The role of vaporization in determining transient diesel spray structure

Abstract

This paper examines how vaporization changes the structure of a non-vaporizing spray. The larger question addressed in this work is the following: How are the penetration, spreading rate, and entrainment of a vaporizing spray different from those of a non-vaporizing spray under diesel engine operating conditions, when the injected mass and momentum flow rates are the same for both? Correlations for these parameters derived for non-vaporizing sprays from measurements in constant-volume chamber are often employed to explain the behavior of vaporizing sprays in diesel engines. In fact, they appear to explain results rather well. How then, if at all, does vaporization influence penetration, spreading rate, and entrainment? Similarly, vapor jets injected with the same mass and momentum flow rates as the vaporizing sprays have been shown to give penetration and spreading rates which are comparable to those of the sprays although the latent heat of vaporization has not been included in specifying the internal energy of the vapor. Why? These questions are addressed in this work carried out in a constant-volume chamber for the SANDIA baseline n-heptane spray (http://www.ca.sandia.gov/ecn). This spray has been the subject of a prior study by the authors. Details of vaporizing and non-vaporizing sprays, and corresponding vapor jets, are examined in order to provide insight into the physics of the spray, whose effects are ultimately reflected in the global parameters such as penetration and spreading. The influence of drop size, and ambient conditions on the conclusions is critically evaluated. The interplay of changes in temperature and density in the core of the spray with momentum exchange and entrainment is discussed as part of the explanation of the observed results.