BSA Flow Software Installation and User's Guide - CSI

Note An important point is that, in order to maximize the accuracy of the concentration
measurement, the main flow direction should be parallel to the X axis.
7.2.2 The concept of the DualPDA
Although phase Doppler anemometry is a well established technique for
measuring size and velocity of spherical particles, there exist some principle
difficulties with the measurement of large particles, in particular when the
system is set up to detect refractively scattered light, as is usually the case in
spray investigations. Large in this connection means that the particle
diameter exceeds about one third of the diameter of the measurement
volume. In such cases erroneous size measurements may lead to a
significant reduction of accuracy in estimating volume weighted flux and
concentration, since the volume is proportional to the third power of
diameter.
Two effects have been identified as potentially leading to incorrect size
measurements in such applications:
• The trajectory effect (or Gaussian beam effect) and
• The slit effect.
Both effects arise when the PDA system is set up to receive refractively
scattered light, as is typical for applications to sprays, but instead receives
reflectively scattered light. Those signals are then also processed with the
phase-diameter relationship based on refraction and thus lead to incorrect
size measurements.
Note The use of three detectors will not ensure that these effects are suppressed.
Furthermore, it is not possible to generalize the magnitude or sense of the error,
i.e. it may be positive or negative, depending on the exact operating conditions
and droplet diameter.
Since the droplet diameter enters the mass flux computation to the third
power, erroneous size measurements may seriously affect the accuracy of the
mass flux computation, even if the size of only very few droplets is
measured incorrectly.
In a number of applications the performance and the accuracy of the
conventional PDA systems (ClassicPDA and FiberPDA) are therefore not
satisfying. This is particularily the case for spray investigations. As already
mentioned, the physical reasons for the shift between refractively and
reflectively scattered light in such applications are the trajectory effect and
the slit effect.
7-30 BSA Flow Software: Reference guide