Renzo Piazzesi

Combustion Simulation of an Exhaust Gas Recirculation Operated Micro-gas Turbine

Following their recent experiences in the search of methods for reducing the nitric oxide emissions from a micro-gas turbine, the authors discuss in this paper the results of the combustion simulation under different conditions induced by the activation of an exhaust recirculation system. The theoretical approach starts with a matching analysis of the exhaust gas recirculation equipped microturbine, and then proceeds with the computational fluid dynamics analysis of the combustor. Different combustion models are compared in order to validate the method for NO x reduction by the point of view of a correct development of the chemically reacting process.

Wave packet analysis and break{up length calculations for an accelerating planar liquid jet

This paper examines the process of transition to turbulence within an accelerating planar liquid jet. By calculating the propagation and spatial evolution of disturbance wave packets generated at a nozzle where the jet emerges, we are able to estimate break{up lengths and break{up times for dierent magnitudes of acceleration and dierent liquid to air density ratios. This study uses a basic jet velocity prole which has shear layers in both the air and the liquid either side of the uid interface. The shear layers are constructed as functions of velocity which behave in line with our CFD simulations of injecting Diesel jets. The non{dimensional velocity of the jet along the jet centre{line axis is assumed to take the form V (t) = tanh(at) where the parameter a determines the magnitude of the acceleration. We compare the fully unsteady results obtained by solving the unsteady Rayleigh equation, to those of a quasi{steady jet to determine when the unsteady eects are signicant,

The application of the Global Quasi-Linearisation technique to the analysis of the cyclohexane/air mixture autoignition

The paper is focused on the application of the recently developed Global-Quasi-Linearization (GQL) method to a specific problem of the cyclohexane/air autoignition in a rapid compression machine environment. A simplified autoignition mechanism including 50 species has been used in the analysis based on the customised computational fluid dynamics (CFD) package FLUENT and the new GQL method. The results predicted by FLUENT are shown to be very close to the results predicted by the zero dimensional code SPRINT, developed at the University of Leeds. The application of the GQL method shows that further substantial reduction of the mechanism is possible without significant loss of accuracy. The method is shown to be an efficient tool for identification and approximations of low-dimensional invariant system manifolds.

Stability analysis and break{up length calculations for steady planar liquid jets.

This study uses spatio-temporal stability analysis to investigate the convective and absolute instability properties of a steady unconfined planar liquid jet. The approach uses a piecewise linear velocity profile with a finite-thickness shear layer at the edge of the jet. This study investigates how properties such as the thickness of the shear layer and the value of the fluid velocity at the interface within the shear layer affect the stability properties of the jet. It is found that the presence of a finite-thickness shear layer can lead to an absolute instability for a range of density ratios, not seen when a simpler plug flow velocity profile is considered. It is also found that the inclusion of surface tension has a stabilizing effect on the convective instability but a destabilizing effect on the absolute instability. The stability results are used to obtain estimates for the breakup length of a planar liquid jet as the jet velocity varies. It is found that reducing the shear layer thickness within the jet causes the breakup length to decrease, while increasing the fluid velocity at the fluid interface within the shear layer causes the breakup length to increase. Combining these two effects into a profile, which evolves realistically with velocity, gives results in which the breakup length increases for small velocities and decreases for larger velocities. This behaviour agrees qualitatively with existing experiments on the breakup length of axisymmetric jets.

Combustion Simulation of an EGR Operated Micro-Gas Turbine

Following their recent experiences in the search of methods for reducing the nitric oxide emissions from a micro-gas turbine, the authors discuss in this paper the results of the combustion simulation under different conditions induced by the activation of an exhaust recirculation system. The theoretical approach starts with a matching analysis of the EGR equipped micro-turbine, and then proceeds with the CFD analysis of the combustor. Different combustion models are compared in order to validate the method for NOx reduction by the point of view of a correct development of the chemically reacting process.Copyright

Comparison of External and Internal EGR Concepts for Low Emission Micro Gas Turbines

The external recirculation of exhaust gases represents an effective tool for approaching an almost flameless regime and controlling the nitric oxide formation, like demonstrated by a number of authors’ papers. Such a system penalizes, on the other hand, the micro-gas turbine performance due to the high EGR rates that are needed for the pollutant reduction. Basing on this consideration, the author consider, in this paper, the possibility of exploiting the internal combustion fluid-dynamics for achieving the same level of pollutant abatement. The comparison of the combustion regimes that are induced by either the external or internal EGR adoption are carried out on a CFD basis. The results refer to different load conditions of the micro-gas turbine, with both gaseous and liquid fuel supply.Copyright

Fuelling an EGR Equipped Micro Gas Turbine With Bio-Fuels

The authors discuss in this paper some aspects related to the employment of liquid and gaseous bio-fuels in a micro-gas turbine. Besides the purpose of checking the effectiveness of supplying the micro-turbine with fuels from renewable sources, the attention is focused on the need of controlling the pollutant emission. To this aim, several solutions are experienced by means of CFD numerical simulations. For the liquid fuel supply, a new shape and location of the main fuel injector is combined with a modified position of the pilot injector. In the case of the biogas fuelling, an external EGR option is considered as activated. Both methods aim at the reduction of the thermal and prompt NO formation by approaching the flameless combustion concept.Copyright

Liquid Bio-Fuels in an EGR Equipped Micro Gas Turbine

Following their previous experiences in the search of feasible modifications to an existing lean-premixed combustor for definite pollutant abatement from a micro-gas turbine, the authors extend their analysis to the case of some typical cases of interest when considering the bio-fuel adoption. In particular, this paper focuses the attention to the possibility of a correct combustion close to the flameless regime, even in the presence of lower calorific value fuels like those that can be found with high concentrations in the biomass derived ones. In addition, the authors pay attention to the nitric oxide control by means of an optimal choice of the pilot injector location.Copyright