On the transient flow in the 20-liter explosion sphere

The turbulence level in the 20-liter explosion sphere, equipped with the PerforatedDispersion Ring, was measured by means of laser Doppleranemometry. The spatial homogeneity of the turbulence was investigated by performingvelocity measurements at various locations in the transient flow field. Directionalisotropy was investigated by measuring two independent components of the instantaneousvelocity. The transient turbulence level could be correlated by a decay law of the form
v_rms/v_rms^o = (t/t₀)ⁿ .
in which the exponent, n, assumes a constant value of 1.49 +- 0.02 in the periodbetween 60 ms and 200 ms after the start of the injection process. In this time intervalthe turbulence was also observed to be homogeneous and practically isotropic. The resultsof this investigation imply that the turbulence level in the 20-liter explosion sphereat the prescribed ignition delay time of ms is not equal to the turbulence level in the1 m³-vessel. Hence, these results call into question the widely held belief thatthe cube-root-law may be used to predict the severity of industrial dust explosionson the basis of dust explosion severities measured in laboratory test vessels.
keywords:cube-root-law, dust explosion, laser Doppler anemometry, turbulence decay.