Page 177 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Measurement techniques for liquid metal based nuclear coolants 149

small time-shift of the signal structure between two consecutive bursts. The velocity is
obtained from a correlation analysis between consecutive bursts. The selection of the
ultrasound frequency depends on the desired measurement depth and the maximum
expected velocity. Typically, frequencies between 1 and 8MHz are used. An impor-
tant issue is the acoustic coupling between the transducer and the liquid, determined
by the wetting conditions at the tip of the sensor. Additionally, a balanced concentra-
tion of scattering particles has to be provided to obtain reliable velocity information
from the fluid. On the one hand, a very high concentration attenuates the signal in the
front region to such an extent that the acoustic waves cannot propagate into larger
measurement depths. On the other hand, a lack of scattering particles in certain mea-
surement depths impedes to determine the flow velocity correspondingly.
The ultrasound Doppler technique was applied first in medical applications before
this method was established in physics and fluids engineering by the pioneering work
of Takeda (1986, 1991). The feasibility of velocity profile measurements in liquid
metals using UDV was demonstrated also by Takeda (1987), who measured velocity
profiles in mercury at room temperature. Later, Brito et al. (2001) presented UDV
measurements in liquid gallium. A successful application in a “hot” liquid metal
was published by Eckert and Gerbeth (2002) for liquid sodium at a temperature of
about 150°C. An improved high-temperature UDV transducer and a special mounting
according to Fig. 3.5.2 enable to measure flows up to 230°C. Fig. 3.5.3 shows a result
of a UDV measurement at an LBE pipe flow conducted at the COMPLOT facility at
SCK CEN. Similar measurements were conducted at the SnBi loop of the
l
LIMMCAST facility at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). It could
be proved that the flow profile in a pipe of a liquid metal can be reliably measured;
however, in general, the acoustic signal energy is quite low due to the necessity of an
acoustic access to the flow through a stainless steel wall, and the acoustic transmission
path has to be arranged carefully.
For temperatures higher than 230°C, waveguides have to be used (Eckert et al.,
2003). In this concept, the acoustic energy propagates inside a construction compris-
ing a coiled, thin foil of stainless steel. A sufficient length of the waveguide protects
the piezoceramics from temperatures above its Curie point. It could be demonstrated
that this technique can be used for melts up to 600°C.
New developments regarding UDV use several linear ultrasound arrays, which
enables the measurements of two components of the flow in a two-dimensional mea-
suring plane with a high spatial and temporal resolution (Franke et al., 2013).
A congeneric technique to UDV based on the pulse wave method simply evaluating
the pulse transmission time from an acoustic reflector as a bubble is called UTTT, for
example, allowing for determining the position and diameter of bubbles in a two-
phase flow (Andruszkiewicz et al., 2013). Typically, ultrasound frequencies around
10–15MHz are used to achieve a high spatial resolution. In order to infer the diameter
of the bubble, two ultrasound sensors are mounted on opposite sides of the wall of the
container, which measure the distance of the interface of the bubble from both sides.
Fig. 3.5.4 shows a typical sensor arrangement. By utilizing multiple pairs of sensors,
the trajectory and the diameter of the bubble can be detected. In order to validate this
technique, single argon gas bubbles in water were measured by UTTT and recorded