1-Sentence-Summary: Pre-ignition and post-ignition turbulence increase explosion severity and decrease ignitability of dust clouds in laboratory and industrial settings.
Authors: P.R. Amyotte, S. Chippett, and M.J. Pegg
Read in: Three Minutes
Favorite quote from the paper:
These authors present the state of understanding for the effects of turbulence on dust explosion up to 1988. They present a substantial literature review focusing on the impact of turbulence on flame propagation rate, maximum explosion pressure, maximum rate of pressure rise, flammability or explosion limits, and minimum ignition energy.
The authors discuss both quantitative data and qualitative observations found in the literature. Information from gas explosion literature is reviewed where current understanding for dust explosion is not available. This work focuses on the impact of turbulence on laboratory scale testing and also implications for industry protection against dust explosion.
Three of the main findings from this paper are:
- Both pre-ignition and post-ignition turbulence increase flame propagation rate, maximum pressure, and maximum rate of pressure rise.
- Turbulence tends to reduce the flammability and ignitability limits of dusts and gases.
- Two avenues are recommenced for improving the understanding of turbulence on dust explosions: concurrent investigation with gas explosion, and comparison of laboratory and industry turbulence parameters.
The following sections outline the main findings in more detail. The interested reader is encouraged to view the complete article at the link provided below.
Finding #1: Pre-existing and flame propagation produced turbulence enhance explosion severity
These authors break the turbulence problem into two pieces: turbulence that is existing prior to cloud ignition and turbulence generated during flame propagation after ignition. Due to the impact of gravity some dispersion mechanism is required to generate a dust cloud. This dispersion mechanism causes preexisting turbulence prior to ignition. Post-iginition turbulence occurs due to flame induced flow or direct flame interaction with nearby surroundings or obstacles. Both mechanisms of turbulence generation have significant impact on explosion severity.
In laboratory testing cloud suspension is generally achieved by an air-blast to rise the dust. In industrial situations the dust cloud could be inherent in the processing application (e.g., dryers or dust transport) or due to upset conditions (e.g., primary blast lifting dust). From reviewing experimental data the current authors demonstrate that pre-existing turbulence will tend to increase explosion severity. For example, data from the literature is presented that shows the flame speed in a wheat flour cloud increasing from 1.2 m/s to 3.1 m/s from fan generated turbulence.
Post-ignition turbulence is caused by pressure waves or flame-induced flow interacting with the surrounding environment. The authors review several papers from the dust and gas explosion literature demonstrating this effect. Acoustically enhanced combustion, hydrodynamic instabilities, and flame stretching around obstacles can all increase the turbulence level and cause the flame speed to increase by one to two orders of magnitude.
Finding #2: Flammability limits and dust ignitability decrease with turbulence level
From reviewing experimental gas explosion results and models of flame ignition, the authors demonstrate that turbulence tends to narrow the flammability limits of combustible fuels. For droplet combustion Ballal and Lefebvre, 1977 explain that turbulence tends to remove heat from the ignition kernel making it more difficult to have successful ignition and flame propagation.
The ignition energy required to initiate a dust explosion also increases with turbulence level. Experimental data for cornstarch dust clouds are presented and demonstrate approximately 30% increase in ignition energy moving from 2 m/s to 4 m/s turbulence level.
The authors caution that differences between ignitability and flammability should be kept in mind when interpreting these results. Particularly for dust clouds, an inability to cause an explosion in a laboratory setting may be due to ignitability, where the cloud may actually propagate a flame under different industrial conditions. An excellent example of this are the experimental results from Hertzberg et al., 1988. These results demonstrate an increased “apparent flammability limit” with low-strength chemical ignitors, where lower concentrations can actually explode if higher strength ignition is used.
Finding #3: Two approaches are recommended to increase understanding of turbulence effects on dust explosion
Following others in the literature including Chippett and Britton, 1985, Bond et al., 1986, and Tamanini et al., 1983 (references in the paper and links can not be found), the current authors propose two approaches to further developing the understanding of turbulence effects on dust explosions: concurrent investigations with gas explosion, and in-depth characterization of laboratory and industrial turbulence parameters.
The thought for concurrent investigation is that the understanding of turbulent effects on gas flames is considerably more advanced than on dust flames. Comparing the effect of turbulence on dust explosion to gas explosion may provide insight into the fundamental processes involved. Some advances in turbulent gas flame structures such as combustion diagrams may be extended to explain turbulent dust flames. The authors also caution that there are likely differences between these systems and that understanding of these differences is also needed.
Other authors such as Chatrathi, 1994 have also called for better characterization of turbulence levels in industry processes and laboratory testing. More recently measurements have been made of the turbulent velocity fluctuations in the 20-L chamber (Dahoe, Cant, and Scarlett, 2001). However, an understanding of how this relates to typical industrial turbulence levels, and how to scale flame propagation rates and flammability levels accordingly, remains to be completed.
My Personal Take-Aways From
“Effects of Turbulence on Dust Explosions”
This paper is a must-read for anyone looking to understand how turbulence effects dust flame propagation and flammability limits. It gives a very thorough review of the literature up to 1988. Many of the 113 articles referenced are also very difficult to retrieve, and this paper may be the most readily available copy.
This paper illustrates an expressed need to better correlate turbulence level for experimental testing to industrial processes. Also note that turbulence levels between the 20-L chamber and 1-m3 are markedly different. In general, the 20-L chamber turbulence level has been adjusted to give comparable results to the larger vessel. This may have important implications for scaling of KSt values from one to the other, and eventually to industrial processes. The current authors also give a good review of the history and existing difficulties with the ‘cubic relationship’ that may be valuable to the reader.
Full Citation: [bibtex file=references.bib key=Amyotte1988]
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