Phil Ligrani

Flow structure due to dimple depressions on a channel surface

Instantaneous, dynamic and time-averaged characteristics of the vortex structures which are shed from the dimples placed on one wall of a channel are described. The dimpled test surface contains 13 staggered rows of dimples in the streamwise direction, where each dimple has a print diameter of 5.08 cm, and a ratio of depth to print diameter of 0.2. Considered are Reynolds numbers (based on channel height) ReH from 600 to 11 000, and ratios of channel height to dimple print diameter H/D of 0.25, 0.50, and 1.00. For all three H/D, a primary vortex pair is periodically shed from the central portion of each dimple, including a large upwash region. This shedding occurs periodically and continuously, and is followed by inflow advection into the dimple cavity. The frequency of these events appears to scale on time-averaged bulk velocity and dimple print diameter, which gives nondimensional frequencies of 2.2–3.0 for all three H/D values considered. As H/D decreases, (i) the strength of the primary vortex pair in...

Flow visualization of Dean vortices in a curved channel with 40 to 1 aspect ratio

Results from a flow visualization study of Dean vortex flow are presented. These were obtained over a range of Dean numbers from 40 to 220 using a transparent channel with mild curvature, an aspect ratio of 40 to 1, and an inner to outer radius ratio of 0.979. Observations and photographs show evidence of pairs of counter‐rotating Dean vortices indicated by mushroom‐shaped smoke patterns for Dean numbers greater than 64 and angular positions at least 85° from the start of curvature. Photographs showing nonsymmetric Dean vortices with rocking motion are presented and believed to be evidence of a twisting mode of oscillations. Dean vortices with oscillations mostly in the radial direction are also observed, which are believed to strongly depend on the small amplitude disturbances that trigger initial vortex development. Photographic evidence of small secondary vortex pairs, and vortices with simultaneous radial and spanwise oscillations are also given along with a domain map showing the experimental conditi...

Film Cooling From Shaped Holes

Local and spatially averaged magnitudes of the adiabatic film cooling effectiveness, the iso-energetic Stanton number ratio, and film cooling performance parameter are measured downstream of (i) cylindrical round, simple angle (CYSA) holes, (ii) laterally diffused, simple angle (LDSA) holes, (iii) laterally diffused, compound angle (LDCA) holes, (iv) forward diffused, simple angle (FDSA) holes, and (v) forward diffused, compound angle (FDCA) holes. Data are presented for length-to-inlet metering diameter ratio of 3, blowing ratios from 0.4 to 1.8, momentum flux ratios from 0.17 to 3.5, and density ratios from 0.9 to 1.4. The LDCA and FDCA arrangements produce higher effectiveness magnitudes over much wider ranges of blowing ratio and momentum flux ratio compared to the three simple angle configurations tested

Nusselt Number Behavior on Deep Dimpled Surfaces Within a Channel

Experimental results, measured on a dimpled teat surface placed on one wall of a channel, are given for a ratio of air inlet stagnation temperature to surface temperature of approximately 0.94, and Reynolds numbers R eH from 12,000 to 70,000. These data include friction factors, local Nusselt numbers, spatially-resolved local Nusselt numbers, and globally-averaged Nusselt numbers. The ratio of dimple depth to dimple print diameter δ/D is 0.3, and the ratio of channel height to dimple print diameter is 1.00. These results are compared to measurements from other investigations with different ratios of dimple depth to dimple print diameter δ/D to provide information on the influences of dimple depth. At all Reynolds numbers considered, local and spatially-resolved Nusselt number augmentations increase as dimple depth increases (and all other experimental and geometric parameters are held approximately constant). These are attributed to: (i) increases in the strengths and intensity of vortices and associated secondary flows ejected from the dimples, as well as (ii) increases in the magnitudes of three-dimensional turbulence production and turbulence transport

Subminiature hot-wire sensors: development and use

Subminiature hot-wire sensors have been developed and qualified to measure small-scale turbulent motions. Measurements were made with hot wires having 0.625, 1.25 and 5 mu m nominal diameters and lengths ranging from 50 mu m to 3 mm. The smallest sensor has a spatial resolution about ten times better than typical commercially available probes. The measurements were made in the inner layer of a turbulent boundary layer near a smooth wall where the viscous length scale v/Ur was about 50 mu m. The data show how measurement of time-averaged longitudinal turbulence intensity is dependent on sensor dimensions.

Heat Transfer Augmentation Technologies for Internal Cooling of Turbine Components of Gas Turbine Engines

To provide an overview of the current state of the art of heat transfer augmentation schemes employed for internal cooling of turbine blades and components, results from an extensive literature review are presented with data from internal cooling channels, both with and without rotation. According to this survey, a very small number of existing investigations consider the use of combination devices for internal passage heat transfer augmentation. Examples are rib turbulators, pin fins, and dimples together, a combination of pin fins and dimples, and rib turbulators and pin fins in combination. The results of such studies are compared with data obtained prior to 2003 without rotation influences. Those data are comprised of heat transfer augmentation results for internal cooling channels, with rib turbulators, pin fins, dimpled surfaces, surfaces with protrusions, swirl chambers, or surface roughness. This comparison reveals that all of the new data, obtained since 2003, collect within the distribution of globally averaged data obtained from investigations conducted prior to 2003 (without rotation influences). The same conclusion in regard to data distributions is also reached in regard to globally averaged thermal performance parameters as they vary with friction factor ratio. These comparisons, made on the basis of such judgment criteria, lead to the conclusion that improvements in our ability to provide better spatially-averaged thermal protection have been minimal since 2003. When rotation is present, existing investigations provide little evidence of overall increases or decreases in overall thermal performance characteristics with rotation, at any value of rotation number, buoyancy parameter, density ratio, or Reynolds number. Comparisons between existing rotating channel experimental data and the results obtained prior to 2003, without rotation influences, also show that rotation has little effect on overall spatially-averaged thermal performance as a function of friction factor.

An infrared thermography imaging system for convective heat transfer measurements in complex flows

An infrared thermography imaging system is described for spatially resolved convective heat transfer measurements when used in conjunction with thermocouples, energy balances, digital image processing, zinc-selenide windows, and unique in situ calibration procedures. The usefulness of the system and the techniques developed are demonstrated by measurements made in two different environments with complex, three-dimensional flow features. First, spatial variations of surface Nusselt numbers are measured along the concave surfaces of a swirl chamber whose geometry models an internal passage used to cool the leading edge of a turbine blade. Second, spatially resolved distributions of the adiabatic film-cooling effectiveness are measured downstream of film-cooling holes on a symmetric turbine blade in transonic flow.

Analysis and Experiments on Three-Dimensional, Irregular Surface Roughness

Randomly placed, nonuniform, three-dimensional roughness with irregular geometry and arrangement is analyzed. New correlations are presented for such roughness for determination of magnitudes of equivalent sand grain roughness size k s from a modified version of the Sigal and Danberg parameter Λ s . Also described are the numerical procedures employed to determine Λ s from three-dimensional profilometry data. The sand grain roughness values determined with this approach are then compared with and verified by k s magnitudes determined using: (i) analytic geometry for uniformly shaped roughness elements arranged in a regular pattern on a test surface, and (ii) measurements made with nonuniform, three-dimensional, irregular roughness with irregular geometry and arrangement. The experiments to obtain these measurements are conducted using this latter type of roughness placed on the walls of a two-dimensional channel. Skin friction coefficients are measured in this channel with three different types of rough surfaces on the top and bottom walls

Miniature five-hole pressure probe for measurement of three mean velocity components in low-speed flows

A miniature five-hole pressure probe is described which has a probe tip diameter of 1.22 mm. The probe was developed to measure three mean velocity components using a non-nulling procedure at individual locations in a curved channel where differential pressures are low, flow is three-dimensional and laminar, and spatial resolution and flow blockage effects are important considerations. Calibration coefficients are presented for Reynolds numbers based on probe tip diameter ranging from 80 to 400. Coefficients for yaw angle, pitch angle, total pressure and total minus static pressure are independent of Reynolds number within experimental uncertainties. Experimental details unique to the use of the miniature probe are also given along with distributions of mean velocity and mean vorticity over a portion of the 12.7 mm*508.0 mm cross section of a curved channel. At a Dean number of 119 (mean streamwise velocity approximately 1.2 m s-1), structural characteristics of Dean vortices are evident from contours of the streamwise velocity, the total pressure and the streamwise component of mean vorticity determined from secondary flow vectors.

Spatial resolution and downwash velocity corrections for multiple-hole pressure probes in complex flows

This work was sponsored by the US Army Office of Aviation Research and Development, NASA-Defense Purchase Request No. C-80019-F. Dr. K. Civinskas was the program monitor.