A new instrument is described for measuring concentrations of submicrometer particulates with diameters ranging from 0.005 to 1.0 μm and concentrations as low as 750 particles per cm3 at small diameters and 25 particles per cm3 at the larger diameters. Developed by the Bureau of Mines, the instrument is capable of obtaining size distributions of the sampled particulates using an electrical mobility classification which requires voltages of less than 1000 V over the entire size range. The instrument is normally operated by 115 V ac, but it contains an internal rechargeable battery pack for up to 25 h of continuous dc operation. It has been designed primarily as an incipient fire detector, but can be used as a suspended particulate monitor and size analyzer or as a laboratory instrument for studying combustion aerosols. Representative data are presented, indicating potential uses of the instrument.

This US Bureau of Mines report on experimental and theoretical studies of diffusion flames in free convection (1) explores the cross-coupling between buoyancy-induced flows and the combustion process as it relates to the size, structure, and radiative balance of flames, (2) presents new data for the size-dependent radiance oscillations of diffusion flames and for the flame, and (3) compares both amplitude and freqency of the measured oscillations with the theoretical predictions that are based on a new view of the structure of diffusion flames. This new view is at variance with the traditional one, which assumes uniform reaction in the diffusive mixing zone. It recognizes the significance of the role played by convective buoyancy and its duality in both aiding and impeding flame propagation. Buoyancy quenches propagation at low burning velocities, introducing a real discontinuity in the combustion rate. A flammable volume is thus defined as that bound between lean- and rich-limit contours. However, in the flow disturbances generated by combustion waves that consume that volume, buoyancy aids propagation by promoting the convective mixing of a new flammable volume. The measured radiance oscillations are manifestations of these propagation and remixing cycles.

The early and reliable detection of the self-heating of combustible substances in mines is of considerable importance to the mining community. This Bureau of Mines report describes the results of a variety of studies of incipient combustion and its detection. Heated samples of coal, wood, and various polymers were used to evaluate the relative sensitivities of several types of detectors. The laboratory results indicate that submicron particulate detection is the most universal and sensitive method. Carbon monoxide detection was as sensitive for pyrolyzing coal, but was relatively insensitive to pyrolyzing wood or plastics. The conventional type of ionization smoke detector was the least sensitive for coal and of intermediate sensitivity for wood and plastics. Self-heating experiments with Pittsburgh Seam coal piles of varying particle sizes gave ignition temperatures of 170C for 4-7micrometers dust, which increased monotonically with particle size. The measured heating rates were compared with data from an adiabatic calorimeter to obtain preexponential rate constants. These varied linearly with the surface area per unit mass of sample.