The Effect of Impingement Jet Heat Transfer on Casing Contraction in a Turbine Case Cooling System

Abstract

This paper reports full local Nusselt number distributions under an array of impinging jets typical of those used for thermal tip clearance control through casing contraction. Characteristic features of this type of application are sparse arrays of short cooling holes flowing at low jet Reynolds numbers (700–11,000) and large stand-off distances from the surface into a semi-confined passage with multiple exits. These features are captured in a large scale model, approximately ten times engine scale. Heat transfer measurements are made using the transient thermochromic liquid crystal technique. The measurement domain was extended far downstream of the impingement array. This allowed the entire heat transfer coefficient distribution contributing to the contraction of the liner around the rotor blades to be captured. CFD studies were conducted to characterize the flow field obtained, which in turn is helpful in understanding the drivers of heat transfer. The results are compared to existing industry standard correlations, which are generally outside the geometric and Reynolds number range of interest. It was shown that, for the tested geometries, the heat transfer was sensibly unaffected by whether the flow was exhausted through one side of the exit passage or equally in both directions, and the bulk flow field could be predicted using a modified distributed injection model. The heat transfer coefficient distributions are linked to a thermal-mechanical finite element model to provide thermal boundary conditions on an idealized representation of the casing for casing contraction in the presence of cooling scheme. For one of the geometries tested, data from an engine casing thermocouple survey have been compared to predictions of casing temperature determined using the measured heat transfer coefficient distributions and these show reasonable agreement.

Enhancement of Impingement Heat Transfer With the Crossflow Normal to Ribs and Pins Between Each Row of Holes

“…This work was concerned with the enhancement of combustor impingement backside cooling using obstacles in the gap. Impingement cooling creates a crossflow [3][4][5][6] which can interact with obstacles to enhance the overall cooling effectiveness. However, this is at the expense of increased impingement cooling pressure loss.…”

“…Studies of the influence of the impingement jet cross-flow with no obstacles present [3][4][5][6] shows that at high X/D with no flow maldistribution there is enhanced heat transfer to the cooled surface at the leading edge and reduced heat transfer at the trailing edge. These trends were predicted by El-Jummah et al [4,[9][10][11] using 3D CHT CFD.…”

Experimental and Numerical Investigation of Annular Casing Impingement Arrays for Faster Casing Response

“…The majority of studies related to tip clearance control for aero-engines use scaled down facilities with cold flow, coupled with computational fluid dynamics (CFD) [8,9] to investigate case cooling. However, the casing transient response that influences tip clearance is also heavily dependent on the heat transferred to the casing inner surface.…”