Unsteady Flow Processes Research Articles

About one model of non-stationary flows of a weakly compressible liquid in a pipeline

The process of unsteady flow of a weakly compressible fluid through a pipeline, described by a two-dimensional model in the form of a nonlinear system of partial differential equations, is considered. The model is averaged over the cross section of the pipeline and the dependence of the friction stress on the pipe wall on the flow velocity is proposed. Within the framework of the model obtained, the inverse problem was posed to determine the flow velocity on the pipeline wall. A discrete analogue of the problem is constructed and a method for its splitting into simple difference problems with respect to the approximate values of the unknown functions is proposed. Refs.: 14 titles.Keywords: pipeline; two-dimensional model; friction stress on the pipe wall; difference problem; inverse problem.

Numerical investigation on wellbore temperature and pressure during carbon dioxide fracturing

In the present study, a transient 2D model considering the factors of CO2 thermal properties, viscous heating source, Joule-Thomson effect, axial and radial heat transfer is proposed to investigate the wellbore temperature and pressure. The temperature governing equations are solved numerically by using the method of Alternating Direction Implicit (ADI). Based on the law of conservation of energy, the viscous heating source is reasonably considered as the thermal energy converted by mechanical energy. The wellbore temperature and pressure are analyzed based on the investigation of the tubing and annulus injection well with the diameter of 50.3 mm and depth of 3600 m. The simulation results of our model are in good agreement with the field data. The analysis results show that the bottom hole temperature increases first and then decreases as injection time increases. Large injection rate results in low bottom hole pressure and negative axial pressure gradient. The critical depth increases as injection time increases. Compared with tubing injection, tubing and annulus injection decreases critical depth and increases the duration of wellbore unsteady flow and heat transfer process. The solid heat conduction in axial direction significantly affects the wellbore temperature and pressure distributions.

Hydraulic Balance Adjustment Analysis and Optimization Method of Farmland Water Conservancy Pipe Network

The hydraulic balance of farmland water conservancy pipe network is a common problem in the current large-scale farmland irrigation system. It is of great practical significance to study the different types of unsteady flow hydraulic calculations in farmland water pipe network for the safe and stable operation of farmland irrigation. In this paper, based on the characteristic’s method, the mathematical model of the unsteady flow for farmland water pipe network was established, and the boundary conditions of the constructed model and the processing method for calculating the time step were given. Then, the proposed model was verified by actual examples, and the unsteady flow process of the irrigation pipe network was also calculated and analysed. The research results show that the method of characteristics can better optimize the boundary problem of water pump, nozzle and t-branch pipe in the unsteady flow of farmland irrigation pipe network. After the valve is closed, the water hammer pressure will appear in the irrigation pipe network system and propagate inside. The water hammer pressure at the valve is the largest; the greater the distance from the closing valve, the smaller the water hammer pressure. When closing multiple valves at the same time, complex water superimposing of hammer wave occurs in the pipe network, and the maximum water hammer pressure after superimposing is much larger than the single valve closing. The longer the valve is closed, the smaller the maximum head pressure at the same monitoring point. The research conclusion can provide a theoretical reference for the engineering treatment of the unsteady flow in the farmland water conservancy pipe network.

Effects of the Second-Stage of Rotor with Single Abnormal Blade Angle on Rotating Stall of a Two-Stage Variable Pitch Axial Fan

It is of great value to study the impact of abnormal blade installation angle on the inducement mechanism of rotating stall to achieve the active control of rotating stall in an axial fan. Based on throttle value function and SST k-ω turbulence model, numerical simulations of the unsteady flow process in stall condition of an axial flow fan with adjustable vanes were carried out, and the influence mechanism of abnormal stagger angle of a single blade in the second stage rotor on induced position and type of stall inception and evolution process of rotating stall were analyzed. The results show that compared with synchronous adjustment of blade angle, the blade with abnormal stagger angle will cause the increase of flow rate at the beginning of stall and make the fan fall into an unstable condition in advance. The existence of blade with abnormal angle does not cause the change of the induced position and type of stall inception and the inducement mechanism of rotating stall, which are the same as the axial fan with normal blade angle. Moreover, the single blade with abnormal deviation angle has important impacts on the 3D unsteady evolution process from stall inception to stall cell formation in two rotors.

Fractal research of transverse sections of a long river channel below a dam

Research on the propagation law of unsteady flow from dam discharge has significance for the engineering problems of shipping and urban flood control. Due to the problem of many factors influencing the process of unsteady flow, we propose using fractal dimensions to replace various hydraulic factors. In this paper, we choose a navigation-obstructing beach section in a river as the research object, establish a two-dimensional unsteady fractal mathematical model to obtain statistics on the channel width at the minimum maintenance water depth and then calculate the fractal dimension of the river cross section. The results show the first-order feature, and it is also shown that the fractal dimension of the river width has a strong relationship with the depth at the beach section and the shape of the cross section. In general, the shoal’s fractal dimension increases with the depth. The fractal dimension of the transverse sections has the trend of gradually reducing as a whole with the velocity. Compared to the fractal dimension of the beach in the same river section, the river cross-section topography is smoother and the section fractal dimension is smaller when the upstream flow is the same.

Accuracy of single-jet water meters during filling of the pipe network in intermittent water supply

In intermittently operated water distribution systems the air intruded into the pipe network through the filling and emptying process must inevitably exit the system at some point. Part of this air is discharged through service connections and thus through water meters. Due to this air flow, water meters are being used contrary to their actual conception. This article presents a study in which the measurement error of single-jet water meters due to the filling process of an empty service connection is experimentally examined. It shows that this error is caused mainly by the air flow before the water front reaches the water meter. The correlation between the measurement error and the air volume in front of the water meter is almost linear. The impact of the water front, the presence of water-air mixtures or unsteady flow processes however, is shown to have very little influence on the results.

ЧИСЛЕННЫЙ МЕТОД РЕШЕНИЯ ОДНОЙ НЕЛОКАЛЬНОЙ ЗАДАЧИ ТРУБОПРОВОДНОГО ТРАНСПОРТА ВЯЗКИХ ЖИДКОСТЕЙ

Х.М. Гамзаев, Азербайджанский государственный университет нефти и промышленности, г. Баку, Азербайджан E-mail: xan.h@rambler.ru. K.M. Gamzaev Azerbaijan State Oil and Industry University, Baku, Azerbaijan E-mail: xan.h@rambler.ru

Permeate flux decrease due to concentration polarization in a closed roto-dynamic reverse osmosis filtration system

The efficiency of water purification system is largely affected by the concentration polarization. This can be reduced by increasing the mixing, mass transfer above the membrane and shear rate over the membrane surface. It is easy to increase shear rate and mixing in a roto-dynamic system than spiral wound system. A closed roto-dynamic filtration system without retentate outlet can be used when the supply of feed water is limited and reuse of water should be maximized. In a closed system, concentration inside the rotor cavity increases with time. We simulated unsteady flow and transport processes along with reverse osmosis through the membrane using ANSYS-Fluent for different values of rotor speed, feed pressure and aspect ratio. We compared the effects of these parameters on concentration polarization and permeate flux reduction. We also estimated the energy input for rotating the rotor and pressure driven flow. The specific energy consumptions for all cases were compared. The permeate flux increases with rotor speed but specific energy consumption also increases rapidly with rotor speed. So high rotor speed may not be always very effective. The present study and computational framework would be useful for optimum design of roto-dynamics cross-flow RO system.

Investigation on the gas pockets in a rotodynamic multiphase pump

The appearance of gas pockets has an obvious impact on the performance of the rotodynamic multiphase pump. In order to study the formation of gas pockets in the pump and its effects on pump's performance, the unsteady numerical simulation and the visualization experiments were done to investigate gas pockets in a three-stage rotodynamic multiphase pump developed by authors. Meanwhile, the mixture of water and air was selected as the medium. According to the distributions of pressure, gas volume fraction and velocity vector in three compression cells in unsteady flow process, the process of the formation of gas pockets in the pump were analysed generally. The visualization experiments were used to verify the validity of the numerical simulation. The results will be benefit for the hydraulic design of the compression cell of rotodynamic multiphase pump.

Investigation on the dynamic characteristics of the counter-current flow for liquid desiccant dehumidification

The dynamic characteristics of solution film flow with counter-current air flow, especially the two phase contact area, are important for the optimal design and operation of LDD (liquid desiccant dehumidifier). However, the unsteady interfacial information, including the interfacial pressure, velocity and film thickness, is hardly to be examined accurately by theoretical prediction or experimental measurement due to its complexity. In this study, CFD (computational fluid dynamic) models for the counter-current flow of the LDD were established based on the VOF (volume of fraction) and RNG (Renormalization group) k-ε turbulence model. Experimental research had been conducted to validate the liquid film thickness and typical interfacial waves obtained in simulation. With the established model, the dynamic formation process of unsteady counter-current flow is evaluated. Moreover, the effects of various parameters on the liquid film waves, pressure drop, liquid film thickness and interfacial information were obtained and analyzed by the built models. With the increase of liquid flow rate, the roll waves appear and the wave amplitudes are increased significantly, which may obviously enhance the performance in the LDD absorption and regeneration process due to the increased contact area.

Analysis of influence design parameters ejector pipe at the detonation specifications engine

The engines that use a detonation cycle have greater thermal efficiency than engines with constant-pressure combustion cycle. The mechanisms of ejector augmentation are evident in secondary air entrainment. The impulse characteristics of a pulse detonation engine (PDE) with ejector were investigated under different parameters. The propagation of a detonation wave in an ejector-augmented pulse detonation rocket engine fueled with a hydrogen-oxygen methane-oxygen and hydrocarbon-oxygen mixture is studied. The straight cylindrical ejector was coaxially installed at different axial locations relative to the exit of the detonation chamber. The use of a non-steady ejector is modeled for pulse detonation engine performance improvement. In this paper, a two-dimensional numerical model is developed to help design the basic geometry and operating parameters of the device. The unsteady flow processes are simulated and compared with a baseline PDE without ejector enhancement. Various features including detonation-shock interaction, detonation diffraction and vortex formation are observed. It is observed that the design of detonation wave flow path in detonation tube, ejector and operating parameters such as Mach numbers are mainly responsible for improving the propulsion performance of PDE.Object оf numerical simulation is the cylindrical detonation chamber with a diameter of Dc = 25 mm and a length of Lc = 100 mm. The cylindrical ejectors with a length of Le = 100 mm and diameters De = 35, 40, 45, 50 that 55 mm were attached to the chamber. Also ejectors with a diameter of De = 35 mm and lengths of Le = 50, 100, 150, 200 that a 250 mm respectively were attached to the chamber. The calculation was performed with a computer program that implements a numerical TVD-scheme. The results of calculations were presented as the dependence of specific thrust impulse Isp/Ispo ratio (Isp ­- specific thrust impulse with ejector nozzle; Ispo - specific thrust impulse of a chamber only) to the ejector-to-PDE diameter ratio (De/Dc), to the ejector-to-PDE length ratio (Le/Lc) and to the displacement of an ejector-to-PDE length ratio ((Ld +Lc)/Lc), where Ld – distance from the input section of the nozzle to the thrust wall of a detonation chamber. The method of a numerical simulation proved the possibility of a significant increase of pulse detonation engine performance through the use of ejector nozzle.

Prediction–Correction Method for Parallelizing Implicit 2D Hydrodynamic Models. II: Application

AbstractA two-dimensional (2D) hydrodynamic model is developed for the Jing-Dongting river–lake system, with the computation domain (3,900 km2) covered by a channel-refined grid of 300,000 cells. The model provides good descriptions of the river–lake characteristics (strongly coupled, annular branches, varying flow regimes). In a simulation of a 1-year unsteady flow process, the mean absolute error in simulated water levels is 0.11–0.15 m, and the mean absolute relative error in simulated cross-section discharges is 4.3–8.5%, compared with field data. The accuracy of the 2D model is obviously better than those of existing one-dimensional (1D) and 1D–2D nested models. The prediction–correction parallelization (PCP) method is then tested by simulating the river–lake system using 64 subdomains. The complexities of real rivers are revealed to have almost no negative effects on the quasi-coupled solutions and accuracy of the PCP method. In tests, the mean absolute error in water level is found to be approxima...

Water Hammer Simulation Using Explicit–Implicit Coupling Methods

AbstractThe method of characteristics (MOC) with the limitation of Courant’s stability condition is widely used in simulation of unsteady flow in a pipeline. However, the relatively complex method of implicit (MOI) provides the advantages of unconditional convergence and mutual independence between time and space mesh parameters. This study combines the MOC and MOI to simulate pipeline unsteady flow and hydropower transient processes. The boundary conditions for the coupled method are introduced and validated by simulating the water hammer in uniform and variable area duct, and the water-level fluctuation in a surge tank. Subsequently, the coupled methods are applied to study the transient processes in two hydropower stations: one is to determine the water-level fluctuation in the surge tank and investigate the effect of water inertia in the connecting pipe on the water hammer pressure, and the other is to determine the solution for the water hammer in a variable-area draft in a pump turbine system by com...

Time-Resolved Flow Properties in a Turbine Driven by Pulsed Detonations

Theoretically, pressure-gain combustion like that of a pulsed-detonation combustor allows heat addition with reduced entropy loss compared to a typical steady deflagration combustor. The pulsed-detonation combustor is inherently unsteady, and time-resolved measurements are required to identify unsteady flow processes at the turbine inlet and exit. In this study, the radial turbine of a Garrett automotive turbocharger was coupled directly to and driven, full admission, by a pulsed-detonation combustor fueled by hydrogen or ethylene. Full-cycle time-resolved turbine inlet and exit pressures were obtained from pressure transducers mounted to wall static pressure ports. Tunable diode laser absorption spectroscopy was used to obtain full-cycle time-resolved turbine inlet and exit temperatures and velocities. Optical access permitted the use of high-speed video for particle streak velocimetry to obtain time-resolved turbine inlet velocities during blowdown with a 20 Hz ethylene-fueled pulsed-detonation combustor with 0.9 fueled fraction and 0.5 purge fraction. A comparison of laser-based and video-based measurements was made for the blowdown portion of the pulsed-detonation combustor cycle. First-ever full-cycle time histories of turbine inlet and exit velocities, total temperature, total pressure, mass flow, and total enthalpy rate revealed reflected pressure waves and showed momentary reverse flow, and thus unsteady accumulation and expulsion of mass and enthalpy.

Air–water flow measurements in a flat slope pooled stepped waterway

Air–water flows on stepped spillways were investigated experimentally in the last decades with a focus on steep slope chutes equipped with flat horizontal steps. Detailed air–water flow properties were recorded herein with three stepped geometries down a slope of θ = 8.9° with: flat horizontal steps, pooled steps, and a combination of flat and pooled steps. The data included the distributions of basic air–water flow properties, as well as the energy dissipation and flow resistance data deduced from the air–water flow measurements. The results on the flat slope showed that the pooled stepped design enabled a greater rate of energy dissipation, but the pooled stepped geometries were affected by some flow instabilities and unsteady flow processes for a range of flow rates.

Novel Oscillatory Patterns of Hypersonic Inlet Buzz

Unstart flows of a hypersonic inlet were experimentally studied at freestream Mach number of 4.5–6.0. With the aid of high-speed schlieren and time-accurate pressuremeasurements, the unsteady flow processes of the entire inlet, including the shock systemmotions and the surface pressure fluctuations, were recorded and discussed. The started flowfield analysis was conducted first, and then the unstarted flowfield was analyzed by using the unsteady pressure signals and schlieren pictures. Results indicate that two novel oscillatory patterns were observed in comparison with the past reported inlet buzz patterns. One is a mixed oscillatory pattern that mixes the “big buzz”with “little buzz,” and the other is a nonoscillatory violent pattern. These novel findings on oscillatory patterns of hypersonic inlet can provide more insight on inlet buzz mechanism, prediction, and control.

Rotary wave-ejector enhanced pulse detonation engine

The use of a non-steady ejector based on wave rotor technology is modeled for pulse detonation engine performance improvement and for compatibility with turbomachinery components in hybrid propulsion systems. The rotary wave ejector device integrates a pulse detonation process with an efficient momentum transfer process in specially shaped channels of a single wave-rotor component. In this paper, a quasi-one-dimensional numerical model is developed to help design the basic geometry and operating parameters of the device. The unsteady combustion and flow processes are simulated and compared with a baseline PDE without ejector enhancement. A preliminary performance assessment is presented for the wave ejector configuration, considering the effect of key geometric parameters, which are selected for high specific impulse. It is shown that the rotary wave ejector concept has significant potential for thrust augmentation relative to a basic pulse detonation engine.

Numerical Modeling of Type I Circular Sedimentation Tank

A numerical model for Type I circular sedimentation tanks (center feed) has been developed to provide useful information for circular tank operation. The unsteady flow process in circular sedimentation tanks is divided by numerous time intervals in which flow and sediment transport are considered to be steady. The other feature for this model is that particle size distribution of raw water is nonuniform. A numerical experiment based on the proposed model is carried out, and the results were compared with results from other models. The comparison shows that this model can provide more information, such as variations of water elevation, overall removal efficiency, bottom sludge thickness, and particle size distribution at outlet. In addition, this model had the advantage of optimizing the tank dimensions based on the sludge raking frequency and preferred removal efficiency. After simplification of the model, this model is practical in determining the overall removal efficiency and dimension of the tank base...

Three-dimensional nature of the glottal jet

The factors contributing to human voice production are not yet fully understood. Even normal human phonation with a symmetric glottal opening area is still the subject of extensive investigation. Among others, it has already been shown that fluid dynamics has a strong influence on the vocal process. The full characterization of the glottal jet has not been accomplished yet. Time-resolved measurement and visualization of the three-dimensional (3D) flow downstream the human vocal folds are difficult if not impossible to perform in vivo. Therefore, it is common to use mechanical and numerical models with a simplified shape and motion profile of the vocal folds. In this article, further results regarding the 3D flow structure obtained in a 3:1 up-scaled dynamic glottis model (cam model) in a water circuit are given, extending earlier work [M. Triep et al. (2005). Exp. Fluids 39, 232-245]. The model mimics the temporal variation in the 3D contour of the glottal gap while water flow reduces the characteristic frequencies by the order of 1/140. The unsteady flow processes downstream of the vocal folds are visualized in slow motion and analyzed in detail via particle imaging techniques. The visualization results show complex 3D flow behavior of lengthwise jet contraction and axis switching. In addition, the time-dependent flow rate during the phonatory oscillation cycle is measured in detail. It is shown that the pressure loss is decreased in the presence of a second constriction downstream of the glottis in form of ventricular folds and it is observed that for this case the jet is stabilized in the divergent phase of the cycle.

Gate Regulation Speed and Transition Process of Unsteady Flow in Channel

The operation methods of channel and the speed of gate regulation have great influence on the transformation of flow in water conveyance channels. Based on characteristics method, a 1-D unsteady flow numerical model for gate regulation was established in this study. The process of water flow was simulated under different boundary conditions. The influence of gate regulation speed and channel operation methods on flow transition process was analyzed. The numerical results show that under the same conditions, with increasing regulation speed of the gate, the change rates of discharge and water level increase, while the response time of channel becomes shorter, and ultimately the discharge and water level will transit to the same equilibrium states. Moreover, the flow is easier to reach stable state, if the water level in front of the sluice is kept constant, instead of behind the sluice. This study will be important to the scheme design of automatic operation control in water conveyance channels.