Double wall cooling, consisting of internal impingement cooling and external film cooling, is an advanced cooling method of gas turbines. In this paper, the flow and conjugate heat transfer characteristics of double wall cooling which has a film plate with gradient thickness are analyzed numerically. The detailed overall cooling effectiveness distributions are obtained by solving steady three dimensional Reynolds-averaged Navier-Stokes equations.

In the double wall cooling scheme, seven vertical film holes and six impingement holes are staggered with same diameter (*D*), and the hole pitch of them are both set to 6*D* in flow direction and lateral direction. The gradient thickness along the flow direction is realized by setting the angle (*α*) between the lower surface of the film plate and the horizontal plane at -1.5 deg and 1.5 deg respectively. By comparing the results of four broadly used turbulence models with experimental data, SST *k-ω* is selected as the optimal turbulence model for double wall cooling analysis in this paper. In addition, the number of grids are finally determined to be 5.2 million by grid sensitivity calculation. The influence of the thickness gradient on the overall cooling effectiveness is revealed by comparing with the constant thickness film plate (Baseline 1 and 2), and all the cases are performed of four various coolant mass flow rates, which correspond to blowing ratios ranging from 0.25 to 1.5.

The calculated results show that the thickening of the film plate downstream is beneficial to the improvement of the overall cooling effectiveness at low blowing ratio, which is benefit from two aspects. One is the thicken film plate weakens the flow separation in film hole and velocity of film hole outlet, another is the thicken film plate makes the impingement channels convergence, and impingement cooling is strengthened to some extent. However, with the increase of blowing ratio, the increasing trend gradually weakens due to the jet-off and limited impinge ability. For thickening film plate, the variations of the double wall cooling configurations are considered at initial film plate thickness *t*~f~ of 2*D* and 3*D*, it is found that the ability to improve the overall cooling effectiveness by thickening the film plate downstream decrease as the initial film plate thickness increases, which is due to the increase of heat transfer resistance, and another finding is the cooling effectiveness of downstream thickening film plate with initial thickness of 2*D* is higher than that of 3*D* constant thickness film plate, which will provide a theoretical foundation both for improving cooling performance and reducing turbine blade weight at the same time. The influence of initial impingement gap *H* is also observed, and the study come to the fact that the best cooling performance occurred in *H*=2*D*.


Conjugate Heat Transfer Characteristics of Double Wall Cooling on a Film Plate With Gradient Thickness

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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