Combustion instabilites in Rockets | Date: 13 April 2011, 12:25
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Gas turbines have made substantial gains in performance since their initial demonstration
in jet powered aircraft and power turbines. The performance, noise characteristics,
and pollutant emissions of gas turbines for propulsive applications
continue to improve. On the ground, contemporary gas turbines produce higher
operating efficiencies and emit fewer pollutants than other major chemical-energy
conversion devices. In addition, the low capital investment, ease of permitting, and
quick installation have made them attractive to investors. As a result, gas turbines
have become a dominant technology for new power generating capacity in the
United States and worldwide.
A variety of factors have contributed to the popularity of gas turbine technology.
Financing considerations are the key high-level driver. Pollutant emissions play
another important role, particularly in motivating the specific technology improvements
and innovations over the last decade. For example, in the United States, the
Clean Air Act Amendments of 1990 imposed strict guidelines on the control of
nitrogen oxides, NOx, which, along with SO2, is a major contributor to acid rain
This book focuses on a particularly serious difficulty in low emissions gas turbines:
combustion-driven oscillations. These instabilities routinely constrain the
operating envelope and power output of fielded machines and, in some cases, lead to
serious damage of hot section components. Gas turbine users have found that components
such as combustor liners, transition pieces, and fuel nozzles need routine
examination for part cracking or excessive wearing because of vibration-induced
fretting. At a minimum, this requires downtime for inspections and part repair,
thereby reducing machine availability. At the worst, a cracked piece may be liberated
into the hot gas path, potentially requiring replacement of expensive turbine
components. In addition, users in certain geographic areas have found that engines
must be seasonally retuned to eliminate oscillations due to ambient temperature
changes. The cost for the repair and replacement of hot section components, much
of which is directly attributable to the combustion instability problem, exceeds
$1 billion annually and constitutes up to 70% of the nonfuel costs of F-class gas
turbines. Major power generating companies have suffered losses in the hundreds
of millions of dollars because of lost revenue from forced outages, resulting in a
number of lawsuits.
Although instabilities have not been nearly as severe a problem in nonpremixed
aero engine combustors, they have appeared in a few cases and posed serious
challenges in the development stage. Military engines, however, have experienced
major problems with low-frequency instabilities in augmentors. A large-scale effort
is currently underway at several gas turbine manufacturers in the United States,
in cooperation with the U.S. Air Force, to overcome such difficulties.
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