Untitled - Aerobib - Universidad Politécnica de Madrid

178 CHAPTER 6. LAMINAR FLAMES
With the experiments made by Murray and Hall it is not possible to verify the
first conclusion since they were all performed at ambient pressure, and the second
conclusion was not checked by the authors. In order to verify both, Adams and Stock
[36] measured the combustion velocity of hydrazine at different pressures. For this
purpose, they stabilized the flame over a column of liquid hydrazine contained in a
capillary tube and measured the recession velocity of the liquid level as its vapour
burnt. This method is not practical to measure the absolute propagation velocity of
the flame due to the irregularity of its front but is very convenient for the comparative
study intended by these authors. The results of their experiments show that the influence
of pressure is approximately that corresponding to a first order reaction. The
discrepancies could be due to the experimental method used.
As for the apparent activation energy it decreases with the drop in flame temperature
and it is in all cases way under the 60 000 cal/mol that correspond to the
process proposed by Szwarc and applied to the theoretical studies mentioned herein.
Such result led Adams and Stocks to conclude that the decomposition of hydrazine
should occur through a complicated system of chain reactions of which (6.164)
is only the initial one. As a result of a detailed discussion of all possible reactions the
above mentioned authors proposed a simplified kinetic scheme [36], which summarizes
the actual process. This proposed scheme considers only three chemical species:
reacting species A (hydrazine), stable products C of the combustion (mixture of NH 3 ,
N 2 , H 2 , etc.), and one radical B, that propagates the chain (which could be NH 2 , H,
etc). The Adams and Stock simplified scheme is a follows
A → 2 B, (6.166)
B + A → B + 2 C, (6.167)
B + B + X → 2 C + X. (6.168)
Of the three reactions, the first initiates the chain, the second is the propagation reaction
and the third the chain breaking reaction.
Lately, Spalding [37] has calculated the propagation velocity of the flame with
the scheme proposed by Adams and Stocks and he applied an interesting method of
numerical integration developed by him. Spalding calculates two cases corresponding,
respectively, to a cold flame and to one of the hot flames experimented by Murray
and Hall. In both cases he gets results in close agreement with those experimentally
obtained by Murray and Hall. These results are summarized in Table 6.4.