Scalar Dissipation Rate Field Research Articles

Relationship between soot and scalar dissipation rate in the soot-inception region of turbulent non-premixed jet flames

Abstract In previous work we employed the Krypton Planar Laser-Induced Fluorescence technique combined with simultaneous Laser Induced Incandescence to obtain measurements of mixture fraction, temperature and soot volume fraction in the soot inception region of turbulent non-premixed ethylene-air flames (Park et al., 2017). This paper extends that work and reports the first simultaneous measurements in turbulent flames of the scalar dissipation rate and soot-volume fraction. The specific jet composition was 50% C2H4, 46% N2 and 4% Kr and the jet Reynolds number was 8300. The measurements were made 10 cm downstream of the jet exit where the soot first begins to form. The present study uses the same data set to derive the scalar dissipation rate fields and relate the dissipation to mixture fraction, soot volume fraction and temperature. The peak mean soot volume fraction was found to be well inside of the stoichiometric surface near values of mixture fraction from 0.3 to 0.4, which roughly corresponded to the regions of peak mean scalar dissipation. The polycyclic aromatic hydrocarbon field was also imaged, but not simultaneously with the other quantities. Although, the mean soot location is correlated to the mean dissipation, this result is misleading as shown by joint probability density function (PDF) between soot volume fraction and scalar dissipation. The joint PDF shows that soot is in fact no typically spatially coincident with regions of high scalar dissipation; in fact, higher dissipation is correlated with lower values of soot. These latter observations are consistent with results from recent numerical simulations that soot was preferentially found in the areas of low scalar dissipation rate and soot is locally inhibited by high scalar dissipation rates.

Measurements and statistics of mixture fraction and scalar dissipation rates in turbulent non-premixed jet flames

The present study adds to the limited database of mixture fraction and scalar dissipation rate measurements within turbulent flames. One noteworthy aspect is that the current measurements are two-dimensional images, which provide the physical structure of the scalar dissipation rate field. Two turbulent carbon monoxide flames are studied at two different jet-exit velocities, where the latter flame is near blowoff. The effects of increasing levels of local extinction (by increasing the jet velocities) on the structure and statistics of the mixture fraction fluctuation and scalar dissipation rate fields are investigated in detail. In general, the topology of the scalar dissipation rate layers is similar to that found previously in turbulent non-reacting flows, with some notable exceptions. For the lower jet-velocity case, the dissipation layers were somewhat smooth, isolated, and preferentially aligned in the axial direction, while for the higher jet-velocity case (near global extinction), the scalar dissipation rate field was characterized by an increased number of shorter, thinner, and highly wrinkled dissipation layers. It appears that as the flame nears blowoff, the scalar dissipation rate field exhibits an increasing level of isotropy, resembling turbulent non-reacting flows. In addition to providing new insights on the physical nature of the dissipation fields, the two-dimensional statistics of the mixture fraction and scalar dissipation rate are examined in detail, in the form of conditional averaging and through the construction of probability density functions. It is found, that although the two flames have apparent differences in physical structure, the corresponding statistical properties of the scalar dissipation rate were similar. Probability density functions (pdfs) of the scalar dissipation rate exhibited large departures from strict log-normality and the pdfs of the scalar dissipation rate were found to be significantly different if only the values of the scalar dissipation conditioned on the stoichiometric contour were considered compared to the entire flowfield. This has implications for modeling assumptions that seek to describe the entire turbulent flowfield by a single pdf. The nature of the scalar dissipation rate in the current lower-Reynolds number turbulent flames, as revealed through the statistics, is compared to previous, well-known data sets obtained in the Sandia series of piloted jet flames as well as other piloted non-premixed jet flames at higher Reynolds numbers. The present flame conditions are significantly different from previous work, yet the general structure, statistical trends, and general effects of increasing levels of local extinction agree favorably with the previous experimental data.

A method to simultaneously image two-dimensional mixture fraction, scalar dissipation rate, temperature and fuel consumption rate fields in a turbulent non-premixed jet flame

A new imaging technique was developed that provides two-dimensional images of the mixture fraction (ξ), scalar dissipation rate (χ), temperature (T), and fuel consumption rate \((\dot{\omega}_{\rm F})\) in a turbulent non-premixed jet flame. The new method is based on “seeding” nitric oxide (NO) into a particular carbon monoxide–air flame in which it remains passive. It is first demonstrated that the mass fraction of NO is a conserved scalar in the present carbon monoxide–air flame configuration, using both laminar flame calibration experiments and computations with full chemistry. Simultaneous planar laser-induced fluorescence (PLIF) and planar Rayleigh scattering temperature imaging allow a quantitative determination of the local NO mass fraction and hence mixture fraction in the turbulent jet flame. The instantaneous mixture fraction fields in conjunction with the local temperature fields are then used to determine quantitative scalar dissipation rate fields. Advantages of the present technique include an improved signal-to-noise ratio over previous Raman scattering techniques, improved accuracy near the stoichiometric contour because simplifying chemistry assumptions are not required, and the ability to measure ξ and χ in flames experiencing localized extinction. However, the method of measuring ξ based on the passive NO is restricted to dry carbon monoxide–air flames due to the well-controlled flame chemistry. Sample imaging results for ξ, χ, T, and \(\dot{\omega}_{\rm F}\) are presented that show high levels of signal-to-noise while resolving the smallest mixing scales of the turbulent flowfield. The application, accuracy, and limitations of the present technique are discussed.

Direct numerical simulation of autoignition in non-homogeneous hydrogen-air mixtures

The autoignition of spatially non-homogeneous hydrogen-air mixtures in 2-D random turbulence and mixture fraction fields is studied using the Direct Numerical Simulation (DNS) approach coupled with detailed kinetics. The coupling between chemistry and the unsteady scalar dissipation rate field is investigated over a wide range of different autoignition scenarios. The simulations show that autoignition is initiated at discrete spatially localized sites, referred to as kernels, by radical build-up in high-temperature, fuel-lean mixtures, and at relatively low dissipation rates. Detailed analysis of the dominant chemistry and the relative roles of reaction and diffusion is implemented by tracking the evolution of four representative kernels that characterize the range of ignition behaviors observed in the simulation. This evolution yields different autoignition delay scenarios as well as extinction at the different sites based on the local dissipation rates and their temporal histories. Where significant autoignition delay and extinction are observed, a shift in the relative roles of dominant reactions that contribute to radical production and consumption during this induction phase is observed. This shift is particularly characterized by an increased role of termination reactions during the intermediate stages of the induction period, which results in extinction in approximately two thirds of the ignition kernels in the computational domain. The fate of the different kernels is associated with: (1) the dissipation of heat that contributes to a slowdown in chemical reactions and a shift in the balance between chain-branching and chain-termination reactions; (2) the dissipation of mass that keeps the radical pool growth in check, and that is promoted by slower reaction rates; and (3) counter to the effects of dissipation of heat and intermediate species, the preferential diffusion of H 2 relative to both heat and its diluent, N 2, that promotes ignition. Ultimately, the balance between radical production and dissipation determines the success or failure of a given kernel to ignite. A new criterion for unsteady ignition is presented based on the instantaneous balance between radical production and dissipation. A Damköhler number, so defined, must remain above a critical value of unity at all times during the induction period if the kernel is to eventually ignite. Inherent in a multi-step kinetic description of ignition phenomena is the disparate time scales associated with different elementary reactions that, coupled with the characteristic scales of heat and mass dissipation, may yield different dominant chemistries at different stages of the induction process for a given kernel. To capture the strong history effects associated with radical build-up, new ignition progress variables based on key radical species are investigated.

High-temperature combustion in autoigniting non-homogeneous hydrogen/air mixtures

The burning modes (premixed vs. diffusion burning) in autoigniting non-homogeneous mixtures of hydrogen in heated air are studied using direct numerical simulations (DNS). The simulations show that high-temperature combustion follows an initial autoignition stage in fuel-lean, low-dissipation kernels. These kernels propagate initially as lean premixed fronts. As they expand into richer mixtures, diffusion flames develop in the wake of rich premixed flames along stoichiometric isocontours. These flames are initially stabilized by diffusion of radicals (H) and excess fuel from the rich premixed flames' side against excess radicals (O and OH) and oxidizer from the earlier passage of lean premixed fronts. In time, diffusion flames detach from the rich premixed flames, and their burning intensity is reduced accordingly. Triple flames also form at the interfaces of the rich and lean premixed flames with the stoichiometric mixture isocontours. However, their contribution to the stabilization and burning intensity of the diffusion branches is insignificant. Analysis of the contribution of lean and rich premixed flames and that of the diffusion flames to the volumetric heat release show that the dominant contribution is attributed mainly to the premixed flames: while the dominant contribution to NO formation is attributed to diffusion flames. The results also show that the relative contribution of the different burning modes is strongly dependent on the mixture distribution and the scalar dissipation rate field. We believe that these parameters affect the diffusion flames' structures and their rates of detachment from the rich premixed flames.

Experimental assessment of fractal scale similarity in turbulent flows. Part 4. Effects of Reynolds and Schmidt numbers

Experimental results are presented for the influence of Reynolds number on multifractal scale similarity in turbulent flows. These are obtained from single-point measurements of a dynamically passive Sc≈1 conserved scalar quantity ζ(t) in a turbulent shear flow at outer-scale Reynolds numbers of 14000[les ]Reδ[les ]110000. Statistical criteria based on the maximum allowable scale-to-scale variation L1(ε) in multiplier distributions P(Mε) from multifractal gauge sets allow accurate discrimination between multifractal and non-multifractal scaling. Results show that the surrogate scalar energy dissipation rate χs(t)≡(dζ/dt)2is found to display a scale-invariant similarity consistent with a random multiplicative cascade characterized by a bilinear multiplier distribution P(Mε) over a range of scales extending downward from the outer scaleTδ. For a range of scales extending upward from the inner (diffusive) scale TD, the dissipation rate displays a different scale-invariant similarity characterized by a uniform multiplier distribution. The former scale-invariance becomes evident in the present Sc≈1 data only when Reδ is sufficiently large. Comparisons with results from Sc 1 data indicate that this scale-invariant similarity applies when the outer-to-inner scale ratio Tδ/TD≈0.09 Re3/4δSc1/2 is greater than about 400. In contrast to the scalar dissipation rate field, the scalar field is found to lack any multifractal scale similarity.

Experimental study of the fine-scale structure of conserved scalar mixing in turbulent shear flows. Part 1. Sc [Gt ] 1

We present results from an experimental investigation into the fine-scale structure associated with the mixing of a dynamically passive conserved scalar quantity on the inner scales of turbulent shear flows. The present study was based on highly resolved two- and three-dimensional spatio-temporal imaging measurements. For the conditions studied, the Schmidt number (Sc ≡ v/D) was approximately 2000 and the local outerscale Reynolds number (Reσ≡ uσ/v) ranged from 2000 to 10000. The resolution and signal quality allow direct differentiation of the measured scalar field ζ(x, t) to give the instantaneous scalar energy dissipation rate field (Re Sc)−1 ∇ζċ∇ζ(x, t). The results show that the fine-scale structure of the scalar dissipation field, when viewed on the inner-flow scales for Sc ≡ 1, consists entirely of thin strained laminar sheet-like diffusion layers. The internal structure of these scalar dissipation sheets agrees with the one-dimensional self-similar solution for the local strain–diffusion competition in the presence of a spatially uniform but time-varying strain rate field. This similarity solution also shows that line-like structures in the scalar dissipation field decay exponentially in time, while in the vorticity field both line-like and sheet-like structures can be sustained. This sheet-like structure produces a high level of intermittency in the scalar dissipation field – at these conditions approximately 4% of the flow volume accounts for nearly 25% of the total mixing achieved. The scalar gradient vector field ∇ζ(x, t) for large Sc is found to be nearly isotropic, with a weak tendency for the dissipation sheets to align with the principal axes of the mean flow strain rate tensor. Joint probability densities of the conserved scalar and scalar dissipation rate have a shape consistent with this canonical layer-like fine-scale structure. Statistics of the conserved scalar and scalar dissipation rate fields are found to demonstrate similarity on inner-scale variables even at the relatively low Reynolds numbers investigated.