Transient Pressure Research Articles

3D numerical investigation on flow boiling characteristics in large length-to-diameter ratio microchannel

To meet the demand of thermal management with large power and high heat flux in the future engineering applications, more and more attentions have been draw to develop the two-phase microchannel cooling technology. In this work, a three dimensional numerical investigation is performed to study the influence of mass flux and heat flux on the two phase flow boiling characteristics in a single microchannel with large length-to-diameter ratio. The related calculation results show that the Volume of Fluid (VOF) model coupled with Lee model can predict the two phase pressure drop and heat transfer coefficient of the microchannel well. And the flow patterns in the microchannel are classified as bubbly flow, slug flow and annular flow, which can be captured by this numerical model precisely. Besides, the transient pressure drop augments with the increase of mass flux. But the effect of mass flux on the amplitude of quasi-steady pressure drop and the peak value of time-averaged heat transfer coefficient along the microchannel is small. At last, the amplitude of quasi-steady pressure drop increases with the augmentation of heat flux. The maximum and fully developed time-averaged heat transfer coefficients along the microchannel increase with the heat flux.

Numerical studies on applications of cavitation models in water hammer-induced cavitating flows in pipelines

Hydraulic transients caused by a fast-closing needle valve and accompanying water hammer-induced cavitation-vortex flow are numerically investigated using the shear stress transport (SST) k–ω model with a pressure–density equation considering the weak compressibility of water and the new cavitation models based on the Rayleigh–Plesset equation that takes into account the surface tension and kinematic viscosity effect on the bubble radius change rate, respectively. The predicted pressure evolutions exhibit reasonable agreement with the experimental data, and the interfacial surface tension-based model is the most accurate in predicting transient pressure evolutions, followed by the kinematic viscosity-based model and the Singhal cavitation model, based on three proposed error indices with minimum of 0.0515, 0.1337, and 0.0035. The wave speed in the upper part of the cavitation trailing edge is higher than that in the lower part, resulting in the evolution of inclination angle of the trailing edge. According to the convective transport mechanism of vorticity, the change rate of vorticity in non-cavitation regions is mainly dominated by the stretching–twisting term and the turbulent fluctuation–diffusion term. In cavitation regions, the dilatation–shrink term makes significant contributions to the change rate of vorticity, and the baroclinic-torque term mainly acts near the edge of the cavitation region.

How different regional approaches to the network design result in key differences in burst event severity and failure vulnerability

Abstract The pipe burst response of an innovative Dutch water distribution network is compared to a traditional looped North American network. Dutch networks focus on water quality and use smaller diameter pipes in branches. The branched network discharges much less water after a burst, which may reduce local flooding, traffic disruption, and product loss. In addition, high velocities and transient pressures are shown to be much localized in the branched Dutch network after a burst, reducing the risks associated with the intrusion of contaminants. However, despite improved water quality, less water loss, and more localized transients, the branched network cannot meet water demands downstream of the burst until the pipe is repaired, unlike a traditional looped network. For modern buildings that meet current design guidelines, the Dutch are content with much lower fire-flow requirements that provide the flexibility to improve water quality and reduce the consequences of pipe bursts, especially water loss.

In operando characterization of the ionic conductivity dependence on liquid transient phase and microstructure of cold-sintered Bi2O3-doped Li1.3Al0.3Ti1.7(PO4)3 solid-state electrolyte

A disruptive sintering technique, Cold Sintering Process (CSP), has been used to produce cold-sintered samples of Bi2O3-doped Li1.3Al0.3Ti1.7(PO4)3 (LATP) as solid-state electrolyte (SSE). An in operando impedance study has been performed to shed light on this sintering process. In this work, Bi2O3 and 3 m acetic acid solution sintering aids were synergistically added to LATP powders to sinter at low temperature. Effects of Transient Liquid Phase (TLP) content on the sintering behavior, phase composition, microstructure, and electrochemical properties were all investigated for a LATP doped with 2 wt% Bi2O3 (optimal content) solid-state electrolyte (SSE). The data revealed that the final electrical properties, which are the key parameter decisive for their application, are defined by the following sintering parameters: Transient Liquid Phase (TLP) content, temperature, pressure, and dwell time. Using acetic acid 3 m as TLP, LATP-based samples can be cold-sintered at optimized sintering conditions of 150 °C and 700 MPa of uniaxial pressure for 90 min. The resultant SSE shows a high ionic conductivity (4.5·10−5 S cm−1) at room temperature (RT) and relative density (∼95%), which demonstrates the effectiveness of the low-temperature sintering process by optimizing the ionic conductivity of LATP-based SSE, also reducing preparation costs and CO2 emissions.

Modeling isovolumetric phases in cardiac flows by an Augmented Resistive Immersed Implicit Surface method.

A major challenge in the computational fluid dynamics modeling of the heart function is the simulation of isovolumetric phases when the hemodynamics problem is driven by a prescribed boundary displacement. During such phases, both atrioventricular and semilunar valves are closed: consequently, the ventricular pressure may not be uniquely defined, and spurious oscillations may arise in numerical simulations. These oscillations can strongly affect valve dynamics models driven by the blood flow, making unlikely to recovering physiological dynamics. Hence, prescribed opening and closing times are usually employed, or the isovolumetric phases are neglected altogether. In this article, we propose a suitable modification of the Resistive Immersed Implicit Surface (RIIS) method (Fedele et al., Biomech Model Mechanobiol 2017, 16, 1779-1803) by introducing a reaction term to correctly capture the pressure transients during isovolumetric phases. The method, that we call Augmented RIIS (ARIIS) method, extends the previously proposed ARIS method (This et al., Int J Numer Methods Biomed Eng 2020, 36, e3223) to the case of a mesh which is not body-fitted to the valves. We test the proposed method on two different benchmark problems, including a new simplified problem that retains all the characteristics of a heart cycle. We apply the ARIIS method to a fluid dynamics simulation of a realistic left heart geometry, and we show that ARIIS allows to correctly simulate isovolumetric phases, differently from standard RIIS method. Finally, we demonstrate that by the new method the cardiac valves can open and close without prescribing any opening/closing times.

Experimental study of the ignition chamber with accelerating cavity applying to methanol lean combustion

Owing to the high laminar flame velocity and broad flammability limit of methanol (CH3OH), it is ideal for lean burning. How to obtain high energy and reliable ignition is one of the biggest challenges with lean or ultra-low combustion. To gain good ultra-lean burn performance, one solution is utilizing an turbulent jet ignition (TJI)system. Moreover, the methanol can be directly used in the jet ignition chamber for its low vaporization temperature and volatile characteristics. To enhance the methanol jet ignition performance, the two-stage accelerating cavity was applied to accelerate the combustion and extend the lean-burn limit in this research. In a optical experimental apparatus, jet ignition and combustion process of premixed methanol in the main combustion chamber (MCC) were investigated using the shadowgraph and transient pressure data acquisition methods. The experiment results showed that the pressure of the MCC emerged an obvious two-stage rising process to use the two-stage accelerating cavity (SAC). In addition, when the SAC was applied in the ignition chamber (IC), the proportion and peak value of the first stage heat release in the MCC were clearly higher than those of the single straight hole (SSH). Moreover, the ignition chamber compounded with SAC exhibited a short ignition delay and long jet ignition duration. Compared to SSH, the methanol combustion duration in the MCC by using SAC shortened by approximately 28% under both stoichiometric and lean conditions. In other words, The SAC accelerated the combustion process of methanol. More significantly, it was found that the proper secondary acceleration hole diameter (SAHD) play an crucial role in the jet ignition and combustion process, especially under ultra-lean conditions. In addition, by utilizing SAC-3-4.5, the combustion duration of was shortened by 41.9% than that of using SAC-3-3 at the equivalence ratio of 0.6. Finally, the ultra-lean burn limit could be extended to 0.35 by using SAC-3-4.5, which greatly expanded the methanol lean combustion limit.

Influence and multi-objective optimization on three-stage guide vane closure scheme of a pumped storage power plant

Selection of the guide vane closure scheme was a critical issue for the load rejection transients of a pumped-storage power plant. To fully and accurately assess the influences of the guide vane closure scheme on the transient performance and pressure fluctuation, the load rejection transients adopting the original two-stage and optimized three-stage guide vane closure schemes were simulated adopting the one- and three-dimensional coupled approach. Results suggested that the guide vane closure schemes significantly influenced the transient performances and pressure fluctuations during load rejection transients. Compared with adopting the original two-stage guide vane closure scheme, adopting the optimized three-stage guide vane closure scheme could improve the maximum rotation speed, maximum and minimum fluctuating pressures in the pumped-storage power plant, and suppress the local backflow vortexes close to the high-pressure inlet of runner and their induced low-frequency and high-amplitude pressure fluctuation close to the no-load conditions. The study results provided theoretical reference for selecting the optimized guide vane closure scheme.

New design and transient flow analysis of a claw vacuum pump with novel gear-claw rotors

The claw vacuum pump is regarded as the most potential pump type used as the vacuum system due to its remarkable advantages of oil-free, structure compact and high reliability. During operation, serious gas leakage between two conventional claw rotors occurs and then the pump efficiency decreases. With the aim for reducing gas leakage, a novel pair of gear-claw rotors were proposed, in which a new gear-type structure was generated to produce special twisting clearance between two claw rotors for preventing gas leakage, and then a new claw vacuum pump with high comprehensive performance was developed. A geometric model of the gear-claw rotor was established, and the impact of design parameters on the performance of the proposed rotor was analyzed. Furthermore, the transient flow and pressure distribution of the conventional and the proposed claw vacuum pumps were compared by means of numerical simulations. It is found that the volumetric efficiency and gas leakage velocity between two claw rotors in the conventional claw vacuum pump are 69.5% and 347.665 m/s, respectively; while the volumetric efficiency and gas leakage velocity between two gear-claw rotors in the proposed claw vacuum pump are 77.4% and 274.020 m/s, respectively. Compared with conventional claw rotors, the gas leakage velocity between two gear-claw rotors is reduced by 21.18%, and its volumetric efficiency is increased by 7.9%. The efficiency and sealing performance of the proposed gear-claw vacuum pump are significantly superior to those of the conventional claw pump.

A plasma-based pressure pulse generator for simulating pellet-cladding mechanical interaction during reactivity-initiated accident.

This paper describes the characteristics of a plasma-based pressure pulse generator and its potential use as a simulator for studying pellet-cladding mechanical interaction (PCMI) during reactivity-initiated accidents (RIAs). In this device, a transient pressure pulse is generated by rapid heating and expansion of hot, dense plasma inside a nuclear fuel cladding. Thus, the parameters of a pressure pulse, such as peak pressure and pressure rise-rate, can be controlled by modifying the electrical parameters of a pulse discharge circuit. The pulse discharge circuit utilizes a capacitor bank comprising several energy storage capacitors connected in parallel and a high-power solid-state switch. A pressure loading system is attached as a load to the pulse discharge circuit. The power and energy delivered to the load are calculated by measuring the voltage and current waveforms at one end of the loading system. A piezoelectric sensor is connected at the other end of the loading system to simultaneously measure the pressure pulse inside the cladding tube. Preliminary experiments are carried out with a stainless-steel tube to characterize the performance of the device as well as with a pre-hydrided ZIRLO™ cladding tube to demonstrate the potential of the device as a simulator for studying the failure characteristics of the cladding as a result of an RIA. The high pressurization rate of the device is expected to offer unique advantages for studying the PCMI mechanism.

Technology Focus: Formation Evaluation (August 2023)

Fractures, whether natural or hydraulic, serve as crucial pathways for fluid flow within rock formations. Because natural and hydraulic fractures are generated hundreds of meters underground, their parameters always seem to be a black box. Their influence on pressure propagation and well productivity, however, cannot be overstated. So how well are we doing in fracture evaluation and modeling? Fracture evaluation can be regarded as an inverse problem, wherein the structural characteristics of a system are deduced based on input and output signals. Our objective is to ascertain the distribution, orientation, connectivity, and properties of fractures through the mathematical and physical methods applied to field data. Generally, we can use fracture-evaluation techniques such as surface tilt fracture mapping, downhole tilt fracture mapping, microseismic fracture mapping, tracer testing, temperature logs, production logs, borehole image logs, downhole video, caliper logs, production analysis, and well testing. In parallel to fracture evaluation, fracture modeling assumes a computational approach to simulate fractures within subsurface reservoirs. We have various methodologies such as analytical, semianalytical, or numerical approaches to develop fracture models, including dual-porosity models, discrete fracture models, embedded discrete fracture models, and flow-net models. By means of fracture simulations, we can glean valuable insights pertaining to transient pressure behaviors, optimal production schemes, and estimation of ultimate recovery. Despite the significant progress made, a gap still exists between theoretical advancements and practical field application in fracture evaluation and modeling, because underground fractures are often more complex than initially expected and nonunique solutions for fracture evaluation are commonplace. Thus, fracture evaluation and modeling represent complex and interdisciplinary fields, requiring expertise in geology, reservoir engineering, geomechanics, simulation, and related disciplines. We also need continuous research endeavors and advancements in fracture evaluation and modeling techniques to augment our understanding of fracture behavior and reservoir performance in the oil and gas industry, as demonstrated in the selected papers and recommended additional reading. Recommended additional reading at OnePetro: www.onepetro.org. SPE 212296 A Simple Analytical Model for Oil Production From Partially Fractured Reservoirs To Estimate Size of Finite Fracture Networks by Sait I. Ozkaya, Consultant SPE 209624 A Flow-Net-Based Method for History Matching and Production Prediction of Shale or Tight Reservoirs With Fracturing Treatment by Hui Zhao, Yangtze University, et al. SPE 209293 A Hybrid Embedded Discrete Fracture Model and Dual-Porosity, Dual-Permeability Work Flow for Hierarchical Treatment of Fractures in Practical Field Studies by Mun-Hong Hui, Chevron, et al.

Extending the Application of Connectivity Metrics for Characterizing the Dynamic Behavior of Water Distribution Networks

AbstractWater distribution networks (WDNs) are complex combinations of nodes and links, and the current tendency is to modify their topological structure through the closure of isolation valves for monitoring and water quality reasons. For their analysis, several approaches based on graph theory have recently been proposed, mainly considering steady‐state flow conditions. However, in their real functioning, WDNs are continuously subjected to pressure transients generated by maneuvers on regulation devices or by users' activity. This study investigates the application of some metrics from graph theory, already used in the context of steady‐state analysis, for assessing the effects of changes in the topological structure of a network—due for example, to sectorization or branching operations—on its transient response when subjected to maneuvers on devices such as hydrants, pumps, etc. or users' activity. The analysis shows that some connectivity metrics can effectively reflect the dynamic pressure behavior of the network and, thus, provide useful indications for design and management operations taking into account unsteady flow features.

Estimation of deposit thickness in single-phase liquid flow pipeline using finite volume modelling

A significant problem with liquid pipelines can be the internal deposition of wax, sediments, tuberculation and biofilms which results in decreased flow assurance of the pipelines and higher pumping costs in addition to safety issues. Therefore, it is very important to monitor the deposits and roughness in the pipelines for the timely restoration of the intended supply. This paper proposes a finite volume model (FVM) based on an implicit flux limiter-Riemann solver (CLAWPACK) to estimate deposit thickness in oil and water pipelines. The novelty of the proposed method is that a snapshot of transient pressures along the pipeline only at a particular time instant (end of simulation) is considered in contrast to the widely used pressure time series for comparison between computed and measured pressures to detect changes in internal diameter and wall friction. The Newton-Raphson Method is used to iteratively estimate the deposition thickness to produce an equivalent response obtained from the pressure measurements of the physical pipeline system. The proposed methodology is applied to three operating crude oil pipelines and the maximum error found between computed and measured pressures and deposit thicknesses are 0.035% and 5 mm respectively. The results indicate that the proposed model is an accurate, efficient, faster and cost-saving alternative to complex invasive type pipeline assessment techniques.

Finite Volume Method for Transient Pipe Flow with an Air Cushion Surge Chamber Considering Unsteady Friction and Experimental Validation

In various water transmission systems such as long-distance water transfer projects and hydropower stations, accurate simulation of water hammer is extremely important for safe and stable operation and the realization of intelligent operations. Previous water hammer calculations usually consider only steady-state friction, underestimating the decay of transient pressure. A second-order Finite Volume Method (FVM) considering the effect of unsteady friction factor is developed to simulate the water hammer and the dynamic behavior of air cushion surge chamber in a water pipeline system, while an experimental pipe system is conducted to validate the proposed numerical model. Two unsteady friction models, Brunone and TVB models, were incorporated into the water hammer equations, which are solved by the MUSCL–Hancock method. One virtual boundary method was proposed to realize the FVM simulation of Air Cushion Surge Chamber. Comparisons with water hammer experimental results show that, while the steady friction model only accurately predicts the first pressure peak, it seriously underestimates pressure attenuation in later stages. Incorporating an unsteady friction factor can better predict the entire pressure attenuation process; in particular, the TVB unsteady friction model more accurately reproduces the pressure peaks and the whole pressure oscillation periods. For water pipeline systems with an air cushion surge chamber, energy attenuation of the elastic pipe water hammer is primarily due to pipe friction and the air cushion. The experimental results with the air cushion surge chamber demonstrate that the proposed FVM model with the TVB unsteady friction model and the air chamber polytropic exponent near 1.0 can well reproduce the experimental pressure oscillations.

Research on Managed-Pressure Running Casing in Oil and Gas Wells with the Negative Pressure Window

The failure of managed-pressure running casing in oil and gas wells may lead to complex accidents such as overflow or leakage. The technique of using multi-density gradient drilling fluids in wellbores with negative pressure windows (NPWs) is often used to deal with this situation. Therefore, it is vital to analyze the dynamic slurry column structure and calculate the wellbore pressure during casing running. For this issue, the model of transient surge pressure is established during casing running. The calculation equation of the model is proposed, and the calculations of the wellbore pressure are carried out with the exploration of Well LT-X1, located in the Xinjiang oil field. A circulation scheme is designed as follows: Circulate 125 m3 of drilling fluid with a density of 2.45 g/cm3 and 155 m3 of drilling fluid with a density of 2.35 g/cm3 at a depth of 3560 m. From there, circulate 164 m3 of drilling fluid with a density of 2.35 g/cm3 at a depth of 5900 m. Finally, at a depth of 7050 m, circulate 250 m3 of drilling fluid with a density of 2.30 g/cm3. The casing running speeds and back-pressure values were designed as follows for the respective well sections: 0–1523 m: 0.160 m/s casing speed, 0 MPa back pressure; 1523–3560 m: 0.160 m/s casing speed, 1.641 MPa back pressure; 3560–5900 m: 0.145 m/s casing speed, 2.427 MPa back pressure; 5900–6674 m: 0.137 m/s casing speed, 4.041 MPa back pressure; 6674–7050 m: 0.124 m/s casing speed, 4.457 MPa back pressure. The results show that optimizing structure of the multi-density gradient drilling fluid with different densities and applying annular back pressure in stages, with the accurate calculation of wellbore pressure, can achieve the goals of leak-proofing and pressure-stabilization. It is concluded that this result may serve as the foundation for managed-pressure running casing technology.

Modelling LOFA scenario for WCLL BB concept using MELCOR1.8.5 code

In the framework of the EUROfusion activities for the DEMO project, the evaluation of the Breeding Blanket (BB) performance and its ancillary systems is a compulsory activity. The European liquid BB concept, named Water Cooled Lithium Lead (WCLL), makes use of the liquid metal alloy PbLi as a tritium breeder/neutron multiplier and water as a coolant. A dedicated ancillary system guarantees the recirculation of the PbLi between the BBs and the tritium extraction/purification system. The BBs and the ancillary system must operate safely and reliably under the designed conditions, while accidental scenarios have to be identified for risk assessment and to define safety plans.In the present work an analysis of the In-Board Loss Of Flow Accident (IB LOFA) in the PbLi ancillary loop of the WCLL BB is conducted. This numerical analysis is performed by employing the fully integrated code MELCOR1.8.5, the release for fusion application. A LOFA scenario is considered by assuming the electromagnetic pump in the IB BB loop, which provides forced circulation to the PbLi, stops working. The nodalisation and numerical models are performed by applying geometries and design parameters using the 2020 WCLL PbLi design. The analysis is performed to develop a suitable numerical model, which simulates transients by studying several factors such as: tritium mobilization and transport inside the loop, fast plasma shutdown leading to pressure and temperature transients, and maximum temperatures inside the First Wall (FW).The preliminarily results demonstrate the maximum temperature inside the FW does not exceed the defined temperature thresholds for Tungsten and EUROFER (1000 °C and 500 °C, respectively). Furthermore, the pressure transient is appropriately accommodated within the ancillary loop. The initial tritium inventory in breeding modules is relocated and accrued in the tritium extraction system's free volume. This work is the first step in the safety analyses of the PbLi ancillary system aiming to support the R&D works carried out in the framework of the DEMO project.

Experimental and Mechanism Study of Aerodynamic Noise Emission Characteristics from a Turbocharger Compressor of Heavy-Duty Diesel Engine Based on Full Operating Range

Heavy-duty diesel engines equipped with turbochargers is an effective way to alleviate energy shortage and reduce gas emissions, but their compressor aerodynamic noise emissions have become an important issue that needs to be addressed urgently. Therefore, to study the aerodynamic noise emission characteristics of a compressor during the full operating range, experimental and numerical simulation methods were used to analyze the aerodynamic noise emissions. The results showed that aerodynamic noise’s total sound pressure level (SPL) increased with increased speed under the test conditions. At low speeds, the total SPL of aerodynamic noise was affected by the mass flow of the compressor more obviously. The maximum difference of aerodynamic noise total SPL was 1.55 dB at 60,000 r/min under different mass flows. At the same speed, the compressor could achieve lower aerodynamic noise emissions by operating in the high-efficiency region (middle mass flows). In the compressor aerodynamic noises, the blade passing frequency (BPF) noise played a dominant role. The transient acoustic-vibration spectral characteristics and fluctuation pressure analysis indicated that BPF and its harmonic frequency noises were mainly caused by the unsteady fluctuation pressure. As the speed increased, the BPF noise contributed more to the total SPL of the aerodynamic noise, and its percentage was up to 75.35%. The novelty of this study was the analysis of the relationship between compressor aerodynamic noise and internal flow characteristics at full operating conditions. It provided a theoretical basis for reducing the heavy-duty diesel engine turbocharger compressor aerodynamic noise emissions.

Using in-situ strain measurements to evaluate the accuracy of stress estimation procedures from fracture injection/shut-in tests

Fracture injection/shut-in tests are commonly used to measure the state of stress in the subsurface. Injection creates a hydraulic fracture (or in some cases, opens a preexisting fracture), and then the pressure after shut-in is monitored to identify fracture closure. Different interpretation procedures have been proposed for estimating closure, and the procedures sometimes yield significantly different results. In this study, direct, in-situ strain measurements are used to observe fracture reopening and closure. The tests were performed as part of the EGS Collab project, a mesoscale project performed at 1.25 and 1.5 km depth at the Sanford Underground Research Facility. The tests were instrumented with the SIMFIP tool, a double-packer probe with a high-resolution three-dimensional borehole displacement sensor. The measurements provide a direct observation of the fracture closure signature, enabling a high-fidelity estimate of the fracture closure stress (ie, the normal stress on the fracture). In two of the four tests, injection created an opening mode fracture, and so the closure stress can be interpreted as the minimum principal stress. In the other two tests, injection probably opened preexisting natural fractures, and so the closure stress can be interpreted as the normal stress on the fractures. The strain measurements are compared against different proposed methods for estimating closure stress from pressure transients. The shut-in transients are analyzed with two techniques that are widely used in the field of petroleum engineering – the ‘tangent’ method and the ‘compliance’ method. In three of the four tests, the tangent method significantly underestimates the closure stress. The compliance method is reasonably accurate in all four tests. Closure stress is also interpreted using two other commonly-used methods – ‘first deviation from linearity’ and the method of (Hayashi and Haimson, 1991). In comparison with the SIMFIP data, these methods tend to overestimate the closure stress, evidently because they identify closure from early-time transient effects, such as near-wellbore tortuosity. In two of the tests, microseismic imaging provides an independent estimate of the size of the fracture created by injection. When combined with a simple mass balance calculation, the SIMFIP stress measurements yield predictions of fracture size that are reasonably consistent with the estimates from microseismic. The calculations imply an apparent fracture toughness 2-3x higher than typical laboratory-derived values.

Field Test and Numerical Investigation of Tunnel Aerodynamic Effect Induced by High-Speed Trains Running at Higher Speeds

After decades of research in the field of high-speed railway technique, technology of running high-speed trains at the velocity level of 350 km/h gradually become mature. It is of great importance to capture the variation regular of aerodynamic parameters in the situation that the high-speed train runs at a higher speed level. The present paper is motivated by this knowledge gap, both field tests and numerical simulations were conducted to help illustrate the basic characteristic of transient pressure loads, micro-pressure wave, as well as the wave propagation inside the tunnel regrading train’s passage and intersection. Results present the major findings as: (1) Transient pressure loads acting at tunnel surface and train body unevenly distributes along the longitudinal, transverse, and vertical directions. Pressure peak along the longitudinal direction occurs nearly at tunnel center and fast decreases from the radiated center to the remote positions. (2) Variation of pressure peak near the tunnel portal in the situation of train’s passage and intersection is limited while its value becomes doubled at the intersection location. Field measurements suggest the maximum pressure load acting at tunnel sidewall at xtin = 200 m and tunnel center being 4.29 and 5.63 kPa, respectively; (3) The maximum value of micro-pressure wave (namely MPW) detected in the field test is approximately 36.73 Pa. Amplitude of MPW at tunnel portal is inversely proportional to its attenuated distance. Through data fitting, an empirical prediction model was established. Outcomes of this paper is anticipated to improve the understanding of tunnel aerodynamic effect regarding higher speed level and its associated problems. Besides, findings of this paper are useful for the future tunnel design.

Study on water hammer effect and tubing string vibration in high-pressure high-production gas wells

The transient internal flow in high-pressure high-production gas wells generates severe water hammer and tubing string vibration, which accelerates fatigue damage, joint wear, thread loosening, and other wellbore integrity problems. Based on the analysis of transient flow in gas wells, a mathematical model for fluid pressure surge and the induced tubing string vibration is developed and validated by similarity experiments. This method considers fluid-structure interaction, initial loads of tubing string, and compressibility of natural gas, and can fill the gap in quantification of water hammer pressure and tubing sting dynamic load in high-pressure high-production gas wells. On this basis, the effects of valve closure time, production rate, liquid content, and tubing string structure on transient flow and tubing string characteristics are analyzed systematically. The calculations show that wellhead pressure increases rapidly, and pressure wave propagates along tubing string to induce vibrations in the string after an instantaneous shut-in at the wellhead. During this transient process, fluid pressure and axial stress at wellhead reach their maximum valves. Owing to the fixed constraint effect of wellhead and downhole packer, the lack of restrictions on tubing distant from wellhead and packer enhances the severity of axial vibration. As valve closure time increases, the maximum wellhead pressure, tubing vibration displacement, and velocity gradually decrease. Therefore, the most direct way to control water hammer effect and tubing string vibration is to extend the shut-in time or prevent a rapid shut-in of gas well. The methods used in this study and the results obtained therein can serve as both a theoretical framework and practical foundation for the prediction and control of water hammer effect and tubing string vibration in high-pressure high-production gas wells.

Research on spectral method of lost circulation layer located by transient pressure wave

AbstractLeakage in the wellbore annulus during drilling operations can affect normal production operations, resulting in a severe waste of resources and economic loss, so it is crucial to adopt a fast and effective leak identification method for subsequent plugging operations. For the complex problem of judging the location of the leakage layer, we proposed the method of excitation pressure wave to identify the location of the leakage layer. By analyzing the transfer of pressure waves within the annular pipe system and the pressure head response spectrum, the leak's location is identified based on the location of the resonance point and the change in resonance amplitude. The pressure wave signal contains too much noise. The variational mode decomposition (VMD) algorithm and the Hilbert joint spectrum were used to extract the main frequency components to reconstruct the signal to achieve the denoising effect. On this basis, the reconstructed signal is processed by fast Fourier transform (FFT) to obtain the pressure wave response spectrum, analyze the frequency domain features, and then determine the location of the leakage layer. The experimental results verify that: ① When a leak occurs in the wellbore annulus, the pressure wave will generate additional resonance points in the frequency domain due to the presence of the leak point. ② The combination of the VMD algorithm, FFT, and Hilbert joint spectrum can effectively remove the noise of the pressure wave signal. ③ The method effectively avoids the difficulty of identifying negative pressure waves in the time domain analysis of pressure wave signals. It can effectively locate the leaky layer in the frequency domain analysis. It is concluded that the principle of the method is feasible and has practical significance for field application.