Preparación y definición de los temas

Figure 5.7 shows the predicted change in the impingement wall pressure loss as a function of the hole number along the gap for Z/D from 0.76 to 6.42. Figure 5.7 shows that there was a significant increase in the pressure loss for Z/D < 3.06. At very low Z/D there was a major component of the pressure loss due to the flow along the gap. At Z/D = 0.76 the pressure loss through the wall at the first hole was 0.7% and increased to nearly 6% by the tenth hole. For Z/D > 3.06 the pressure loss across the impingement wall dominated the pressure loss. These results show that any increase in the impingement heat transfer at low Z/D occurs with an increase in pressure loss. If comparison was made at the same pressure loss the impingement wall at Z/D > 3.06 should have a smaller hole size to increase the pressure loss to that of the low Z/D impingement geometry. This has not been done in any experimental work on the influence of Z/D on impingement heat transfer. Thus the apparently significant influence of low Z/D on impingement heat transfer is mainly due to the increased pressure loss and hence increased turbulence levels that occurs at low Z/D.

This predicted pressure loss for varied X/D from 1.86 - 11.04 is shown in Figure 5.8, as a function of the axial distance along the impingement gap in terms of the number of holes. Equations 2.11 and 2.12 show the strong link between the pressure loss and X/D and the impingement wall porosity A. Thus at high X/D of 11.04, the pressure loss at the high coolant mass flux of 1.93 kg/sm2bar was predicted to be very high which is at an unrealistic

Figure 5.7: Impingement jet holes predicted pressure loss for varied Z/D at constant G

Figure 5.8: Impingement holes predicted pressure loss for varied X/D at constant G

level for gas turbine applications. This is because an X/D of 11.04 would not be used with all the compressor flow. It is the design choice for local hot spot cooling of combustor or turbine blade walls, where a small proportion of the total compressor air flow is used but at a low local G with a 3 - 4% pressure loss. However, the variation of X/D at constant G was the objective of the present CFD investigation.

Figure 5.8 show that the pressure loss along the cross-flow gap was predicted to be small relative to the impingement jet wall pressure loss at high X/D. However, as X/D is reduced and the pressure loss reduces, the cross-flow pressure loss becomes more significant, especially for X/D < 3.78. It is in this region that the flow-maldistribution becomes significant as shown in Figure 5.3. The pressure loss was experimentally measured as the static pressure difference between the plenum chamber and the external ambient air. This was then corrected for the small pressure loss of the cross-flow discharge from the impingement gap. This was computed as one dynamic head pressure loss, based on the

mean impingement gap flow Vc of 23.92 [25]. This correction was 0.34% and was the same for all X/D. The CFD predictions did not predict the pressure loss of the dump expansion from the gap and predicted the pressure loss to the upstream wall static pressure 25.4 mm downstream of the last row of impingement holes.

The predicted pressure losses as a function of the air hole number are shown in Figure 5.9 for five coolant mass flux G. This shows the increase in pressure loss along the duct due to the interaction of impingement jets with the cross-flow. Figure 5.9 shows that the major part of the pressure loss is that through the impingement wall and that the pressure loss due to flow along the impingement gap is much smaller, but still significant. At the highest G, 73% of the total pressure loss occurs across the impingement jet wall. At the next highest G, 77% of the pressure loss was across the impingement wall. Thus around three quarters of the pressure loss is across the wall and one quarter due to cross-flow along the gap.

Figure 5.9: Impingement holes predicted pressure loss for varied G at fixed X/D and Z/D

Figure 5.11: Impingement holes predicted pressure loss for varied n and X/D at constant G

The predicted pressure loss results are shown as a function of the distance along the impingement gap in Figure 5.10 Figure 5.11, which are the predictions for G of 1.93 and 1.08 kg/sm2bar, respectively. Figure 5.10 show that the difference in pressure loss was nearly constant along the impingement gap, hence the difference in the overall pressure loss was due to that pressure loss in the holes, not due to any effect of the cross-flow. Equations 2.11 and 2.12 only give the same pressure loss of Figure 5.10, if the hole Cd is constant. With the largest number of holes, Table 5.3 shows that Cd decreased from 0.91 for N = 10 to 0.81 for N = 25 and this is an 11% reduction in Cd, which will give a 23% increase in the pressure loss for the same air mass flux and wall porosity. Figure 5.10 shows that at the start of the impingement gap, the impingement walls with N = 10 and 15 had a predicted pressure loss of about 1.67% and this increased to 2% for the N = 25 wall. This is a 20% increase in the predicted pressure loss, which is close to that measured experimental data [22]. This shows that the flow inside the holes and the cross-flow aerodynamics along the impingement gap flow were adequately predicted. But for a reduced G of 1.08 kg/sm2bar and varied large n from 9688 - 26910 m-2 as shown in Figure 5.11, the pressure loss increases from 0.9% for N = 25 to 1.27% for N = 20 and finally to 3.72% for N =15. This should be based on the differences in increased X/D as Table 5.4 show, which also show that the reduced hole size with N influences the increased in pressure loss, which is contrary to Figure 5.10.

The reason for the higher pressure loss for N = 25 in Figure 5.10 and for all N in Figure 5.11 walls, was the increase in hole L/D ~ 4.5 and the differences in the method of manufacture of the wall. Spark erosion for the N = 25 wall and is the same for all n of Figure 5.11, while drilling was used for the larger holes [45], which gave a different wall surface finish. The CFD predictions do not specifically include any effect of the wall roughness on the

predictions. However, as the wall function approach was used for the boundary layer, this occupied more of the flow when the hole diameter was small and this acts as a pseudo wall roughness effect and gives a good prediction of the increased pressure loss.