/,,,u,--7o NASATechnical Memorandum_!044ii ::: /_,,2 _ - AIAA-911=2_460 .... _..................... _.................... _...... ............. .. ,. ...... . , ±.... - :.... 7 .... :2..... ± : =_: -,:_-_:-:2-7:Y _-::-= i ,_--=_-.... _ ° LT: 2_7- L _ ,2_-=i-=_-: i , A CFD__Study of Jet Mixing in Reduced Flow ___e-asfor Lower Combustor Emissions =- C2E. Smith and M.V. Tal_Ilikar CFD Re_search Corporation Huntsville, Alabama - --k: ± -_ L - _L_ :2 - L :L !L and J.D_Holdeman ........................... Lewis Research Center ............. Cleveland, Ohio ' _ _ _ -_ :: _ : .... :_::_::_-i _:: _ _ -_,_ Prepared f6r-ih_- ..... :/-::- ::-7-7 i-:_ ----i-: _7_J , : . " " " 27th Joint Propul_0n Corffe_rence_ ................. cosponsored by the AIAA, SAE, ASME,-and ASEE "" .......... Sacramento, Ca!ifomg j_e.24-27, i99:!-_=]:_:_ ,=. :_ =_2_--___,,=__:=_. : _-- It - z -: = := _ __ .= N91-23185 (NASA-TM-I0441I) A CFD aTUDY OF JET MIXING IN R_DUCED FLnW APEAS FOR LOWER COMBUSTnR . <.g EMISSIONS (NASA) 17 p CSCE 21E Unclas =: : 63/0? 0014928 A CFD STUDY OF JET MIXING IN REDUCED FLOW AREAS FOR LOWER COMBUSTOR EMISSIONS C. E. Smith* M. V. Talpallikar** CFD Research Corporation Huntsville, AL 35805 J. D. Holdeman*** NASA Lewis Research Center Cleveland, OH 44135 Abstract 1. Introduction The Rich-burn/Quick-mlx/Lean-burn (RQL) The design of low NOx combustors isa subject combustor has the potential of significantly of ongoing research at NASA Lewis Research reducing NOx emissions in combustion chambers Center as applied to High Speed Civil Transport of High Speed Civil Transport (HSCT) aircraft. (HSCT) aircraft. One combustor design presently Previous work on RQL combustors for industrial under studyis the Rich-burn/Quick-mix/Lean-burn applications suggested the benefit of "necking (RQL) combustor. Originally conceived and down" the mixing section. In this study, a 3D developed for industrial combustors I-2, the RQL numerical investigation was performed to study the concept utilizes staged burning, as shown in effects of neckdown on NOx emissions and to Figure 1. Combustion isinitiatedinafuel richzone develop a correlation for optimum mixing designs atequivalence ratiosbetween 1.2 and 1.8, thereby interms of neckdown area ratio. The results of the reducing NOx formation by depleting the available study showed that jet mixing inreduced flow areas oxygen. Bypass air is introduced in a quick-mix does not enhance mixing, but does decrease section and lean combustion occurs downstream residence time at high flame temperatures, thus at equivalence ratios between 0.5 and 0.7. A key reducing NOx formation. By necking down the design technology required for the RQL mixing flow area by four, apotential NOx reduction combustor isamethod of rapidly mixingbypass air of sixteen-to-one is possible for annular withrich-bum gases. Rapid and uniform mixingis combustors. However, there is a penalty that required for producing low amounts of NOx while accompanies the mixing neckdown: reduced oxidizingCO produced inthe rich-burnsection. pressure drop across the combustor swirler. At conventional combustor loading parameters, the Generic research on dilution jet mixing ingas pressure drop penalty does not appear to be turbine combustors has been performed in the excessive. past3,and is applicable to RQL combustors. Good * Manager, ApplicationProjects Member AIAA ** Project Engineer Member AIAA *** Senior Research Engineer Member AIAA engineeaingcorrelationswere developedfor inches) was performed to understand the cause of optimummixingofdilutionjetsincan,rectangular NOx reductioninreduced flow area. And finally, a andannulargeometries4.Insearchofimproved 1-D computer code was used to calculate the mixingschemesr,ecentworkhasbeenperformed overall pressure lossofacombustor and toassess on staggered dilution jets in rectangular the penalty ofmixinginaneckdown section. Each geometriess,asymmetrijcetsincangeometrie6s, part ofthe studywill be discussed inthe following andslotsincangeometrie7s. sections. An important aspect of jet mixing that warranted further investigation was the effect of "necking down" the mixing flow area. The mixing Geometry section has been typically necked down in RQL combustors to promote better mixing and prevent A "no neckdown" case was selected as the backflow8-11. In Reference 2 itwas experimentally baseline. The baseline configuration (see Figure shown that neckdown of the mixing section 2) consisted of three components: inlet pipe, produced lower NOx emissions. The experiments converging section, and mixing section. The inlet did not provide the data base to identify why pipe was 6.0 inches (0.152 m) indiameter and 3.0 neckdown produced lower NOx emissions or how inches (0.0762 m) in length. The convergence to optimize NOx reduction. Hence, this study was section connected the inlet pipe to the mixing undertaken to investigate the effects of area section and was 0.866 inches (0.022 m) in length. reduction on NOx formation inthe mixing section, The mixing section had a diameter of six inches and to develop design correlations to optimize (Le. no neckdown_ a_ad twelve equally-spaced mixing inreduced areas. slots located on its perimeter. The slots' centerlines were located one mixing section diameter downstream of the exit plane of the converging section. The aspect ratio of each slot Parametric numerical calculations were was 4-to-1, with the largest dimension of 1.31 performed to quantify potential improvement from inches (0.033 m) positioned in the direction of the neckdown and to understand the physical mainstream. The mixing section extended two mechanisms causing low NOx. Both 3-D CFD mixing section diameters downstream of the jet numerical analysis and 1-D analysis were centerline. employed. The 3-D numerical calculations were made using the CFD code named REFLEQS. Grid REFLEQS has been developed to analyze turbulent reacting flows12, and has undergone a The baseline grid had 20,160 cells (56x20x1 8 considerable amount of systematic quantitative cells in x,r, 0directions). Figure 3 shows two views validation for both incompressible and of the baseline grid. The grid distribution is compressibleflows. Over 30validationcases have non-uniform with greater grid density inthe vicinity been performed to date, and good to excellent of the slot as well as the combustor wall. The agreement between data and predictions has domain in the 0-direction extends from the jet been shown13-14. Further, ithas been shownthat centerline to between the jets. Only a pie section REFLEQS is a viable tool in modelling complex with a central angle of 15" was analyzed to geometries and intricate flow patterns involved in conserve grid points. The grid distribution in each mixingconcepts of lowemissioncombustors5-7. direction is described below. The studywas divided into four parts. First, a baseline mixing configuration was analyzed and assessed for grid independence. Second, the X0 < x < X1 inlet pipe 4 cells uniform baseline configuration was optimized interms of numberofslots. Third,aparametricvariationofthe Xl

A CFD study of jet mixing in reduced flow areas for lower combustor emissions

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