Heat transfer study of a triple row impingement channel at large impingement heights

Advanced cooling techniques are required to increase the Brayton cycle temperature ratio necessary for the increase of the overall cycle\u27s efficiency. Current turbine components are cooled with an array of internal cooling channels in the midchord section of the blade, pin fin arrays at the trailing edge and impingement channels in the leading edge. Impingement channels provide the designer with high convective coefficients on the target surface. Increasing the heat transfer coefficient of these channels has been a subject of research for the past 20 years. In the current study, a triple row impingement channel is studied with a jet to target spacing of 6, 8 and 10. The effects of sidewalls are also analyzed. Temperature sensitive paint alongside thin foil heaters are used to obtain heat transfer distributions throughout the target and side walls of the three different channels. Thermal performances were also calculated for the two largest channels. It was found that the side walls provide a significant amount of cooling especially when the channels are mounted side by side so that their sidewalls behave as fins. Similar to literature it was found that an increase in Z/D decreases heat transfer coefficient and provides a more uniform profile. It was also found that the Z/D = 6 and 8 target wall heat transfer profiles are very similar, hinting to the fact that successful potential core impingement may have occurred at height of eight diameters. A Computational Fluid Dynamics, or CFD, study was also performed to provide better insight into the flow field that creates such characteristic heat transfer profiles. The Realizable k-µ solution with enhanced wall functions gave surface heat transfer coefficients 30% off from the experimental data

Heat And Fluid Flow Characterization Of A Single-hole-per-row Impingement Channel At Multiple Impingement Heights

The present work studies the relationship between target and sidewall surfaces of a multirow, narrow impingement channel at various jet heights with one impingement hole per row. Temperature sensitive paint and constant flux heaters are used to gather heat transfer data on the target and side walls. Jet-to-target distance is set to 1, 2, 3, 5, 7 and 9 jet diameters. The channel width is 4 jet diameters and the jet stream wise spacing is 5 jet diameters. All cases were run at Reynolds numbers ranging from 5,000 to 30,000. Pressure data is also gathered and used to calculate the channel mass flux profiles, used to better understand the flow characteristics of the impingement channel. While target plate heat transfer profiles have been thoroughly studied in the literature, side wall data has only recently begun to be studied. The present work shows the significant impact the side walls provide to the overall heat transfer capabilities of the impingement channel. It was shown that the side walls provide a significant amount of heat transfer to the channel. A channel height of three diameters was found to be the optimum height in order to achieve the largest heat transfer rates out of all channels

Comparison Of Heat Transfer Coefficient And Thermal Performance Of A Narrow Impingement Channel

Impingement channel cooling shows great potential for removing large amounts of heat from turbine airfoils. This comes at the expense of a large pressure drop required across the jet plate. The consideration of both heat transfer levels and pressure losses is then important when considering the implementation of this cooling scheme. This paper examines the local and averaged effects of channel height in the presence of side walls on heat transfer coefficients and the additional work required to achieve these levels. Local temperature distributions on target and side wall surfaces were measured using temperature sensitive paint and recorded via a scientific grade charge-coupled device (CCD) camera, and compared to available literature. Streamwise pressure distributions were recorded and used to explain heat transfer trends and determining pumping power requirements. Results are presented for average jet based Reynolds numbers between 17,000 and 45,000. All experiments were carried out on a large scale single row, 5 hole impingement channel, with X/D of 15, Yc/D of 4, and Z/D of 1, 3 and 5. Results showed that the channel with a medium channel height, and high Reynolds number yields both high heat transfer coefficients, at a minimal relative pumping power penalty. On the other hand, the smallest channel height performs best considering constant pressure conditions

Channel Height And Jet Spacing Effect On Heat Transfer And Uniformity Coefficient On An Inline Row Impingement Channel

Future high performance turbine airfoils will likely be cooled in a near wall configuration, potentially employing a combination of narrow, distributed internal cooling channels and impingement. In such applications, the jets impinge against a target surface, and then exit along the channel formed by the jet plate, target plate, and side walls. Local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets and the complex interaction between the jet and the cross flow. Numerous studies have explored the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients, yet with little consideration of thermal stress related effects. A detailed study on the uniformity coefficient that these jets and cross flow generate on the surface is carried out. It is important to maintain a high uniformity coefficient while still having a high heat transfer coefficients to reduce thermal stresses. It is also important to use as little flow as possible while maintaining a high heat transfer coefficient. The study presented experimentally investigates the effects of wall height, jet Reynolds number, and jet spacing on the Nusselt number and uniformity of a narrow inline row impingement channel. The channel height was set at 1, 3, and 5 diameters, jet spacing was 5 and 15 diameters, and the channel width was kept constant at 4 diameters. Although heat transfer coefficients are highly sensitive to the jet Reynolds number and channel height, the uniformity of the distribution is mainly governed by the channel height and jet spacing. A channel height of 3 jet diameters tended to produce the best uniformity coefficients, regardless of the jet to jet spacing; with side walls out performing target surfaces. Copyright © 2010 byASME

Feasibility Study Of Film Source Coupling Transpiration And A Row Of Cylindrical Film Holes

The present study assesses the performance of a film cooling source, composed of a segment of permeable wallinstalled near a row of cylindrical film holes. Coolant is introduced through both the permeable wall and the film holes resulting in a downstream film composed of both transpired and discretely injected coolant. The aerodynamic performance and cooling downstream of this coupled sourceare studied analytically, experimentally andnumerically. The sensitivity of transpiration blowing ratio, discrete hole blowing ratio, discrete hole pitch, transpiration slot width, and the relative positioning of the two sources on these global parameters of interest is the end result. The analytical results are used to get a basic understanding of the issue and design the test matrix for the numerical study. A range of design parameters are studied numerically in a Box-Behnken test matrix to yield global results which are then combined into one parameter, β. A second order response surface of the parameter βallows for a more complete understanding of the coupled source. This numerical analysis is compared with experimental results from two different coupled sources in order to assess the predictions. The results indicate that coupling of the two sources allows a more efficient use of coolant by generating a more uniform initial film. One coupled case in particular shows a 170% increase in cooling over the baseline cylindrical row, a 22% reduction in aerodynamic losses over the baseline transpiration source, all with a 51% reduction in mass flow as compared with the same cylindrical row baseline.Further study and optimization will allow this technique to provide more effective thermal protection at a lower cost of aerodynamic losses and spent coolant. Copyright © 2012 by ASME

Heat Transfer Characteristics of Jet Array Impingement at Low Streamwise Spacing

The present work studies the effect of low streamwise jet-to-jet spacing and uneven spanwise jet-to-jet spacing on target wall heat transfer coefficient in impingement cooling systems. Temperature sensitive paint alongside constant flux heaters were used to gather heat transfer data on the target wall. Two different geometries have been tested with varying jet-to-jet spanwise distance. The streamwise jet spacing was set to 3 jet diameters, the spanwise jet spacing was set to 3, 8 and 13 jet diameters while the jet-to-target spacing was set to 3 jet diameters. The tests were run at three average jet Reynolds numbers of 10,000, 13,000 and 16,000. Results show little effect of crossflow on the target wall heat transfer. Nusselt number profiles are compared to the Florschuetz prediction, the area averaged Nusselt number matches closely; however, the Florschuetz correlation shows a decreasing trend in Nusselt number as a function of streamwise distance while the data shows a Nusselt number profile that remains relatively constant as a function of streamwise distance, x. To better understand the flow physics behind this trend, a CFD run was set up using the v2-f turbulence model for all cases. Computational and experimental results display a strong similarity of their heat transfer trends. The crossflow is seen to not be able to reattach behind each jet due to their proximity to one another. Copyright © 2013 by ASME

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 (TSP) on low thermal conductivity film cooled surfaces. The effects of blowing ratio, surface angle, and hole spacing are observed by testing four fullcoverage arrays composed of cylindrical staggered holes all compounded at 45 deg, which parametrically vary the inclination angles, 30 deg and 45 deg, and the spacing of the holes 14.5 and 19.8 times the diameter. Local film cooling effectiveness is obtained throughout these largely spaced arrays to 23 rows for the 19.8 diameter spacing array and 30 rows for the 14.5 diameter 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 merges laterally and provides high effectiveness in the gaps between injections. At low blowing, each individual jet remains discrete throughout the array. At higher blowing rates, the profile is far more uniform due to jets spreading as they reattach with the wall. Laterally averaged values of effectiveness approach 0.3 in most cases with some high blowing low spacing, even reaching 0.5

Heat Transfer Characteristics Of Jet Array Impingement At Low Streamwise Spacing

The present work studies the effect of low streamwise jet-to-jet spacing and uneven spanwise jet-to-jet spacing on target wall heat transfer coefficient in impingement cooling systems. Temperature sensitive paint alongside constant flux heaters were used to gather heat transfer data on the target wall. Two different geometries have been tested with varying jet-to-jet spanwise distance. The streamwise jet spacing was set to 3 jet diameters, the spanwise jet spacing was set to 3, 8 and 13 jet diameters while the jet-to-target spacing was set to 3 jet diameters. The tests were run at three average jet Reynolds numbers of 10,000, 13,000 and 16,000. Results show little effect of crossflow on the target wall heat transfer. Nusselt number profiles are compared to the Florschuetz prediction, the area averaged Nusselt number matches closely; however, the Florschuetz correlation shows a decreasing trend in Nusselt number as a function of streamwise distance while the data shows a Nusselt number profile that remains relatively constant as a function of streamwise distance, x. To better understand the flow physics behind this trend, a CFD run was set up using the v2-f turbulence model for all cases. Computational and experimental results display a strong similarity of their heat transfer trends. The crossflow is seen to not be able to reattach behind each jet due to their proximity to one another. Copyright © 2013 by ASME

Experimental Evaluation Of Large Spacing Compound Angle Full Coverage Film Cooling Arrays: Heat Transfer Augmentation

Heat transfer augmentation values are obtained experimentally with the use of temperature sensitive paint on constant flux heaters attached to full coverage film cooled surfaces as a function of blowing ratio. The effects of blowing ratio, surface angle and hole spacing are observed by testing four full coverage arrays of round 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 heat transfer augmentation is obtained throughout these largely spaced arrays over 20 rows for the 19.8 spacing array and 30 rows for the 14.5 spacing array. The first five to six rows show low heat transfer enhancement between holes with peaks in augmentation occurring directly downstream of the hole. Heat transfer enhancement is seen to be close to unity at the leading edge of the arrays. Laterally averaged values of heat transfer augmentation increase every row, leveling out to values between 20 and 30% augmentation with peaks reaching the 40% mark. Copyright © 2012 by ASME

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 © 2012 by ASME