Perspectives on statistical modeling of turbulent liquid jet breakup in combustion systems

The problem of turbulent liquid breakup refers to the disintegration of a turbulent liquid jet owing to competing effects of aerodynamic forces, surface tension forces and turbulence when injected into a stationary or moving ambient gas. Owing to the difficulty in predictively modeling breakup of turbulent liquid jets, this phenomenon is considered to be an unsolved problem in the field of fluid mechanics, and is perhaps more challenging to model than the problem of single-phase turbulence. Analytical approaches to model liquid jet breakup have focussed on attempting to correlate the wavelength of interfacial instabilities on characteristic drop sizes that are produced upon breakup. Performing high-fidelity experimental studies of liquid jet breakup has been challenging as the flow field inside and in the vicinity of the injector has been inaccessible to accurate measurements. Still, experiments have been able to quantify the role of various non-dimensional groups associated with liquid jet breakup on the break up regimes and characteristic drop sizes produced upon liquid breakup. In addition, experiments have reported that nozzle geometry and in-nozzle cavitation affect the break up behavior of the liquid jet. However, such experimental and analytical studies are yet to result in a predictive model for liquid jet breakup that can be employed in numerical calculations of multiphase turbulent reactive flows with sprays. The goal of a liquid breakup model depends closely on the application under consideration. For instance, in the ink-jet printing or spray painting industry the objective is to achieve precise control on the final drop size, and so the goal of a liquid breakup model in this context is appropriately a drop-size distribution, for a given set of flow parameters inside the injector. The effect of each droplet or stream of droplets on the ambient environment may be of secondary importance. Historically, the goal of a liquid breakup model for fuel sprays in combustion systems such as internal combustion engines and gas turbine combustors has been to model the distribution of drop sizes that is observed in experiments. Several models have been proposed which, after considering various purported mechanisms of breakup of a coherent liquid jet, predict a characteristic drop size. However, in internal combustion engines, the evolution of the fuel spray and the gas phase are strongly coupled through the interphase transfer processes. These processes can have a non-negligible influence on the ambient gas in the vicinity of the spray which could also modify the flow field several diameters downstream. A breakup model that provides drop sizes as an output may not necessarily capture these important interphase transfer processes in a predictive manner. Breakup of liquid jets is normally classified into two distinct phases: a primary breakup phase where the first set of dispersed phase elements (DPEs) – a dispersed phase element is a generic term used to denote separated liquid structures, ligaments or drops – are