Novel PDA-Systems

The dual-mode PDA therefore allows the measurement volume to be made much smaller without danger of compromising measurement accuracy. This in turn leads to the possibility of measuring in flows of higher densities of the dispersed phase, for example near injection spray nozzles. Due to the increased reliability of the particle diameter measurement and the availability of two velocity components, the dual-mode PDA results in improved estimations of the mass flux (Dullenkopf et al. 1996). 

The mass flux of the dispersed phase is in fact an essential measurement quantity in many experimental investigations. The accuracy of mass flux measurements will depend not only on the instrumentation, but also on the flow field and the size distribution of the dispersed phase, so that a general accuracy estimate is not feasible. In simple spray flows however, an accuracy of 10% on the local mass flux can be expected (Sommerfeld and Qiu 1995, Mundo 1996). 

lens 160mm Receiv. lens 160mm Mask 1 

One pre-requisite for such estimates is that all measured particles are spherical. This may be obtainable in modeled flows with selected particles, but is certainly not the rule in practical situations. The instrumentation problem is therefore two-fold. If the PDA system can detect non-sphericity, as indicated above using a three-detector receiver, then as a minimum the mass contained in all rejected non-spherical particles will be missed. If on the other hand, many non-spherical particles are in fact accepted as spherical particles, their computed size may differ from the volume equivalent diameter of a spherical droplet, thus also falsifying the measured mass flux. 

Presently most commercial PDA systems assume that the non-sphericity validation is reliable and the measured mass flux is adjusted according to the 

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