Effects of Chamber Diameter on the Flowfield in Unielement Rocket Combustors

U NI-ELEMENT combustors embodying a single fuel oxidizer, injector pair constitute the smallest scale at which combustors can be tested in a rocket engine development program. These unielement systems provide fundamental information on combustor operation that cannot be obtained on larger sized rigs. The small scale provides an economical and safe test bed that allows more detailed instrumentation and much faster turn-around time than that for a practical larger scale combustor. The relatively simple geometry and construction is conducive to the application of advanced diagnostics while also lending itself to detailed computational fluid dynamics (CFD) analyses, both of which can aid in understanding and improving this most fundamental engine building block. Unielement testing is also useful for screening candidate injector types for a new engine, for diagnosing the performance of existing engines, or for fundamental academic studies into high-intensity combustion phenomena. A primary goal of unielement testing in an engine development program is to replicate as closely as possible the environment that a stream tube from an individual element will experience in the fullscale engine. Accordingly, unielement tests must be done with injector elements that are exact scaled copies of those to be used in the full-scale engine, both in terms of kinetic/kinematic and geometrical details. Additionally, the flow rates, oxidizer-to-fuel (O/F) ratios, and incoming propellant temperatures and pressures must match engine conditions, whereas the nozzle must be sized to ensure the proper chamber pressure. Finally, the length of the chamber should be matched to the distance from the injector face to the nozzle throat in the engine to ensure similar characteristic flow times. Clearly, interelement interactions and the intricate recirculation regions adjacent to the face of the full-scale engine cannot be replicated in unielement studies, but unielement combustors are widely used and accepted as an effective means for understanding rocket combustors and as an important precursor to subscale engine studies. A point of continuing controversy in unielement testing, however, concerns the cross-sectional size and shape of the chamber. A key argument has been that the unielement chamber should be sized to provide the cross-sectional area occupied by the stream tube from a single injector element in the full-scale engine, thereby reproducing the proper mean flowMach number. The impact of the cross section on instrumentation and optical diagnostics as well as on CFD modeling is, however, also an important concern that affects shape and size. Experimental configurations with optical access (which enables much more detailed quantitative measurements) drive the chamber cross section to larger sizes. Square chambers, such as those used in early experiments by Moser et al. [1], Foust et al. [2], and De Groot et al. [3], are most convenient for optical diagnostics but potentially introduce geometry-specific corner flows that are not present in engines and are difficult to represent in CFD analyses. Circular chambers eliminate concerns of corner flows and are also friendlier toward CFD modeling, but even with round chambers larger diameters facilitate optical access and advanced diagnostics. Increased chamber diameters, however, give rise to stronger recirculation regions adjacent to the injector face that eventually dominate flame attachment and the ensuing combustion processes. Despite these uncertainties, definitive experiments or computations concerning the effect of chamber Mach number and combustor diameter have never been attempted. The present paper represents a first attempt to address some of these issues. In the past decade, unielement combustors have been studied intensively, both experimentally [1–13] and computationally [14– 22]. The numerous experimental activities have provided detailed insights into, and visualization of, the flowmixing and combustion in unielement combustors. Important relationships between the physical phenomena and operating conditions, such as the momentum ratio and properties of the propellants, the density and velocity ratios of the jets, the temperature and pressure of the combustion chamber, and the role of the detailed local geometry, have been identified. In addition to physical understanding, there has been an emphasis on obtaining detailed experimental data for use in validating CFD models. Presented as Paper 2009-3897 at the 39th AIAA Fluid Dynamics Conference, San Antonio, TX, 22–25 June 2009; received 26 May 2011; revision received 12 December 2011; accepted for publication 14 December 2011. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0748-4658/12 and $10.00 in correspondence with the CCC. Postdoctoral Research Fellow; currently Staff Scientist, IBM Corporation, Inc., Hudson Valley Research Park, Hopewell Junction, NY 15233; chenzhoulian@gmail.com (Corresponding Author). Reilly Professor of Engineering. Senior Research Associate, School of Mechanical Engineering. JOURNAL OF PROPULSION AND POWER Vol. 28, No. 3, May–June 2012