Laser Supported Combustion Waves Research Articles

Study of the length and influencing factors of air plasma ignition time

Abstract When a high-energy laser acts on a film surface, plasma flashes of both the air and film can be generated simultaneously. However, when the conventional plasma flash method is used to identify thin film damage, there is a misjudgment problem caused by the inability to distinguish the air and film plasma flashes. In order to solve the problem of misjudgment, the ignition times of air and thin film plasma flashes can be obtained, respectively. If the ignition times of air and thin film plasma flashes are not equal, they can be distinguished from the time difference. In this paper, a nanosecond Nd:YAG pulse laser is used to break down the air at room temperature and pressure, and the theoretical and experimental values of the ignition time of air plasma flash are obtained. The curves of the ignition time of air plasma flash with the laser wavelength, incident energy, focusing spot, and pulse width are simulated. The reasons for the changes are analyzed from the perspectives of multiphoton absorption, cascade ionization theory, and electromagnetic theory of laser breakdown gas. The results show that when the laser pulse width is 10 ns, the energy is 160 mJ, and the spot radius is 0.015 cm. The theoretical and experimental values of the ignition time of air plasma flash are 2.146 and 2 ns, respectively, which are in good agreement. Larger values of laser focus spot size and pulse width relate to a longer ignition time of the air plasma flash, whereas larger values of laser wavelength and incident energy are related to a shorter ignition time. The research reflects the characteristics and electronic transition of air plasma, as well as the micromorphological evolution of the interaction between laser and air, presents the process of air plasma flash generation and growth, and reveals the ignition mechanism of air plasma. It not only provides a basis for improving the traditional plasma flash identification method to identify film damage but also has a certain scientific significance for studying the generation mechanism of laser-supported combustion waves and detonation waves.

Time-resolved temperature field and fluid velocity field numerical simulation of plasma generated by long pulse laser induced transparent medium

Laser supported combustion waves(LSCW)exist in the process of laser induced damage to the fused silica. Based on the combustion wave supported by a single-pulse laser, the combustion process of pulsed laser is modeled and simulated in stages, taking into account the effects of the temperature residuals of the first and multiple laser pulses, changes in the target morphology, the distribution of the splash material, and the distribution of the target surface airflow conditions. The laser energy transfer process including inverse bremsstrahlung radiation, thermal radiation, heat conduction and convection processes were simulated by establishing a two-dimensional axisymmetric gas dynamic model. The simulation results show that: under the action of the parallel laser beam, the propagation of the combustion wave is steady-state and the gas dynamics behavior is stable, under the action of the focused laser beam, the propagation of the combustion wave is unsteady. The expanding velocity and the gas dynamic structure of the combustion wave are in good agreement with the experimental results and theoretical results, which verifies the correctness of the theoretical model.

Laser-supported combustion wave in the field of gravity

Numerical simulation is used for studying the propagation of slow laser-supported combustion waves (LCW) in atmospheric air. The gasdynamic structure is investigated of the flow field which accompanies the movement of laser-supported combustion wave. A special feature of the problem being solved is the inclusion of gravity; this results in the emergence of convective motions in the thermal wake of LCW. It is demonstrated that different modes of thermal gravitational convection are observed depending on the conditions of LCW movement. A two-dimensional radiation-gasdynamic model is given, which includes the continuity and Navier-Stokes equations, the energy equation, and the equations of laser and thermal radiation transport in a multi-group approximation. The real thermal and spectral optical properties of air in the temperature range from 300 to 20 000 K are taken into account.

Numerical analysis of subsonic laser-supported combustion waves

A nonstationary model of radiation gas dynamics for subsonic laser-supported combustion waves is presented. A systematic numerical simulation of the temperature and gas-dynamic structure of such waves under the conditions of experiments of E L Klosterman and S R Byron is carried out. It is shown that the dominant process governing the propagation of subsonic laser-supported combustion waves in this case is the reabsorption of heat radiation ahead of the wave front, whereas the gas-dynamic processes are of minor importance.

Plasma formation resulting from the interaction of a laser beam with a solid metal target in an ambient gas

The formation and role of a plasma resulting from the interaction between a laser beam and a metal target in an ambient gas are reviewed in this paper. The plasma is initiated after the production of primary electrons by multiphoton absorption and thermionic photoemission mechanisms. Vaporization of the surface then occurs at a lower threshold than theoretically deduced, due to surface defects and impurities. Vapour ionization is first a thermal process, primary electrons gaining energy by inverse Bremsstrahlung than electron cascade growth occurs. The plasma propagates with absorption waves; laser-supported combustion or detonation waves depending on the laser irradiance regime. The ablation process is favoured with the shortest wavelengths, whereas ambient gas breakdown occurs with the largest wavelengths. The nature of the ambient gas must be adapted for the chosen application. Plasma development can be an inconvenience with target shielding effect when the ionization degree is too high and with laser-supported detonation wave generation occurring in this case. However, transparent plasmas provide optimum laser machining, as good surface coupling with plasma formation occurs which is useful for drilling or welding.

Analytical Study of Laser-Supported Combustion Waves in Hydrogen

A one-dimensional energy equation, with constant pressure and area, was used to model the LSC wave. This equation balances convection, conduction, laser energy absorption, radiation energy loss and radiation energy transport. Solutions of this energy equation were obtained to give profiles of temperature and other properties, as well as the relation between laser intensity and mass flux through the wave. The flow through the LSC wave was then conducted through a variable pressure, variable area streamtube to accelerate it to high speed, with the propulsion application in mind. A numerical method for coupling the LSC wave model to the streamtube flow was developed, and a sample calculation was performed. The result shows that 42% of the laser power has been radiated away by the time the gas reaches the throat. It was concluded that in the radially confined flows of interest for propulsion applications, transverse velocities would be less important than in the unconfined flows where air experiments have been conducted.

Ignition and maintenance of subsonic plasma waves in atmospheric pressure air by cw CO2 laser radiation and their effect on laser beam propagation

Laser supported combustion (LSC) waves, subsonically propagating plasma, are ignited from solid targets and maintained in atmospheric pressure air using focused high−power, as high as 15 kW, cw CO2 laser radiation. The intensity necessary to ignite LSC waves, Ii, is measured for steel and aluminum targets as a function of beam focal spot diameter and wind velocity transverse to the beam axis as is the intensity needed to maintain such waves in atmospheric pressure air, Im. In the absence of a transverse wind, the quantities Ii and Im are essentially identical for diameters smaller than 0.1 cm, but Ii is substantially larger at larger diameters. The reason for this is discussed in terms of the ignition mechanism itself. In the presence of a 20−m/sec transverse wind, Im and Ii are essentially identical at 0.2 cm, the largest diameter for which Ii was determined. The measurements of Im extend to 0.5 cm beam diameter and indicate, in the absence of the transverse wind, that while thermal conduction determines the value of Im at small (0.007 cm) beam diameters, radiative losses are important at the larger diameters considered. An interferometric technique is utilized to obtain spatially resolved LSC wave plasma properties, and the latter are used to calculate the decrease in focal spot laser intensity due to absorption and refraction by the plasma. The calculated results, as well as those obtained from independent experiments, indicate that while inverse bremsstrahlung absorption can account typically for a 50% loss in focal spot intensity, scattering or beam blooming by the plasma has a significantly greater effect as the LSC wave propagates away from the focal spot.

Role of Radiative Transport in the Propagation of Laser Supported Combustion Waves

We report results of an investigation into the roles played by thermal and radiative transport in the propagation of Laser Supported Combustion (LSC) waves. We replace the volume radiation losses assumed by Raizer with detailed treatment of radiation in 19 frequency groups, the flux of each being calculated from the temperature in discrete cells along the LSC wave axis and allowed to deposit in other cells according to temperature dependent absorption coefficients for air. Calculations of LSC wave structure and propagation velocity as a function of laser intensity are compatible with available experimental data. a = c = CP = f = h = h = hi(a) = K = k = KV =

Experimental studies of laser-supported absorption waves with 5-ms pulses of 10.6-μ radiation

The response of materials to ≈5-ms pulses of 10.6-μ laser radiation at peak power densities of 1 to 2 MW/cm2 has been observed with simultaneous determination of (i) both front and back surface temperature rises, (ii) pressure delivered to the specimen as a function of time, (iii) total impulse delivered to the specimen, and (iv) absorption wave effects via high-speed framing camera coverage. The materials studied are 2024 aluminum alloy, 304 stainless steel, graphite, a Fiberglas-epoxy composite, alumina, MgO, and Pyroceram. It was found that Fiberglas-epoxy and graphite exhibit continuous coupling of the laser energy during the entire pulse, whereas the metals showed strong initial coupling which gradually diminished to near zero and then recoupled at the end of the pulse. The ceramics exhibited repetitive decoupling and recoupling during the entire pulse. The photographic evidence indicates the formation of laser-supported plasmatrons in the case of the metals and laser-supported combustion waves with the ceramic materials.