Computational Study of the Unsteady Flow Structure of the Buoyancy-Driven Rotating Cavity With Axial Throughflow of Cooling Air

Abstract

Buoyancy driven flows such as the one that occurs in the inter-disk space of an axial compressor spool plays a major role in determining the gas turbine engine projected life and performance. Details of the developed flow structure inside these spaces largely impact the operating temperatures on the rotating walls of the compressor hardware and therefore impact the life of the machine. In this paper the impact of engine power condition (Idle, Highpower, and Shutdown) on the flow structure for these rotating cavities is studied under a wide range of operating conditions encountered by realistic turbomachines. A computational analysis is performed using commercially available computational tools for grid generation (ICEM-CFD) and turbulent-flow simulation (CFX). A computational test case was developed to imitate the rig-test conditions of Owen and Powell, and computed results were assessed and validated by comparison with their experimental results. A total of fifteen unsteady CFD cases covering a wide range of operating conditions (Rossby Number Ro, Rotational Rayleigh Number Raφ, and axial Reynolds Number Rez) were analyzed. The computed flow results revealed that the flow structure evolution, starting from a steady state solution, is such that radial arms of different number (according to the engine power condition), surrounded by a co-rotating (cyclonic) and counter-rotating (anti-cyclonic) pair of vortices, start to form at different locations. Cold air from the central jet enters the cavity in these arms under the combined action of the centrifugal buoyancy and the Coriolis forces. As time proceeds, the flow structure tends to become virtually invariant with time in a repeatable pattern. The number of radial arms, strength of recirculation zones, and the degree of invasion of the central cooling air toward the shroud are all dependent on the engine power condition. The computations also revealed that at high rotational speed the flow stabilizes, and the unsteady features of the flow structure (cyclonic and anti-cyclonic recirculation zones surrounding the radial arms, radial invasion of the cooling air in the radial arms, and its final impingement upon the shroud surface) eventually disappear after a threshold value of the rotational speed is reached.

Experimental and Computational Investigation of Flow Structure in Buoyancy-Dominated Rotating Cavities

“…The method of Dweik et al [18] was used to create 25 points in the first 2 mm normal to the surface with the first point located 0.005 mm from the walls. A logarithmic bunching algorithm is then used in order to reach a high resolution boundary layer with y + < 1 on the cavity walls and shroud region.…”

Experimental and Computational Investigation of Flow Structure of Buoyancy Induced Flow in Heated Rotating Cavities

“…Each cavity domain includes around 9 million nodes. In order to have a high resolution within the boundary layers the method employed by Dweik et al (Dweik, et al, 2009b) is used to create 25 points in the first 2 mm normal to the wall with the first point located 0.005 mm away from the walls. A logarithmic bunching algorithm with a growth ratio of 1.1 is then used to reach a high resolution boundary layer resulting in a y + <1 on the cavity walls and shroud region.…”

“…The meridional view of the computational domain within a cavity is shown in Figure 6. The turbulence model used was the − SST model that combines both − and − models and benefits from automatic near wall treatment that switches to low-Reynolds number formulation in the viscous sub-layer (Dweik, et al, 2009b). The converged steady-state results were used to kick-start the transient simulations presented in this paper.…”

“…The frequency spectrum obtained ( Figure 2) from tangential velocity data for mid-plane of cavity 3 at radial location of r/b ≈ 0.765 showed the evidence of the dominant frequency at approximately twice that of the rotational speed of the flow implying the presence of two vortex pairs around that specific location. Dweik et al (Dweik, et al, 2009b) conducted a total of fifteen unsteady computational case studies for an open rotating cavity for idle, high-power and shutdown conditions. The test rig conditions in Owen and Powell (Owen & Powell, 2004) was imitated to develop a numerical test case.…”

Experimental and computational investigation of flow structure of buoyancy induced flow in heated rotating cavities

“…Buoyancy parameter βΔT ¼ (T sh À T in )=T in spectrum obtained (Figure 2) from tangential velocity data for mid-plane of cavity 3 at radial location of r/b ≈ 0.765 showed the evidence of the dominant frequency at approximately twice that of the rotational speed of the flow implying the presence of two vortex pairs around that specific location. Dweik et al (2009b) conducted a total of fifteen unsteady computational case studies for an open rotating cavity for idle, high-power and shutdown conditions. The test rig conditions in Owen and Powell (2004) was imitated to develop a numerical test case.…”