CHAPTER 2. CONFIGURATION AND CONSTRUCTION
200. GENERAL DESIGN CONSIDERATIONS. The primary function of the engine test cell (facility) is to provide a controlled environment for engine testing that is compatible with the engine under test and, thus, will not hinder engine operation. It is therefore necessary to conduct tests in a facility that can accurately and consistently provide a measurement of engine performance relative to the performance that would have been obtained if the engine had been tested at the OEM’s baseline facility. All test facilities affect the testing environment and influence the data obtained during testing. This is particularly true of indoor test cells. An OEM’s document, such as a test cell facility planning manual, may be used as a design guide. If a document is not available or if the facility elects not to use it, the information contained in this AC may be used as a general guideline in planning and operating a test cell facility. This AC can be useful in evaluating the design of a test cell, troubleshooting correlation results, and in maintaining a satisfactory correlation status of the facility.
a. Generally, the cell construction is of reinforced concrete for primary support structures with control room construction that consists of either reinforced concrete or concrete block. Secondary cell support areas, such as preparation rooms, may be of any industrial construction. Additional information on design can be found in reference 1-2(a)(2) of Appendix 1.
b. The following system elements are common to most cell configurations. The design of these components may affect engine performance and data accuracy and, thus, the correlation results.
/g183/g32 Intake section
/g183/g32 Engine test and mount system section
/g183/g32 Augmentor/diffuser section
/g183/g32 Exhaust section
/g183/g32 Data acquisition and reduction system
/g183/g32 Instrumentation system and control room
/g183/g32 Engine fuel supply system
/g183/g32 Auxiliary system (may be required by a particular powerplant being evaluated)
201. TEST CELL CONFIGURATIONS. The presence of vortices, turbulence, and non-uniform temperatures and pressures in the area surrounding the engine under test can drastically affect engine performance and test repeatability. Therefore, all test cell configurations should be designed to provide stable testing conditions and minimize turbulent flow by minimizing pressure loss and temperature and pressure variations. Under most environmental conditions, a test cell configuration should not allow recirculation of engine exhaust gases from the cell exhaust stack into the cell inlet. It should also prevent reingestion of engine exhaust gases at the rear of the engine back into the engine inlet. Test cell designs are typically of the following three general configurations:
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a. “L” Type. This design has a horizontal intake and a vertical exhaust. Airflow treatment may include at least one turning vane to turn the intake air uniformly. A grid may be installed on the horizontal inlet to assist with airflow straightening and provide increased noise reduction. The type “L” is the simplest design and the least costly to construct. A horizontal inlet will generally have good flow distribution and a reduced Pcd when the external wind is directly entering the inlet. This configuration is sensitive to prevailing wind conditions and loses efficiency and repeatability when the wind changes direction. This configuration requires a relatively large, unobstructed test area at the engine inlet station for maximum performance (see Figure 2-1 for details).
FIGURE 2-1. TEST CELL CONFIGURATION: “L” TYPE
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b. “U” Type. This design incorporates a vertical stack for both inlet and exhaust. The vertical intake tends to have a more uniform inlet air velocity. This design is less subject to wind disturbances.
Airflow treatment may include at least one turning vane to turn the intake air uniformly. The inlet may be designed to produce a uniform airflow within the test area at the engine inlet station, as well as a uniform engine inlet temperature. A grid may be installed on the vertical inlet to assist with airflow straightening and provide increased noise reduction (see Figure 2-2 for details).
ExhaustAugment orPrimary Test Chamber Screen Test St and w it h Thrust Frame Inlet Acoust ic Baffl es
FIGURE 2-2. TEST CELL CONFIGURATION: “U” TYPE
c. Folded Inlet Type. This design usually incorporates vertical inlets on the building sides, resulting in a more accessible test section because the engine entry door can be located directly in front of the thrust stand. Another variant of this design includes a horizontal inlet on the building top. In both cases, the exhaust is configured vertically. Using a series of turning vanes, the inlet air is uniformly drawn through a winding flow path to a large plenum, which then supplies air to the test section. This configuration is also not significantly affected by wind direction (see Figure 2-3 for details).
ExhaustAugmentorPrimary Test Chamber Test Stand with Thrust Frame Reverse Flow Inlet Acoustic Baffles Retractable Turning Vanes
FIGURE 2-3. TEST CELL CONFIGURATION: FOLDED INLET TYPE Par 201 Page 7
202. ENGINE TEST CELL MAJOR COMPONENTS/SYSTEMS.
a. Engine Test Section. The engine test section is the area immediately approaching the engine under test. Generally, this area will be of a sufficient cross section so that the air velocity approaching the engine inlet will not exceed approximately 50 feet per second (fps). In this section of a well-designed test cell, the airflow tends to have uniform pressure distribution. Test section design and construction may incorporate tapered or concave corners at the rear section where the air flows into the augmentor.
b. Interior Treatment.
(1) Steep floor grades should be avoided
(2) All drains, fittings, and other floor access areas should be flush with the floor
(3) Floor surfaces should be clean, dry, and sealed to prevent foreign object ingestion into the engine and degradation of the test cell surfaces due to leakage of engine oils, jet fuel, and other engine-related substances.
(4) Interior walls and ceilings should be smooth and free of protrusions.
(5) Moveable mechanical equipment should be removed from the engine test area during testing.
c. Engine Mounts. The engine mounts support the engine during testing and permit engine thrust to be accurately measured. Thrust of the engine is usually produced at the engine centerline and transmitted through the mounts to a thrust frame. The thrust frame then pushes against or pulls on a load cell. Thus, it enables the reaction to be measured.
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NOTE: The most common method of engine mounting is overhead suspension.
However, at some engine test facilities, the engine is mounted on a pedestal supported by the test cell floor. The overhead mount more closely simulates the mounting in many aircraft, and more easily accommodates the cleaning of the engine test section and the accessibility of bottom-mounted engine accessories. The engine mount should be designed to prevent transverse motion, fishtailing, or any type of lateral instability. With turbofans, poor lateral stability due to mount flexibility can result in severe engine oscillations during testing. Thrust mount designs should also ensure that engine axial alignment is maintained during testing.
See Figure 2-4 for a schematic representation of a typical overhead mount installation.
FIGURE 2-4. TYPICAL THRUST MOUNT DESIGN
d. Test Cell Inlet System. The test cell inlet system conditions incoming air to reduce the effects of wind speed, direction, and extreme temperatures. This system consists of flow straighteners, heaters, screens, and noise suppressers. These components tend to create a pressure loss or Pcd. The following test cell inlet design features may have an influence on engine performance:
/g183/g32 Area
/g183/g32 Blockage
/g183/g32 Turning angle
/g183/g32 Axial distance between engine inlet and exit of the cell inlet
/g183/g32 The angle of inlet turning vanes
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e. Test Cell Exhaust System. The test cell exhaust system augmentor removes engine exhaust gases from the engine test section, induces the flow of secondary air for cooling, and provides some noise abatement. The mix of exhaust gases and the cooling secondary airflow that goes through the augmentor is then directed through an exhaust stack prior to exiting the facility. The following test cell exhaust design features may have an influence on engine performance:
/g183/g32 Augmentor configuration (i.e., convergent or divergent)
/g183/g32 Augmentor tube length and diameter
/g183/g32 Exhaust inlet tube diameter
/g183/g32 Axial distance between engine exhaust and augmentor inlet
/g183/g32 Area ratio of the engine exhaust to the augmentor area Page 10 Par 202
CHAPTER 3. MEASUREMENT SYSTEMS, CALIBRATION, AND CORRELATION
300. MEASUREMENT SYSTEMS. The following paragraphs provide a general description of typical measurement systems. Instrumentation quantity, position, range, and accuracy are usually specified in an OEM’s document, such as a facility planning manual or overhaul manual.
NOTE: End-to-end checks of each of the systems addressed in this section are extremely important to assure the validity and accuracy of the data being collected.
These checks involve the comparison of data from a particular readout with its associated primary or secondary standard or with a known source. During these system checks, many physical, electrical, or flow discrepancies can be detected and corrected before correlation testing begins.
a. Pressure Measurement Systems. Pressure measurement systems usually use simple pressure gauges, individual dedicated pressure transducers, or a scanning-valve system. Scanning-valve systems are typically a single pressure transducer that alternately measures many pressure lines of similar pressure range.
NOTE: High humidity or rain drawn into the test cell can result in water getting into the pressure lines and producing erroneous pressure measurements for both types of pressure management systems. (See section 305a(2) of this AC.)
b. Temperature Measurement Systems. Temperature measurement systems should be capable of independently evaluating “low and medium” and “high” temperatures. Commonly, low and medium temperature measurement systems utilize E- or J-type thermocouples to measure engine cold section and facility temperatures. High temperature systems typically use K-type thermocouples to measure exhaust gas temperatures. The parameters to be measured will depend on the OEM’s requirements for testing.
Thermocouple junction locations should be minimized within the test cell environment to reduce system noise and error.
c. Engine Speed Measurement System. The engine speed measurement system must be capable of measuring all engine speeds. A typical system may consist of a digital electronic frequency counter device and a readout device, or an analog type with a graduated display dial readout. Both systems provide engine speeds in revolutions per minute or percentages.
d. Fuel Flow Measurement Systems. A typical fuel flow measurement system may consist of one or more turbine flow meters (or comparable flow measuring device(s)), each connected to a digital frequency counter and display gauge. This total measurement system, from the flow meter through the frequency counter to the display gauge, must be checked end-to-end for the entire operating range of the system. For example, a defective frequency counter could provide an acceptable error in the low flow range, while generating a divergently increasing unacceptable error in the high range.
NOTE: Fuel flow measurement becomes increasingly important when it is used in turbine gas temperature calculations. This calculation is sometimes required by OEMs for acceptance testing.
e. Vibration Measurement System. Vibration is not typically evaluated during test cell correlation testing because it does not normally affect engine performance. However, turbine engines cannot be properly evaluated during in-service acceptance testing, as required by the overhaul manual, without employing a system to provide vibration measurement. System components can include vibration pickups, amplifiers, tracking filters, spectrum analyzers, band pass filters, and audio oscillators. Engine Par 300 Page 11
pickup locations can include the fan case, intermediate case, diffuser case, and turbine exhaust case. Hot section vibration pickups may require insulation to prevent high-temperature damage.
f. Trim Balance System. Although trim balancing of the low speed compressor is not required for test cell correlation, performing such balancing before correlation testing is acceptable. In addition, the OEM may specify a trim balance during in-service engine acceptance testing to determine imbalance moment values and, thus, the location and weights of balance counterweights. Generally, trim balancing is accomplished with the aid of a trim balance analyzer. However, procedures are also available for trim balancing with trial balance weights.
g. Thrust Measurement Systems. Thrust measurement is typically accomplished by either an electronic strain gauge or a hydraulic measurement system. The required system accuracy should be specified by the OEM.
301. INSTRUMENT CALIBRATION.
a. Engine Test Facility. The test cell can be considered a component of an engine’s test equipment.
As such, it requires instrumentation calibration and an end-to-end check of the measurement systems.
Instrument calibration plus system checks ensure acceptable measurement accuracy within engine acceptance testing limits specified in the OEM’s overhaul manual. The OEM’s overhaul manual and other documents, such as a facility planning manual, reflect the procedures and test equipment that are required to test engines and the parameters to be measured. Test cell instrumentation must be calibrated and traceable to an accepted, known national standard.
b. Standards. In the United States, the NIST represents the United States at the International Conference on Weights and Measurements and has the primary responsibility for maintaining the national and international standard units of length, mass, time, temperature, and electrical quantities. Other nations have comparable standards bodies. An acceptable secondary or transfer standard has been generally regarded as any standard that is at least four times more accurate than the accuracy requirement for the instrument being calibrated and is traceable to a national standard, such as those maintained by NIST.
c. Calibration of Instruments and a Check of Associated Systems. Calibration is used to check an instrument’s measurements against an accepted known standard and to eliminate unacceptable measurement errors. Instruments should be calibrated over the expected range of operation. Periodic calibration of individual instruments is necessary and can be performed in place or in the instrumentation shop.
d. Traceability. Traceability of each instrument calibration is important to show where all possible error contributions occur between the instrument and the standard. Traceability should be well documented. The calibration hierarchy, with inherent accuracies at each point, must be documented.
Traceability back to a specific standard does not reduce the uncertainty of a measurement; instead, it provides clear documentation of the process of its determination.
302. THRUST CALIBRATION. To calibrate the load cell, a common procedure is to apply known centerline forces using a centerline pull rig. This calibration procedure results in the development of a calibration curve of the known force and the measured load cell reaction. This curve is used during the correlation to obtain a true force from the load cell measurement. Another method is the use of a calibration device built into the thrust stand.
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303. FACTORS INFLUENCING CORRELATION. The following factors may influence the correlation of an engine test cell:
/g183/g32 Configuration of the test cell (particularly the test cell inlet, engine test section, and augmentor tube and exhaust stack configurations)
/g183/g32 Engine position in the test cell
/g183/g32 Ambient conditions and surrounding area configurations
/g183/g32 Instrumentation (including the calibration, location, measurement accuracy, and quantity)
/g183/g32 Test cell thrust measurement system
/g183/g32 Test procedures
/g183/g32 Data acquisition system
/g183/g32 Fuel properties
/g183/g32 Cell airflow
NOTE: In addition, engine test hardware, such as cowling inlets, bellmouths, QEC packages, cowling thrust reversers and noise suppressers, and exhaust nozzles installed on the engine during testing may greatly influence the data. These components should be serialized for tracking control.
304. PERFORMANCE MEASUREMENT.
a. The primary function of the engine test facility is to obtain a proper performance evaluation of an engine. The test facility must provide a stable test environment conducive to a smooth, surge free operation of the specific engine model. The environment of test facilities always has an influence on the test data. This is particularly true of indoor test cells. In addition, the engine test hardware configuration influences the test data. The combined effects of the test facility and engine test configuration on performance parameters are generally the reason for differences in an engine’s performance between test cells. The test cell correlation provides a means to:
/g183/g32 Quantify these differences with respect to a standard
/g183/g32 Understand these differences
/g183/g32 Eliminate or reduce these differences, whenever possible
/g183/g32 Establish appropriate cell correlation factors
b. Performance Parameters. During a test cell correlation, a variety of engine performance parameters should be examined, including those parameters specified in the overhaul manual for engine acceptance testing and any values or factors used to correct primary parameters. Parameters may include, but are not limited to, the following:
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/g183/g32 Engine fuel flow
/g183/g32 Engine thrust
/g183/g32 Engine rotor speeds
/g183/g32 Engine airflows
/g183/g32 Engine pressures and temperatures
/g183/g32 Engine inlet conditions
/g183/g32 Test cell temperatures and pressures
/g183/g32 Test cell inlet airflow
/g183/g32 Engine control systems and variable geometry
/g183/g32 Barometric pressure
/g183/g32 Ambient humidity
NOTE: Sufficient data must be obtained to establish the performance of the engine.
Secondary parameters used in the derivation of any of the above items must also be measured and recorded. As an example, a fuel sample should be taken during the test and accurately analyzed for a lower heating value (LHV) and specific gravity value. In addition, redundant instrumentation can be useful for data verification and troubleshooting purposes.
305. FACTORS AFFECTING PERFORMANCE. To compare performance parameters between various test facilities or conditions within the same test facility, correcting or normalizing the performance parameters to common reference conditions is necessary. These reference conditions fall into three groups: Ambient (humidity, temperature, and pressure); fuel properties (relative density, viscosity, and LHV); and aerodynamic (ram pressure ratio, cell depression, and cell bypass airflow interaction).
a. Humidity. While it has been recognized for a long time that high humidity levels will affect the performance of gas turbine engines, no consensus exists on how to handle the associated effects or problems. Water vapor contained in the air may have several influences on the engine and its performance. Although the consequences are complex, they fall into two major categories: engine inlet condensation and changes in engine gas properties. While the relative humidity is related to the extent of engine inlet condensation, the absolute or specific humidity affects the gas properties of the engine cycle and, hence, the performance elements.
(1) Actual condensation in an engine inlet depends on a series of factors, such as relative humidity, air temperature, air pressure, inlet Mach number, and dwell time. For given humidity conditions, the probability for condensation is higher in long inlet ducts and lower in bellmouth intakes.
Bellmouths should be sized per the OEM’s specifications. For example, an undersized bellmouth could result in a high local inlet Mach number that might cause inlet condensation at lower relative humidities.
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(2) Humidity corrections for most performance parameters have been found to be small.
However, humidity can have an impact on performance and should be considered when accurate performance measurements are required. To minimize this effect, some test facilities and OEMs choose to impose limits on test cell humidity during correlation testing for particular engine models. A relative humidity restriction of 75 percent is not uncommon for correlation testing.
b. Engine Inlet Temperature and Pressure. All gas turbine engines are affected by the ambient conditions in which they operate (primarily air temperatures and pressures). Also, engine operation and correlation can be affected by an engine inlet temperature distortion or gradient due to exhaust gas recirculation or other sources of heat. This gradient should be minimized. Unstable and low pressures in the test chamber or engine inlet also affect engine operation and correlation. Distorted/non-uniform cell airflow can cause vortices and asymmetric pressure distribution in the engine inlet. These conditions can also be caused by exhaust gas recirculation, poorly designed test cell inlet and test chamber sections, too many items protruding into the inlet airflow stream, and marginal chamber secondary airflow. In most cases, unacceptable temperature or pressure conditions are obvious because of their effect on engine performance. Their cause should be corrected to the extent possible before correlation testing.
(1) It is usually not possible to control the engine inlet air temperature and pressure to sea level, static, standard day values. Therefore, to compare one engine run to another, the measured engine performance parameters must be adjusted to the values they would have with standard day inlet air. This process is called normalizing or making standard day corrections to the measured data.
(2) The method for correcting performance parameter data often varies between different engine types and models. The procedures to correct parameters for a particular engine model can be found in a specification, Technical Standard Order, or the OEM’s engine overhaul manual.
(3) Certain primary operating variables of a gas turbine engine are normalized or corrected as functions of total temperature and total pressure at the engine inlet. The basic normalizing parameters are as follows:
(a) θ (theta) = (observed inlet total absolute temperature) divided by (absolute temperature of International Standards Organization [ISO] sea level, static, standard day reference atmosphere).
(b) δ (delta) = (observed inlet total absolute pressure) divided by (absolute pressure of ISO sea level, static, standard day reference atmosphere).
NOTE: These ratios require the use of consistent units and absolute values (e.g., temperatures in degrees Kelvin or degrees Rankine and pressures in psia, in-HgA, or kPa).
NOTE: In some cases, measured data from certain gas turbine engines are corrected to conditions other than ISO standard day values. In these cases, refer to the applicable engine model specification, Technical Standard Order, or engine overhaul manual.
(4) The correction or normalizing of the major engine performance parameters requires the use of θ and δ, as follows:
(a) Rotor speed, N, is normalized when divided by θx (i.e., N/θx) where X is dependent on the engine type and defined by the manufacturer (commonly, 0.5 is used).
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(b) Thrust, F, is normalized when divided by δ (i.e., F/δ).
(c) Airflow rate, Wa, is normalized when multiplied by √(θx/δ) (i.e., Wa√(θx/δ)).
(d) Fuel flow rate, Wf, is normalized when divided by δθy (i.e., Wf/δθy) where Y is dependent on the engine type and defined by the manufacturer (typical values of Y range from 0.5 to 0.7).
(e) Engine cycle absolute temperatures (such as T3, T4, T5, and T7) are normalized when divided by θz (i.e., T5/θz) where Z is dependent on the location within the engine and the engine type.
Z is usually defined by the OEM (typical values of Z range from 0.85 to 1.1).
(f) Engine cycle pressures (e.g., compressor discharge, turbine discharge, nozzle exit) are normalized when divided by δ (i.e., P3/δ).
NOTE: The subscripts “3, 4, 5, 7” are designators of specific axial stations in the engine. They are peculiar to each manufacturer. Those shown in this AC are for illustrative purposes only.
c. Fuel Properties. Experience has shown that fuel selected to a particular specification from a single supply source maintains reasonably stable properties. However, fuel taken from various sources, even when selected to the same specification, can vary in properties by several percent. When comparing fuel property data taken from different sources, the factors in the following paragraphs should be taken into consideration.
(1) Fuel Relative Density. The fuel relative density, also referred to as specific gravity, may be utilized in the fuel flow calculation as a first order correction. Therefore, accuracy of the specific gravity measurement is of prime importance when using volumetric flow measurement devices, such as turbine flowmeters. Relative density calculations are made in two steps: the relative density at a given reference temperature, then the relative density at the temperature of the fuel measured at the flowmeter. Relative density determined at the reference temperature (usually 60.0 degrees Fahrenheit [F] or 15.6 degrees Celsius [C], as provided within references of section 1-2 of Appendix 1 of this AC) is then corrected to the actual fuel temperature, using either a graph or an empirical equation. Since the relative density is a direct multiplier on the fuel flow, any error in the former will be transferred to the latter.
(2) Fuel Viscosity. Depending on the type of fuel flow measuring device utilized, the viscosity of the fuel can play a role in the calibration of that device. For example, fuel flow device calibration factors for Jet A/A1 and Jet B fuels are significantly different due to their differences in viscosity. Also, viscosity is a significant function of temperature. The specific effect can be found in the SAE ARP Report No. 4990, Turbine Flowmeter Fuel Flow Calculations.
(3) Fuel Lower Heating Value.
(a) Fuel flow measurements are corrected to a common fuel LHV base. This base is obtained by applying the direct ratio of the fuel’s LHV to the LHV of the engine model specification or to the LHV of the baseline test fuel (i.e., industry standard).
(b) Analysis of the LHV for a given batch of fuel can usually be obtained from the fuel supplier. An acceptable accuracy for LHV can be obtained using the aniline point gravity product method of a fuel sample. The determination of the aniline point and the fuel density requires a minimum of laboratory equipment. A more accurate fuel LHV can be determined in a laboratory by a precision bomb calorimeter.
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