Published versio

A rotating cavity with an axial throughflow of cooling air is used to provide a simplified model for the flow that occurs between adjacent corotating compressor disks inside a gas turbine engine. Flow visualization and laser-Doppler anemometry are employed to study the flow structure inside isothermal and heated rotating cavities for a wide range of axial-gap ratios, G, rotational Reynolds number, Reφ, axial Reynolds numbers, Rez, and temperature distributions. For the isothermal case, the superposed axial flow of air generates a powerful toroidal vortex inside cavities with large gap ratios (G ≳ 0.400) and weak counterrotating toroidal vortices for cavities with small gap ratios. Depending on the gap ratio and the Rossby number, ε (where ε ∝ Rez/Reφ), axisymmetric and nonaxisymmetric vortex breakdown can occur, but circulation inside the cavity becomes weaker as e is reduced. For the case where one or both disks of the cavity are heated, the flow becomes nonaxisymmetric: Cold air enters the cavity in a “radial arm” on either side of which is a vortex. The cyclonic and anticyclonic circulations inside the two vortices are presumed to create the circumferential pressure gradient necessary for the air to enter the cavity (in the radial arm) and to leave (in Ekman layers on the disks). The core of fluid between the Ekman layers precesses with an angular speed close to that of the disks, and vortex breakdown appears to reduce the relative speed of precession.

Heat transfer measurements were made in two rotating cavity rigs, in which cooling air passed axially through the center of the disks, for a wide range of flow rates, rotational speeds, and temperature distributions. For the case of a symmetrically heated cavity (in which both disks have the same temperature distribution), it was found that the distributions of local Nusselt numbers were similar for both disks and the effects of radiation were negligible. For an asymmetrically heated cavity (in which one disk is hotter than the other), the Nusselt numbers on the hotter disk were similar to those in the symmetrically heated cavity but greater in magnitude than those on the colder disks; for this case, radiation from the hot to the cold disk was the same magnitude as the convective heat transfer. Although the two rigs had different gap ratios (G = 0.138 and 0.267), and one rig contained a central drive shaft, there was little difference between the measured Nusselt numbers. For the case of “increasing temperature distribution” (where the temperature of the disks increases radially), the local Nusselt numbers increase radially; for a “decreasing temperature distribution,” the Nusselt numbers decrease radially and become negative at the outer radii. For the increasing temperature case, a simple correlation was obtained between the local Nusselt numbers and the local Grashof numbers and the axial Reynolds number.

A relatively simple theory is presented which can be used to model the flow and pressure distributions in a brush seal matrix. The model assumes laminar, compressible, isothermal flow and requires knowledge of an empirical constant: the seal porosity value. Measurements of the mass flowrate together with radial and axial distributions of pressure were taken on a non-rotating experimental rig. These were obtained using a 122 mm bore brush seal with 0.25 mm radial interference. The experimental data are used to estimate the seal porosity. Measurements of the pressure distributions along the backing ring and under the bristle tips are discussed. Predicted mass flows are compared with those actually measured and there is reasonable agreement considering the limitations of the model.

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