# Review on the Recent Numerical Studies of Liquid Atomization
## 8 Summary and Outlook  
Theoretically, the interface capturing method coupled with the DNS turbulence model is able to precisely replicate the observed atomization phenomena in any scale of all kinds of liquid and gas layout and shapes of nozzle. This is because it is based upon conservative fundamentals of mass, momentum, and energy, relying on no additional assumptions or empirical correlations. To acquire precise enough interface structures, the turbulence in the Kolmogorov scale has to be resolved, and the grid size must be in the same magnitude. The turbulence of both near and far away from the interface may produce a strong enough disturbance to transform interface topology and shape, so the method of adaptive mesh refinement based on the gradient of interface area density is sometimes not adequate. Moreover, the Kolmogorov scale decreases with increasing flow velocity, which further brings down the scale that has to be involved for liquid atomization as a well atomized flow intrinsically need high speed. Limited by the current computer technology, the interface capturing method can only cover a small range that is a portion of the primary region near the nozzle exit. This method is now mainly for exploring atomization details, which are too fast and small scale to conduct experiment observation, and currently not suitable for engineering design purposes even with the supercomputer center. As a result, one research orientation of this method is attributed to developing high-performance computers, building clusters, and improving efficiency of parallel computing. Another research orientation is improving the computer iteration algorithms in order to save computational effort. For the particle tracking method, the gas is treated in a Eulerian manner usually with a RANS-type turbulence model, requiring much coarser grids than the interface capturing method. The liquid, whether large liquid core or small liquid ligaments, is treated as blobs, parcels, or droplets tracked in the Lagrangian framework, which does not need grids. Therefore, it consumes realizable computational effort even with a personal computer and can predict full primary and secondary atomization in a broader range within a realizable amount of time. This method has been applied during the design of nozzles in the cooling tower, irrigation, and dust removal. However, since the instantaneous local interface is not constructed, the primary and secondary breakup of the liquid phase must be realized through constitutive models, which are actually semi-empirical formulations defining the transition rates between liquid core and droplets as well as among droplets of varying sizes. It has been clarified that turbulence kinetic energy, surface tension, viscosity, and aerodynamic forces are the main factors determining the disintegrating rates, upon which various models were established. However, there remain two aspects opening for further research. Firstly, the accuracy of those constitutive models generating primary and secondary breakup rates needs to be improved to fit various layouts of atomization, speeds, and properties of liquid and gas and nozzle shapes because if those disintegrating and coalescences models need to be tuned for each kind of atomization, their level of confidence would be greatly compromised. This can be performed via thorough quantitative understanding of the controversial mechanisms that promote or suppress the breakup potential. Secondly, as the particle tracking method is able to cover a wider region of atomization, the collisions and coalescences of droplets far downstream probably occur and influence the distribution of droplet velocity and size severely. This phenomenon has not been fully considered and there are rarely models involved in the atomization process. The hybrid method of interface capturing and particle tracking avoids a majority of the empirical models that particle tracking requires while making use of the initial droplets distribution from the interface capturing method.