<p>Electrical properties of a propane flame was investigated to improve detection of the flame in absorption refrigerators. The principle of flame rectification, which uses the diode property of the flame, was studied. A DC voltage in the range 0–130V was applied, between the burner and an electrode in the flame, and the current through the flame in the forward and reverse direction was measured. This measurements were performed with the electrode top in different horizontal and vertical positions. AC voltages at various frequencies was also applied and the average current through the flame was measured.</p><p>A linear relation was found between the applied DC voltage and the current through the flame which means that the resistance, in the investigated voltage range, is independent of the applied voltage. The resistance in the forward direction was almost constant for different electrode positions but the reverse resistance varied many hundred MOhm­ when the electrode was moved vertically away from the burner. The gas flow also influenced the reverse resistance to a large extent.</p> / <p>Elektriska egenskaper hos en propanlåga undersöktes i syfte att förbättra detekteringen av lågan i absorptionskylskåp. Rektifieringsprincipen, vilken utnyttjar lågans diodegenskap, undersöktes. En likspänning i intervallet 0–130V lades på, mellan brännaren och en elektrod i lågan, och strömmen genom lågan i fram- och backriktningen mättes. Dessa mätningar gjordes med elektroden i olika horisontella och vertikala positioner. Växelspänning med olika frekvenser lades också på och medelvärdet av strömmen genom lågan mättes.</p><p>Ett linjärt samband upptäcktes mellan pålagd likspänning och strömmen genom lågan vilket betyder att resistansen, i det undersökta spänningsintervallet, är oberoende av pålagd spänning. Resistansen i framriktningen var i princip konstant vid olika elektrodplaceringar medan backresistansen varierade flera hundra MOhm när elektroden flyttades bort från brännaren vertikalt. Gasflödet påverkade också backresistansen i stor utsträckning.</p>

Electrical properties of a propane flame was investigated to improve detection of the flame in absorption refrigerators. The principle of flame rectification, which uses the diode property of the flame, was studied. A DC voltage in the range 0–130V was applied, between the burner and an electrode in the flame, and the current through the flame in the forward and reverse direction was measured. This measurements were performed with the electrode top in different horizontal and vertical positions. AC voltages at various frequencies was also applied and the average current through the flame was measured. A linear relation was found between the applied DC voltage and the current through the flame which means that the resistance, in the investigated voltage range, is independent of the applied voltage. The resistance in the forward direction was almost constant for different electrode positions but the reverse resistance varied many hundred MOhm­ when the electrode was moved vertically away from the burner. The gas flow also influenced the reverse resistance to a large extent. / Elektriska egenskaper hos en propanlåga undersöktes i syfte att förbättra detekteringen av lågan i absorptionskylskåp. Rektifieringsprincipen, vilken utnyttjar lågans diodegenskap, undersöktes. En likspänning i intervallet 0–130V lades på, mellan brännaren och en elektrod i lågan, och strömmen genom lågan i fram- och backriktningen mättes. Dessa mätningar gjordes med elektroden i olika horisontella och vertikala positioner. Växelspänning med olika frekvenser lades också på och medelvärdet av strömmen genom lågan mättes. Ett linjärt samband upptäcktes mellan pålagd likspänning och strömmen genom lågan vilket betyder att resistansen, i det undersökta spänningsintervallet, är oberoende av pålagd spänning. Resistansen i framriktningen var i princip konstant vid olika elektrodplaceringar medan backresistansen varierade flera hundra MOhm när elektroden flyttades bort från brännaren vertikalt. Gasflödet påverkade också backresistansen i stor utsträckning.

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A full-scale ASTM E 1321 Lateral ignition and Flame Transport (LIFT) apparatus was constructed and compared with a Reduced scale Ignition and Flame spread Test apparatus (RIFT) adaptation of the cone calorimeter in the vertical position. The objective was to find a low cost and simple alternative to the LIFT apparatus for measuring opposed flow flame spread. Ignition tests were conducted using the LIFT, RIFT and ISO 5657 ignitability apparatus and flame spread experiments were conducted in the LIFT and RIFT. Nine different types of timber based products were tested for ignition and flame spread, and Quintiere's flame spread model was applied to the results to obtain material properties, such as thermal inertia, flame spread parameter and the minimum heat flux required for flame spread. These materials included plywood, medium density fibreboard (MDF), hardboard, particle board flooring, Melamine (Melteca) covered MDF, New Zealand Rimu, and Beech and New Zealand grown Macrocarpa and Radiata (Monterey) Pine. Further limited tests were conducted on Melteca covered particle board, and a second brand of particle board. The materials in the RIFT were tried with and without preheating to equilibrium. In addition, a view factor for the RIFT was developed, based on earlier work for the cone calorimeter element. The view factor equation was experimentally tested against measured values, and the calculated value was consistently lower than the experimental values, with similar flux profile. The standard procedure is for the material to be preheated before ignition, so that the surface is at equilibrium. The spread of the flame front past points on the sample surface after ignition is recorded, and from the flame front velocity and the model by Quintiere, material specific properties can be derived. The lack of preheating was found to affect the final results, by reducing the flame spread velocity and increasing the scatter in the experimental results. The RIFT gives comparable results to the same materials tested in the LIFT and to the published literature. The results the flame spread parameter and the minimum flux for flame spread are usually higher for the RIFT against the same material in the LIFT. There proved to be an effective limit on suitable materials able to be successfully tested in the RIFT to those that have a minimum flux for flame spread of less than 7kW/m2, with this limitation is dictated by the flux profile along the sample, and the lower resolution dictated by the smaller size. It is approximately equivalent to a minimum ignition flux of 18kW/m2.

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The present thesis investigates the interaction of a shock wave with a cellular flame and the ensuing mechanisms on the dynamics of the subsequent flame deformation. The inter- action is known to disrupt the flame surface through the Richtmyer-Meshkov instability, hence potentially enhancing the local combustion rates. This study aims to clarify the evolution of a flame when perturbed head-on by a shock wave. Two novel series of experiments were conducted in a vertically-oriented Hele-Shaw cell, which could successfully isolate a quasi-bidimensional cellular flame structure at ambient conditions. In the first configuration, the passage of the shock wave arising in the burned products of a deflagration wave was investigated, while both waves propagated in the same outward direction. In the other configuration, the shock wave centrally emerged in the unburned gases and collided with a cellular flame front traveling in the opposite direction. The event was captured using a Z-type Schlieren imaging system to visualize the growth of the flame cells. Shock characterization was determined in the Hele-Shaw apparatus to estimate the strength of the blast wave generated by energy deposition using a high-voltage igniter or by decoupled detonation from a detonation tube. A combustion study was also performed to determine the laminar flame speed in a mixture of hydrogen-air according to different equivalence ratios in the apparatus. The experiments revealed that inherent cellular flame instabilities are well developed in the observation scale of the Hele-Shaw geometry. The shock-flame complex was therefore analyzed experimentally for selected mixtures. As the shock wave traversed the interface separating the burned and unburned gases, the flame became more corrugated. Following the interaction, the flame cusps were stretched and/or flattened. At later times, the wrinkled interface was reversed and developed finer scales. A time scale analysis was performed to identify the contribution of the competing effects of Richtmyer-Meshkov and Rayleigh-Taylor instabilities on the flame interface deformation. For the case of a shock wave traversing the flame interface from the unburned to the burned side, the early perturbations were mainly governed by the Richtmyer-Meshkov instability. Finally, Rayleigh-Taylor instability resulted from the decaying pressure profile of the blast wave and tended to stabilize the perturbed interface to eventually reverse the cellular structure. Experimental and inert numerical results on the flame cell’s amplitude growth were found to be in good agreement.

Mining, process and energy industries suffer from billions of dollars of worldwide losses every year due to Vapour Cloud Explosions (VCE). Moreover, explosion accidents are often tragic and lead to a high number of severe injuries and fatalities. The VCE scenario is complex and controlled by various mechanisms. The interplay among them is still not entirely understood. Understanding all these intricate processes is of vital importance and requires detailed experimental diagnostics. Coupling accurate numerical simulations to well documented experiments can allow an elaborate description of these phenomena. This thesis focuses on explosions occurring on configurations that are either semi-confined or confined. In such configurations, the explosion is generally initiated by a mild ignition and a subsonic flame front emerges from the ignition source. An important feature of self-propagating flames lies in their intrinsically unstable nature. When they propagate in an environment with high levels of confinement and congestion, which is the case in most industrial sites, a Flame Acceleration (FA) process is often observed that can give rise to very fast flames, known for their destructive potential. In some cases, the FA process can create the appropriate conditions for the initiation of detonations, which corresponds to a rapid escalation of the explosion hazard. To reproduce the confinement and congestion conditions that one can find in industrial sites, the university of Munich TUM equipped a confined chamber with a series of obstacles and analysed the influence of repeated obstructions on the propagation of hydrogen/air deflagrations. This experimental study showed a strong influence of the mixture composition on the acceleration process. A Deflagration to Detonation Transition (DDT) has also been observed for a certain range of equivalence ratio. This configuration is therefore ideal to study the mechanisms of flame acceleration as well as the intricate DDT process. A numerical study of both scenarios is performed in this thesis: -First for a lean premixed hydrogen/air mixture, a strong flame acceleration is observed experimentally without DDT. The characteristic features of the explosion are well reproduced numerically using a Large Eddy Simulation (LES) approach. The crucial importance of confinement and repeated flame-obstacle interactions in producing very fast deflagrations is highlighted. -DDT is observed experimentally for a stoichiometric hydrogen/air mixture. This thesis focuses on the instants surrounding the DDT event, using Direct Numerical Simulations (DNS). Particular attention is drawn to the impact of the chemistry modelling on the detonation scenario. The failure of preventive measures is often observed in many explosion accidents. To avoid a rapid escalation of the explosion scenario, mitigative procedures must be triggered when a gas leak or an ignition is detected. Metal salts (like potassium bicarbonate and sodium bicarbonate) have received considerable attention recently because well-controlled experiments showed their high efficiency in inhibiting fires. The last part of the thesis focused on the mechanism of flame inhibition by sodium bicarbonate particles. First, criteria based on the particle sizes are established to characterize the inhibition efficiency of the particles. Second, two dimensional numerical simulations of a planar flame propagating in a stratified layer of very fine sodium bicarbonate particles showed that under certain conditions these powders can act as combustion enhancers. These results echo a number of experimental observations on the possible counter-effects of the inhibitors.