Stratified and Buoyancy-Induced Flow in Closed Compressor Rotors

Abstract

The radial growth of compressor discs is strongly influenced by conjugate heat transfer between conduction in the co-rotating discs and buoyancy-driven convection in the rotating fluid core between the discs. An accurate prediction of metal temperatures of these discs is an important issue in thermo-mechanical design. This paper presents an experimental study of the fluid dynamics and heat transfer in a closed rotating cavity, comparing results with theoretical models and introducing a new compressibility parameter χ . At large values of χ, where compressibility effects are significant, the air temperature approaches that of the shroud; such conditions suppress buoyancy effects and the flow in the rotating cavity becomes stratified. The experiments reveal that there is a critical value of χ where the convective heat flux to the shroud is zero. The radial distribution of disc temperature was that expected from pure conduction in a cylinder. A new heat transfer correlation based on measured shroud heat flux and the theoretical core temperature is presented. The unsteady flow characteristics in the cavity were also investigated, identifying coherent rotating structures across a range of experimental conditions. These cyclonic / anti-cyclonic vortex pairs generate the non-dimensional circumferential pressure difference necessary for the radial outflow (of cold fluid) and inflow (of hot fluid) through the rotating core. The experiments show the magnitude of these pressure variations can be correlated against Grashof number and that at high values of χ the structures do not exist.

A Model of Mass and Heat Transfer for Disc Temperature Prediction in Open Compressor Cavities

“…For these cases, 𝜃 𝑑 is slightly underpredicted at higher radii, but the agreement is generally good. In the closed cavity, as shown by Tang and Owen [20] and Lock et al [22], the distribution of 𝜃 𝑑 was insensitive to Re 𝜙 . However, in the open cavity where there is an exchange of mass between the cavity and the throughflow and angular momentum is not conserved, an increase in Re 𝜙 significantly reduces 𝜃 𝑑 at lower radii.…”

“…where Δ𝑝 is the pressure difference between the anti-cyclonic and cyclonic vortices in the cavity. Lock et al [22] correlated 𝜓 𝑝 with the shroud Grashof number,…”

“…Pernak et al [23] corrected the correlation from [22] for the nonadiabatic boundary conditions, and generated a correlation for the open cavity of the Bath Compressor Cavity Rig, shown as follows:…”

“…This is analogous to Rayleigh-Bénard free convection correlations with gravitational acceleration replaced by centripetal acceleration. Lock et al [22] developed a correlation for the closed cavity using heat flux measurements and the plume model. Nicholas et al [21] extended the plume model to transient operating conditions and applied a similar correlation to the prediction of transient shroud heat fluxes.…”

Use of Bayesian statistics to calculate transient heat fluxes on compressor disks

“…There is a critical χ where the core temperature becomes higher than the shroud temperature, the buoyancy-induced flow structures disappear and the flow becomes stratified. The model was validated by experimental data presented in Lock et al 25 . Both the theory and experiment concluded that the steady-state disc temperatures and fluxes are governed by three non-dimensional parameters, Re φ , β ∆T and χ.…”