Lean Premixed Prevaporised Combustion for Microturbines

Abstract

There has been growing interest in the microturbine because of its potential application as a range extender in hybrid electric vehicles. Unlike the existing microturbines used for other purposes, these microturbines will need to operate on liquid fuels with highly preheated air while maintaining low pollutant emissions. A promising strategy to meet these requirements is the lean premixed prevaporised (LPP) concept. Thus, this thesis explores the fuel flexibility, flame stability, structures, gas emissions, and droplet behaviour in LPP combustion at conditions relevant for microturbines. The first part of the thesis examines the effect of fuel choice using zero- and one-dimensional combustion simulations. The fundamental flame properties of gaseous mixtures of air and diesel, gasoline, and kerosene respectively are calculated at typical microturbine operating conditions. The calculations find that the three fuels differ significantly in their ignition delay times and extinction strain rates but are relatively similar in their laminar flame speeds and adiabatic flame temperatures. These results highlight the importance of fuel choice in microturbine combustion systems that rely on autoignition or non-premixed flames. In the second part of the thesis, simulations on single droplet combustion are conducted. The spontaneous ignition of isolated n-heptane droplets with initial diameters of 20–100 μm in air at 4 atm and 700–1200 K is modelled, which includes the typical operating conditions of microturbines. Because some fuel droplets in a combustor may be sprayed or carried to near the recirculation zone, the simulations use a mixture of pure air and hot combustion products as the oxidiser. The flame structures, evaporation times, and autoignition times in both physical and mixture fraction spaces for the different conditions are presented and compared. The variables examined include the air preheat temperature, amount of dilution with hot products, initial fuel droplet diameter, oxidiser temperature, and oxygen concentration. The results show that droplets in pure air at microturbine conditions fully evaporate before ignition, suggesting that a prevapourised concept is suitable for microturbines. The dilution with hot combustion products decreases the ignition delay time mainly by raising the oxidiser temperature. Low-temperature chemistry does not have a significant effect on droplet ignition because adding even a small amount of hot combustion products can increase the oxidiser temperature to higher than the temperatures favorable for low-temperature kinetics. The cool flame is only observed for 100 μm droplets at low temperatures, but two-stage ignition is not observed. The last part experimentally explores the stability and structure of a turbulent swirling n-heptane spray flame under various degrees of prevaporisation at ambient pressure. The results show that preheating the air to 343 and 393 K has little effect on the lean blow-off (LBO) velocity, but recessing the fuel injection significantly decreases the lean stability limit. To correlate these limits, various attempts to define a Damköhler number were made, but unlike previous studies with no prevaporisation, the difficulty in defining laminar flame speed in the present case does not allow a single correlation to work for all degrees of prevaporisation. Four stable cases that differ in equivalence ratio, air preheat temperature, and fuel injection recess are investigated using a gas analyser, 1D PDA, OH* chemiluminescence, and CH₂O-PLIF. Decreasing the global equivalence ratio, preheating the air, and recessing the fuel injection all reduced the NOₓ emissions. Cases without fuel injection recess or air preheat exhibit a conical-shaped heat release zone near the shear layers. Preheating the air to 393 K reduced the Sauter mean diameter (SMD), increased prevaporisation, and enabled a second heat release zone downstream of the fuel injection. Recessing the fuel injection by 25 mm reduced droplet velocities and led to a semi-spherical instead of a conical heat release zone. The CH₂O-PLIF signal without injection recess was high along the central axis and its distribution resembled that observed for spray jet flames. In contrast, with recessed spray injection, CH₂O was mainly found outside the central recirculation zone and only appeared inside during LBO; similar to previous work with premixed flames. Single droplet evaporation was also modelled at the experimental conditions using the single droplet combustion code. The results agree with the experimental data that preheating the air was the most effective for obtaining small droplet diameter. These findings show that different methods of prevaporisation, which only differ by subtle changes in droplet characteristics, strongly impact flame stability. These data can be used for turbulent flame modelling focusing on sprays and finite-rate kinetics.