Thermal energy recovery is being investigated by leading aerospace companies as a means to improve the efficiency of next-generation propulsion systems. The organic Rankine cycle (ORC) system, due to the flexibility of the concept, is arguably the best technology for waste heat recovery and, thus, a promising solution to develop recuperated engines. In such systems, heat exchangers are arguably the most critical components, as their design must balance thermal performance with constraints on weight and volume. Consequently, integrating the optimization of heat exchangers into the overall system design may lead to substantial performance enhancement compared to more traditional iterative design methods. The objective of this study was the development of a systematic methodology for optimizing airborne thermal systems, with a focus on addressing the computational challenges of integrated design. Three design strategies are compared: (i) optimization of the sole cycle parameters while performing heat exchanger sizing for values of the geometrical characteristics defined a priori based on a preliminary investigation of the design space of these components, (ii) concurrent optimization of both the thermodynamic cycle and of the most critical heat exchanger, e.g., the condenser, and, (iii) use of a data-driven surrogate model of the condenser to predict the optimal heat exchanger geometry as a function of any feasible thermodynamic conditions to reduce the number of optimization variables of the integrated design problem. The surrogate model is constructed based on datasets of Pareto-optimal HX designs in the objective space defined by heat exchanger weight and pressure drops. The three design strategies are applied to two case studies featuring supercritical ORC systems utilizing cyclopentane as the working fluid: a combined cycle auxiliary power unit (CC-APU) and a combined cycle turboshaft (CC-TS) engine. Findings indicate that integrated optimization yields performance gains that vary depending on the heat exchanger topology, application, and thermodynamic cycle. For instance, CC-APU designs obtained with the integrated design optimization method are up to 15% lighter than designs obtained with the optimization of the thermodynamic cycle parameters alone, for the same net power output. Microchannel condenser designs with offset strip fins allow for obtaining a better performance than louvered fin-based designs if a low-pressure drop is targeted, whereas louvered fins are advantageous if a higher pressure drop is allowed. The design strategy employing the surrogate model considerably reduces the computational cost, without significantly affecting accuracy: the relative deviation between the Pareto front obtained with the surrogate model and that obtained with the integrated optimization strategy ranges between 1% and 2.9%. These values are comparable to the uncertainty of the predictions of the heat exchanger model. The reduction in computational time required to generate the Pareto fronts associated with the two case studies is up to 200%.

Waste heat recovery (WHR) from aeroengines via compact organic Rankine cycle (ORC) units may increase the fuel efficiency of air transportation. Heat exchangers are arguably the key components of ORC systems for aeronautical applications and their design must be optimized to guarantee the best trade-off between fluid pressure drop, weight and induced aircraft drag. At present, no heat exchangers design guidelines are available for waste heat recovery systems aboard aircraft. This study, thus, contributes to defining a proper design methodology for ORC systems of such applications. The chosen test case is a supercritical ORC system with cyclopentane as the working fluid, which recovers waste heat from the auxiliary power unit of an aircraft. The exhaust gas temperature and mass flow rate of the power unit are known and kept constant in the analysis, and so are the ambient conditions, which define the cold sink of the ORC turbogenerator. Three design strategies targeting minimum mass and maximum net power output of the ORC unit have been assessed. In the first one, the multi-objective optimization is performed by prescribing a priori the geometry and frontal area of the heat exchangers. Thus, only the cycle parameters are optimized. The second method tackles, instead, the simultaneous optimization of the geometric parameters of the condenser and the cycle parameters. It was found that the integrated design allows for system mass reduction by 10 - 12% for a given ORC power output, highlighting the importance of performing the simultaneous optimization of the thermodynamic process and the heat exchanger geometry. Finally, the third method addresses the same optimal design problem by leveraging a reduced-order model of the condenser to predict the optimal design space of this component. The generated Pareto front obtained with this method is very similar to that found by optimizing simultaneously the complete condenser geometry and the cycle parameters. The mean deviation is about 2%. With just one heat exchanger surrogate model, the Pareto front was generated in one fourth of the computational time. This is due to the lower number of optimization variables and the faster objective function evaluation.

This paper describes an experiment conducted within the nozzle test section of the Organic Rankine Cycle Hybrid Integrated Device (ORCHID) aimed at providing accurate data for the validation of NICFD flow solvers [5]. A supersonic flow of the dense vapor siloxane MM established in the nozzle of the setup was characterized by means of the schlieren technique and by pressure taps along the nozzle profile. The nozzle inlet conditions corresponded to a stagnation temperature and pressure of T0=253∘C and P0=18.36bara. At these inlet conditions, the compressibility factor of the fluid is Z₀= 0.58. The nozzle backpressure was equal to Pb=2.2bara. The experimental data-set includes: 1) the average mid-plane local Mach number, which was derived from the schlieren images by estimating the angle of the Mach waves originating from the roughness of the upper and lower nozzle surfaces, 2) the angle of a shock wave generated by a 5^∘ wedge placed at the nozzle exit, also detectable in the schlieren images, and 3) the static pressure distribution along the flow expansion acquired with a Scanivalve DSA3218 pressure scanner device. The Mach number at the nozzle exit estimated based on the schlieren images is M= 1.95 ± 0.05, very close to the expected value of M= 2 according to the design conditions of the experiment. The static pressure measurements have a maximum absolute uncertainty amounting to ± 1.80 kPa in the initial stages of the expansion. This information was used to assess the capability of the open-source SU2 flow solver in evaluating the NICFD effects in a supersonic flow of MM when the fluid thermodynamic properties are modeled with a cubic equation of state. For this purpose, two-dimensional Euler simulations were carried out with SU2 for the operating conditions achieved in the experiment. The numerical results are in good agreement with the experimental data. The largest deviation between the simulation and experiment is observed in the nozzle uniform region, where two dips in the Mach number occur due to a slight local decrease in flow velocity owing to two weak shock waves. The shock wave generated by the wedge located at the nozzle outlet propagates with two different angles, namely, β_above= 37. 6^∘± 0.86, and β_below= 31. 6^∘± 0.64, due to the axial misalignment of the wedge with respect to the flow.

This paper reports one of the initial NICFD experiments in the nozzle test section of the ORCHID aimed at providing accurate data for the validation of flow solvers, albeit, at this stage of the research, the focus is limited to inviscid phenomena. Notably, a series of schlieren photographs displaying Mach waves in the supersonic flow of the dense vapor of siloxane MM were obtained and are documented here for the commissioning experiment, namely, for inlet conditions corresponding to a stagnation temperature and pressure of T0=252∘C and P0=18.4bara. At these inlet conditions the compressibility factor of the fluid is Z₀= 0.58. The digital processing of the schlieren images allowed to estimate multiple angles formed by the Mach waves stemming from the upper and lower nozzle surfaces because of the infinitesimal density perturbations generated by the, albeit small, roughness of the metal surfaces. These values are related to the local Mach number by a simple geometric relation. Moreover, the total expanded uncertainty in the Mach number was computed. This information together with the estimate of the average Mach number was used for a first assessment of the capability of evaluating NICFD effects occurring in a dense organic vapor flow of MM by comparison with the results of CFD simulations. The outcome of the comparison was satisfactory. It can thus be inferred that the nozzle test section has been commissioned and it is ready for experimental campaigns in which its full potential in terms of measurements accuracy, repeatability, and operational flexibility will be exploited.