Seeding of high temperature air ﬂow
D. Rusch Æ A. Kempe Æ T. Roesgen
Received: 7 April 2006 / Revised: 21 February 2007 / Accepted: 26 February 2007 / Published online: 20 March 2007
Abstract Seeding of high temperature air ﬂows is prob-
lematic because common seeding materials can ignite and
may pose a threat for human health in case of inhalation.
Sodium chloride is proposed as a seeding material to avoid
these problems. The generation of crystals using an ultra-
sonic atomizer is described and the measured particle size
distribution is presented. The described method has the
additional advantage that the particle size can easily be
adjusted. This allows the operator to comply with the
experimental requirements in terms of resolvable frequen-
cies. The described method has been successfully applied
to LDA measurements in a 500°C air ﬂow.
In LDA measurements, particles have to be introduced into
the ﬂow in order to have light-scattering objects. The au-
thors faced the speciﬁc problem to seed a hot-jet-in-
crossﬂow conﬁguration where the maximum temperature is
about 500°C. In addition, the tunnel is open to the envi-
ronment and a volume ﬂow rate of up to about 1 m
to be continuously seeded for several hours. The descrip-
tion of the experimental setup can be found in Menter et al.
(2006). For measurements in air ﬂows, mainly oil droplets
produced by atomizers or solid hydrophobic powders, such
as metal oxides, are used. For high temperature applica-
tions, such as in air ﬂows with temperatures around 500°C,
oil droplets cannot be used since the temperature is higher
than both the ﬂash point and the auto-ignition temperature.
Thus, such oil droplets would evaporate quickly and even
start to burn spontaneously.
The ﬂash point and auto-ignition temperature at ambient
pressure for different oils are given in Table 1. The ﬂash
point temperature is deﬁned as the minimum temperature at
which the vapor pressure of a liquid is sufﬁcient to form an
ignitable mixture with air near the surface of the liquid.
The auto-ignition temperature in contrast is the minimum
temperature required for self-sustained combustion in the
absence of an external ignition source.
Metal oxide seeding particles can be used for tempera-
tures up to more than 2,000°C according to Albrecht et al.
(2003). Since many experiments are performed in open
wind tunnel environments, such powders cannot be used
for health safety reasons: Roessler et al. (2001) discusses
the absorption rate of particles in the human body during
breathing. Figure 1 shows the total absorption rate and the
absorption rate within the lungs. It can be seen that the total
absorption rate for particles with a diameter >10 lmis
nearly 100% but only less than about 10% reach the lungs
as these particles are mainly absorbed in the throat and
nose region through sedimentation. Particles with a diam-
eter <10 lm, such as typical seeding particles, reach the
lungs and are ﬁnally absorbed by a diffusion process. The
smaller the particles the higher the absorption rate within
the lungs. Particles <0.1 lm do not only reach the lungs but
can also enter the bloodstream. The total absorption rate
shows a minimum at particle diameters of about 0.3 lm. A
large portion of particles of this size are not absorbed but
Dependent on particle concentration, shape, size and
chemical behavior, the human body reacts in different ways
D. Rusch Á A. Kempe Á T. Roesgen (&)
Institute of Fluid Dynamics, ETH Zurich,
8092 Zurich, Switzerland
Exp Fluids (2007) 42:761–765