Study of a boiling bubble in uniform approaching flow at high bubble Reynolds numbers

Abstract

The process of the bubble growth and detachment from a wall with uniform up°ow parallel to the wall has been studied in this thesis. Experiments have been designed and performed in such a way that the assumptions of an existing analytical model were met. The aim of this study was to validate the predictions and to quantify the hydrodynamic lift force on a boiling bubble. This kind of detailed modeling of detachment can in principle be combined with numerical modeling of °ow with heat transfer in evaporator tubes, e.g. ri°ed tubes, with the aid of commercially available packages. The ¯ndings can be used to predict boiling characteristics in complicated geometries, such as ri°ed tubes, for conditions that occur during di®erent operation regimes of conventionally ¯red power plants. In the corresponding EC-project, all major European electricity companies join e®orts to design a power plant operating under high-temperature steam conditions to achieve 5% increase of the net e±ciency. The model presented yields analytical expressions for the forces acting on the bub- ble. A fully closed solution of the added mass forces involved in motion and growth of bubbles footed on the wall was obtained. Validation and quanti¯cation experiments corresponding to this model are described in this thesis. Nucleate boiling experiments were performed using demineralized water at near- saturated °ow boiling conditions. Both the thermal boundary condition and the approaching °ow (inlet) condition have been prescribed. The bubble substrate tem- perature and approach velocity were both constant. A microscale heater and a Wheat- stone bridge were used to maintain a constant wall temperature around an arti¯cial cavity. This cavity was used as the nucleation site where bubble would appear on a vertical part of the wall. This bubble generator has been given a special shape to minimize the velocity boundary layer thickness at the location of the arti¯cial cavity. The bubble generator intrudes a pipe and positions the arti¯cial site at the center of the pipe. As a result, a (nearly) homogeneous liquid velocity pro¯le approaches the bubble at the boiling site. Extensive image processing analysis enabled the determination of bubble geometrical parameters and to make a 3D reconstruction of the bubble volume history. The main non-hydrodynamic force components in the direction perpendicular to the vertical wall were determined from the measured quantities. The sum of these forces should equal the hydrodynamic force that is independently assessed using model predictions and measured geometrical parameter histories. This comparison was made, and used to draw conclusions on how much the deformations in°uence comparison with the model predictions and how the model should be ex- tended. In addition to model validation, temperature and power measurements have been used in a heat transfer analysis. It is shown that a signi¯cant part (60-70%) of the heat needed to make a bubble of certain volume in °ow boiling experiments arises from the superheated liquid layer in front of the heating element in respect to the heat delivered through the micro layer beneath the bubble foot from the electrical heater. These ¯ndings are con¯rmed by comparing the heater area and e®ective area, and by investigating the area of the bubble in°uence. The main conclusion drawn is that the assumption of a truncated sphere shape of the bubble does not correspond to actual bubble shapes at early times of bubble growth. The bubble is °attened parallel to the wall in this stage of growth. Therefore the volume equivalent radius does not yield an accurate representation of the actual frontal area of the bubble. That is the reason why the prediction of the hydrodynamic lift force is not good for this case. In the second half of the growth time the bubble becomes elongated away from the wall. It is not growing any more, but it moves away from the wall and pushes the surrounding liquid. This results in the negative hydrodynamic force. Before the de- tachment the neck is formed and this phenomenon is not considered by the model. Deformations should be accounted for by introducing more than one geometrical pa- rameter to describe the shape. Other recommendations given in this thesis consider the improvement of the experimental set-up and using some other measuring tech- niques. A brief description of how the model can be combined with commercially available packages has been provided. Boiling is the most e±cient, yet least understood, phase change process. The work presented in this thesis leads to an increased understanding of the physical phe- nomenon of the bubble growing and detaching in °ow boiling. The obtained knowl- edge should be used to improve modeling of this process and to be applied in industry.