Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 165

6

C-O reaction: B =× 41 10 .

.

m s, E = 179 kj mol

2 S , 0 S , 0

8

C-CO reaction: B =× 11 10 m s, E = 270 kj mol

2 SP , SP ,

The solid and dotted curves shown in Figures 2.129 and 2.130 represent the
combustion rates calculated from the values indicated above. The results of tests
correspond comparatively well with the results of analysis. As for the air concen-
tration and coefficient of viscosity used in the calculation of combustion rates, ρ ∞
= 1.09 kg/m 3 and µ ∞ = 1.95 × 10 -5 Pa·s at 320 K were used since the analysis was
based on the condition that ρ µ is constant.
Since the mass transfer number β that determines the nondimensional combus-
tion rate [(- fs ) = {ln(1 + β)}/ K ] is of almost the same value for both room temperature
airflow and high temperature airflow, the difference in the combustion rate can be
attributed to the concentration or coefficient of viscosity, or both, which are physical
property values. With the mass flow rate of the air fixed at the same level, the flow
velocity increases in high temperature airflow as a result of thermal expansion. This
then increases the velocity gradient to finally raise the combustion rate. With the
velocity gradient fixed at the same level, the concentration is lowered in high
temperature airflow as a result of thermal expansion and the mass flow rate of oxygen
to be transported to the surface decreases, thus lowering the combustion rate.

0.12

Two-dimensional Flow

a=40000s -1
a=10000s -1 (Cold air)

0.1

Yo =0.233

T =1280K
Yp =0.0

0.08

Yo =0.15

0.06

Cold air

Yo =0.10

0.04

Yo =0.07
Yo =0.05

0.02

0

1000 1500 2000 2500

Surface temperature, K

FIGURE 2.131 Influence of the concentration of the oxygen in oxidizing agent flow on the
combustion rate of a solid carbon in high temperature airflow.