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Abstract An experimental apparatus was designed to study the impacts of wettability on evaporation of water from Ottawa sand. Evaporation rates were measured for: (1) a 5.7-cm-thick layer of hydrophilic Ottawa sand; (2) a 5.7-cm-thick layer with 12% hydrophobic content, consisting of a 0.7-cm-layer of n-Octyltriethoxysilane-coated hydrophobic sand buried 1.8 cm below the surface of hydrophilic sand; and (3) a 5.7-cm-thick layer with mixed wettabilities, consisting of 12% n-Octyltriethoxysilane-coated hydrophobic sand mixed into hydrophilic sand. The sand–water mixtures experienced forced convection above and through the sand layer, while a simulated solar flux (i.e., 112±20 W/m2) was applied. Evaporation from homogeneous porous media is classified into the constant-rate, falling-rate, and slow-rate periods. Wettability affected the observed evaporation mechanisms, including the transition from constant-rate to falling-rate periods. Evaporation entered the falling-rate period at 12%, 20%, and 24% saturations for the all hydrophilic sand, hydrophobic layer, and hydrophobic mixture, respectively. Wettability affected the duration of the experiments, as the all hydrophilic sand, hydrophobic layer, and hydrophobic mixture lasted 17, 20, and 26 trials, respectively. Both experiments with hydrophobic particles lasted longer than the all hydrophilic experiment and had shorter constant-rate evaporation periods, suggesting hydrophobic material interrupts capillary action of water to the soil surface and reduces evaporation. Sand temperatures suggest more evaporation occurred near the test section inlet for higher saturations and the hydrophobic layer experienced more evaporation occur near the outlet. Evaporation fluxes were up to 12× higher than the vapor diffusion flux due to enhanced vapor diffusion and forced convection. 

Abstract Drying front propagation and coupled heat and mass transfer analysis from porous media is critical for soil–water dynamics, electronics cooling, and evaporative drying. In this study, de-ionized water was evaporated from three 3D printed porous structures (with 0.41 mm, 0.41 mm, and 0.16 mm effective radii, respectively) created out of acrylonitrile butadiene styrene (ABS) plastic using stereolithography technology. The structures were immersed in water until all the pores were invaded and then placed on the top of a sensitive scale to record evaporative mass loss. A 1000 W/m2 heat flux was applied with a solar simulator to the top of each structure to accelerate evaporation. The evaporative mass losses were recorded at 15 min time intervals and plotted against time to compare evaporation rates from the three structures. The evaporation phenomena were captured with a high-speed camera from the side of the structures to observe the drying front propagation during evaporation, and a high-resolution thermal camera was used to capture images to visualize the thermal gradients during evaporation. The 3D-structure with the smallest effective pore radius (i.e., 0.16 mm) experienced the sharpest decrease in the mass loss as the water evaporated from 0.8 g to 0.1 g within 180 min. The designed pore structures influenced hydraulic linkages, and therefore, evaporation processes. A coupled heat-and-mass-transfer model modeled constant rate evaporation, and the falling rate period was modeled through the normalized evaporation rate. 

In 2021, the White House proposed a 50-52% reduction in greenhouse gas emissions by the year 2030; therefore, there is significant interest in energy sources and processes that reduce carbon dioxide emissions. This paper presents a sensitivity analysis of a nuclear microreactor-powered design for concurrent hydrogen (H2) and ammonia (NH3) production, with a focus on wastewater treatment plant applications. Wastewater with organic materials (e.g., municipal wastewater, swine lagoon waste, and food waste) are the analyzed feedstocks. The system integrates the anaerobic digestion of wastewater sludge with a Brayton cycle-based power generation unit heated by the microreactor. Using empirical data and an analytical model, the paper investigates the system's response to variations in key operational parameters. The sensitivity analysis explores the influence of parameters such as the chemical oxygen demand of the feedstock, compressor isentropic efficiency, and reactor temperature and pressure on H2 and NH3 production rates, Brayton cycle efficiency, and carbon dioxide emissions. Highlights from this analysis show a nonlinear dependence for Brayton efficiency on reactor temperature, the proportionality of ammonia and hydrogen production on chemical oxygen demand values, the major impact of compressor isentropic efficiency, and the minimal response from changing the pressure of steam methane reforming. These results signify opportunities to improve the system and ultimately lead to lowered greenhouse gas emissions. 

Introduction:Agriculture is the largest user of water globally (i.e., 70% of freshwater use) and within the United States (i.e., 42% of freshwater use); irrigation ensures crops receive adequate water, thereby increasing crop yields. Surfactants have been used in various agricultural spray products to increase spray stability and alter droplet sizes. Methods:The effects of the addition of surfactant (0.1 wt% Surfactin; surface tension of 29.2 mN/m) to distilled water (72.79 mN/m) on spray dynamics and droplet formation were investigated in four flat fan (206.8–413.7 kPa), one full cone (137.9–413.7 kPa), and three LEPA bubbler (41.4–103.4 kPa) nozzles via imaging. Results and discussion:The flat fan and cone nozzles experienced second wind-induced breakup (i.e., unstable wavelengths drive breakup) of the liquid sheets exiting the nozzle; the addition of surfactant resulted in an increased breakup length and a decreased droplet size. The fan nozzles volumetric median droplet diameter decreased with the addition of surfactant (e.g., decreased by 26.3–65.6 μm in one nozzle). The full cone nozzle volumetric median droplet diameter decreased initially with the addition of surfactant (27.8, 14.3, and 13.4 μm at 137.9, 206.8, and 310.3 kPa respectively), but increased at 413.7 kPa (24.3 μm). Sprays from the bubbler nozzles were measured and observed to experience Rayleigh (i.e., the droplets form via capillary pinching at the end of the jet) and first wind-induced breakup (i.e., air impacts breakup along with capillary pinching). The effect of Surfactin on droplet size was minimal for the 41.4 kPa bubbler nozzle. The addition of surfactant increased the diameter of the jet or ligament formed from the bubbler plate, thereby increasing the breakup length and the droplet size at 68.9 and 103.4 kPa (droplet size increased by 750.6 and 4,462.7 μm, respectively). 

The primary source of water for crops and livestock in the United States Central High Plains is irrigation from the Ogallala Aquifer. Due to the semi-arid climate of this region, little rainfall contributes to watering crops, thereby resulting in water scarcity. Reducing the evaporation from soil is one approach to conserve the water. In this study, a soil evaporation chamber was designed and constructed to study the impacts of environmental conditions on evaporation from Ottawa sand. Prior to entering the sand test section, compressed air flow was dried in a desiccator then split in two flows before entering the 57mmx228mmx838mm test section, with one airflow flowing above the 57mm thick sand layer and the other below and, subsequently, flowing through the moist sand layer. The percent relative humidity (RH) was measured at the entrance and exit to record the change in relative humidity and, therefore, water content removed from the sand. Using inlet air mass flow rates of air of approximately 1E-4kg/s–2E-4kg/s, temperatures of 28–31oC, and dry air (i.e. 0–1%RH), exit flows of 19–20oC and 80–85%RH were measured. Measured evaporation rates ranging from 3.0E-6kg/s to 5.0E-6kg/s for soil saturation levels of 55–80.5%. 

Heat pipes are passive heat transfer devices crucial for systems on spacecraft; however, they can freeze when exposed to extreme cold temperatures. The research on freezing mechanisms on wicked surfaces, such as those found in heat pipes, is limited. Surface characteristics, including surface topography, have been found to impact freezing. This work investigates freezing mechanisms on wicks during condensation freezing. Experiments were conducted in an environmental chamber at 22 °C and 60% relative humidity on three types of surfaces (i.e., plain copper, sintered heat pipe wicks, and grooved heat pipe wicks). The plain copper surface tended to freeze via ice bridging—consistent with other literature—before the grooved and sintered wicks at an average freezing time of 4.6 min with an average droplet diameter of 141.9 ± 58.1  μm at freezing. The grooved surface also froze via ice bridging but required, on average, almost double the length of time the plain copper surface took to freeze, 8.3 min with an average droplet diameter of 60.5 ± 27.9  μm at freezing. Bridges could not form between grooves, so initial freezing for each groove was stochastic. The sintered wick's surface could not propagate solely by ice bridging due to its topography, but also employed stochastic freezing and cascade freezing, which prompted more varied freezing times and an average of 10.9 min with an average droplet diameter of 97.4 ± 32.9  μm at freezing. The topography of the wicked surfaces influenced the location of droplet nucleation and, therefore, the ability for the droplet-to-droplet interaction during the freezing process.