The Environmental Durability Branch, Materials Div., NASA Glenn Research Center, is charged to bring research materials to a higher level of technology readiness for use in advanced propulsion and power systems. Developing strategies to enhance and predict component durability in gas turbine engine requires a fundamental understanding of the high-temperature degradation mechanisms in advanced materials. This requires that materials are studied under conditions similar to those under which they will operate. NASA's high-pressure burner rig (HPBR) facility offers a way to simulate the harsh environments to which these materials will be exposed. For example, HPBR test data enabled researchers to make major contributions to the knowledge base of advanced materials such as the durability of silicon-base monolithic and composite ceramics. Researchers also studied important issues such as the effects of water vapor and scale volatility, providing vital mechanistic and performance information. In addition, collaborations with government and industry have provided insight into the development of innovative component concepts and advanced sensor technologies.
Test facility description
HPBR provides a relatively inexpensive, yet sophisticated way to study high-temperature oxidation of advanced materials. The facility can operate under both fuel-lean and fuel-rich gas mixtures, using a fume incinerator to eliminate any harmful byproduct emissions (carbon monoxide, or CO, and hydrogen sulfide, or H2S, for example) of rich-burn operation. Test samples are easily accessible for ongoing inspection and documentation of material weight change and thickness, cracking, and other metrics. Temperature measurement can be performed via thermocouples and optical pyrometry, and quartz windows allow observation and videotaping.
Operating conditions include:
- Capability of 1.0 kg/sec (2.0 lb/sec) combustion and secondary cooling airflow
- Equivalence ratios (ERs) of 0.5 - 1.0 (lean) to 1.5 - 2.0 (rich), typically with 10% H2O vapor pressure
- Gas temperature ranging from 700 to 1650C (1290 to 3000F)
- Test pressure ranging 4 to 12 atm
- Gas flow velocities ranging 10 to 30 m/s (30 to 100 ft/sec)
- Capability of cyclic and steady-state exposure
The facility historically has been used to test coupon-size specimens, including metals and ceramics, but complex-shaped components, such as cylinders, airfoils, and film-cooled end walls, also have been tested. The facility also has been used to develop thin-film temperature measurement sensors.
A schematic of the HPBR is shown in Fig. 1. Combustion air provided by a dedicated 400-hp compressor enters the combustor and flows over the outside of the liner; the air provides cooling and also is preheated for more efficient combustion. The air is directed through a swirler in the combustor dome, mixed with jet fuel supplied by an air-blast fuel nozzle, and ignited using a spark plug and hydrogen. Combustion products flow downstream through a water-cooled turbulator orifice and optional transition section(s), incurring some heat loss before passing into the test section.
Combustion gas flows downstream into the test section and over specimen(s) held in a fixture. This specimen holder is mounted on a shaft, which is accessible to the gas path through a "T" section (figure 2). Mass flow, gas chemistry, velocity, and pressure are controlled in the test section, and temperature is measured optically and using thermocouples. Combustion gases are quenched downstream by a water spray before passing through an exit valve that maintains system pressure. A second orifice located between the test chamber and quench section is used to create a pressure drop, which prevents quench water from coming upstream.
Upon exiting the rig, combustion products, condensed water, and steam pass through a particle separator and natural-gas stack burner before being vented to the atmosphere. This removes water and environmentally unacceptable soot, and byproduct emissions. The modular design of the HPBR makes each section easily accessible for assembly and maintenance.
The combustor consists mainly of a housing, fin-cooled liner, swirler, and fuel injector (figure 3). At the inlet, a manifold system distributes the airflow uniformly onto the liner and creates a pressure seal as the liner flange is pushed against the turbulator. Air passes through the annulus between the housing and outside diameter of the liner, entering the combustor dome through the swirl plate and air-blast fuel nozzle. The swirl plate and nozzle are secured (pinned) in the housing, lending to a free-floating metal liner, which is completely unconstrained to expand and contract during operation. The liner is slotted to allow for growth over the swirler and past the pins, which eliminates severe buckling. The combustor liner also uses a plasma-sprayed thermal barrier coating (TBC) to improve durability.
The test section is a 150-mm (6-in.) diameter T-shaped chamber through which the combustion gases flow while being directed over the specimen holder. The T section provides access for the specimen holder and can accommodate specimen sizes (including holder) of approximately 100 mm diameter by 150 mm long (4 by 6 in.) when inserted (figure 4).
Candidate material coupons are tested in a standard specimen configuration (figure 5). Test samples arranged in a wedge configuration are loosely contained in slotted superalloy grips within the holder. A standard sample size of 76 mm long by 13 mm wide by 3 mm thick (3 by 0.5 by 0.12 in.) is preferred, but sample width and thickness can vary slightly; the 13 mm wide sample size allows testing of 4 coupons in the fixture (figure 5a). A thermocouple probe measures the gas temperature at a position centered 20 to 30 mm (0.78 to 1.12 on.) behind the wedge, and sample temperature is measured via optical pyrometry through the viewport.
Downstream of the test area, there are component sections and systems critical to the operation of the HPBR including a water-quench section and liquid/gas separator to cool the effluent and remove any condensate and a natural gas fume incinerator to burn off any environmentally hazardous emissions (typically CO). In addition, the facility has a fully automated data acquisition and control system, which is critical to maintain a reliable, well-documented test. These support systems are described in [1-2].
The primary variables of interest to simulate gas-turbine conditions for materials test purposes are temperature, pressure, gas flow velocity, and most importantly, gas composition. Table 1 summarizes the operating envelope of the HPBR for both lean and rich-burn operation. In the standard test, the fuel-to-air equivalence ratio is controlled at a fixed test pressure (6 atm) and mass airflow (0.5 kg/sec, or 1 lb/sec). This airflow and pressure is recommended to provide adequate cooling and optimal durability to the combustor liner over the entire operating range. However, lower airflow can be used for moderate combustion temperatures. Specimens are pneumatically inserted into the gas stream when preferred test conditions are achieved.
While test conditions vary depending on requirements, there are limitations for varying parameters independently. Operation at slightly higher airflow is possible for lean-burn conditions, but the CO fume incinerator is limited to approximately 0.6 kg/sec (1.3 lb/sec) under rich-burn conditions. Regarding fuel flow, the region around stoichiometry (ER = 1.0 - 1.5) is avoided due to the high temperatures associated with this region, while ERs in excess of 2.0 are avoided because of sooting. Minimum pressure (4 to 5 atm) is driven by the minimum airflow mentioned, as well as the maximum exit area. Maximum pressure (10 to 15 atm) is limited by the requirement to maintain a sufficient pressure drop across the swirler for proper fuel-to-air mixing, which may necessitate an increase in the mass flow. Accordingly, an increase in pressure decreases the velocity, while an increase in mass flow produces a proportional increase in velocity.
Test coupon temperature
Because of the specimen holder's wedge-like configuration, the two inside samples generally run hotter than the outside samples due to significant axial and radial temperature differences. The inside samples are referred to as leading edge (LE) samples and the outside samples are referred to as trailing edge (TE) samples. Test results verify that gas and sample temperatures are dependent on the fuel-to-air ratio, with maximum temperatures observed near stoichiometry. Axial and radial temperature gradients between LE and TE samples are dependent on pressure and velocity. Figure 6 shows that sensors on both LE and TE samples measure temperatures (in agreement with pyrometry measurements) relative to their position as well as the combustion gas temperatures.
Test coupons are not visible under rich-burn conditions due to the intense luminosity of the flame, and, therefore, sample temperature cannot be measured directly using pyrometry. Instead, sample temperature is calculated using the correlation between pyrometer and combustion gas temperatures measured in lean-burn operation. The correlation (a least-squares, straight- line regression) is repeatable and can be input into the computer to estimate real-time temperatures during the test.
NASA and leading engine makers were eager to identify a primary method for hot gas testing during initialization of the Enabling Propulsion Materials (EPM) Program, NASA's government-industry partnership. Generally, burner rigs offer certain advantages over furnace testing and other methods in the evaluation of materials for use in high-temperature engine applications. Complex conditions, such as thermal stresses, moisture, and oxidizing atmospheres are more realistically simulated in the harsh environment combustion rigs such as the HPBR. The HPBR was recognized as unique among existing facilities with respect to gas chemistry, operating regime, and user friendliness, and was adopted as one of the EPM program's primary materials development test facilities. The HPBR has a wide range of capabilities to conduct various types of studies (from thermal shock to long-term exposure), but the main focus of testing has been oxidation studies.
HPBR testing has been instrumental in making significant contributions into the understanding of high-temperature oxidation of SiC. Specifically, environmental durability studies benefit from the high pressure, water-laden atmospheres present in the HPBR. For example, SiC recession was identified as a primary concern in both fuel-lean and fuel-rich environments. To augment analytical and furnace data, an extensive series of tests was performed in both gas mixtures to determine the SiC recession rates due to SiO2 scale volatility in the presence of water vapor.
Coupon testing is the primary focus of the HPBR, specifically concerning high temperature oxidation behavior in the presence of water vapor. Materials tested in addition to SiC include metals, silicon nitride (Si3N4) and numerous coating systems.
In addition to standard coupon testing, the HPBR can accommodate component testing for aeronautical programs requiring complex geometries, transpiration cooling, and even flight cycle simulation. A number of successful studies have been completed in the facility, each requiring specific modifications to rig hardware and software to accomplish specific goals. In each case, the HPBR's versatility enabled development and implementation of effective solutions in a timely manner.
As part of a turbine airfoil program, a series of fiber-reinforced ceramic matrix composite (CMC) airfoils were exposed in the HPBR to simulated gas turbine conditions. The SiC/SiC CMC airfoils featured transpiration cooling air holes and a complex geometry including a twist (figure 7). Alternate specimen holders were fabricated to secure the airfoils and deliver the required cooling to the internal blades. The cooling air also can be preheated and delivered at pressures as high as the external pressures.
A concept for a combustor liner application was creatively installed and evaluated within another useful test area of the HPBR. A large CMC cylinder was mounted in a modified transition section designed to place the cylinder in the open area between the combustor and test sections. The 100 mm diam by 200 mm long (4 by 8 in.) C/SiC tube was suspended on six water-cooled legs used to support the structure and accommodate thermocouple probes (figure 8a). The cylinder shows some cracking and slight oxidation (figure 8b) after 50 hr exposure at material temperatures near 1370C (2500F). Three thin-film sensors were used to monitor materials temperature and three probes monitored gas temperature.
In a current project, the HPBR is being used to cycle a new SiC/SiC leading-edge airfoil between simulated idle, lift, and cruise flight conditions to determine conceptual durability and temperature benefits. The second stage high pressure turbine vane of the Pegasus F402-RR-406 engine, powerplant of the U.S. Navy's Harrier fighter, was equipped with a ceramic matrix composite insert made by AlliedSignal Composites Inc. (figure 9a). The CMC airfoil, together with a baseline metal vane (figure 9b), was air-cooled, uniquely instrumented, and exposed to flight cycles intended to simulate the Harrier mission cycle. Testing successfully reproduced failures on the metal vane similar to those experienced in service, while demonstrating the durability of the SiC/SiC insert and reduced leading-edge temperatures. The cycle also included step changes in external gas pressure and velocity, further demonstrating the extended capabilities of the HPBR.
For more information: Craig Robinson is materials test engineer, NASA Glenn Research Center, Mail Stop 24-1, 21000 Brookpark Rd., Cleveland, OH 44135; tel: 216-433-5547; e-mail: raymond. c.robinson@ grc.nasa.gov