Experimental study of the formation and collapse of an overhang in the lateral spread of smouldering peat fires

Smouldering combustion is the driving phenomenon of wildfires in peatlands, and is responsible for large amounts of carbon emissions and haze episodes world wide. Compared to flaming fires, smouldering is slow, low-temperature, flameless, and most persistent, yet it is poorly understood. Peat, as a typical organic soil, is a porous and charring natural fuel, thus prone to smouldering. The spread of smouldering peat fire is a multidimensional phenomenon, including two main components: in-depth vertical and surface lateral spread. In this study, we investigate the lateral spread of peat fire under various moisture and wind conditions. Visual and infrared cameras as well as a thermocouple array are used to measure the temperature profile and the spread rate. For the first time the overhang, where smouldering spreads fastest beneath the free surface, is observed in the laboratory, which helps understand the interaction between oxygen supply and heat losses. The periodic formation and collapse of overhangs is observed. The overhang thickness is found to increase with moisture and wind speed, while the spread rate decreases with moisture and increases with wind speed. A simple theoretical analysis is proposed and shows that the formation of overhang is caused by the spread rate difference between the top and lower peat layers as well as the competition between oxygen supply and heat losses

Comprehensive review and application of particle image velocimetry

For a fluid dynamics experimental flow measurement technique, particle image velocimetry (PIV) provides significant advantages over other measurement techniques in its field. In contrast to temperature and pressure based probe measurements or other laser diagnostic techniques including laser Doppler velocimetry (LDV) and phase Doppler particle analysis (PDPA), PIV is unique due to its whole field measurement capability, non-intrusive nature, and ability to collect a vast amount of experimental data in a short time frame providing both quantitative and qualitative insight. These properties make PIV a desirable measurement technique for studies encompassing a broad range of fluid dynamics applications. However, as an optical measurement technique, PIV also requires a substantial technical understanding and application experience to acquire consistent, reliable results. Both a technical understanding of particle image velocimetry and practical application experience are gained by applying a planar PIV system at Michigan Technological University’s Combustion Science Exploration Laboratory (CSEL) and Alternative Fuels Combustion Laboratory (AFCL). Here a PIV system was applied to non-reacting and reacting gaseous environments to make two component planar PIV as well as three component stereographic PIV flow field velocity measurements in conjunction with chemiluminescence imaging in the case of reacting flows. This thesis outlines near surface flow field characteristics in a tumble strip lined channel, three component velocity profiles of non-reacting and reacting swirled flow in a swirl stabilized lean condition premixed/prevaporized-fuel model gas turbine combustor operating on methane at 5-7 kW, and two component planar PIV measurements characterizing the AFCL’s 1.1 liter closed combustion chamber under dual fan driven turbulent mixing flow

Flame-flow interaction during premixed and stratified swirl flame flashback in an annular swirl combustor

The interaction between a propagating flame and the approach flow is critical to the understanding of boundary layer flashback of swirling flames. In this work, I investigated this interaction during flashback using high-speed luminosity imaging and simultaneous three-dimensional particle image velocimetry. The mean axial velocity through the mixing tube is kept at 2.5 m/s while the hydrogen enrichment of the fuel is varied up to 87%. These flashback experiments are conducted at pressures ranging from 1 to 5 atm. To understand the flame-flow interaction physics, I developed a novel analysis methodology for low-turbulence fully-premixed methane-air swirl flame flashback, by stacking the planar flame profiles and three-dimensional velocity data. In the quasi-reconstructed velocity field, the motion of an approaching fluid parcel is analyzed in the frame-of-reference of the propagating flame. For the first time, the role of inertial forces in swirling flame-flow interaction is revealed. Subsequently, I investigated the effect of fuel-air partial premixing on the flashback behavior at atmospheric and elevated pressures. A swirler-based fuel-injection system was used to create fuel-air stratification in the radial direction. For elevated pressure measurements, an optically accessible elevated pressure chamber was designed and constructed to conduct flashback experiments up to 5 atm. The spatial distribution of the equivalence ratio under non-reacting conditions was investigated using planar laser-induced fluorescence with acetone as the fuel tracer. It was observed that fuel-air pockets were distributed across the mixing tube width, although in an average sense, the fuel-air mixture was radially stratified. The global behavior of upstream flame propagation is reported for different levels of hydrogen-enrichment. For stratified hydrogen-rich flashback, the propagation path of the flame changes from the inner wall to outer wall induced by the faster chemistry of stoichiometric mixtures that are frequently present near the outer wall. This behavior of hydrogen-rich flashback persists even at elevated pressures up to 5 atm, although the propagation of the flame occurs as a wide flame tongue as opposed to the acute-tipped flame structures present in the atmospheric cases.Aerospace Engineerin