Flow Control in Y-Duct

7/31/2019 Flow Control in Y-Duct

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SEMINAR REPORTANALYSIS OF FLOW CONTROLS IN Y-DUCT DEFFUSER

NITHIN HEGDE

2011FE12

DEPARTMENT OF APPLIED MECHANICS

MOTI LAL NEHRU NATIONAL INSTITUTE OF

TECHNOLOGY, ALLAHABAD


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CFDANALYSIS OFFLOWCONTROLSINY-DUCTDIFFUSER

INTRODUCTION

Aircraft propulsion systems often use Y-shaped subsonic diffusing duct as air-

intakes to supply the ambient air into the engine compressor to compress the air, so

the thrust generation. For many military applications the inlet geometry isimportant for stealth requirement.A serpentine inlet can be used to hide the line ofsight to the compressor face in order to reduce the infra-red signature.

Source of losses in intake diffuser:

Losses due to Friction on the walls of the duct.

Flow separation due to adverse pressure gradient as well as due bends. Flow separation causes total pressure distortion, this cause flow non-

uniformity at compressor inlet so the asymmetric loading on compressor

blade.

The distortion is a significant cause of premature engine surge.Major separation control techniques used in intake diffuser:

Tangential blowing to directly energize the low momentum region near thewall.

Wall suction to remove low momentum region. Vortex generator (VG) in the form of vanes. Forced excitation devices.

1st

and 2nd

are very effective in controlling separation. However, these strategies

requires a high mass flux source, thus they are rarely used.

In VG produces strong vortices which enhance the mixing between high

momentum core flow and low momentum boundary layer flow, thus energizing the

boundary layer flow.


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Most of single engine military aircrafts consists of a Y-shaped twin air-

intake duct, which is mounted on either sides of the fuselage and carries

atmospheric air in to the compressor. The air intake of the aircraft supplies the

mass flow demand of the engine over a range of aircraft speeds and altitudes with

high pressure recovery and at minimum flow distortion. Due to space constraint,the diffusers need to be curved, which causes severe flow non-uniformity at the

engine face and separation on the curved surface causes flow distortion. Total

pressure distortion at the engine face is one of the parameters that contribute to

intake losses (Seddon and Goldsmith, 1995). The distortion causes premature

engine surge and flow non-uniformity which may cause range of undesirable

effects including asymmetric loading of the compressor blades.

OBJECTIVE

The present study attempt to control flow in a mild-curved Y-duct diffuser

using trapezoidal-shaped submerged co-rotating vortex generator (VG) attached on

the top and bottom surface of inflection plane.

Which ensures duct with co-rotating VG higher static pressure recovery, lower

total pressure loss, minimum flow distortion and less flow non-uniformity as

compared to bare Y-duct diffuser.

Geometry:

Bare duct:

Y-duct diffuser with 20turning angle (plane-B). Straight length of 75mm is added to outlet for proper boundary layer growth. Inlet area of diffuser was chosen as 75*75 mm2 (plane-A&F). The duct is tapered to an outlet width of 200mm (plane-D). The area ratio for the Y-duct is 1.33.


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Geometry of Y-duct with co-rotating VG at top and bottom surface at inflexion

plane:

Type h1 (mm) h2 (mm) L (mm) l (mm)

Co-VG 13.5 2.0 4.0 11.0 10.7

Top view of Y-duct

diffuser

Isometric view of Y-

duct diffuser

Schematic diagram of VG


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Grid generation:

Grid Independency test:

Sl. No. No. of GridElements

Csp Total CPU Time

1 60984 0.1313053 1hr. 30min.

2 108000 0.127469 2hr. 15min.

3 164052 0.124053 2hr. 30min

4 239250 0.122255 3hr

5 261120 0.121075 3hr 20min

6 493848 0.121018 4 hr. 15min

CPU CAPACITY: 2.2GHz DUAL CORE PROCESSORS WITH 4GB RAM.

Isometric view of Y-duct with

counter rotating VG


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Skewness test:

1. For bare Duct:

Hexahedral meshing is done on whole domain. Total 261120 elements are crated. Boundary layer is created near the wall to capture near wall effects. Boundary conditions are given as;

o Velocity inlet at plane A and F.o Pressure outlet,o Planes B, C, D, E are interiors.o And remaining walls.


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2. Duct with Co-rotating VG:

Mesh Quality:

Applying quality criteria for hexahedra cells.

Maximum cell squish = 6.07381e-001

Maximum aspect ratio = 6.23260e+00

Solver: For validation of result RNG k- turbulence model is used. In this model

transport equation solved for turbulent Kinetic energy (k) and rate of

dissipation (). This model is derived from instantaneous Navier-stokes

equations, using a mathematical technique called renormalization group

(RNG) method. The effect of swirl on turbulence is included in the RNG

model, so enhancing accuracy for swirling flows.

Transport equations for RNG - model:

Where:


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o Gk and Gb are generation of turbulent K.E. due to mean velocitygradients and buoyancy resp.

o YM is represents the contribution of the fluctuating dilatation incompressible turbulence to the overall dissipation rate.

o k and are the inverse effective Prandtl numbers for and kresp.

o S and S are user-defined source terms for and k resp.

Enhanced wall treatment method is used to account for boundary layersformed during grid generation.

Second order upwind discretization scheme is employed. For pressure and velocity coupling, SIMPLE (semi-implicit method for

pressure linked equations) scheme is employed.

Convergence criteria are set as 10-5 for all solutions. No-slip condition is set for duct walls.

Boundary conditions:

Turbulent kinetic energy k =1.5*(Uavi * I)2,

Turbulence dissipation rate = (C3/4

k3/2

)/L

Where;

L=turbulence length scale=0.07*Lc,

Lc=characteristic length,

I = turbulent intensity = 0.16 (Re)-1/8

,

C = turbulent model constant.


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Velocity inlet conditions;

Velocity profile is given at inlet.

Re=1.009526*105, Lc =0.075m, C =0.0845.

I=0.037897,

k =0.83283 m2/s

2,

= 22.689 m2/s

3.

Pressure outlet conditions;

Zero gauge pressure is set at exit condition.

Lc =0.2m, C =0.0845.

I=0.037897, k =0.83283 m2/s

2, = 8.5084 m

2/s

3.

Residual curve:


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Contour plots:

1. Contour of total pressure at plane-D:

Without VG

With VG


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2. Static pressure contour at mid-plane of Y-duct diffuser:

3. Contour of velocity magnitude at mid-plane of the Y-duct diffuser:

Without VG With Co-VG

Without VG With Co-VG


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Results and Discussion:

1. Static pressure recovery coefficient(CPR ):It is described as the ratio of rise in average static pressure with respect to

the inlet to the average dynamic pressure at inlet.

i.e.

CPR = (PSPSi)/(0.5Uavi2)

Where, PS and Psi are the static pressure at any point and at inlet respectively.

2. Total pressure loss coefficient (CPR) :It is defined as ratio of total pressure loss with respect to the inlet to the

average dynamic pressure at inlet.

i.e.

CTL = (PtiPt)/(0.5Uavi2)

Where, Pti and Pt are total pressure at inlet and at any point.

Validation of CPR and CTL for bare duct:


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* A.R Paul, P Ranjan, R.R upadhyay, A jain. Flow control in Y-shaped air-intakes using vortex generators.37th National & 4

thInternational conference on fluid mechanics and fluid power

Comparison of CPR and CTL for duct with co-rotating VG on top and

bottom surface of the wall and Bare duct:

Static pressure recovery is improved in case of diffuser with co-rotating VGas compared to bare duct.

It is noted that total pressure loss coefficient CTL variation is less in case ofVG attached on top and bottom walls.

3.Secondary flow Non-uniformity (Sio):The non-uniformity index (Sio) at duct outlet (plane-D) i.e.

Aerodynamic Inlet Plane (AIP) can be defined as the average of

the sum of secondary velocities (Uyz in y-z plane), non-

dimensionalized by dividing by the average velocity at the inlet.

CTL


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i.e.

where, Uyz=2 2

( )y z

u u , n=number of computed data point.

Uavi=19.67m/s=average velocity at the duct inlet.

o Below data is calculated by taking the data of velocities at y and z directionin plane-D.

Bare duct;Sum (Uyz) = 732.7766203,

n = 3360,

Uavi=19.67 m/s.

i.e.Sio = 732.7766203/(3360*19.67)

Sio = 0.011087355.

Duct with co-VG:Sum (Uyz) = 596.1312887,

n = 2520,

Uavi=19.67 m/s.

i.e.Sio = 596.1312887/(2520*19.67)

Sio = 0.0120264.

Validation of Sio:My result Paper result

*

Bare duct Co-vg Bare duct Co-vg

0.01108 0.012026 0.010 0.011

From above table it is noted that S io value records only a slight increase incase of VG.

Sio=


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* A.R Paul, P Ranjan, R.R upadhyay, A jain. Flow control in Y-shaped air-intakes using vortexgenerators.37

thNational & 4

thInternational conference on fluid mechanics and fluid power.

4. Axial flow Non-uniformity:It is the mean standard deviation in the axial velocity measured at a cross-

plane of air-intake and is expressed as,

Where,

Ux is the longitudinal velocity, Uxav is the average of longitudinal velocity and n is

the number of computed data points.

o Below data is calculated by taking the x-velocity component at plane-D. Bare duct:

Uxav=8.7684927,

(Sum (Ux-Uxav))2

= 117093.8112,

n=3360,=(117093.8112/3360)

0.5

= 5.903333

Duct with co-vg:Uxav = 10.97623351,

(Sum (Ux-Uxav))2

= 77044.24644,

n=2520

=(77044.24644/2520)0.5

= 5.5292959.

From above it is noted that, axial flow non-uniformity( ) is decreases incase co-rotating VG attached on top and bottom surface of the duct as

compared to bare duct.


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Validation of axial flow non-uniformity( ):My result Paper result

Bare duct Duct with Co-vg Bare duct Duct with Co-vg

5.903 5.529 5.420 5.208

* A.R Paul, P Ranjan, R.R upadhyay, A jain. Flow control in Y-shaped air-intakes using vortex generators.37th National & 4

thInternational conference on fluid mechanics and fluid power.

5.Momentum thickness ():Momentum thickness () value is an indicative of momentum deficit in the

boundary layer. This deficit increase the chances of the flow separation in y-

duct diffusers.

Calculations:o For bare duct:

Velocity profile at plane-D along y-axis line


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Boundary layer thickness =0.021104mm

Then, momentum thickness () = (2/15)*

i.e.

=2.813867mm

o For duct with co-vg:


Then, momentum thickness () = (2/15)*

i.e.

=2.672mm

Velocity profile at plane-D along y-axis line


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Validation of momentum thickness ():My result Paper result

Bare duct Duct with co-vg Bare duct Duct with co-vg

2.814 2.672 2.60 2.48

6. Distortion pressure coefficient (DC60):Since the total pressure distortion causes surge or buzz at the exitof the y-duct and is responsible for the intake losses. This

phenomenon leads to a range of undesirable effects including

asymmetric loading of the compressor blades.

The distortion is designed in terms of distortion coefficient (DC60) in worst

600

sectors.

i.e.

Where Pte is the total at the duct exit (plane-D) and P60 represents

the total pressure at the duct exit.

DC60 = (PteP60)/(0.5Uavi2)


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Bare duct total pressure contour map at exit plane-D

Worst 600 sector:

Avg. total pressure at exit plane-D = 103.42638 pa

Avg. total pressure at worst 600

sector at exit = 41.15 pa

Now,

DC60 = (103.42638-41.15)/(0.5*1.225*19.672)

DC60 = 0.2627898


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For duct with co-VGtotal pressure contour map at exit plane-D:

At worst 600 sector:

Avg. total pressure at exit plane-D = 101.26767 pa

Avg. total pressure at worst 600

sector at exit = 45.62256 pa

Now,

DC60 = (101.26767-45.62256)/(0.5*1.225*19.672)

DC60 = 0.2348


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The use of VG on the inner surface of the duct promotes better mixing oftwo flow fields and results in decrease in DC60 values in the Y-duct diffuser.

Validation of DC60:

My result Paper result

Bare duct Duct with co-vg Bare duct Duct with co-vg

0.2627898 0.2348 0.259 0.223

Boundary layer at inflexion plane:

u= 0.99*Uavg = 0.99*19.5

u = 19.305m/s,


Hence height of VG used less than 4.448 mm, therefore we can say that

VG used is submerged vortex generator type (SVG).


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Summary of results:

Type

Bare ductCo-VG at top and bottom

surface of the duct

My

result

paper

result% error My

result

paper

result% error

DC60 0.263 0.259 1.52 0.2348 0.223 5.02

Sio 0.01108 0.010 9.74 0.01203 0.011 8.56

5.903 5.420 8.18 5.53 5.208 5.82

2.814 2.60 7.6 2.672 2.48 7.18

Conclusion:

VG attached to top and bottom walls of the Y-duct diffuser have aneffect on flow uniformity at its outlet.

It is noted that axial flow non-uniformity( ) is decreases in caseco-rotating VG attached on top and bottom surface of the duct as

compared to bare duct.

Static pressure recovery is improved in case of diffuser with co-rotating VG as compared to bare duct.

Momentum thickness is decreases in case co-rotating VG attachedon top and bottom surface of the duct as compared to bare duct,

which indicates delay in flow separation.

Attaching the VG distortion at the exit of the duct is decreases ascompared to bare duct.


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REFERRENCES:

1. A.R Paul, P Ranjan, R.R upadhyay, A jain. Flow control in Y-shaped air-intakes using vortex generators.37

thNational & 4

thInternational conference

on fluid mechanics and fluid power December 16-18, 2010, IIT Madras,

Chennai, India.

2. Control of Compressor Face Total Pressure Distortion on a High BypassTurbofan Intake using Air-Jet Vortex Generators. S.D. Erbsloh and Dr. W.J.

Crowther The University of Manchester, Manchester, UK, M13 9PL J.R.Frutos FEMTO-ST Institute, LPMO Department, 25044 Besanon, France.

3. Ansys Fluent 12.0 theory guide, April 2009.4. Flow Control Analysis of S-duct Diffuser Inlet. Lian.Xiaochun Zhang.Lifen

Wu.Dingyi School of Power and Energy, Northwestern PolytechnicalUniversity, Shaanxi 710072.5. Flow Improvement in Rectangular Air Intake by Submerged Vortex

Generators. A.R. Paul, K. Kuppa, M.S. Yadav and U. Dutta,Journal ofAppliedFluidMechanics, Vol. 4, No. 2, Issue 1, pp. 77-86, 2011.


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Remarks:

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Flow Control in Y-Duct