Flow and Heat Transfer Characteristics of Impinging Transient Diesel Sprays

The spatial and temporal characteristics of transient diesel sprays impinging on unheated and heated walls were investigated by phase-Doppler anemometry (PDA) and the heat-transfer distribution in the vicinity of the impingement region was determined by fast response thermocouples. The results have provided quantitative evidence about the effect that the presence of the flat wall exerts on the spray characteristics. For example, independent of the thickness of the liquid film, the wall rearranges the droplet size distribution of the free spray with droplet collision and coalescence playing an important role in both the droplet redistribution and in the development of the wall-jet. Droplet sizes were reduced and mean tangential velocities increased with wall temperature at the upstream side and at the front of the wall-jet, respectively. The wall temperature also affected the rearrangement of the droplet size distribution by reducing droplet coalescence at the leading edge of the wall-jet and enhancing it after the passage of the head vortex. The pre-impingement droplet velocities were found to play the dominant role on the local, time-resolved heat transfer rates; the higher the pre-impingement axial droplet velocities are, the higher the heat transfer rate. A correlation for the instantaneous and spatially-resolved spray/wall heat transfer was thus developed, based on a Nu-Re-Pr-We number relationship, which can be useful for multi-dimensional computer simulations of diesel engine combustion.