Combustion and Flame 148 (2007) 62–75 www.elsevier.com/l ocate/combustflame Dissipation length scales in turbulent nonpremixed jet flames Guanghua Wang a , Adonio s N. Karpe tis b , Robert S. Barlow a,∗ a Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA b Department of Aerospace Engineering, Te xas A&M University , College Station, TX 77843-3141, USA Received 24 March 2006; received in revised form 30 August 2006; accepted 12 September 2006 Available online 28 November 2006 Abstract Line imaging of Raman/Rayleigh/CO-LIF is used to investigate the energy and dissipation spectra of turbulent fluctu ations in tempe ratur e and mixture fract ion in sev eral flames, inclu ding CH 4 /H 2 /N 2 jet flames at Reynolds numbers of 15,200 and 22,800 (DLR-A and DLR-B) and piloted CH 4 /air jet flames at Reynolds numbers of 13,400, 22,400, and 33,600 (Sandia flames C, D, and E). The high signal-to-noise ratio of the 1D Rayleigh scattering images enables determination of the turbulent cutoff wavenumber from 1D dissipation spectra. The local length scale inferred from this cutoff is analogous to the Batchelor scale in nonreacting flows. The measured thermal dissipation spectra in the turbulent flames are shown to be similar to the model spectrum of Pope for turbulent kinetic energy dissipation. Furthermore, for flames with Lewis number near unity, the 1D dissipation spectra for temperature and mixture fraction are shown to follow nearly the same rolloff in the high-wavenumber range, such that the cutoff length scale for thermal dissipation is equal to or slightly smaller than the cutoff length scale for mixture fraction dissipation. Measurements from the piloted CH 4 /air flames are used to demonstrate that a surro- gate cutoff scale may be obtained from the dissipation spectrum of the inverse of the Rayleigh signal itself, even when the Rayleigh scattering cross section varies through the flame. This suggests that the cutoff length scale deter mined from Rayleigh scatteri ng measuremen ts may be used to define the local resolut ion requirements and optimal data processing procedures for accurate determination of the mean mixture fraction dissipation, based upon Raman scattering measurements or other multiscalar imaging techniques. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Turbulent flames; Scalar dissipation; Dissipation length scales; Raman scattering; Rayleigh scattering 1. Intr oducti on The accurate measurement of scalar gradients is important in a wide range of turbulent flow applica- tions, including those that involve mixing, combus- * Corresponding author. E-mail address: [email protected] (R.S. Barlow). tion, and heat transfer [1–3]. For example, the mixture fraction dissipation rate χ , defined as (1) χ = 2D ξ ∇ ξ · ∇ ξ , where ξ is the mixture fraction and D ξ is the mixture averaged mass diffusivity, is important in scalar mixing because it is a measure of the rate at which inhomogeneities in the scalar property are removed by diffusion. Due to the importance of scalar gradi- 0010-2180/$ – see front matter © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2006.09.005

CFD Dissipation length scales in turbulent nonpremixed jet flames.pdf

Download PDF Report

Upload
mojicap
View
217
Download
0

Embed Size (px)

Citation preview