Investigation on disk friction loss and leakage effect on performance in a Francis model turbine

Abstract

In this article, computational fluid dynamics technique has been applied to examine the disk friction loss and leakage effect on performance of a Francis model turbine, and a model test for obtaining hydraulic efficiency of the turbine was also conducted. The analysis of the results shows that near the optimal operating point, computational fluid dynamics can predict accurately the turbine hydraulic efficiency compared with experimental value, validating the applied computational fluid dynamics method. The disk friction loss of the runner is observed to increase with the increment of unit speed, and the band surface of the runner produces more disk friction losses than the runner crown surface. Furthermore, the disk friction loss from the inner surfaces of the band and crown is basically equivalent to the one from the outer surfaces. Doubling the clearance size reduces the total friction loss but decreases the turbine efficiency as well. It is also observed that the pressure field on the runner blade suction side is more sensitive to the clearance size than on the pressure side. Moreover, the leakage flow from the clearance near the runner band deteriorates the inflow condition for the downstream draft tube and the flow pattern inside resultantly. It is also observed that the phenomenon of rotor-stator interaction has very limited effects on both the disk friction loss and leakage loss.

Flow Inside the Sidewall Gaps of Hydraulic Machines: A Review

“…Čelič and Ondračka [63] quantified the difference between CFD taking into consideration sidewall gaps in the case of Francis turbine and reported an acceptable 3% difference for BEP; however, in off-design conditions the discrepancies rise dramatically. According to Feng et al [66], the error due to neglecting disk friction in the CFD model of Francis turbine is about 10%. Losses related to sidewall gaps flow are the most pronounced while operating at low specific speed as demonstrates Figure 11.…”

“…Feng et al [66] observed that even the leakage through the crown seal gap propagates downstream in a way that can influence the flow in the draft tube. The meridional velocity near the crown is reduced due to the presence of throughflow and, as a consequence, the velocity gradient along with the blade height increases.…”

“…However, it has to be noted that the influence of axial clearance in a turbulent regime is insignificant when compared to the other effects, e.g., surface roughness, affecting the value of resulting c m much more, as documented in Figure 17. Feng et al [66] investigated the effect of axial clearance on disk friction of Francis turbine. It was found out that an increase in the sidewall gap clearance brings about a decrease in the disk friction losses.…”

Numerical analysis of transient characteristics of a bulb hydraulic turbine during runaway transient process

“…In recent decades, computational fluid dynamics (CFD) technology has developed greatly, which has been widely applied in internal flow simulations for hydraulic turbines. 7,8 Three-dimensional CFD method was introduced by some researchers to study transient processes in hydraulic turbines. Liu et al.…”

“…In recent decades, computational fluid dynamics (CFD) technology has developed greatly, which has been widely applied in internal flow simulations for hydraulic turbines. 7,8 Three-dimensional CFD method was introduced by some researchers to study transient processes in hydraulic turbines. Liu et al 9 utilized 3D simulations with dynamic mesh method for a pump-turbine to study the transient process of load rejection, determining that the pressure fluctuations increased during the pump braking process.…”