Laser-induced Plasma Ignition Research Articles

Temperature Distribution Within an Ignition Kernel Initiated by a Laser-Induced Plasma

The sizes of, and temperature distributions within, ignition kernels initiated by a Q-switched neodymium-doped yttrium aluminum garnet laser-induced plasma in an unconfined lean premixed hydrogen-air upward jet flow are investigated. The experiments involved a range of jet velocities and a range of deposited laser energies at a fixed height above the exit along the axis of a burner. The growth of, and the temperature distributions within, the ignition kernels, as affected by the size and the energy distribution of the laser-induced plasma, are monitored with an infrared camera. The initial ignition kernels’ areas are larger with higher laser pulse energies and remain unchanged up to 20 μs and then increase by factors of up to 3 at 100 μs. The change in the kernel area caused by the jet velocities is less than 1.5%. An increase of the bulk velocity by 190% decreases the ignition kernel temperature by 6%. This reduction in the ignition kernel temperatures is because of an increase in energy losses by a factor of 2 and decreases in heat releases by 2% at 10 μs and by 11% at 100 μs. The present contributions are: measurements of and insights into temperature distributions and kernel development rates during the laser-induced plasma ignition process at different deposited energies and flow velocities.

Research on laser induced plasma ignition of gas oxygen/methane

Laser-induced plasma ignition is a new ignition method to achieve stable and reliable ignition of liquid rocket engines. It has the advantages of accurate and controllable ignition position, broadening ignition lean-burn limit and small disturbance to flow field. In this paper, the laser ignition process and combustion characteristics of gas oxygen/gas methane under rocket engine conditions are studied from four aspects, namely different ignition positions, ignition energy, fuel-oxygen momentum ratio and chamber pressure. Using a high-speed camera, the dynamic process of ignition was captured by the shadow method, and the generation and propagation of laser-induced flame nuclei were recorded. The results show that at the airflow velocity of 300–400 m/s, the time required for the flame kernel to develop to the entire combustion chamber is about 300 μs, and the time for the flame to reach a steady state is tens of milliseconds. When the ignition energy is increased by 1.5 times, the time required for the flame kernel to develop from the recirculation zone to the mainstream zone is shortened to 1/3 of the original; with the increase of the fuel-oxygen momentum ratio, the shape of the initial flame kernel changes from round to oval, and the development speed of the flame kernel also increases. The fuel-oxygen momentum ratio increases from 0.59 to 2.75, and the time required for the flame kernel to develop from the recirculation zone to the mainstream zone decreases from 167 μs to 56 μs＀ Except chamber pressure, other factors have no effect on the pressure rise rate of the ignition process, and the higher the chamber pressure, the faster the pressure rise rate; in order to avoid deflagration, the shear layer is more suitable as the ignition position. With the increase of ignition energy, fuel-oxygen momentum ratio and chamber pressure, the development speed of flame kernel increases, but the shape and development process of flame kernel will be different under the influence of different factors.

Laser ignition of iso-octane and n-heptane jets under compression-ignition conditions

This work aims to investigate the effect of laser-induced plasma ignition (LI) on combustion behaviours of iso-octane (a gasoline surrogate) at compression-ignition (CI) conditions. A high-energy laser was used to force the fuel ignition at a quiescent-steady environment inside an optically accessible constant-volume combustion chamber with 900K ambient gas temperature, 22.8kg/m3 ambient gas density and 21 vol.% O2 concentration. A diesel surrogate (n-heptane) was tested at a lower charge temperature of 735K to offset its higher fuel reactivity than the iso-octane, such that the flames of both fuels can have a similar lift-off length. Forced laser ignition was introduced either before or after the natural autoignition timing of the fuels. The laser was focused at the jet axis 15mm and 30mm from the nozzle. High-speed schlieren imaging, heat release analysis and flame luminosity measurement were applied to the flames. The high-speed schlieren imaging was used to monitor the flame structure evolution of the natural ignition and LI cases. Due to laser ignition, the flame lift-off lengths decrease, with which the uncertainties in the lift-off distances reduce by more than 80%. The laser-affected flame bases return back to the natural flame base locations. The uncertainties in the lift-off lengths also increase, as the flame stabilisation locations approach the natural lift-off distances. Under the test conditions of this work, the rates at which the iso-octane flames shift downstream are slower than the n-heptane cases. The heat release rate profiles show high heat release from the flames following the LI events, before transitioning to lower steady values. The flame luminosity measurements indicate a strong correlation between the LI affected lift-off length and increased soot formation. The luminosity levels decrease as the flame base shifts downstream over time.

Shock Wave Propagation and Flame Kernel Morphology in Laser-Induced Plasma Ignition of CH4/O2/N2 Mixture

The application of laser ignition in the aerospace field has promising prospects. Based on the constant volume combustion chamber, the laser ignition of CH4/O2/N2 mixture with different initial pressure, different laser energy, different equivalence ratio and different oxygen content has been carried out. The development characteristics of the flame kernel and shock wave under different conditions are analyzed. In addition, the Taylor model and Jones model are also used to simulate the development process of the shock wave, and a new modified model is proposed based on the Jones model. The experimental results show that under pure oxygen conditions, the chemical reaction rate of the mixture is too fast, which makes it difficult for the flame kernel to form the ring and third-lobe structure. However, the ring structure is easier to form with the pressure and laser energy degraded; the flame kernel morphology is easier to maintain at a rich equivalence ratio, which is caused by the influence of the movement of hot air flow and a clearer boundary between the ring and the third-lobe. The decrease of the initial pressure or the increase of the laser energy leads to the increase in shock wave velocity, while the change of the equivalence ratio and oxygen content has less influence on the shock wave.

Future gasoline engine ignition: A review on advanced concepts

To meet the future requirements of fuel economy and exhaust emissions, high-efficiency gasoline engines tend to employ diluted combustion concepts along with intensified charge motion and stratified mixtures. Securing the ignition of such mixtures over the full engine operation range is challenging, because of the lowered mixture reactivity and increased discrepancy of stoichiometry. In recent years, increasing research efforts have been spending on innovations of ignition technologies to tackle the challenges. In this paper, the directions of ignition improvement are highlighted based on the fundamental understanding of the ignition mechanisms. The working principles of the primary types of advanced ignition systems are introduced; and relevant engine and combustion vessel test results are reviewed. The ignition systems are categorized as: (1) high-energy spark ignition, (2) pulsed nanosecond discharge ignition, (3) radio-frequency plasma ignition, (4) laser-induced plasma ignition, and (5) pre-chamber ignition. The advanced ignition systems are commented, regarding the ignition effectiveness and the implementation challenges, according to the literatures and the extensive empirical work at the authors’ laboratory.

Effects of microwave radiation on laser induced plasma ignition of n-butane/air mixture under atmospheric conditions

This study presents the effect of the microwave radiation control parameters (i.e. pulse width and pulse repetition frequency (PRF)) on laser-induced plasma (LIP) ignition in a constant volume combustion chamber (CVCC) of the n-butane-air mixture (ϕ = 0.7) under atmospheric conditions. The ignition behaviors were investigated using ignition success rate, high-speed imaging, net heat release, and plasma temperatures. The result shows that the ignition success rate (ISR) defined in this study gets increased with an increase in the pulse width. Moreover, ISR was found insensitive to PRF at higher pulse width. This implies that the combustion advancement occurred at a faster rate on the coupling of high PRF and longer pulse width microwave radiation with the LIP. The ignition delay evaluated based on the net heat release gets reduced; consequently, the ignition advances much faster with an increase in the pulse width as compared to the small pulse width. The direct images of early combustion revealed an expansion of the plasma when the coupling between microwave radiation and the LIP occurs. This expansion of the plasma is the generation of active radicals due to the direct collisions of electrons with ionized molecules via interaction between the electromagnetic field and free electrons. Besides, the combustion advancement mechanism was investigated using plasma emission spectroscopy. The plasma temperatures which are closely related to the chemical reaction rates were calculated by fitting the homo-nuclear N2(2+) system. The results show that the ignition success rate enhances with an increase in the rotational temperature. This might be due to much faster rotational-transnational energy transfer as compare to vibrational-transnational energy transfer. This implies that the bulk gas temperature increases quickly since the rotational temperature is approximately equal to the gas temperature.

Combustion improvement and pollutants reduction with diesel-gasoline blends by means of a highly tunable laser plasma induced ignition system

The use of alternative fuels in compression ignition engines, either completely or partially replacing the conventional ones, have potential to reduce pollutant emissions (especially soot). However, some of these fuels do not provide good ignition features under diesel engine like conditions, which affects engine efficiency. Thus, in order to extend the application of alternative fuels, the current research proposes the use of a laser induced plasma ignition system to assist on the combustion of blends of fuels with less reactivity than pure diesel. This fuel has been chosen as the base component and it has been mixed with gasoline (as the low-reactivity fuel) in different ratios as an example of fuels with very different reactivity properties. Tests have been performed in a single cylinder optically accessible engine, allowing deeper study of combustion development and soot formation. For different in-cylinder conditions and fuel blends, the effect of laser induced plasma ignition system has been evaluated at different crank angle degrees and locations inside the combustion chamber. The application of these blends under low-reactivity engine conditions show that combustion efficiency is dramatically affected. However, the study proves that it is possible to control blend ignition delay and flame lift-off length by means of laser induced plasma. Besides, using the proper ignition system configuration, combustion characteristics similar to those of diesel fuel autoignition can be achieved for high gasoline substitution rates. They lead to similar energy release rates, which confirms that diesel-gasoline blends can reach a combustion efficiency close to pure diesel, while a strong reduction on soot formation was also obtained. These results open a door to efficiency improvement and pollutant reduction by means of a highly tunable ignition of alternative fuel blends.

Laser-Induced Plasma Ignition Experiments in a Direct-Connect Supersonic Combustor at Mach 3

A new direct-connect supersonic combustor has been designed and tested for the study of plasma-assisted combustion in the ACT-II supersonic combustion tunnel. Laser-induced plasma ignition was investigated in an ethylene-fueled cavity flame holder at Mach 3. Combustor design, geometry, and flow characteristics are discussed in the first part of the paper. The supersonic combustor includes a cavity flame holder, where ethylene fuel is injected, mixed with entrained air, and ignited. Two injector configurations are included, one horizontal and one vertical, which were designed in such a way as to enhance the cavity flow recirculation. Two different plasma ignition mechanisms were considered: 1) a laser-induced plasma located at the center of the cavity, and 2) a spark plug placed on the bottom surface of the cavity. The diagnostics tools tested to characterize the flow include time-resolved pressure measurements, high-speed schlieren visualization and chemiluminescence visualization, and instantaneous planar laser-induced fluorescence of the CH radical. The second part of the paper discusses the results of the comparison between horizontal and vertical fuel injection. The larger backpressurization of the isolator and a more uniform distribution within the cavity suggest a greater heat release and combustion efficiency when the horizontal jet is used.

Laser-Induced Plasma Ignition in a Cavity-Based Scramjet Combustor

Experimental investigations of laser-induced plasma (LIP) ignition processes were conducted in an ethylene-fueled scramjet combustor with a rear-wall-expansion cavity. The results were obtained under the inflow conditions of supersonic inflow with a stagnation pressure of 2.6 MPa and a stagnation temperature of 1530 K. chemiluminescence recorded at a flame rate of 50 kHz was used to characterize the ignition and flame stabilization processes. Large-eddy simulations were carried out to characterize the mixing process and the flow structure before LIP. It was found that, with the LIP ignition in the foreside of the cavity, the initial plasma flame kernel first anchored at the cavity leading edge and then moved away along the cavity leading edge toward the high-speed shear layer, where the flame could be stabilized or quenched, depending on the global equivalence ratio. On the other hand, after a LIP ignition in the rearward of the cavity, the initial flame in the rearward of the cavity was propagating along the cavity floor to the foreside of the cavity, and eventually to the shear layer where a stable flame could be established. The numerical results showed that a more successful LIP ignition in a cavity-based scramjet combustor could be achieved inside the subsonic zones and in the rear side of the cavity. The ignition process could be characterized as four stages: an initial plasma ignition stage; followed by the plasma-quenching stage, the reignition stage, and, finally, the stable flame stage.

Experimental study of the stabilization mechanism of a lifted Diesel-type flame using combined optical diagnostics and laser-induced plasma ignition

The understanding of the stabilization process of Diesel spray flames is a key challenge because of its effect on pollutant emissions. In particular, the close relationship between lift-off length and soot production is now well established. However, different stabilization mechanisms have been proposed and are still under debate. The objective of this paper is to provide an experimental contribution to the investigation of these governing mechanisms. Combustion of a Diesel spray issued from a single-hole nozzle (90 µm orifice, ECN spray A injector) was studied in a constant-volume precombustion vessel using a combination of optical diagnostic techniques. Simultaneous high frame rate (6kfps) schlieren, 355 LIF (excitation at 355 nm and maximum collection at 430 nm) and high-temperature chemiluminescence (collection from 400 nm to 490 nm) or OH* chemiluminescence (collection at 310 nm and frame rate at 60kfps) are respectively used to follow the evolution of the gaseous jet envelope, formaldehyde location and lift-off position. Additional experiments are performed where the ignition of the mixture is forced at a location upstream of the natural lift off position by laser-induced plasma ignition (at 1064 nm). The evolution of the lift-off position until its return to the natural steady-state position is then studied for different ambient temperatures (800 K to 850 K), densities (11 kg/m3 to 14.8 kg/m3) and rail pressures (100 MPa to 150 MPa) using the same set of optical diagnostics. The analysis of the evolution of the lift off position without laser ignition reveals two main types of behaviors: sudden jumps in the upstream direction and more progressive displacement towards the downstream direction. While the former is attributed to auto-ignition events, the latter is studied through the forced laser ignition results. It is found that the location of formaldehyde greatly impacts the return velocity of the lift-off position: if laser ignition occurs upstream of the zone where formaldehyde is naturally present, the lift-off position convects rapidly until it reaches the region where formaldehyde is present and then returns more slowly towards its natural position, suggesting that cool-flame products greatly assist lift-off stabilization. The average return velocity in this second stage depends on the operating conditions.

Simulation of Laser-Induced Plasma Ignition in a Hypersonic Crossflow

A numerical study of a laser-induced plasma ignition system in a scramjet-like geometry is conducted. Simulations are based on experiments in which hydrogen is injected into a hypersonic air-crossflow, compressed by a ramp from a freestream Mach number of 8 to below hydrogen autoignition conditions. Ignition of the jet is attempted using a laser pulse, modeled in the simulations as a small region of high temperature and pressure corresponding to the energy input measured in the experiments. To capture the highly unsteady nature of the fuel-air interface and its coupling with the dynamics of the expanding laser spark, wall-modeled large-eddy Simulations are used to resolve the majority of turbulent motion. Thermal nonequilibrium and detailed finite-rate chemistry models are included in the calculations. The simulations indicate that the cloud of hydroxyl radicals detected in the experiments is created on the jet’s forward surface, as the expanding plasma kernel drives a blast wave into the bowshock surrounding the jet. This event raises the temperature and pressure in the upstream mixing layer, causing spontaneous ignition of the mixture. Within tens of microseconds the laser-induced combustion detaches from the jet and combustion ceases due to unfavorable conditions for flame anchoring in this area. Downstream of the jet, the simulations suggest that the plasma kernel remains can initiate radical formation in the wake region, but no stable flame forms due to nominally low pressures in the experimental condition studied.

Effect of cavity fueling schemes on the laser-induced plasma ignition process in a scramjet combustor

Laser-induced plasma (LIP) ignition processes in a cavity-based scramjet combustor were investigated experimentally in this study. CH⁎ spontaneous emission recorded at a flame rate of 50 kHz was used to characterize the ignition and flame stabilization processes. Numerical calculation was also performed to characterize the non-reacting flow-field structures. Effect of cavity fueling schemes on the LIP ignition process in the rear-wall-expansion cavity was then examined. It is found that the cavity fueling scheme acts as a dominant factor in a LIP ignition process. After a LIP ignition in the cavity rearward, the initial flame kernel is likely to anchor and grow directly in the rearward of the cavity under cavity upstream fueling schemes. However, the flame kernel will propagate towards the cavity leading edge and grow there under cavity direct fueling schemes. It is concluded that both chemical and turbulent issues of the flow-field affect the LIP ignition process. Local fuel-rich environment inside the cavity is favorable for a LIP ignition process, and the fueling scheme with parallel fuel injectors upstream the cavity is most favorable for a LIP ignition which presents a stable combustion process.

激光诱导等离子体点火方法研究进展

激光诱导等离子体点火方法研究进展

Characterization of ignition transient processes in kerosene-fueled model scramjet engine by dual-pulse laser-induced plasma

Dual-pulse laser-induced plasma ignition of kerosene in cavity at model scramjet engine is studied. The simulated flight condition is Ma 6 at 30 km, and the isolator entrance has a Mach number of 2.92, a total pressure of 2.6 MPa and a stagnation temperature of 1650 K. Two independent laser pulses at 532 nm with a pulse width of 10 ns, a diameter of 12 mm and a maximum energy of 300 mJ are focused into cavity for ignition. The flame structure and propagation during transient ignition processes are captured by simultaneous CH* and OH* chemiluminescence imaging. The entire ignition process of kerosene can be divided into five stages, which are referred as turbulent dissipation stage, quasi-stable state, combustion enhancement stage, reverting stage and combustion stabilization stage. A local closed loop of propagations of the burning mixtures from the shear layer into the recirculation zone of cavity is revealed, which the large-scale eddy in the shear layer plays a key role. The enhancement of mass exchange between shear layer and the recirculation zone of cavity could promote the flame propagation process and enhance the ignition capability as well as extend the ignition limits. A cavity shear-layer stabilized combustion of kerosene is established in the supersonic flow roughly 3.3 ms after the laser pulse. Chemical reactions mainly occur in the shear layer and the near-wall zone downstream of the cavity. The distribution of OH* is thicker than CH* at stable combustion condition.

Repetitive Laser-Induced Plasma Ignition and Assisted Combustion of Premixed Methane/Air Flame

ABSTRACTLaser-induced plasma ignition and assisted combustion of premixed methane/air flames are investigated using a repetitive nanosecond Nd:YAG laser. The flame kernels formed in the ignition process break up into two sub-kernels, which results from the hydrodynamic stretch induced by the toroidal structures generated in the breakdown process. The flame propagation speeds of the upper and lower fronts of the flame kernels are enhanced under low flow speed conditions. Faster propagation of the upper fronts than the lower fronts enables the catch-up of consecutive flame kernels and flameholding using high repetition laser-induced plasmas. The laser repetition rate (frep) to realize cost-effective flameholding is determined from the temporal propagation of the flame fronts. The minimum frep to achieve flameholding at 1–5 mm above the plasma are 387–534 Hz, 454–612 Hz, and 610–869 Hz for the flows of 5.5 m/s, 6.3 m/s, and 9.0 m/s, respectively, and the corresponding up limits for the frep are 704 Hz, 775 Hz, and 1030 Hz, respectively.

Experimental investigation on laser-induced plasma ignition of hydrocarbon fuel in scramjet engine at takeover flight conditions

Laser-induced plasma ignition of an ethylene fuelled cavity is successfully conducted in a model scramjet engine combustor with dual cavities. The simulated flight condition corresponds to takeover flight Mach 4, with isolator entrance Mach number of 2.1, the total pressure of 0.65 MPa and stagnation temperature of 947 K. Ethylene is injected 35 mm upstream of cavity flameholder from four orifices with 2-mm-diameter. The 1064 nm laser beam, from a Q-switched Nd:YAG laser source running at 10 Hz and 940 mJ per pulse, is focused into cavity for ignition. High speed photography is used to capture the transient ignition process. The laser-induced gas breakdown, flame kernel generation and propagation are all recorded and ensuing stable supersonic combustion is established in cavity. The highly ionized plasma zone is almost round at starting, and then the surface of the flame kernel is wrinkled severely in 150 μs after the laser pulse due to the strong turbulence flow in cavity. The flame kernel is found rotating anti-clockwise and gradually moves upstream as the entrainment of circulation flow in cavity. The flame is stabilized at the corner of the cavity for about 200 μs, and then spreads from leading edge to trailing edge via the under part of shear layer to fully fill the entire cavity. The corner recirculation zone of cavity is of great importance for flame spreading. Eventually, a cavity shear-layer stabilized combustion is established in the supersonic flow roughly 2.9 ms after the laser pulse. Both the temporal evolution of normalized chemiluminescence intensity and normalized flame area show that the entire ignition process can be divided into four stages, which are referred as turbulent dissipation stage, combustion enhancement stage, reverting stage and combustion stabilization stage. The results show promising potentials of laser induced plasma for ignition in real scramjets.

Stabilization of a premixed methane-air flame with a high repetition nanosecond laser-induced plasma

Laser-induced plasma ignition has been applied in various combustion systems, however, work on flame stabilization with repetitive laser-induced plasma (LIP) is rather limited. In this paper, stabilization of a premixed methane-air flame with a high repetition nanosecond LIP is reported. The plasma energy coupling and the temporal evolution of the flame kernels generated by the LIPs are investigated with different laser repetition rates, i.e., 1Hz, 100Hz and 250Hz, respectively. The plasma energy coupling is not affected in the air flow and in the premixed methane-air flow with the applied laser repetition rates. Continuous combustion flame stabilization has been achieved with LIPs of 100Hz and 250Hz, in terms of catch-up and merging of the consecutive flame kernels. The flame kernel formed by the last LIP does not affect the evolution of the newly formed flame kernel by the next LIP. The catch-up distance, defined as the distance from the LIP initiation site to the flame kernel catch-up position, is estimated for different laser repetition rates based on the temporal evolution of the flame kernels. A higher laser repetition rate will lead to a shorter catch-up distance which is beneficial for flame stabilization. The up limit for the laser repetition rate to realize effective flame stabilization is determined from the critical inter-pulse delay defined from the onset of the LIP to the return of the initially contraflow propagating lower front to the LIP initiation site. The up limit is 377Hz under the flow conditions of this work (equivalence ratio of 1, flow speed of 2m/s, and Reynolds number of 1316).

Effect of laser induced plasma ignition timing and location on Diesel spray combustion

An experimental study about the influence of the local conditions at the ignition location on combustion development of a direct injection spray is carried out in an optical engine. A laser induced plasma ignition system has been used to force the spray ignition, allowing comparison of combustion’s evolution and stability with the case of conventional autoignition on the Diesel fuel in terms of ignition delay, rate of heat release, spray penetration and soot location evolution. The local equivalence ratio variation along the spray axis during the injection process was determined with a 1D spray model, previously calibrated and validated. Upper equivalence ratios limits for the ignition event of a direct injected Diesel spray, both in terms of ignition success possibilities and stability of the phenomena, could been determined thanks to application of the laser plasma ignition system. In all laser plasma induced ignition cases, heat release was found to be higher than for the autoignition reference cases, and it was found to be linked to a decrease of ignition delay, with the premixed peak in the rate of heat release curve progressively disappearing as the ignition delay time gets shorter. Ignition delay has been analyzed as a function of the laser position, too. It was found that ignition delay increases for plasma positions closer to the nozzle, indicating that the amount of energy introduced by the laser induced plasma is not the only parameter affecting combustion initiation, but local equivalence ratio plays a major role, too.

Laser induced plasma methodology for ignition control in direct injection sprays

New combustion modes for internal combustion engines represent one of the main fields of investigation for emissions control in transportation Industry. However, the implementation of lean fuel mixture condition and low temperature combustion in real engines is limited by different unsolved practical issues. To achieve an appropriate combustion phasing and cycle-to-cycle control of the process, the laser plasma ignition system arises as a valid alternative to the traditional electrical spark ignition system. This paper proposes a methodology to set-up and optimize a laser induced plasma ignition system that allows ensuring reliability through the quantification of the system effectiveness in the plasma generation and positional stability, in order to reach optimal ignition performance. For this purpose, experimental tests have been carried out in an optical test rig. At first the system has been optimized in an atmospheric environment, based on the statistical analysis of the plasma records taken with a high speed camera to evaluate the induction effectiveness and consequently regulate and control the system settings. The same optimization method has then been applied under engine-like conditions, analyzing the effect of thermodynamic ambient conditions on the plasma induction success and repeatability, which have shown to depend mainly on ambient density. Once optimized for selected engine conditions, the laser plasma induction system has been used to ignite a direct injection Diesel spray, and to compare the evolution of combustion with that of a conventional auto-ignited Diesel spray.

用于激光诱导等离子体点火技术的激光源研究进展

用于激光诱导等离子体点火技术的激光源研究进展