## 1 Introduction
The prewhirl configuration, incorporating positive whirl angles, demonstrated a reduction in losses within the pump impeller. Furthermore, [PERSON] et al. demonstrated the positive impact of an optimised design of the IGV blades on the efficiency of the pump [11]. [PERSON] et al. showed that improvements in efficiency of 2% can be achieved by proper prewhirl control [12]. In addition to the design of the IGV, [PERSON] et al. (2012) demonstrated that its positioning in the suction line also plays an important role [13]. An empirical equation was derived to determine the optimal distance between the suction nozzle of the pump and the IGV. Furthermore, [PERSON] et al. pointed out the positive effect of IGV on the pump's characteristic curves when it is positioned at an optimal distance from the suction nozzle [14]. It was demonstrated that a sufficient distance between the IGV and the pump resulted in a 35% reduction in pressure pulsations at the leading edge of the impeller blade [14]. In addition to its impact on the head, the prewhirl exerts a significant influence on the power consumption and efficiency of pumps. Furthermore, it plays a pivotal role in the cavitation behaviour of pumps. This was investigated by [PERSON] et al. (2014) on a pump with a specific speed of \(n_{\text{q}}=35\) min\({}^{-1}\)[15]. The results demonstrated that the Net Positive Section Head (\(NPSH\)) of the pump exhibited only a slight influence from the prewhirl. [PERSON] also investigated the cavitation behaviour under prewhirl, but in comparison to the findings of [PERSON] (2014), he concluded that prewhirl has a more significant impact on the \(NPSH\) of the pumps [9]. The studies mentioned above have demonstrated that prewhirl is not solely a consequence of IGV control but can also arise from the suboptimal dimensioning of the pump's suction area. It became evident that this has an impact on the characteristic curves of the pumps. For this reason, several studies have already analysed the behaviour of the hydraulic losses when prewhirl occurs in the pump. [PERSON] et al. studied numerically the influence of prewhirl on the inlet flow at an impeller of an axial pump operating at different flow rates [16]. [PERSON] et al. demonstrated in their numerical investigations that the Best-Efficiency-Point (BEP) of the pump shifts depending on the prewhirl direction [17]. [PERSON] has analysed the behaviour of the characteristic curve under the influence of prewhirl [18]. He shows that the prewhirl direction affects the specific work. In a numerical investigation, [PERSON] et al. examined the impact of prewhirl on partial load recirculation and hydraulic losses in a radial centrifugal pump [19]. The reduction in the head characteristics associated with partial load recirculation was compensated by utilising an IGV. Furthermore, the occurrence of losses at the impeller inlet during partial load operation was diminished, thereby enhancing efficiency. Furthermore, [PERSON] et al. demonstrated that by utilising an IGV, the distributions of velocity and turbulent kinetic energy (TKE) within the impeller were equalized, thereby enhancing the smoothness of pump operation. [PERSON] et al. investigated numerically and experimentally the effects of prewhirl and specially profiled blades on the operating and cavitation behaviour of a centrifugal pump [20]. The results demonstrated that the magnitude of friction losses is contingent upon the orientation of the prewhirl angle. This has either a positive or negative effect on the hydraulic efficiency. [PERSON] et al. conducted an analysis of the flow and pressure pulsations on a mixed flow pump at varying prewhirl angles [21]. It was also demonstrated that the amplitudes of the pressure pulsations between the impeller and the guide vane could be diminished with an increase in positive prewhirl angles. Furthermore, [PERSON] et al. mentioned that the flow within the impeller becomes increasingly uniform at positive prewhirl angles, resulting in a reduction of secondary flows.