Flow and heat transfer characteristics in a pre-swirl rotor-stator cavity

Abstract

The present study investigates flow and heat transfer characteristics in a rotor-stator pre-swirl system. Considered are rotational Reynolds numbers Re_Ф ranging from 3.4 × 10⁵ to 6.8 × 10⁵, with jet Reynolds numbers Re_w of approximately 8.8 × 10⁴ to 3.81 × 10⁵, and associated turbulent flow parameter λ_T = C_w/Re_Ф^0.8 varying from 0 to 0.4. Experimentally-measured and numerically-predicted results illustrate the combined and separate influences of jet impingement and rotor entrainment, as they affect and govern flow and heat transfer characteristics within the cavity. These results show that the relative strength of the two effects varies with Re_Ф and Re_w, with similar performance for flow conditions with similar magnitudes of the turbulent flow parameter λ_T. When 0<λ_T < 0.27, jet impingement and rotor entrainment both have important influences on flow and heat transfer characteristics. However, when λ_T > 0.27, the effects of rotor entrainment are less pronounced.

Introduction

Rotor-stator cavities in gas turbines are important parts of secondary air systems. Such systems provide cooling air from the compressor to different turbine components, either for cooling or for sealing. In the first stages of a high-pressure turbines, pre-swirl systems within rotor-stator cavities, use angled nozzles to inject and swirl cooling air. When employed in this manner, of importance are the pressure drop penalty and cooling efficiency as jet flow air passes through the cavities.

A number of related investigations consider different parameters which characterize flow and heat transfer phenomena, such as static pressure drop and heat transfer coefficient magnitudes. Of interest are parameter variations which result from different operating conditions. Of these investigations, Meierhofer and Franklin [1] confirm that pre-swirl of cool air is useful to minimize total temperature increases, and to improve local cooling efficiency. Bricaud and Bricaud [2] present local temperature distributions along rotating disc surfaces, and indicate that local heat transfer rates are governed by viscous effects or jet impingement, depending on the magnitude of impingement mass flow rate. Lock et al. [3] and Kakade et al. [4] employ transient liquid crystal measurement technology [5] to investigate local convection heat transfer characteristics along rotating disc surfaces. Two-dimensional Nusselt number distributions are observed near receiver hole entrances, whereas axisymmetric variations are present for other rotor disc surface regions. Yan et al. [6] investigate a “direct-transfer” pre-swirl system computationally and experimentally, and show that pressure and tangential velocity, predicted using a simplified, three-dimensional steady model, are in good agreement with experimental data. The investigators also show that total pressure decreases significantly between pre-swirl nozzles and rotating fluid core regions. Javiya et al. [7] assess different turbulence models, with reasonable quantitative agreement with experimental flow dynamics characteristics for all models considered. Predicted heat transfer magnitudes, near and around receiver holes, evidence important sensitivity to the type of near-wall model which is employed. Karabay et al. [8] consider a cover-plate pre-swirl system, and provide evidence of a critical value of pre-swirl ratio, which corresponds to zero frictional moment. Also indicated is an optimal pre-swirl ratio, which is associated with a minimum value of spatially-averaged Nusselt number along the rotor disk surface. Xiang et al. [9] experimentally investigate a rotor-stator cavity with a 30-degree pre-swirl angle to indicate that rotational Reynolds number is more influential than jet Reynolds number, in regard to spatially-averaged Nusselt number alterations along the rotating disc surface. Lewis et al. [10] show that maximum discharge coefficients for the receiver holes decreases, and that the adiabatic effectiveness for the pre-swirl system generally increases, as the inlet radius ratio becomes larger.

Other investigations address flow and heat transfer mechanisms associated with varied operating conditions, as they are related to parameters such as rotor circumferential speed, and inlet mass flow rate. Kakade et al. [4] discuss changes of fluid dynamics and heat transfer characteristics as coolant flow rates are altered. At relatively low flow rates, flow characteristics are governed by the rotating speed of the rotor. Impingement jets are then most influential when associated mass flow rates are relatively large. The effects of the rotating speed of the rotor are not investigated. Lewis et al. [11] and Wilson et al. [12] analyze flow characteristics within pre-swirl systems to characterize coolant streamline paths from pre-swirl nozzles to receiver holes. Considered are “indirect” coolant trajectories, which are initiated within the core in the rotor-stator cavity. Effects responsible for local high heat transfer rates along surfaces, especially near receiver hole entrances, are also discussed.

Additional related investigations are described by Chattopadhyay et al. [13], Pal et al. [14], Murmu et al. [15], and Chattopadhyay and Murmu [16]. Of these investigations, Chattopadhyay et al. [13] and Pal et al. [14] consider heat transfer characteristics of laminar-turbulent annular jets impinging on a surface. The latter of these investigations addresses annular nozzle impingement heat transfer enhancement, using angled and standard axial jet configurations. Here, heat transfer augmentations are most significant when higher levels of turbulent fluctuations are produced by an angled jet arrangement. Murmu et al. [15] consider the effects of inlet turbulent intensity on heat transfer characteristics for different bluff body arrangements. According to these investigators, the transition SST Model provides accurate predictions of heat transfer characteristics in both laminar and turbulent flow regimes. In addition, heat transfer augmentations increase with larger values of inlet turbulence intensity and as Reynolds number increases. Chattopadhyay and Murmu [16] provide additional information on the effects of turbulence intensity on transport phenomena within flows over two-dimensional bluff bodies. Of note is the observation that drag and pressure coefficients are not affected by variations of turbulence intensity magnitude.

The present investigation provides new data and understanding of the flow and heat transfer characteristics for a simplified pre-swirl cavity with a radial outlet. Considered are rotational Reynold numbers Re_Ф from 3.4 × 10⁵ to 6.8 × 10⁵, jet Reynold numbers Re_w from 8.8 × 10⁴ to 3.81 × 10⁵, and turbulent flow parameter λ_T = C_w/Re_Ф^0.8 magnitudes from 0 to 0.4. The present investigation is innovative and unique because a combined experimental and numerical study provides new insight into the combined and separate influences of jet impingement and rotor entrainment, as they affect and govern flow and heat transfer characteristics within the cavity. Also unique is information regarding the performance of the turbulent flow parameter λ_T = C_w/Re_Ф^0.8, whose value indicates similar flow and heat transfer characteristics, regardless of the magnitudes of rotational Reynold number and jet Reynold number.