Influence of the fluid-dynamic properties of organic fluids on pump performance

In the industrial sector, medium, low and ultra-low temperature waste heat represents a significant source of energy loss as well as constitutes a harmful environmental effect which must be avoided. In this context, the Organic Rankine Cycle (ORC) technology is a proven solution because, being the working fluid an organic substance with low boiling temperature, it is better than water when low grade heat needs to be recovered. However, the recovery process presents several challenges when dealing with low and ultra-low temperature (<150°C) heat sources and numerous studies have been already carried out with the aim of, from the one hand, improving the ORC performance from a global point of view and, on the other hand, focusing on the performance of key components like heat exchangers and turbines. Only few works focused on ORC pumps performance because it is considered an established component. The result is that ORC models available in literature completely disregarded the influence of different properties (i.e. density, viscosity, …) of the considered working fluid on the pump performance by assuming a constant value in the range from 65% to 85%. This simplified approach to the pump models generally brings, on one side, to an overestimation of the achievable ORC efficiency and, on the other side, to an unfair comparison between different working fluids behavior. This paper presents an in-depth analysis of the influence of the organic substances fluid-dynamic properties on the pump performance. The performance of a multi-stage centrifugal pump, designed to serve with water but suitable for ORC applications, is experimentally and numerically investigated by means of the commercial CFD code, Ansys CFX. Flow fields and performance of the pump operating with eight organic fluids typically used in ORC applications (R134a, R141b, R245fa, R152a, R142b, Acetone, Benzene and Toluene) are investigated by properly combining the CFD code with CoolProp. All the fluids are assumed to be sucked by the pump at a condensing temperature of 30°C in pure liquid condition. A negligible heat transfer from the machine to the environment is considered during the numerical simulation. Besides the expected density factor, which modifies the best efficiency point in terms of design mass flow rate and head, the comparison of the pump dimensionless performance, within the frame work of the similarity laws, highlighted a clear influence of the Reynolds number that is greatly affected by the different fluid properties (density and viscosity). Differences in efficiency of approximately 2%, being the flow coefficient equal, are detected due to different viscous losses. This finding influences the ORC system efficiency, stuck around 10-12% in case of low-grade waste heat recovery applications. Possible developments of prediction methods as well as of new design strategies, based on the different working fluid properties and, hence, on the expected flow regime, will be foreseen.