Onset Criteria for Bulk-Mode Thermoacoustic Instabilities in Supercritical Hydrocarbon Fuels

We have investigated supercritical-p (p > 1192 psi (8.22 MPa)) methanol at pressures up to 1645 psi (11.3 MPa) flowing through a heated tube at flow rates of 4-7 lb/h (1.8-3.2 kg/h). Tube heated lengths have been varied from 4 to 6 in (10 to 15 cm), internal diameters from 0.027 to 0.069 in (0.069 to 0.175 cm), and heat inputs between zero and 800 W. Fluid temperature at the tube inlet remained subcritical (T < 464 degrees F (513 K)); outlet temperatures were transcritical or supercritical. Two phenomena were observed: system-wide bulk-mode oscillations and localized acoustic modes. In this study, predictive efforts are undertaken to characterize system-wide bulk-mode oscillations. The parameter space has been nondimensionalized, yielding four dimensionless variables. Stability criteria based on these dimensionless groups have been established for two separate test articles and fluids; both criteria suggest that the heat required for the onset of oscillations is proportional to the mass flow rate times the mean pressure and inversely proportional to the fuel density.