Fundamental Study Of Transient High Frequency Nanosecond Surface Dielectric Barrier Discharge Multi-Channel Ignition Technology

The utilisation of lean burn and exhaust gas recirculation technologies will lower the ignition stability of internal combustion engines and increase their cyclic variation.Low temperature combustion can boost the engine efficiency while reducing the emissions,but need more effective methods to improve the controllability of combustion phase and combustion rate,so as to expand its working conditions and fuel adaptability.An important method for addressing these issues is the improvement of ignition performance.Since lean limit can be broadened by non-equilibrium plasma,and ignition stability and combustion speed can be promoted through volume ignition,discharges which can generate large volume non-equilibrium plasma are promising candidates for advanced combustion control technologies.Nanosecond Surface Dielectric Barrier Discharge(nSDBD)can generate multi-channel non-equilibrium plasma,but nSDBD ignition in previous research required a voltage pulse higher than 30 kV,and can’t control the discharge energy and plasma characteristics.This dissertation proposed an original method-transient high frequency nSDBD ignition,which may provide better modulation of discharge plasma characteristics and improve the feasibility of combustion control by multi-channel plasma.1)A high frequency nSDBD ignition system was designed.The nSDBD ignition system consists of a nanosecond pulse generator and a SDBD electrode.Since increasing the pulse repetition frequency(PRF)can lower the nSDBD initiating voltage,resonant charging is used to achieve faster charging and higher PRF;and the charging voltage and interval can be program-controlled,thus the pulse parameters can be modified in real time.Then,a program-controlled compact ns pulse generator based on magnetic compression and resonant charging was proposed.Its operating principles were demonstrated using transient circuit simulation,and the effects of the pulse generator’s electronic component parameters were investigated.The outputs of the pulse generator were tested after the construction of the hardware and the control software.Results show that the output pulses can have two kinds of rise time(20 ns/100 ns),and the voltage amplitude of the pulses and PRF can exceed 10 kV and 10 kHz,respectively.A coaxial SDBD electrode was constructed,and the influence of electrode parameters on the electric field distribution was analyzed through electric field simulation.Multi-channel discharges were observed when high voltage high frequency ns pulses were applied to the SDBD electrode.The discharge area can cover most of the 40 mm diameter dielectric surface.2)The discharge characteristics of transient repetitive nSDBD were studied.A detailed picture of the time-spatial characteristics of the nSDBD energy deposition can aid in the understanding of its ignition performance.Based on the measured voltage-current signals and discharge images,the electrical characteristics such as current,power,energy,and the morphological characteristics such as the number,length,and distribution of discharge channels are calculated using the equivalent circuit model,as well as the transformation of these characteristics during the transient nSDBD process.Single-pulse nSDBD generates a discharge at the rising edge and the falling edge of the pulse respectively.The energy of the rising edge discharge accounts for about 70%of the single discharge energy,which is more than twice the energy of the falling edge discharge.The discharge characteristics can be changed by adjusting pulse parameters:as pulse number increases,the discharge energy rises and filaments grow longer before stabilizing.At PRF=10kHz,the discharge energy increases from about 3mJ to 6mJ;The higher the PRF,the faster the discharge energy and the the filament length increase with the pulse number,and the larger the discharge energy value when reaching the steady state.The filaments of successive pulses are partially overlapped with divergent roots and a concentrated middle-part.This results in multiple tree-like discharge patterns—Filament Tree(FT)in repetitive nSDBD.The accumulated luminance in each FT trunk is stronger than that in the FT root,which is different from the energy distribution of a single pule nSDBD.3)The flame kernel initiation process of the nSDBD was investigated.The ignition probability of nSDBD,the characteristics of initial flame kernels and the effect of continuous discharge on the growth of flame kernels were analyzed by ignition test in a constant volume combustion vessel.Compared with pin-pin electrode(SND),the ignition probability of repetitive nSDBD increased much faster with discharge energy;the energy corresponding to 50%ignition probability(MIE)of nSDBD is slightly higher than ignition with SND,while the energy corresponding to 90%ignition probability is lower than SND.Each FT of nSDBD can generate independent ignition spot,and the overall ignition probability increases exponentially with the number of ignition spot.The initial flame kernels tend to locate at the trunk of each FT,this is consistent with the distribution of the highest cumulative luminance along FT.The adjacent FTs show a competitive relationship,which leads to the uneven distribution of discharge energy among FTs and the non-simultaneous generation of flame kernels;With the increase of pulse number,the number of kernels increases gradually.After the flame kernels are generated,subsequent discharges can greatly promote the kernels’expansion in the radial direction,reshaping the round kernels into long strips.During the continuous discharge,spatial relationship between the filament and the kernel can be divided into three stages:1)the kernel is penetrated by the filament from the near and far side,2)the filament only exists the far side,and 3)the filament is completely covered by the flame.Because the energy deposited by the filament lasts for a longer time in the far side region of the kernel,the expansion of this region dominants the growth of the kernel in the radial direction.With the increase of discharge energy,the increase of the total flame equivalent radius presents a two-stage characteristic.4)The final part of this work is about the combustion process of the multi-channel ignition.According to the flame propagation images and cylinder pressure profiles in the constant volume combustion vessel,the flame development process and combustion characteristics under different discharge parameters were studied;combined with numerical simulation,the influences of the number and distribution of initial flame kernels on the combustion characteristic parameters were analyzed.By regulating the number and size of kernels,the change of discharge parameters can significantly affect the development of kernels and combustion characteristic parameters;especially in the case of small clearance,the combustion phase and combustion rate can be controlled simultaneously by regulating the kernels’ parameters.The increase of kernel number not only increase the flame area,but also increase the kernel expansion speed.When the kernel number is near zero,the increase of kernel number can lead to a significant reduction of FDT and FRT;when the kernel number reach a certain value,the influence of kernel number on the combustion process is apparently reduced due to the fusion of kernels.When the number of kernels is fixed,the influence of the kernels’ radial position on the combustion process is more complex.When the radial position is at a certain intermediate value,the shortest FRT can be expected;If the radial position is too large or too small,the FRT will increase due to the wall constraint or the fusion of the kernels.In addition,the fusion time of kernels is greatly affected by their spatial distribution,so the FRT with concentrated kernels is significantly longer than that with dispersed kernels.In this dissertation,multi-channel ignition based on transient repetitive nSDBD discharge is proposed and realized,and the key process of nSDBD ignition is systematically analyzed and illustrated,which confirms the great potential of multi-channel ignition for combustion control.The results can provide guidance and support for the development of nSDBD ignition system and the further research and application of transient high-frequency nSDBD ignition for combustion control in engines.