Simulation of flow pattern transitions in the Euler-Euler framework

Two-phase flows occurring in nature or industrial applications frequently involve gas-liquid interfaces which vary over a wide range of scales. Simultaneously, within one flow domain there might be very small bubbles or droplets, but also large interfaces as e.g. caused by stratification due to gravity. In addition transitions between these different morphologies may occur such as bubble entrainment by jets or breaking waves, droplet generation from wave crests or the generation of large gas structures out of smaller ones by coalescence. Such flow situations are very challenging from the modelling point of view. At least for medium and large size flow domains it is not possible to resolve all interfacial scales down to the smallest ones because this would lead to a number of cells of the numerical grid which would exceed todays computing capacity by far. Consequently there will be interfaces smaller and larger than the computational grid. Clearly the smaller ones should be considered by appropriate sub-grid models while the larger ones should be simulated.
Up to now there is no CFD approach established for such flow situations. One promising approach is the so-called GENeralized TwO-Phase Flow concept (GENTOP) which was recently developed at Helmholtz-Zentrum Dresden – Rossendorf. It bases on the two-fluid multi-field approach. Beside one or several fields representing the dispersed morphologies of gas and/or liquid potentially continuous phases for gas and liquid are introduced. Interfaces between these potentially continuous fields are statistically resolved if the local volume fraction is large enough. If this is not the case, closure models for the disperse phase are applied. For this reason it is called potentially continuous phase. The coupling of the dispersed and potentially continuous fields is done basing on a population balance. The knowledge on the typical length scale of a gas or liquid structure allows its presentation in the corresponding field. Transitions can be modelled as coalescence and breakup processes which are in agreement with the involved physical phenomena.
The concept was previously implemented in the CFX-code of ANSYS and tested considering only one continuous field for liquid, but disperse fields and a potentially continuous field for gas. Demonstration cases involve the bubble entrainment by a plunging liquid jet, generation of large bubbles out of small ones due to coalescence in a bubble column, collapse of a water column with transitions from continuous to disperse morphologies of the gas in the beginning and the vice versa process in the later phase and the simulation churn-turbulent pipe flows. Recently first simulations were done for boiling in a side wall heated vertical pipe. Single phase liquid enters the pipe from below with slight sub-cooling. Steam bubbles are generated at the wall and continue to increase and coalesce producing large bubbles which migrate to the pipe center caused by the inversion of the lateral lift force. Finally large gas structures are observed in the pipe center leasing to a transition to annular flow. The simulation involves the transition between bubbly flow and churn-turbulent flow regime and a starting transition to annular flow. In the talk the GENTOP concept and selected demonstration cases with focus on the new simulations on boiling in the heated pipe are presented. Also some recent developments to implement a similar approach in OpenFOAM are presented.