3D CFD Analysis of a Twin Screw Expander With Different Real Gas Models for R245fa

Abstract

With the increasing importance of minimizing primary energy usage and complying with emission restrictions, a significant interest has been developed towards waste heat recovery from industrial processes. A large portion of this energy is available at low temperatures (350K–400K) but it can be relatively efficiently converted into mechanical power using an Organic Rankine Cycle (ORC). Twin screw expanders can be used as an alternative to turbines with their cheap production costs and well proven efficiencies. In this paper, 3D CFD simulations of a twin screw expander using R245fa as the working fluid are performed. Since the fluid properties show big deviations when using the ideal gas equation of state (EoS), the flow problem has been evaluated using different real gas models. Thermodynamic parameters for the ideal gas EoS, the cubic Aungier Redlich-Kwong EoS and the CoolProp fluid database (open source) were compared in a preliminary study. After that, the models have been included through user-defined functions (UDFs) in ANSYS Fluent and were tested on 3D CFD calculations of a twin screw expander and a simplified expansion model. Several performance indicators such as mass flow rates, pressure-volume diagrams and power output are used to compare different fluid models for R245fa. From the results of this study, it can be concluded that the ideal gas EoS shows big deviations going closer to the saturation vapor line and the deviation in power comparing to the Aungier Redlich-Kwong EoS is around 8%. Conversely, the Aungier Redlich-Kwong EoS and the CoolProp database present very similar results for this case.

Nonlinear Processes in Safety Systems for Substances with Parameters Close to a Critical State

“…Nevertheless, the most commonly used are equations of state for real gas, such as the Van der Waals, Redlich -Kwong, Peng -Robinson, Kamerlingh Onnes, Martin Howe equations, etc. [2,3,17,20,23,30]. The best-known equation of state for real gas is the Van der Waals equation, which takes into account intermolecular interactions and the real (nonzero) intrinsic volume of molecules.…”

“…Based on the digital solution to a system of Navier -Stokes equations with the closure of the system by the Van der Waals equation of state, theoretical studies of flows and heat transfer in near-critical media under space flight conditions are carried out in [21]. A modified Van der Waals gas model is used to analyze the mass flow rate and opening parameters of safety valves at operating pressures up to 3,600 bar [2], and to study flows in engineering devices used to implement the organic Rankine cycle [20].…”