Showing papers by "Christopher J. Elkins published in 2015"

Three‑dimensional inspiratory flow in the upper and central human airways

Abstract: The steady inspiratory flow through an anatomically accurate model of the human airways was studied experimentally at a regime relevant to deep inspiration for aerosol drug delivery. Magnetic resonance velocimetry was used to obtain the three-component, mean velocity field. A strong, single-sided streamwise swirl was found in the trachea and persists up to the first bifurcation. There, the swirl and the asymmetric anatomy impact both the streamwise momentum distribution and the secondary flows in the main bronchi, with large differences compared to what is found in idealized branching tubes. In further generations, the streamwise velocity never recovers a symmetric profile and the relative intensity of the secondary flows remains strong. Overall, the results suggest that, in real human airways, both streamwise dispersion (due to streamwise gradients) and lateral dispersion (due to secondary flows) are very effective transport mechanisms. Neglecting the extrathoracic airways and idealizing the bronchial tree may lead to qualitatively different conclusions.

Film Cooling Effectiveness Improvements Using a Nondiffusing Oval Hole

Abstract: The need for improvements in film cooling effectiveness over traditional cylindrical film cooling holes has led to varied shaped hole and sister hole designs of increasing complexity. This paper presents a simpler shaped-hole design which shows improved film cooling effectiveness over both cylindrical holes and diffusing fan-shaped holes without the geometric complexity of the latter. Magnetic resonance imaging measurement techniques are used to reveal the coupled 3D velocity and coolant mixing from film cooling holes which are of a constant oval cross-section as opposed to round. The oval shaped hole yielded an area-averaged adiabatic effectiveness twice that of the diffusing fan-shaped hole tested. Three component mean velocity measurements within the channel and cooling hole showed the flow features and vorticity fields which explain the improved performance of the oval shaped hole. As compared to the round hole, the oval hole leads to a more complex vorticity field which reduces the strength of the main counter-rotating vortex pair. The counter-rotating vortex pair acts to lift the coolant away from the turbine blade surface and thus strongly reduces the film cooling effectiveness. The weaker vortices allow coolant to stay closer to the blade surface and to remain relatively unmixed with the main flow over a longer distance. Thus, the oval-shaped film cooling hole provides a simpler solution for improving film cooling effectiveness beyond circular hole and diffusing hole designs.Copyright © 2015 by ASME

Quantitative MRI Measurements of Hot Streak Development in a Turbine Vane Cascade

Abstract: Hot streaks from the combustor and cool streaks from nozzle vane film cooling impose strong inlet temperature variations on high pressure turbine blades, which can lead to local hot or cold spots, high thermal stresses, and fatigue failures. Furthermore, the complex three dimensional flows around the vane may act to concentrate cool or hot fluid exiting the vane row. In order to optimize the cooling design of the turbine blades, the designer must be able to predict the temperature distribution entering the turbine rotor. Therefore, it is important to understand and predict how combustor hot streaks are dispersed as they pass through the vane row. The goal of the present work is to provide detailed three dimensional velocity and temperature data for simulated combustor hot streaks developing through a film cooled vane cascade using the Magnetic Resonance Velocity/Concentration experimental technique. The measurements show that the hot streaks are thinned by acceleration through the vane cascade and diffused by turbulence. The turbulent diffusivity is suppressed by acceleration and leaves significant temperature nonuniformity in the vane wake.Copyright © 2015 by ASME

Building Block Experiments in Discrete Hole Film Cooling

Abstract: This paper reports a series of building block experiments for discrete hole film cooling Seven different configurations, including variations in injection wall curvature, mainstream pressure gradient, and boundary layer thickness are measured for a round film cooling hole, inclined 30 degrees at injection, and operated at a blowing ratio of unity Full three dimensional, three component velocity fields and scalar coolant concentration fields are acquired using Magnetic Resonance Imaging (MRI) techniques The results show the effect of varying the mainstream condition on the mean coolant concentration distribution and mean velocity field, including the counter-rotating vortex pair (CVP), a dominant feature of jet in crossflow type flows The present study focuses on an analysis of the building block configurations only possible with full three dimensional velocity and concentration fields Several scalar parameters including normalized perimeter, jet trajectory, maximum coolant concentration, and coolant concentration spread are extracted from the collected data and compared across the different configurations The results indicate that the pressure gradient variations have the strongest effect on the calculated quantities, the boundary layer slightly less, and the curvature very little© 2015 ASME

Turbulent Dispersion of Film Coolant and Hot Streaks in a Turbine Vane Cascade

Abstract: : Magnetic resonance imaging techniques were used to investigate the 3D mean flow and turbulent mixing around a film cooled turbine vane. The overall objective was to understand the turbulent mixing in a complex flow and develop tools to determine the non uniform temperature distribution incident on a downstream turbine rotor. Magnetic resonance velocimetry provided the three component velocity distribution throughout a double passage vane cascade. The magnetic resonance concentration technique was used to measure the concentration of film coolant injected from trailing edge slots. The turbulent dispersion was strongly affected by vortex structures produced by film cooling slots. The passage vortex increased the spread of coolant near the end walls. Combustor hot streaks injected upstream of the cascade dispersed very slowly because turbulence is strongly suppressed by the acceleration through the cascade. A separate experiment examined the relevance of magnetic resonance experiments in water to turbine flows at high subsonic Mach numbers in air. Identical 3D mixing layer experiments were performed with low speed water mixing a chemical agent and high speed air flows mixing temperature. The dimensionless concentration/temperature profiles were nearly identical between the experiments.