Mixing of multiple jets with a confined subsonic crossflow

Mixing of gaseous jets in a cross ow has signi cant applications in combustion science and engineering, one example of which is the mixing zone of a rich-burn/quick-mix/lean-burn gas turbine combustor. A major design question is the jet ori ce shape and jet ori ce number that optimizes the mixing performance. To delineate the optimal ori ce features for a given axial distance and momentum-ux ratio, a statistical design of experiments test matrix was established around three variables: the number of ori ces, the ori ce aspect ratio, and the ori ce angle (in circumferential plane). A jet-to-mainstream momentum-ux ratio of 40 and a mass-ow ratio of 2.5 were selected as representative of a practical design. To yield an interpolating equation that predicts the mixing performance of ori ce geometry combinationswithin the range of the test matrix parameters, a regression analysis was conducted on the data. The results reveal that 1) mixture uniformity is a nonlinear function of the number of ori ces, the ori ce aspect ratio, and the ori ce angle and that 2) optimum mixing occurs when the mean jet trajectories are in the range of 0.30 < /R < 0.5 (where = R ¡ r) at x/R = 1. At the optimum number of ori ces, the difference between shallow-angled slots with large aspect ratios and round holes is minimal and either geometry produces optimal mixing performance. Introduction MIXING of air jets into a cross ow is a fundamentalpart of gas turbine engine technology. Combustor ef ciency, exit plane

Section: Introductionsupporting

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Optimization of Orifice Geometry for Crossflow Mixing in a Cylindrical Duct

Kroll

Sowa

Samuelsen

et al. 2000

“…For multiple jet injectioninto a nonreactingcross ow, Holdeman 3 notes that optimal jet penetration and mixing is determined by J and the ori ce spacing to duct height ratio S/ H . The following relationship,…”

Section: Optimum Number Of Round Holesmentioning

confidence: 99%

“…An extensivelisting of these documented jet-in-cross ow studies performed in the past few decades can be found in Refs. [2][3][4][5]. Note that many of the studies cited in these summaries are of a single jet in an unboundedcross ow or are otherwise inappropriate for direct application.…”

Section: Introductionmentioning

confidence: 99%

Optimization of Jet Mixing into a Rich, Reacting Crossflow

Leong

Samuelsen

Holdeman

2000

Radial jet mixing of pure air into a fuel-rich, reacting cross ow con ned to a cylindrical geometry is addressed with a focus on establishing an optimal number of jet ori ces. Note that the optimum would not be expected to be universal. That is, the optimum mixer that one would identify will depend on both the momentum-ux ratio and an axial distance appropriate to the application.The number of round holes that most ef ciently mixes the jets with the mainstream ow, and thereby minimizes the residence time of near-stoichiometric and unreacted packets, was determined. Such a condition might reduce pollutant formation in axially staged, gas turbine combustor systems. Five different con gurations consisting of 8, 10, 12, 14, and 18 round holes are reported. An optimum number of jet ori ces is found for a jet to mainstream momentum-ux ratio J of 57 and a mass-ow ratio (MR) of 2.5. For this condition, the 14-ori ce case produces the lowest spatial unmixedness and the most uniformly distributed species concentration and temperature pro les at a plane located an axial distance equal to one duct diameter from the jet ori ce inlet. Note that this is the same con guration that would be identi ed from non-reacting experiments. Nomenclature d= ori ce axial length, or round hole diameter J = jet to mainstream momentum-ux ratio, (qV 2 ) jets / (qV 2 ) main MR = jet to mainstream mass-ow ratio, (qVA) jets / (qVA) main n = number of round holes in quick-mix module R = radius of the quick-mix module r = radial distance from the module center T = ow eld temperature T jet = average temperature of the jets T main = average mainstream temperature at a plane one duct radius upstream of the jets U s = spatial unmixedness V = velocity V ref = reference velocity de ned at nonreacting inlet conditions x = axial distance from the leading edge of the ori ces q = density u = equivalence ratio, (fuel/air) local /(fuel/air) stoichiometric Introduction VARIOUS systems, such as fuel injection and exhaust temperature control processes, rely on a rapid and thorough jet mixing with a cross ow of gas to mix streams of uid. Jet mixing in a cross ow, in fact, may play a fundamentalrole in the next generation of low-pollutant-emittingengines such as the rich-burn/quick-mix/ lean-burn (RQL) combustion concept. The success of this combustor over conventional gas turbine combustors in lowering pollutant production depends on the mixing section between the fuel-rich (u > 1) and fuel-lean (u < 1) stages of combustion. In this combustor design, the jets of air introduced through the quick-mixing section mix with the rich reacting cross ow as quickly as possible to bring the reaction to an overall lean equivalenceratio. The formation of various pollutants is driven by high temperaturesattained in near- stoichiometric reactions. Therefore, the strategy in this combustor design lies in minimizing the lifetime of, as well as preventing the formation of, near-stoichiometric uid packets. As passing through stoichiometric regions is inevitable, rapid mixing reduces ...

“…Shown in Figure 12 are the results of the combustoronly calculations for three specified uniform inlet velocities: 1) jet velocity corresponding to the overall pressure drop velocity, 155 rn/sec; 2) jet velocity corresponding to the average pressure drop velocity, 135 m/sec; and 3) jet velocity corresponding to the massflow through the orifice geometric area, 92 rn/sec. Compared to the baseline calculation (Figure 3), each combustor-only case predicted jet overpenetration.…”

Section: Combustor-only Calculationsmentioning

confidence: 99%

Flow Coupling Effects in Jet-in-Crossflow Flowfields

Bain

Smith

Liscinsky³

et al. 1999

The combustor designer is typically required to design liner orifices that effectively mix air jets with crossflow effluent. CFD combustor analysis is typically used in the design process; however the jets are usually assumed to enter the combustor with a uniform velocity and turbulence profile. The jet-mainstream flow coupling is usually neglected because of the computational expense. This CFD study was performed to understand the effect of jet-mainstream flow coupling, and to assess the accuracy of jet boundary conditions that are commonly used in combustor internal calculations.