Computational fluid dynamics analysis for effect of length to diameter ratio of nozzles on performance of pre-swirl systems

Abstract

In a direct-transfer pre-swirl system, cooling air expands through stationary pre-swirl nozzles and flows through the cavity to the receiver holes located in the rotating turbine disc for blade cooling. This paper investigates the effect of the length-to-diameter ratios of pre-swirl nozzles on the performance of a direct-transfer pre-swirl system in a rotor-stator cavity. The commercial code CFX 12.1 is used to solve the Reynolds-averaged Navier–Stokes equations using the SST turbulence model. Computations are performed for seven length-to-diameter ratios, L/ D = 1, 2, 3, 4, 5, 6, and 7, a range of pre-swirl ratios, 0.5 <  β p<2.0, and varying turbulent flow parameters, 0.12 <  λ T < 0.36. The rotational Reynolds number for each case is 106. The computational fluid dynamics model presented in this paper is validated with the experimental results available in the literature. The nozzle exit flow angle α decreases as length-to-diameter ratio L/ D increases for L/ D < 1/tan θ (θ is pre-swirl nozzle angle). When L/ D > 1/tan θ, α is approximately invariant and below θ. The discharge coefficient C d,b for the receiver holes reaches a peak as the fluid in the rotating core is in synchronous rotation with the receiver holes. For small turbulent flow parameters λ T, a peak of C d,b can be observed as L/ D = 3. For large turbulent flow parameters λ T, the shift of the position for a peak occurs. When L/ D = 3, the synchronous rotation can be achieved with the smallest value for turbulent flow parameters λ T. The adiabatic effectiveness of the system increases with turbulent flow parameters λ T. A peak of Θ b,ad is seen as L/ D = 3 for each case. When L/ D < 1/tan θ, there is a significant increase in Θ b,ad, especially for L/ D < 2, with an increase in L/ D. When L/ D is increased further, a slight decrease occurs. Both performance parameters show that the optimum value for all cases can be achieved as L/ D = 3, which is slightly above 1/tan θ.