Michael W. Reeks School of Mechanical and Systems Engineering Newcastle University, Newcastle upon Tyne, UK Presentation Title: The development and application of the PDF approach for modelling dispersed particle flows Abstract: Dispersed flows of droplets and small particles abound in nature from clouds, mists and fogs to the long-range transport of fine dust released in desert storms or in volcanic eruptions. They control the weather and influence the climate. They play a key role in many industrial energy processes from spray drying, pneumatic conveying and fluidized beds, to coal gasification and mixing and combustion processes. They can have a profound effect on our health and quality of life. Understanding their behaviour through modelling and experiment is therefore important in our control of the environment, improving our health, and in the design and improvement of industrial processes. An elegant and computationally efficient way of modelling such flows is to treat the dispersed phase of particles as a fluid in much the same way as the carrier phase is treated as a fluid in terms of a set of continuum equations and constitutive relations. This Freeman Scholar review lecture is about the development and application of a PDF approach for the treatment of such flows as fluids and how it at the same time provides a general theoretical framework in which the behaviour of a reacting turbulent dispersed flow of particles can be formulated. So in addition to providing the continuum equations of the dispersed phase, it can properly account for particle-wall interactions that constitute the natural boundary conditions of the flow, to the growth of the size distribution of individual particles in a turbulent flow, and to two-way momentum coupling between the dispersed and carrier phases. Over the last 15 years, there has been a considerable interest in the way particles interact with the small scale structures in a turbulent flow and how this leads to segregation and demixing of the particles and enhanced particle collision rates. Whilst DNS studies have identified a number of interesting features associated with this demixing process e.g. the existence of caustics and singularities in the particle concentration field and the occurrence of random uncorrelated motion (RUM), these features are not an intrinsic part of a traditional two-fluid model. I will show how these features form part of a generalised PDF formulation by extending the PDF approach from one particle to two particle transport, leading eventually to the calculation of enhanced collision rates in a similar way to deposition of inertial particles in turbulent boundary layers. In this review I will trace its historical development over the last 30 years, showing how it has resolved some of the outstanding problems in particle transport in turbulent flows and how it has been used to tackle practical engineering problems as diverse as emissions of radioactive aerosols from nuclear reactors to the collision and precipitation of water droplets in clouds. Indeed the approach provides a basic framework for formulating a whole range of industrial and environmental particulate flows. In the process I will highlight its successes, where it has added insight and understanding and where the challenges lie in the future. Biography: Since 2004 Michael Reeks has been Professor in Multiphase Flow in the School of Mechanical & Systems Engineering at Newcastle University, UK. He received his B.Sc. (1967) and Ph(1971) in Physics at the University of Birmingham. Most of his career has been spent in the UK Nuclear Industry (1973-1998) as a research engineer at CEGB Berkeley Nuclear Labs where he was head of aerosol physics and containment thermal hydraulics and latterly, university research programs manager. Throughout that time he worked on safety related problems involving the release of in-circuit fission products and radioactive aerosols in gas cooled reactors and PWRs; in particular he pioneered the development of a kinetic theory for particle transport in turbulent gas flows forming the basis of nuclear industry approved safety codes for radioactive releases (PDF modeling being an underlying theme to all his theoretical work since that time). From 1998-2004 he was a visiting scientist / senior Marie Curie Research Fellow at the European Joint Research Centre (Italy) being involved in large scale nuclear aerosol experiments (PHEBUS and STORM). During his time at Newcastle, he has extended his kinetic theory to include the influence of turbulence on particle collisions and its application to non-nuclear processes from lung deposition, to droplet growth and transport in clouds. He is a Fellow of the Institutes of Physics and Mechanical Engineers, and a Member of ASME. He was formerly an Assoc Editor of J. Fluids Eng. and is currently an Editor J. Flow Turbulence & Combustion, a member of Multiphase Flow Governing Board and a visiting professor at Ecole Central de Lyon, IMFT Toulouse, U Tokyo and U. Florida. He has given seminars/ key note/ plenary lectures at numerous conferences, research institutes (he recently gave the Knox Millsapps Memorial Lecture at Florida University). He has organized international conferences, workshops, summer schools, is a member of PHEBUS scientific committee, a consultant to British Energy and UK Nuclear Decommissioning Authority. He is the author of over 150 peer reviewed journal papers, reports, and contributed to articles in an encyclopedia and handbook on multiphase flow.