The focus of the Trapped Vortex Combustor (TVC) program was to develop gas turbine engine technology which can simplify the military’s efforts to comply with the Clean Air Act (CAA). Although all military aircraft are exempt from emission standards applied to commercial aircraft, the military bases where these aircraft are stationed are required to comply with the CAA such that the military must monitor all emissions from new stationary and mobile sources, including aircraft operations up to 3,000 feet above the runway. If the total amount of criteria pollutants [ozone, carbon monoxide (CO), or particulate matter (PM)] or precursors of ozone such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) exceed the Environmental Protection Agency (EPA) established minimum levels, the military must demonstrate how they will conform to the State Implementation Plan for the area. This has proven to be difficult with the deployment of new weapons systems and in cases when aircraft were transferred from one base to another. Future weapons systems may require emissions reductions in order to avoid deployment issues in areas of nonattainment. This program focused on the demonstration of an engine quality low emission TVC design. The TVC program goals were to reduce at high power conditions the NOx emissions by 50% and at low power conditions both CO emissions by 60% and unburned hydrocarbons (HC) by 80% from the baseline engine. With the TVC technology, the demonstration has enabled future deployment of military aircraft that can simplify the ability to conform to the CAA in nonattainment regions.
A joint ESTCP, Navy, Air Force, Department of Energy (DOE), and industry program was proposed to demonstrate the capabilities of the TVC technology for reducing emissions in gas turbine engines in aircraft, ships, and stationary power facilities. The TVC concept had been successfully evaluated on a prior joint SERDP program with Naval Sea Systems Command (NAVSEA), General Electric Aircraft Engines (GEAE), and Air Force Research Laboratory (AFRL). This ESTCP program leveraged the prior work and included Naval Air Systems Command (NAVAIR) to produce engine quality hardware compatible with the F414 engine. Significant reductions in NOx, HC and CO emissions and high performance capabilities have been demonstrated with a TVC sector rig operating at high pressures and temperatures. This program extended the technology base established on the SERDP program to the development and demonstration of a full annular, low emission, high performance TVC for the F414 engine. Sector rig experiments were used to determine an optimized TVC low emissions design. The low emissions, high performance characteristics of the F414 TVC were demonstrated in a test cell at conditions corresponding to current and projected growth of the F414, as well as at F-35 and LM2500 engine conditions as appropriate.
During the course of this program two (2) Full Annular Rigs (FAR) were fabricated for potential insertion into an existing engine. Tests were conducted at full engine pressures, temperatures, and airflows. The FAR tests demonstrated a 42% reduction in NOx at full power conditions. The TVC at low power was comparable to the current baseline design with a 16% reduction in HC, but the design had a 6.8% increase in CO. The low power emissions performance did not meet the program goals and requires more development and test. Secondary performance characteristics that exceeded the baseline engine were an increase in the altitude re-light, a decrease in the fuel flow at ground start, and an improvement in the Lean Blow-Out (LBO). The concept showed no acoustic issues during the tests and the liner and dome surface temperatures were well below the metal temperature limits. These positive characteristics were combined with the potential of a shorter combustor length and lower airflow requirements to cool the temperatures on the combustor liners.
In summary, the TVC has maintained good performance capabilities while significantly decreasing the high power NOx generation. While additional work is needed to improve the low power emissions, the TVC design has demonstrated a significant potential for current and future military engine applications. The TVC has achieved a Technology Readiness Level (TRL) of 5 with the completion of the full scale 360-degree combustor rig tests.
The successful demonstration of the TVC low emissions technology has provided the Department of Defense (DoD) with a technical alternative for meeting CAA in future gas turbine engine based weapon systems. To be production ready and achieve a TRL 6 level, this technology still needs to complete an engine demonstration, which is currently not planned, to be fully qualified and validated. At this time, the TVC still requires further testing to improve the low power characteristics without impacting the existing durability and high power performance characteristics.