SIMULACIÓN NUMÉRICA DE UN FLUJO AGUA-PETROLEO EN UN INYECTOR DE TURBINA A GAS

Abstract

Con el objetivo de disminuir las emisiones de NOx térmico y cumplir asi con lasnuevas normas de emisiones de contaminantes para las centrales térmicas, una de lasalternativas que se presentan corresponde a realizar un pre-mezclado entre el combustibley agua con el fin que esta absorba parte de la energía liberada durante la combustión,disminuyendo la temperatura de la combustión y los gases de escape e inhibiendoel mecanismo de formación de NOx térmico. Pero se ha observado que este procesogenera una inestabilidad termoacústica en la cámara de combustión, la cual genera dañossobre el inyector provocando su destrucción. En el presente trabajo se presenta unasimulación numérica del proceso de mezclado de ambos fluidos al interior de la cámarade pre-mezclado del inyector para distintos caudales mediante la implementación deun código basado en el software libre OpenFOAM para flujos turbulentos multifásicosacoplado a la ecuación de energía. se analizo las estructuras coherentes del campoturbulento desarrollado así como también se realizo una descripción de la morfología dela fase dispersa.In order to decrease the total emissions of NOx, gas turbines has the posibility touse a mixture of fuel with water, this way can reach lower combustion temperatureand colder exhaust gases. Has been noticed the presence of termoacustic inestabilitysin the combustion chamber when this mixture is used, resulting in searious structuraldamage in the unit. At the begining, methodology for multiphase simulation and disperseturbulents multiphase models are reviewed. Then the mathematical basis is introduced:Navier-Stokes ecuations for modeling incompressible flows, Turbulence modelling usingLarge-Eddy simulation and sub-grid model for kinetic turbulent energy, energy transportecuation for model the internal temperature distribution and finally volume fractionecuations used in “Volume of fluids” multiphase model. Next the Finite Volume Method(FVM) is introduced in order to discretize this set of partial diferential equations andalgorithms to solve the corresponding matricial equations. Next chapter presents theproblem of interest treated in this work, phisical dimensions and the standard operatingconditions of the injector. The main goals of the work are to study and fully understandthis phenomenon through biphasic numerical simulation, inmiscible two-liquid flow. Tomake this, a code was developed by modifying preexisting multiphase solver included inOpenFOAM and validated later using two cases: first, the heat exchange across circularpipes and second, Rayleigh-Bénard inestability. The obtained results of the simulationsallow to determine stagnation zones where water phase tend to acumulate, this zones aredependent on the inlet flows conditions, moving upstream when the oil flux is increasedand growing when the water flux raise. Temperature fields also seems to be influencedby the inlets flows, the lowest temperatures occur near the axial axis and the maximumtemperatures are developed on the plane of 45o degress, increasing the inlet temperatureat 18%. Finally, spectral analysis was performed in order to determine the main frecuency of oscillation in the pressure field inside the mix chamber. It concludes that the instablitycan’t be explained only taking accounts the hidrodynamics conditions,being necessary toconsider the process of combustion of the mixture.