Numerical investigation of the aerodynamic breakup of droplets in tandem

Abstract

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights Numerical examination of the aerodynamic breakup of four Diesel droplets in tandem formation at representative engine conditions. Identification of a new breakup mode in the multi-mode regime, termed as "shuttlecock". Droplet proximity becomes important for non-dimensional distances between the droplets lower than 9. Drag coefficient and liquid surface area of the "representative chain droplet" are lower than the corresponding ones of the isolated droplet; breakup initiation time is longer and the critical We number is higher. Proposed correlations to predict the drag coefficient, liquid surface area, breakup initiation time and critical We of the "representative chain droplet".

Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster

“…As it is observed, the droplet in the cluster formation initially deforms into a disklike shape (t/tsh=0.4), followed by a semi-spherical shape (t/tsh=0.8); breakup occurs with stripping of liquid from its periphery (t/tsh=1.5). This breakup mode is called shuttlecock and has been identified in the authors' previous work for the breakup of droplets in tandem formation [4]; it is characterized by a large streamwise droplet deformation, while most of the liquid volume remains at its core. Turning now to the isolated droplet, it experiences the well-known multi-bag breakup regime, in which the droplet gradually deforms into a disk-like shape followed by the creation of a bag at its periphery (not shown here).…”

“…These conditions correspond to those encountered in Diesel enginesas presented in Table 1, along with the corresponding references used for their estimation. It should be noted that at such high air temperature, heating and evaporation of the droplets takes also place, but these were neglected in the current work, since its primary scope lies on the investigation of the effect of droplet proximity (similar to our previous work [38]). In view of that, any variations of droplet physical properties with temperature, including that of surface tension, were neglected, as the flow was considered to be isothermal.…”

“…Two-dimensional (2-D) axisymmetric and three-dimensional (3-D) simulations are performed with the commercial CFD tool ANSYS FLUENT v16 [22] along with the use of various User Defined Functions (UDFs); these account for the following: i) adaptive local grid refinement technique around the liquid-gas interface [23], ii) adaptive time-step scheme for the implicit VOF solver based on the velocity at the droplet interface [24], and iii) moving mesh technique based on the average velocity of the droplets. The CFD model has been developed and validated in previous works of the authors for a number of applications; among them are the free fall of a droplet [23], the droplet impingement on a flat wall [25] or a spherical particle [26][27][28], the aerodynamic droplet breakup [4,24,[29][30][31][32][33][34][35] and the droplet evaporation [24,31,36]. It should be noted that he extension of the model validation for the case of droplet clusters is not possible since, to the author's best of knowledge, there are no experimental studies in the literature with droplet clusters, only a few featuring two droplets [5,15,16].…”

“…The droplet surface area is an important quantity for spray applications and is calculated in the CFD simulations as = ∑ |∇ |, which has been utilized also in [36,38,45,46] and is derived using the divergence theorem (or Gauss theorem) for the volume fraction at the interface cells. The ratio of the maximum surface area of a droplet in a parallel moving cluster, to the maximum surface area of an isolated droplet (Smax,cl/Smax,is) is presented in Figure 11.…”

“…(2) The aerodynamic breakup of isolated droplets has been thoroughly investigated (see for example [2] among many others); more recently the breakup of droplets in tandem formations has been also reported [4][5][6]. On the other hand, not much focus has been paid to the breakup of droplets moving in parallel with respect to the surrounding air; such collective droplet cluster motions can be considered more representative for the conditions typically realised in fuel sprays consisting of a large number of droplets [1].…”