Effects of Bubbles, Droplets or Particles on Heat Transfer in Turbulent Channel Flows

Adding small immiscible substances of low volume to turbulent channel water flow modifies turbulence and turbulent heat transfer. We examine the effects on the modifications of the ratios of the physical properties of substances to the physical properties of the carrier fluid, the particle Reynolds number, the Stokes number and the Weber number. We discuss the applicability and importance of the local, instantaneous values of these non-dimensional numbers. In particular, the maxima and minima for the time change in the Stokes number are found to correspond to the minima and maxima respectively for the time change of the wall-normal distance of a heavy, solid particle in turbulent flow in a vertical channel. We also investigate the effectiveness of the recent developments of particle-tracking velocimetry and particle-resolved direct numerical simulation, in increasing the understanding of changes in turbulence.