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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

atw Vol. 62 (2017) | Issue 7 ı July impeller inlet diameter d 1 54 mm impeller outlet diameter d 2 136 mm number of blades 6 impeller inlet blade angle β 1 18,2 ° impeller outlet blade angle β 2 26,8 ° | | Tab. 1. Schematic view of the experimental setup. reservoir, which also acts as a gasliquid separator. The temperature of the liquid was measured using a PT100 temperature sensor. It amounts 21 ˚C and was constant during the experiments. The liquid flow rate is measured upstream the mixing module by a magnetic inductive liquid flow meter (MAG 1100, Siemens). The injected gas volume fraction ε in (0≤ ε in