Flow control through an airplane's intake Andreea Cristina Petcu* INCD Turbomotoare COMOTI, B-dul Iuliu Maniu nr. 220D, sector 6, cod 061126, OP 76, CP174, Bucuresti, Romania Abstract: In this paper is presented a study regarding the control of air flow. For this purpose it was considered the study of the air flow through a supersonic airplane’s intake. Aircraft engine intakes for upersonic flight speeds have sharp edged inlet cowls, and in consequence considerable losses in total pressure of the entering air occur at take-off conditions. To reduce this loss we opened an auxiliary air inlet in the side of the intake. Our purpose was to find a relationship between the medium total pressure at the entrance in the compressor and the dimension of the auxiliary inlet knowing that at the compressor’s entrance the Mach number of the air flow must be 0.5. The air flow numerical simulations are realized using FLUENT. 2010 Mathematics Subject Classification: 76 Fluid mechanics Keywords: flow control, intake, take-off conditions, inviscid fluid, compressible fluid, total pressure 1. The intake device The air intake is that part of an aircraft structure by means of which the aircraft engine is supplied with air taken from the outside atmosphere. The air flow enters the intake and is required to reach the engine face with optimum levels of total pressure and flow uniformity. These properties are vital to the performance and stability of engine operation [1]. The main conditions to be met by the intake device are [2]: - To ensure at the entry into the compressor a uniform distribution of speeds in time and space; thus there shouldn’t be any pulse, local turbulence or variations of speed in radial and angular directions. A uniform distribution of speeds will generate a uniform distribution of static and total pressures, condition for stable operation of the compressor; - To partly transform the kinetic energy of the air flow into mechanical work of compression - The evolution of the air into the intake device to generate a minimum possible total pressure drop, for a 1% decrease in pressure results in a decrease in traction force of 0.5-1.2% depending on the engine design solution and flight regime; _____________________________________________ *Tel: 0214340240, fax: 0214340241 E-mail adress: [email protected]; URL: http.//www.comoti.ro 1
Transcript
Flow control through an airplane's intake
Andreea Cristina Petcu*INCD Turbomotoare COMOTI, B-dul Iuliu Maniu nr. 220D, sector 6, cod 061126, OP 76, CP174, Bucuresti, Romania
Abstract: In this paper is presented a study regarding the control of air flow. For this purpose it was considered the study of the air flow through a supersonic airplane’s intake. Aircraft engine intakes for upersonic flight speeds have sharp edged inlet cowls, and in consequence considerable losses in total pressure of the entering air occur at take-off conditions. To reduce this loss we opened an auxiliary air inlet in the side of the intake. Our purpose was to find a relationship between the medium total pressure at the entrance in the compressor and the dimension of the auxiliary inlet knowing that at the compressor’s entrance the Mach number of the air flow must be 0.5. The air flow numerical simulations are realized using FLUENT.2010 Mathematics Subject Classification: 76 Fluid mechanicsKeywords: flow control, intake, take-off conditions, inviscid fluid, compressible fluid, total pressure
1. The intake device The air intake is that part of an aircraft structure by means of which the aircraft engine is supplied with
air taken from the outside atmosphere. The air flow enters the intake and is required to reach the engine face with optimum levels of total pressure and flow uniformity. These properties are vital to the performance and stability of engine operation [1].
The main conditions to be met by the intake device are [2]:- To ensure at the entry into the compressor a uniform distribution of speeds in time and space; thus there shouldn’t be any pulse, local turbulence or variations of speed in radial and angular directions. A uniform distribution of speeds will generate a uniform distribution of static and total pressures, condition for stable operation of the compressor;- To partly transform the kinetic energy of the air flow into mechanical work of compression - The evolution of the air into the intake device to generate a minimum possible total pressure drop, for a 1% decrease in pressure results in a decrease in traction force of 0.5-1.2% depending on the engine design solution and flight regime;- The plane’s drag induced by the intake device to be as little as possible.
Therefore an optimal intake device cannot be achieved for all flight regimes, but a compromise solution can be developed corresponding to the main flight regimes depending on the type of aircraft used.
The intake device constructive solution depends mainly on the flight speed of the aircraft (subsonic or supersonic regime), as well as the location of the engine (fuselage or external gondola). If the engine is embedded in the fuselage the air intake stands outside, while the intake’s sewerage is a constructive element of the plane’s fuselage. In this case the total pressure loss may depend on the constructive solution of the intake’s sewerage, which varies from plane to plane, and the drag is determined only by the air intake. If the engine is located on an external gondola the admission device is part of the engine, whose constructive solution must be analyzed in terms of maximum performance of the entire propulsion system.
The air’s velocity leaving the intake device is subsonic. For example at turbo-reactors the axial component of the velocity at the compressor’s entry is between 120-160 m/s in the case of centrifugal compressors and 150-220 m/s in the case of axial compressors.
After the flight speeds of the plane for which the air intake was designed we distinguish [3]:1. M < 0.7 – subsonic admission device, in any point of the flow the critical regime is not reached.
2. 0.7 1.2M - admission device with one normal shock wave ( Pitot type).
3. 1.2 1.5M - supersonic admission device with one oblique shock wave and one normal shock wave.
4. 1.5 2.5M - supersonic admission device with two oblique shock waves and one normal shock wave.
2. The physical problem Flight at low speeds represents the worst working regime for the plane’s intake from the point of view of
the functioning of the plane’s engine. That’s why the analyze of the total loss pressure in the intake effect’s upon the compressor is very important [4].
Aircraft engine intakes for supersonic flight have sharp-edged inlet cowls and in consequence considerable losses in total pressure of the entering air occur when the aircraft is moving at low forward speeds, particularly under take-off conditions. One way of reducing this loss is to open auxiliary air inlets in the side of the intake so that the inlet is partially bypassed at low forward speeds.
Knowing that the flow’s Mach number at the compressor’s entry is 0.5 we what to find a relationship between the opening of the auxiliary inlet and the air’s total pressure at the compressor’s entry. The studied intake is presented in Figure 1.
Figure 1
It has a single cone centrebody of semi-angle positioned for shock on lip operation at M=4. The dimensions of the intake are shown in Figure 2.
Figure 2
2
Two ways of constructing the auxiliary inlet were considered:1. by sliding forward the exterior part of the admission system.
Figure 32. by sliding backward the exterior part of the admission system.
Figure 4For the simulation of air flow through the plane’s intake device was used FLUENT 6.1. The calculus
grid was realized in GAMBIT. The air flow through the considered intake was studied for three different plane’s speed values, namely
speeds corresponding to Mach numbers 0.1, 0.2 and 0.3.
3. Governing equationsConsidering that the air that flows through the plane’s intake is compressible and inviscid the flow
phisycal phenomenon is described by the axisymmetric Euler model [5]:
( ).U F G
S Ut r z
(1)where:
2 2
2
1, ,
( ) ( ) ( )
u u u
uvu u p puG
v ruv v p uv
E u E p v E p u E p
U F S
(2)So that the physical phenomena to be completely described we associate to the model above a state
equation
2 2
( 1)2
u vp RT E
(3)which characterizes the flow’s thermodynamic variables (pressure, temperature, density). Also we impose the initial and boundary conditions.
3
4. FLUENT Simulations4.1. Used settings:1. The chosen solver is of coupled type, explicit, axisymmetric and steady. 2. The working fluid is considered air, inviscid and that it has the properties of the ideal gas (density,
thermal conductivity, caloric capacity at constant pressure).3. Operating conditions: P=101325 Pa and T=288K4. Boundary conditions:
Figure 5
4.2. FLUENT simulation results
4.2.1. Simulation results with the auxiliary inlet closed. The Mach number coresponding to the plane’s flying speed has the following values: 0.1, 0.2, 0.3.
Flying speed Mach number
Mach Number field Static pressure field
0.1
0.2
0.3
4
4.2.2. Results with the auxiliary inlet open. In these simulations the Mach number coresponding to the plane’s flying speed is 0.1.
The opening of
the auxiliary inlet (inches)
The auxiliary inlet of type 1(sliding type) M=0.1
The auxiliary inlet of type 2(valve type) M=0.1
0.5
Mac
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mbe
r fi
eld
Sta
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pres
sure
fi
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1
Mac
h nu
mbe
r fi
eld
Sta
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sure
fi
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4.2.3. Results with the auxiliary inlet of type 2 (valve type) open. In these simulations the Mach number coresponding to the plane’s flying speed is 0.2 and 0.3.
The opening of the auxiliary inlet
(inches)M=0.2 M=0.3
0.5
Mac
h nu
mbe
r fi
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Sta
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pres
sure
fi
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Mac
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mer
fie
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tati
c pr
essu
re
fiel
d
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5. Results Analyse
Results obtained for the forward sliding type of auxiliary inlet and for a plane’s speed of 0.1 Mach.
Auxiliary inlet openingx (inch)
Average total pressure at the compressor’s entry P (Pa)
By interpolating the Fluent simulations results (using Curve fitting toolbox from Matlab) the function that describes the relationship between the average total pressure at the compressor’s entry and the auxiliary inlet opening was obtained:
5 0.04868 4 0.9545( ) 1.15 10 2.538 10x xP x e e (4)
Results obtained for the backward sliding type auxiliary inlet and for a plane’s speed of 0.1 Mach.
Auxiliary inlet openingx (inch)
Average total pressure at the compressor’s entry P (Pa)
By interpolating the Fluent simulations results (using Curve fitting toolbox from Matlab) the function that describes the relationship between the average total pressure at the compressor’s entry and the auxiliary inlet opening was obtained:
5 0.0722 4 0.574( ) 1.312 10 4.17 10x xP x e e (5)
7
Using the Fluent simulations results and the ones obtained by interpolation we were able to create the graphic of the average total pressure’s - at the compressor’s entry – dependence on the auxiliary inlet’s opening. The both types of auxiliary inlet were considered. (Figure 6)
Figure 6
Results obtained for the backward sliding type auxiliary inlet and for a plane’s speed of 0.2 Mach.
Auxiliary inlet openingx (inch)
Average total pressure at the compressor’s entry P (Pa)
By interpolating the Fluent simulations results (using Curve fitting toolbox from Matlab) the function that describes the relationship between the average total pressure at the compressor’s entry and the auxiliary inlet opening was obtained:
5 0.0655 4 0.5437( ) 1.306 10 3.538 10x xP x e e (6)
8
Results obtained for the backward sliding type auxiliary inlet and for a plane’s speed of 0.3 Mach.
Auxiliary inlet openingx (inch)
Average total pressure at the compressor’s entry P (Pa)
By interpolating the Fluent simulations results (using Curve fitting toolbox from Matlab) the function that describes the relationship between the average total pressure at the compressor’s entry and the auxiliary inlet opening was obtained:
5 0.05204 4 0.5119( ) 1.269 10 2.545 10x xP x e e (7)
Having the results obtained above we represented graphic the relationship between the average total pressure at the compressor’s entry and the auxiliary inlet opening (the backward sliding type auxiliary inlet) for the three considered plane’s speeds. (Figure 7)
Figure 7
9
From the graphic above it can be observed that small openings of the auxiliary inlet – between 0 and 0.2 inches – don’t diminish the loss of pressure in the intake, the contrary they are energy consumers.
Based on the interpolation results above we determined the dependence of the auxiliary inlet opening - corresponding to the maximum average pressure at the compressor’s entry - of the plane’s take-off speed Mach number:
(8)
In figure 8 is represented graphic this dependence.
Figure 8
From figure 7 it can be observed that from a value of the auxiliary inlet opening the total pressure increase is insignificant. The relationship between the value of the auxiliary inlet opening from which the total pressure increase is insignificant and the plane’s take-off speed Mach number is given be equation (9):
(9)
In figure 9 is represented graphic this dependence.
Figure 9
10
6.ConclusionsBy realizing the flow simulations in Fluent and then interpolating these results we were able to find a
mathematical relationship between the auxiliarly inlet opening and the average total pressure at the compressor’s entry.
We have studyed the air flow throught the considered intake for three different values of the plane’s flying speed – namely Mach 0.1, 0.2 and 0.3. We were able to determine the dependence of the auxiliary inlet opening corresponding to the maximum average total pressure at the compressor’s entry of the plane’s take-off speed Mach number.
We have observed that from certain values of the auxiliary inlet opening the total pressure increase at the compressor’s entry is insignificant. Also we reached the conclusion that small openings of the auxiliary inlet – between 0 and 0.2 inches - don’t diminish the pressure losses along the intake, instead they are energy consumers.
References[1] C. D. Soni, Study of air intake configuration in aircraft, MVJ College of Engineering, Bangalore, 2009[2] V. Pimsner, Motoare aeroreactoare, vol. I, Ed. Didactică şi Pedagogică, Bucureşti, 1983[3] V.Stanciu, Motoare Aeroreactoare, Ed. Institutului Politehnic din Bucuresti, Bucuresti, 1992[4] M. Cox, Static Tests on a Conical Centerbody Supersonic Air Intake with an Auxiliary Airinlet Slot, London, 1960[5] M. Stoia-Djeska, A Practical Introduction to Computational Fluid Dynamics, Ed. Didactica si Pedagogica, Bucuresti, 2005
