Dirk Roekaerts

Spectral radiative effects and turbulence/radiation interaction in a non-luminous turbulent jet diffusion flame

A non-luminous turbulent jet diffusion flame is numerically simulated using a Reynolds stress second-order closure, the steady laminar flamelet model, and different approaches for radiative transfer. The commonly used optically thin approximation is compared with the discrete ordinates method. Calculations using the Planck mean absorption coefficient are compared with computations performed using the spectral line-based weighted-sum-of-gray-gases model. The interaction between turbulence and radiation is simulated, and its influence on the predicted results is investigated. It is shown that the discrete ordinates method and the optically thin approximation yield relatively close results for the present flame if the medium is modelled as gray using the Planck mean absorption coefficient. In both cases, the predicted fraction of radiative heat loss is significantly overestimated. However, if the spectral nature of gaseous radiation is accounted for, the computed radiation loss is closer to the experimental data. The fluctuations of the species have a minor role in the interaction between turbulence and radiation, which is mainly due to the temperature fluctuations.

Application of a New Cubic Turbulence Model to Piloted and Bluff-Body Diffusion Flames

A new two-equation turbulence model is described. It combines an algebraic, non-linear expression of the Reynolds stresses in terms of strain rate and vorticity tensor components, with a modified transport equation for the dissipation rate. Thanks to the cubic law for the Reynolds stresses, the influence on turbulence from streamline curvature is accounted for, while the increase in computational costs is small. The classical transport equation for the dissipation rate is altered, in order to bring more physics into this equation. As a result, more realistic values for the turbulence quantities are obtained. A new low-Reynolds source term has been introduced and a model parameter is written in terms of dimensionless strain rate and vorticity. The resulting model is firstly applied to the inert turbulent flow over a backward-facing step, demonstrating the quality of the turbulence model. Next, application to an inertly mixing round jet reveals that the spreading rate of the mixture fraction is correctly predicted. Afterwards, a piloted-jet diffusion flame is considered. Finally, inert and reacting flows in a bluff-body burner are addressed. It is illustrated for both reacting test cases that the turbulence model is important with respect to the flame structure. It is more important than the chemistry model for the chosen test cases. Results are compared to what is obtained by linear turbulence models. For the reacting test cases, the conserved scalar approach with pre-assumed β-probability density function (PDF) is used.

Experimental and Numerical Investigation of a FLOX Combustor Firing Low Calorific Value Gases

A prototype flameless oxidation (FLOX) gas turbine combustor has been investigated experimentally and numerically. The combustor was operated with various Low Calorific Value gases. At the outlet main component and emission measurements (CO and NO) have been performed. The influence of several parameters (i.e., fuel composition, outlet temperature, and nozzle diameter) on the emissions have been investigated. Ultralow emissions (single digit) have been achieved. Moreover, axial temperature profiles in the combustion chamber have been measured with a suction pyrometer. The combustor has been simulated with a commercial CFD code (FLUENT 6.3) to gain insight in the combustion characteristics. Using the Eddy Dissipation Concept model for turbulence-chemistry interaction in combination with the Reynolds Stress model for turbulence and two different chemistry mechanisms, the measured temperature profiles have been reasonably well reproduced. In postprocessing mode different NO formation paths have been studied.

Numerical Investigation of a Bluff-Body Stabilised Nonpremixed Flame with Differential Reynolds-Stress Models

A numerical investigation of a bluff-body stabilised nonpremixedflame, and the corresponding nonreacting flow, has been performed withdifferential Reynolds-stress models (DRSMs). The equilibrium chemistry model is employed and an assumed-shape beta function PDFapproach is used to represent the interaction between turbulence andchemistry. The Reynolds flux of the mixture fraction is obtained from atransport equation, hence a full second moment closure is used. Toclarify the applicability of the existing DRSMs in this complex flame,several models, including LRR-IP model, JM model, SSG model as well as amodified LRR-IP model, have been applied and evaluated. The existingmodels, with default values of the coefficients, cannot provide overallsatisfactory predictions for this challenging test case. The standardLRR-IP model over predicts the centreline velocity decay rate, andtherefore does not perform satisfactory. The modified LRR-IP model, withmodel constant C∈1 = 1.6 instead of the standard value1.44 (here named BM-M1), gives better results for the mean velocity.However in the nonreacting case this does not lead to improvement inpredicting rms fluctuating velocities especially downstream of therecirculation zone. Motivated by the need to improve the prediction, anew modification of the LRR-IP model is proposed (BM-M2), with modelconstant C2 = 0.7in the pressure strain correlation rather thanthe standard value 0.6. With the new modified model, a verysignificant improvement of the prediction of flow field is obtained inthe nonreacting case, whereas in the reacting case the prediction ofthe flow field is of the same overall quality as with BM-M1. This showsthat some DRSMs have different behaviour in the nonreacting case andthe reacting case. In the reacting case also the mean and variance ofmixture fraction are considered and it is found that the best resultsare obtained with the BM-M1 model, with SSG as second best. Combiningthe results for flow field and mixture fraction field it is concludedthat the BM-M1 model is recommended for further studies of thisbluff-body stabilised flame. Grid independence of the result isdemonstrated.

On the accuracy of temperature measurements in turbulent jet diffusion flames by coherent anti-stokes-raman spectroscopy

Coherent anti-Stokes-Raman spectroscopy (CARS) has been applied to determine profiles of mean and standard deviation of temperature and of probability density functions for the Delft turbulent jet diffusion flames fed with Dutch natural gas. Several improvements to the CARS data reduction are introduced and special attention is paid to the effect of spatial averaging. It is shown that the error due to the limited resolution is at most 40 K for mean temperatures around 750 K. The CARS results are compared to the Raman-Rayleigh results obtained earlier for the same flames. Within experimental uncertainty, the results from the two independent nonintrusive laser diagnostic techniques are the same, except for the low central part of the flames. The difference can be a consequence of the occurrence of large methane concentrations and of the sensitive dependence on upstream boundary conditions.

Thermometry for turbulent flames by coherent anti-Stokes Raman spectroscopy with simultaneous referencing to the modeless excitation profile

An optimal system for temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) in turbulent flames and flows is presented. In addition to a single-mode pump laser and a modeless dye laser, an echelle spectrometer with a cross disperser is used. This system permits simultaneous measurement of the N2 CARS spectrum and the broadband dye laser profile. A procedure is developed to use software to transform this profile into the excitation profile by which the spectrum is referenced. Simultaneous shot-to-shot referencing is compared to sequential averaged referencing for data obtained in flat flames and in room air. At flame temperatures, the resultant 1.5% imprecision is limited by flame fluctuations, indicating that the system may have a single-shot imprecision below 1%. At room temperature, the 3.8% single-shot imprecision is of the same order as the best values reported for dual-broadband pure-rotational CARS. Using the unique shot-to-shot excitation profiles, simultaneous referencing eliminates systematic errors. At 2000 and 300 K, the 95% confidence intervals are estimated to be +/- 20 and +/- 10 K, respectively.

EMISSION AND EFFICIENCY COMPARISON OF DIFFERENT FIRING MODES IN A FURNACE WITH FOUR HiTAC BURNERS

Combustion in a furnace equipped with two HiTAC burner pairs, with a thermal power of 100 kWth each, has been investigated experimentally and computationally. The objective of this study is (1) to observe differences in the performance of the furnace operating in two different firing modes, parallel and staggered, and (2) to explain these differences using detailed CFD simulations. Besides the permanent measurements of temperature, flow and pressure, in-furnace probe measurements of temperature, oxygen and emissions (NO and CO) have been performed. Experimental results show that the efficiency of the furnace was higher in parallel mode compared to staggered mode, 48% and 41% respectively. The values of CO emitted were equal for both firing modes. However, in parallel mode the NOx production was 39 ppm v @3%O2, whereas in staggered mode 53 ppm v @3%O2 NOx was produced. Considering both efficiency and emissions, parallel firing mode performs better than staggered mode. Next, CFD simulations of the furnace were performed in order to explain the observed differences. The simulations were validated with the in-furnace measurements. It was confirmed that the furnace firing in parallel mode achieved a higher efficiency. The radiative heat transfer was higher due to formation of a larger zone with gases with improved radiative properties. In addition, higher velocities along the cooling tubes, due to lower momentum destruction, led to higher convective heat transfer. Also, the lower production of NOx in parallel mode was reproduced by the simulations. This is due to the fact that in parallel mode the fuel jets are merging slower with the combustion air jet, leading to less intense combustion zones. Thus, lower peak temperatures and radical concentrations are achieved, and the NOx production via the thermal and N2O pathways was lower.

Interaction between chemistry and micro-mixing modeling in transported PDF simulations of turbulent non-premixed flames

A comparative study is presented of the impact of chemistry modeling on the behavior of 2 micro-mixing models in transported scalar PDF simulations of turbulent non-premixed flames. The micro-mixing models are CD (modified Curls Coalescence/Dispersion) and EMST (Euclidean Minimum Spanning Tree). A first order non-linear k-ε turbulence model is applied for the turbulent flow and mixing fields. Three C1 chemistry models are considered: 2 skeletal schemes containing 16 species and 31, resp. 41 elementary reactions, and one augmented reaction scheme (ARM), consisting of 9 independent species and 5 global reaction steps. The test cases considered are the piloted turbulent Delft Flame III jet diffusion flame and the bluff-body stabilized turbulent non-premixed Sydney HM1 flame. With EMST the chemistry model choice has a negligible effect on the micro-mixing model behavior or on the results in physical space. With CD on the other hand, larger differences appear.

MODEL OF CHEMICAL REACTION KINETICS FOR CALCULATING DETONATION PROCESSES IN GAS AND HETEROGENEOUS MIXTURES CONTAINING HYDROGEN PEROXIDE

ABSTRACT An approximate two-stage kinetic model of the chemical reaction in hydrogen-oxygen mixtures containing hydrogen peroxide, water, and inert diluents is developed. The model includes one differential equation for the calculation of the molar mass of the gas after the induction period and algebraic formulas for the calculation of the heat release, internal energy and thermodynamic parameters of the mixture. Based on this model, 2D numerical simulations of a multifront gas detonation wave are performed. These pioneering simulations correspond to the stoichiometric hydrogen-oxygen mixture with hydrogen peroxide and argon additions. An approximate model of chemical equilibrium in heterogeneous gas-condensed phase systems containing hydrogen peroxide is presented.

Effects of micro-mixing in gas-phase turbulent jets

Abstract Scalar mixing in turbulent jets is modeled using a hybrid Monte Carlo PDF method which solves for the joint velocity-scalar PDF. Molecular scalar mixing is modeled by: Interaction by Exchange with the Mean, Coalescence/Dispersion (C/D), Binomial Langevin and Mapping Closure models. The concentration field statistics adopt a self-similar solution along rays emerging from the jets virtual origin. Predicted PDFs of the normalized concentration are compared to measurements. All mixing models yield good agreement with the measurements at all radial and axial positions. The C/D model gives the best results close to the centerline but at the edge of the jet, the model predicts unrealistic PDF shapes.