Data set of evaporation from Ottawa sand with mixed wettabilities under simulated solar flux and forced convection

Description

This data set contains data collected during multiple evaporation experiments of Ottawa sand. In this study, a soil-water evaporation experimental apparatus was designed and constructed to study evaporation of water from Ottawa sand, with a forced air flow convection above and through the sand layer. A simulated solar flux i.e. (112 ± 20 W/m2) was applied to the sand, while varying soil wettability. Evaporation rates were measured for three cases: 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 evaporation rate for each trial of the three experiments was calculated using the air flow temperature, pressure, and relative humidity recorded at the inlets and outlet to the test section. These values were used to calculate the mass flow rate of the air and the humidity ratio which were used to calculate the evaporation rate. The saturation ratio was calculated using a scale that measured the change in mass of the test section throughout experimentation. The steady-state value of these trials was determined when the temperature of the outlet air flow remained constant. This data set displays the average values recorded when the steady state air temperature was reached for each trial of each experiment. The uncertainty of each trial was calculated using the uncertainties of the instrumentation used for measurements and the uncertainty in the preparation of the sand-water mixture. The calculated uncertainties for each trial are also presented in this data set. Evaporation from homogeneous porous media is frequently classified into three, distinct periods: 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 hydrophilic, hydrophobic layer, and hydrophobic mixture, respectively. The duration of the experiments was affected by the wettability, as the hydrophilic, hydrophobic layer, and hydrophobic mixture lasted 17, 20, and 26 trials, respectively. The temperatures measured in the sand suggest more evaporation occurring in the sand near the inlet to the test section for higher saturations. The evaporation flux was found to be 0.1-12 times higher than the vapor diffusion flux calculated, due to enhanced vapor diffusion and forced convection above and through the sand.

Steps to reproduce

To study the impacts of wettability on evaporation under a simulated solar flux, with forced convection above and below – and, thus, through – the porous media, an air-tight test section was required that could suspend the wet sand above an air flow, allow a solar heat flux to be applied, and be used to calculate a mass balance on the flows entering and leaving the test section to determine the evaporation rate. A test section was constructed out of 0.006-m-thick aluminum plates welded and bolted together with rubber gaskets. The middle section included a metal crossed support with a single layer of nylon, semi-permeable membrane, attached with adhesive tape, to hold the sand layer and allow air to pass through the sand. The porous media studied was a layer of 0.057-m-thick Ottawa sand. Clear polycarbonate was attached to the side and top of the test section with industrial adhesive for viewing of the sand and a solar heat flux to be applied from above. Reflective heat tape was used on the top of the polycarbonate window to shield the glue from direct heat flux from the solar simulator. Holes were drilled in the side of the test section for nine thermocouples to measure the temperature of the porous media throughout trials. The thermocouples were placed at locations A, B, and C, 0.079 m, 0.419 m, 0.759 m from the inlet, respectively. The depth below the surface of the sand was at position 1, 2, and 3, 0.008 m, 0.02 m, and 0.035 m, respectively. Building supply air at 827 kPa was regulated and sent through a desiccator to dry it to near-zero relative humidity. The air travelled through a heat exchanger submerged in a water bath to heat the dry air to the desired inlet temperature. The air was brought to a tee junction and split to travel above and below the sand layer at flow rates of 2.0E-4 kg/s, respectively. Swagelok disconnects were added after the tee junction and on the air lines. The air flows were controlled by multiple valves (Swagelok Valve SS-45S8) and measurements for pressure and volumetric flow rate were recorded by pressure transducers and volumetric flow meters, mass flow rates were calculated in Engineering Equation Solver. The two inlet air flows passed through humidity sensors and thermocouple probes. After the air passed through the test section, air passed through a Swagelok valve to ensure mixing, and temperature and relative humidity were recorded by sensor and thermocouple probe. Data were acquired in LabVIEW. Seventeen halogen lights were placed 0.25 m above the sand layer and were cooled by five fans; heat fluxes were measured using a LICOR light meter and pyranometer. A heated blanket was wrapped around the test section to allow for even heating of the apparatus for proper operating conditions and to insulate throughout trials; additional fiberglass insulation was installed to reduce heat loss. The entire apparatus was placed on a scale, with capacity of 250 ± 0.01 kg, to measure the gravimetric water content.

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