B URNING V ELOCITY
2.4. RECIRCULATION OF HEAT FROM BURNING AND BURNED GAS
the limits of the peninsula shown in the figure. It was found that at sufficiently high values ofe, greater than about 80, the detonations could propagate outside the hot spot with the Chapman–Jouguet detonation velocity.
A variety of propagation modes of the reaction front are possible and these are discussed in more detail by Bradley et al. (2002) and Gu et al. (2003). Thermal explosionsoccur whenx¼0. At slightly higher values ofx, but less thanxl(regime P on the figure),supersonic autoignitive frontsoccur. Abovexu, in regime B, the fronts propagate subsonically. As fdecreases,teincreases,edecreases and, as can be seen from Figure 2.16, the detonation regime narrows appreciably. Conversely, as f increases from such low lean values, the detonation peninsula broadens appreciably.
This corresponds to increases infabove about 0.6.
2.4. RECIRCULATION OF HEAT FROM BURNING
Referring to Figure 2.17(i), in a simple burner without any recirculation, the flowbis zero. With recuperative heat recirculation solely via a heat exchanger, as indicated by the dashed lines,b<aandd¼0. A compact heat exchanger working on this principle is the double spiral“Swiss roll”burner of Weinberg (1986). In this, the reactants flowed inwards through an outer channel of the heat exchanger to the combustion chamber, which was at the center of the spiral. Thereafter, the combustion products spiraled outwards and transferred heat to the reactants in the counter-flow heat exchanger. The increased tempera- ture of the reactants enabled leaner mixtures to be burned. A flow that was too high led to flame blow-off, while one that was too low led to flash-back. Such an atmospheric pressure burner was able to burn CH4-air in a stable flame withfas low as 0.16. This was lower than the flammability limit off¼0:53 at atmospheric temperature and pressure.
A combination of both heat and hot gas recirculation appears to be necessary to achieve sufficiently low values oftifor autoignitive, flameless combustion in very lean mixtures. This corresponds to the conditions b<a,d¼b, andc¼0 in Figure 2.17(i).
The burner described by Plessing et al. (1998) operated in this mode with hot gas recirculation within the combustion chamber. At start-up, CH4 and air were first fed separately to the burner, with heat transfer from exhaust gas to the incoming air, as well as internal hot gas recirculation. This enabled stable, lean flames to be established, with up to 30% of the fuel and air flow recirculating. This regime is indicated by S in Figure 2.18. Transition to the flameless mode involved switching the fuel flow to a central nozzle and increasing the internal recirculation. Eventually, the increased dilution quenched the turbulent flame, in the regime indicated by Q, as the recirculation, d, increased. At temperatures above 1000 K, the mixture re-ignited in the stable“flameless oxidation”mode, regime F in Figure 2.18, indicated by the shading. For the idealized conditions of a perfectly stirred reactor, a necessary condition for combustion would be that the residence time in the reactor must be greater than the autoignition delay time.
The nature of the flameless oxidation in the burner of Plessinget al. was diagnosed with OH laser-induced pre-dissociative fluorescence (LIPF) and Rayleigh thermometry.
Images of the reaction zone are compared with those of a highly turbulent flame in Figure 2.19. Color-coded temperatures are on the left and OH intensities are on the right. The upper, highly turbulent flame images indicate a thin, convoluted flame reaction sheet. The lower images of flameless oxidation indicate a very different spotty structure, with reaction at small disconnected hot spots and with smaller increases in temperature. The OH concentrations are fairly uniform in both cases. For the highly
F
S
Q
Very Slow Reaction autoignition temperature
Temperature (K)
Recirculation (d)
Figure 2.18 Burner of Plessinget al. (1998). Transition from flame to autoignitive combustion. S is regime of stabilized flame, Q that of flame quenching, and F that of stable autoignitive burning.
turbulent flame, this was because of the turbulence. For the less turbulent, flameless oxidation, it was because of the more uniformly distributed reaction. Maximum tem- peratures in the flameless oxidation were about 1650 K, with a temperature rise due to reaction of about 200–400 K and a NOxconcentration of about 10 ppm. The latter value was below the 30 ppm found in the stabilized flame. Chemical times were greater than turbulent times and reaction was likened to that in a well-stirred reactor.
Recirculation of hot gas also is important in explosive combustion, as in HCCI engines. Recirculation both dilutes the mixture further and increases its temperature.
Dilution with inert gases reduces the rate of burning. On the other hand, the increased temperature and any recirculated active radicals increase the rate of burning. This increase tends to predominate over the dilution decrease much more in the autoignitive mode, than in the flame propagation mode. It is one reason why the autoignitive mode is so often the only means of achieving very lean burn.
Weinberg (1986) has described a variety of heat-recirculating burners, including the spouted bed burner shown in Figure 2.17(ii). In this, a central jet of gaseous reactants causes chemically inert particles in a bed at the bottom of the reactor to rise in a fountain. A conical flame stabilizes where the fountain emerges from the top of the bed.
The particles are heated as they pass through the flame, and the particle bed reaches a temperature in the region of 1000 K. The particles recirculate due to gravity and transfer
Highly turbulent premixed methane flame (Φ =1)
Flameless oxidation (Φ =1) 18.0
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Figure 2.19 Simultaneous temperature and OH-LIPF images of premixed highly turbulent flame and flameless oxidation (Plessinget al., 1998, p. 3202). (See color insert.)
heat to the incoming gaseous reactants. Stable, lean limit combustion of CH4–air mixtures was observed down tof¼0:28.
With regard to the filtrational combustion of Figure 2.17(iii), combustion within porous inert media has been reviewed by Howell et al. (1996). Different regimes of flame propagation have been identified by Babkin (1993). A flame established in a tube filled with an inert, porous medium, through which a homogeneous mixture flows, can move either with, or against, the direction of flow. Figure 2.20 shows temperature profiles in the gas and the solid porous medium in alow velocity regime. Heat conducts through the porous medium in the direction upstream from the flame and this heats the cooler flowing mixture. Sufficient heat is transferred to initiate reaction in the gas. As a result, this becomes hotter than the solid, but cools as it transfers heat to it. The high thermal conductivity of the solid effectively conducts sufficient heat from hot gases to cold reactants. The porous medium acts as a recuperator, transferring heat from hot gas to cold reactants, in a not-dissimilar fashion to what occurs in the Swiss roll and the spouted bed burners. Hsuet al. (1993) were able to burn CH4-air down tof¼0:41 in a porous ceramic burner.
With increasing pore size there is a transition to ahigh velocity regimeof the reaction front, with reduced thermal coupling of gas and solid in the reaction zone and no heat recuperation. There is an increasing dominance of aerodynamic and turbulence effects.
A pressure wave builds up in front of the combustion zone and asonic velocity regimeis entered, with a sudden jump in the reaction wave velocity. Finally, there is a transition to a low velocity, subsonic detonation, which evolves spontaneously from deflagrative combustion (Brailovsky and Sivashinsky, 2000).