Greg Natsui

A Detailed Uncertainty Analysis of Adiabatic Film Cooling Effectiveness Measurements Using Pressure Sensitive Paint

Pressure sensitive paint (PSP) can be a powerful tool in measuring the adiabatic film cooling effectiveness. There are two distinct sources of error for this measurement technique; the ability to experimentally obtain the data and the validity of the heat and mass transfer analogy for the problem being studied. This paper will assess the experimental aspect of this PSP measurement specifically for film cooling applications.Experiments are conducted in an effort to quantifiably bound expected errors associated with temperature non-uniformities in testing and photo-degradation effects. Results show that if careful experimental procedures are put in place, both of these effects can be maintained to have less than 0.022 impact on effectiveness.Through accurate semi-in-situ calibration down to 4% atmospheric pressure, the near-hole distribution of effectiveness is measured with high accuracy. PSP calibrations are performed for multiple coupons, over multiple days. In addition, to reach a partial pressure of 0 the calibration vessel was purged of all air by flowing CO2.The primary contribution of this paper lies in the uncertainty analysis performed on the PSP measurement technique. A thorough uncertainty analysis is conducted and described, in order to completely understand the presented measurements and any shortcomings of the PSP technique. This quantification results in larger, albeit more realistic, values of uncertainty, and helps provide a better understanding of film cooling effectiveness measurements taken using the PSP technique. The presented uncertainty analysis takes into account all random error sources associated with sampling and calibration, from intensities to effectiveness.Adiabatic film cooling effectiveness measurements are obtained for a single row of film cooling holes inclined at 20 degrees, with CO2 used as coolant. Data is obtained for six blowing ratios. Maps of uncertainty corresponding to each effectiveness profile are available for each test case. These maps show that the uncertainty varies spatially over the test surface, high effectiveness corresponds to low uncertainty. The noise floors can be as high as 0.04 at effectiveness levels of 0. Day-to-day repeatability is presented for each blowing ratio and shows that laterally averaged effectiveness data is repeatable within 0.02 effectiveness.Copyright

A Detailed Uncertainty Analysis of Adiabatic Film Cooling Effectiveness Measurements Using Pressure-Sensitive Paint

Pressure-sensitive paint (PSP) can be a powerful tool in measuring the adiabatic film cooling effectiveness. There are two distinct sources of error for this measurement technique: the ability to experimentally obtain the data and the validity of the heat and mass transfer analogy for the problem being studied. This paper will assess the experimental aspect of this PSP measurement specifically for film cooling applications. Experiments are conducted in an effort to quantifiably bound expected errors associated with temperature nonuniformities in testing and photodegradation effects. Results show that if careful experimental procedures are put in place, both of these effects can be maintained to have less than 0.022 impact on effectiveness. Through accurate semi in situ calibration down to 4% atmospheric pressure, the near-hole distribution of effectiveness is measured with high accuracy. PSP calibrations are performed for multiple coupons, over multiple days. In addition, to reach a partial pressure of zero the calibration vessel was purged of all air by flowing CO2. The primary contribution of this paper lies in the uncertainty analysis performed on the PSP measurement technique. A thorough uncertainty analysis is conducted and described, in order to completely understand the presented measurements and any shortcomings of the PSP technique. This quantification results in larger, albeit more realistic, values of uncertainty and helps provide a better understanding of film cooling effectiveness measurements taken using the PSP technique. The presented uncertainty analysis takes into account all random error sources associated with sampling and calibration, from intensities to effectiveness. Adiabatic film cooling effectiveness measurements are obtained for a single row of film cooling holes inclined at 20 deg, with CO2 used as coolant. Data are obtained for six blowing ratios. Maps of uncertainty corresponding to each effectiveness profile are available for each test case. These maps show that the uncertainty varies spatially over the test surface and high effectiveness corresponds to low uncertainty. The noise floors can be as high as 0.04 at effectiveness levels of 0. Day-to-day repeatability is presented for each blowing ratio and shows that laterally averaged effectiveness data are repeatable within 0.02 effectiveness.

The Effect of Transpiration on Discrete Injection for Film Cooling

A segment of permeable wall is installed near a row of cylindrical film holes, parallel to the flow and inclined at 35 degrees. Coolant is forced through both the permeable wall and the film holes resulting in a downstream film composed of both transpired and discretely injected coolant. The permeable wall extends 1.5 cylindrical hole diameters in the flow direction. The effects on the aerodynamic performance and cooling downstream of the row of cylindrical holes in the presence of transpiration is studied numerically with a procedure validated by hot-wire anemometer and temperature sensitive paint measurements. The hydrodynamic boundary layer in the presence of film and adiabatic film cooling effectiveness downstream of single and coupled film sources are compared with numerical predictions. The performance of the coolant film is predicted in order to understand the sensitivity of cooling and aerodynamic losses on the relative positioning of the two sources at each blowing ratio. The results indicate that a coupling of the two sources allows a more efficient use of coolant by generating a more uniform initial film. With careful optimization the discrete holes can be placed farther apart laterally and operate at a lower blowing ratio with a transpiration segment making the large deficits in cooling effectiveness mid-pitch less severe, overall minimizing coolant usage. Comparisons of linear superposition predictions of the two independent sources with the corresponding coupled scenario indicate the two films positively influence one another and surpass additive predictions of cooling. All relative placements have an overall beneficial effect on the cooling seen by the protected wall. Some cases show an increase in area-averaged film cooling effectiveness of 300% along with a 50% increase in aerodynamic loss coefficient by injecting an additional 10% coolant. In this study the downstream transpiration placement is found to perform best of the three geometries tested while considering cooling, aerodynamic losses, local uniformity and manufacturing feasibility. With further study and optimization this technique can potentially provide more effective thermal protection at a lower cost of aerodynamic losses and spent coolant.Copyright

Hot-Wire Study on the Impact of Porous Structure on Mean and Turbulent Velocity Profiles in the Near-Field of a High Aspect Ratio Porous Filled Slot Jet

A 2-D rectangular slot jet (AR=61) with a porous blockage is experimentally tested for mean velocity and turbulence profiles in the near field. Porosities of the blockages tested are nominally 0, 0.40, 0.50, and 0.60, all crushed aluminum foam. The presence of the porous blockage can be seen in the deformed mean profile and in the lower magnitudes of turbulence intensity. The porous blockage acts to change the relevant length scale to that more on the order of the pore size rather than the slot width. Using a method meant for standardizing the turbulent length scale calculation, a length scale for each case is calculated and found to vary weakly with respect to porosity. The three length scales are calculated for each case and are compared. The length scale based on the zero frequency extrapolation of the power spectral density gave the most reasonable results. Adiabatic film cooling effectiveness values are given for three blowing ratios for the 0.60 porosity insert. Film effectiveness is seen to increase with mass injected over the entire test surface.

Experimental study of transverse jet mapping using PLIF

Planar Laser Induced Fluorescence (PLIF) with acetone seeding is applied to measure the scalar fields of an axisymmetric freejet and an inclined jet-in-crossflow as applicable to film cooling. From the scalar fields, jet-mixing and trajectory characteristics are obtained. In order to validate the technique, the canonical example of a nonreacting freejet of Reynolds Numbers 900-9000 is investigated. After validating the technique with the axisymmetric jet, the jet-in-crossflow was tested with various velocity ratios and jet injection angles. Results indicate the degree of wall separation for different injection angles and demonstrate both the time-averaged trajectories as well as near-wall concentration results for varying jet momentum fluxes. Consistent with literature findings, the orthogonal jet trajectory for varying blowing ratios collapses when scaled by the jet-to-freestream velocity ratio and hole diameter, r_d. Similar collapsing is demonstrated in the case of a nonorthogonal jet. Computational Fluid Dynamic (CFD) simualtions using the OpenFOAM software is used to compare predictions with a select experimental case, and yields reasonable agreement.

Experimental Evaluation of Large Spacing Compound Angle Full Coverage Film Cooling Arrays: Adiabatic Film-Cooling Effectiveness

Adiabatic film cooling effectiveness contours are obtained experimentally with the use of temperature sensitive paint on low thermal conductivity full coverage film cooled surfaces. The effects of blowing ratio, surface angle and hole spacing are observed by testing four full coverage arrays composed of cylindrical staggered holes all compounded at 45°, which parametrically vary the inclination angle, 30° and 45°, and the spacing of the holes, 14.5 and 19.8 diameters. Local film cooling effectiveness is obtained throughout these largely spaced arrays over up to 23 rows for the 19.8 spacing array and 30 rows for the 14.5 spacing array. The coolant takes several rows to merge and begin to interact with lateral holes at these large spacings, however; at downstream rows the film builds and provides high effectiveness in the gaps between injection. At low blowing, each individual jet throughout the entire array can be seen in the effectiveness profiles. At higher blowing rates, the profile is far more uniform due to jets spreading as they reattach with the cooled wall. Laterally averaged values of effectiveness easily approach 0.3 in most cases with some, high blowing low spacing, even reaching 0.5.Copyright

Planar Laser-Induced Fluorescence Experiments and Modeling Study of Jets in Crossflow

Planar Laser Induced Fluorescence (PLIF) with acetone seeding was applied to measure the scalar fields of an axisymmetric freejet and an inclined jet-in-crossflow as applicable to film cooling. From the scalar fields, jet-mixing and trajectory characteristics were obtained. In order to validate the technique, the canonical example of a nonreacting freejet of Reynolds Numbers 900-9000 was investigated. Desired structural characteristics were observed and showed strong agreement with computational modeling. After validating the technique with the axisymmetric jet, the jet-in-crossflow was tested with various velocity ratios and jet injection angles. Results indicated the degree of wall separation for different injection angles and demonstrate both the time-averaged trajectories as well as select near-wall concentration results for varying jet momentum fluxes. Consistent with literature findings, the orthogonal jet trajectory for varying blowing ratios collapsed when scaled by the jet-to-freestream velocity ratio and hole diameter, rd. Similar collapsing was demonstrated in the cases of a non-orthogonal jets. Computational Fluid Dynamic (CFD) simulations using the OpenFOAM software were used to compare predictions with select experimental cases, and yielded reasonable agreement. Insight into the importance and structure of the counter rotating vortex pair and general flowfield turbulence was highlighted by cross validation between CFD and experimental results.

Adiabatic Film Cooling Effectiveness Measurements Throughout Multi-Row Film Cooling Arrays

Adiabatic film cooling effectiveness measurements are obtained using pressure-sensitive paint (PSP) on a flat film cooled surface. The effects of blowing ratio and hole spacing are investigated for four multi-row arrays comprised of 8 rows containing 52 holes of 3.8 mm diameter with 20° inclination angles and hole length-to-diameter ratio of 11.2.The four arrays investigated have two different hole-to-hole spacings composed of cylindrical and diffuser holes. For the first case, lateral and streamwise pitches are 7.5 times the diameter. For the second case, pitch-to-diameter ratio is 14 in lateral direction and 10 in the streamwise direction. The holes are in a staggered arrangement. Adiabatic effectiveness measurements are taken for a blowing ratio range of 0.3 to 1.2 and a density ratio of 1.5, with CO2 injected as the coolant.A thorough boundary layer analysis is presented, and data was taken using hotwire anemometry with air injection, with boundary layer and turbulence measurements taken at multiple locations in order to characterize the boundary layer. Local effectiveness, laterally averaged effectiveness, boundary layer thickness, momentum thickness, turbulence intensity and turbulence length scale are presented. For the cylindrical holes, at the first row of injection, the film jets are still attached at a blowing ratio of 0.3. By a blowing ratio of 0.5, the jet is observed to lift off, and then impinge back onto the test surface. At a blowing ratio of 1.2, the jets lift off, but reattach much further downstream, spreading the coolant further along the test surface. A thorough uncertainty analysis has been conducted in order to fully understand the presented measurements and any shortcomings of the measurement technique. The maximum uncertainty of effectiveness and blowing ratio is 0.02 counts of effectiveness and 3 percent respectively.Copyright

The Effect of Inclination Angle on Turbulent Quantities of a Single Row of Cylindrical Jets in Crossflow

This is an experimental investigation into the effect of inclination angle and lateral spacing on the variation of mean/turbulent velocity profiles for a single row of cylindrical jets issuing into a turbulent boundary layer. Extensive velocity measurements are made in the region near injection for a single row of film cooling holes, to determine how hole orientation affects turbulent length scales and aerodynamic performance. Inclination angles of 30°, 35°, 45°, 60°, 75°, and 90° are tested, while the film cooling hole’s length (L/D) and lateral spacing’s (P/D) are fixed for each angle at 4.25 and 3, respectively. The measurement domain exists from x/D=0 to 3, y/D=-1.5 to 1.5 and z/D=0 to 2 at nominal blowing ratios of M=0.5, 1, and 1.5. This data represents an extension of the flow measurements previously available in literature, and supplements companion heat transfer studies performed with the same experimental setup and test specimen. Experimental measurements of turbulence will be expressed by mean and fluctuating velocity, length and time-scales, spectral analysis, and flat plate boundary layer thickness calculations. Such turbulence quantification for a single row of cylindrical holes at various inclination angles will provide assistance for better formulation of boundary conditions and turbulence specification in Computational Fluid Dynamics (CFD) simulations.

Heat Transfer in a Coupled Impingement-Effusion Cooling System

Modern research on gas turbine cooling continues to focus on the optimization of different cooling designs, and better understanding of the underlying flow physics so that cooling schemes can be coupled together. The current study focuses on one particular coupled cooling design: an impingement-effusion cooling system, which combines impingement cooling on the backside of the cooled component and full coverage effusion cooling on the exposed surface.The goal of this study is to explore a wide range of geometrical parameters outside the ranges normally reported in the available literature. Particular attention is given to the total coolant spent per unit surface area cooled. Through determination of impingement heat transfer, film cooling effectiveness, and film cooling heat transfer on the target wall, a simplified heat transfer model of the cooled component is developed to show the relative impact of each parameter on the overall cooling effectiveness.The use of Temperature Sensitive Paint (TSP) for data acquisition allows for high resolution local heat transfer and effectiveness results. Impingement arrays with local extraction of coolant via effusion are able to produce higher overall heat transfer, as no significant cross flow is present to deflect the impinging jets. Low jet-to-target-plate spacing produces the highest yet most non-uniform heat transfer distribution; at high spacing the heat transfer rate is much less sensitive to impingement height.Arrays with high hole-to-hole spacing and high jet Reynold’s number are more effective (per mass of coolant used) than tightly spaced holes at low jet Reynold’s number. On the effusion side, staggered hole arrangements provide significantly higher film cooling effectiveness than their in-line counterparts as the staggered arrangement minimizes jet interactions and promotes a more even lateral distribution of coolant.Copyright