Heat transfer in laser pulse interaction with reactive substances

The purpose of this paper is to analyze thermal regime of a laser-pulse interaction with a substance reacting in bulk and on the surface. Emphasis is on the critical phenomena in non-resonant interaction of substance with laser beam, and how they depend on thermo-chemical and optical characteristics of system. It is shown that irradiation of exothermically reacting substance can induce its ignition, if the laser-beam diameter exceeds a critical one. This critical diameter depends on the pulse intensity and duration, as on the thermo-chemical characteristics of system. The cases of short-pulse and quasi-steady irradiation are considered in details. A role of soot particles or other absorbing micro-inclusions in the interaction is investigated. In certain field of the system parameters such micro-inclusions cause local heat explosion, due to exothermic reaction in environment. In the case of quasi-steady irradiation the explosion is induced if the particle diameter exceeds a critical one. This critical diameter depends on the initial temperature, reaction rate constants, as on radiation intensity. It is directly proportional to the characteristic space-scale of reaction in the system. The product of the particle critical diameter and the radiation intensity is approximately constant. In the case of short-pulse irradiation the highest temperature disturbance in the substance is caused by particles having certain medium diameter. This critical diameter increases with pulse duration and with heat-conductivity of the substance. The main results of consideration are illustrated by instances with reacting gas, condensed fuels, and explosives. The results application to typical problems of laser based diagnostics are also demonstrated.