Abstract
Abstract Single- and multicomponent liquid fuels are injected in a jet-in-coflow configuration at elevated temperatures and pressures with both a custom plain orifice nozzle and a commercial pressure-swirl atomizer. The transitions in spray morphology from mechanical breakup to superheated/supercritical regimes are characterized qualitatively by laser shadowgraphy and evaluated based on quantitative measures of superheat. Although fuel preheating exhibits no discernible effect in the mechanical breakup regime, dramatic jet-to-plume transition as well as build-up of fuel vapor in the spray chamber is observed with increasing level of superheat. The difference between two different atomizers in terms of spray behavior diminishes at high levels of superheat, suggesting the predominant role of thermal effect on spray morphology in superheated/supercritical regimes. For a mutlicomponent fuel such as Jet A-1, the transition into a fully flashing spray occurs at temperatures lower than expected values, which are calculated by treating Jet A-1 as a single-component fuel. Additionally, pressure drop is shown as a sensitive indicator for the departure from mechanical breakup and the onset of thermal effect on the spray. Comparisons between measured and estimated pressure drop also reveal the differences in susceptibility to thermal effects between the plain orifice and the pressure-swirl atomizers.
Highlights
As the operating temperature and pressure of aero engines continue to be raised for improved overall efficiency and reduced CO2 emission, it becomes increasingly likely that liquid fuels will be injected into the combustion chamber under superheated or supercritical conditions
Superheated and supercritical injection of cyclohexane with a plain orifice nozzle Figures 2 to 4 display collages of single-shot shadowgraphs of cyclohexane injected at m F=1 g/s and a chamber pressure of 1.5, 3 or 6 bars, using the custom plain orifice nozzle described in the previous section
Note that the chart is cropped above the Pc of cyclohexane at about 40 bars, such that the end of the right end of the solid black line represents the critical point of the liquid fuel
Summary
The commercial aviation sector is expected to continue its robust growth with doubled passenger numbers in the two decades [1]. As the operating temperature and pressure of aero engines continue to be raised for improved overall efficiency and reduced CO2 emission, it becomes increasingly likely that liquid fuels will be injected into the combustion chamber under superheated or supercritical conditions. As the fuel temperature and pressure prior to injection approach its supercritical limits, the liquid fuel behaves more and more like its gaseous counterpart Under these conditions, fuel sprays can attain finer droplets, wider opening angles [3] and smaller penetration depths [4] comparing to injections at normal conditions [5, 6, 7]. Fuel sprays can attain finer droplets, wider opening angles [3] and smaller penetration depths [4] comparing to injections at normal conditions [5, 6, 7] These advantages, when fully leveraged, could enable novel strategies for achieving compact flames and low pollutant emission [8,9] that are not accessible through conventional means of combustor modification. Many applications of superheated/supercritical injections can be found in wide-ranging industries, relatively few work exists with regard to gas turbine combustion that focuses on multi-component fuels and atomizers with complex geometries
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