Lean Stability Research Articles

Effect of vitamin C concentration and co-injection with calcium chloride on beef retail display color.

The objectives of these experiments were to determine 1) the most effective vitamin C concentration to stabilize color of beef cuts during retail display and 2) the effect on color of incorporating vitamin C into a calcium chloride (CaCl2) injection solution. Top round cuts (semimembranosus and adductor) were injected with 5% by weight of a 0, .25, .5, 1, 2, or 4% sodium ascorbate solution (Exp. 1) or a 0, .5, 1, or 1.5% sodium ascorbate solution (Exp. 2). In Exp. 1, vitamin C resulted in more (P < .05) stable lean color during 9 degrees C display, and .5, 1, and 2% vitamin C were most (P < .05) effective. In Exp. 2, all concentrations of vitamin C maintained redder (P < .05) steaks after 3, 5, and 7 d of display than control steaks and had a lower (P < .05) percentage of surface discoloration after 5 and 7 d of display at 1 degree C. Experiment 3 used beef bottom round cuts (biceps femoris) to compare control, vitamin C- (1%), CaCl2-(200 mM) and vitamin C + CaCl2-treated steaks displayed at 1 degree C. Calcium chloride-treated steaks were more (P < .05) brown and had a higher (P < .05) percentage of surface discoloration on d 5 and 7 than control steaks, whereas steaks treated with vitamin C or vitamin C + CaCl2 were more (P < .05) red and had lower (P < .05) discoloration on d 5 and 7 than control steaks. Vitamin C can be injected into beef subprimals to enhance lean color stability and extend retail display life. Vitamin C also can be used in combination with CaCl2 to offset potential color deterioration, after 5 d of display, due to salt-induced oxidation.

NO x and CO emissions from a pulse combustor operating in a lean premixed mode

Control over the combustion fluid dynamics was used to minimize the emission of NO x and CO. The combustion kinetics were controlled by operating the combustor premixed and were varied by modifying the equivalence ratio over the lean stability envelope. A wide dynamic range in fluid dynamic mixing characteristics was also investigated by modifying the degree of macroscopic mixing and microscopic mixing. The residence time at high temperature was controlled by modifying the frequency of the periodic reacting flow in a pulse combustor. It was found that controlling the flame temperature, chemical kinetics, and residence time at high temperature was best accomplished by controlling the equivalence ratio and the degree of macroscopic mixing rather than controlling the microscopic mixing over the dynamic range obtainable by the techniques used in this study. Emission levels below 5.0 ppm NO x , with corresponding levels of 5.0 ppm CO (corrected to 3% O 2), were achieved in a pulse combustor operating in a lean premixed mode, without the use of any postcombustion cleanup technologies. Both NO x and CO emissions were invariant to changes in the power input.

Development and assessment of correlations for predicting stability limits of swirling flames

Swirling flow systems are being used in many industrial processes like separation of particles in gas flows (cyclones), atomisation and spreading of liquids (water, oil) and fast mixing and high-intensity reaction in reactor systems as, e.g. stable and intense flames of swirl burners. The widespread use of swirl burners in the process and energy industries and, in particular, new concepts for the reduction of NO x-emissions (ultralean premixed combustion) raise the need for simple-to-use models for predicting lean stability limits of highly turbulent flames stabilized by internal recirculation. Based on recently published experimental data of the first author concerning the reaction structures of swirling flames operated near to the extinction limit, different methods for predicting lean blow-off limits have been developed and tested for different burner sizes and fuel gas compositions. The aim of the investigations was to find stabilization criteria that allow predictions of blow-off limits of highly turbulent recirculating flames without the requirement for expensive and time-consuming measurements in those flames. Several similarity criteria based on volumetric flowrates, burner size and material parameters of the cold gases, were found to be capable to predict stability limits of premixed and (in some cases) nonpremixed flames at varying swirl intensities, burner scales and fuel compositions. A previously developed numerical field model, combining a k,ε-model with a combined “assumed-shape Joint-PDF”/Eddy-Dissipation reaction model for the determination of the time mean reaction rates in turbulent flows was also tested for its potential for stability prediction. All the methods presented have specific advantages and limitations: the similarity criteria are restricted to geometrically similar systems, but they are easy to use, fairly precise and can take the detailled chemistry into account in an integral manner. The numerical field model necessitates large computational effort and is limited to very much simplified, global reaction mechanisms, but it offers the opportunity to make predictions for different burner geometries and swirl conditions.

Stability of pulverized coal burners

Stability limits of pulverized fuels, burned with the support of natural gas in quarl burners with swirling secondary air, are quantified. The potential heat release of the solid fuels was up to about 14 kW while the gas support was reduced, typically to less than about 12 kW, to find the extinction limits as a function of the swirl number, quarl length, secondary average axial velocity, downstream confinement and for Welsh anthracite, English bituminous coal and Greek lignite (the latter with 12% moisture). In unconfined operation, attached and lifted flames were produced at swirl numbers between 0.3 and 0.6, depending on the quarl length and the air velocity. The bituminous coal provided the broadest stability limits: dried lignite gave comparable lean stability limits but also introduced rich stability limits as the solid feed rate was increased. In confined operation, common rich stability limits were found for attached flames of all coal types: the lean limits were also common for swirl numbers less than than about 0.7. As lean extinction of confined bituminous-fired flames was approached, attached flames were followed by extensive lifted regions. As burner confinement increased, attached flames were observed, for all coal types, at lower values of swirl numbers.

The influence of burner geometry and flow rates on the stability and symmetry of swirl-stabilized nonpremixed flames

The flame characteristics of a swirl-stabilized, nonpremixed natural gas burner have been established as a function of equivalence ratio and swirl number for three quarls and three fuel nozzle arrangements over a heat-release range of about 5–85 kW and combustion-air Reynolds numbers of 29,000 and 45,000, corresponding to bulk air velocities of 8.5 and 13.2 m/s. The quarl half-angles were 20° and 25°, the ratios of length to the throat diameter were L D = 1 and 1.9, and the maximum swirl number was 1.8. The lean stability limits were narrower for the higher bulk air velocity and the 20°, L D = 1 quarl supported the leanest flames; the poorest stablility was associated with the 25°, L D = 1.9 quarl. The stability limits generally reduced as the swirl number increased above 0.2, although for the higher bulk air velocity and for the L D = 1 quarls there was a local “bulge” towards lean equivalence ratios at swirl numbers between about 0.9 and 1.6. Flame lengths decreased with increasing swirl and bulk air velocity but the recirculation zone length was largely unaffected by changes in these two quantities. Axial fuel injection through a centrally located nozzle resulted in symmetric flows only for fuel-lean flames as swirl numbers below 0.6 and for near stoichiometric flames at swirl numbers above unity. The asymmetric was not due to asymmetric inlet velocity profiles. Angled annular or radial injection of the natural gas extended the range of operation for which symmetrical flames could be obtained, without greatly changing the flame lengths or stability characteristics.

Response of a pulse combustor to changes in fuel composition

The response of a valved pulse combustor to changes in the homogeneous chemical ignition delay time has been examined. Guided by detailed chemical kinetic modeling, experiments were performed which measured the response of this combustor using two fuels having significantly different ignition delay times. It was found that differences in the reaction kinetics of the fuel caused changes in the coupling between the heat release and the resonant acoustic waves which control the pulsating combustion process. Furthermore, the change in this coupling caused corresponding changes in the fuel and air injection process—hence, the strength of the resonant pressure wave. This complicated interaction resulted in a modification of the combustor's lean stability limit which was contrary to predictions based on chemical kinetics arguments alone.

Emission Characteristics of a Combustor Burning Simulated Partial-Oxidation Product Gas

A TWO-STAGE gas turbine combustor concept based upon a very fuel-rich partial-oxidation stage is being explored for broadening the combustion margin between ultralow emissions and the lean stability limit. Combustion and emission results are presented for a series of experiments where a simulated partial-oxidation product gas was used in a premix combustor operated with inlet air state conditions typical of cruise power for high-performance aviation engines (1.2 MPa and 728 K). Ultralow NOX, CO, and HC emissions and an extended lean burning limit (relative to single-stage baseline operation) were achieved simultaneously.

The combustion characteristics of spouted beds

The behaviour of spouted beds as combustors was studied to determine the interaction between bed fluid mechanics and the combustion process. A cylindrical geometry, particles in the 0.5–2 mm range, and methane/air reactants were employed. Combustion reduces the entrance jet velocity at which a spout can be maintained, and increases particle circulation rate, pressure drop and spout height in comparison with operation at identical conditions without combustion. It is shown that events in the spout determine flame stability. Measurement of bed temperatures reveal that the particle temperature remains nearly uniform and an approximate theoretical analysis of heat transfer in the bed suggests that particle-gas contact times are inadequate to allow these two phases to attain thermal equilibrium. This restricts lean stability limits, in the present system, to approximately 2/3 of the normal flammability limit, as compared with the 1/2 which would be the minimum possible if the particle temperature were to reach the exit gas temperature. Various methods of improving particle-gas heat transfer are considered and the thermal disequilibrium in the spout is shown to be greatly decreased. Leanest burning occurs in deep beds which also manifest fast flow rates with velocities at the inlet exceeding half the speed of sound.

Lean Stability Augmentation for Premixing, Prevaporizing Combustors

An experimental program was conducted to investigate techniques for improving the lean combustion limits of premixing, prevaporizing combustors applicable to gas turbine engine main burners. Augmented flameholders employing recessed perforated plates, catalyzed tube bundles, and configurations in which pilot fuel was injected into the wake of V-gutters or perforated plates were designed and tested. Stable operation of the piloted designs was achieved at equivalence ratios as low as 0.25; NOX emissions of less than 1.0 g/kg at simulated turbine engine cruise conditions were obtained. A piloted perforated plate employing 4°'o pilot fuel flow produced the best performance while meeting severe NOX constraints. Nomenclature d = diameter El = emission index, g/kg P = pressure, atm; ratio of pilot fuel flow to total fuel flow PPM = concentration in parts per million by volume T = temperature, K V = velocity, ms w = flow rate, kg/s rjc = combustion efficiency (chemical) 0 = equivalence ratio T = residence time, s Subscripts ad = adiabatic flame bo = blowout = design point = approach flow condition = pilot stream des o P