Evaporation Transfer Coefficient Research Articles

Assessment of the water vapor permeability: Effect of the total pressure

This paper shows that the theory describing gas diffusion with no consideration of the total gas pressure leads to an underestimation of the water vapor diffusive permeability. This experimental method, called cup method, can be considered as one of the most popular tests in the domain of heat and mass transfer for building applications. Thus, it is likely that the water vapor permeability defined for most current building materials is underestimated. Thanks to the approach proposed in this paper, provided that there is an advective gas permeability, it is possible to know whether a given water vapor permeability has been underestimated or not. However, it is not possible to estimate the magnitude of this measurement error. In the last part, a new experimental procedure is proposed, integrating a total gas pressure measurement on both sides of the sample. This modified test method makes it possible to simultaneously determine the advective gas permeability and the diffusive vapor transfer coefficient.

Air‐Lake Momentum and Heat Exchange in Very Young Waves Using Energy and Water Budget Closure

AbstractTurbulent momentum and heat exchanges at the air‐water interface of lakes are complex processes due to diverse characteristics such as climate, fetch limitation, and geometry; however, their parameterizations are necessary for hydro‐ and thermodynamic modeling. In this study, we performed comprehensive research on turbulent exchanges of a shallow, large but highly fetch limited lake employing 5 months of simultaneous eddy‐covariance (EC), wave, energy, and water budget measurements. Based on the Monin‐Obukhov similarity theory, wind and wave age‐related drag coefficients and roughness lengths were derived for very young wave ages. The new drag functions showed an increased water surface resistance compared to marine conditions. In terms of water vapor roughness length and transfer coefficients, an apparent intra‐seasonal variation was observed. Turbulent heat fluxes were evaluated by closing the lake's energy budget using the Bowen ratio method. Furthermore, we aimed to close the energy budget by assuring the water balance closure as well through the evaporation (latent heat flux), resulting in significant deviations from the EC‐based results. EC‐based heat fluxes and transfer coefficients were found to be underestimated on average by 29% and 33%, respectively. The water balance closure suggested that the energy residual should not be distributed based on the Bowen ratio, and sensible heat fluxes were more underestimated than latent heat fluxes. In the case of Lake Balaton, atmospheric stability plays a minor role; however, our analysis revealed significant effects caused by the surrounding topography and water side stratification.

Intra-Seasonal and Intra-Annual Variation of the Latent Heat Flux Transfer Coefficient for a Freshwater Lake

In the case of lakes, evaporation is one of the most significant losses of water and energy. Based on high-frequency eddy-covariance (EC) measurements between May and September of 2019, the offshore heat and water vapor exchanges are evaluated for the large (~600 km2) but shallow (~3.2 m deep) Lake Balaton (Transdanubian region, Hungary). The role of local driving forces of evaporation in different time scales (from 20 min to one month) is explored, such as water surface and air temperatures, humidity, atmospheric stability, net radiation, and energy budget components. EC-derived water vapor roughness lengths and transfer coefficients (Cq) show an apparent intra-seasonal variation. Different energy balance-based evaporation estimation methods (such as the Priestley-Taylor and the Penman-Monteith) confirm this observation. Furthermore, this has suggested the existence of an intra-annual variation in these parameters. This hypothesis is verified using ten years of water balance measurements, from which, as a first step, evaporation rates and, second, transfer coefficients are derived on a monthly scale. Cq is highly reduced in winter months (~1 × 10−3) compared to summer months (~2.5 × 10−3) and strongly correlated with net radiation. The application of time-varying Cq significantly increases the accuracy of evaporation estimation when the Monin-Obukhov similarity theory-based aerodynamic method is applied. The determination coefficient increases to 0.84 compared to 0.52 when a constant Cq is employed.

Investigation of coupled vapor and heat transport in hygroscopic material during adsorption and desorption

Vapor sorption in hygroscopic porous materials is accompanied by latent heat release/storage, which can influence indoor thermal comfort and building heating and cooling energy consumption. There is a need to better understand the coupled vapor and heat transport during adsorption and desorption. In this study, longitudinal spruce samples are exposed to adsorption and desorption experiments. Neutron radiography provides accurate measurement of moisture content variations spatially and temporally. Wireless thermocouples provide accurate measurements of temperature at different locations. Large changes in moisture content and temperature are observed during both adsorption and desorption experiments. Both moisture content and temperature variations seen in experiments are well simulated with hygrothermal modeling. The latent heat associated with vapor sorption is found to be the source of the large variations in temperature. It is found that vapor permeability influences both vapor and thermal transport while thermal conductivity influences only thermal transport. The vapor transfer coefficient has a small influence on vapor transport while the convective heat transfer coefficient has an influence on heat transport. The validated hygrothermal model is further used to simulate the coupled vapor and heat transport occurring in moisture buffering tests. It is found that moisture buffering values are different by up to 14% depending on the presence or absence of thermal insulation around the samples. For more hygroscopic materials, the difference can be even much larger. It is recommended not only to seal and but also to insulate samples for moisture buffering tests.

Parameterization of Sea Surface Drag Coefficient for All Wind Regimes Using 11 Aircraft Eddy-Covariance Measurement Databases

The drag coefficient is essential for calculating the aerodynamic friction between air and sea. In this study, we regress a set of relationships between the drag coefficient and the wind speed for different wind ranges using an observational dataset that consists of 5941 estimates of the mean flow and fluxes from 11 aircraft turbulent measurements over the sea surface. Results show that: (1) the drag coefficient is a power function of wind speed over smooth sea surface when it is no greater than 4.5 ms−1, and the drag coefficient decreases with the increase of wind speed; and (2) for rough sea surface, when the wind speed is greater than 4.5 ms−1 and less than or equal to 10.5 ms−1, the drag coefficient increases linearly with the increase of horizontal wind speed; when the wind speed is greater than 10.5 ms−1 and less than or equal to 33.5 ms−1, the drag coefficient changes parabolically with the increase of wind speed; when the wind speed is greater than 33.5 ms−1, the drag coefficient is constant. Additionally, regressed from drag coefficient, the saturated wind speed threshold is 23 ms−1. Parameterizations of turbulent heat transfer coefficient (Ch) and water vapor transfer coefficient (Ce) are also investigated.

Study of the effective thickness of the water-intrudable hydrophilic layer in dual-layer hydrophilic-hydrophobic hollow fiber membranes for direct contact membrane distillation

Dual-layer hydrophilic-hydrophobic hollow fiber membrane is a promising membrane that simultaneously shows enhanced vapor transfer coefficient and inhibited conductive heat loss in direct contact membrane distillation (DCMD). The presence of an effective water-intrudable hydrophilic layer enables to reduce the vapor transportation distance and improve the water flux in DCMD. The aim of this study is to estimate the water-intrudable hydrophilic layer thickness in a dual-layer hydrophilic-hydrophobic hollow fiber membrane comprising a PVDF/PEG inner layer and a PVDF outer layer. The water-intrudable hydrophilic layer thickness was precisely measured by a dye solution permeation method under simulated DCMD conditions. The results showed that the water-intrudable hydrophilic layer thickness was highly determined by the pore size and effective porosity of the PVDF/PEG inner layer. The applied hydraulic pressure contributed significantly to the difference in the thickness of the water-intrudable hydrophilic layer. The membranes with higher thickness percentages of the water-intrudable hydrophilic layer exhibited simultaneously enhanced permeate water flux and energy efficiency in DCMD. This study offered a simple and reliable approach for the estimation of the effective thickness of the water-intrudable hydrophilic layer for the dual-layer hydrophilic-hydrophobic hollow fiber membrane that contributes substantially to the desalination performance enhancement in DCMD.

Evaporation-induced soil water flux to design suction drain for low-carbon ground stabilisation: Experimental investigation and modelling

The suction drain is a novel concept for low-carbon temporary ground stabilisation in clayey soils alternative to jet grouting and ground freezing. Boreholes are drilled into the ground and air is injected to the borehole end through a delivery pipe. The air flowing through the gap between the pipe and the borehole surface backward towards the borehole entry removes water by evaporation and, hence, increases the undrained shear strength of the soil surrounding the drain. There are no studies that allow quantifying soil water evaporation generated by tangential airflow for the case of ‘wind tunnel’ only a few centimetres high over an evaporating surface a few meters long. This paper first presents an experimental investigation on water evaporation induced by air flow. A 3 m long wet surface was subjected to tangential air flow into a 40 mm gap. Tests were carried out by considering different air velocities and inlet air relative humidities. A model was then formulated to quantify the water evaporation rate for any length of the wet surface. The model parameters were calibrated against one experimental dataset and the model was then validated against an independent dataset. Finally, an empirical equation is proposed to estimate model parameters without the need of carrying out experimental tests. This is based on the vapour transfer coefficient established empirically for evaporation from open water (external air flow), which was found to remain valid for confined evaporation (internal air flow). The paper therefore provides a tool to estimate airflow-induced evaporation to successfully design the suction drains.

A multi-scale approach to optimize vapor-liquid mass transfer layer in structured catalytic packing

In order to implement the multi-scale model to optimize the structure of vapor-liquid mass transfer layer, vapor and liquid mass transfer coefficients that embrace the effects of structure parameters of mass transfer layer should be obtained in advance. In the study, the influences of four structure parameters on mass transfer performance were studied by the tracer method. Moreover, the corresponding mass transfer coefficients were correlated. By comparing with the experimental data of different types of structured catalytic packings, it has been found that the correlations exhibit good universality. Furthermore, the correlations were applied to the non-equilibrium model for the hydrolysis of methyl acetate (MA), so as to investigate the influences of four structure parameters on catalytic distillation process. The results indicated that wavy height and the area ratio of mass transfer layer to catalyst layer have more significant influences on the conversion of MA than inclined angle and base length.

Influence of inner vapor transfer property of a freeze dryer on the design space of drying process

AbstractA freeze dryer with a radiative heating device was constructed and the inner vapor transfer property was regulated by separating the drying chamber and the condenser with an orifice. A mathematical model was used to obtain the contour diagrams of the selected parameters (e.g., product temperature and total drying period) during drying as a function of the heating condition and chamber pressure. Furthermore, the inner vapor transfer coefficients of the freeze dryer were used to draw the operation lines on the diagrams. The combinations of contour diagrams and operation lines clearly depict the design spaces. It was confirmed that the inner vapor transfer property substantially affected the acceptable operation range, and it was revealed that this coefficient is key to quantify the performance of a freeze dryer. This coefficient could also be used to predict the occurrence of choked flow during a freeze‐drying run.

Evaporation variability of Nam Co Lake in the Tibetan Plateau and its role in recent rapid lake expansion

Previous studies have shown that the majority of the lakes in the Tibetan Plateau (TP) started to expand rapidly since the late 1990s. However, the causes are still not well known. For Nam Co, being a closed lake with no outflow, evaporation (EL) over the lake surface is the only way water may leave the lake. Therefore, quantifying EL is key for investigating the mechanism of lake expansion in the TP. EL can be quantified by Penman- and/or bulk-transfer-type models, requiring only net radiation, temperature, humidity and wind speed for inputs. However, interpolation of wind speed data may be laden with great uncertainty due to extremely sparse ground meteorological observations, the highly heterogeneous landscape and lake-land breeze effects. Here, evaporation of Nam Co Lake was investigated within the 1979–2012 period at a monthly time-scale using the complementary relationship lake evaporation (CRLE) model which does not require wind speed data. Validations by in-situ observations of E601B pan evaporation rates at the shore of Nam Co Lake as well as measured EL over an adjacent small lake using eddy covariance technique suggest that CRLE is capable of simulating EL well since it implicitly considers wind effects on evaporation via its vapor transfer coefficient. The multi-year average of annual evaporation of Nam Co Lake is 635 mm. From 1979 to 2012, annual evaporation of Nam Co Lake expressed a very slight decreasing trend. However, a more significant decrease in EL occurred during 1998–2008 at a rate of −12 mm yr−1. Based on water-level readings, this significant decrease in lake evaporation was found to be responsible for approximately 4% of the reported rapid water level increase and areal expansion of Nam Co Lake during the same period.

Expanding the applicability of remotely sensed soil evaporation transfer coefficient to the estimation of soil moisture over bare soil and partially vegetated areas

ABSTRACTSoil evaporation transfer coefficient (ha) is an effective means of estimating surface soil moisture from moderate-resolution imaging spectroradiometer (MODIS) imagery. This coefficient is a function of three variables: air temperature, land surface temperature and dry soil temperature. The first two variables can easily be obtained from different sources, whereas dry soil temperature cannot be determined as easily as the other ones, particularly over partially vegetated areas. In this paper, to enhance the capability of ha in estimating soil moisture, we propose to use the combination of land surface temperature (LST) and normalized difference vegetation index (NDVI) to estimate dry soil temperature over partially vegetated areas as well as bare soil areas; this combination is known as LST-NDVI feature space in the literature. The underlying assumption of the proposed method is that at any given pixel over partially vegetated or bare soil areas, dry soil temperature is approximately equivalent to the maximum LST derived from LST-NDVI triangle space. The results showed that calculating ha using dry soil temperature derived from the triangle space can result in more reliable estimation of soil moisture over bare soil and partially vegetated areas.

Applicability of the Bulk-Transfer Approach to Estimate Evapotranspiration from Boreal Peatlands

AbstractIn northern landscapes, peatlands are widespread and their hydrological processes are complex. Furthermore, they are typically remote, limiting the amount and accuracy of in situ measurements. This is especially the case for evapotranspiration ET, which strongly influences watershed hydrology. The objective of this paper is to demonstrate the validity of the bulk-transfer approach to estimate ET over boreal peatlands. The simplicity of the model relies on four assumptions: (i) near-neutral atmospheric conditions; (ii) wet surface; (iii) constant momentum roughness length depending on vegetation height; and (iv) constant water vapor roughness length, with the last two assumptions implying a constant water vapor transfer coefficient CE. Using eddy covariance data from three Canadian peatlands—Necopastic (James Bay, Québec), Mer Bleue (Ottawa, Ontario), and Western Peatland (Athabasca, Alberta)—this paper shows that these sites are characterized by frequent occurrences of near-neutral atmospheric conditions, especially the Necopastic site, with nearly 76% of the 30-min data segments occurring under near-neutral stratification. The analysis suggests these near-neutral conditions occur as a result of strong mechanical turbulence and weak buoyancy effects. The bulk-transfer approach gives promising results for 30-min and daily ET in terms of mean error and correlation, with performances similar to the Penman equation, without requiring net radiation data. The accuracy of the approach is likely related to the number of near-neutral periods and the elevated position of the water table, which backs up the wet surface assumption.

Continuous stand-alone controllable aerosol/cloud droplet dryer for atmospheric sampling

Abstract. We describe a general-purpose dryer designed for continuous sampling of atmospheric aerosol, where a specified relative humidity (RH) of the sample flow (lower than the atmospheric humidity) is required. It is often prescribed to measure the properties of dried aerosol, for instance for monitoring networks. The specific purpose of our dryer is to dry cloud droplets (maximum diameter approximately 25 μm, highly charged, up to 5 × 102 charges). One criterion is to minimise losses from the droplet size distribution entering the dryer as well as on the residual dry particle size distribution exiting the dryer. This is achieved by using a straight vertical downwards path from the aerosol inlet mounted above the dryer, and removing humidity to a dry, closed loop airflow on the other side of a semi-permeable GORE-TEX membrane (total area 0.134 m2). The water vapour transfer coefficient, k, was measured to be 4.6 × 10-7 kg m−2 s−1% RH−1 in the laboratory (temperature 294 K) and is used for design purposes. A net water vapour transfer rate of up to 1.2 × 10-6 kg s−1 was achieved in the field. This corresponds to drying a 5.7 L min−1 (0.35 m3 h−1) aerosol sample flow from 100% RH to 27% RH at 293 K (with a drying air total flow of 8.7 L min−1). The system was used outdoors from 9 May until 20 October 2010, on the mountain Brocken (51.80° N, 10.67° E, 1142 m a.s.l.) in the Harz region in central Germany. Sample air relative humidity of less than 30% was obtained 72% of the time period. The total availability of the measurement system was >94% during these five months.

Remote detection of bare soil moisture using a surface-temperature-based soil evaporation transfer coefficient

An approach for estimating soil moisture is presented and tested by using surface-temperature-based soil evaporation transfer coefficient (ha), a coefficient recently proposed through the equation ha=(Ts−Ta)/(Tsd−Ta), where Ts, Tsd, and Ta are land surface temperature (LST), reference soil (dry soil without evaporation) surface temperature, and air temperature respectively. Our analysis and controllable experiment indicated that ha closely related to soil moisture, and therefore, a relationship between field soil moisture and ha could be developed for soil moisture estimation. Field experiments were carried out to test the relationship between ha and soil moisture. Time series Aqua-MODIS images were acquired between 11 Sep. 2006 and 1 Nov. 2007. Then, MODIS derived ha and simultaneous measured soil moisture for different soil depths were used to establish the relations between the two variables. Results showed that there was a logarithmic relationship between soil moisture and ha (P<0.01). These logarithmic models were further validated by introducing another ground-truth data gathered from 46 meteorological stations in Hebei Province. Good agreement was observed between the measured and estimated soil moisture with RMSE of 0.0374cm3/cm3 and 0.0503cm3/cm3 for surface energy balance method at two soil depths (10cm and 20cm), with RMSE of 0.0467cm3/cm3 and 0.0581cm3/cm3 for maximum temperature method at two soil depths. For vegetated surfaces, the ratio of ha and NDVI suggested to be considered. The proposed approach has a great potential for soil moisture and drought evaluation by remote sensing.

Experimental Determination of Soil Evaporation Stages with Soil Surface Temperature

Soil evaporation process can be divided into three stages: the constant‐rate, the falling‐rate, and the low‐rate stages. Soil surface temperature is closely related to the soil evaporation rate and soil evaporation stage. The objectives of this study were: (i) to propose an approach to determine the soil evaporation stage by using infrared (IR) radiometers, the three‐temperature (3T) model, and the Priestley–Taylor (PT) model; (ii) to test it on three soil types (coarse sand, sand, and clay), and (iii) to apply it using remote sensing. A drying experiment to address these objectives was continuously performed for 539 h at constant air temperature (25°C) and relative humidity (50%). The results clearly indicated that the combination of the IR radiometers and the PT model had improved the performance of the 3T model. Soil evaporation stages could be determined with the soil evaporation transfer coefficient (ha), a key parameter in the 3T model. At the constant‐rate stage, ha was a constant with low value, while cumulative evaporation (Ec) rapidly increased. At the falling‐rate stage, ha increased with Ec and the relationship between them was curvilinear. At the end of the falling‐rate stage, ha could be &gt;0.90. Our results showed that the application of thermal imaging for the estimation of ha and the evaporation rate was promising for applying this technique in remote sensing.

Remotely monitoring evaporation rate and soil water status using thermal imaging and “three-temperatures model (3T Model)” under field-scale conditions

Remote monitoring of soil evaporation and soil water status is necessary for water resource and environment management. Ground based remote sensing can be the bridge between satellite remote sensing and ground-based point measurement. The primary object of this study is to provide an algorithm to estimate evaporation and soil water status by remote sensing and to verify its accuracy. Observations were carried out in a flat field with varied soil water content. High-resolution thermal images were taken with a thermal camera; soil evaporation was measured with a weighing lysimeter; weather data were recorded at a nearby meteorological station. Based on the thermal imaging and the three-temperatures model (3T model), we developed an algorithm to estimate soil evaporation and soil water status. The required parameters of the proposed method were soil surface temperature, air temperature, and solar radiation. By using the proposed method, daily variation in soil evaporation was estimated. Meanwhile, soil water status was remotely monitored by using the soil evaporation transfer coefficient. Results showed that the daily variation trends of measured and estimated evaporation agreed with each other, with a regression line of y = 0.92x and coefficient of determination R(2) = 0.69. The simplicity of the proposed method makes the 3T model a potentially valuable tool for remote sensing.

An experimental study and model validation of a membrane humidifier for PEM fuel cell humidification control

This paper presents an experimental study and model validation of an external membrane humidifier for PEM fuel cell humidification control. Membrane humidification behavior was investigated with steady-state and dynamic tests. Steady-state test results show that the membrane vapor transfer rate increases significantly with water channel temperature, air channel temperature, and air flow rate. Water channel pressure has little effect on the vapor transfer rate and thus can be neglected in the system modeling. Dynamic test results reveal that the membrane humidifier has a non-minimum phase (NMP) behavior, which presents extra challenges for control system design. Based on the test data, a new water vapor transfer coefficient for Nafion membrane was obtained. This coefficient increases exponentially with the membrane temperature. The test results were also used to validate a thermodynamic model for membrane humidification. It is shown that the model prediction agrees well with the experimental results. The validated model provides an important tool for external humidifier design and fuel cell humidification control.

Dynamic model for vacuum freeze-drying of waterlogged archaeological wooden artefacts

Abstract A mathematical model for vacuum freeze-drying of waterlogged and PEG impregnated wooden objects has been developed. The model is based on basic physical laws for heat and mass transfer in vacuum and porous objects. The heat transfer to the object is considered to be radiation and the drying to take place as an ice front retreating parallel to the surface of the object. The model was used to determine the heat and water vapour transfer coefficient for waterlogged oak, birch and balsa wood, and for frozen aqueous solutions of PEG 4000. Based on the aforementioned data, theoretical transfer coefficients for PEG treated wood were derived. A numerical model based on linear drying in finite time intervals was developed. The model, which has as its output the position of the ice front, surface and ice temperatures, was found to predict vacuum freeze-drying processes in waterlogged wood and PEG with excellent accuracy. The model can be used as a tool for development of freeze-drying equipment especially designed for waterlogged archaeological wooden objects and for new and less resource demanding freeze-drying procedures.

Theoretical analysis of a remotely measurable soil evaporation transfer coefficient

Reducing soil evaporation ( E) is an important way to increase water use efficiency for agriculture and sustainable water use. To achieve this goal, an accurate estimate of soil evaporation at the macro-scale level is necessary. By including the surface temperature of dry soil, the three temperatures model (3T model) was proposed as an estimate of E, and its temperature term was defined as the soil evaporation transfer coefficient ( h a). Although it has been primarily shown that h a determines the boundaries of E, further studies of its properties are necessary because h a has the application potential for remote sensing. The objectives of this study are to (1) investigate the relationships between h a and those parameters that are important for E estimation but are not able to be remotely measured (aerodynamic resistance, soil surface resistance ( r s), and cumulative evaporation ( E c)); and (2) discuss the possibility for remote sensing application. Two experiments were carried out for these purposes: one in an open field and the other one in a closed growth chamber at constant temperature, humidity, and radiation. The following results were obtained: (1) Given the boundaries of 0 ≤ h a ≤ 1, E was determined between its maximum value (potential evaporation rate) and minimum value (zero); (2) During the period when evaporation is controlled by r s, a linear relation between h a and log ( r s) is observed with a coefficient of determination r 2 = 0.76. Because h a and r s are independent variables with significant differences in properties and magnitudes, these results indicate that h a and r s are well related to each other; (3) During stage 1 of evaporation, cumulative evaporation ( E c ) increased with time while h a remained constant. Afterwards, h a linearly increased with E c. The relationships among h a, E c, r a, and r s would be useful for estimation of evapotranspiration by remote sensing.

乾燥，湿潤，氷板および圧雪路面と大気との間の熱・水蒸気移動

This paper describes water vapor transfer coefficients related to sublimation and evaporation, and heat transfer coefficients between atmosphere and pavement surface under dry, wet, ice plates and packed snow, obtained from wind tunnel tests and a heat energy balance model. In particular, the contact surface area between an asphalt pavement and a snow/ice layer was taken into consideration in the heat conduction flux supplied from the pavement.Consequently, the roughness heights of four kinds of pavement surfaces mentioned above were obtained. Moreover, the heat and water vapor transfer coefficients were formulated as a function of the friction velocity for each surface.