Spray Diagnostics of a Low NOx Air Blast Atomizer for NASA ERA N+2 Program

Spray Diagnostics Measurements of an air blast atomizer at ambient pressure conditions are presented. The atomizer was developed by United Technologies Aerospace Systems as part of a combustor design for NASA's Environmentally Responsible Aviation (ERA) N+2 advanced, low NOx combustor technologies program. The goals of this combustor design is to meet or exceed the N+2 LTO NOx goal of 75% reduction from the ICAO standard adopted by CAEP 6 in an engine in the 70,000 lb. thrust class with an engine pressure ratio of at least 55:1. The combustor was developed to include three independent fuel circuits (pilot, intermediate, and outer) which are radially staged to improve performance and stability. The main objective of the presented research is to probe the performance of the outer air blast atomizer design utilized in this combustion system. The atomizer design was developed extensively by utilizing CFD with the intent of rapidly and thoroughly mixing fuel and air to minimize NOx predictions. It is the intent of the currently presented research to confirm that the actual atomizers created behave as predicted. This paper provides comparisons of the air flow field predicted by CFD to that measured by Particle Image Velocimetry (PIV) at ambient air-only test conditions for a 5% ∆P pressure drop through the atomizer. Additionally, the atomizer’s behavior with both air and fuel was investigated using Laser Induced Fluorescence (LIF). In these tests, a laser dye (Pyromethene 597) was added to MIL-PRF-7024 Type II test fluid and excited by a laser sheet to produce the LIF results. These air and liquid PIV/LIF results are presented and compared to air-only PIV results at the same pressure drop. Mass flux measurements calculated for the fuel by performing liquid PIV measurements and utilizing the LIF signal are also presented. Corresponding author: [email protected] Introduction As part of NASA’s “Environmentally Responsible Aircraft” (ERA) N+2 initiative, United Technologies Aerospace Systems (UTAS) is working to develop combustion technologies for a low-NOx combustor [1][2]. This program builds on UTAS’s history of Multipoint Lean Direct Injection (MLDI) concepts [3] which have been shown to have very low NOx emissions index (EI). Recent developments from UTAS [4] include radial staging of injectors, converging combustor geometries, and the use of air-blast fuel injectors in addition to the traditional air-assist fuel injectors which were solely used in previous MLDI concepts. It is the intent of these recent developments to advance the practicality of the combustion technologies by increasing the operating range of the combustor, reducing the number of nozzles, as well as improving low-NOx emissions to a goal of 75% reduction from the ICAO standard adopted by CAEP 6 at engine pressure ratios of at least 55. UTAS is working with NASA and the University of Cincinnati to demonstrate concepts with these capabilities [9-11]. In the effort to design the current MLDI combustor array and constituent fuel injectors, CFD was heavily used to predict the NOx emissions associated with a number of injector concepts. One limitation of this approach is that assumptions were made in terms of how the fuel was distributed by the fuel injectors. In an airblast atomizer, the distribution of fuel is heavily influenced by the momentum of the air. Due to the nature of the environment in which the fuel nozzles operate, it is extremely difficult to experimentally measure the droplet size and distribution of fuel. Therefore, it is the intent of the present work to experimentally measure the flow field and fuel distribution at atmospheric conditions to be used to correlate to CFD predictions. With this correlation established, it is the hope that future CFD predictions will be more able to predict the fuel and air distributions more accurately.