Search

The multiscale boiling project, also known as RUBI, is an experimental project supported by the European Space Agency (ESA) focusing on the nucleate boiling within the framework of the utilization of the International Space Station (ISS). In order to investigate the operational behavior of the experimental components and determine a promising parameter range, an experimental setup with the same heater design is constructed and tested within the similar parameter range in the framework of the ESA 70th parabolic flight campaign. The present work introduces the experimental setup as well as the measurement methodologies. It concentrates on the results obtained for the nucleate boiling in the absence of external forces such as electric field or shear flow, which are also part of the multiscale boiling project. A barium fluoride glass sputtered with chromium on the top of a chromium nitride layer served as the heater. The manufacturing process of the heater substrate is presented in detail. In order to generate a vapor bubble in the subcooled liquid pool, a laser beam is used to provide the necessary heat at an artificial nucleation site situated at the center of the glass. The experimental setup allows for time adjustment between switch-on of the heater and shooting the laser which is defined as preheating time. The effect of the preheating time on the nucleation and behavior of the bubbles is investigated. Furthermore, by selecting a long laser pulse duration the influence of the laser thermal energy on the growth of the bubbles is studied. It is observed that the preheating time has a significant effect on the bubble growth. However, no influence of the laser shot is detected on the bubble growth, even for a long pulse duration.

We present a calibration method for the measurement of temperature using infrared cameras that enables high-accuracy thermography of moving targets with locally varying emissivity. Correlation-based image matching techniques are applied to calculate position-dependent calibration functions with sub-pixel accuracy. The calibration method is described in detail and demonstrated using an experimental setup consisting of a rotating cube. The measurement uncertainty is analysed in comparison to a conventional pixel-wise in situ method used for non-moving measurements. An analysis is carried out for varying levels of sub-pixel resolution and within different regions of the field of view. The results show that the measurement uncertainty decreases with increasing sub-pixel resolution and is comparable to levels obtained with a pixel-wise in situ calibration applied on a non-moving target. The error introduced by the motion of the sample was reduced by 75 on a high emissivity surface and by 96 within an area of the field of view, which experienced significant emissivity changes due to the motion of the sample.

We present experimental results on the transition between contact line evaporation and microlayer evaporation during the dewetting of a superheated wall. The length and thickness of the liquid microlayer is measured, both showing a dependency on the wall superheat, dewetting velocity, and heating power. This work falls within the continuing efforts of understanding the formation of the so called microlayer: a thin liquid film, that is observed for example during boiling underneath a growing vapor bubble. The short lived and small scale nature of the phenomenon make accurate measurements difficult, therefore a novel experimental set-up is proposed that allows high resolution measurements in a steady-state manner. The time resolved formation of the microlayer is described in detail and the influence of the dewetting velocity, wall superheat, and heating power is discussed. Microlayer formation is observed only if a critical velocity is exceeded for a given combination of wall superheat and heating power. Below this threshold velocity, contact line evaporation dominates the heat transfer. The dependency of the microlayer length and thickness on the dewetting velocity is described using power laws. Instability effects near the contact line are observed at high dewetting velocities and high wall superheat. It is concluded that microlayer evaporation and contact line evaporation exist in separate regimes, obviously separated by a parameter combination of dewetting velocity, wall superheat, and heating power.