Small Particle Heat Research Articles

Initiation and propagation of spherical premixed flames with inert solid particles

Spherical flame initiation and propagation in particle-laden mixtures are investigated theoretically in this work. Within the framework of constant density, large activation energy and quasi-steady assumptions, a correlation describing spherical flame propagation speed as a function of flame radius is derived. This correlation is used to assess the influence of gas and particle properties on initiation and propagation of premixed spherical flames. Spherical flame initiation and propagation are shown to be influenced noticeably by the appearance of inert solid particles. It is found that the flame propagation speed and temperature both decrease with increased particle heat capacity and thermal relaxation time. A non-monotonic change of the flame propagation speed with flame radius is observed when there are particles with large heat capacity. Furthermore, the bifurcation of flame propagation speed is observed for particles with large heat capacity and thermal relaxation time. Within a certain flame radius range, there are both strong and weak flame solutions. The abrupt jump from the strong flame to weak flame results from the excessive heat loss caused by the solid particles and the energy balance is re-established along the weak flame branch. The Lewis number strongly affects the flame propagation speed, particularly for small thermal response time and high particle heat capacity. Additionally, the minimum ignition energy of the particle-laden spherical flames is found to increase with the Lewis number. At higher Lewis number, the difference of minimum ignition energy between gaseous and particle-laden situations becomes larger. To validate the theoretical results, one-dimensional transient simulations of particle-laden spherical flames with detailed chemistry have been conducted. Qualitative agreement is achieved for results from numerical simulations and theoretical analysis.

Critical behavior of the specific heat of small magnetic Cr2O3 particles

The critical behavior of the specific heat of small magnetic particles of the real antiferromagnet Cr2O3 is investigated by the Monte-Carlo method. The critical indices α=−0.17±0.03 and the ratio of the critical amplitudes A/A′=1.03±0.07 are calculated for particles containing N=286 to 2502 spins.

Transport and heating of small particles in high density plasma sources

Calculations of the forces experienced by particles in high density plasma sources such as an electron cyclotron resonance system indicate that particles can accelerate in the plasma and travel through sheaths. This is due primarily to the very high ion fluxes in these systems and relatively low sheath potentials. The net result is less particle trapping and growth in the plasma compared to conventional plasma sources. The temperature of the particles is calculated from an energy balance. The particles heat due to electron-ion recombination on their surface, and they cool by radiation and conduction to the neutral gas. Because of the high plasma density and low neutral pressure, the particles may reach high temperatures (T∼1000 K).

Thermodynamic transition of small superconducting particles

Measurements are presented which show that the specific heat of small tin particles exhibits a BCS-like structure at the superconducting transition even if the particle size is much smaller than the superconducting coherence length. This structure remains, broadened by fluctuation effects, down to dimensions so small that size quantization produces an electronic energy level spacing larger than the superconducting energy gap. Our results suggest that this energy ratio determines the crossover from bulk behavior (with a well-defined phase transition) to zero-dimensional behavior (with no superconducting order).

Alleged Specific Effects of High-Frequency Fields on Biological Substances

VARIOUS claims for the demonstration of specific electric actions of high-frequency fields on biological substrates, that is, non-thermal effects dependent on wave-length, have already been refuted1. Attenuation of some bacterial toxins exposed to 1·9–3·7 m. fields, under conditions which appeared to eliminate the influence of the heat developed, seemed nevertheless to indicate the existence of such an athermal action (Szymanowski and Hicks2). Subsequent negative results with different bacteria, etc., led these authors3 to suspect that their previous results might have been due to an undetected local rise of temperature, although a selective heating of small dispersed particles (bacteria, micellae, macromolecules) is improbable because of the rapid heat exchange with the medium4.