Melanie M. Derby

Droplet Coalescence and Freezing on Hydrophilic, Hydrophobic, and Biphilic Surfaces

Frost and ice formation can have severe negative consequences, such as aircraft safety and reliability. At atmospheric pressure, water heterogeneously condenses and then freezes at low temperatures. To alter this freezing process, this research examines the effects of biphilic surfaces (surfaces which combine hydrophilic and hydrophobic regions) on heterogeneous water nucleation, growth, and freezing. Silicon wafers were coated with a self-assembled monolayer and patterned to create biphilic surfaces. Samples were placed on a freezing stage in an environmental chamber at atmospheric pressure, at a temperature of 295 K, and relative humidities of 30%, 60%, and 75%. Biphilic surfaces had a significant effect on droplet dynamics and freezing behavior. The addition of biphilic patterns decreased the temperature required for freezing by 6 K. Biphilic surfaces also changed the size and number of droplets on a surface at freezing and delayed the time required for a surface to freeze. The main mechanism affecting freezing characteristics was the coalescence behavior.

Exploration of Experimental Techniques to Determine the Condensation Heat Flux in Microchannels and Minichannels

This study seeks to analyze and explore experimental methods to study condensation heat transfer in micro- and mini-channel. Following, an experimental setup was built and initial results are presented. Several experimental techniques were reviewed, while two, thermoelectric coolers and a copper-heat-flux-sensor were analyzed in detail for condensation heat flux. It was concluded that thermoelectric coolers were not suitable as heat flux sensors for single-phase validation, but the copper-heat-flux-sensor was appropriate to measure heat transfer coefficients at the mini-scale. Condensation heat transfer coefficients were obtained experimentally in seven parallel square mini-channels of diameter 1mm. Existing condensation correlations were applied to these data; micro- and mini-scale correlations captured the appropriate trends, but the macro-scale Shah (1979) correlation fit the data best.Copyright

Update of the scientific evidence for specifying lower limit relative humidity levels for comfort, health, and indoor environmental quality in occupied spaces (RP-1630)

Nearly 600 articles were located in citation and keyword searches regarding the effects of humidity on comfort, health, and indoor environmental quality. Of these, around 70 articles reported the effects of low humidity (relative humidity ≤ 40%) and were analyzed in detail. Information in some categories was well chronicled, while other categories had significant knowledge gaps. Low humidity decreased house dust mite allergens. Due to different envelopes, generalizations could not be made for all bacteria and viruses. However, lower humidity increased virus survival for influenza. For comfort, low humidity had little effect on thermal comfort, but skin dryness, eye irritation, and static electricity increased as humidity decreased. For indoor environmental quality, low humidity had nonuniform effects on volatile organic compound emissions and perceived indoor air quality. Across many low humidity studies, ventilation rates and exposure times were noted as confounding variables. A majority of studies that used human subjects utilized exposure times of 3 h or less with adult subjects; few studies used children, adolescents, or elderly subjects.

Combined Visualization and Heat Transfer Measurements for Steam Flow Condensation in Hydrophilic and Hydrophobic Mini-Gaps

Citation: Chen X, Derby MM. Combined Visualization and Heat Transfer Measurements for Steam Flow Condensation in Hydrophilic and Hydrophobic Mini-Gaps. ASME. J. Heat Transfer. 2016;138(9):091503-091503-11. doi:10.1115/1.4033496.

Droplet ejection and sliding on a flapping film

Water recovery and subsequent reuse are required for human consumption as well as industrial, and agriculture applications. Moist air streams, such as cooling tower plumes and fog, represent opportunities for water harvesting. In this work, we investigate a flapping mechanism to increase droplet shedding on thin, hydrophobic films for two vibrational cases (e.g., ± 9 mm and 11 Hz; ± 2 mm and 100 Hz). Two main mechanisms removed water droplets from the flapping film: vibrational-induced coalescence/sliding and droplet ejection from the surface. Vibrations mobilized droplets on the flapping film, increasing the probability of coalescence with neighboring droplets leading to faster droplet growth. Droplet departure sizes of 1–2 mm were observed for flapping films, compared to 3–4 mm on stationary films, which solely relied on gravity for droplet removal. Additionally, flapping films exhibited lower percentage area coverage by water after a few seconds. The second removal mechanism, droplet ejection was analy...

Evapotranspiration from Spider and Jade Plants Can Improve Relative Humidity in an Interior Environment

Citation: Kerschen, E., Garten, C., Williams, K., & Derby, M. (2016). Evapotranspiration from Spider and Jade Plants Can Improve Relative Humidity in an Interior Environment. HortTechnology, 26(6), 803-810. doi: 10.21273/HORTTECH03473-16

Visualization of Steam Flow Condensation in Hydrophobic and Hydrophilic Mini-Gaps

An experimental, flow visualization study was conducted for flow condensation in 1-mm diameter mini-gaps with a visualization window. The hydrophilic channel was composed of bare copper, while the hydrophobic channel was coated with Teflon AF™. An open loop steam experimental apparatus was constructed, and single-phase validation tests are presented. For flow condensation in the hydrophilic gap, filmwise condensation was observed for all cases with a mass flux range of 30–100 kg/m2s. In contrast, the hydrophobic channel promoted dropwise condensation at mass fluxes of 30 and 50 kg/m2s. Cycles of droplet nucleation, growth and coalescence, and sweeping were identified at both mass fluxes in the hydrophobic channel at steady state. Droplets departed at a smaller sizes and more frequently at the higher mass flux, demonstrating the effects of shear forces on droplet sweeping. Based on flow visualizations, three droplet sweeping methods were identified: interfacial shear stress, upstream droplets, and absorbing into rivulets.Copyright