• Energy Research

The paper presents the application of an optimization procedure to the aero-thermal design of high performance turbomachinery cascades. The blade profiles are parameterized with third-order Bezier polynomials for the pressure and suction sides, and quadratic functions for the leading and trailing edges, according to the composite curve approach. Evidence is given that fairly complicated blade shapes are well approximated by the present parameterization. The method is applied to an existing nozzle guide vane for which extensive and detailed experimental data are available, with the objective of decreasing in an integral sense the thermal load, maintaining or increasing the blade efficiency and the aerodynamic load. The results demonstrate that, despite the excellence of the initial design, there is room for improvements.

Abstract The paper presents a multi-disciplinary multi-objective design optimization methodology of a combustion chamber effusion cooling system. The optimizer drives an Artificial Neural Network and a Manufacturing Time Model in a repeated analysis scheme in order to increase the combustor liner LCF life and to reduce the liner cooling system manufacturing time, simultaneously. The ANN is trained with a set of fluid/structure/lifing simulations arranged in a three-levels dataset based on a properly designed DOE approach. The CFD simulations are carried out with a reliable and robust in-house developed three-dimensional high resolution reactive viscous flow solver, accounting for conjugate heat transfer approach; the liner structural analysis is performed with a standard FEM code while the liner life assessments are obtained through an in-house developed software operating on the temperature/stress fields. Results demonstrate the validity of the overall approach in a five-dimensional state space with truly moderate computational costs.

AbstractThe paper presents a generalized semi-analytical actuator disk model as applied to the analysis of the flow around ducted propellers at different operating conditions. The model strongly couples the non-linear actuator disk method of Conway[1] (J. Fluid Mech. 1998; 365: 235-267) and the vortex element method of Martensen[2] (Arch. Rat. Mech. 1959; 3: 235-270) and it returns the exact solution, although in an implicit formulation, for incompressible, axisymmetric and inviscid flows. The solution is made explicit through a semi-analytical procedure developed and validated by Bontempo and Manna[3] (J. Fluid Mech. 2013;728:163-195). Moreover the method duly accounts for non-uniform load radial distribution, slipstream contraction, mutual non-linear interaction between duct and propeller, wake rotation, and ducts of general shape. Thanks to its extremely reduced computational cost it can easily be integrated into design systems based on the repeated analysis scheme of hierarchical type. A comparison between open and ducted rotors is carried out in order to quantify the effects of the duct on the overall performance of the device. Emphasis is given to the appropriate matching between the duct geometry and the propeller load to exploit the benefits that could arise ducting the propeller.

Diffuser-augmented wind turbines are known for the potential improvement in power extraction in comparison with open wind turbines. Despite the large number of research works dealing with this subject and unlike the open rotor case, an optimum ducted rotor model is still missing. Since the Joukowsky (free-vortex) optimum rotor exhibits the best power coefficient in the open configuration, this paper presents a newly developed ring-vortex free-wake approach for the performance evaluation of an optimum Joukowsky rotor enclosed in a duct of general shape. The method, which is extensively verified, relies on the exact solution of the steady, incompressible, inviscid and axisymmetric flow, and it naturally takes into account the wake divergence and rotation. The procedure is used to obtain, for the first time, the maximum-power-coefficient/tip-speed-ratio characteristic curve for a diffuser augmented wind turbine. The proposed ducted rotor beats the Betz limit by 14.5% when the power coefficient is referred to the device frontal (exit) area. Additionally, the device experiences a slower decrease in performance with the reduction of the tip-speed ratio, thus extending the design range of ducted rotors in comparison with the open ones. Finally, taking into account the mutual influence of the disk and duct, a new rotor design strategy, capable to evaluate the optimum distribution of the chord and pitch-angle along the blade span, is also proposed. A complete design exercise is carried out and the rotor geometry is obtained for three different values of the nominal tip speed ratio. The paper also proves that a two-dimensional design procedure, which strongly couples the duct and the rotor induced flow, is mandatory to properly evaluate the optimum rotor geometry.

Abstract The paper presents an innovative data processing methodology for the analysis of the surge phenomenon occurring in a compressor. Since the dynamic of the surge cycle does not have a deterministic character, its proper description can only be obtained through a statistical approach. To this aim, the temporally resolved traces of the pressure and mass flow rate signals are processed through a phase averaged decomposition technique. Furthermore, the shape of the oscillating surge cycle is detected and quantified by introducing the joint probability density function of the aforementioned signals which are reported in the pressure ratio versus mass flow rate plane. This probabilistic approach offers two significant advantages over the conventional deterministic approach, namely the possibility to quantify the time of residence of all individual unstable states in a statistical sense, as well as the possibility to carry out a proper code-to-experiments or experiments-to-experiments comparison of such an unstable phenomenon. In this paper, the proposed statistical approach is used to process the experimental data related to the surge phenomenon occurring in a small-sized free spool centrifugal compressor for automotive applications. However, the methodology can be applied both to numerical and experimental surge data from either centrifugal or axial compressors.

Abstract The paper presents a theoretical analysis of advanced gas-turbine cycles. Specifically, three cycles are investigated, that is the intercooled, the reheat, and the intercooled and reheat cycles. The internal irreversibilities, which characterise the compression and expansion processes, are taken into account through the polytropic efficiencies of the compressors and turbines. New analytical formulations for the overall and intermediate pressure-ratios which maximise the net work of the three aforementioned cycles are proposed along with an order relation between these optimum pressure-ratios. Moreover, the thermal efficiency of these cycles is also analysed providing, among other findings, the ranges of the intermediate pressure-ratios returning a benefit in the thermal efficiency in comparison with the simple cycle. Finally, for the sole intercooled and reheat cycle, a novel analytical expression for the maximum point of the thermal efficiency is given. It is also shown that, for the intercooled and reheat cycle, there is a unique value of the overall pressure-ratio which simultaneously maximises the net work and the thermal efficiency. To give some quantitative information, consider a maximum to minimum cycle-temperature ratio equal to 1573/300 and a compressor (resp. turbine) polytropic-efficiency equal to 0.8 (resp. 0.88). The net work and the thermal efficiency are maximised by a set of overall pressure-ratios obeying an order relation. The simple, the reheat, the intercooled, and the intercooled and reheat cycles reach the maximum net-work (resp. thermal efficiency) for increasing values of the overall pressure-ratio, that is 8.550 (resp. 18.260), 14.950 (resp. 25.039), 20.846 (resp. 39.984), and 73.109 (resp. 73.109). The reheat cycle achieves a 34.899% (resp. 6.077%) gain in the net work (resp. thermal efficiency), while the intercooled cycle returns a 31.477% (resp. 15.970%) increment. The maximum net-work of the intercooled and reheat cycle exactly doubles that of the simple cycle. Finally, the maximum thermal efficiency of the intercooled and reheat cycle yields a 24.966% improvement in comparison with the simple one.

Abstract The paper describes the steady and unsteady performance characteristics of a small size turbocharger typically employed in automotive downsized engine applications. The analysis is carried out by experimental means using an innovative hot gas generator system specifically designed for turbocharger testing which is capable of delivering a wide range of flow rates with adequate thermodynamic characteristics. More in detail, the gas generator consists of a medium size direct injection compression ignition Internal Combustion Engine (ICE) feeding the turbine of the test article. To independently set the hot gas mass flow rate and the turbine inlet temperature, the operating parameters of the aforementioned ICE are specified through an electronic control unit in a fully automated manner. Compared to previously presented data [1] (Energy Procedia, vol. 45, pp 1116-1125, 2014), those reported herein have been collected with the help of newly installed equipment and controlling software allowing for the estimation of the thermal power transferred from the turbocharger to the environment. In particular, thanks to a first law analysis, the collected measurements have shown that the algebraic sum of the thermal power transferred to the lubricating oil as well as to the environment is roughly speaking 20–30% of the compressor total enthalpy change per unit time. Moreover, it has been shown that evaluating the compressor efficiency through classical expression based on the adiabatic assumption leads to a 5–10% relative error. The improved experimental set up also allows for higher precision transient analysis both on the cold and hot side branch of the test article. While the steady-state performance maps of the turbocharger are readily obtained with the semi-automated testing procedure, the detailed analysis of the unsteady phenomena related for instance to the occurrence of mild and deep compressor surge events, are reproduced and thoroughly analysed using the rig in more advanced operating modes.

AbstractThe paper provides a verification of the well‐known momentum theories by comparing their results with those of a semi‐analytical method based on the exact solution of the flow through an actuator disk. In fact, many error sources are disseminated in these theories. Specifically, the axial momentum balance is diffusely applied in an approximate differential form, and several linearization are customarily introduced in the equations of the motion. In the present study, an analytical formulation of these two kinds of errors is provided and a set of data generated with controlled accuracy is used to quantify them both in terms of global (power coefficient) and local (axial velocity at the disk plane) quantities. The overall errors increase by increasing the thrust coefficient and decreasing the tip speed ratio. Although the errors can be generally considered small in terms of global performance coefficients, the differences between the two methods are significant when looking at local quantities. In particular, for the cases presented herein, the momentum theory is shown to overestimate the axial velocity at the tip and at the disk centre regions and to underestimate this velocity elsewhere. For low values of the tip speed ratio, an underestimation also occurs in the zone near to the disk centre. Copyright © 2017 John Wiley & Sons, Ltd.