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  1. 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%. 
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  2. Semi-arid regions faced with increasingly scarce freshwater resources must manage competing demands in the food-energy-water nexus. A possible solution modifies soil hydrologic properties using biosurfactants to reduce evaporation and improve water retention. In this study, two different soil textures representative of agricultural soils in Kansas were treated with a direct application of the biosurfactant, Surfactin, and an indirect application via inoculation of Bacillus subtilis . Evaporation rates of the wetted soils were measured when exposed to artificial sunlight (1000 W/m 2 ) and compared to non-treated control soils. Experimental results indicate that both treatments alter soil moisture dynamics by increasing evaporation rates by when soil moisture is plentiful (i.e., constant rate period) and decreasing evaporation rates by when moisture is scarce (i.e., slower rate period). Furthermore, both treatments significantly reduced the soil moisture content at which the soil transitioned from constant rate to slower rate evaporation. Out of the two treatments, inoculation with B. subtilis generally produced greater changes in evaporation dynamics; for example, the treatment with B. subtilis in sandy loam soils increased constant rate periods of evaporation by 43% and decreased slower rate evaporation by 49%. In comparing the two soil textures, the sandy loam soil exhibited a larger treatment effect than the loam soil. To evaluate the potential significance of the treatment effects, a System Dynamics Model operationalized the evaporation rate results and simulated soil moisture dynamics under typical daily precipitation conditions. The results from this model indicate both treatment methods significantly altered soil moisture dynamics in the sandy loam soils and increased the probability of the soil exhibiting constant rate evaporation relative to the control soils. Overall, these findings suggest that the decrease in soil moisture threshold observed in the experimental setting could increase soil moisture availability by prolonging the constant rate stage of evaporation. As inoculation with B. subtilis in the sandy loam soil had the most pronounced effects in both the experimental and simulated contexts, future work should focus on testing this treatment in field trials with similar soil textures. 
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  3. This paper investigates the effects of hemispherical mounds on filmwise condensation heat transfer in micro-channels. Also investigated were the impacts that spatial orientation of the three-sided condensation surface (i.e., gravitational effects) on steam condensation, where the cooled surfaces were either the lower surface (i.e., gravity pulls liquid towards the condensing surfaces) or upper surface (i.e., gravity pulls liquid away from the condensing surfaces). Two test coupons were used with 1.9-mm hydraulic diameters and either a plain copper surface or a copper surface modified with 2-mm diameter hemispherical mounds. Heat transfer coefficients, film visualization, and pressure drop measurements were recorded for both coupons in both orientations at mass fluxes of 50 kg/m 2 s and 125 kg/m 2 s. For all test conditions, the mounds were found to increase condensation heat transfer coefficients by at minimum 13% and at maximum 79%. When the test section was inverted (i.e., condensing surface on the top of flowing steam), minimal differences were found in mound performance, while the plain coupon reduces heat transfer coefficients by as much as 14%. Flow visualization suggests that the mounds enhanced heat transfer due to the disruption of the film as well as by reducing the thermal resistance of the film. Pressure drops followed parabolic behavior with quality, being higher in the mound coupon than the plain coupon. No significant pressure drop differences in the inverted orientation were observed. 
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  4. Evaporative drying from porous media is influenced by wettability and porous structures; altering these parameters impacts capillary effects and hydraulic connectivity, thereby achieving slower or faster evaporation. In this study, water was evaporated from a homogeneous porous column created with ~1165 glass (i.e., hydrophilic) or Teflon (i.e., hydrophobic) 2.38-mm-diameter spheres with an applied heat flux of 1000 W/m2 supplied via a solar simulator; each experiment was replicated five times and lasted seven days. This study investigates the combination of altered wettability on evaporation with an imposed heat flux to drive evaporation, while deploying X-ray imaging to measure evaporation fronts. Initial evaporation rates were faster (i.e., ~1.5 times) in glass than in Teflon. Traditionally, evaporation from porous media is categorized into three periods: constant rate, subsequent falling rate and slower rate period. Due to homogeneous porous structure and similar characteristic pore size (i.e., 0.453 mm), capillary effects were limited, resulting in an insignificant constant evaporation rate period. A sharp decrease in evaporation rate (i.e., falling rate period) was observed, followed by the slower rate period characterized by Fick’s law of diffusion. Teflon samples entered the slower rate period after 70 hours compared to 90 hours in glass, and combined with X-ray visualization, implying a lower rate of liquid island formation in the Teflon samples than the glass samples. The evaporative drying front, visualized by X-rays, propagated faster in glass with a final depth (after seven days) of ~30 mm, compared to ~24 mm in Teflon. Permeability was modeled based on the geometry [e.g., 3.163E-9 m2 (Revil, Glover, Pezard, and Zamora model), 3.287E-9 m2 (Critical Path Analysis)] and experimentally measured for both glass (9.5E-10 m2) and Teflon (8.9E-10 m2) samples. Rayleigh numbers (Ra=2380) and Nusselt (Nu=4.1) numbers were calculated for quantifying natural evaporation of water from fully saturated porous media, Bond (Bo=193E-3) and Capillary (Ca=6.203E-8) numbers were calculated and compared with previous studies. 
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  5. Abstract Engineering innovations—including those in heat and mass transfer—are needed to provide food, water, and power to a growing population (i.e., projected to be 9.8 × 109 by 2050) with limited resources. The interweaving of these resources is embodied in the food, energy, and water (FEW) nexus. This review paper focuses on heat and mass transfer applications which involve at least two aspects of the FEW nexus. Energy and water topics include energy extraction of natural gas hydrates and shale gas; power production (e.g., nuclear and solar); power plant cooling (e.g., wet, dry, and hybrid cooling); water desalination and purification; and building energy/water use, including heating, ventilation, air conditioning, and refrigeration technology. Subsequently, this review considers agricultural thermal fluids applications, such as the food and water nexus (e.g., evapotranspiration and evaporation) and the FEW nexus (e.g., greenhouses and food storage, including granaries and freezing/drying). As part of this review, over 100 review papers on thermal and fluid topics relevant to the FEW nexus were tabulated and over 350 research journal articles were discussed. Each section discusses previous research and highlights future opportunities regarding heat and mass transfer research. Several cross-cutting themes emerged from the literature and represent future directions for thermal fluids research: the need for fundamental, thermal fluids knowledge; scaling up from the laboratory to large-scale, integrated systems; increasing economic viability; and increasing efficiency when utilizing resources, especially using waste products. 
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  6. Altering soil wettability by inclusion of hydrophobicity could be an effective way to restrict evaporation from soil, thereby conserving water resources. In this study, 4-μL sessile water droplets were evaporated from an artificial soil millipore comprised of three glass (i.e. hydrophilic) and Teflon (i.e. hydrophobic) 2.38-mm-diameter beads. The distance between the beads were kept constant (i.e. center-to-center spacing of 3.1 mm). Experiments were conducted in an environmental chamber at an air temperature of 20°C and 30% and 75% relative humidity (RH). Evaporation rates were faster (i.e. ∼19 minutes and ∼49 minutes at 30% and 75% RH) from hydrophilic pores than the Teflon one (i.e. ∼24 minutes and ∼52 minutes at 30% and 75% RH) due in part to greater air-water contact area. Rupture of liquid droplets during evaporation was analyzed and predictions were made on rupture based on contact line pinning and depinning, projected surface area just before rupture, and pressure difference across liquid-vapor interface. It was observed that, in hydrophilic pore, the liquid droplet was pinned on one bead and the contact line on the other beads continuously decreased by deforming the liquid-vapor interface, though all three gas-liquid-solid contact lines decreased at a marginal rate in hydrophobic pore. For hydrophilic and hydrophobic pores, approximately 1.7 mm2 and 1.8–2 mm2 projected area of the droplet was predicted at 30% and 75% RH just before rupture occurs. Associated pressure difference responsible for rupture was estimated based on the deformation of curvature of liquid-vapor interface. 
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  7. The food-energy-water nexus considers critical resource challenges which must be resolved in order to meet the needs of a growing population. Agriculture is the largest global water user, accounting for two-thirds of global water withdrawals, including water for crop irrigation. Understanding and therefore reducing evaporation of water from soil is an approach to conserve water resources globally. This work studies evaporation of water from a simulated soil column and employs x-ray imaging to determine the location of water in the porous media. A 30-mL beaker was filled with approximately 1700 2-mm hydrophilic glass beads. Water (i.e., 5.5 mL) was added to the simulated soil, comprised of glass beads and a heat flux (i.e., 1500 W/m2) was applied to the beaker using a solar simulator and the intensity was measured with a light meter. Real-time mass measurements were recorded during evaporation and X-ray imaging was utilized to capture liquid transport during evaporation. Images were post-processed using Matlab; the position of the liquid front was determined from this imaging. Across three replications, it took 47 hours on average to evaporate 5 mL of the total 5.5 mL of water. The transitions between evaporation Stage I, II, and III evaporation rates were determined using mass data and x-ray imaging; transition between Stages I and II occurred between approximately 4 and 9 hours, and the transition from Stage II to III evaporation occurred between approximately 18 and 24 hours. The result of this experiment will be useful to understand the liquid transport and formation of liquid bridges during evaporation from soil. 
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  8. For this experimental study on evaporation of water from graphene, two graphene samples with different thickness and microstructure were used. Figure 1 shows the representative optical and scanning electron microscope (SEM) images of the two samples. Sample 1, shown in Figure 1a-b, is a 3 to 4 atomic layer of continuous graphene sheet grown on copper substrate via chemical vapor deposition (CVD) and was subsequently transferred to a quartz substrate using a wet chemical method reported previously [5]. The graphene thickness is at 1.2 nm to 1.4 nm, as measured by Atomic Force Microscopy. Sample 2, shown in Figure 1c-d, represents an inkjet-printed reduced graphene oxide on silicon and subsequently treated with a direct pulsed laser writing (DPLW) process for surface 3D-nanostructuring. The layer thickness is between 6 µm and 7 µm. 
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  9. Reduction of irrigation is a pressing issue in the food-water-energy nexus. Around two-third of global water withdrawals are used for irrigation in the areas with insufficient rainfall. In the U.S. Central High Plains, the Ogallala Aquifer is responsible for providing water for the production of corn, wheat, soybeans, andreducing the evaporation of water from soil provides an excellent opportunity to decrease the need for irrigation. In this paper, evaporation of sessile 4-μl water droplets from a single simulated soil pore was observed. Soil pores were created using three 2.35-mm hydrophilic glass or hydrophobic Teflon beads of the same size. The experiments were conducted at the same temperature (20° C) and two relative humidity levels, 45% and 60% RH. Evaporation times were recorded and the transport phenomena were captured using a high-speed camera. Relative humidity directly affected evaporation; evaporation times were lower at the lower RH. The glass surface had higher wettability and therefore the droplets were more stretched on the glass beads, more droplet-air areas were created and evaporation times were approximately 30 minutes at 60% RH. The Teflon surface was hydrophobic, for which air-water contact areas were lower, and evaporation times were longer – approximately 40 minutes at 60% RH. As evaporation progressed, a liquid island formed between two beads at both 45% and 60% RH in for glass and Teflon pores. The rate of decrease of the radius of the liquid island was shorter in Teflon than glass beads, which corresponded to lower evaporation rates from Teflon. 
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  10. Worldwide, agriculture is responsible for two-thirds of water withdrawals because many productive, food-producing areas lack sufficient rainfall to grow crops without irrigation. In much of the Great Plains, the Ogallala Aquifer is the primary water source for food production, and diminishing water levels require improvements in sustainable agriculture. Reductions in soil evaporation rates will reduce irrigation demands and overall water consumption for crop production, thereby conserving water in areas such as the Ogallala Aquifer. In this study, evaporation of water is studied in a single pore comprised of three 2.38-mm diameter beads to simulate a soil pore. Evaporation times and high-speed imaging were recorded for hydrophilic (i.e., glass) and hydrophobic (i.e., Teflon) beads. Experiments were conducted with moist air at approximately 22.5 °C and approximately 60% RH. Water evaporated faster from the hydrophilic beads; contact line and angle dynamics were documented for hydrophobic and hydrophilic cases. The study found that for droplets on hydrophobic beads the evaporation times were on average 55 minutes and contact area decreased with evaporation. In contrast, water droplets on hydrophilic beads averaged evaporation times of 40 minutes and decreasing contact angle occurred during evaporation. 
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