ISABE-2017-22643 (RAeS Review, Subject Area: Engineering Sciences), Sept. 3-8, 2017, Manchester, UK
(Draft)
Study of a conceptual design for cooled cooling air in a Preswirl Cavity
Zixiang Sun and John W. ChewThermo-Fluid Systems UTC
Dept. of Mech. Eng. Sciences
University of Surrey
Guildford, Surrey, GU2 7XH
UK
Abstract
To achieve enhanced cooling of hot components in the high pressure (HP) section of an aeroengine, application of cooled cooling air (CCA) has been proposed. Here a “two row preswirl feed” arrangement is considered to accommodate the CCA, together with the uncooled cooling air (UCA) in high pressure turbine (HPT) preswirl cavity. The CCA and UCA inflows are introduced into the preswirl cavity at two different radii. Most of the cooling air leaves the preswirl cavity from the receiver holes. To assess the CCA behavior in the preswirl cavity, a definition of feeding effectiveness is introduced based on the relative total temperature at the exit of the receiver hole. The CFD investigation for the preswirl cavity was conducted in a systematic way by altering both the radial position of the receiver hole and inflows of the CCA and UCA, while keeping other conditions unchanged. It was found that the feeding effectiveness increases as the radial position of the receiver hole decreases. An optimal feeding effectiveness close to a minimum mixing condition was achieved by adjusting the CCA and UCA inflows. Unsteady CFD investigations gave a similar prediction for the overall performance of the CCA in the preswirl cavity, but with a lower feeding effectiveness compared with its steady CFD counterpart. The reduction in the feeding effectiveness was attributed to an enhanced mixing from the discrete CCA and UCA inflows and associated unsteady effects.
Key Word: Cooled cooling air (CCA); Enhanced cooling; Conceptual Design.
Nomenclature
Cp specific molar heat capacity at constant pressure
dh hydraulic diameter at inlet = 4*Area/Wetted-perimeter
I Rothalpy
mass flow rate
PCR pitch circle radius
pout outlet pressure at receiver hole
Rebulk inflow Reynolds number = rUbulk dh/m
ReW rotational Reynolds number = rW ro2/m
r radius
r1,2 entrance and exit of receiver hole, respectively
ro outer radius of the cavity
T temperature
Tref reference temperature
T*rot rotary stagnation temperature
Tt total temperature
Ttrel relative total temperature
Ubulk bulk velocity at inlet
Vq swirl velocity
y+ dimensionless wall distance = r(tw/r)0.5yp/m
yp wall distance
Greek
hrel feeding effectiveness, Equation 3
m dynamic viscosity
W angular velocity of rotor or disc
r density
tw wall shear stress
Subscripts
baseline baseline test case
CCA cooled cooling air inflow
Full full mixing
UCA uncooled cooling air inflow
i inner radius
in_seal inner labyrinth seal
m middle radius
min minimum or minimum mixing
RecHol receiver hole
Sum total mass flow rate through the cavity
o outer radius
w wall
1. introduction
Use of cooled cooling air to achieve enhanced cooling of hot components in aeroengine applications has been considered in recent years. A CCA concept was investigated as part of the “active core” research under the European Framework 6 collaborative project NEW Aero engine Core concepts (NEWAC, 2006~2011). An early publication about the CCA option in the “active core” task under the NEWAC consortium was given by Wilfert et al. in 2007 [1]. Relevant progress on the CCA work was reported by Sturm [2] and Ebert et al. [3] at the European Workshop on New Aero Engine Concepts held in Munich, Germany, 2010. A further description of the CCA study in the “active core” research was published by Bock et al. in 2008 [4]. Other ideas for aeroengine applications of CCA were claimed in patents, such as European patent EP2275656A2 by Chir and Edwards in 2011 [5] and US patent 6612114 by Hermann in 2003 [6] amongst others.
Most of the previous CCA researches have been limited to conceptual studies and assessments of the CCA technology. In the present study, a more detailed aspect of CCA application is considered. In particular, a “two row preswirl feed” arrangement to accommodate the CCA in a HPT preswirl cavity is investigated. The temperature of the air delivered to the blade cooling feed is represented through a feeding effectiveness, and it is shown that this depends on the preswirl arrangement.
Fig. 1 Illustration of a Cooled Cooling Air Application
The proposed cooled cooling air application is illustrated in Figure 1. A portion of hot air is extracted from the main gas path of the high pressure compressor (HPC), and cooled by the bypass air through a heat exchanger. The cooled air is then fed into the secondary air system through the HPC drive cone cavity and HPT preswirl cavity to provide an enhanced cooling for hot components of the engine, such as the HPC rear cone, HPT blade and disc rim, etc.
A possible two row preswirl feed arrangement of the CCA and UCA streams is described in the next section. The CFD model used, the scope of the investigation, definition of the feeding effectiveness, results and discussion are then presented in the following sections.
2. two row PRESWIRL feed arrangement
A sectional view of a “two row preswirl feed” to the HPT preswirl cavity for the CCA application is shown in Figure 2. The geometry is based on an industrial configuration, and was modified to accommodate the CCA. It can be seen that the preswirl cavity is bounded by stationary structures on the left and the rotating disc on the right. The receiver holes on the right also rotate. The boundaries separating the stationary and rotating walls are the inner and outer labyrinth seals, located at the inner and outer radii of the preswirl cavity, respectively. In addition to the seal inflow and outflow, there are two more inflows and one further outflow in the cavity. The CCA and UCA inflows were arranged at two different radii in the stationary structures on the left, with the CCA being placed at a slightly lower pitch circle radius (PCR). Such an arrangement for the CCA is thus called “two row preswirl feed”. Most of the cooling air outflow leaves the cavity through the receiver holes on the right.
Fig. 2 Two row preswirl feed
It may be noted that the seal flows are significant and have an important influence on the temperature of the cooling air delivered to the receiver holes. Mixing of the three inlet streams will also affect the cooling air delivery temperature.
3. CFD model AND NUMERICAL ISSUES
The CFD model for the engine representative preswirl cavity employed in the present study is shown in Figure 3. It is a 2.73° sector assuming circumferential periodicity, with one discrete receiver hole. A contraction was added to the exit of the receiver hole to eliminate reverse flow there. The inner and outer seals plus the CCA and UCA inflows were approximated with annular slots. All the walls are axi-symmetric except the receiver hole. The mesh was generated using ICEM CFD software. All the mesh cells are of hexahedral type, with 634,020 cells. The mesh resolution follows previous mesh-dependency investigations and is similar to the authors’ two previous publications [7, 8] regarding preswirl cavities. The mesh was thus considered adequate. The dimensionless wall distance y+ obtained was generally kept between 50 and 100.
Fig. 3 The Preswirl Cavity CFD Model (2.73° Sector)
The boundary condition types of the CFD model are also shown in Figure 3. All the three inlets are specified as subsonic inlet with assigned mass flow, total temperatures and swirl velocities. In the figure,, Tt and Vq denote mass flow rate, total temperature and swirl velocity, respectively. The outer labyrinth seal and receiver hole are treated as a subsonic outlet with specified flow () and an outlet with assigned pressure (), respectively. The mass flow rate () at the receiver hole is determined by the CFD solution, and may slightly deviate from a mass balance analysis. The right rotor walls are under rotation with an angular speed of W. All the walls were assumed no-slip and adiabatic in the CFD simulations.
(4-a) at PCRo (4-b) at PCRm (4-c) at PCRi
Fig. 4 The Preswirl Box CFD Models (2.73° Sector)
Three simplified models were further constructed for a comparative study using the 2.73° sector with a discrete receiver hole. These are shown in Figure 4. It can be seen that most geometrical features of the preswirl cavity, such as inlets and outlets were kept in these simplified “box” models. In addition, all the three box models are identical apart from the radial position of the receiver hole. The pitch circle radii (PCR) of the receiver hole for the three box models are at the outer, middle and inner radii PCRo, PCRm and PCRi, respectively. Contractions were again added to the exit of the receiver hole to eliminate reverse flow there. A box model with a discrete preswirl nozzle rather than a slot was also considered and is described later.
The CFD simulations were conducted using the standard k-e turbulence model and wall functions. Turbulence modeling is fully justified as the Reynolds number of the flow is high. Taking the baseline test case for example, its rotational Reynolds number is equal to ReW=rWro2/m=3.13x107, where r and m stand for density and viscosity, respectively, and ro denotes the outer radius of the preswirl cavity. The through-flow Reynolds numbers are equal to Rebulk=rVbulkdh/m =1.10x107, 7.28x104 and 4.96x104 for the UCA, CCA and the inner labyrinth seal inflows, where Vbulk and dh denote the bulk velocity of the inflows and the hydraulic diameter of the inlets, respectively.
The CFD solver employed in the investigation was the Rolls-Royce Hydra code. The numerical scheme used is of 2nd order accuracy. Both steady and unsteady CFD simulations were performed.
Convergence of all CFD simulations was monitored according to the standard practice. For example, in the preswirl cavity model under the baseline condition, the overall balance of mass, angular momentum and total enthalpy are 0.1%, 2% and 3% in terms of total mass inflow rate, rotor shaft torque and rotor windage, respectively.
4. test case matrix
Seven test conditions including the baseline condition, were employed in the present parametric study. The baseline test case represents a maximum take-off (MTO) condition. The remaining six test conditions were obtained from the baseline by altering the CCA and UCA inflows systematically, while keeping all other boundary conditions unchanged. In fact, only three variables of the boundary conditions for the CCA and UCA inflows were changed. They are the mass flow rates for both the CCA and UCA, plus the total temperature of the CCA. The change of the above three items was further constrained by the total mass and inflow energy requirements. For all the seven test cases, the total mass inflows and total energy (enthalpy) influx were kept unchanged.
(1)
(2)
where Cp denotes specific heat at constant pressure, which was also assumed constant in the present investigation.
Fig. 5 Changes of Boundary Conditions
A summary of alterations in boundary conditions for CCA and UCA is given in Table 1. The corresponding graphic representation of the boundary condition changes is shown in Figure 5. It can be seen that as the mass flow from CCA decreases, the mass flow from UCA increases accordingly to keep the total mass inflow unchanged. The total temperature of the UCA was kept constant for all the test cases. As a result, the total temperature of the CCA has to be reduced to keep the total enthalpy influx from inlets constant when the mass flow from CCA decreases. In addition, the swirl velocities for all the inflows were kept unchanged.
Table 1 Test Case Matrix
Test Case / / CCA (Vq=Const.) / UCA (Tt, Vq=Const.)/ Tt/Tref /
Baseline / 1.0 / 0.280 / 2.462 / 0.521
No. 1 / 1.1 / 0.308 / 2.539 / 0.493
No. 2 / 0.9 / 0.252 / 2.367 / 0.549
No. 3 / 0.8 / 0.224 / 2.248 / 0.577
No. 4 / 0.75 / 0.210 / 2.177 / 0.591
No. 5 / 0.7 / 0.196 / 2.096 / 0.605
No. 6 / 0.5 / 0.140 / 1.608 / 0.661
Where mSum (=mCCA+mUCA+mIn-Seal) and Tref denote the total mass flow through the cavity and a reference temperature of the engine.
5. definition of feeding effectiveness
To facilitate the assessment of the CCA behavior in the preswirl cavity, a definition of feeding effectiveness is given as follows:
(3)
where hrel stands for the feeding effectiveness based on relative total temperature. Ttrel_RecHole denotes the relative total temperature at the receiver hole exit obtained in the CFD modelling. Ttrel_RecHole@Min_mixing and Ttrel_RecHole@Full_mixing represent the relative total temperatures at receiver hole obtained by an analytical estimation at the idealized minimum and full mixing situations, respectively.
An illustration of the idealized minimum and maximum mixing assumptions is given in Figure 6. In the idealized full mixing situation, all the inflows are assumed to be fully mixed in the preswirl cavity before entering the receiver hole and the outer labyrinth seal. In the minimum mixing condition, it is assumed that the inflows from CCA and the inner labyrinth seal enter the receiver hole without mixing with the UCA. Part of the UCA joins the CCA and the inflow from the inner labyrinth seal to go through the receiver hole, while rest of the UCA is isolated from other inflows and leaves the cavity from the outer labyrinth seal. It may be noted that fully mixed preswirl chamber models have previously been used successfully in convectional systems [9].