Experimental and Numerical Investigation of the Precessing Helical Vortex in a Conical Diffuser, With Rotor–Stator Interaction

Abstract

The flow unsteadiness generated in a swirl apparatus is investigated experimentally and numerically. The swirl apparatus has two parts: a swirl generator and a test section. The swirl generator which includes two blade rows, one stationary and one rotating, is designed such that the emanating flow at free runner rotational speed resembles that of a Francis hydroturbine operated at partial discharge. The test section consists of a conical diffuser similar to the draft tube cone of a Francis turbine. Several swirling flow regimes are produced, and the laser Doppler anemometry (LDA) measurements are performed along three survey axes in the test section for different runner rotational speeds (400–920 rpm), with a constant flow rate, 30 l/s. The measured mean velocity components and its fluctuating parts are used to validate the results of unsteady numerical simulations, conducted using the foam-extend-3.0 CFD code. Furthermore, phase-averaged pressure measured at two positions in the draft tube is compared with those of numerical simulations. A dynamic mesh is used together with the sliding general grid interfaces (GGIs) to mimic the effect of the rotating runner. The delayed detached-eddy simulation method, conjugated with the Spalart–Allmaras turbulence model (DDES–SA), is applied to achieve a deep insight about the ability of this advanced modeling technique and the physics of the flow. The RNG k−ε model is also used to represent state-of-the-art of industrial turbulence modeling. Both models predict the mean velocity reasonably well while DDES–SA presents more realistic flow features at the highest and lowest rotational speeds. The highest level of turbulence occurs at the highest and lowest rotational speeds which DDES–SA is able to predict well in the conical diffuser. The special shape of the blade plays more prominent role at lower rotational speeds and creates coherent structures with opposite sign of vorticity. The vortex rope is captured by both turbulence models while DDES–SA presents more realistic one at higher rotational speeds.

Active flow control of the vortex rope and pressure pulsations in a swirl generator

“…If the runner rotational speed decreases from 920 via 700 down to 600 rpm, the swirl vanishes and the vortex rope disappears. If the rotational speed further decreases to 400 rpm, the swirl changes direction and the vortex breaks down (Javadi, Bosioc, Nilsson, Muntean, & Susan-Resiga, 2014;Javadi et al, 2016). At these two regimes, 920 and 400 rpm, the vortex rope is well developed and generates large pressure pulsations.…”

“…The flow in water turbines operating at the best efficiency point condition has a moderately low level of swirl forming a stable on-axis structure (Javadi, Bosioc, Nilsson, Muntean, & Susan-Resiga, 2016;Javadi & Nilsson, 2015a). The flow at conditions away from the best efficiency point often contains a high level of swirl which may cause a recirculation region.…”

“…The jets are injected in a direction opposite to that of the main swirl of the flow. The study is performed with the DDES-SA turbulence model and the results are compared with the results of Javadi and Nilsson (2015a), and Javadi et al (2016).…”

“…Javadi and Nilsson (2014a) studied the Scale-Adaptive Simulation (SAS) (Menter & Egorov, 2010) approach in highly swirling flows and reported that the approach predicts the massively separated swirling flows very well. Detached-Eddy Simulation (DES) (Spalart, Jou, Strelets, & Allmaras, 1997) is another promising hybrid URANS-LES strategy which is studied for application to swirling flows (Javadi & Nilsson, 2015a, 2015bJavadi et al, 2016). The Delayed DES (DDES) methodology was proposed by Spalart et al (2006) to identify the boundary layer thickness and to extend the URANS mode compared to that of the initially proposed DES methodology.…”

Vortex rope instabilities in a model of conical draft tube

“…Thus, we manage the runner by controlling the motor rotational speed. This swirl device was designed similarly the swirl generator designed at Politehnical University of Timisoara [7]. The main idea of using the simplified draft tube model is not a complete reproduction of whole hydro turbine unit but the reconstruction of velocity profile close to distribution under nature turbine runner.…”