Optical procedures for the measurement of flow velocities do not measure directly the velocity of the flowing medium, but the velocity of optically detectable tracer particles, which must be present in the flow.
The follow-up behaviour of the tracer particles in the flow as well as the light-scattering characteristics of these particles are of greatest importance. Thus, choosing a suitable particle material and a suitable particle generator is essential for the successful solution of a measuring task. Too little attention is usually given to this fact, although the financial expenditure for the generation of tracer particles is considerably lower than e.g. a complete LDV system with cross-beam equipment.
For already many years, the Palas® aerosol generators are used also for these applications worldwide with great success.
Necessary characteristics of tracer particles in gases:
- good following behaviour, depending on the particle diameter, form and density
- good scattering behaviour, depending on the particle diameter, form and refractive index. The laser power, the wave length of the laser as well as the optical setup, e.g. forward or backward scattering are to be considered.
- determined particle size distribution and concentration
- particles, e.g. with flame measurements, must be inert
- physiologically harmless
- easy handling
- low maintenance
- high reliability
- long dosing time
- best reproducibility
- dosing against overpressure and in hot gases
- long lifetime
- reduces your operating expenses
We offer aerosol generators for dispersion and dosage of:
- powders and materials
- pure liquids
also against pressures up to 10 bar and high temperatures up to approx. 1000°C as e.g. in engines.
The Palas® aerosol generators produce aerosols in different concentrations and different particle size distributions as e.g. monodisperse, quasimonodisperse or polydisperse.
Important parameters for tracer particles are:
- the diameter
- the refractive index
- the material
- the material density.
The particle diameter must not change over the measurement time from measurement place to measurement place, e.g. by vaporisation. Big particles are quite easy to be detected, but the following behaviour must be considered.
The following behaviour of monodisperse DEHS droplets of 2 and 4 µm size is being visualized in the figures 1 and 2 by optical sectioning at a delta wing. Just the particle diameter changed, not the flow. The turbulence is created by the leading edge of the delta wing. The smaller the particle, the smaller the turbulence core. By well-directed tracer particle feeding at the edge of the delta wing and with particles of dp ≈ 0,6 µm, particles can also be brought into the turbulence core.
Fig. 1: dp = 4 µm Fig. 2: dp = 2 µm