Detailed measurements on a row of jets in a crossflow : With applications
Creators: name: Ajersch, Peter
Publisher: name: University of British Columbia
Year: 2009

description: The arrangement of multiple jets issuing perpendicularly to a crossflow is a fundamental fluid flow problem relevant to several engineering applications. One such application is the film cooling of gas turbine blades by discrete hole injection; another is the injection of combustion air in the lower levels of a Kraft recovery boiler. Both have been investigated experimentally and computationally. As the associated flow fields are three-dimensional and highly complex, and the effects of system parameters — such as fluid injection rate and hole spacing — on system performance are not fully appreciated, two experimental studies were conducted. The first was designed to amass detailed measurements about a single row of jets, and to analyze the data with regard to film cooling. The second was designed to determine the characteristics of the flow field in a Kraft recovery boiler. In the first set of experiments, a row of six square jets spaced at 3.0 jet widths was arranged in a low-speed wind tunnel, and air was injected into the free stream at jet-to-crossflow velocity ratios (R) of 0.5, 1.0, and 1.5. The row was aligned at 90° to the direction of the crossflow, and the jet axes were perpendicular to the tunnel floor. Jet and crossflow air were maintained at approximately equal temperatures. Measurements of the mean velocities and six flow stresses were acquired by three-component, coincidence-mode, laser Doppler velocimetry (LDV). Seed particles necessary for this technique were generated in both the jet and crossflow air by means of commercially available smoke machines. To complement the detailed measurements, flow visualization was performed by transmitting the laser light through a cylindrical lens, thereby brightly illuminating a given portion of the test section with a 5 mm wide sheet of light. Only the jets were seeded with smoke; therefore it was possible to acquire video recordings showing the penetration of the jets in various cross-tunnel planes. Results of the L D V measurements revealed distinctive trends of jets in a crossflow such as 3-D separation on the lee side of injection and counter-rotating vortex pairs; however, these were less evident in the case of the weakest (R=0.5) jets. For R&gt;1.0, the jets penetrated the turbulent free stream boundary layer and interacted with the crossflow, while in the case of R=0.5, they did not. Flow visualization clearly demonstrated the turbulent fluctuations in the flow, as well as possible unsteadiness. Mixing of the jets with the crossflow and with one another, as indicated by the penetration of smoke, increased dramatically with velocity ratio. In the second set of experiments, water was injected through three levels of injection ports in a 1/28 scale model of a Kraft recovery boiler, thereby simulating the isothermal flow of combustion air. Flow velocities and normal stresses in selected cross-sections were measured with two-component random-mode L D V . A second optical measurement method, particle image velocimetry (PIV), was used to examine the large-scale unsteadiness thought to be present in the boiler. For the conditions investigated, results showed that the flow was distributed neither evenly nor symmetrically across the boiler, and that day-to-day variations in the mean flow patterns were possible. The flow was generally characterized by a vertically rising core surrounded by regions of recirculation in the lower furnace, and a more even velocity distribution higher up. The core tended to drift toward one of the boiler walls, as opposed to rising vertically up the center. PIV results revealed the presence of large-scale low-frequency fluctuations in the flow, which motivates one to reconsider the meaning of mean and fluctuating velocities in such an arrangement. These unsteady flow conditions were largely considered to be a factor of the non-uniform distribution of fluid injection, and the instabilities this may have induced.