The second work that was awarded the Hermann von Helmholtz Prize in 1990 in the field of "Chemical and Physical Safety Technology" dealt with "Heterogeneous detonations and indirect ignition processes". The author was Dr. Bodo Plewinsky of the BAM Bundesanstalt für Materialforschung und -prüfung (BAM Federal Institute for Materials Research and Testing) in Berlin. Heterogeneous detonations take place in heterogeneous systems that consist of two or more phases. They occur on liquid films as well as on the surfaces of compact liquid layers, in aerosols, in powders and on wicks.
The reason for Dr. Plewinsky's investigations had been an accident in the laboratory of a German chemical plant. In this laboratory, scientists intended to burn tetramethyl-dihydrogen-disiloxane (TMDS), which moistened a wick, in a calorimetric bomb in pure oxygen at a pressure of about 40 bar. Although there was no intrinsic explosion hazard for the gas surrounding the wick since the composition of the gas phase was above the upper explosion limit, the ignition caused a heavy detonation within the bomb and destroyed it entirely.
Dr. Plewinsky repeated the experiment in a modified form. With a high-velocity camera that was able to take up to 24000 pictures/s, he watched the flame spread along a TMS-moistened cotton wick after the ignition in pure oxygen, and saw how this led to a detonation of the wick. First, a conical flame front spread along the wick at a speed of about 1300 m/s, but after about 1/5000 s, the combustion reaction spread to the surrounding headspace although its composition was above the upper explosion level (UEL) so that, even in this case, a homogeneous gas detonation was impossible. The cause of the above-mentioned laboratory accident had, hence, been a wick detonation. In addition, Dr. Plewinsky was able to observe the phenomenon of the indirect ignition process. The ignition of a flame on a flammable liquid is possible even if the composition of the gas phase is above the UEL and if the direct path between the ignition source and the liquid surface is blocked by a barrier.
Dr. Bodo Plewinsky stated on this occasion that the systematic investigation of surface detonation and indirect ignition is of great importance for safety engineering processes of chemical plants where, for example, the oxidation or chlorination of hydrocarbons are performed. Later, Dr. Plewinsky and his colleagues worked in the field of surface detonations. They discovered that surface detonations cannot be prevented by mechanical barriers. They further investigated how heterogeneous detonations occur in model foams and how the bubbles of a gaseous oxidant in an organic solvent are made to explode by shock waves.