  A one-dimensional, unsteady nozzle flow is modelled to identify the sources
of indirect noise in multicomponent gases. First, from non-equilibrium
thermodynamics relations, it is shown that a compositional inhomogeneity
advected in an accelerating flow is a source of sound induced by
inhomogeneities in the mixture (i) chemical potentials and (ii) specific heat
capacities. Second, it is shown that the acoustic, entropy and compositional
linear perturbations evolve independently from each other and they become
coupled through mean-flow gradients and/or at the boundaries. Third, the
equations are cast in invariant formulation and a mathematical solution is
found by asymptotic expansion of path-ordered integrals with an infinite radius
of convergence. Finally, the transfer functions are calculated for a supersonic
nozzle with finite spatial extent perturbed by a methane-air compositional
inhomogeneity. The proposed framework will help identify and quantify the
sources of sound in nozzles with relevance, for example, to aeronautical gas
turbines.
