale internal pressure built up quickly and reached peak value after combustion began. Then the pressure declined close to ambient pressure within a short period. Under 400 °C, the pressure of inner and outer holes were close to each other; the peak pressure could rise up to 7000 Pa. Both the gas generated from the thermal cracking of kerogen and gas desorbed from the pore surface contributed to pressure rising. However, at lower combustion temperature (280 °C), the rising of the pressure is mainly owing to the desorption gas from the surface to the pore volume. The peak pressure was only 780 Pa, much lower than that under 400 °C. By selecting an appropriate exponential m in the power law of scaling pressure, pressure rising and declining processes inside shale samples could be well simulated.

Hyporheic exchange in a gravel bed flume with and without traveling surface waves

Solute exchange between the pore water of a streambed and overlying flowing surface water can have important effects on the chemical mass balance and biological productivity in the aquatic environment. The base mechanism for driving this hyporheic exchange in a moving current is turbulence at the water- sediment interface and the water surface slope. Many previous studies have shown that the nature of the exchange can be significantly modified when spatial pressure variations, induced by standing surface waves or bed forms, are present in the system. In contrast to these studies, we consider how transient pressure variations induced by moving waves modify hypothetic exchange. This is achieved experimentally by using a vertical array of electrical conductivity probes to track the movement of a conservative solute tracer in the gravel bed of a recirculating laboratory flume under a variety of flow conditions. Our experimental observations indicate that the addition of traveling waves greatly increases the initial rate and depth of solute penetration into the bed. When the analytical solution of a one-dimensional (vertical) advection–dispersion model is fit to the experimental measurements it appears that the presence of traveling surface waves results in a more dispersive hyporheic transport. Through experimental observation, model fitting, and direct simulation we demonstrate that this apparent dispersion dominated transport is due to the vertical “pumping” of interstitial fluid in the presence of traveling surface waves. From these simulations we develop a relation that for a given set of flow and wave characteristics allows us to estimate the apparent dispersion due to traveling waves.

Flow transportation inside shale rocks at low-temperature combustion condition: A simple scaling law

The gas flow inside the shale undergoes a complicated process from continuum-flow to Knudsen diffusion during combustion. In this study, a shale sample obtained from Shanxi province with burial depth of 643 m was combusted at a constant temperature to study the transient internal pressure variation. In the beginning, a cylinder-shaped shale was combusted above an electric heater at a constant temperature 450 °C. The transient internal pressures and surface temperatures of the shale sample were recorded every 15 s. With the thermal wave propagating from the electric heater to the top of shale, the shale surface temperature gradually increased. The pressure inside the sample was quickly built up because of the thermal cracking of kerogen. It reached a peak value of 12,000 Pa. Afterwards, the pressure gradually declined with the improvement of the permeability of the shale. By the observations during the experiment, a combustion model based on the scaling power law was used to simulate the transient internal pressure changes of a cylinder shaped shale sample during the combustion process. The simulated results showed a good agreement with experimental data. Based on the pressure variation data, it was found that not only the original trapped/storage gas (free state gas, adsorbed state gas, and dissolved state gas) but also the thermal cracking hydrocarbon gases produced from kerogen could be extracted from tight shale formation for gas recovery. Thus, combustion can further improve the production rate of gas from shale reservoirs.

Subsurface Fluid Pressure and Rock Deformation Monitoring Using Seismic Velocity Observations

AbstractFluid pressure within the Earth's crust is a key driver for triggering natural and human‐induced seismicity. Measuring fluid pressure evolution would be highly beneficial for understanding the underlying driving mechanisms and supporting seismic hazard assessment. Here we show that seismic velocities monitored on the 20‐m scale respond directly to changes in fluid pressure. Our data show that volumetric strain resulting from effective stress changes is sensed by seismic velocity, while shear dislocation is not. We are able to calibrate seismic velocity evolution against fluid pressure and strain with in situ measurements during a decameter‐scale fluid injection experiment in crystalline rock. Thus, our 4‐D seismic tomograms enable tracking of fluid pressure and strain evolution. Our findings demonstrate a strong potential toward monitoring transient fluid pressure variations and stress changes for well‐instrumented field sites and could be extended to monitoring hydraulic stimulations in deep reservoirs.

Erratum to: Experimental investigations of transient pressure variations in a high head model Francis turbine during start-up and shutdown

This corrects the article “Experimental investigations of transient pressure variations in a high head model Francis turbine during start-up and shutdown” in volume 26, issue 2 on page 277. In the published article “Vaginal adhesions in a woman with the history of dystocia. Journal of Hydrodynamics, 2014, 26(2): 277–290. 10.1016/S1001-6058(14)60031-7,” the spell of author's name is given incorrectly. The Editorial Office of Journal of Hydrodynamics would like to correct the author's name. The Editorial Office apologizes for any inconvenience that it may have caused. Chirag Trivedi, Michel J. Cervantes, B. K. Gandhi, Dahlhaug G. Ole The author list should be corrected as follows: Chirag Trivedi, Michel J. Cervantes, B. K. Gandhi, Ole Gunnar Dahlhaug

Transient Pressure Measurements in the Vaneless Space of a Francis Turbine during Load Acceptances from Minimum Load

Increased penetration of solar and the wind impels the designers of the hydroelectric power generation unit to provide more flexibility in operation for the stability of the grid. The power generating unit includes turbine which needs to sustain sudden change in its operating conditions. Thus, the hydraulic turbine experiences more transients per day which result in chronic problems such as fatigue to the runner, instrument malfunctioning, vibrations, wear and tear etc. This paper describes experiments performed on a high model (1.5:1) Francis turbine for load acceptances from the minimum load. The experiments presented in the paper are the part of Francis-99 workshop which aims to determine the performance of numerical models in simulations of model Francis turbine under steady and transient operating conditions. The aim of the paper is to present the transient pressure variation in the vaneless space of a Francis turbine where high-frequency pulsations are normally expected. For this, two pressure sensors, VL1 and VL2, are mounted at the vaneless space, one near the beginning of the spiral casing and the other before the end of the spiral casing. Both are used to capture the unsteady pressure field developed in the space between guide vanes and runner inlet. The time-resolved pressure signals are analyzed and presented during the transient to observe the pressure variation and dominant frequencies of pulsations.

Experimental investigation of passive thermodynamic vent system (TVS) with liquid nitrogen

Thermodynamic vent system (TVS) is an attractive technology to maintain an allowable pressure level of a cryogenic propellant storage in a spacecraft under micro-gravity condition. There are two types of TVS; active or passive. In this paper, the passive TVS which does not utilize a cryogenic liquid circulation pump is experimentally investigated with liquid nitrogen and numerically analyzed by thermodynamic and heat transfer model. A cylindrical copper tank, which is 198 mm in inner diameter and 216 mm in height, is utilized to suppress a thermal-stratification effect of inside cryogenic fluid. A coil heat exchanger, which is 3 m in length and 6.35 mm in outer diameter, and a fixed size orifice of which diameter is 0.4 mm are fabricated to remove heat from the stored fluid to the vented flow. Each vent process is initiated at 140 kPa and ended at 120 kPa with liquid nitrogen fill levels which are 30%, 50% and 70%, respectively. In the numerical model, the fluid in the tank is assumed to be homogeneous saturated liquid-vapor. Mass and energy balance equations with heat transfer conditions suggested in this research are considered to calculate the transient pressure variation in the tank and the amount of heat transfer across the heat exchanger. We achieve the average heat rejection rate of more than 9 W by TVS and conclude that the passive TVS operates satisfactorily. In addition, the prediction model is verified by experimental results. Although the model has limitation in providing accurate results, it can surely predict the tendency of pressure and temperature changes in the tank. Furthermore, the model can suggest how we can improve the heat exchanger design to enhance an overall efficiency of passive TVS. Moreover, the performance of passive TVS is compared with other cryogenic vent systems (direct vent system and active TVS) by suggested performance indicator.

Numerical analysis of transient pressure variation in the condenser of a nuclear power station

To research the characteristics of the transient variation of pressure in a nuclear power station condenser under accident condition, a mathematical model was established which simulated the cycling cooling water, heat transfer and pressure in the condenser. The calculation program of transient variation characteristics was established in Fortran language. The pump’s parameter, cooling line’s organization, check valve’s feature and the parameter of siphonic water-collecting well are involved in the cooling water flow’s mathematical model. The initial conditions of control volume are determined by the steady state of the condenser. The transient characteristics of a 1000 MW nuclear power station’s condenser and cooling water system were examined. The results show that at the condition of plant-powersuspension of pump, the cooling water flow rate decreases rapidly and refluxes, then fluctuates to 0. The variation of heat transfer coefficient in the condenser has three stages: at start it decreases sharply, then increases and decreases, and keeps constant in the end. Under three conditions (design, water and summer), the condenser pressure goes up in fluctuation. The time intervals between condenser’s pressure signals under three conditions are about 26.4 s, which can fulfill the requirement for safe operation of nuclear power station.

SIMMER-III Analyses of Local Fuel-Coolant Interactions in a Simulated Molten Fuel Pool: Effect of Coolant Quantity

Studies on local fuel-coolant interactions (FCI) in a molten pool are important for the analyses of severe accidents that could occur for sodium-cooled fast reactors (SFRs). To clarify the mechanisms underlying this interaction, in recent years, several experimental tests, with comparatively larger difference in coolant volumes, were conducted at the Japan Atomic Energy Agency by delivering a given quantity of water into a molten pool formed with a low-melting-point alloy. In this study, to further understand this interaction, interaction characteristics including the pressure buildup as well as mechanical energy release and its conversion efficiency are investigated using the SIMMER-III, an advanced fast reactor safety analysis code. It is found that the SIMMER-III code not only reasonably simulates the transient pressure and temperature variations during local FCIs, but also supports the limited tendency of pressurization and resultant mechanical energy release as observed from experiments when the volume of water delivered into the pool increases. The performed analyses also suggest that the most probable reason leading to such limited tendency should be primarily due to an isolation effect of vapor bubbles generated at the water-melt interface.