The abstract of the third part of this paper is included in the abstract of part 1.
Table 2: Compilation of precompression factors found for all tests conducted. The tests are listed in the same order as in Table 1. Omitted are those tests without DDT and those with a DDT location upstream of the first pressure sensor (tests 29, 26, 24). Note that tests 22, 32 and 35 were tests in the short pipe configuration.
num-berof testinitial pressure of the gas mixture [bar abs]value of last pressure measurement of precompressed zone upstream of DDT location [bar abs]precom-pression factor[ - ]axial position of last pressure measurement [mm] location of DDT as inferred from pipe deformation [mm]speed of precompressed zone as inferred from the two last pressure signals upstream of DDT location [m/s]distance between two last pressure sensors upstream of DDT location
The full width at half maximum of the pressure peaks corresponding to the compressed zone of unreacted gas ahead of the accelerating deflagrative flame front is of the order of 30 micros (see Figures 9, 10, 11) in the final stage of the run-up to detonation. This also holds for the other mixtures investigated in the past. If a final speed of 1150 m/s relative to the pipe wall is assumed, the extension of the compressed zone in axial direction is about 35 mm (34.5 mm = 1150 m/s * 30micros). Figure 12 provides a schematic sketch.
To allow the precompressed unreacted gas to autoignite before it is consumed by the extremely fast deflagrative flame front, its ignition delay time must drop to values of about 10 micros or less. This results from the fact that in the final stage of flame acceleration the deflagrative flame front propagates into the unreacted, heated mixture with speeds of the order of 1000 m/s, meaning that within 10 micros a 1 cm thick slice (in axial direction) of the shock front is converted into hot reaction products. If the thin slice of unreacted mixture still remaining ahead of the front would need longer than about 10 micros to autoignite a and thereby generating the DDT a it would be consumed in the next 10 micros in deflagrative manner and the DDT would not occur.
Figure 12: Schematic sketch of the pressure distribution in a long pipe when the pressure peak corresponding to the compressed unreacted mixture ahead of the deflagrative flame front has attained its maximum height, i.e. an instant just before DDT occurrence.