A good design of the regenerator of a Stirling engine is required to obtain high performance and efficiency of such an engine. The regenerator is basically a heat-exchanger placed between the hot and cold working streams. It usually consists of stacked woven wires. The fluid pressure drop and heat transfer are the main parameters of the regenerator influencing the engine performance. In this work, friction coefficient, thermal efficiency and Nusselt number are numerically evaluated in order to assess the performance of the regenerator. The open-source software OpenFOAM is used to analyse the thermo-fluid dynamic behaviour of a regenerator wire netting at different Reynolds numbers. Firstly, isothermal air flows and adiabatic wire matrices are considered, by assuming the fluid flowing through the regenerator as incompressible. Then, air flows with a fluid temperature of 500 K and wires at a temperature of 300 K are analysed. The results are compared with those obtained by means of the commercial software Ansys Fluent.

The Stirling engine design process is mostly focused on the study of the engine regenerator, whose thermal efficiency is strictly related to the global efficiency of the engine. The present work aims to setup a numerical model by means of OpenFOAM libraries to simulate the engine regenerator as a porous media, in order to reduce the overall computational cost of the simulations and to obtain a suitable model that can be eventually used for topological optimization. The solid part of the numerical domain, i.e. the stoked wires, is represented as a zone with the highest porosity, whereas intermediate areas are defined in order to simulate the boundary layers in proximity of the walls. The presence of porous media is modelled by introducing a Darcy-type pressure drop in the momentum equation, whereas in the energy equation the thermal properties of the solid, intermediate and fluid materials are modelled by means of a linear function of the porosity value in the domain. After a process of tuning, the results obtained with this model are reported for different flow conditions in terms of both aerodynamic and thermal performances, thus showing a good agreement with the numerical results obtained with a more classical CFD methodology.

The scavenging process of two-stroke engines plays a fundamental role in cylinder flow patterns and in the overall engine performance. In this work, 3D CFD simulations of the scavenging in a uniflow, two-stroke, compression ignition engine for general aviation, named GF56, have been performed by using a 3D finite-volume FANS equations solver with k-ϵ closure. The GF56 engine consists of six cylinders, separated into two quasi-symmetric banks. Both the right and the left banks, together with the corresponding cylinders, are carefully analyzed. Charging and trapping efficiencies are computed as a function of the delivery ratio for different mass flow rates entering into the plenum, and the influence of the exhaust pressure and of the cylinder’s location in the bank are analyzed. The results show that the fresh air trapped during the scavenging process is quite similar for each cylinder of the right bank and it is about 92% of the in-cylinder mass. The cylinder’s location in the bank by itself slightly affects the scavenging performance, whereas the pressure profile at the outlet section has a major role. The design of the intake ports is fundamental for establishing the in-cylinder flow field and a new ports configuration is proposed to enhance the swirl ratio and, consequently, the scavenging performance with high delivery ratios.

The present work deals with an analysis of the cooling system for a two-stroke aircraft engine with compression ignition. This analysis is carried out by means of a 3D finite-volume RANS equations solver with k- ϵ closure. Three different cooling system geometries are critically compared with a discussion on the capabilities and limitations of each technical solution. A first configuration of such a system is considered and analyzed by evaluating the pressure loss across the system as a function of the inlet mass-flow rate. Moreover, the velocity and vorticity patterns are analyzed to highlight the features of the flow structure. Thermal effects on the engine structure are also taken into account and the cooling system performance is assessed as a function of both the inlet mass-flow rate and the cylinder jackets temperatures. Then, by considering the main thermo-fluid dynamics features obtained in the case of the first configuration, two geometrical modifications are proposed to improve the efficiency of the system. As regards the first modification, the fluid intake is split in two manifolds by keeping the same total mass-flow rate. As regards the second configuration, a new single-inlet geometry is designed by inserting restrictions and enlargements within the cooling system to constrain the coolant flow through the cylinder jackets and by moving downstream the outflow section. It is shown that the second geometry modification achieves the best performances by improving the overall transferred heat of about 20% with respect to the first one, while keeping the three cylinders only slightly unevenly cooled. However, an increase of the flow characteristic loads occurs due to the geometrical restrictions and enlargements of the cooling system.

It is well known that spark ignition internal combustion engines for aeronautical applications operate within a specific temperature range to avoid structural damages, detonations and loss of efficiency of the combustion process. An accurate assessment of the cooling system performance is a crucial aspect in order to guarantee broad operating conditions of the engine. In this framework, the use of a Conjugate Heat Transfer method is a proper choice, since it allows to estimate both the heat fluxes between the engine walls and the cooling air and the temperature distribution along the outer wall surfaces of the engine, and to perform parametric analyses by varying the engine operating conditions. In this work, the air-cooling system of a 4-cylinder spark ignition engine, designed by CMD Engine Company for aeronautical applications, is analysed in order to evaluate the amount of the air mass flow rate to guarantee the heat transfer under full load operating conditions. A preliminary validation of the model is performed by comparing the results with available experimental data. A parametric study is also performed to assess the influence of the controlling parameters on the cooling system efficiency. This study is carried out by varying the inlet air mass flow rate from 1.0 kg/s to 1.5 kg/s and the temperature of the inner wall surfaces of the engine combustion chambers from 390 K to 430 K.