Abstract
The effect of pressure on the evaporation lifetime, the ignition delay, and the burning and combustion time of single liquid fuel drops on a heated surface are investigated in a pressure chamber; the fuels used are the four engine reference fuels, viz., n-Heptane, n-Hexadecane, 2,2,4-Trimethylpentane (iso-octane), and a-methylnaphthalene; the pressures used range from atmospheric pressure to 69 atmospheres. Also, the effect of pressure on the 'cleanliness' of combustion of single n-Hexadecane drops burning on a heated surface is investigated. Theoretical expressions are derived to predict the evaporation lifetime of a liquid drop on a heated surface in three important modes of evaporation, viz., true contact evaporation, evaporation in the Maximum Evaporation Rate Range and spheroidal evaporation The Transition Temperature is defined as the surface temperature at which physical and chemical factors have the same influence on the ignition delay of a fuel drop. Allowing for the variation of the S.I.T. with pressure, in no case is a Transition Temperature of the fuels used more than 100 deg.C above the S.I.T. At higher surface temperatures than a Transition Temperature, the ignition delay is largely determined by physical factors. A new theoretical concept of the spontaneous ignition and ignition delay of a fuel drop is introduced; diffusion is an integral part of this concept. This concept predicts the effect of pressure on ignition delay. Excepting spheroidal evaporation, a flame presence has a small influence on the evaporation lifetime of a drop on a heated surface. It is shown that the evaporation lifetime of a drop on a heated surface may be much less than the combustion time. Physical factors are primarily responsible for the dirty combustion of a n-Hexadecane drop. It is shown that the dirtiest combustion is obtained when the air temperature and pressure are close to and below the critical temperature and pressure of the fuel; clean combustion is obtained at reduced pressures greater than approximately four.