Chemistry Reference
In-Depth Information
Direct contact of the transducer with the fluid in the pipe ('wetted')
was described by several authors (Shekarriz and Sheen, 1998; Choi
et al.
,
2002; Ouriev and Windhab, 2003). Mounting the transducer in flush with
the pipe wall makes it usually impossible to have a realistic measurement
in the wall region, as the ringing of the transducer from the pulse emis-
sion is superimposed on the first received echo. Depending on the design
of the electronics, the receiving part is also oversteered for some time
after sending a pulse. In addition, as mentioned by Hoeks
et al.
(1991),
measurements inside the near field are affected by an inhomogeneous
pressure distribution and a variation in the Doppler angle (Bascom
et al.
,
1986). If the transducer is pulled away from the flow to have the focal
point of the pressure field in the region of the pipe wall, a cavity is
formed which can accumulate air bubbles or sediments. This method
has additional disadvantages if the fluid is highly attenuating, as the
available penetration depth is reduced accordingly. Improved set-ups
were shown by Lemmin and Rolland (1997) and Wiklund and Stading
(2006) involving a film in front of the cavity to minimise influences on
the pipe flow streamlines. In this case it is also possible to fill the cavity
between the transducer and the film with ultrasound gel or another fluid
matched acoustically to the fluid in the pipe to minimise the refraction
and Doppler angle change at the interface between the two fluids. It
is important to avoid the inclusion of air bubbles in the liquid in front
of the transducer, as this would lead to unreproducible artefacts in the
measured profile.
It is also possible to measure through the pipe wall, especially for
materials such as Plexiglas (Yamanaka
et al.
, 2002). The effects of a
layer of Plexiglas or polyethylene on the ultrasound beam as a function
of the incidence angle for longitudinal and shear waves were investigated
in detail, theoretically and experimentally, by Thompson
et al.
(2000).
The influence of the curvature of the pipe on the pressure field was
investigated by Thompson and Aldis (1996) and Tortoli
et al.
(1999).
Regarding flow adapter material selection, Nowak (2002) and Hung
and Goldstein (1983) are of interest as they give sound velocities and
densities for various relevant materials (e.g. nylon, PEHD, PET (Mylar),
PMMA (Plexiglas), POM (Delrin), PVC and Teflon).
Steel pipes (Kishiro
et al.
, 2004; Wada
et al.
, 2004) are quite diffi-
cult to penetrate; many authors used, for example, a wedge transducer
(Tezuka
et al.
, 2006) to improve the measurements.
3.3.1
Doppler angle
Two different definitions of the Doppler angle are used in the literature.
In the medical field, it is common to use the angle between flow direc-
tion and ultrasound direction, as indicated in Fig. 3.3. This is also the
