Ignition Thresholds and Flame Propagation of Methane/Air Mixtures Ignited via Radiatively Heated Inert Particles

The prevention and evaluation of explosions requires suitable standards of measurement. As such, for this study two ignition thresholds, the ignition temperature and the minimum ignition irradiance were selected as the assessment criteria. These ignition threshold values were experimentally determined by heating stationary inert silicon carbide particles via thermal radiation with a large spot size in order to ignite quiescent methane-air fuel mixtures. A high-speed Schlieren camera was used to capture the progression of the formation and propagation of the flames throughout the experiments. The results of the experiments show that the irradiance and temperature threshold are directly and inversely proportional to the particle size, respectively. Furthermore, the irradiance and temperature thresholds have similar tendencies within the flammability limits; wherein, the minimum value corresponds to fuel mixtures at a stoichiometric ratio, and increases as the equivalence ratio shifts toward the flammability limits. Irradiance thresholds, though, are more sensitive to changes in equivalence ratio than temperature. The temperature histories of the heated particle determined that when the irradiance is lower than its ignition threshold value, the heated particle-fuel mixture system will arrive at a thermal equilibrium, rather than ignition, due to the inability of the particle to reach the ignition temperature. This study also found that longer ignition times will result in a more drastic deformation of the flame fronts caused by natural convection.

Towards Measuring the Maxwell–Boltzmann Distribution of a Single Heated Particle

The Maxwell–Boltzmann distribution is a hallmark of statistical physics in thermodynamic equilibrium linking the probability density of a particle’s kinetic energies to the temperature of the system that also determines its configurational fluctuations. This unique relation is lost for Hot Brownian Motion, e.g., when the Brownian particle is constantly heated to create an inhomogeneous temperature in the surrounding liquid. While the fluctuations of the particle in this case can be described with an effective temperature, it is not unique for all degrees of freedom and suggested to be different at different timescales. In this work, we report on our progress to measure the effective temperature of Hot Brownian Motion in the ballistic regime. We have constructed an optical setup to measure the displacement of a heated Brownian particle with a temporal resolution of 10 ns giving a corresponding spatial resolution of about 23 pm for a 0.92 μm PMMA particle in water. Using a gold-coated polystyrene (AuPS) particle of 2.15 μm diameter we determine the mean squared displacement of the particle over more than six orders of magnitude in time. Our data recovers the trends for the effective temperature at long timescales, yet shows also clear effects in the region of hydrodynamic long time tails.

Фото- и термофорез нагретых умеренно крупных аэрозольных частиц сферической формы

A theory of photo- and thermophoresis of a heated medium-size spherical aerosol particle is proposed in the quasi-stationary Stokes approximation at relatively small Reynolds and Peclet numbers. Gas-dynamic equations are solved with allowance for power dependences of the molecular transport (viscosity and thermal conductivity) coefficients and density of the gas medium on temperature. Boundary conditions take into account effects that are linear with respect to the Knudsen number. Expressions for the total force and velocity are derived. It is shown than the above effects may substantially affect the motion of a heated particle.

Settling of heated particles in homogeneous turbulence

We study the case of inertial particles heated by thermal radiation while settling by gravity through a turbulent transparent gas. We consider dilute and optically thin regimes in which each particle receives the same heat flux. Numerical simulations of forced homogeneous turbulence are performed taking into account the two-way coupling of both momentum and temperature between the dispersed and continuous phases. Particles much smaller than the smallest flow scales are considered and the point-particle approximation is adopted. The particle Stokes number (based on the Kolmogorov time scale) is of order unity, while the nominal settling velocity is up to an order of magnitude larger than the Kolmogorov velocity, marking a critical difference with previous two-way coupled simulations. It is found that non-heated particles enhance turbulence when their settling velocity is sufficiently high compared to the Kolmogorov velocity. Energy spectra show that the non-heated particle settling impacts both the very small and very large flow scales, while the intermediate scales are weakly affected. When heated, particles shed plumes of buoyant gas, further modifying the turbulence structure. At the considered radiation intensities, clustering is strong but the classic mechanism of preferential concentration is modified, while preferential sweeping is eliminated or even reversed. Particle heating also causes a significant reduction of the mean settling velocity, which is caused by rising buoyant plumes in the vicinity of particle clusters. The turbulent kinetic energy is affected non-monotonically as the radiation intensity is increased due to the competing effects of the downward gravitational force and the upward buoyancy force. The thermal radiation influences all scales of the turbulence. The effects of settling and buoyancy on the turbulence anisotropy are also discussed.

Laser-Induced Emission of Ultrafine Particulates Evolved by Pulverized Coal Pyrolysis

The importance of on line measurement of ultrafine particulates in pulverized coal flames is mainly due to the detection of ultrafine particulate in the effluent for pollution control, and the quantification of fuel burnout in real time within a boiler for improved understanding of the flame heat transfer soot modeling as well. A method has been investigated using laser-heated emission within an O2-free flame which provides a continuous in situ measurement of ultrafine particles during high-temperature pulverized coal pyrolysis. Bituminous coal particles are entrained by nitrogen along the centerline of a laminar flow flat flame burner, where a hydrogen-air flame under fuel-rich condition is used as a heat source. The temperatures of the hydrogen flame were measured by a finite-wire silica-coated Platinum-Rhodium type B Thermocouple. Volatiles released during the coal pyrolysis form a cloud of ultrafine particles at high temperature. A pulse laser sheet introduced to the flame heats the ultrafine particles to incandescent temperatures. The time-resolved laser-induced emission signals with different incident laser-pulse fluences were evaluated. The volume faction of ultrafine particles was associated with the peak value of the signals, and the mean particle size characterized by a time constant of the exponential signal decay. A strong dependence of the characteristic peak value and emission time constant during laser-heated particle cooling from the measured coal particle class could be determined. Specialties in signal evaluation due to residence time in the hydrogen flame for two sizes of coal particles are discussed.