Improving the validation of turbulent jet breakup models

Abstract

Understanding the physics of the breakup of turbulent liquid jets is important for a variety of applications including engine sprays, fire suppression systems, and water jet cutting. Models of turbulent jet breakup allow predictions of quantities of interest like the droplet size distribution and breakup length of the jet. These models are compared against experimental data in a process called validation. If the model predictions are within the experimental uncertainty, then the model is "validated" and believed to be accurate, and possibly can explain the physics. Uncertainty quantification is necessary for model validation. While unfortunately relatively few experimental studies quantify uncertainty, that is not the most pressing validation issue in turbulent jet breakup. I detail 3 additional problems that can make the apparent validation of a model actually an illusion, regardless of how well the model appears to match the data. These problems include: 1. important variables being omitted or guessed in experiments and models, 2. confounding between independent variables, that is, two variables changing simultaneously, making determining cause and effect impossible, and 3. testing only combinations of submodels and not each submodel in isolation. To avoid these problems and others, I developed validation guidelines that are detailed in this work. Following these guidelines, I compiled a large experimental database. Only 28 out of 47 experimental studies considered met my data quality guidelines. Only 18 studies had quantified uncertainty, and only 3 studies had substantial variation in the turbulence intensity.

Reevaluating the jet breakup regime diagram

“…Many past researchers believe that breakup in these regimes is caused mainly by ambient gas effects (Dumouchel, 2008;Lin and Reitz, 1998;and Reitz, 1978), but my own analyses cast doubt on these assertions. To be more specific, in the "second wind-induced" (i.e., turbulent surface breakup) regime I find no influence of the ambient gas at all Trettel (2019aTrettel ( , 2020a, which makes the "wind-induced" name seem inappropriate. Sallam (2002, p. 94) agrees with this assessment.…”

“…The regressions developed in this work will be presented without discussing how they were obtained. The reader is referred to Trettel (2019aTrettel ( , 2020a to understand the choice of the functional forms. Grant and Middleman (1966, fig.…”

“…3), which reduces the breakup length, just like low density ratios. Problems like this are called "confounding" and is discussed more in Trettel (2019a).…”

“…Using the turbulent kinetic energy, the turbulence intensity can be defined as Tu 0 ≡ (2k 0 /3) 1/2 /U 0 . The motivation behind this particular definition of the turbulence intensity is described by Trettel (2019aTrettel ( , 2020a.…”

Turbulent theory of velocity-profile-induced jet breakup

“…(A "plane-averaged" turbulence intensity in this work is defined as Tu 0 ≡ 2k 0 /(3U 2 0 ) where k 0 is a planeaveraged turbulent kinetic energy.) I discuss confounding in detail in another paper at this conference [18]. Essentially, both the turbulence intensity and velocity profile are changing when the nozzle length changes [19].…”