The Oxford Science Lecture Series

PROFESSOR ANN DOWLING

Department of Mechanical Engineering, University of Cambridge

"Singing Flames that Break... Power Stations and Jet Engines"

University Museum, Oxford, 15th October 1998

The seventh lecture in the Oxford Science Lectures series was given on October 15 by Ann Dowling, Professor of Mechanical Engineering at Cambridge University. It was the first lecture on a purely engineering subject, and was attended by a strong contingent of Sixth-form boys. The theme of Professor Dowling's research is 'unstable flows'. It involves studies of the conditions which cause apparently stable systems to develop instabilities, and finding routes for calming or preventing such situations from reaching dangerous levels. She is currently applying the theory and techniques to investigations of flames in specific cases: in the afterburner of a jet engine, and in gas-fired power stations.

Jet afterburners are effectively applied to bring about acceleration in aircraft; the greater the velocity of gas issuing from the back, the greater the forward thrust on the vehicle carrying the engine. The process is not an efficient one in acceleration per fluid consumed, so is only applied for short periods. However, the conditions under which the afterburner operates can cause the gas to burn in a highly unstable mode; large pressure oscillations can build up, capable of causing severe structural damage to the engine itself. Gas-fired power stations work at a very high efficiency level and are therefore extremely desirable as energy generators, but all are prone to commence unsteady combustion. The pressure waves, or sound, thus generated have already caused many such power stations to be damaged - linings crack and whole buildings may eventually collapse - so there is an urgent need to analyze the conditions responsible for the onset of these combustion instabilities and to devise a suitable remedy. (The damage that sound waves can cause can be demonstrated in the home; it is said that you can cause a wine glass to shatter by playing the right note on a violin.)

Instabilities occur through interaction between sound wave and unsteady heating. Unsteady combustion generates sound (acoustic) waves, and conversely sound can cause flames to become unsteady. The principles involve the properties of an organ pipe (i.e. an open or closed tube that is made to vibrate), and what makes it resound. By heating a gauze placed at one of the nodes in a Rijke Tube (a large metal `organ pipe' with a diameter of about two inches) Professor Dowling showed how the tube resounded loudly when held with the gauze below the middle of the tube, but ceased to do when when it was turned the other way up. Indeed, it was surprising that only a few calories of heat energy could be responsible for producing so much sound energy; the equivalent effect when scaled up to the size of a power station was difficult to imagine...

Unsteady burning is often traced back to methods of gas injection and air mixing, and instabilities can sometimes be damped by applying suitable interruptions to the input flow. In simple situations one can determine the frequencies which the unsteady burning causes, and design the relevant structure so as to avoid resonances from building up. In other cases the boundary conditions may be changed by electronic interference; this was again demonstrated convincingly by lowering a glass tube over a flame until it `sang' loudly, and then controlling it via a feedback mechanism. The feedback was achieved by using a microphone to record the sound being produced at the top of the tube, analysing its frequency, shifting its phase electronically by 180 degrees, and then playing the new sound back at the level of the flame using a loudspeaker. The difference between the sound with and without feedback was appreciable.

We were also shown video films of experiments in which flames were 'attacked' by acoustic waves, and heard about models for analyzing problematic situations and current efforts at controlling them. The stable gas-fired power station is not yet a reality, nor is the silent afterburner, but by pursuing experiments and theories such as these the Cambridge team is surely on the right road to finding solutions.

Professor Dowling has already made history as the first woman to be appointed to a Professorship of Engineering at Cambridge University. Someone with such natural talent and clear insight, such fascination for mechanical problems and evident zeal for every facet of the science, must be on course to bring down important scientific barriers as well. We wish her all the best in the years ahead.

Elizabeth Griffin