Jacob E. Temme

Unsteady aspects of lean premixed prevaporized gas turbine combustors: Flame-flame interactions

This work quantifies several sources of unsteadiness that exist within a Lean PremixedPrevaporized (LPP) gas turbine combustor that was operated at elevated pressures using Jet-A fuel. Flame-flame interactions and shear layer vortex shedding, which can be sources of combustion instabilities, are quantified with PIV and PLIF diagnostics. Flame-flame interactions occur because LPP aircraft combustors employ a premixed Main flame that is anchored by the non-premixed Pilot flame. The measured degree of unsteadiness is the standard deviation of: a) flame surface density, b) flame length, c) vorticity in shear layer, and d) recirculation zone size. The flame surface density profile was broad, indicating that large flame motions occur. Flame length increases non-linearly with fuel flowrate. Intense vortices in the shear layer are more than twice the average vorticity, indicating the need for unsteady modeling. Chamber pressure and liquid fuel flow rates were varied. Velocity fields for the five reacting cases were similar but they differed from the two non-reacting cases. Heat release causes the recirculation zone shape to change from ellipsoidal (for the reacting cases) to toroidal (for the non-reacting cases). Methods were developed to image Jet-A spray flames at 3 atm. using formaldehyde fluorescence.

Measurements of premixed turbulent combustion regimes of high reynolds number flames

The goal of this research is to empirically identify the boundaries between different regimes of premixed turbulent combustion that appear on the diagrams of Borghi and Williams. To date, four conditions have been extensively studied. The most intense of the four conditions possesses a turbulence level (u’/SL) of 185, an integral length scale (λ/δF,L) of 46, and a turbulent Reynolds number of 69,000. At present, the data set is too limited to plot boundaries on the regime diagrams. However, the four conditions have been categorized into their appropriate regimes. The structure and the thicknesses of the reaction zones were determined from simultaneous PLIF images of formaldehyde (CH2O) and OH. Locally distributed reactions and shredded (i.e. broken) flamelets were observed in these images. The burning fraction varied between 0.75 and 1.0, indicating that up to 25% of the reaction layer was locally extinguished where “holes” were formed. The reaction or preheat zones associated with a particular condition were classified as being “globally distributed” if the mean thickness for that condition exceeded four times the laminar value. If a particular reaction zone is both four times thicker than the laminar value and its length to thickness ratio is less than four it is identified as being “locally distributed.” In contrast, if this ratio exceeds four or the zone is not locally four times thicker than the laminar value it is considered to be thickened. While none of the cases were identified as being “globally distributed;” some of the cases were “partially distributed;” this is defined to occur when more than 25% of the reaction surface consists of “locally distributed” reaction zones. The preheat zone thickness was deduced from the CH2O PLIF images. Three of the four conditions, in which the turbulent Reynolds number exceeded 20,000, were found to have “globally distributed” preheat zones. Thickening of the preheat zone is believed to be enhanced when “holes” allow hot products to rapidly mix with the reactants. Previous studies conducted at much lower turbulent Reynolds numbers rarely observed local extinction within the reaction layer.

Role of Swirl in Flame Stabilization

This paper first reviews recent ideas that explain why swirl has a strong stabilizing effect on a flame. Then some measurements are discussed that were obtained using a complex gas turbine fuel injector/mixer operated at realistic levels of swirl and multiple recirculation zones. While swirl is known to have several beneficial effects that improve the mixing and flame stabilization within a gas turbine combustor, swirl also can lead to some undesirable effects. A precessing vortex core can be a source that drives a combustion instability. In addition, swirl affects the unsteady anchoring location of a flame, which also can lead to combustion instabilities, as are observed in our experiment. Interactions between the recirculation zones are observed. The observed large scale unsteady motions cause serious problems for CFD simulations, since the measured mean velocities and turbulence levels on the combustor centerline are much larger than the computed values. Reasons for this difference are associated with unsteady motions.

Experimental assessment of premixed flames subjected to extreme turbulence

Structural features of highly turbulent piloted flames were acquired from simultaneous PLIF images of formaldehyde (CH2O) and OH. Both lean and near-stoichiometric (equivalence ratio φ = 0.75 and 1.05, respectively) methane-air flames were studied under twelve different flow conditions and at two different interrogation regions. The non-reacting conditions for these flames consist of turbulent Reynolds numbers (ReT), turbulence intensities (u’/SL), and integral length scales that range from 520 to 80,000; 5 to 185; and 6 mm to 37 mm, respectively. Eight of the twelve cases have u’/SL > 25 and thus are classified into a regime of extreme turbulence. Preheat and reaction zone thicknesses were measured in all twelve cases. The preheat zone thickness was interpreted from the CH2O PLIF images and the reaction zone thicknesses were obtained from the profiles derived from the pixel-by-pixel product of the OH and CH2O PLIF images. The preheat zones associated with a particular condition were classified as being “thickened” if the mean thickness for that condition exceeded two but not four times the measured laminar value (0.42 and 0.39 mm for lean and rich flames, respectively). If the average thickness was greater than four times the measured laminar value that preheat zone was deemed “primarily distributed.” Ten of the twelve cases possessed “primarily distributed” preheat zones, while those in the two least turbulent cases were “thickened.” The majority of the cases possessed average reaction layer thicknesses that are no thicker than twice the measured laminar value (0.39 and 0.38 mm for lean and rich flames, respectively); hence, they were identified as having “thin” reaction layers. Regardless of being categorized as “thin,” the reaction zones in each case exhibited regions of both relatively thin and thick reaction layers. In fact the appearance of the observed reaction zones can best be described as resembling “chicken noodle soup.” That is, in any given instantaneous image relatively thin, “noodle-like” reaction layers are generally accompanied by thicker “chunky-chicken-like” reaction regions. Furthermore, the observed reaction zone structures in a particular case often fail to correspond to those predicted by the turbulent premixed combustion regime diagram. This suggests that the regime diagram requires alterations if it is to properly forecast the appearance of a flame based on a simple set of operating conditions. The data set presented here is currently too limited to enable a thorough re-mapping of the regime diagram. However, based on their structural features, the cases considered here were categorized into appropriate regimes of combustion.

Evaluation of Cooling Potential and Tool Life in Turning Using Metalworking Fluids Delivered in Supercritical Carbon Dioxide

Carbon Dioxide is an industrial byproduct that has been proposed as an alternative metalworking fluid (MWF) carrier with lower environmental impacts and better cooling potential than existing MWFs. This paper investigates the heat removal and tool life effects of rapidly expanding supercritical CO2 (scCO2 )-based MWFs relative to MWFs delivered as a flood of semi-synthetic emulsion or as minimum quantity lubrication (MQL) sprays. When cutting both compacted graphite iron (CGI) and titanium, tool wear was most effectively controlled using the scCO2 -based MWF compared with the other MWFs. Analysis in this paper suggests that the performance benefit imparted by rapidly expanding scCO2 appears to be related to both the cooling potential and penetration of the sprays into the cutting zone. High-pressure gas sprays have lower viscosity and higher velocity than conventional MWFs. An experiment in which the spray direction was varied clearly demonstrated the importance of spray penetration in tool wear suppression. The type of gas spray is also a significant factor in tool wear suppression. For instance, a spray of N2 delivered under similar conditions to CO2 effectively reduced tool wear relative to water based fluids, but not as much as CO2 . This result is particularly relevant for MQL sprays which are shown to not cool nearly as effectively as scCO2 MWFs. These results inform development of scCO2 -based MWFs in other machining operations, and provide insight into the optimization of scCO2 MWF delivery.© 2009 ASME

Experimental investigation of premixed turbulent combustion in high reynolds number regimes using PLIF

Premixed turbulent flame structures are imaged with simultaneous formaldehyde and OH PLIF. A new piloted burner was designed to achieve high turbulent Reynolds numbers (Ret) up to 68,000 and low Damkohler numbers (Dat). Primary reaction zones are identified by the overlap of the OH and formaldehyde signals and preheat zones of low temperature secondary reactions are identified from the formaldehyde signal. At low Ret of 600, the primary reaction zones are continuous and products do not mix with reactants. This results in thin preheat layers and relatively thin flamelets. As Ret increases, the primary reaction zones become shredded and disconnected. This allows mixing of the hot products with the reactants and broadens the preheat/secondary reaction zones. Additionally, the reaction layers are typically 4-5 times thicker than those in a laminar flamelet. Interestingly, as Ret increases further, the thickness of the reaction layers only increases slowly, but the total area of reaction regions grows rapidly.

Low frequency combustion instabilities imaged in a gas turbine combustor flame tube

An “equivalence ratio oscillation” was studied that produced a strong low frequency combustion instability (at 80 and 160 Hz) when a commercial lean premixed prevaporized (LPP) fuel injector was operated using Jet-A fuel at realistic conditions. The elevated pressures, temperatures, air flow rates and overall equivalence ratios were close to conditions at engine idle. However the fraction of fuel diverted to the various injector ports was sufficiently off-design to create an instability. High speed movies showed that the flame base violently moves upstream and downstream at 80 Hz, and that flashback plays a role in the rapid upstream movements. Flame shape also changes at 80 Hz from a flat flame to a more elongated flame, which affects the heat release pattern and causes oscillations in the air flow rate. A regime diagram is reported which contains the boundary that marks the onset of the instability. Other higher frequency instabilities were also present and were due to longitudinal and azimuthal organ tones, but their magnitudes were more than ten times smaller than the pressure fluctuations caused by the equivalence ratio oscillation.