Dietl, Jochen (2015)
Numerical Simulation of Pool Boiling from Reentrant Type Structured Surfaces.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Enhancement of heat transfer in pool boiling can be achieved by employing a structured surface. So called reentrant type surfaces, consisting of subsurface tunnels connected through pores with the pool, were found to strongly improve the performance of heat exchanger tubes. Although employed since decades, several of the processes within the tunnel are not understood and the presented models are not able to predict the different boiling modes. With the rapid development of numerical methods in the last years, the simulation of boiling is possible by today, allowing to study the processes with high temporal and spatial resolution.

In the presented thesis, numerical simulations of boiling from reentrant type structured surfaces are performed. Processes are studied at single reentrant cavities and at a piece of subsurface tunnel with two pores. The dimensions and shapes of the cavity, tunnel, and pore are varied to obtain the influence of geometric properties on the process. Furthermore, a simplified model is created to calculate flow into the subsurface tunnel with reduced computational effort, in order to study a wider parameter range.

The employed numerical model is based on the VOF method and was validated in earlier works. In this work, the solver is adapted to work with capillary flows at low contact angles. The simplified model is based on solving the Young-Laplace equation to obtain the pressure jump at the bubble and the liquid film inside the structure. With the pressure differences the evolution of the liquid film during bubble growth and departure can be predicted. The results for the amount of liquid in the tunnel obtained from the numerical simulation and the simplified model are in good agreement. The results of the simulations show that with single reentrant cavities obtaining thin film evaporation inside the cavity is difficult if only one pore exists at each cavity. Introducing an additional channel next to the pore, connecting the liquid pool with the liquid film, similar processes can be observed inside the cavity as observed with subsurface tunnels. Simulations with the section of the subsurface tunnel indicate a strong dependence of the processes on pore size but also pore shape. With the given geometry, very high heat transfer coefficients are obtained, caused by the evaporation of the thin liquid films inside the structure.

From the parameter study with the simplified model, the influence of geometric properties on the operation range of the surface can be deduced. Furthermore, the model gives characteristic dimensionless parameters governing the process. Small bubble diameters as well as wide and deep tunnels are beneficial to prevent flooding of the structures. The point of dryout is delayed with a large open pore area. The introduction of two additional sub-stages of the boiling modes is suggested, namely the vapor expansion stage and the partial dryout stage, which should be considered in future modeling approaches.

In summary, numerical simulations and analysis of the boiling process from structured surfaces performed in this work, improve the understanding of the interrelations of important parameters and lead to suggestions regarding the design of the surfaces and modeling of the heat transfer.

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