The use of mixtures as working fluids of organic Rankine cycle (ORC) waste heat recovery (WHR) power plants has been proposed in the past to improve the matching between the temperature profile of the hot and the cold streams of condensers and evaporators, thus to possibly increase the energy conversion efficiency of the system. The goal of this study is to assess the benefits in terms of efficiency, environmental (GWP) and operational safety (flammability) that can be obtained by selecting optimal binary mixtures as working fluids of air-cooled ORC bottoming power plants of medium-capacity industrial gas turbines. Furthermore, two thermodynamic cycle configurations are analyzed, namely the simple recuperated cycle and the so-called split-cycle configurations. The benchmark case is a combined cycle power plant formed by an industrial gas turbine and an air-cooled recuperated ORC power unit with cyclopentane as the working fluid. The results of this study indicate that binary mixtures provide the designer with a wider choice of optimal working fluids, however, in the case of the recuperated-cycle configuration, no improvement in terms of combined cycle efficiency over the benchmark case can be achieved. The split-cycle configuration leads to an increase of combined cycle efficiency of the order of 1.5%, both in case of pure and blended working fluids. Furthermore, for this cycle configuration the use of Novec 649 as working fluid is advantageous because it is environmentally and operationally safe, and it does not involve any penalty in terms of combined cycle efficiency if compared to the benchmark case. Additionally, the use of this fluid would lead to a more compact turbine, as the corresponding thermodynamic cycle would determine a turbine volume flow ratio that is half of the value of the benchmark case and a specific enthalpy difference over the expansion that is one fifth.

This work proposes and assesses two numerical models for solving high-speed condensing flows in metastable conditions. Each model involves a set of governing equations (mass, momentum, and energy) for the mixture or the continuum phase, i.e. the vapor, and two additional transport equations to characterize the dispersed phase. Such relations are formulated through the so-called method of moments that allows to represent the wetness fraction and the number of droplets of the liquid. The transport relations are discretized in space by means of a new coupled up-wind scheme. A segregated implicit time integration strategy is exploited to hasten the convergence of the full system to steady-state. The performance and accuracy of both models are thoroughly investigated on a reference quasi-1D problem and confronted against experimental data and more advanced two-phase flow models. Results show that experimental observations are adequately predicted, especially concerning the droplets dimension. It is additionally inferred that the new upwind flux is beneficial to improve robustness of the underlying numerical methods. Finally, it is demonstrated that the continuum phase model outperforms the mixture one in terms of numerical stability and computational cost, thereby making it very promising for the extension to multi-dimensional problems.