Starting characteristics of supersonic inlets

The starting characteristics of a small-scale rectangular inlet with a thick ingested boundary layer were investigated at nominal Mach 3 conditions. Parameters investigated included Reynolds number, cowl length, and cowl height. Measurements of the maximum and restart contraction ratios were made. Depending on the test configuration, the unstarts were classified into two broad categories as either "hard" or "soft." The hard unstarts appear to occur when the flow at the inlet throat chokes. The soft unstarts occur as large-scale separation develops within the inlet. The ability of the classical Kantrowitz limit to predict the restart contraction ratio was assessed, and it was shown to be applicable for the hard unstart/ restart configurations. The role of fluid injection upstream of the unstarted inlet was also assessed. The use of this injection may ultimately lead to improving the starting characteristics of inlets. Nomenclature A Area hc Cowl height Lc Cowl length rh Mass flow M Mach number M Mass-averaged Mach number P Pressure Pt Freestream total pressure x, y Cartesian coordinates y Ratio of specific heats 6 Boundary layer thickness 6* Boundary layer displacement thickness 0 Boundary layer momentum thickness 9C Cowl angle p Density * Principal staff engineer, senior member AIAA ** Senior staff engineer, member AIAA ^ Associate staff engineer, member AIAA Copyright © American Institute of Aeronautics and Astronautics, Inc., 1996. All rights reserved. Subscripts 0 Freestream 2 Entrance to cowl 4 Inlet throat cl Wall conditions at cowl lip inj Injectant Introduction Airbreathing engines that operate at supersonic and hypersonic speeds require inlets to capture and compress air for processing by the remainder of the engine. The goal in the design of any inlet is to define a minimum weight geometry that provides an efficient compression process, generates minimum drag, produces nearly uniform flow entering the compressor or combustor, and provides these characteristics over a wide range of flight and engine operating conditions. For efficient operation and moderate induced drag, most inlets use a combination of external and internal compression. The introduction of internal contraction in an inlet adds complexity in the design and analysis process in that the starting of the inlet must be ensured. For efficient operation, supersonic and hypersonic inlets must operate in a started mode. The process of inlet starting and unstarting is well understood at a conceptual level, although significant details remain to be resolved. Some variation exists in the very definition of a started inlet. One convention states that a started inlet is one with supersonic flow in the inlet throat, but it is well known that some unstarted inlets can have complex internal flowfields with a significant fraction of supersonic flow in the inlet throat. In the present work, the term "started" is used to denote operation under conditions where flow phenomena in the internal portions of the inlet do not alter the air capture characteristics of the inlet. (Reduction in the captured mass flow through the use of bleed holes or bypass channels is not considered in assessing whether an inlet is started.) An inlet can be unstarted by either over-contracting to the point where the flow chokes at the inlet throat or by raising the back pressure beyond the level that can be sustained by the inlet. Currently, a significant uncertainty exists regarding the conditions under which an inlet will unstart or restart. Part of this uncertainty is due to the large variety of 1 American Institute of Aeronautics and Astronautics geometries that have been considered in designing engines. The design of an inlet is strongly affected by vehicle considerations, and a variety of two-dimensional planar, axisymmetric, and three-dimensional inlet designs have been investigated. The diversity found in inlet designs can be seen in the sample inlets shown in Fig. I.'" Preliminary estimates of the internal contraction that will self-start can be obtained from the Kantrowitz limit. This limit is determined by assuming a normal shock wave at the beginning of the internal contraction and calculating the one-dimensional, isentropic internal area ratio that will produce sonic flow at the inlet throat. For a perfect gas, the Kantrowitz limit can be calculated as follows: Oswatisch inlet HRE-type inlet A2] /KANTROWITZ M, l)MJ