Document
15ICE-0019
Ice Crystal Icing Engine Testing in the NASA Glenn Research Center’s
Propulsion Systems Laboratory: Altitude Investigation
Michael J. Oliver
NASA Glenn Research Center Copyright © 2014 SAE International the PSL, an existing altitude simulation engine test facility [4]
Abstract
with an ice crystal cloud generation system. The PSL cloud generation system/facility was calibrated to duplicate the ice The National Aeronautics and Space Administration (NASA) crystal icing field event from the BAe146 flight test campaign conducted a full scale ice crystal icing turbofan engine test [5]. NASA conducted an inaugural full scale ice crystal icing using an obsolete Allied Signal ALF502-R5 engine in the turbofan engine test in the modified PSL in February 2013. The Propulsion Systems Laboratory (PSL) at NASA Glenn instrumented engine that experienced the flight test event is Research Center. The test article used was the exact engine the same serial number (LF01) engine that was used for the that experienced a loss of power event after the ingestion of NASA PSL inaugural ice crystal icing engine testing. This ice crystals while operating at high altitude during a 1997 testing is documented and detailed in reference [6].
Honeywell flight test campaign investigating the turbofan engine ice crystal icing phenomena. The test plan included The data generated during this testing contained three test points conducted at the known flight test campaign field subsets: known high altitude event conditions, altitude scaling event pressure altitude and at various pressure altitudes conditions and a design of experiment (DOE) data set. The ranging from low to high throughout the engine operating flight test data supplied by Honeywell Engines was the basis envelope. The test article experienced a loss of power event for the known event conditions portion of the test plan. The at each of the altitudes tested. For each pressure altitude test altitude scaling portion of the test plan was designed to point conducted the ambient static temperature was predicted duplicate the known flight test event at subsequently lower using a NASA engine icing risk computer model for the given altitudes. This was done to generate engine test data in order ambient static pressure while maintaining the engine speed.
to investigate the feasibility of altitude scaling testing of ice crystal icing in a turbofan engine environment. The DOE
Introduction
portion of the test plan was developed by Honeywell engines in order to investigate sensitivity of the engine Pilots have experienced uncommanded thrust reductions while performance/operability to various test parameters. The operating near convective storms during high altitude flights known conditions portion of the test plan is discussed in since the early 1950’s [1]. These events are suspected as due reference [6] and the DOE portion of the test plan is discussed to the ingestion of ice crystals into the turbofan engines in reference [7]. The data generated during the altitude scaling powering the aircraft. The theory is that a phenomena termed portion of the test plan is the basis for this manuscript.
ice crystal icing is occurring on warmer than freezing surfaces inside the engines operating at high altitudes due to the
PSL Ice Crystal Icing Altitude Scaling
ingestion of ice crystals [2]. Several loss of thrust events
Engine Testing
occurred during the 1990’s on BAe 146 aircraft powered by AlliedSignal ALF502-R5 turbofan engines. This led to an investigation and resolution program conducted by Honeywell Roll Back Testing at Lower Altitudes Engines [3]. The investigation included flight testing of an instrumented engine and aircraft that allowed for the Testing Objectives continuous monitoring of engine and ambient flight conditions as the aircraft flew through ice crystal icing conditions near The objective of these altitude scaling test points was to convective storms. During these flight tests a loss of thrust generate engine data to assist researchers in investigating the event occurred while both engine operating parameters and feasibility of developing altitude scaling laws that would assist ambient flight conditions were being recorded. This data was engine manufacturers in designing test plans simulating high made available to NASA by Honeywell Engines.
altitude ice crystal icing tests but conducted at existing sea level engine test facilities [8]. This is required to allow for the To better understand this ice crystal icing phenomenon and to testing of large, high air mass flow turbofan engines that develop full scale engine, driven rig and component test cannot be tested in altitude simulation facilities like PSL due to capability in high altitude ice crystal icing conditions, NASA maximum airflow limitations.
Glenn Research Center in Cleveland, OH modified test cell 3 in Page 1 of 7 indicators charts for selected pressure altitude test conditions Altitude Scaling Test Plan Development conducted on this test article. The plots are presented in decreasing altitude order: Altitude-1 > Altitude-2 > Altitude-3 > The altitude scaling test points were conducted to generate Altitude-4 > Altiude-5. Altitude-1 in Figure 1, is the called roll data of a full scale turbofan engine ingesting ice crystals and back of the known flight test condition at high altitude tested experiencing a loss of thrust event while operating at altitudes during the flight test campaign and duplicated in PSL.
ranging from high to low. The high altitude flight test ice crystal icing event duplicated in PSL was based on a known set of environmental and engine operating conditions. For the Altitude Scaling Test Points altitude scaling portion of the test plan, potential ice crystal icing conditions at lower altitudes leading to an ice crystal icing Each of the altitude scaling test points shown in Figures 1-5 event were unknown. This presented a difficult task for the underwent an uncommanded loss of power due to operation in development of the altitude scaling portion of the test plan. To an ice crystal cloud environment produced in PSL. Though the address this issue, NASA developed an engine icing risk test points conducted are not intended to represent a feasible model based on the geometry and performance maps of the flying condition the objective of the tests was to produce an ice test article and the prediction of icing risk as a function of crystal icing related loss of power event. A major benefit of ingested ice crystal melt ratio and wet-bulb temperature within using the PSL facility is that any test point a researcher wishes the flow path. This icing risk model was used to predict the to run can be conducted whether it represents a feasible ambient environmental operating temperature for each altitude condition for flight or not. This capability to generate altitude scaling test point conducted [9]. To facilitate testing by limiting scaling data was not possible before the PSL modifications. It unknowns, the engine operating parameters were not changed can now be used to investigate the feasibility of developing between the various altitude scaling test points if possible.
altitude scaling laws [8].
Total water content of the cloud and tweaks to engine speed were required changes for some test points.
General Discussion about the Plot Scales Key Roll Back Indicating Parameter A detailed description is presented in the following sections for each of the altitude scaling test points presented in Figures 1 During the PSL ice crystal icing testing, the key indicating to 5. The brief notes listed immediately following this parameter of the ice crystal icing event, a compressor roll paragraph apply to the scales of all the charts in the figures. It back, was found to be the initial reduction of the measured should be noted by the reader that the plots are all sanitized at load parameter, the average of two measured load cells the request of the engine owner in efforts to protect data mounted on the thrust stand [6]. Based on the investigation deemed proprietary in nature.
and resolutions report [3] and the PSL test data analyzed [6], the theory is that ice build-up or ice crystal icing occurs in the Load low pressure compression (LPC) system for this test article.
The ice buildup restricts the core air flow or flow through the The load data was normalized by the maximum value in each high pressure compressor, combustor and high pressure data set before it was plotted. Each load data point represents turbine of the engine. The restriction is theorized to be either a a relative value rather than an absolute value. The data point physical or aerodynamically induced blockage that limits the is referenced to the maximum value of load measured during airflow and thus the energy able to be produced in the engine the respective test to ascertain whether the load is in general core. The reduction in energy produced in the core results decreasing or increasing as the test point ensues. In all of the initially in a reduction of the core component of engine thrust plots the load increases slightly at cloud on since the additional seen as a small reduction in the measured load parameter and mass of the water entrained in the airflow goes to increasing subsequently an uncommanded reduction in N1 or fan speed the change in momentum as the air and water mixture passes leading to a reduction in the by-pass component of engine through the various stages of the engine. For each plot a label thrust seen as a larger reduction in measured load and if is placed showing in general the load decreasing. This label is allowed to further continue an eventual sudden uncommanded not intended to identify specifically the onset of load reduction.
reduction in N2 or a full roll back of the high pressure compression system. This is seen as a sudden drop off of N2, Time N1 and measured load.
The time scale is not equal for all of the plots. In an effort to Called Roll Back facilitate time scale comparisons between the test points the time scale for the reference Altitude-1 test condition is called X.
A full engine roll back exposes the test article to a high risk of All the other time scales are referenced as some factor of X.
hardware damage due to the shedding of ice buildup in the This allows the reader to easily compare time scales on the flow path. In order to mitigate this risk a called roll back sanitized charts.
procedure was developed. The called roll back procedure requires close monitoring of the measured load parameter, Cloud flow path static pressure, metal temperatures, N1 and N2.
Once a certain threshold of reduced measured load is met The cloud plot line is intended to be an on/off indication. When accompanied by observed static pressure, temperature, N1 the cloud plot line is not on the X-axis the cloud is turned on for and N2 losses, a rollback is deemed imminent, the cloud is the test point. The cloud was turned off for every test point at turned off and the engine is allowed to recover or return to the instant a decision was made to call the roll back condition.
initial cloud off operating condition. The plots shown in Figures 1 to 5 show plots of these parameters or called roll back Page 2 of 7 N1, N2, Ps, PLA and Temperature altitude than Altitude-1. From the chart it can be seen that the time scale for the roll back condition is nearly 4X longer than the Altitude-1 reference condition. The cloud total water The plot scales for all the plots are identical for the parameters content was held constant for these two test points. The only of N1, N2, static pressure (Ps), power lever angle (PLA) and flow path metal temperatures. There is a significant difference changes between them were the ambient static temperature and pressure. The plane temperatures do not converge as between cloud on and cloud off metal temperatures measured quickly as they did for the reference condition. N1 and Ps are in the flow path. For each test point presented the load parameter appears to reduce as the metal temperatures reduced slightly and the N2 reduction is not visible on the plot.
PLA remains constant. This result supports the theory that the appear to converge as the test point ensues. The temperature planes referenced go deeper into the engine as the respective large temperature drop at cloud on for the average plane temperatures may be related to the wet-bulb temperature at plane number increases. They are all located in the low the surface as water on the surface where the thermocouples pressure compression system. Plane 5 temperature converges the fastest for all test points. are embedded evaporates into the airflow and the air temperature approaches the adiabatic saturation temperature.
The increased time to cool down the metal temperatures at the Altitude-1 lower altitude may be related to the fact that at the higher pressures of the lower altitude the evaporation potential is The chart, Altitude-1, in Figure 1 plots the called roll back data lower and is not as readily occurring from the surface. Indeed generated in PSL that duplicated the known high altitude ice it can be seen that the static pressure is higher along the flow crystal icing roll back event which occurred during actual flight path for altitude-2 than for the reference condition.
testing conducted by Honeywell Engines using the LF01 turbofan engine [3]. It is noted that this is the only known environmental and operational roll back condition for any turbofan engine that exists for an ice crystal icing event. The parameters of this figure are used as reference for the altitude scaling test points conducted at lower altitudes.
Figure 2. Altitude-2 Roll back indicator chart for the altitude scaling test condition tested in PSL. Altitude-2 < Altitude-1 Altitude-3 The chart, Altitude-3, in Figure 3 plots the PSL data generated Figure 1. Altitude-1, Roll back indicator chart for the known high altitude flight test condition tested in PSL. during an ice crystal icing test point conducted at a lower altitude than Altitude-2. It should be noted that the cloud total water content was 2.4 times higher than that of the reference The Load parameter begins to decrease as the average plane condition for this altitude scaling test point. From the plot it is temperatures appear to converge. N1 is slightly reduced seen that the roll back condition took 2.4X longer than the though PLA is steady for this called roll back test point and an reference condition. The static pressure along the flow path is N2 reduction is not noticeable to the naked eye. Note that the slightly higher than Altitude-2 but in general Altitude-3 looks load parameter begins to reduce prior to the reduction of N1 and N2. It is also seen that there is a slight reduction in Ps as similar to both the reference condition and Altitude-2. The main difference is the time to roll back. The total water content the temperatures converge and the load begins to decrease.
is known to be a major factor in roll back time [6]. Altitude-2 This plot supports the theory that ice buildup is leading to restricted flow which is manifested as a reduction in load and a and Altitude-3 plots suggest that both altitude and total water content influence the roll back time for this test article.
reduced static pressure as the air mass flow rate begins to pass through a smaller area.
Altitude-2 The chart, Altitude-2, in Figure 2 plots the PSL data generated during an ice crystal icing test point conducted at a lower Page 3 of 7 Figure 4. Altitude-4 Roll back indicator chart for the altitude scaling test Figure 3. Altitude-3 Roll back indicator chart for the altitude scaling test condition tested in PSL. Altitude-4 < Altitude-3 condition tested in PSL. Altitude-3 < Altitude-2 Altitude-5 Altitude-4 The chart, Altitude-5, in Figure 5 plots the PSL data generated The chart, Altitude-4, in Figure 4 plots the PSL data generated during an ice crystal icing test point conducted at a lower during an ice crystal icing test point conducted at a lower altitude than Altitude-4. The total water content for this test altitude than Altitude-3. The total water content for this test point is the same as for Altitudes -3, and -4. It is 2.4 times point is the same as for Altitude-3 and is 2.4 times higher than higher than the reference condition. The static pressure along the reference condition. The initial run shows no changes the flow path is higher than for Altitude-4 but in general the test other than the initial drop in plane temperature with cloud on.
point looks similar to the Altitude-4 test point in that it required There is no convergence of plane temperatures and Ps, N1, two PLA adjustments to induce the roll back. Once the N2 and PLA remain constant. A slight reduction in PLA results appropriate PLA position was sufficiently adjusted, the roll back in a slight reduction in all the parameters but again they settle occurred 2.5 times longer than the roll back in Altitude-4. This out at a steady state. A second reduction in PLA results in a reinforces that altitude has a significant influence on ice slight reduction again in all the parameters but this time the buildup leading to roll back for this test article.
plane temperatures converge and the load parameters begin to reduce over time indicating an imminent roll back condition.
The step reduction in load at each of the PLA adjustments is a result of decreased N1 and not due to reduced flow in the core.
The Ps begins to reduce at the second PLA adjustment. It should be noted that the PLA adjustments were very minimal on the order of small perturbations and not significant changes in power lever position. The NASA model was successful at predicting the proper ambient temperature for the lower altitude conditions of Altitudes-2, and -3 but needed a little tweaking for Altitude-4. This suggests that altitude has a significant influence on ice crystal icing especially at significantly lower altitudes. Once the final PLA adjustment was made the called roll back took 1.5 times longer to occur than the reference condition which had a little more than half as much total water content in the cloud on condition.
Figure 5. Altitude-5 Roll back indicator chart for the altitude scaling test condition tested in PSL. Altitude-5 < Altitude-4 Discussion: Altitude Scaling Feasibility The PSL testing clearly demonstrated that full scale altitude scaling testing does work and is feasible for this test article.
The ice crystal icing event was duplicated easily at Page 4 of 7 consecutively lower altitudes. Future testing with internal made by researchers to frame the procedures to develop cameras will provide additional evidence and insight as to the altitude scaling laws for this test article [8]. The modifications location and characteristics of the ice crystal icing between the giving PSL a high altitude, ice crystal icing capabilities have high and low altitude test points. However, for some of the been built and demonstrated to work. Researchers are lower altitude test points it was necessary to hypothesize and developing PSL test plans and fundamental experiments to tweak test parameters in order to replicate the ice crystal icing better understand the physics of ice crystal icing and Industry event. This implies that there are some physics that need to is beginning to develop test methodologies and rigs that allow be better understood for altitude scaling. It could be said that it to better understand the effects of high altitude ice crystal given enough time, additional full scale testing in PSL would ingestion into their engines. It remains to be seen if altitude allow for a realistic understanding of how to simulate the high scaling laws are feasible to be developed that generally apply altitude ice crystal icing conditions and event at a lower altitude across the board for all engines experiencing ice crystal icing test condition. This does not necessarily prove that altitude and allow industry to develop their engines at sea level test scaling is feasible when applied to other turbofan engines facilities or if individual engines will require specific altitude operating in the field or on current or future the drawing scaling laws based on both module testing in a facility like PSL boards. The objective of this altitude scaling testing conducted that translate to sea level testing to demonstrate regulatory in PSL is to provide experimental data that allows researchers compliance.
to investigate the feasibility of developing altitude scaling laws.
References
The concept of altitude scaling implies that a series of scaling laws can be developed that allow ice crystal icing test plans to 1. Hooker, Stanley, J., Not Much of an Engineer , Airlife be implemented on full scale turbofan engines at existing sea Publishing imprint of The Crowwood Press Ltd, Ramsbury, level test facilities or lower altitudes in general that duplicate Marlborough Wiltshire SN8 2HR, 2002, Chap. 7, pp. 136- the high altitude ice crystal icing flow path conditions and 37.
subsequent events—based on the altitude scaling laws [8]. By 2. Mason, J.G., Strapp, J.W. and Chow, P., The Ice Particle mitigating these events or showing the engines are not th Threat to Engines in Flight, 44 Aerospace Sciences susceptible to them via full scale testing at the sea level facility Meeting and Exhibit, AIAA, January 9-12, 2006, Reno, NV industry could demonstrate regulatory compliance.
3. Goodwin, Ronald, V. and Dischinger, David, G., Turbofan Ice Crystal Rollback Investigation and Preparations Taken literally, one thought to this end is to exhaustively Leading to Inaugural Ice Crystal Engine Test at NASA investigate and understand fully all of the physics, engine and PSL-3 Test Facility, 6th Atmospheric and Space environmental parameters influencing ice crystal icing at high Environments Conference, AIAA, Atlanta, GA, June 16-20, altitude operation and duplicate the exact results in a full scale engine test at a sea level test facility by manipulating the 4. Griffin, T.A., Dicki, D. J.; Lizanich, P. J.; “PSL Icing Facility various parameters to reproduce the high altitude ice crystal Upgrade Overview”, 6th AIAA Atmospheric and Space icing operational environment. Indeed this is the path of many Environments Conference, American Institute of current worldwide sponsored research efforts. Another thought Aeronautics and Astronautics, June 2014, Atlanta, Ga.
is to use a facility like PSL as a research and development tool 5. Van Zante, Judith, F. and Rosine, Bryan, M., NASA Glenn to identify key areas of an individual engine flow path that may Propulsion Systems Lab: 2012 Inaugural Ice Crystal Cloud be susceptible to ice crystal icing and work to mitigate these Calibration Procedure and Results, 6th Atmospheric and areas in a research and development environment. These Space Environments Conference, AIAA, Atlanta, GA, June lessons could then be incorporated into current and/or future 16-20, 2014.
designs. The main issue with both options however is how to 6. Oliver, M.J., “Validation Ice Crystal Icing Engine Test in ultimately demonstrate regulatory compliance.
the Propulsion Systems Laboratory at NASA Glenn Research Center”, 6th AIAA Atmospheric and Space The main driver for sea level testing is that larger fan high Environments Conference, American Institute of bypass engines require orders of magnitude more bypass Aeronautics and Astronautics, June 2014, Atlanta, Ga.
airflow than PSL can deliver under high altitude ice crystal icing 7. Hauser, K. and Oliver, M.J., “Ice Crystal Icing Engine Test test conditions. Since the ice crystal icing affects the airflow Model Development and Sensitivity Analysis”, SAE 2015 through the engine compression system and not the bypass International Conference on Icing of Aircraft, Engines and airflow, it is conceivable that researchers could use Structures, Prague, Czech Republic, June 22-25, 2015.
compression system rigs as their research and development 8. Tsao, Jen-Ching, Struk, Peter, M. and Oliver, Michael, J., test articles for the larger turbofan engines. PSL can handle “Possible Mechanisms for Turbofan Jet Engine Ice Crystal th the compression system airflows of even the largest turbofan icing at High Altitude”, 6 Atmospheric and Space engines in existence or on current drawing boards. Testing Environments Conference, AIAA, Atlanta, GA, June 16-20, compression modules separated from the rest of the engine brings with it many issues to deal with in and of itself. These 9. Veres, Joseph, P. and Jorgenson, Philip, C., “Modeling test issues are not insurmountable but they are beyond the Commercial Turbofan Engine Icing Risk with Ice Crystal scope of this manuscript. Suffice it to say the engine Contact Information companies in addition to engineering these test rigs would need to garner support of the process from the various regulatory authorities worldwide.
The feasibility of altitude scaling testing has been demonstrated for this test article. A first attempt has been Page 5 of 7
Definitions/Abbreviations
DOE Design of Experiment NASA National Aeronautics and Space Administration N1 Fan Speed N2 High Pressure Compressor Speed PLA Power lever angle Ps Static Pressure PSL Propulsion Systems Laboratory Page 6 of 7
Appendix
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