| 1. |
- Liu, Lipeng, 1990-, et al.
(author)
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A Parallel Projection Method for the solution of Incompressible Navier-Stokes Equations based on Position-State Separation Method
- 2015
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Conference paper (other academic/artistic)abstract
- A simple parallel numerical algorithm with P1/P1 velocity-pressure elements for solving the incompressible Navier-Stokes equations is proposed. In order to circumvent the Babuška-Brezzi condition (i.e. inf-sup condition), a velocity-pressure separation strategy following the idea of projection method is presented. Thus, the velocity and pressure terms are separated using operator-splitting, dividing the problem into two individual governing equations. The velocity equation is solved using an extension form of the position-state separation method (POSS), which has very good performance to solve convection-diffusion-reaction equations. The parallel computation of the method is easy to implement. Classical numerical experiments are conducted which show the efficiency, robustness and accuracy of the method.
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| 2. |
- Liu, Lipeng, 1990-, et al.
(author)
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An efficient model to simulate stable glow corona discharges and their transition into streamers
- 2017
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In: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 50:10, s. 105204-
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Journal article (peer-reviewed)abstract
- A computationally efficient model to evaluate stable glow corona discharges and their transition into streamers is proposed. The simplified physical model referred to as the SPM is based on the classic hydrodynamic model of charge particles and a quasi-steady state approximation for electrons. The solution follows a two-step segregated procedure, which solves sequentially the stationary continuity equation for electrons and then time-dependent continuity equations for ions. The validity of using the SPM to simulate glow corona discharges and their transition into streamers is demonstrated by performing comparisons with a fully coupled physical model (FPM) and with experimental data available in the literature for air under atmospheric conditions. It is shown that the SPM can obtain estimates similar to those calculated with the FPM and those measured in experiments but using significantly less computation time. Since the proposed model simulates efficiently the ionization layer without prior knowledge of the surface electric field or the discharge current, it is a computationally efficient alternative to calculations of glow corona discharges based on Kaptzov's approximation (KAM). The model can also be employed to efficiently calculate the conditions for the transition of glow corona into streamers, overcoming the limitations of KAM to provide such estimates.
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| 3. |
- Liu, Lipeng, 1990-, et al.
(author)
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An Efficient Semi-Lagrangian Algorithm for Simulation of Corona Discharges: The Position-State Separation Method
- 2016
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In: IEEE Transactions on Plasma Science. - : IEEE Press. - 0093-3813 .- 1939-9375. ; 44:11, s. 1-10
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Journal article (peer-reviewed)abstract
- An efficient algorithm without flux correction for simulation of corona discharges is proposed. The algorithm referred to as the position-state separation method (POSS) is used to solve convection-dominated continuity equations commonly present in corona discharges modelling. The proposed solution method combines an Eulerian scheme for the solution of the convective acceleration, the diffusion and the reaction subproblems, and a Lagrangian scheme for the solution of the linear convection subproblem. Several classical numerical experiments in different dimensions and coordinate systems are conducted to demonstrate the excellent performance of POSS regarding low computational cost, robustness, and high-resolution. It is shown that the time complexity of the method when dealing with the convection of charged particles increases linearly with the number of unknowns. For the simulation of corona discharges where local electric fields do not change strongly in time, the time step of POSS could be much larger than the Courant–Friedrichs–Lewy (CFL) time step. These special features enable POSS to have great potential in modeling of corona discharges in long interelectrode gaps and for long simulation times.
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| 4. |
- Liu, Lipeng, 1990-, et al.
(author)
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Application of the Position-State Separation Method to Simulate Streamer Discharges in Arbitrary Geometries
- 2017
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In: IEEE Transactions on Plasma Science. - : IEEE Press. - 0093-3813 .- 1939-9375.
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Journal article (peer-reviewed)abstract
- In this paper, we extended the recent work of Liu and Becerra to employ the position-state separation (POSS) method to simulate filamentary streamer discharges. POSS is a semi-Lagrangian method, which solves convection-dominated continuity equations without numerical flux correction. An improved interpolation strategy for POSS is here introduced to overcome the excessive numerical diffusion of the method when very small time step is used. Several benchmark tests in the literature are used to validate the improved method. Numerical experiments show that POSS is an accurate, efficient, and robust numerical method to simulate streamer discharges in arbitrary geometries when combined with finite-element method.
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| 5. |
- Liu, Lipeng, 1990-, et al.
(author)
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Gas heating dynamics during leader inception in long air gaps at atmospheric pressure
- 2017
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In: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 50:34
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Journal article (peer-reviewed)abstract
- The inception of leader discharges in long air gaps at atmospheric pressure is simulated with a thermo-hydrodynamic model and a detailed kinetic scheme for N2/O2/H2O mixtures. In order to investigate the effect of humidity, the kinetic scheme includes the most important reactions with the H2O molecule and its derivatives, resulting in a scheme with 45 species and 192 chemical reactions. The heating of a thin plasma channel in front of an anode electrode during the streamer to leader transition is evaluated with a detailed 1D radial model. The analysis includes the simulation of the corresponding streamer bursts, dark periods and aborted leaders that may occur prior to the inception of a propagating leader discharge. The simulations are performed using the time-varying discharge current in two laboratory discharge events of positive polarity reported in the literature as input. Excellent agreement between the simulated and the experimental time variation of the thermal radius for a 1m rod-plate air gap discharge event reported in the literature has been found. The role of different energy transfer and loss mechanisms prior to the inception of a stable leader is also discussed. It is found that although a small percentage of water molecules can accelerate the vibrational-translational relaxation to some extent, this effect leads to a negligible temperature increase during the streamer-to-leader transition. It is also found that the gas temperature should significantly exceed 2000 K for the transition to lead to the inception of a propagating leader. Otherwise, the strong convection loss produced by the gas expansion during the transition causes a drop in the translational temperature below 2000 K, aborting the incepted leader. Furthermore, it is shown that the assumptions used by the widely-used model of Gallimberti do not hold when evaluating the streamer-to-leader transition.
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| 6. |
- Liu, Lipeng, 1990-, et al.
(author)
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On the critical charge required for positive leader inception in long air gaps
- 2018
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In: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 51:3
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Journal article (peer-reviewed)abstract
- The amount of the electric charge injected by the streamer corona bursts during the stage of leader inception determines the energy deposited to thermalize the corona stem into a leader segment. This paper is aimed at investigating the critical charge required for positive leader inception in air by using a thermo-hydrodynamic model with a detailed kinetic scheme. In order to simplify the analysis and to speed up the simulation, a reduced kinetic scheme for air is proposed. Numerical comparisons show that the reduced scheme can obtain almost the same results as the previous comprehensive kinetic scheme but with only half of the number of species and reactions. The thermo-hydrodynamic model with the reduced kinetics is then used to solve the radial dynamics of a single stem heated by current pulses typical of streamer corona bursts. The critical charge necessary for the direct transition of a first streamer corona into a leader under electrodes with large curvature radius is estimated between 0.08 and 0.5 uC per stem. Furthermore, the simulation shows that the gas heating of corona stem formed from electrodes with small curvature radius is mainly determined by the total accumulated charge injected by previous streamer corona bursts and the length of the dark periods in between the current pulses. The shape and the number of the corona current pulses in the discharge also play a role and their effects are discussed. It is suggested that the transition into a leader is triggered when a secondary streamer burst is initiated after the gas temperature is increased by the heating of previous streamers to about 1200 K. In addition, it is found that the heating produced by the charge injected by previous streamer corona bursts can be neglected if the dark period to the next burst is larger than few hundreds of for a corona stem with moderate initial stem radius. This indicates that the critical charge criterion obtained from laboratory experiments does not hold to evaluate the inception of positive leaders under conditions when long dark periods are present.
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| 7. |
- Liu, Lipeng, 1990-, et al.
(author)
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On the dynamics of streamer-to-leader transition in long air gap discharges
- 2016
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Conference paper (peer-reviewed)abstract
- In this paper, a self-consistent numerical model is employed to investigate the ionization kinetics of the streamer-to-leader transition in long air gap discharges. The model focuses on the dynamics of the discharge channel in the radial direction using a 1D radial system by taking into account 21 species and 106 chemical reactions. The detailed dynamics of the streamer-to-leader transition after a sequence of streamer bursts, dark periods and aborted leaders are discussed. The calculations are performed for a 1 m long rod-plate configuration using the current measured in experiments reported in the literature. The results show that the average central temperature of a newly formed and a self-propagating stable leader segment is around 2000K and 3000K, respectively. The instant temperature in the gap as a leader segment is incepted can reach 5000K. The predictions of temperature and thermal radius of the leader channel agree well with previous experimental studies.
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| 8. |
- Liu, Lipeng, 1990-, et al.
(author)
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On the transition from positive glow corona to streamers in air
- 2014
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Conference paper (peer-reviewed)abstract
- The transition phase from glow to streamer corona is of significant importance for the evaluation of leader discharges initiated from ultra-high voltage power transmission lines under thunderstorms. In order to study the condition required for streamer inception from a glow-corona generating wire, the continuity equations for electrons, positive ions and negative ions coupled with Poisson’s equation are solved. The calculations are performed for a wire-cylinder coaxial configuration in one dimension. The analysis is performed by considering the generation of glow corona under a DC electric field, followed by the transition to a streamer-like ionization wave under a fastchanging electric field ramp. Thus, the critical rate of rise of the applied voltage on the wire surface required for streamer-like structures to initiate in the presence of a stable glow corona is evaluated for different radii and DC voltages.
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| 9. |
- Liu, Lipeng, 1990-, et al.
(author)
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On the transition from stable positive glow corona to streamers
- 2016
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In: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 49:22, s. 225202-
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Journal article (peer-reviewed)abstract
- A 2D numerical simulation of the transition from stable positive glow corona to streamers in coaxial cylindrical configuration is presented. The hydrodynamic model with several convection-dominated continuity equations together with Poisson equation are solved with consideration of the ionization layer. The transition from a stable positive glow corona produced under a DC voltage to streamers is investigated under a sudden change of the applied voltage. The critical rate of rise of voltage required for the transition from positive glow to streamer corona is evaluated with a voltage ramp. By introducing either physical or numerical instabilities into the model, streamers with filamentary structures are observed, which produce a sudden increase of the discharge current by more than two orders of magnitude. It is also found that the surface electric field of the corona-generating conductor deviates from the onset electric field, casting doubts about the validity of Kaptzov's approximation to evaluate the transition from stable glow to streamers.
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| 10. |
- Liu, Lipeng, 1990-
(author)
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Physics of Electrical Discharge Transitions in Air
- 2017
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Doctoral thesis (other academic/artistic)abstract
- Electrical discharges with a variety of different forms (streamers, glow corona, leaders, etc.) broadly exist in nature and in industrial applications. Under certain conditions, one electrical discharge can be transformed into another form. This thesis is aimed to develop and use numerical simulation models in order to provide a better physical understanding of two of such transitions, namely the glow-to-streamer and the streamer-to-leader transitions in air.In the first part, the thesis includes the two-dimensional simulation of the glow-to-streamer transition under a fast changing background electric field. The simulation is performed with a fluid model taking into account electrons. An efficient semi-Lagrangian algorithm is proposed to solve the convection-dominated continuity equations present in the model. The condition required for the glow-to-streamer transition is evaluated and discussed. In order to enable such simulations for configurations with large interelectrode gaps and long simulation times, an efficient simplified model for glow corona discharges and their transition into streamers is also proposed.The second part of the thesis is dedicated to investigate the dynamics of the streamer-to-leader transition in long air gaps at atmospheric pressure. The transition is studied with a one-dimensional thermo-hydrodynamic model and a detailed kinetic scheme for N2/O2/H2O mixtures. In order to evaluate the effect of humidity, the kinetic scheme includes the most important reactions with the H2O molecule and its derivatives. The analysis includes the simulation of the corresponding streamer bursts, dark periods and aborted leaders that may occur prior to the inception of a stable leader. The comparison between the proposed model and the widely-used model of Gallimberti is also presented.
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