Hot-film anemometer measurements in a starting turbulent jet

Introduction I has become common practice when modeling diesel engine combustion to assume that a two-phase fuel spray can be treated as a homogeneous, quasisteady turbulent gas jet.The physical description used to support this assumption proposes that the atomization process is rapid when the liquid-jet velocity is sufficiently high. Since finely dispersed droplets will decelerate rapidly, the velocity of the droplets will quickly become accommodated to the velocity of the air entrained by the jet. Thus, at a short distance from the jet orifice the fuel spray will behave as a classical quasisteady free-air jet that is carrying with it continually evaporating fuel droplets. Considering how often this simple model is used, it is surprising that quantitative verification has not been even partially established. It is admittedly difficult to confirm an analogy between two-phase sprays and homogeneous gas jets; however, the assumption of treating a suddenly started transient jet as a quasisteady free jet is amenable to verification. The experiment briefly described in this Note addresses this question, in addition to providing some measurements on the tip penetration rate of suddenly started turbulent air jets. Further details can be found in Ref. 3.