Influence of Zonal Surface Roughness on the Pump Characteristics of a Turbopump

Authors

Benjamin Torner  - Institute of Turbomachinery, University of Rostock, Germany
Frank-Hendrik Wurm - Institute of Turbomachinery, University of Rostock, Germany

Abstract

The consideration of rough surfaces in flow simulations of turbopumps can be an important factor in adequately computing the pump characteristics (pressure heads, efficiencies). This is especially the case with radial turbopumps of low specific speeds, where dominant roughness effects can be present in the long, narrow blade channels and side chambers of the pump. These roughness effects must be adequately accounted for in the flow simulations. For this purpose, a roughness modeling is mostly used in the common flow solvers, for which an equivalent sand roughness has to be defined at the specific rough surface. This study investigates the roughness effect on the pump characteristics of a radial turbopump with different zonal roughnesses by a simulative approach. For this purpose, pump simulations are carried out in which appropriate equivalent sand roughnesses are defined on the respective rough walls in the pump. Then, the extent to which the different roughness regions have an influence on the pump characteristics is investigated in detail. To carry out this procedure, it was first necessary to determine the equivalent sand roughnesses at various locations of the pump. For this purpose, the surface roughness at various walls (impeller, volute, front side chamber, rear side chamber) were scanned and digitized. Subsequently, the specific equivalent sand roughness of the scanned surfaces could be determined using simple rough channel flow simulations and a self-developed simulation method, which takes the roughness effect into account. Afterwards, pump simulations were performed, with roughness modelling applied to the volute and the front side chamber region at first. These simulations were successfully validated by experiments. For this purpose, plastic films, in which the roughness surface was engraved, were glued to the corresponding walls in the experimental pump, whereby the same roughness effect were present in simulation and experiment. The pump simulations with roughness consideration at the volute and the front side chamber showed a significant reduction of head and efficiency compared to pump simulations with hydraulically smooth walls. Subsequently, additional areas were defined as rough walls in the pump simulation, and the pump characteristics were then determined. Interestingly, the additional roughness effects led to a further reduction of pump's efficiency in each case, while the head did not necessarily decrease. In conclusion, the study shows the importance of numerical consideration of rough walls in flow computations for turbopumps. Significant discrepancies in pump characteristics can exist between experiments on a manufactured pump and flow simulations of a computer-designed pump if these roughnesses are not accounted for in the simulations.