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The paper deals with the optimisation of the charging efficiency in a small two-stroke high-speed engine according to well-established design guidelines. The aim is to present the method successfully used in the preliminary screening of the performance improvement attainable in a crank-case-compression engine by design modifications to the transfer ports and manifolds. The method applies a basic CFD model of the steady-state flow across the cylinder block validated against experimental tests at the discharge flow bench. This model is used to approximate the actual scavenging process through a transient simulation of the cold flow across the cylinder inside which the piston is fixed at the bottom centre. A fast assessment of the charging efficiency is permitted by a transported passive scalar implemented in the model to easily estimate all the parameters needed for monitoring the effectiveness of the scavenging process at each crankangle. This practical design approach has been applied to the geometry of a Schnürle-type loop-scavenged 125cc single-cylinder engine compliant with the 2018 FIA homologation form for the KF2 karting competition category. The maximum increase of indicated mean effective pressure expected according to the comparison between the CFD simulations of the original and the modified design of the transfer ports is approximately equal to 4%. This result demonstrates that the CFD analyses are sensitive to the limited modifications commonly needed to tune two-stroke racing-engines and confirms that the suggested design approach can be profitably employed by engineers and technicians involved in the design of small two-stroke highspeed engines.

An original method for formulating surrogate fuels from actual syngas mixtures is presented and formalised. The method is the first example in the scientific literature of a rather complete tool for planning and setting up a laboratory syngasfuelled engine test when some components of the syngas mixture are not available. Basically, the method allows a map to be built that provides the composition for a surrogate fuel once the composition of a syngas mixture is assigned, the components of a surrogate fuel are selected and the equivalence parameters are defined. The laminar flame speed, the energy density of the fuel-air mixture and the methane number are identified as equivalence parameters in the study. In particular, the proper laminar flame speed and energy density ensure that an engine fuelled by the surrogate mixture produces the same indicated power as it would when fuelled by the original syngas. Instead, the methane number allows for checking the fact that the tendency of the engine to knock is the same or greater than the knock tendency during syngas operation. In this article, the method is used to determine the hydrogen-methane-nitrogen mixtures corresponding to six five-component syngas mixtures, resulting from actual gasification processes. The laminar flame speed and methane number of each syngas mixture are estimated by means of simple original models aimed at either improving the predicting capabilities of existing models or allowing for a prompt application of the procedure. The results show that four of the six surrogate fuels are equally or more knock-prone than the original syngas mixtures, whereas only one of the two remaining surrogate fuels likely imposes a retardation of the spark advance in the final setup of the engine for actual syngas operation. [ABSTRACT FROM AUTHOR]

Abstract: Gas turbine performance is strongly dependent on the flow field inside the combustor. In the primary zone, the recirculation of hot products stabilises the flame and completes the fuel oxidation. In the dilution zone, the mixing process allows to obtain the suitable temperature profile at turbine inlet. This paper presents an experimental and computational analysis of both the isothermal and the reactive flow field inside a gas turbine combustor designed to be fed with natural gas and hydrogen. The study aims at evaluating the capability of a coarse grid CFD model, already validated in previous reactive calculations, in predicting the flow field and NOx emissions. An experimental campaign was performed on an isothermal flow test rig to investigate the combustion air splitting and the penetration of both primary and dilution air jets. These experimental data are used to validate the isothermal computations. The impact of combustion on the calculated flow field and on air splitting is investigated as well. Finally, NOx emission trend estimated by a post-processing technique is presented. The numerical NOx concentrations at the combustor discharge are compared with experimental measurements acquired during operation with different fuel burnt (natural gas or hydrogen) and different amount of steam injected. [Copyright &y& Elsevier]

Proper cooling of the hot components and an optimal temperature distribution at the turbine inlet are fundamental targets for gas turbine combustors. In particular; the temperature distribution at the combustor discharge is a critical issue for the durability of the turbine blades and the high performance of the engine. At present, CFD is a widely used tool to simulate the reacting flow inside gas turbine combustors. This paper presents a numerical analysis of a single can type combustor designed to be fed both with hydrogen and natural gas. The combustor also features a steam injection system to restrain the NOx pollutants. The simulations were carried out to quantify the effect of fuel type and steam injection on the temperature field. The CFD model employs a computationally low cost approach, thus the physical domain is meshed with a coarse grid. A full-scale test campaign was performed on the combustor: temperatures at the liner wall and the combustor outlet were acquired at different operating conditions. These experimental data, which are discussed, were used to evaluate the capability of the present CFD model to predict temperature values for combustor operation with different fuels and steam to fuel ratios. [ABSTRACT FROM AUTHOR]