A comprehensive assessment on detached eddy simulation and delayed detached eddy simulation approaches in large-scale rotating separated flow in a centrifugal impeller under small flow rate

The occurrence and development of centrifugal pump rotating stall is closely related to disturbance in the incoming flow. The seal clearance leakage is an inherent disturbance in the impeller incoming flow of centrifugal pumps. In order to investigate the seal clearance leakage influence on the rotating stall characteristics, the flow field numerical simulation of a centrifugal pump under rotating stall flow conditions with and without seal clearance are carried out. It is found that under the 0.4Q0 rotating stall condition with seal clearance leakage, the “λ” special shaped high-velocity region distribution along the blade leading edges are strengthened, which lead to larger scale of stall vortices. And the periodicity of the separation vortex evolution is more obvious under the condition with leakage. For a specific stall vortex development process, at the initial format stage, there is a notable disturbance characteristic of medium frequencies band that of 2–3 times the rotation frequency in the pressure fluctuation near the blade leading edges, which is significantly different from that under the condition without leakage. The vorticity analysis shows that this disturbance is closely related to the leakage induced vortices. Affected by the seal clearance leakage disturbance, the stall rotating propagation frequency is reduced from 0.143 times the impeller rotational frequency to 0.103 times. Meanwhile, the fluctuation intensity in the impeller channels is greatly enhanced. Therefore, by judging the special fluctuation frequencies, regularity, and intensity trend in the monitoring signal, it can not only be used to guide the safe operation of centrifugal pumps under off-design conditions but also serve as the basis for judging the wear of the seal, which has important practical significance.

Investigation on the influence of seal clearance leakage on the rotating stall characteristics for a centrifugal pump

Tang,

Wang,

Wang

et al. 2024

Numerical study on the influence of turbine swirl on exhaust system

Hou,

Wang,

et al. 2024

This article employed numerical methods to investigate the aerodynamic, thermal, and infrared characteristics of last-stage turbine swirl on a two-dimensional convergent divergent (2D-CD) exhaust system with an afterburner under varying bypass ratios. The research results indicate that the swirl diminishes the thrust coefficient and flow coefficient of the 2D-CD exhaust system with an afterburner. At a bypass ratio of 0.2, the thrust coefficient and flow coefficient decrease by 0.65% and 1.07%, respectively. When the bypass ratio is relatively small, the swirl flow leads to a decrease in the temperature of the afterburner heat shield. Conversely, when the bypass ratio is relatively large, the opposite occurs. The maximum temperature of the heat shield increases by up to 7.2% (bypass ratio = 0.35), while the average temperature decreases by up to 7.1% (bypass ratio = 0.2). The swirl causes an increase in the temperature of the divergent section heat shield, with the most significant deterioration observed at a bypass ratio of 0.25, resulting in a maximum temperature increase in 12.2%. Swirling flow shortens the length of the jet flow, and as the bypass ratio reduces, this attenuation effect becomes more pronounced. When the bypass ratio is 0.2, the length of the core area decreases by 40.3%, and the infrared intensity of the narrow-side jet flow decreases by 12.5%. Overall, on the XOY detection plane, the maximum decrease in infrared intensity is 11.5%, and the maximum increase is 11.7%. On the XOZ detection plane, the maximum decrease in infrared intensity is 15.9%, and the maximum increase is 5.7%.

Spatiotemporal characteristics of flow in the bend and return channel perturbed by rotor–stator interaction

Zhang,

Gu,

Zhang

et al. 2024

The flows in turbomachinery systems are strongly perturbed by the rotor–stator interaction (RSI) between the rotating and stationary through-flow components, generating the fluctuating flows. This work presents a numerical investigation of a flow in a test centrifugal pump system consisting of an impeller, diffuser, bend, and return channel at the designed and low flow rates. Both vaneless and vaned diffuser models are considered to determine the perturbation of the diffuser vanes on the flow. The objective is to quantitatively assess the impacts of two influential factors, i.e., the diffuser type and the flow rate, on the spatiotemporal characteristics of the transient flow in the various through-flow components. Particular attention is paid to the generation of the reversed flow, circulating vortices, and temporal fluctuations of the velocity field. It is found from the numerical data that both the flow rate and the diffuser type affect the temporal fluctuations of the flow, primarily via the impact of the RSI-induced large-scale vortices generated in the passages of the impeller and the vaned diffuser. The flow in the bend experiences the streamline curvature on the solid walls; thus, the peak values of the fluctuating amplitude of the streamwise and transverse velocities and the reversed flow may shift between the two halves of the channel close to the inner and outer wall. The boundary layer flow patterns are mainly determined by the flow rate at the low flow rate and by the diffuser vanes at the designed flow rate. This work comprehensively reveals the transient behaviors and unsteadiness of the flow which was not provided in the experimental investigation for the test centrifugal pump system which was widely studied since the essential flow physics are included. The numerical data in the stationary bend and return channel are first provided, which are of engineering significance for this type of centrifugal turbomachinery.