Elena Maja Slomski

Local Heat Flux Investigation during Pool Boiling Single Bubble Cycles under Reduced Gravity

In the present work the bubble diameter, heater surface temperature distribution, and local heat flux during different stages of single bubble cycles during pool boiling of PF-5060 at a pressure of p = 600 mbar have been investigated in several stable low g levels during the 1st Joint European Partial-g Parabolic Flight (JEPPF) Campaign. In previous parabolic flight campaigns, microgravity conditions were achieved by following a parabolic trajectory with the specially equipped A-300-Zero-G Aircraft. In this recent JEPPF campaign, the parabolic trajectories were slightly shifted, to establish—apart from microgravity conditions—also stable gravity levels of 0.16 g (lunar gravity) and 0.38 g (Martian gravity). High-resolution measurements of the heater surface temperature were performed using high-speed infrared thermography. An infrared (IR)-transparent sputtered heater design was employed in order to allow temperature measurements by IR thermography at a distance of approximately 800 nm to the heater/fluid interface. From the acquired temperature data, the local heat flux distribution was calculated numerically. Bubble shape and interaction were recorded with a high-speed black-and-white camera. In contrast to previous investigations, the stable low gravity levels enabled performance of measurements during single bubble (ebullition) cycles without the influence of residual flows induced by boiling under a different gravity level, as is the case in the beginning of a regular microgravity parabola. The accuracy of the measurement technique could be drastically enhanced compared to earlier publications. A local temperature drop and corresponding heat flux peak have been observed close to the three-phase contact line.

Enhancement of nucleate boiling heat transfer by micro-structured chromium nitride surfaces

In the present work, the influence of micro-structured chromium nitride (CrN) surfaces on boiling heat transfer during nucleate boiling of FC-72 at saturation temperature was investigated. The CrN thin coatings of 4.5 μm thickness in average were deposited by Physical Vapor Deposition (PVD) as a combination of High Power Impulse Magnetron Sputtering (HiPIMS) and Direct Current Magnetron Sputtering (DCMS) on polished copper surfaces. Due to variation of the applied bias voltage to the substrates different sizes and orientations of the CrN crystals were realized. The typical dimensions of the hereby generated surface structures are in the order of a few hundred nm to a few μm. It was found that all employed micro-structured CrN surfaces yield an increase of the critical heat flux (CHF) compared to an uncoated reference surface. The maximum ratio of the CHF of a CrN structured surface to the CHF of the uncoated copper surface reached within this study was CHFmax/CHFref = 2.5.

Design of Control Concepts for a Smart Beam Structure with Sensitivity Analysis of the System

A smart structure is a structure that can reduce a structural vibration by means of the integration of one or more sensor, actuator, and controller. A sensor detects a vibration in the structure and transfers a signal to a controller. The controller, designed to compensate the structural vibration, then computes the desired control signal and sends it to an actuator. A piezoelectric ceramic patch is often used, as in the present study, as a sensor or an actuator in a smart structure. A smart structure with a properly designed controller can reduce the structural vibration without changing the structure’s physical dimensions. Since a smart structure contains more uncertainty factors, not least due to the additional interfaces in comparison to a passive structure, the smart structure should be well analyzed to ensure its reliability and robustness. This paper focuses on how to set up a numerical model for a smart beam structure, how to design its control concept, and how to investigate the controller’s robustness by means of the Design of Experiments (DoE) method (In this paper experiment refers to numerical simulation.). A full factorial design is used, in which the parameters of the smart structure are either varied in their distributed range or held constant, so that several structures are designed with slight variations. This study aims at determining, which controller is the most robust by comparing the performance for different structural variations of the smart structure. Only if a smart structure is connected to a robust controller, its reliability can be analyzed, which is the aim of further research.