For self-entrainment venturi nozzles, the effects of nozzle shapes and operating conditions on the water–air two-phase flow pattern, and the characteristics of the air entrainment rate have been ...investigated. A rectangular venturi nozzle with width and height dimensions of 3 mm and 0.5 mm was used with a vertically downward flow direction. The pressure ratio, which is the ratio of the inlet and outlet pressures, water flow rate, and diverging angle were set as experimental parameters. From the flow visualization, annular and bubbly flows were observed. In the case of bubbly flow, the more bubbles that are generated with a higher water flow rate, the smaller the pressure ratio. In the case of annular flow, the increased pressure ratio and water flow rate induce the breakup of air core in the diverging area and make the interfacial oscillation stronger, which finally causes the flow transition from annular to bubbly flow, by accompanying a sharp increase in the air entrainment rate. During this flow transition, the frictional pressure drop of the two-phase flow is reduced, showing that a two-phase multiplier gets smaller.
Purpose
The purpose of this paper is to propose a prediction method of gas-liquid two-phase flow patterns and reveal the flow characteristics in the suction chamber of a centrifugal pump.
...Design/methodology/approach
A transparent model pump was experimentally studied, and the gas-liquid two-phase flow in the pump was numerically simulated based on the Eulerian–Eulerian heterogeneous flow model. The numerical simulation method was verified from three aspects: the flow pattern in the suction chamber, the gas spiral length and the external characteristics of the pump. The two-phase flow in the suction chamber was studied in detail by using the numerical simulation method.
Findings
There are up to eight flow patterns in the suction chamber. However, at a certain rotational speed, only six flow patterns are observed at the most. At some rotational speeds, only four flow patterns appear. The gas spiral length has little relationship with the gas flow rate. It decreases with the increase of the liquid flow rate and increases with the increase of the rotational speed. The spiral flow greatly increases the turbulence intensity in the suction chamber.
Originality/value
A method for predicting the flow pattern was proposed. Eight flow patterns in the suction chamber were identified. The mechanism of gas-liquid two-phase flow in the suction chamber was revealed. The research results have reference values for the stable operation of two-phase flow pumps and the optimization of suction chambers.
In this work, a transition process in a hypersonic flow over a cold-wall compression ramp is studied using direct numerical simulation (DNS) and global stability analysis (GSA). The free-stream Mach ...number and the Reynolds number based on the flat-plate length are 7.7 and $8.6 \times 10^5$, respectively. The shock-induced pressure rise causes the boundary layer to separate on the flat plate, forming a separation bubble around the corner. Without introducing any external disturbances, the DNS captures the transition to turbulence downstream of flow reattachment. The DNS results agree well with the experimental data as well as theoretical predictions. To uncover the intrinsic instability in the flow system, GSA is employed to investigate the three-dimensionality of the two-dimensional base flow. Several stationary and oscillatory unstable modes are revealed, which result in spanwise periodicity inside and downstream of the separation bubble. The GSA and DNS results indicate that the intrinsic instability of the flow system triggers the formation of streamwise counter-rotating vortices and boundary-layer streaks near reattachment. The downstream transition to turbulence starts from the breakdown of the streamwise vortices and streaks. Moreover, the second harmonic of the most unstable global mode and a broadband low-frequency unsteadiness occur in the saturated flow, which has a significant influence on the transition process. In summary, the present study demonstrates a transition process in a hypersonic compression-ramp flow as a result of the intrinsic instability of the flow system.
Oil-water two-phase flow exists widely in the industrial process. At present, it is very difficult to describe the oil-water two-phase flow system theoretically. The mechanism of droplet ...fragmentation and coalescence, which restricts droplet size and flow stability, is still unclear. Accurate identification of oil-water two-phase flow patterns is of great significance to flow control and measurement. In this study, a refined composite multi-scale rescaled range fractional permutation entropy (MRSFPE) is proposed for identifying the flow pattern of oil-water two-phase flow in vertical pipes. Firstly, the proposed algorithm is simulated numerically to demonstrate its ability to uncover the complexity of nonlinear time series. Then, the conductance sensor signals are collected in the flow loop test facility of vertical upward oil-water two-phase flow. In the experiment, the observed flow patterns, captured by a high-speed camera, include very fine dispersed oil-in-water flow (VFD O/W), dispersed oil-in-water flow (D O/W), dispersed oil-in-water slug flow (D OS/W), and transition flow (TF). Finally, the proposed algorithm is applied to the experimental signal analysis, extracting two scale indices of the entropy rate at high scale (MRSFPE_rate) and the mean value entropy at low scale (MRSFPE_ave). The results show that the four flow patterns can be identified in the joint distribution of the two scale indices. Furthermore, the MRSFPE can be a useful approach to characterize the nonlinear dynamics of multiphase flow based on sensor measurement signals.
This article presents a massively parallel and robust strategy to perform the simulation of turbulent incompressible two-phase flows on unstructured grids in complex geometries. This strategy relies ...on a combination of a narrow-band accurate conservative level set (ACLS)/ghost-fluid framework with isotropic adaptive mesh refinement. This combination enables to accurately capture interface dynamics and topology, and the small physical scales at the liquid-gas interface are resolved at an affordable cost. The ACLS method, even if not strictly mass-conserving, ensures the exact conservation of a smoothed phase indicator, which minimizes the liquid mass conservation errors. In the accurate conservative level set framework, presented first in Desjardins et al. (2008) 25, the interface is defined as the iso-contour of a hyperbolic tangent function, which is advected by the fluid, and then reshaped using a reinitialization equation. Several forms of this reinitialization exist: the original ACLS form proposed by Desjardins et al. involves numerical estimation of the hyperbolic tangent gradient, which is difficult to compute accurately on unstructured meshes. It is thus susceptible to induce artificial deformation of the interface. A new form has been recently proposed in Chiodi and Desjardins (2017) 29, which takes advantage of a mapping onto a classical distance level set while much better preserving the interface shape. Nevertheless, the implementation of this new form on unstructured grids requires special care. In this work, a robust implementation of this new form on unstructured meshes is proposed and implemented in the YALES2 low-Mach flow solver. In order to compute interface normals and curvature, the signed-distance function is reconstructed in parallel at nodes in the narrow band around the interface using a Geometric-Projection Marker Method (GPMM). This method relies on the triangulation of the level set iso-contour and exact geometric projection to the closest surface elements. Spatial convergence, robustness and efficiency of the overall procedure are firstly demonstrated through classical interface transport test cases and two-phase flow examples. Eventually, to emphasize the significant computational gain using adaptive mesh refinement and the ability to compute complex turbulent flows with large density ratios, two Large-Eddy Simulations (LES) of atomizing liquid jets in air are presented, each one at various resolutions. The first one is a low-pressure water jet in quiescent air from a compound nozzle with full computation of the internal injector flow, while the latter is a high-pressure kerosene jet in crossflow. Both simulations are validated against experiments, demonstrating the potential of the method to access a deep numerical insight into jet instabilities and internal flow dynamics with 3D unstructured meshes.
•Convergence and robustness of the unstructured ACLS interface-capturing technique are demonstrated.•The present ACLS method is implemented in an unstructured incompressible finite-volume solver.•Capillary-driven flows are correctly computed on triangular and tetrahedral meshes.•Massively parallel LES simulations of atomization are performed on adaptive unstructured grids.•Adaptive mesh refinement proved to provide significant computational savings.
Despite the significant progress over the last 50 years in simulating flow problems using numerical discretization of the Navier–Stokes equations (NSE), we still cannot incorporate seamlessly noisy ...data into existing algorithms, mesh-generation is complex, and we cannot tackle high-dimensional problems governed by parametrized NSE. Moreover, solving
inverse flow problems
is often prohibitively expensive and requires complex and expensive formulations and new computer codes. Here, we review
flow physics-informed learning
, integrating seamlessly data and mathematical models, and implement them using physics-informed neural networks (PINNs). We demonstrate the effectiveness of PINNs for inverse problems related to three-dimensional wake flows, supersonic flows, and biomedical flows.
Graphical abstract
•Flow patterns for zeotropic hydrocarbon mixtures were observed.•A new flow pattern transition line between annular and non-annular flow pattern was proposed.•An improved heat transfer correlation ...was proposed.
An experimental and simulation investigation of condensation flow pattern and heat transfer characteristics of zeotropic hydrocarbon mixtures methane/propane and ethane/propane in a helical tube was presented. Flow visualization was conducted to capture flow pattern during flow condensation. The flow mechanisms were categorized into six different flow patterns: slug flow, stratified flow, transition flow, wavy flow, half-annular flow and annular flow. In addition, several existing two-phase flow pattern maps in horizontal tube were compared with the flow pattern data. Then a new flow pattern transition line between annular and non-annular flow pattern was proposed. Based on the flow pattern transition line, the heat transfer data under different flow pattern were compared with the existing heat transfer corrections. Finally, an improved heat transfer correlation was proposed and well coincided with the data with a mean absolute relative deviation (MARD) of 13.8%.
Surface flow and subsurface flow constitute a naturally linked hydrologic
continuum that has not traditionally been simulated in an integrated
fashion. Recognizing the interactions between these ...systems has encouraged
the development of integrated hydrologic models (IHMs) capable of treating
surface and subsurface systems as a single integrated resource. IHMs are
dynamically evolving with improvements in technology, and the extent of their
current capabilities are often only known to the developers and not general
users. This article provides an overview of the core functionality,
capability, applications, and ongoing development of one open-source IHM,
ParFlow. ParFlow is a parallel, integrated, hydrologic model that simulates
surface and subsurface flows. ParFlow solves the Richards equation for
three-dimensional variably saturated groundwater flow and the
two-dimensional kinematic wave approximation of the shallow water equations
for overland flow. The model employs a conservative centered finite-difference scheme and a conservative finite-volume method for subsurface
flow and transport, respectively. ParFlow uses multigrid-preconditioned
Krylov and Newton–Krylov methods to solve the linear and nonlinear systems
within each time step of the flow simulations. The code has demonstrated
very efficient parallel solution capabilities. ParFlow has been coupled to
geochemical reaction, land surface (e.g., the Common Land Model), and atmospheric
models to study the interactions among the subsurface, land surface, and
atmosphere systems across different spatial scales. This overview focuses on
the current capabilities of the code, the core simulation engine, and the
primary couplings of the subsurface model to other codes, taking a
high-level perspective.
In a mixed-flow pump, the energy performance curve always appears as an unsteady region under rotating stall condition. In this paper, computational fluid dynamics (CFD) technology is employed to ...study the effect of variation in blade thickness of the impeller for the purpose of improving the stall characteristics of the mixed-flow pump. The local loss caused by the turbulent flow is investigated using the entropy generation method that considers the wall effects. The results show that the stall region of mixed-flow pump is closely related to the blade thickness, and the total entropy generation (TEG) in impeller and guide vane are affected. At design flow rate, the tip leakage vortex (TLV) accounts for most of the hydraulic loss in the impeller and the change in the blade thickness does not influence the pump head or efficiency significantly. Whereas, within stall region, the stall vortex and TLV determine the high TEG region in the impeller, the backflow and secondary flow are the source of major hydraulic loss in the guide vane. A small increase in the blade thickness reduces the loss caused by the stall vortex which delays the stall point. However, once the blade thickness increases significantly, the stall vortex becomes bigger which deteriorates the flow fields in the impeller and guide vane resulting in rotating stall. Thus, the blade thickness can be considered as a key parameter to minimize the occurrence of rotating stall in a mixed-flow pump for improving its efficiency and safety.
•Effect of blade thickness on energy performance of mixed-flow pump is investigated.•Proper increase in blade thickness will delay the stall.•Large increase in blade thickness will deteriorate the rotating stall and make it happened in advance.•Tip leakage flow, back flow, secondary flow and stall determine the high loss region in impeller.
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