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This paper compares the thermoacoustic limit cycles of a premixed laboratory combustor with acoustically open and choked exits. It is shown that the form of the downstream boundary condition can have a significant effect on the combustion chamber acoustics, with both the dominant limit cycle frequencies and the acoustic mode shapes being very different for the different combustor exits. The fundamental limit cycle frequency with the choked exit in place agrees closely with that determined by the convective time scales of entropy disturbances, as argued by other authors. A novel experimental method is then developed to examine the acoustic response of an arbitrary duct termination to incident pressure and entropy perturbations, and used to measure the response of the combustor downstream boundary condition during thermoacoustic limit cycle. The reflection coefficient for the acoustically open exit matches closely the classical result for zero mean flow. It is also shown that a choked nozzle downstream of the flame generates significant sound due to the interaction of the convected entropy perturbation with the nozzle. This final result is qualitatively in keeping with an existing analytic boundary condition for a choked nozzle, even though quantitative agreement is not observed. Reasons for this discrepancy are then suggested. Nomenclature A = Fourier transform of the nondimensional incident pressure fluctuation AR = downstream nozzle area ratio a = amplitude of the incident nondimensional pressure fluctuation B = Fourier transform of the nondimensional reflected pressure fluctuation b = amplitude of the reflected nondimensional pressure fluctuation C ij = real part of the cross-spectral density between i and j c = speed of sound c p = specific heat at constant pressure f = frequency H = transfer function k = wave number L = length of rig downstream of the flame holder M = Mach number n = mode number P = Fourier transform of the nondimensional pressure fluctuation p = pressure Q ij = imaginary part of the cross-spectral density between i and j R = reflection coefficient S = Fourier transform of the nondimensional entropy fluctuation S ij = cross-spectral density between i and j s = entropy, amplitude of the nondimensional entropy fluctuation T = temperature, Fourier transform of the nondimensional temperature, measurement period t = time u = velocity x = spatial coordinate = ratio of specific heats = density = equivalence ratio ! = angular frequency Subscripts H = hot flow quantity i = incident n = mode number p = pressure r = reflected s = entropy 1,2,. . . = measurement location, transfer function number Modifiers g = mean component of the quantity g g 0 = fluctuating component of the quantity g g = complex conjugate of the complex number g

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Experimental and numerical investigations of primary atomization in a high-pressure diesel jet are presented. Information on flow processes and structures inside and near nozzle exit are described at early and quasi-steady stages of injection. The numerical method is based on the Volume Of Fluid (VOF) phase-fraction interface capturing technique, in an Eulerian framework. The influence of grid resolution, convection interpolation scheme and temporal integration scheme on the modelling of jet physics are investigated. The present flow setup includes in-nozzle disturbances with the no-slip condition at the walls. All experimental operating conditions are replicated in the numerical models. The early stage liquid jet leading edge demonstrates an umbrella-shaped structure in the numerical results which is in qualitative agreement with experimental imaging. Data obtained provide insight into the flow behavior in the dense region including commencement of fragmentation and early spray angle formation. Experimental images show a cloud of air-fuel mixture at the early stage of injection. The existence of ingested air inside the injector after needle closure could be the source of the observed deviation between experimental and numerical results. The results show that the jet break-up rate and liquid core length increase in cases with higher grid resolutions. The early spray angle from the numerical results at the quasi-steady stage, shows good agreement with experimental data.