Study of two-phase flow for a ramjet combustor

Abstract

A reactive two-phase flow is studied experimentally by a special laser Doppler anemometry (LDA) technique and numerically by solving the elliptic governing equations. The relation between the liquid fuel injection and the aerodynamic flameholding mechanism of a dump combustor was investigated. The flow geometry chosen was an axisymmetric sudden expansion with small diameter ratio. The measurements included pressure, temperature, gas species concentration, as well as velocities of both phases and droplet size. A special processing technique enabled the simultaneous measurement of the gas velocity represented by the smallest droplets and of the size-velocity correlation of the droplets in the range of 5-500 |xm. The specific geometry of the sudden expansion combustor creates an annular flame stabilized just downstream of the step. The geometry of the flame depended very much on the equivalence ratio, with larger values creating longer and thicker flames. The combustion shortened the recirculation length from about 11 times the step height to about four times. There was no direct influence of the droplets on the average gas velocity in cold flow, whereas the size distribution of the droplets had significant influence on the combusting flow pattern. Two computational case studies were performed for isothermal and reactive two-phase flow situations. The numerical results indicated the fast dynamic and thermal response of the small droplets to the surrounding gas flow. Moreover, the droplets did not penetrate into the fuel-rich recirculation zone. The shear layer, which separates it from the core, was characterized by a local equivalence ratio close to unity. The numerical predictions and the experimental results exhibited fairly good agreement, provided that the initial conditions of the spray and the thermal boundary conditions are determined experimentally.