Internal Flow Modeling and Compressor Inlet Distortion

Internal flow represents an important class of fluid dynamics problems involving flow through an object or device with one or more inflows and exit flows. Many exact or approximate analytical solutions are known for certain simplified conditions that provides the basis for understanding internal flow physics. This research focuses on two aspects of internal flows: analytical solutions for steady flow and an experimental investigation of unsteady flow.

There is no known solution for the general case of steady internal flow. However, certain assumptions sufficiently simplify the problem so that solutions can be obtained. One of these common simplifying assumptions is incompressibility, which limits the applicability of the solution. However, few cases have known solutions for compressible flow. The simplest of these use a quasi-one-dimensional assumption so that the problem simplifies to a single spatial dimension. Further, these cases usually only include a single effect of interest, e.g. heat transfer, friction, area change, etc. The present work describes a new, exact, closed-form solution for steady, quasi-one-dimensional, compressible internal flow problems that includes arbitrary forces (including friction), heat transfer, area change, flow non-uniformity, and even multiple inlet/exit flows.

The analytical study of unsteady internal flows is significantly more challenging relative to steady conditions. For example, consider internal flow through a component in which the upstream stagnation pressure varies periodically in time. The flow within the volume will have a transient response to the changing inlet condition. However, determining an the 'effective response time' is not simple. A somewhat analogous problem can be found can be found in the compressor component of a gas turbine engine. Specifically, the problem of non-uniform inlet conditions on a jet engine compressor can be thought of as unsteady internal flow.

This work shows the results of an experimental investigation of a transonic axial compressor subjected to an approximate 120 degree circumferential inlet stagnation pressure distortion. The goal of the investigation is to experimentally determine the rotor performance when exposed to the distorted and undistorted inlet flow. Measurements of the rotor performance used an inlet rake and stator leading-edge instrumentation. The upstream grid pattern, used to generate the stagnation pressure non-uniformity, was rotated to obtain measurements at different effective circumferential positions. A high sampling frequency stagnation pressure measurement was also used to determine if the rotor was stalled when exposed to the inlet distortion. The resulting rotor performance characteristics are shown for 70%, 90%, and 100% corrected speeds. The distorted and undistorted rotor performance was found to be on two characteristic curves, which were different than the rotor's uniform inlet characteristic. The orbits of the highest flow and near stall conditions were found to be change from the highest flow condition to a near stall condition. It was hypothesized that the orbits are related to the amount of pre-swirl and counter-swirl created by the inlet circumferential static pressure gradient across the distorted-undistorted flow boundary.