Ingmar Schoegl

Numerical Investigation of Ultra-Rich Combustion in Counter Flow Heat Exchangers

Recirculation of heat has been used for decades to react mixtures beyond the conventional flammability limits. One means of obtaining this recirculation is through counter-flow heat exchange. In contrast to filtration combustion in which a reaction front propagates through a packed bed, counter-flow heat exchange results in stationary reactions zones. The objective of this study is the numerical investigation of reaction zone characteristics of ultra-rich methane/air combustion with counter-flow heat exchange. The flow channels of a counter-flow heat exchanger are modeled in two dimensions, and detailed reaction chemistry is described by the kinetics mechanism GRI 2.11. The results show that combustion starts in the vicinity of the channel walls, resulting in tulip-shaped reaction zones. The impact of operating conditions on combustion characteristics is studied in terms of inlet velocity and equivalence ratio, and findings are related to recently published experimental data.

Tomographic Imaging of Flames: Assessment of Reconstruction Error Based on Simulated Results

Temperature and concentration distributions of a horizontal section through a simulated stationary test flame were reconstructed with the help of computer tomography. The study uses temperature and concentration distributions equal to published time-averaged data for the Sandia D flame. Projection values are obtained by simulated line-of-sight integration for two water vapor (H2O) absorption lines in the 6930–6940  cm−1 range, where direct absorption spectroscopy and wavelength modulation spectroscopy with second harmonic frequency detection are considered. The projected values obtained from line-of-sight measurements are deconvoluted by tomographic methods, where Abel inversion and filtered backprojection are investigated. The tomographic reconstruction yields spatially resolved spectroscopic data, which is then used to calculate distributions for temperature and concentrations. Results illustrate that tunable diode laser absorption spectroscopy artifacts are caused by relatively small errors in reconstr...

Numerical analysis of flame instabilities in narrow channels: Laminar premixed methane/air combustion

Premixed flames propagating within small channels show complex combustion phenomena that differ from flame propagation at conventional scales. Available experimental and numerical studies have documented stationary, non-stationary, or asymmetric modes that depend on properties of the incoming reactant flow as well as channel geometry and wall temperatures. This work seeks to illuminate mechanisms leading to symmetry breaking and limit cycle behavior that are fundamental to these combustion modes. Specifically, four cases of lean premixed methane/air combustion—two equivalence ratios (0.53 and 0.7) and two channel widths (2 mm and 5 mm)—are investigated in a 2D configuration with constant channel length and bulk inlet velocity, where numerical simulations are performed using detailed chemistry. External wall heating is simulated by imposing a linear temperature gradient as a boundary condition on both walls. In the 2 mm channel, both equivalence ratios produce flames that stabilize with symmetric flame fronts after propagating upstream. In the 5 mm channel, flame fronts start symmetrically, although symmetry is broken almost immediately after ignition. Further, 5 mm channels produce non-stationary combustion modes with dramatically different limit cycles: in the leaner case (φ = 0.53), the asymmetric flame front flops periodically, whereas in the richer case (φ = 0.7), flames with repetitive extinctions and ignitions (FREI) are observed. To further understand the flame dynamics, reaction fronts and flame fronts are captured and differentiated. Results show that the loss of flame front symmetry originates in a region close to the flame cusp, where flow and chemical characteristics exhibit large gradients and curvatures. Limit cycle behavior is illuminated by investigating flame edges that are formed along the wall, and accompany local or global ignition and extinction processes. In the flopping mode (φ = 0.53), local ignition and extinction in regions adjacent to the wall result in oblique fronts that advance and recede along the wall and redirect the flow ahead of the flame. In the FREI mode, asymmetric flames propagate much farther upstream, where they experience global extinction due to heat losses, and re-ignite far downstream with opposite flame front orientation. In both cases, an interaction of flow and chemical effects drives the asymmetric limit cycles. The lack of instabilities and asymmetries for the 2mm cases is attributed to insufficient wall separation, which is of the same order of magnitude as the flame thickness.

Limited View Tomography of Combustion Zones Using Tunable Diode Laser Absorption Spectroscopy: Simulation of an Algebraic Reconstruction Technique

Temperature and concentration distributions of a simulated flame were reconstructed with the help of computer tomography and tunable diode laser absorption spectroscopy (TDLAS). Reconstructions were based on the simulated numerical values of temperature and concentration of a stationary flame. Integrated absorption measurements along the line-of-sight (LOS) across the flames due to absorption by water vapor (H2O) in the near infra-red (NIR) region, specifically the 6930–6940 cm−1 range, were simulated to obtain the projection values for tomography. Spectroscopic parameters for the absorptions transitions, such as line-strengths, transition wavenumbers, collisional broadening coefficients and coefficients for their temperature dependency were selected from the HITRAN 2004 database. Simulated LOS data are inverted using a multiplicative algebraic reconstruction technique (MART), which are known to outperform traditional filtered back projection methods for cases with limited numbers of views. Based on spatially resolved reconstructions of spectroscopic data, temperature and concentration distributions are calculated using the wavelength modulation spectroscopy with second harmonic detection (WMS-2f) technique. A parametric study based on the number of views, orientation of views and number of rays per view required by the ART is performed in order to assess requirements for an acceptable reconstruction.Copyright

Processing and Analysis of Ceramic Mesoscale Combustors Fabricated by Co-Extrusion

In this work, millimeter-scale tubular combustion channels were fabricated from ceramic precursor materials. Co-extrusion of structured feedrods holds promise for the development of multi-layered, functionally graded and/or textured combustor walls, but requires a polymer binder that is difficult to remove before structures can be sintered to full density. In conventional thermal debinding, cracking is a major issue, where crack formation is attributed to a lack of pore space for outgassing of pyrolysis products. The main focus of this study is to validate a manufacturing process that uses a combination of solvent de-binding and thermal debinding, which is applied to a simple combustor geometry. Alumina powder was batched with a mixture of polyethylene butyl-acrylate (PEBA) and polyethylene glycol (PEG) in a torque rheometer. A 19mm feedrod, consisting of a carbon-black/binder mixture as core, and a surrounding ceramic/binder mixture forming the wall, was extruded through a 5.84 mm die. The binder removal involves two processing steps, where the PEG content was removed by solvent extraction (SE) to initiate pore formation, after which thermal de-binding by pyrolysis removes the remaining binder and carbon-black. Solvent extraction was performed in water at three different temperatures for various times. The 1:1 mixture of PEG:PEBA showed the highest PEG removal of 80wt% for 6 hrs extraction. The thermal de-binding cycle was designed based on thermo-gravimetric analysis (TGA) and successfully performed with a ramping rate of 1.25°C/min to 1000°C without any crack formation. After de-binding, samples were sintered at 1600°C for 1 hr. SEM analysis showed some void spaces in the solvent extracted samples but confirmed that solvent extraction followed by thermal de-binding yielded the best results. The viability of sintered ceramic tubes was tested for conditions typical for thermal cycling in a combustion environment.Copyright

Tomographic Imaging of Combustion Zones: Impact of Measurement Noise and Reconstruction Error

Temperate and concentration distributions of a horizontal section through a stationary test flame were reconstructed with the help of computer tomography. Simulated reconstructions are based on temperature and concentrations distributions equal to published time-averaged data of the Sandia D flame. Based on property distributions, line integrated absorption due to water vapor (H 2 O) in the 6930-6940 cm 1 range was simulated to obtain projection values for tomographic reconstructions, where spectroscopic parameters for absorption features were obtained from the HITRAN 2004 1 database. Simulated projection values were calculated for direct absorption spectroscopy (DA) and wavelength modulation spectroscopy with second harmonic frequency detection (WMS-2f). The resulting projections were de-convoluted by tomographic methods, where Abel inversion and filtered backprojection were investigated. Based on the reconstructed spatial distribution of spectroscopic data, temperature and concentration profiles were obtained. Results highlight the impact of uncertainties introduced by measurement noise, deviations of reconstructed values, and tomographic artifacts. The comparisons of reconstructed results indicate that WMS-2f in conjunction with tomography is a viable method to reconstruct spatially resolved temperature and concentration distributions for stationary flames.