Similarity and scale effect in ramjet combustors

Abstract

In view of the successful application of “pressure scaling” to gas turbine and reheat combustors, there is interest in using the technique for ramjets. However, as the fuel distribution process in typical ramjet combustors was suspected of causing scaling difficulties, the present work set out to determine the value of pressure scaling under these circumstances and to establish the appropriate fuel injector scaling criterion. The work was carried out using a current design of practical ramjet combustor in sizes of approximately 3-, 5-, and 8-inch duct diameter. Besides direct scaling comparisons, it was found necessary to investigate the detailed performance of this type of combustor and to study the fuel distribution process, both from isolated injectors and within the actual combustor. An important irregularity of combustor performance noted was the phenomenon of primary fuel penetration into the secondary zone, this giving rise to a high sensitivity of performance to fuel distribution effects. The injector scaling conditions necessary to preserve similarity of the penetration process under these circumstances have been determined in the form of an empirical law. Assesment of the comparative performance of the three combustors shows that, under those operating conditions in which the above fuel penetration does not occur, combustion similarity is achieved by pressure scaling, broadly independent of fuel injector size. For conditions in which fuel penetration is marginal, use of the empirical law found for injector scaling results in reasonable similarity. The only scale effect noted has been in the tendency of the primary zone stability limits to decrease in mixture strength as scale increases. This appears to be related to dissimilarity of air flow distribution, an effect which is being further investigated.