atw 2017-07

atw Vol. 62 (2017) | Issue 7 ı July
474
AMNT 2017 ı COMPETENCE PRIZE
Competence
Prize
WINNER
Thomas Schäfer
is the winner of the
Competence Prize,
48 th Annual Meeting
on Nuclear Technology
(AMNT 2017).
Ultrafast X-ray Tomography for Twophase
Flow Analysis in Centrifugal Pumps
Thomas Schäfer and Uwe Hampel
Motivation Centrifugal pumps are main components of the emergency core cooling systems (ECCS) of nuclear
power stations with light water reactors. During a loss-of-coolant accident (LOCA), they must continuously convey the
coolant, to steadily discharge all decay heat that is produced inside the core. To guarantee a sufficient cooling capacity,
additional coolant is taken from large reservoirs, for example, the condensation chambers or the reactor sump. Here,
the coolant is in contact with air, on its free surface, where gas entrainment may occur due to hollow vortex formation.
The vortex formation is initiated by small surface vortices, which are always present in such reservoirs, and can lead to
large developed gas entraining hollow vortices. This process is depending on the suction rate of the coolant, the critical
submergence of the intake and also on its geometry, in addition to the fluid properties [1, 2]. The gas entrainment into
the coolant, results in a gas-liquid two-phase flow, which may enter the intake and even the centrifugal pump. This
leads to unexpected operation states or even damages, since coolant pumps are usually designed for single phase liquid
flow. Furthermore, the efficiency of the ECCS coolant loops strongly depends on the performance curves of the operating
pumps, which changes drastically under two-phase flow conditions [3 to 6].
To distinguish and handle such
critical situations, it is necessary
to investigate and understand the
hydro dynamics of the two-phase flow
inside the pump. Furthermore, it is
required to find out simplified model
approaches, depending on general
and easy measureable properties of
the present inlet flow conditions and
given pump geometries and characteristics
as well.
In the past, several experimental
and numerical studies have been performed,
to investigate the operating
behavior of centrifugal pumps under
various operating conditions. Some
observations on centrifugal pumps
under two phase flow conditions have
been reported [6 to 11]. Here, the pump
performance of a full size nuclear
reactor pump under two-phase flow
conditions has been studied [6]. Also,
in comparison to experimental results,
the gas void fraction, pressure and
velocity in the impeller of a centrifugal
pump were numerically calculated,
applying Reynolds-averaged Navier-
Stokes equa tions [7, 8], or consequences
of two phase flow due to
cavitation were identified [9 to 11].
Another numerical study was focused
on the influence of bubble diameter
and void fraction of entrained gas on
the pump operation [12]. Furthermore,
a few experimental investigations have
been done on the behavior of single or
multiple bubbles in centrifugal pumps
[13 to 15]. Recently, the gas accumulation
inside a closed impeller of an
industrial centrifugal pump under
various gas entrainment conditions
has been quantified, and the corresponding
gas holdup areas have
been visualized, using high-resolution
gamma- ray computed tomography
[16, 17]. Nevertheless, there is a leak
of knowledge about the non-steady
behavior of the two-phase flow inside
the impeller. To overcome this, ultrafast
X-ray computed tomography [18]
was used in this study. Ultrafast X-ray
computed tomography was already
successfully applied on two-phase
flows in opaque objects like heated
rod bundles [19], structured packings
[20], helical static mixers [21] and
monoliths [22]. The application of
ultrafast X-ray computed tomography
provides the opportunity to measure
the gas-liquid phase distribution in a
closed radial multi vane impeller with
high temporal and spatial resolution
and under gas entrainment conditions
with high gas fractions.
Materials and methods
The experimental setup is shown in
Figure 1. It consists of a flow loop
| | Fig. 1.
Schematic view of the experimental setup.
wherein a gas-liquid two-phase flow is
conveyed by a centrifugal pump. The
impeller of this pump is the core of the
facility and the object under investigation.
The material of the impeller must
be suitable since ultrafast X-ray tomography
was chosen to investigate the
two-phase flow inside the impeller.
Thus, the impeller was made from
polyamide and was manufactured by
rapid prototyping. The geometry of
the impeller is based on the impeller
of an industrial centrifugal pump
(Etachrom BC 032-160/074 C11,
KSB). Important geometric data can
be found in Table 1.
An adjustable gas-liquid two-phase
flow is generated upstream the
impeller in a gas-liquid mixing
module. Here, liquid (tap water) and
gas (de-oiled pressurized air) are
mixed. The liquid comes from a 170 l
AMNT 2017
Ultrafast X-ray Tomography for Two-phase Flow Analysis in Centrifugal Pumps ı Thomas Schäfer and Uwe Hampel