J.F. Zevenbergen

Dust explosions in spherical vessels: The role of flame thickness in the validity of the ‘cube-root law’

Abstract A well known limitation of the ‘cube-root law’ is that it becomes invalid when the flame thickness is significant with respect to the vessel radius. In the literature, flame thicknesses in dust-air mixtures ranging from 15 to 80 cm have been reported, which exceed the radii of the 20 litre sphere and the 1 m3 vessel. Therefore, we have developed a model (the three-zone model) for the pressure evolution of confined dust explosions in spherical vessels which takes the flame thickness into account. The pressure-time curves that are generated with this model show a good resemblance with those measured in practice. It is shown by numerical simulations that the maximum rate of pressure rise can be normalized with respect to the vessel volume as well as to the flame thickness and that the ‘cube-root law’ becomes inaccurate for relative flame thicknesses exceeding 1%. Furthermore, the actual burning velocity and the flame thickness during real dust explosions can be obtained by fitting the model to the experimental pressure-time curve.

The relation of cool flames and auto-ignition phenomena to process safety at elevated pressure and temperature

The cool-flame phenomenon can occur in fuel-oxygen (air) mixtures within the flammable range and outside the flammable range, at fuel-rich compositions, at temperatures below the auto-ignition temperature (AIT). It is caused by chemical reactions occurring spontaneously at relatively low temperatures and is favoured by elevated pressure. The hazards that cool flames generate are described. These vary from spoiling a product specification through contamination and explosive decomposition of condensed peroxides to the appearance of unexpected normal (hot) flame (two-stage ignition).

The influence of injector design on the decay of pre-ignition turbulence in a spherical explosion chamber

Abstract This paper reports on an experimental study to characterize the turbulent flow field inside a 20 l Siwek chamber during the pre-ignition period. An acrylonitrile–butadiene–styrene model of the chamber was constructed with optical quality windows enabling laser Doppler anemometry (LDA) to be used for turbulence measurements. Alumina (Al 2 O 3 ) particles were used as the seed dust for measuring the gas-phase turbulence. Three specific dust dispersion systems have been investigated: (1) the deflector plate (also referred to as the rebound nozzle); (2) the perforated annular nozzle; and (3) the circular “Dahoe” nozzle. It is assumed that changing the method of dust dispersion alters the turbulence characteristics. The flow field is non-stationary, i.e., the mean (or predominant fluid flow) and superimposed velocity fluctuations upon the mean decrease with time. Furthermore, there are variations from injection to injection. A procedure has been developed to analyze this non-stationary signal to extract the mean and fluctuating components of velocity, thereby paving the way for decay “laws” to be determined for a particular nozzle configuration.

Laser Ignition of Single Magnesium Particles

The minimum ignition temperature and minimum ignition energy of single magnesium particles was determined as a function of particle diameter. The particle was levitated ultrasonically and was ignited by a short laser pulse. The temperature transient of the particle was captured using a fast, optical fiber thermometer. It is shown that the minimum ignition temperature and the normalized minimum ignition energy are material constants of magnesium and are independent of particle diameter in the size range measured.

Process safety at elevated temperatures and pressures: Cool flames and auto-ignition phenomena

Publisher Summary The chapter explains the concept of process safety at elevated temperatures and pressures. The hazards due to cool flames generation are also described. Partial oxidation processes, carried out at elevated conditions are used in the chemical industry. Detailed knowledge about relevant explosion indices is essential for a safe and economic operation in the most efficient way. Such explosion indices must be known under realistic process conditions—high temperature, high pressure, and high turbulence conditions. The cool flame phenomenon can occur in fuel (-air) -oxygen mixtures, within the flammable range and outside the flammable range, at fuel-rich composition and at temperatures below the auto-ignition temperature. It is caused by chemical reactions occurring spontaneously at relatively low temperatures and is favored by elevated pressure. The hazards of cool flames vary from spoiling a product specification through contamination and the appearance of unexpected normal (hot) flame (two-stage ignition) to explosive decomposition of condensed peroxides. Therefore, cool flame temperature and limits should be considered as a safety parameter for processes operating at elevated temperatures and pressures.