The air we breathe is polluted by many small particles, particularly in urban industrial areas. These particles are usually too small to be seen individually but in sufficient concentration (104 - 106 particles per cm3) they can cause the atmosphere to become hazy. Then, buildings become discoloured by the deposition of airborne particles and people suffer respiratory illnesses. In order to tackle these problems, it is necessary to understand how the particles form, how they are transported from place to place and how they are deposited on solid surfaces.
Research is being undertaken to look at how droplets and particles are nucleated from supersaturated vapours. The physics of nucleation is not too well understood even for single component substances (like pure water droplets nucleating from supersaturated steam) and the problems are compounded when other chemicals, such as sulphuric acid, are present. Nevertheless, the very small particles that do form are extremely detrimental when inhaled as they are acidic and of a size that can penetrate deep into the lungs. We are studying the theory of nucleation to try and provide better theoretical and computational models to describe the formation and initial stages of growth of these tiny particles.
In many industrial processes, particularly those involving turbomachinery, combustion equipment and heat exchangers, very small particles are transported by a flow of gas. Sometimes this is deliberate and sometimes it is not but in either case some particles become deposited onto solid surfaces. The economic implications of particle deposition, particularly in engineering equipment used for power generation, are considerable. Gas turbines, for example, must be fired by natural gas which is a clean fuel. Coal cannot be used because the ash particles deposit on the turbine blades causing blockage, reduced efficiency and erosion damage. If this problem could be overcome then the world's plentiful supply of coal could be used for gas turbine power generation in preference to the more precious reserves of natural gas. Another example concerns particle deposition in heat transfer equipment, a process which invariably results in reduced performance and often in serious corrosion damage. In the UK, the cost of cleaning and de-scaling heat transfer equipment is estimated to be about 0.5% of the nation's GDP.
In order to improve our understanding of the physical mechanisms responsible for deposition, research is being carried out on the behaviour of small particles in turbulent flows near solid surfaces. This is an extremely complicated and challenging two-phase flow situation and the rate of deposition depends on many factors including the size, shape and composition of the particles, the structure of the turbulence in the gas flow near the surface and the way it interacts with the particles, the roughness of the surface itself, and the local temperature gradients if the surface is heated or cooled. We are tackling the problem by mathematical modelling of the physical processes involved, analytical and numerical solution of the resulting equations, and verification by experimental measurements.