Theoretical Investigation of the Compression Augmentation Effects of Variable Area Ejectors

A theoretical analysis of a variable area ejector is presented. The flowfield is solved using a steady quasi-one-dimensional, inviscid control volume formulation assuming a fully mixed exit flow. A three parameter analytical wall pressure distribution is chosen to best match numerical results for ejector compression. While two of the parameters are pressure boundary conditions, the third is the minimum rocket stream pressure, and is found by using either a control volume or Riemann based approach. Both methods give very similar results for the minimum rocket pressure and nearly identical trends for its variation with total air pressure. The complete analysis procedure is used to study the performance of the constricted ejector at various operating conditions. Exit area constriction is found to have an overall beneficial effect on ejector compression. Although the theoretically predicted compression factors are about 30-40% higher than similar computational values, the exponential increase in compression with increasing constriction is well captured. Decreasing the exit area tended to increase thrust augmentation, with performance gains as high as 9% for the case with the highest area constriction.