In this section the adiabatic film effectiveness performance of the different hole geometries are compared directly. Due to differences in the design and manufacture of each test plate over time the first row of cooling holes are not located in the same place in all designs, with the cylindrical holes starting closer to the upstream edge of the test plate than the other designs. For direct comparison purposes therefore, the spanwise averaged data in each case are shifted so the leading edge of the first row of hole exits are always located at the same point. The geometries are compared at the high freestream turbulence intensity condition and at the highest and lowest momentum ratio conditions equivalent to wall pressure drops of 0.5% and 2.5% at engine cycle conditions. Surface contour data for the ≅ 15 momentum ratio condition is given in Figure 85 and a spanwise averaged plot in Figure 86, similar data for the ≅ 3 momentum ratio condition is given in Figures 87 and 88.
It is immediately apparent that all of the more complex geometries produce a superior film to the Cylindrical hole case at all observed conditions, with the Cylindrical design only reaching a spanwise averaged effectiveness of 0.5 by the last row of the cooling array. In comparison to the remaining geometries which reach this level by the second row of coolant holes. The major factor causing this superior performance is the tendency for the coolant to remain close to the surface through a reduction in jet velocity as a result of flow diffusion within a fan-like geometry located at the effusion hole exit. Through fanning the exit and reducing the jet velocity the actual blowing and momentum ratios are reduced, a factor which is critical to creating a good film. A second contributor to the increased performance comes as a result of ensuring the coolant covers a wider lateral area; the two methods of accomplishing this are lateral fanning of the hole exit, as employed by the Spey, Modified, Circular and Rectilinear Helical geometries, and decreased lateral pitch, as used by the Slotted geometry. Both methods give rise to much better coolant coverage from the first row onwards.
As the coolant delivery holes in the Slotted design are more tightly packed in the lateral direction, a smaller surface area is exposed to the mainstream flow. This gives rise to the much higher spanwise averaged adiabatic effectiveness of 0.5 in line with the first row as roughly half the area is taken up by the coolant holes. This then drops to around 0.4 as the coolant jet spreads over the surface before being replenished by the second row. In order to keep the
increased, resulting in the coolant having to cover a larger streamwise area before being augmented by the successive row. As a result, despite starting at a higher initial average adiabatic film effectiveness than the other designs, the Slotted geometry is out performed by all the laterally fanned designs as their second row of coolant is injected much sooner, allowing the film to be more incrementally enhanced. The lack of lateral fanning results in much less spanwise spreading of the coolant after exiting the hole resulting in a much less uniform effectiveness pattern.
From the 2D surface contours it can be seen that the Spey and Modified fanned geometries suffer from poor hole definitions caused by the DLD manufacturing technique. Due to the inherent build direction there is an obvious asymmetry within the holes of both these and the cylindrical hole test plates. The other test plates indicate no obvious signs of asymmetric manufacture on the surface contour data. The Slotted geometry is designed with an asymmetric inlet and flow passage which appears to minimise the effect of build direction. Any exit flow asymmetry for the Slotted or Helical designs is a result of the asymmetrical design of the flow passages. Insensitivity to build direction is an important feature of any DLD enabled design.
The two helical geometries show a slight bias towards one side of the fanned section; this is thought to be due to the direction of the internal helix presenting the fan with a biased flow at its entry. It can also be seen that despite similar span averaged results, the Rectilinear Helical design shows a less uniform surface effectiveness distribution than the Circular Helical geometry with the coolant favouring the corners of the fan. The performance of these two designs is similar with the Circular Helical design showing slightly better spanwise averaged performance over the majority of the plate. The spanwise averaged plot for the rectilinear design shows that the area upstream of the first coolant hole is at a higher effectiveness level of 0.1 compared to the other designs which are all in the region of 0.02-0.05. This is thought to be a result of either illumination inconsistency between images or slight temperature difference of the plate surface. This has the effect of increasing the spanwise average upstream of and in line with the first row of cooling holes and as a result, the data for the rectilinear design appears to be higher than the Circular Helical case. In the absence of this effect the performance of the two designs in this region is considered to be comparable.
The Spey, Slotted, Circular and Rectilinear Helical designs all show similar performance after the first row with spanwise averages of around 0.35-0.4 but with the Circular Helical design showing slightly better performance than the other geometries. The Modified fan design returns around 0.3 in this area. This design is hampered by the lack of lateral spreading of coolant as it emerges from the plate due to the very low divergence angle of the fanned portion of the hole. A similarly reduced spanwise averaged effectiveness would be expected of the Slotted design had the lateral spacing of the holes been in line with the other hole arrays.
As number of cooling rows increase the importance of the internal geometry and fan design become apparent as the Spey design diverges from the two helical geometries. This indicates that the exit velocity from the hole and hence jet penetration into the mainstream flow are an important factor in determining the maximum film effectiveness of a given array. Due to the increased pressure drop within the internal length of the helical geometries the flow has less momentum as it emerges from the hole and hence cannot penetrate as far into the mainstream flow. Thus increasing the proportion of coolant remaining close to the wall and hence film effectiveness. The fans of the helical designs have also been defined with reduced fan angle compared to the Spey geometry and as a result, the coolant fills the fan more successfully, avoiding coolant separation within the fanned section. Conversely the Modified fan does not diffuse aggressively enough to promote spanwise spreading of the coolant and as a result shows poorer distribution of coolant than the other laterally fanned geometries.
It is also worth noting that the porosity of the Spey fan is reduced compared with the other geometries as a result of the flat spot created during manufacture, resulting in a reduced amount of coolant emerging from the holes compared with the other designs. However, as seen in the previous section, this design is insensitive to coolant flow rate at the range of momentum, ratio conditions considered, therefore this decreased porosity is unlikely to have much effect on the ranking of this design in terms of adiabatic effectiveness.
𝜼𝜼𝒎𝒎𝒂𝒂
Cylindrical Spey Fan
Modified Fan Slotted
Circular Helix Rectilinear Helix
Figure 85 – Geometry comparison of adiabatic effectiveness surface map, 𝑴𝑴𝑫𝑫 ≅ 𝟏𝟏𝟏𝟏 and 𝑻𝑻𝒖𝒖 = 𝟐𝟐𝟎𝟎%
𝜼𝜼𝒎𝒎𝒂𝒂
Cylindrical Spey Fan
Modified Fan Slotted
Circular Helix Rectilinear Helix
Figure 87 – Geometry comparison of adiabatic effectiveness surface map, 𝑴𝑴𝑫𝑫 ≅ 𝟑𝟑 and 𝑻𝑻𝒖𝒖 = 𝟐𝟐𝟎𝟎%