ERCOFTAC Design Optimization:Methods & Applications

International Conference & Advanced CourseAthens, Greece, March 31- April 2, 2004

ERCODO2004_202

Small Scale Engine Test Bed Design and Optimisation

P. Laskaridis P. Pilidis

V. Pachidis

Cranfield University, UNITED KINGDOM

Conference ProceedingsEditors: K.C. Giannakoglou (NTUA), W. Haase (EADS-M)

P. Laskaridis, P.Pilidis, V. Pachidis

Design Optimization International Conference March 31-April 2, 2004, Athens, Greece

SMALL SCALE ENGINE TEST BED DESIGN AND OPTIMISATION

P. Laskaridis, P. Pilidis, V. Pachidis

Gas Turbine Engineering Group, Department of Power Propulsion and Aerospace Engineering, Cranfield University, Bedfordshire UK

Keywords: Design, Optimisation, CFD, Engine Testing, Performance, Micro Turbine, Test Bed, Experimental Facility.

Abstract: Recently, Cranfield University purchased a micro gas turbine and invested in the design and construction of a small test bed facility that is used to provide experimental data for the support of analytical CFD studies concentrating on gas turbine performance within enclosed test bed facilities. The purpose of this micro-turbojet and test bed facility is to mimic full-scale test beds and provide a better understanding of the aerodynamic and engine performance issues involved. A number of different engine arrangements are considered and optimised to minimise pressure losses and ensure stable engine operation. Commercial CFD tools and in-house engine performance codes are used to predict the performance of the engine and the results are compared against experimental data. This paper presents the use of commercial CFD software for understanding and solving practical problems. Its other purpose is to describe the experimental facility and the present state of technical progress, which is under development. 1 INTRODUCTION � DESCRIPTION OF THE PROBLEM

Engine testing is a necessary but nonetheless expensive and time consuming process. One of the main performance parameters needing to be measured with great accuracy is the engine thrust. Many hours of engine testing are required during the development stage to ensure that the final design complies with the original specifications. Then, prior to customer delivery the engine has to pass the production acceptance tests. Finally, following a major refurbishment or overhaul, the engine is also tested to ensure that its performance is within the required standards. The present paper concentrates on sea level static tests, which are further divided into outdoor and indoor tests. Outdoor test beds consist of an open air stand where the engine and the necessary instrumentation are mounted. The area around the engine should be free of obstructions to prevent disturbances on the mass flow entering the engine. A large mesh screen can also be fitted around the engine inlet to mitigate the effect of cross wind on engine behaviour [1], as shown in Figure 1.The advantage of such an installation is that under conditions of zero wind the outdoor test facility provides an ideal environment where the air enters the engine without any forms of re-circulation and drag components and therefore the trust measured is the gross thrust produced by the engine, that is, as if it were in an infinite atmosphere. However, such conditions are not always possible to achieve.

Figure 1: Schematics of Typical Outdoor and Indoor Test Facilities

P. Laskaridis, P.Pilidis, V. Pachidis

In practice, the performance of the engine and the accuracy of the thrust measurements depend heavily on the weather conditions; for accurate calibrations, a long wait may be necessary to obtain suitable weather, and this ties up costly capital. In addition noise is a major factor of disturbance and therefore these tests must be done in remote areas to minimise environmental impacts. For these reasons most of the facilities used to measure engine performance are enclosed test beds. In these facilities, the engine is fixed on a cradle in a test chamber and air is provided through an inlet system, as shown in Figure 1. The main advantages of an indoor test bed are the provision of all-weather availability, repeatable and controlled conditions and noise reduction. However, indoor facilities present a number of undesirable effects, such as the existence of a secondary flow bypassing the engine, interferences with the floor and the walls, possible re-ingestion of hot gases, etc. Moreover, the thrust measured in such facility has to be corrected to obtain the real thrust delivered by the engine.

In the case of indoor tests the ejector action of the jet stream from the engine nozzle induces a secondary flow around the engine. This secondary mass flow gives rise to three main drag components, as shown in Figure 2: Inlet momentum drag (IMD), Cradle drag, Nozzle base drag, [18]. These forces are acting on the engine and the cradle, thus affecting the thrust measurements. For this reason, the thrust measured in an indoor test bed can be as much as 10% lower than the thrust measured in an outdoor test bed. Figure 3 plots the thrust measurements for indoor and outdoor tests

Measured Thrust: W1*Vj+(p0-pj) Aj- Fcradle - Fnozzle- IMD- External Drag + ∫ −

1

00 )( dApp

Figure 2: Schematic of Test Bed Drag Components [18] contacted at Cranfield University with a small scale test bed and micro gas turbine engine. Correction factors, therefore, need to be calculated and applied to obtain the true thrust of the engine. Test cell correction factors depend on a number of generic parameters such as the size and geometry of the test bed and the engine, and therefore they must be calculated separately for each engine/bed combination. The calculation of these correction factors relies on many experiments and calibrations that end up being highly costly. Obviously, the calibration method with an outdoor test is expensive, time consuming and can also incur more variation and uncertainty, particularly in an inconsistent UK climate.

Figure 3: Difference in thrust measurements [21]

Cradle Drag

A1

A 0

Aj

(p0-pj)(A1-Aj)

Exterrnal Drag

W1*Vj+(p0-pj)Aj

∫ −1

0 0 )( dApp

IMD= W*V0

P. Laskaridis, P.Pilidis, V. Pachidis

2 EXPERIMENTAL FACILITY

As a result, a lot of research here at Cranfield is concentrating on the use of CFD tools to provide a better understanding of the aerodynamic and engine performance issues involved [3-9]. Given the unstable nature of flow within a test bed, Cranfield University purchased a small gas turbine engine and invested in the design and construction of a small-scale test bed facility shown in Figure 4. The primary aim of building this facility is to study the behaviour of an engine within a test bed and to calculate the required thrust correction factors. The facility is also used for a better understanding of the effects taking place and also to provide experimental data to support a theoretical analysis. This test bed is designed, with the assistance of Rolls-Royce graduate trainees, for a micro turbine. To be able to study the effects of varying test bed geometry it was designed to be of variable proportions, and has a moving cradle to facilitate thrust measurement.

Figure 4: Cranfield�s Test Bed Facility

The small gas turbine used at Cranfield�s test bed was designed and manufactured by AMT Netherlands [19],

showing Figure 5. This engine is mainly used for the propulsion of radio-controlled aircraft. The Olympus consists of a single stage radial compressor, an axial turbine and an annular combustion chamber. The Olympus turbine is controlled and protected by means of a microprocessor controller (ECU) that regulates the engine operation and fuel supply. The ECU uses the gas exhaust temperature, the rpm of the engine and the inputs from the radio control system as input data. The fuel flow supplied to the engine is adjusted to the correct value. The ECU is fully automatic and needs no adjustment. For safety reasons, if the exhaust gas temperature exceeds 700°C, the control system closes the fuel supply to the engine. The engine uses propane as starting gas for the pre-heating of the motor. After the start-up the engine is switched from propane to kerosene. The data of the Olympus engine are given in Table 1 below [19].

Table 1: AMT Olympus engine Characteristics Figure 5: AMT Olympus engine [19]

One of the main parameters defining the performance of an engine within enclosed test bed facilities is the Entrainment Ratio (ER), which is defined as the ratio of secondary mass flow to engine mass flow:

Weight 2.400 kg Length 0.27 m Diameter 0.13 m Thrust 7 N at minimum

power (3000rpm) 190 N at maximum

power (110000rpm) Air mass flow 0.4 kg/s (at maximum

power) Fuel flow 0.009 kg/s (at

maximum power) Exhaust gas temperature 650°C (at maximum

power)

P. Laskaridis, P.Pilidis, V. Pachidis

1

1

WWWER CELL −

=

Where WCELL = Total mass flow in test bed, W1= Engine Mass flow

Because of the size of the engine it was difficult to use instrumentation without disturbing the quality of the flow at the inlet to the engine. Consequently, it was decided to use a venturi pipe at the inlet to the engine to accurately measure the mass flow. However, because of the length of a venturi pipe, it was not possible to fix it in the test chamber. As a result, it was decided to put the engine outside the test bed and to put a dummy engine into the test chamber. Another venturi tube was fixed at the rear of the detuner to measure the mass flow going out of the test bed. Then, by a simple subtraction it was possible to obtain the value of the second mass flow. However, to have accurate measurements in the Venturi tube, it was essential to have a smooth flow entering the device. Furthermore, another requirement was that the flow coming out of the engine should be uniform to simulate the real exhaust flow of an engine. When the configuration shown in Figure 6 was tested, the engine did not run in a stable manner. Fluent, a commercial CFD software was used along with in-house engine performance code to establish the cause of unstable engine operation and optimise the configuration.

Figure 6: Alternative Engine Test Bed Configuration (Dimensions in mm)

Turbomatch, an in house engine performance code, is used together with Gotress, a gas path analysis program developed here at Cranfield University, to perform what is known as analysis by synthesis techniques. An iterative procedure is followed to match the design point modelling and performance of Turbomatch models with the experimental data. With mass flow, pressure ratio, exhaust gas temperature, thrust and nozzle area known, Turbomatch models were developed. Four variables were selected i.e. compressor, turbine and burner efficiency as well as pressure losses downstream the turbine. Engine thrust and nozzle area were selected as the matching variables or dependant variables. Initial values for the efficiency and pressure losses, also called independent variables, were set so that the dependent variables form Turbomatch were close to experimentally measured dependent variables. These initial values together with the target values are then used as inputs to

Figure 7: Effect of Inlet Pressure Losses TET and Mass Flow

AMTEngineInlet Venturi

Plenum

Exhaust Venturi

Dummy Engine Detuner

0.350

0.360

0.370

0.380

0.390

0.400

0% 2% 4% 6% 8% 10% 12%Inlet Losses

Mass flow (kg/s)

900.00

950.00

1000.00

1050.00

1100.00

1150.00

0% 2% 4% 6% 8% 10% 12%

Inlet Losses

K

Turbine Inlet TemperatureExhaust Gases TemperatureMaximun Temperature Allowed

P. Laskaridis, P.Pilidis, V. Pachidis

the gas path analysis tool. The outputs include changes in the four independent variables required to achieve the target values for thrust and nozzle area. With compressor, turbine and combustion efficiency known the Olympus model was run at off-design conditions to simulate the effect of inlet pressure losses on the performance of the engine. For this reason pressure losses were introduced at the inlet of the engine and the fuel flow was kept constant. The effects on mass flow and turbine entry temperature (TET) are shown in figure 7. The same procedure was followed to simulate the effects of outlet pressure losses on TET and mass flow, Figure 8.

Figure 8: Effect of Outlet Pressure Losses on Mass Flow and TET

To calculate the pressure losses at the inlet duct, CFD simulations of the whole inlet system were performed. Initial boundary conditions used a static pressure at the inlet of the engine to give a mass flow of 0.4kg/s. This value of mass flow corresponds to the design point of the engine and is given by the engine manufacturer. The value was also used for the engine design point modelling with Turbomatch. For this conditions and the engine inlet area the required static pressure found to be 93 KPa. With this value as boundary condition at the engine inlet, Fluent predicted a mass flow of 0.25 kg/s. To be able to predict the correct boundary conditions, different values for static pressure at the inlet of the engine were considered and represented different pressure losses. An iterative process was established and the results from CFD were plotted against Turbomatch results to establish the appropriate boundary conditions and provide the correct value of mass flow. The results are plotted in Figure 9. The final values are shown in table 2.

0.280

0.300

0.320

0.340

0.360

0.380

0.400

0 2 4 6 8 10

Pressure losses (%)

Mass flow (kg/s)

Turbomatch simulationFLUENT simulation

Figure 9: Iterative Process for CFD Boundary Conditions

Relative pressure losses (%) Mass flow (kg/s) Fluent: 85500 Pa 7.4 0.37

Turbomatch 7.4 0.372 Table 2: Mass flow and Pressure losses calculated by Turbomatch and Fluent

0.370

0.375

0.380

0.385

0.390

0.395

0.400

0% 1% 2% 3% 4% 5% 6% 7%Outlet Losses

Mass flow (kg/s)

900.00

925.00

950.00

975.00

1000.00

1025.00

1050.00

1075.00

1100.00

0% 1% 2% 3% 4% 5% 6% 7%

Outlet Losses

K

Turbine Inlet TemperatureExhaust Gases TemperatureMaximum Temperature Allowed

P. Laskaridis, P.Pilidis, V. Pachidis

Figure 10: Total pressure on symmetry plan (Pa) Figure 10 includes a diagrammatic representation of the total pressure variation within the inlet system. The

total pressure losses at the inlet system are around 75 KPa. Three main sources of losses are identified:

- Venturi pipe - Plenum chamber - Duct linking plenum to the engine

30% of the losses take place in each component. The pressure losses occur due to the turbulence that exists in the flow. In regions were the velocity is reduced, intense turbulence is generated as velocity pressure is converted to static pressure. This is explaining why the two regions of the inlet system that correspond to a diffusion zone, the divergent section of the Venturi and the plenum chamber, are two of the main sources of pressure losses, Figure 11.

The pressure losses in the duct linking the plenum to the engine can be explained by the fact that numerous re-circulations take place in this region due to the sudden change in area. Figure 11 also presents streamlines of particles in the inlet system and shows the re-circulations that take place.

Figure 11: Total Pressure within Plenum (Pa) � Streamline within Plenum

4 INLET OPTIMISATION

In an attempt to minimise the pressure losses of the configuration it was decided to redesign and alter the existing arrangement. Once again a commercial CFD package was used to visualise, understand and evaluate the flow within the redesigned system. Although a third of the total pressure losses took place at the venturi, it was decided not to alter its configuration. Instead, the following options were considered:

93500

94500

95500

96500

97500

98500

99500

100500

101500

Bellm

outh

Inlet

Exit B

end

Inlet

Ven

turi

Exit V

entur

i

Inlet

Plen

um

Outle

t Plen

um

PreIn

letEn

gine

Engin

e Inle

t

Pa

P. Laskaridis, P.Pilidis, V. Pachidis

- Replace the duct between the venture and the plenum with a diffuser - Replace the plenum with a bend - Replace the duct between the plenum and the engine with a nozzle - Replace the plenum with a bend and connect it to the engine with a nozzle - Use a diffuser, a bend and a nozzle to connect the Venturi to the engine CFD models were developed and used for the above mentioned options, Figure 12. As with the existing

design, an iterative process was followed to calculate boundary conditions for each configuration and evaluate pressure losses. The final results are shown in Figure13.

Figure 12: Inlet Optimisation � Options Considered and Final Design

Figure 13: Pressure Losses at Inlet System

5 OUTLET OPTIMISATION

Fluent was also used to evaluate the magnitude and effect of pressure losses at the exhaust of the system. In the case of the exhaust pressure losses were found to be 7 %. CFD results, Figure 14, show that after the 90 degrees bend, regions of low and high pressure are formed leading to recirculation, pressure losses and non-uniform profiles. Based on the flow patterns provided by fluid several options were considered for minimising pressure losses and introducing more uniform velocity, temperature and pressure profiles at the exhaust nozzle. The redesign and optimisation of the original geometry was based of the findings of the CFD models and the visualisation of the flow patterns within the exhaust system. For this reason turning vanes where introduced, Figure 15.

Duct or Diffuser

Plenum orBend

Duct or Nozzle

Duct or Diffuser

Plenum orBend

Duct or Nozzle

4.55.05.56.06.57.07.5

Des

igne

d

Diff

user

+ple

num

Ben

d

Plen

um+N

ozzl

e

Ben

d+N

ozzl

e

Diff

user

+Ben

d+N

ozzl

e%

P. Laskaridis, P.Pilidis, V. Pachidis

Figure 14: Streamline within Exhaust and contours of Velocity (m/s)

The alternative configurations considered include the following: - Single turning vane - Two turning vanes - Smooth Bend - Smoth turning vanes

Although inclusion of turning vanes did not eliminate flow recirculation and did not establish a uniform outlet profile, the intensity of recirculation reduced significantly, especially in the case of smooth guide vanes, and the pressure losses reduced from 7% to 2.4%.

Figure 15: Alternative Configurations Based on the Flow of the Original Exhaust 6 CONCLUSSIONS

Cranfield University is involved in a research study evaluating the performance of gas turbine engines within enclosed test bed facilities. For this reason a small scale test bed has been designed and constructed to be used with a micro gas turbine engine and provide data for theoretical analysis. Accurate mass flow measurements are required and for this reason a special arrangement has been considered. When tested, the engine did not operate in a stable manner. Fluent, a commercial CFD package, was used along with in-house engine performance and gas path analysis tools to quantify the magnitude and the effect of system pressure losses on the performance and operation of the engine. In addition CFD models where used to re-design the inlet and exhaust system and to minimise pressure losses. The end result is a reduction of pressure losses from 7.4% to 4.6 for the inlet system and a reduction from 7% to 2.6% for the exhaust.

P. Laskaridis, P.Pilidis, V. Pachidis

Exhaust with 1 guide vane Exhaust with 2 guide vanes

Exhaust with a bend Exhaust with smooth guide vanes

Figure 16: Streamlines at Exhaust Configurations

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[2] DNS81496/1, 2001 � Performance Engineering Technical Guide to the Compatibility, Commissioning and Calibration of a Gas Turbine Engine Test Facility, Rolls Royce plc.

[3] Ybarra, C. 2002. Cradle modeling for gas turbine test cell CFD simulation. MSc Thesis, Cranfield University, Cranfield.

[4] Dejean, R. R. 2003. CFD prediction of test cell thrust correction on high by pass ratio turbofan, MSc Thesis, Cranfield University, Cranfield.

[5] Franco, S. 2000. Gas turbine test cell simulation, MSc Thesis, Cranfield University, Cranfield. [6] Kammerer, A. 2002. Initial investigation into simulation of vortex formation in a gas turbine test cell, MSc Thesis Cranfield University, Cranfield. [7] Le Fay, A., 2002, Engine test bed flow path analysis, MSc Thesis, Cranfield University, Cranfield [8] Gunathilake, D, 2002, Engine test cell airflow analysis - CFD validation through experimental model, MSc Thesis, Cranfield University, Cranfield. [9] Cannock, I.P., 1993, A study of sea level gas turbine test bed features and their influence on engine performance, MSc Thesis, Cranfield University, Cranfield [10] Barton J. M., 1984, The Role of Computational Fluid Dynamics in Aeropropulsion Ground Testing, Journal of Aircraft, 21, Number 10.

[11] Cross, M. A. 1987. Application of computational fluid dynamics to analysis of exhaust gas /diffuser interactions in a turbine engine altitude test cell. AIAA 87-2412 23rd Joint propulsion conference, San Diego. [12] Freuler, R. J. 1993. Recent successes in modifying several existing jet engine test cells to accommodate large, high- bypass turbofan engine. AIAA 93-2542 29th Joint propulsion conference and exhibit. Monterey, California. [13] Kodres, C. A. and G. L. Murphy. 1998. Jet-engine test cell augmentor performance. Journal of propulsion and power, 14 number 2, pp 129-134. [14] Kromer, S. L. and D. A. Dietrich. 1985. Flow field analysis of low bypass ratio test cell. Journal of aircraft, 22, number 2, pp 99-100. [15] Power, D. and B. D. Heikkenen. 1993. CFD applications in aeropropulsion test environment. AIAA 93-1924, 29th Joint proulsion conference and exhibit, Monterey, California. [16] Prufert, M. B., M. D. Mclure, and G. D. Power. 1994. Computational support to engine test. SAE 942141.

P. Laskaridis, P.Pilidis, V. Pachidis

[17] Prufert, M. B. and J. W. Williamson. 2000. Computational analysis of turbine engine test cell flow phenomena. AIAA 2000-2210 Aerodynamic measurement and ground testing conference, Denver, Colorado. [18] Rios, R. M. and others. 1998. Thrust correction on jet engine in sea level test facility. AIAA 98-3109 34th AIAA/ASME/SAE/ASEE Joint propulsion conference & exhibition, Cleveland [19] Olympus Manual, 2000,Revision 2.09, AMT Helmond Netherlands.

[20] Rolls Royce, 2003,Thrust Measurement Rig for a Micro Gas Turbine, Assembly, Operation and Maintenance Manual.

[21] Gonzalez S E, 2003, Thrust Correction Factors for Small Gas Turbine Indoor Test Bed, MSc Thesis, Cranfield University,.

[22] Henderson K., 2000, Ascertaining the Flow Coefficient of the PT75 Airmeter, Technical Report, Test005, Rolls-Royce plc.

[23] Lahti D.J, Hamed A., 1993, Verification of the Theoretical Discharge Coefficient of a Subcritical Airflow Meter, Journal of Propulsion and Power, 9, No. 4.