Effects of tip gap height on tip convective heat (mass) transfer characteristics in turbine blade cascades of weak and strong reactions

Abstract

Effects of h/s (tip gap height/blade span) on heat (mass) transfer and tip near-wall flow are investigated in turbine blade cascades of exit-to-inlet velocity ratio (EIVR) = 1.2 (weak reaction) and 2.4 (strong reaction), by using the naphthalene sublimation technique and oil film method. They have almost same inlet flow conditions and major cascade parameters except for EIVR. The results show that for EIVR = 2.4, there are chord-wise periodic changes of tip near-wall flow and heat transfer between the leading edge and mid-chord. On the contrary, the chord-wise periodic change is hardly found for EIVR = 1.2. Regardless of EIVR, $\overline{\mathrm{Sh}}$ (area-averaged tip Sherwood number) has a tendency to increase, to have a peak, and then to decrease, with increasing h/s. The stronger tip leakage flow for higher EIVR tends (i) to augment $\overline{\mathrm{Sh}}$ and (ii) to make the increasing-decreasing tendency of $\overline{\mathrm{Sh}}$ arise in a narrower tip gap range. The ratio of $\overline{\mathrm{Sh}}$ for EIVR = 2.4 to that for EIVR = 1.2 consistently decreases with h/s and is finally lower than unity. However, the mass-averaged loss for EIVR = 2.4 is about 2.5 times as high as that for EIVR = 1.2, regardless of h/s.