Computational investigation of direct noise generated by synthetic hot spots in a duct
Pressure fluctuations associated with combustion, particularly in gas turbines, are often linked to either the direct gas expansion due to heat release (direct noise) or the passage of temperature or composition non-uniformities through an outlet gas nozzle (indirect noise). Experiments using the Cambridge Entropy Generator (CEG) have been able to separate and measure the contributions between direct and indirect noise in a system with well controlled boundary and operating conditions, suitable for direct comparison with models. The CEG consists of a tube through which air flows at a controlled rate. Temperature variations are generated by Joule heating of a thin wire grid, and are accelerated through an orifice plate operated at sub- or supercritical conditions, with pressure fluctuations measured upstream. Current prediction models assume a 1D propagation of the entropy wave, where the input is the measured temperature. The present work provides a compressible, unsteady RANS simulation for the unsteady temperature and pressure distribution of the CEG experiments, including advection and dispersion of entropy spots. The implementation of the model in OpenFOAM shows that it is possible to capture the behaviour of the experiments, including the evolution of the temperature in time and space, as well as the acoustic signature for different boundary conditions. The model further highlights some of the limitations in representing entropy spots as one-dimensional waves.