Browsing by Author "Nuske, Philipp"

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Open Access
Beyond local equilibrium : relaxing local equilibrium assumptions in multiphase flow in porous media
(2014) Nuske, Philipp; Helmig, Rainer (Prof. Dr.-Ing.)
One of the most basic assumptions in the field of multiphase flow in porous media is that of local equilibrium. This work is a contribution to the study and understanding of this assumption. Assuming local equilibrium basically means that heat and mass transfer processes between phases take place instantaneously. For a number of applications, this assumption is questionable for various reasons. In the following, two of these reasons are illustrated by examples. *Supply of Non-Equilibrium* If there are sources present in the porous medium which unbalance the system, assuming local equilibrium for all phases is a very strong assumption in need of justification. Practically, this source can be the supply of non-equilibrated, i.e. not fully water saturated, air bypassing soil. The assumption that the air immediately becomes fully vapor-saturated might be a serious oversimplification. Heat sources in porous media are another reason to doubt the applicability of local equilibrium assumptions. An evident example is the cooling of severely damaged nuclear reactor cores. In this case, the solid phase continuously provides energy and complex phase-change processes occur. *Insufficient Equilibration Time* If the considered porous medium is thin, for example paper during drying processes, the short residence time of air makes the idea of immediate equilibration far-fetched. Another case of questionable applicability of local equilibrium assumptions comes from the field of steam-assisted subsurface remediation. In order to predict remediation success, the correct description of the high flow velocities as well as large temperature gradients that occur are crucial. What the systems described above have in common is that studying them experimentally is hard and/or expensive. Therefore, simulation technologies can be an important tool in this context. This work is a contribution to the study and understanding of such (non-) equilibrium situations in multiphase flow in porous media. In a first application of the non-equilibrium model developed here, a parameter study, motivated by evaporation from a porous medium, is conducted. In order to obtain the necessary input parameters (volume-averaged interfacial areas) a pore-network model is adapted and constitutive relations derived. Varying an unknown model parameter (scaling factor) within a range of values results in the physically expected behavior of the system. In order to put the study of non-equilibrium effects in multiphase flow in porous media on an experimental basis, a new platform is developed in collaboration with colleagues from the University of Utrecht. The experimental setup allows us to study thermal equilibration and immiscible displacement processes simultaneously in a transparent micro-model. This is accomplished by a setup which includes both an optical as well as an infrared camera, recording the same invasion process. Both the versatility and feasibility of the platform are demonstrated in qualitative findings and observations. Building on this, two different experimental runs are simulated by means of the model developed previously. In order to be able to use the non-equilibrium model, a new image-analysis procedure had to be developedfor the determination of constitutive relations. As the proposed scaling factor of heat transfer is still unknown, it had to be calibrated to the experimental observations. Remarkably, both simulations find that the same scaling factor results in the best reproduction of the experimental observations. In another effort to increase confidence in the model, a code intercomparison study is conducted. In this regard, the same technical application, a metallic evaporator, is simulated by two simulators developed independently. These are the model developed in this work and a model describing multiphase flow in porous media based on a mixture description. Although both models are different in terms of mathematics, numerics and the simulation environments employed, they produced very similar results. This was taken as a validation of the developed model. In summary, three things are accomplished in this thesis: a toolbox of models and methods allowing the study of local non-equilibrium effects in multiphase flow in porous media in a volume-averaged sense is developed and successfully applied to a number of different applications. Secondly, new procedures for sustainable software development and source code accessibility are developed. Thirdly, different ways to determine constitutive relations are developed and successfully applied.