| A study has been conducted to investigate secondary droplet breakup under conditions where the flow breaking the drops changes during the breakup process. An acoustically driven wind tunnel produced the periodically fluctuating flows used in this study. It superimposed axial acoustic waves of controlled frequency and amplitude onto a mean flow. The wave's pressure amplitude can exceed 10% of the ambient pressure and its velocity amplitude can be nearly 100% of the mean flow velocity. A droplet generator capable of injecting monodisperse droplets into this strongly fluctuating flow field was also developed.; Droplets of methanol or water were injected into different flow fields. Their breakup response was recorded photographically. The effects of the frequency ratio, fa/fn, (fa = acoustic wave frequency, fn = drop natural frequency) and the acoustic Weber number (We acs = ρu′2d/σ, u′ = acoustic velocity amplitude) on the breakup process were investigated over the experimental domain 0.044 < fa/fn < 0.468 and 2.1 < Weacs < 15.25. Additionally a computer analysis, using the Taylor Analogy Breakup (TAB) model, extended the range of conditions examined beyond those obtainable experimentally. Breakup response was characterized by the mean velocity critical Weber number (WeM = ρU2d/σ, U = wind tunnel mean velocity), and the peak-to-peak variation of WeM, denoted Wep-p.; Drops broke up via previously observed modes (e.g. bag). Their critical WeM depended on the phase (&phis;) of the acoustic wave at which they were observed. The parameter Wep-p was found, both from experiments and TAB modeling, to depend on Weacs and fa/fn . For fa/fn < 0.3 “quasisteady” behavior was observed. For quasisteady response Wep-p scales with Weacs and the drops respond to the total velocity present at the moment of their injection as if it were unchanging, breaking when We T ≈ 12 (WeT = ρ(U+u′) 2d/σ). At higher values of fa/fn the drops respond primarily to the mean velocity and break at WeM ≈ 12 regardless of &phis;. Hence the value of Wep-p is reduced at a given Weacs. In the quasisteady regime the TAB model and experimental results closely match, above fa/fn ≈ 0.3 the agreement decreases. |
