Low-Emission, Fuel-Flexible Combustion of Liquid Fuels - Novel Combustion Concepts for Sustainable Energy Development - page 467

rate (HRR) was kept constant at approximately 7 kW, and the total air (combustion
air plus atomizing air) flow rate (Q) was kept constant at 150 slpm pertaining to

pure glycerol combustion at overall equivalence ratio of 0.75. At these conditions,
50/50 glycerol and methane fuel blend (meaning 50 % heat release from glycerol
and 50 % from methane) was used, while the AA

fl

flow rate through the injector was
varied to change the air-to-liquid mass ratio (ALR). Flame images and emissions
data were acquired to determine the optimum ALR for atomization. This procedure
was repeated for different HRRs to

fl

find the optimum ALR for each case. Finally,

for a fixed heat release rate of 7 kW, the fuel blend was varied between 0 % glycerol

(100 % methane) and 100 % glycerol (0 % methane).

7.1 Effect of Air-to-Liquid Mass Ratio

The visual flame images for 50/50 glycerol-methane fuel, HRR = 7.0 kW, and

Q = 150 slpm showed that the

fl

flame becomes shorter and less luminous as the ALR
is increased. At smaller ALRs, the

fl

fl flame exhibited two distinct regimes; a blue

region in the near field and downstream orange region. The blue region pertains to

combustion of methane and evaporated glycerol from smaller droplets. The larger
glycerol droplets burnt mostly in diffusion mode in the downstream yellow region.
With increasing ALR, these two distinct regions merged to form a fairly homo-
geneous reaction zone. For higher ALR, the FB injector is expected to produce
smaller droplets that evaporate quickly to form a combustible fuel air mixture

-

closer to the injector exit. Thus, the resulting flame becomes shorter, less luminous,

and more premixed. If the ALR was too high, the

fl

flame became unstable and
eventually blew off. If the ALR was too low, unburned reactants were released in
the products indicating incomplete combustion resulting from poor atomization.
Based on these observations, an ALR of 2.6 was determined to be the best choice.
Figure 15 shows the effect of ALR on CO and NO X emissions for test conditions
discussed above. The emissions measurements were taken at the center point of the
combustor exit plane. Figure 15 shows that the CO emissions decrease as the ALR
is decreased. Evidently, the large concentrations of CO emissions produced near the
wall of the quartz combustor were not captured by the single-point measurements
presented in Fig. 15 . The NO X emissions were low, within 2 ppm, for all values of
ALR.

fl

7.2 Effect of Heat Release Rate

Heat release rate is an important parameter affecting the fuel and air fl ow rates,

and thus, the flow mixing processes inside the combustor. Visual images revealed

that at the lowest

fl

fl flow rates (or HRR), the fuels do not burn as a homogenous

fl flame. At higher heat release rates, the fl flames appeared completely homogenous.