Effect of Hole Spacing on Deposition of Fine Coal Flyash Near Film Cooling Holes

Particulate deposition experiments were performed in a turbine accelerated deposition facility to examine the nature of flyash deposits near film cooling holes. Deposition on both bare metal and TBC coupons was studied, with hole spacings (s/d) of 2.25, 3.375, and 4.5. Sub-bituminous coal ash particles (mass mean diameter of 13 microns) were accelerated to a combustor exit flow Mach number of 0.25 and heated to 1183°C before impinging on a target coupon. The particle loading in the 1-hr tests was 310 ppmw. Blowing ratios were varied in these experiments from 0 to 4.0 with the density ratio varied approximately from 1.5 to 2.1. Particle surface temperature maps were measured using two-color pyrometry based on the RGB signals from a camera. For similar hole spacing and blowing ratio, the capture efficiency measured for the TBC surface was much higher than for the bare metal coupon due to the increase of surface temperature. Deposits on the TBC coupon were observed to be more tenacious (i.e., hard to remove) than deposits on bare-metal coupons. The capture efficiency was shown to be a function of both the hole spacing and the blowing ratio (and hence surface temperature). Temperature seemed to be the dominant factor affecting deposition propensity. The average spanwise temperature downstream of the holes for close hole spacing was only slightly lower than for the large hole spacing. Roughness parameters Ra and Rt decreased monotonically with increased blowing ratio for both hole spacings analyzed. The roughness for s/d=3.375 was lower than that for s/d=4.5, especially at high blowing ratio. It is thought that these data will prove useful for designers of turbines using synfuels. NOMENCLATURE CMM coordinate measurement machine d hole diameter D coupon diameter DVC dense vertically cracked HVOF high velocity oxygen fuel M blowing ratio =ρcUc/ρ∞U∞ M Mach number M3 s/d=3.375, metal coupon M4 s/d=4.5, metal coupon ppmw parts per million by weight R coupon radius Ra centerline-averaged roughness value [μm] Rt mean of vertical distance of peak and valley [μm] Rz vertical distance of peak and valley [μm] s hole spacing [mm] slpm standard liters per minute T temperature Tc coolant temperature Ts wall surface temperature Tinf wall surface temperature at adiabatic condtion TADF turbine accelerated deposition facility TBC thermal barrier coating x spanwise coordinate from left edge of coupon y streamwise coordinate from downstream edge of center film cooling hole α impingement angle η Overall film cooling effectiveness ρ density Subscripts free stream condition c coolant s surface