A STUDY OF HEAT TRANSFER AND FLOW CHARACTERISTICS OF RISING TAYLOR BUBBLES

Practical application of flow boiling to ground- and space-based thermal management systems hinges on the ability to predict the system’s heat removal capabilities under expected operating conditions. Research in this field has shown that the heat transfer coefficient within two-phase heat exchangers can be largely dependent on the experienced flow regime. This finding has inspired an effort to develop mechanistic heat transfer models for each flow pattern which are likely to outperform traditional empirical correlations. As a contribution to the effort, this work aimed to identify the heat transfer mechanisms for the slug flow regime through analysis of individual Taylor bubbles. An experimental apparatus was developed to inject single vapor Taylor bubbles into co-currently flowing liquid HFE 7100. The heat transfer was measured as the bubble rose through a 6 mm inner diameter heated tube using an infrared thermography technique. High-speed flow visualization was obtained and the bubble film thickness measured in an adiabatic section. Experiments were conducted at various liquid mass fluxes (43-200 kg/m2s) and gravity levels (0.01g-1.8g) to characterize the effect of bubble drift velocity on the heat transfer mechanisms. Variable gravity testing was conducted during a NASA parabolic flight campaign. Results from the experiments showed that the drift velocity strongly affects the hydrodynamics and heat transfer of single elongated bubbles. At low gravity levels, bubbles exhibited shapes characteristic of capillary flows and the heat transfer enhancement due to the bubble was dominated by conduction through the thin film. At moderate to high gravity, traditional Taylor bubbles provided small values of enhancement within the film, but large peaks in the wake heat transfer occurred due to turbulent vortices induced by the film plunging into the trailing liquid slug. Characteristics of the wake heat transfer profiles were analyzed and related to the predicted velocity field. Results were compared and shown to agree with numerical simulations of colleagues from EPFL, Switzerland. In addition, a preliminary study was completed on the effect of a Taylor bubble passing through nucleate flow boiling, showing that the thinning thermal boundary layer within the film suppressed nucleation, thereby decreasing the heat transfer coefficient.

UNDERSTANDING THE RELATIONSHIP BETWEEN THE BROWN MARMORATED STINK BUG, HALYOMORPHA HALYS (STÅL), AND ITS SYMBIONT, PANTOEA CARBEKII, WITH IMPLICATIONS FOR STINK BUG MANAGEMENT

Symbiotic relationships between insects and beneficial microbes are very common in nature, especially within the Hemiptera. The brown marmorated stink bug, Halyomorpha halys Stål, harbors a symbiont, Pantoea carbekii, within the fourth region of the midgut in specialized crypts. In this dissertation, I explored this insect- microbe relationship. I determined that the brown marmorated stink bug is heavily reliant on its symbiont, and that experimental removal of the symbiont from the egg mass surface prior to nymphal acquisition led to lower survival, longer development, lower fecundity, and aberrant nymphal behavior. Additionally, I determined that even when the symbiont is acquired and housed in the midgut crypts, it is susceptible to stressors. Stink bugs reared at a higher temperature showed lower survival, longer development, and a cease in egg mass production, and when bugs were screened for their symbiont, fewer had successfully retained it while under heat stress. Finally, with the knowledge that the stink bug suffers decreases in fitness when its symbiont is missing or stressed, I wanted to determine if targeting the symbiont was a possible management technique for the stink bug. I tested the efficacy of a number of different insecticidal and antimicrobial products to determine whether prevention of symbiont acquisition from the egg mass was possible, and results indicated that transmission of the symbiont from the egg mass to the newly hatched nymph was negatively impacted when certain products were applied (namely surfactants or products containing surfactants). Additionally, direct effects on hatch rate and survival were reported for certain products, namely the insect growth regulator azadirachtin, which suggests that nymphs can pick up residues from the egg mass surface while probing for the symbiont. I conclude that P. carbekii plays a critically important role in the survival of its host, the brown marmorated stink bug, and its presence on the egg mass surface before nymphal hatch makes it targetable as a potential management technique.

A Thermal Management Solution for Compact Power Converters

Regulated Transformer Rectifier Units contain several power electronic boards to facilitate AC to DC power conversion. As these units become smaller, the number of devices on each board increases while their distance from each other decreases, making active cooling essential to maintaining reliable operation. Although it is widely accepted that liquid is a far superior heat transfer medium to air, the latter is still capable of yielding low device operating temperatures with proper heat sink and airflow design. The purpose of this study is to describe the models and methods used to design and build the thermal management system for one of the power electronic boards in a compact, high power regulated transformer rectifier unit. Maximum device temperature, available pressure drop and manufacturability were assessed when selecting the final design for testing. Once constructed, the thermal management system’s performance was experimentally verified at three different power levels.