Transonic Relief in Fans and Compressors

Compressor blade profiles are usually designed using two-dimensional or quasi-three dimensional methods. In such methods, the streamtube contraction through the compressor is fixed. As the inlet Mach number to a blade approaches the value of one, the design of such blades becomes increasingly more difficult. In fact, at an inlet Mach number of one blade design becomes nearly impossible. In real three-dimensional compressors this is not the case, and though blade design is not easy, profiles can be designed which successfully operate at Mach one. This paper explains why this contradiction occurs. The reason for the difficulty in two-dimensional blade design close to Mach one is explained. It is shown that at Mach one there is only a single value of the upstream stream tube to the geometric throat area ratio at which a profile can operate. This results in the near impossibility of design and even if the blade is designed it has no incidence range. In real three-dimensional compressors a new mechanism, known as transonic compressor relief, is shown to occur. It is shown that as the inlet Mach number approaches one, the ratio between the upstream stream tube area and the geometric throat automatically readjusts itself moving toward the optimal value. This new mechanism is important because it acts as a ‘get out of jail free card’ and allows for the practical design, and manufacture, of transonic compressors.