Ignition by hot gas jets

*English* Ignition of hydrogen/air mixture by hot turbulent jets finds a lot of applications in a wide range of fields starting from nuclear safety, pulse detonation engines, supersonic combustors, explosion protection etc. The current work is conducted on the background of explosion protection. Experimental and numerical investigations on the fundamental mechanisms behind the ignition process are reported in this work. An experimental realisation of such an ignition process using a simple, well-defined and optically accessible combustion cell, where a jet of hot exhaust gas impinges through a narrow nozzle into a quiescent hydrogen/air mixture, possibly initiating ignition and combustion, is studied. Laser diagnostics methods like high-speed LIF sequences of the hydroxyl radical (OH) simultaneous with Laser Schlieren visualisation was used to gain spatial and temporal information about the development of individual ignition events. Simple numerical simulations of the reactive mixing of exhaust gas with H2/air mixture was combined with spectroscopic simulations of the OH-LIF signal to aid in interpretation of the experimental results. A novel method using a combination of LIF of seeded NO, numerical simulations of the underlying physical and chemical processes and spectroscopic simulation of the NO-LIF signal is used for the measurement of quantitative, instantaneous, 2D mixture fraction during the ignition of hydrogen/air mixture by hot jets. The outcome of numerical modelling and experiments indicate the important influence of the hot jet temperature and speed of mixing between the hot and cold gases on the ignition process. The high-speed OH-LIF sequences show the quenching of the flame inside the nozzle and the subsequent ignition of the mixture by the hot exhaust jet. The 2D mixture fraction studies using NO-LIF and numerical analysis focussed mainly on two experimental configurations, namely, with and without ignition of the fuel/air mixture by the hot exhaust gas. In both cases the method employed successfully captures the axial monotonic decay of the jet fluid concentration and the typical axisymmetric radial profiles of jet flows. The mixture fraction plots also highlight dissimilar mixing behaviour at regions near to the jet tip and the radial sides as seen in the OH-LIF experiments. The limitations of the new method and the possible alternative strategies for the measurement of mixture fraction are also discussed in the report. The experimental data obtained from this study will be used as a basis for developing and validating detailed models of the hot jet ignition problem. Understanding this problem is a key in the design and development of the electrical equipment of the protection type „flameproof enclosure“.