Fundamental Principles and Definitions

Drops and bubbles exist in a large number of natural as well as man-made systems. In nearly every situation, these fluid particles, which may lie within a continuum of another fluid, or another state of the same fluid, have an important bearing on the physical behavior of the system. For example, clouds are natural assemblages of tiny water droplets which coalesce due to changes in the atmosphere and lead to rainfall or other forms of precipitation. Natural water systems such as lakes and oceans contain air in dissolved form as well as bubbles, and make up a component that is essential to marine life. On the other hand, with industrial systems, such as some nuclear power plants, one encounters bubbles in a boiling water reactor and drops in spray cooling components. In chemical reactors, drops and bubbles commonly occur as carriers of both reactants and products. Internal combustion engines and jet engines utilize sprays of atomized liquid hydrocarbons as fuel. With most of these systems, important physical phenomena arise through the transport of heat and/or mass which may be accompanied by fluid motion which generally serves to enhance the transport. In cases involving phase change, fluid motion may be additionally generated by the transfer process. With most processes involving drops and bubbles, there is relative motion between particles, as well as between the fluid particles and the surrounding medium. In this book, transfer processes associated with drops and bubbles under various physical circumstances will be discussed.