Computational Study of a High-Expansion Ratio Radial Organic Rankine Cycle Turbine Stator

Abstract

There is a growing interest in organic Rankine cycle (ORC) turbogenerators because they are suitable as sustainable energy converters. ORC turbogenerators can efficiently utilize external heat sources at low to medium temperature in the small to medium power range. ORC turbines typically operate at very high pressure ratio and expand the organic working fluid in the dense-gas thermodynamic region, thus requiring computational fluid dynamics (CFD) solvers coupled with accurate thermodynamic models for their performance assessment and design. This article presents a comparative numerical study on the simulated flow field generated by a stator nozzle of an existing high-expansion ratio radial ORC turbine with toluene as working fluid. The analysis covers the influence on the simulated flow fields of the real-gas flow solvers: FLUENT, FINFLO, and ZFLOW, of two turbulence models and of two accurate thermodynamic models of the fluid. The results show that FLUENT is by far the most dissipative flow solver, resulting in large differences in all flow quantities and appreciably lower predictions of the isentropic nozzle efficiency. If the combination of the k−ω turbulence model and FINFLO solver is adopted, a shock-induced separation bubble appears in the calculated results. The bubble affects, in particular, the variation in the flow velocity and angle along the stator outlet. The accurate thermodynamic models by Lemmon and Span (2006, “Short Fundamental Equations of State for 20 Industrial Fluids,” J. Chem. Eng. Data, 51(3), pp. 785–850) and Goodwin (1989, “Toluene Thermophysical Properties From 178 to 800 K at Pressures to 1000 Bar,” J. Phys. Chem. Ref. Data, 18(4), pp. 1565–1636) lead to small differences in the flow field, especially if compared with the large deviations that would be present if the flow were simulated based on the ideal gas law. However, the older and less accurate thermodynamic model by Goodwin does differ significantly from the more accurate Lemmon–Span thermodynamic model in its prediction of the specific enthalpy difference, which leads to a considerably different value for the specific work and stator isentropic efficiency. The above differences point to a need for experimental validation of flow solvers in real-gas conditions, if CFD tools are to be applied for performance improvements of high-expansion ratio turbines operating partly in the real-gas regime.

A New Criterion to Optimize ORC Design Performance using Efficiency Correlations for Axial and Radial Turbines

“…A radial turbine stage can deliver a greater specific power than an equivalent axial stage, and this may imply smaller and/or fewer stages in a machine (see [14]). The enthalpy drop in the expansion processes of Organic Rankine Cycles is lower than the steam enthalpy drop over the same temperature interval owing to the use of heavy substances as working fluids [18]. Thus, in many low temperature applications the work transfer can be simply achieved by using an axial stage [19].…”

Similarity Theory Based Radial Turbine Performance and Loss Mechanism Comparison between R245fa and Air for Heavy-Duty Diesel Engine Organic Rankine Cycles

“…As the organic fluid properties are totally different from air, these researches are mainly divided into two technical routes. One route focuses on forward design, including preliminary design of geometry parameters and aerodynamic design of blade profiles [12][13][14][15][16][17][18][19][20][21]. Dolz et al [13] propose that the pre-design of turbomachinery must take real gas equations of state into consideration, because the specific energy deviation between real gas and perfect gas can be as large as 100%, which may lead to total wrong turbine preliminary design result.…”

“…After three times of optimization, the best turbine produced 45.6 kW at 56.1% efficiency. Colonna et al [16,17] made the fluid-dynamic design of ORC turbine using CFD tools. The authors numerically investigated the real gas effects occurring in the supersonic ORC stator nozzles.…”

Quasi-1D Compressible Flow of Hydrocarbon Fuel

“…due to its moderate critical pressure and temperature, and low sound speed as Figure 1 shows. 3 It is easy to accelerate supercritical hydrocarbon fuel to supersonic in near-critical region. Another benefits of supercritical hydrocarbon fuel is that they are dry fluid if we regard it as working fluid of an incomplete supercritical Rankine cycle, as Figure 7(a) shows, which means it will not condense during isentropic expansion and avoid some problems associated with two-phase flow.…”