COMBUSTION PEOPLE

Professor Peter Gray, FRS: 80 this year!

Dear Peter

It has not escaped our notice that the year of 2006 not only marks the 80th birthdays of the Queen, Fidel Castro, Margaret Thatcher, George Melly and David Attenborough, but also of our good friend and mentor, Peter Gray. Through this letter, Peter, we pay tribute to all that you have meant to us over what, by definition, is quite a lengthy period. We reflect on your life and times in combustion and related research and thank you for all you have achieved for our community, both in the UK and internationally.

Newport - Cambridge - Leeds - Cambridge

Though not contemporaries, one of us (like you, a Fellow of the Royal Society) is a former pupil of the remarkable, Newport High School, Gwent. That auspicious secondary education took you to Cambridge where, in due course, your first research interest was in explosive substances, which you pursued with Bowden and Yoffe, also with Philip Thomas as a contemporary researcher. One of your achievements, prior to 1950, was to have identified the detonation of liquid explosives under impact as a gas-phase process, resulting from spontaneous ignition of the vapour compressed within bubbles. From your appointment there as Demonstrator in Chemical Engineering, in 1955 you joined the School of Chemistry at Leeds as a lecturer, bringing with you a formidable group of research students and post-doctoral workers from Cambridge.

Related work on explosives was continued by you and your research group, through the study of alkyl nitrates and nitrates, as well as of hydrazine decomposition. However, the move to Leeds heralded a new era insofar that you then began chemical kinetic studies, with Alan Williams as your first "Leeds" postgraduate student. Alan's research project combined studies of the methyl nitrite decomposition flame with kinetic investigations, with specific interest in the reactions of alkoxyl radicals. The latter became the subject of a Chemical Review (vol 59, 1959), jointly authored by you and Alan.

Within a very short period, your academic talent was recognised at Leeds by a meteoric rise from lecturer to reader and professor, with personal chair - at a time when Leeds University (as other UK Universities) was bound by tight quota controls on promotions. The then Professor of Physical Chemistry was Fred Dainton (later Lord Dainton of Hallam Moors). You were appointed as Dainton's successor in 1965, but had already established a very large research group comprising postgraduates and postdoctoral fellows. The chemical kinetic activities continued through measurements of elementary reaction rates using photolytic generation of radicals and reaction product analysis - remembering that chromatography was still then in its infancy as an analytical tool. The rate parameters measured during this period for methyl and alkoxyl radicals have stood the test of time remarkably well. These efforts did not stunt the development of the "explosives" work, which had blossomed into your seminal experimental and theoretical studies of thermal ignition. The importance of the measurements of temperature change using very fine thermocouples for the direct validation of Frank-Kamenetskii, thermal explosion theory in low pressure gases (< 20 torr) cannot be overstated. These first, accurate studies were helped considerably by the contemporary investigations at UMIST by your good friend and one-time Cambridge colleague, Sandy Ashmore. Certainly experience in the tricky techniques was shared between the groups. To make very fine thermocouples using 25 mm platinum wire remains a talent today, but it should be borne in mind also that electronic systems were very rudimentary in the 1960s, such that to obtain fast response, low-noise signals - photographed in real-time on an oscilloscope - was no trivial matter.

We recognise that this was by no means the limit of your combustion interest in the '60s. Flame speed measurements in a closed chamber were being made, recorded by a rotating drum camera, with particular interest in hydrazine and its derivatives, as well as hydrogen and, to investigate isotope effects, deuterium as fuels with oxygen and inert gases. The physical properties did not escape attention either. You set up a group to measure diffusion coefficients and thermal conductivities of binary and ternary mixtures relevant to these "simple" flame systems. One outstanding subsequent achievement of that research group, then under the direction of Tony Clifford, was the measurement of the diffusion coefficient for hydrogen atoms and the remarkable discovery of their adsorption / desorption at a quartz surface, to give a "chromatographic effect". The importance of the measured values of diffusion coefficient for these flighty species carried through into Graham Dixon-Lewis's computations of laminar flame structure, (and subsequent work of others elsewhere), and by this route into Derek Bradley's flamelet modelling in turbulent combustion.

Lest it be thought that the molecular transport studies were independent activities, David Smith was amongst a number of your research students who made both flame speed and thermal conductivity measurements within their PhD research projects. In parallel with these activities, in the Physical Chemistry Department were the thermochemical measurements and thermal analyses by "Carsy" (A.S. Carson) and Peter Laye, which linked to the explosion theory and also experimental studies of solid explosives and pyrotechnics, through Terry Boddington, and continued to do so for many years.

The late '60s constituted a consolidation of the theoretical hold that you had established on "thermal explosion theory" (Gray and Lee, Oxidation and Combustion Reviews, vol 2) and also initiated a development that spawned an enormous diversity in your scientific contributions that continued thereafter for more than 30 years. This was the investigation of cool flame and multiple stage ignition phenomena of hydrocarbons, using the experimental thermal explosion techniques, and the accompanying theoretical development of their understanding and interpretation. The latter is credited to Brian Gray, who was appointed by you to the Physical Chemistry Department in the School of Chemistry at Leeds in 1967. Brian's vision and interpretation (with C.H. Yang) of the highly non-linear, chain-thermal interactions in hydrocarbon combustion rationalised what had hitherto "baffled" chemical kineticists. The relevance was that another very exciting and rewarding period then began at Leeds, with the development of new experiments, involving John Griffiths, such as the closed and flowing stirred reactors (CSTR), and the recognition of how the theory and experiment impinged on the stability issues that were simultaneously exercising chemical engineers' minds in chemical reactor theory. The pioneering application of CSTRs to low temperature hydrocarbon oxidation and cool flames (previous experiments had been performed only by Frank-Kamenetskii and by Denbigh), and the coupled measurements of many key chemical species and the gas temperature, revealed the essential thermokinetic origin of the phenomena and led to early computational modelling of these systems at Leeds. The numerical work continues today, through Alison Tomlin in particular, in the development and application of formal mathematical model reduction techniques which allow the kinetics to be combined with equations for complex fluid flow. Studies of autoignition in a rapid compression machine also began at Leeds during the 1970s, so fashionable now with regard to gasoline combustion and related fuels, but in that first instance connected with spontaneous ignition hazards during the use of isopropyl nitrate as a rocket propellant - so revisiting your earliest combustion research.

1967 became a "vintage year" for combustion in Leeds. It was in that year that the Centre for Combustion came into being in Leeds University as a consortium drawn from the combustion interests within the Departments of Fuel Science, Mechanical Engineering, and Physical Chemistry, for the purpose of post-graduate teaching through the MSc in Combustion, and for joint research. This was something of a "first" and some of us recall you in your formidable way arguing for post graduate education in combustion at an EU Meeting in Brussels. All this would not have happened, but for your considerable efforts as one of the prime movers of the venture. Not only did you draw most of the staff members from Physical Chemistry named above, into it, but your lead was picked up with enthusiasm by those in the other Departments. Throughout most of its existence, 20 - 30 tenured academics have been engaged in the Centre's activities. The Centre has been an outstanding manifestation of the late Vice-Chancellor, Lord Boyle's characterisation of University education as "teaching in an atmosphere of research", and it is a legacy to be cherished in the present climate.

Thanks again to you, the chemical kinetic activities in Physical Chemistry at Leeds also took a remarkable turn. No combustion kinetics modeller anywhere in the world, and others too, would be able to practice his or her craft without invoking the critical assessments of rate data by Don Baulch and numerous co-workers (which are dated 2006 in the most recent publications). However, the lineage goes back to a Research Council review of high temperature processes, which prompted the Office of Scientific and Technical Information (OSTI) to commission the rate data assessments, published as the blue-bound "Butterworth" tomes initially (1972, '73 and '76) with Dougal Drysdale as one of the first authors, subsequently to become Head of the Fire Safety Engineering Group at Edinburgh University. The work was directed by Don from 1967, but it would not have come about had you not seized the opportunity to set up the rate data assessment project. There's a contribution to combustion that is hard to beat!

Your interest and activity in non-linear dynamics, largely in the context of temporal phenomena in the spatially uniform environment, flourished throughout the 1970s and '80s, during which attention was turned to H2 + O2 and CO + O2, the reactions from which the understanding of chain branching theory had originally emerged in the 1930s through the work of Semenov and Hinshelwood. As quite strongly exothermic reactions, both of these systems exhibit interesting non-isothermal phenomena, and these were studied experimentally, using the small-scale, well-stirred flow reactors, and interpreted both analytically and numerically. However, under very carefully controlled conditions the CO + O2 system also displays isothermal oscillations and the Leeds investigations of these phenomena brought clarity where previously a degree of confusion had prevailed (see Chapter 5 in Comprehensive Chemical Kinetics, vol. 35, S.K. Scott).

No doubt the kinetic simplicity of the gas-phase oscillators spurred you into theoretical investigations of the simplest kinetic structures from which oscillations and other instabilities could emerge. This interest coincided with another timely and long-standing association, with Steve Scott, leading to an astonishingly prolific period from which came remarkably lucid insights into stability criteria, bifurcation phenomena and the enormously rich patterns of behaviour that are possible through the elegantly simple kinetic interactions of the "Gray and Scott models". Your interest in travelling waves in autocatalytic systems also developed in this period, and the main achievements culminated in publication of the text book Chemical Oscillations and Instabilities (P. Gray and S.K. Scott). Specifically, you had recognised the similarities between the equations governing the evolution of simple propagating flames and those for isothermal chemical wave fronts, where your familiarity with the Soviet as well as the Western literature (helped by the association with Terry Boddington) paid dividends. You conjectured that the instabilities that arise for flames would appear in autocatalytic chemical fronts and that spatial structures such as target patterns and spirals seen in, for instance, the solution-phase Belouzov-Zhabotinsky reaction might occur in flame systems. This conjecture has subsequently been confirmed theoretically and experimentally by other groups, including that of Ken Showalter (University of West Virginia) who became a regular visitor to Leeds, with his family. The links between Physical Chemistry and Applied Maths, which you had fostored from the early '70s through the connections with David Crighton, John Merkin and John Brindley, added considerably to the achievement during this period.

Although, in 1988, you had accepted the prestigious invitation to leave Leeds and become Master of your former Cambridge College, Gonville and Caius, it did not constitute a swansong to your scientific interest and endeavour. You extended your exploration of non-linear interactions in simple kinetic systems, especially involving spatial structure, and you continued to be productive into the present millennium. Perhaps that you were working on the equations made famous by RA Fisher, the mathematical statistician and geneticist who was also a Fellow of Gonville and Caius College and eventually its President, gave you additional pleasure. During this latter period you have been able also to renew your early association with the Department of Chemical Engineering, through the auspices of John Davidson, and have contributed to post-graduate teaching in the Department. Meanwhile you were also very much involved in the University's administration, being, amongst other things, Chairman of the Faculty Board of Engineering, as well as a member of both the Council of the Senate and the Financial Board.

The Combustion Institute and the British Section

In addition to being invited to present the plenary lecture at the Twenty-third Symposium (1990), entitled "Chemistry and Combustion", you had the considerable distinction of being awarded the Bernard Lewis Gold Medal, in 1978, with its citation "for brilliant research in the field of combustion, particularly on theoretical and experimental thermochemistry of combustion". This honour was bestowed barely more than half way through your prolific career. You have been a consistent contributor to the International Combustion Symposia over a very long period and there are 18 such papers amongst your enormous publications list. The first of these appeared in the Fifth Symposium (1958).

Your first became a member of the British Section Committee in the period 1974 - 82, and this led to your role as "Host Chairman" for the 17th International Combustion Symposium, when it was held at Leeds University in 1978. Felix Weinberg was the Chairman of the British Section at that time. After a brief period of retirement from office, you were re-elected to the Committee as its Chairman, in 1986, and served in this post until 1992. These significant contributions to Section affairs over very many years were recognised through your nomination as a life member of the Committee. In return you continue to attend meetings and to make your usual incisive contributions to the workings of the Section. You have also served on the Editorial Board of Combustion and Flame.

Honours, distinctions and awards

As a mark of your considerable distinction you have received many academic awards of merit from your earliest days as an undergraduate at Gonville and Caius College. Subsequently you have been the recipient of medals from the Royal Society of Chemistry (Meldola medal, 1956) and from the Faraday Society (Marlow medal, 1959). You received the Italgas prize for chemistry in 1988. You were elected as a Fellow of the Royal Society in 1977.

You have performed many professional duties in a wider context as a physical chemist, with a particular commitment to the Faraday Society. You were a Member of Council from 1965, a Vice-President from 1970, its Treasurer from 1973 and President, 1983-5. You have held many visiting professorships, including those with very close friends at the Universities of Göttingen and Louvain-la-Neuve. You have also served on a number of Ministry of Defence committees and as Visitor to the Fire Research Station, BRE

Throughout your career you have been a colleague, mentor and friend to an enormous number of people, and many lives have been enriched by this delightful association. But these recollections are also tinged with sadness because your first wife, Barbara, was also a warm, caring friend of so many of us, and a fine academic herself, who steadfastly supported you in all of your endeavours. Barbara's death in 1992 was a huge loss to you and many others, but we know that support has come in your subsequent marriage to Rachel. We wish you and Rachel every happiness for many years to come.

Don Baulch, Terry Boddington, Derek Bradley, Martin Braithwaite, John Brindley, John Davidson, Graham Dixon-Lewis, Dougal Drysdale, Allan Hayhurst, Clifford Jones, John Griffiths, Chris Lawn, Andy McIntosh, Mike Pilling, Steve Scott, David Smith, Philip Thomas, Pierre van Tiggelen, Heinz Wagner, Felix Weinberg, Alan Williams.

These recollections were compiled mainly through conversations followed up by email exchanges to iterate to the final form. In addition, I did receive very warm letters from Philip Thomas, Graham Dixon-Lewis and David Smith, which are reproduced below.

John Griffiths

Extract from Philip Thomas's letter

I am a contemporary of Peter and I was amongst those studying the subject of hot spots on solid materials. My contact with him on gaseous matters was virtually non-existent. I left the lab and entered the scientific civil service, losing all contact with Peter, so I was pleasantly surprised when I had a letter from him referring to my paper in the Journal of the Faraday society. It was this that led to the long-term relationship that still exists.

From Graham Dixon-Lewis

My association with Peter dates back to the 1930s, when we both attended Newport High School - myself being Peter's senior there by some four years. Our paths after leaving school diverged for some years, when Peter went to Gonville and Caius College, Cambridge in 1944, and myself to Oxford in 1940. Peter's distinguished early career at Cambridge is already recorded above.

In 1955 our paths reconverged when Peter was appointed Lecturer in the School of Chemistry at Leeds. Prior to this I had myself moved to the then Department of Coal Gas and Fuel Industries, in 1953. A further happy circumstance is that our interests have lain in complementary aspects of the field of combustion, so that we now became colleagues and friends for many years.

Peter was appointed Professor of Physical Chemistry in 1965, and his numerous major achievements both before that date and subsequently have already been outlined, both in the matter of the distinguished publications in which he has been involved, and in the initiation of the inter-departmental Centre for Combustion, as well as the kinetic rate data assessment project at Leeds. This last has been of particular value to my own kinetic modelling work, as also have the measurements of the diffusion coefficient of H atoms.

Finally, at the personal level, may I add how helpful Peter has been throughout our association, both as a friend and colleague. For my part, and as a fellow alumnus of earlier years, I am delighted and honoured to be able to take part in this very timely tribute.

Thank you, Peter, and best wishes for the future

From David Smith

When I graduated in 1963 in Chemistry at Leeds, I wanted to stay on to do research and was assigned to Peter Gray's Group. I well remember that first meeting when he outlined possible projects and asked for my preference. I had none, so he put me down for a combustion project on the measurement of flame speeds for binary and ternary mixtures of NH3 and H2 as fuels with O2, N2O and NO as oxidants. Some mixes had already been done by a previous students.

We used spherically propagating flames. The techniques have recently become very popular again, though with the important difference that flame stretch effects, of which we then had no appreciation, are now routinely included. Another difference from current practice is that we used a rotating drum camera, as cine-cameras then were either not fast enough, or hugely expensive. A schlieren method made flame position more distinct and, by placing a slot between flame and camera, flame progress showed as a V-shaped trace whose slope gave the flame speed. Today, computer programs allow computation of flame speeds, permitting comparisons with measured and hence detailed conclusions to be drawn about flame chemistry. Then we could only draw much vaguer conclusions - perhaps more fun and more mentally taxing!

Given his contacts with Government scientists, Peter arranged for me to visit the Rocket Propulsion Establishment at Westcott in Buckinghamshire. (This was at a period when UK aimed to develop its own independent rocket.) I went with another student (Alan Jones), so when we arrived and announced ourselves to Security as Smith and Jones, we received a very dusty look. But a quick check in the register showed we were kosher. Through Peter's contacts, and also because I had a cousin who worked at Westcott, I was able to make a second visit there to witness a rocket test. Boy was it loud - more a pressing on the chest than in the ears (though not quite as loud as a "Who" concert that I went to some years later).

I am grateful to Peter for teaching me research skills and for introducing me to the combustion world. He set me off on the combustion path and so has been responsible for my career and professional interest over 40 years. Thank you, Peter.