Vapor Pressure Gradient Research Articles

A Contribution to the Analysis of Water Vapor Transport in Porous Building Materials

The suitability of a water vapor transport parameter determined by the cup method for the description of water vapor transport in porous building material is analyzed in the paper. The relative humidity profiles are measured by a transient method with different water vapor pressure gradients. Moisture profiles calculated using a mathematical model are compared with the measured profiles. The goodness-of-fit tests are calculated for different experimental setups. The possibility of application of the analyzed water vapor transport parameter is discussed.

Attributes of Lake Okanagan evaporation and development of a mass transfer model for water management purposes

Understanding the water budget of the valley lakes in the water-stressed Okanagan region of British Columbia is important for allocating resources to maximize social well-being, environmental quality and the economy. However, high uncertainty in existing estimates of lake evaporation prevents sound water resource decision making. To address this uncertainty, buoy- and shore-based eddy covariance and meteorological instrumentation were deployed on the largest of the valley lakes, Lake Okanagan, for approximately 3 years. The objectives were to address the uncertainty in existing Lake Okanagan evaporation estimates by describing seasonal cycles and annual rates and the meteorological attributes controlling evaporation, and developing an accurate and useful model suitable for water managers and policy makers. Results indicate that two sites on Lake Okanagan experienced average annual evaporation of 725 and 835 mm over the study period. The difference can be attributed to spatial differences in surface water temperatures, vapour pressure gradients and atmospheric stability across the lake. Good relationships were found between evaporation rates measured with the eddy covariance systems and meteorological conditions at the offshore buoys, specifically between wind speed and the surface–atmospheric vapour pressure differences. From these relationships, a mass transfer model was developed. Accounting for the seasonal cycle in atmospheric stability increased the accuracy of monthly and annual evaporation estimates from this mass transfer model, but it remains inappropriate to predict daily or hourly evaporation. The study period included years that were climatically typical, so the evaporation observations could represent values close to the long-term mean, but this is unknown. The findings of this study highlight that long-term observations of the atmosphere consistently conditioned to the lake surface are needed for water managers and decision makers to have sound data and information on lake evaporation.

Study on mass transfer of droplets in narrow space

Combinatorial or high-throughput experimentation is improving numerous scientific domains intensely, such as biological chemistry, material science, catalysis. In the past research, most of researchers focus on the novel configuration and microfabrication rather than the transfer mechanism in the microcosm. In this study, we proposed a mass transfer model among droplets in a narrow space. In the derivation, the molecular mass flow rate (JM) formula was used to calculate droplet mass transfer rate, which was controlled by the concentration gradient of the surface vapor pressure (PS) near the droplets. Especially, in the simplification of the expression of the simulating results, all droplets were divided into groups following their radii subtly. In order to assess the accuracy and predictive capability of the proposed model, we carried out some designed experiments to investigate distribution and size changes of the droplets at different temperatures and times. After capturing and analyzing the images of the droplets gathering on a hydrophobic glass slide in an airtight narrow space, we verified the simulating results with the experimental data. The less than 5% deviation indicated the proposed mass transfer model could describe the mass transfer phenomenon among droplets in a narrow space reliably. Furthermore, we found that the total droplet number (ND) decreases with the running time, which should be taken into account discreetly when designing the parallel micro-reactors.

Interrelationships Between Flux, Membrane Properties, and Soil Water Transport in a Subsurface Pervaporation Irrigation System

Oil and natural gas-produced waters are by-products of energy development that present unique challenges to the energy industry and impacted landowners. Conventionally, produced waters are typically saline and therefore must be desalinated to some extent before beneficial reuse. Subsurface pervaporation irrigation combines desalination with beneficial reuse in the form of crop irrigation. This investigation studied factors governing water flux for two membranes, one polyether ester (PEE) and one cellulose triacetate (CTA), for use in subsurface pervaporation irrigation. Water flux was determined to be a function of membrane properties (thickness, hydrophilicity) and environmental variables (vapor pressure gradient, soil texture, soil clay content). Specific water fluxes ranged from a maximum of 6.77×10−2 L/[m2·day·Pa] for CTA membranes to a minimum of 7.97×10−3 L/[m2·day·Pa] for PEE membranes. Fluxes increased as water vapor pressure in the soil adjacent to the membrane decreased. Total soil water potential and its specific components, matric potential and osmotic potential, influenced flux. Clayey and peaty soils have a lower matric potential than sand for the same soil water content, which translated to a lower vapor pressure and hence higher flux for clayey/peaty soils.

Numerical analysis of hygrothermal performance of reflective insulated roof coatings

The use of thermal insulation in roofing systems has been progressively increased in the last few years especially in emerging market countries to reduce energy consumption due to the more frequent use of air conditioners. However, moisture can meaningfully affect the performance of the roof and cause deterioration of the insulation as well as increasing the risk of mold growth. Therefore, for the prediction of performance and risks in roofing systems, a fully combined heat and mass transport model through roofing systems used in hot and humid climates is needed as it is barely explored in the literature. In this way, a mathematical model considering the combined two-dimensional heat, air and moisture transport through unsaturated roofing systems is presented in this paper. In the porous domain, the differential governing equations are based on driving potentials of temperature, moist air pressure and water vapor pressure gradients, while, in the air domain, a lumped approach is considered for modeling the heat and mass transfer through the cavity between the roof sheet and insulation. Results are presented in terms of heat flux and temperature and moisture content distributions profiles within the porous roofing structure, indicating the impact of moisture adsorption and desorption on the roofing hygrothermal performance due to additional transport mechanisms. The paper shows the importance of using a more complete mathematical model – even when conductive latent loads are negligible due to an impermeable reflective insulation – since the mass transport and phase change mechanisms modify the temperature distribution, which cannot be observed by purely conductive heat transfer models presently used in most building energy simulation tools. The roofing short and long-wave radiation heat transfer processes, including the multi-reflective effect of aluminum foil in the air cavity, are also presented and discussed.

A Review of the Bowen Ratio Surface Energy Balance Method for Quantifying Evapotranspiration and other Energy Fluxes

<abstract><title><italic>Abstract.</italic></title> The Bowen ratio energy balance (BREB) method of indirect measurement of latent and sensible heat fluxes and other major components of the surface energy balance is accepted as one of the robust approaches in the fields of agricultural engineering, micrometeorology, water resources research, hydrology, and related disciplines. The adoption of this technology in quantification of surface energy fluxes in practical applications is mainly attributed to the method’s performance and robustness in measurement of such fluxes for various agro-ecosystem surfaces in different climates. The method determines the Bowen ratio (β) by means of measured gradients of atmospheric temperature and air moisture content (actual water vapor pressure) and applies β in the energy balance equation to solve for the latent and sensible heat flux. The main assumption in the Bowen ratio theory is that the energy transfer coefficients for latent heat (K_V) and sensible heat fluxes (K_H) are equal. This assumption is made because all energy scalars are carried by the same eddies and, therefore, these scalars are associated at the same boundary layer of the evaporating surface. A basic criterion of this method is that the air temperature and water vapor pressure are measured (above an evaporating surface) at such heights that the horizontal gradient of air temperature and water vapor pressure can be neglected. The equality (or similarity) assumption (K_V = K_H) has been proven to be valid for a range of field and vegetation surfaces in various climates. However, the assumption has also been proven invalid for some heterogeneous vegetation surfaces. Nevertheless, successful application of the BREB method to measure surface energy fluxes has been reported for many different types of terrestrial surfaces, including agricultural fields, grasslands, forestry, lakes, wetlands, ocean, etc. This article reviews the history; the main scientific, theoretical, and technical principles; the operational characteristics; and some of the advances in instrumentation of the BREB method. Some studies utilizing the BREB method are also presented.

Modeling of Moisture Distribution Evolution in High-Performance Concrete under Fire Exposure

High-performance concrete (HPC) will undergo severe damage under fire conditions. It is well known that vapor pressure induced by high temperatures plays an important role in the damaging process. Therefore, the determination of the moisture distribution evolution in concrete is essential to the damage analysis of heated HPC. This paper presents a numerical method for the prediction of the moisture distribution evolution in HPC under fire conditions. In the method, the vapor pressure and the moisture transport induced by the vapor pressure gradient are analyzed. The effect of the thermal decomposition on the moisture distribution and the effects of the slippage flow and the water saturation degree on the permeability are considered. The proposed method is applied to the moisture distribution analysis of a concrete cube with 90% initial moisture content under fire conditions and can be further used for the analysis of the thermal damage of heated HPC.

Experimental highlight of hygrothermal phenomena in hemp concrete wall

This study investigates the hygrothermal behaviour of a timber-framed hemp concrete wall made of precast blocks. Measurements are performed on an uncoated wall as well as on a coated wall. The experimental device consists in two air-conditioned rooms where ambient conditions are selected to induce temperature and/or vapour pressure gradient between the two sides of the wall. The monitoring deals with temperature and relative humidity within the wall. Kinetics of temperature and of vapour pressure are given. Profiles are drawn at several times of the transient phase. In the regular part of the wall, several kinds of hygric behaviours are highlighted such as homogeneous vapour diffusion and huge vapour pressure variations due to evaporation–condensation and/or sorption-desorption phenomena. The results in the line of frame show that the frame doesn't induce disturbances in the hygrothermal behaviour of the wall. It is also shown that the coating reduces and delays vapour diffusion.

Penman's sink-strength model as an improved approach to estimating plant canopy transpiration

The Penman energy balance equation has become a common approach to estimating the evaporation of plant canopies. Unfortunately, implementation of the energy balance equation requires various assumptions and empiricisms, including “crop coefficients”, so that in practice the Penman equation is used as a framework in which to apply empirical approximations. These limitations are reviewed in this paper. An alternative to the energy balance approach suggested by Penman was a “sink strength” model in which the gradient in vapor pressure was considered the prime driver of plant water loss. In this paper, the sink strength model is developed for determining plant canopy transpiration based on explicitly defined properties of the plants. The key variables are the carbon assimilation pathway of photosynthesis and the biochemical composition of the plant material being synthesized. These two parameters define the transpiration efficiency coefficient and radiation use efficiency that are required in the calculation of transpiration. Also, it is necessary to define a weighted vapor pressure gradient for transpiration when making estimates of the amount of transpiration on a daily basis. The derived expression of the sink strength model was used to compare predicted transpiration with measured values of four turf grass species measured over three years. A close linear correlation was observed for all grasses (P<0.0001). However, there was an indication that the grasses may vary slightly in their radiation use efficiencies, which with measurement of photosynthetic capacity would further improve the predicted transpiration rates. The ability to directly compute transpiration from explicitly defined variables based on plant traits offers an attractive alternative to the Penman energy balance approach.

A new method to measure Bowen ratios using high-resolution vertical dry and wet bulb temperature profiles

Abstract. The Bowen ratio surface energy balance method is a relatively simple method to determine the latent heat flux and the actual land surface evaporation. The Bowen ratio method is based on the measurement of air temperature and vapour pressure gradients. If these measurements are performed at only two heights, correctness of data becomes critical. In this paper we present the concept of a new measurement method to estimate the Bowen ratio based on vertical dry and wet bulb temperature profiles with high spatial resolution. A short field experiment with distributed temperature sensing (DTS) in a fibre optic cable with 13 measurement points in the vertical was undertaken. A dry and a wetted section of a fibre optic cable were suspended on a 6 m high tower installed over a sugar beet trial plot near Pietermaritzburg (South Africa). Using the DTS cable as a psychrometer, a near continuous observation of vapour pressure and air temperature at 0.20 m intervals was established. These data allowed the computation of the Bowen ratio with a high spatial and temporal precision. The daytime latent and sensible heat fluxes were estimated by combining the Bowen ratio values from the DTS-based system with independent measurements of net radiation and soil heat flux. The sensible heat flux, which is the relevant term to evaluate, derived from the DTS-based Bowen ratio (BR-DTS) was compared with that derived from co-located eddy covariance (R2 = 0.91), surface layer scintillometer (R2 = 0.81) and surface renewal (R2 = 0.86) systems. By using multiple measurement points instead of two, more confidence in the derived Bowen ratio values is obtained.

The Misuse of Relative Humidity in Ecological Studies of Hibernating Bats

Field biologists almost invariably report relative humidity as a measure of moisture in the air and assume that relative humidity somehow predicts evaporative water loss from an animal. In this paper, I use the vapor pressure gradient to show that, under conditions of constant relative humidity, evaporative water loss from the body surface of a hibernating bat can vary by more than 100%, depending on ambient temperature. Potential evaporative water loss at constant relative humidity is an increasing curvilinear function of ambient temperature for a torpid bat that has a surface temperature equal to surrounding air temperature, but a decreasing curvilinear function of air temperature for an aroused bat in the hibernaculum. Under some circumstances, evaporative loss actually can be greater in a hibernaculum with higher relative humidity than in one with lower relative humidity. When examining potential differences in evaporative water loss between sites, habitats, or treatments, biologists should consider t...

Analysis of Moisture Transfer During the Drying of Clay Tiles with Particular Reference to an Estimation of the Time-Dependent Effective Diffusivity

The description of the drying process was reduced to the establishment of a series of theoretical and empirical drying models. The complex processes of simultaneous moisture and heat transfer, which are often nonstationary, and the distinct nature and properties of the material to be dried further complicate the description of the drying process. Three theories—diffusion theory, capillary flow theory, and evaporation–condensation theory—have won general recognition for the explanation of moisture transfer in porous media. The mechanisms of moisture movement during drying in the constant and especially in the falling drying period are rather complex and, hitherto, there have been no generally accepted explanations that could identify the exact transition between possible drying mechanisms, such as liquid movement due to capillary forces, liquid diffusion due to concentration gradients, liquid and vapor flow due to differences in total pressure, vapor diffusion due to difference in vapor concentration, vapor diffusion due to partial vapor pressure gradients, Knudsen diffusion, thermodiffusion, and the evaporation–condensation mechanism. The goal of this study was to find a way to better understand the different drying mechanisms, to identify the exact transition between them, and to estimate the time-dependent effective diffusivity. The results presented in this article confirmed that the effective diffusivity represents an overall mass transport property of moisture that includes all possible moisture transport mechanisms that are simultaneously controlling the moisture migration process in a material during drying. The experimental investigations were performed on clay tiles in a laboratory recirculation dryer, for which the drying parameters (humidity, temperature, and velocity) could be programmed, controlled, and monitored during drying.

Hygrothermal bridge effects on the performance of buildings

Although the thermal bridge effects on the building energy performance have been presented in the literature, the multidimensional hygrothermal analysis of the building envelope is still a challenge due to many difficulties such as modeling complexity, computer run time, numerical convergence and highly moisture-dependent properties. However, their effects are of paramount importance due to the local increase of heat and mass flux densities so that moisture can be easily accumulated around internal corners, increasing mold growth risk and causing structural damage. Therefore, for analyzing the effects of building lower and upper corners, a multidimensional model has been developed to calculate the coupled heat, air and moisture transfer through building envelopes. The algebraic equations are simultaneously solved for the three driving potentials — temperature, vapor pressure and gas pressure gradients — to improve the numerical stability of the discretized model. In the Results section, the coupling of the upper corner, wall, lower corner (with different types of foundations), ground and floor are analyzed in terms of temperature and relative humidity profiles, vapor flow and heat flux, showing the importance of a detailed hygrothermal analysis for accurately predicting building energy consumption, mold growth and structural damage risks.

Feasibility assessment of pervaporation for desalinating high-salinity brines

Pervaporation, which is a non-pressure driven membrane process, was evaluated to determine its viability for desalinating high-salinity source waters like those originating from oil and natural gas development (produced water). Two types of membrane material chemistries were studied in order to identify the optimal properties for maximizing the permeate flux under a given set of operating conditions. Permeate flux was determined to be a significant function of membrane thickness and the diffusion coefficient of water through the membrane. The diffusion coefficient is in turn a function of the membrane's affinity for water (hydrophilicity) and its fractional free volume space. A cellulose triacetate membrane (Membrane B) achieved fluxes of 0.06 m3m–2day–1 when treating solutions having salt concentrations of 100 g L–1, comparable to fluxes achieved by other types of non-pressure driven membrane processes. The flux increased in a linear fashion with decreasing ionic strength and improved through increases in the vapor pressure gradient and/or inclusion of a feed channel spacer into the test cell. Salt rejection efficiencies by all membranes were &amp;gt;99%; however, co-ions were able to penetrate into the membrane material matrix over time.

Magnesium vapor nucleation in phase transitions and condensation under vacuum conditions

Recent findings related to coagulable magnesium vapor nucleation and growth in vacuum were assessed critically, with emphasis on understanding these processes at a fundamental molecular level. The effects of magnesium vapor pressure, condensation temperature, and condensation zone temperature gradient on magnesium vapor nucleation in phase transitions and condensation from atomic collision and coacervation with collision under vacuum conditions were discussed. Magnesium powder and magnesium lump condensates were produced under different conditions and characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The right condensation zone temperature approach to the liquid transition primarily improved the magnesium vapor concentration rate. The gas–solid phase transition was primarily inhibited by setting a small condenser temperature gradient. Under the right condensation temperature and temperature gradients, increasing magnesium vapor partial pressure improved crystallization and reduced the oxidation rate.

The effectiveness of bed bathing practices on skin integrity and hospital-acquired infections among adult patients: a systematic review protocol

Review question/objective The objective of this systematic review is to identify, appraise and synthesize the best available evidence on the effectiveness of bed bathing practices on skin integrity and hospital-acquired infection (HAI) among adult patients. More specifically, the review question is: What is the effectiveness of traditional towel bed bath practice compared to other innovate bed bath practices on maintaining skin integrity, skin barrier function and reduction of pathogen microbial counts on skin among adult patients in all settings? Inclusion criteria Types of participants This review will consider studies that include patients over the age of 18 with no upper age limit, regardless of ethnicity and gender, in all hospital wards. Patients with skin allergies, visible damage to their skin, a chronic skin condition (e.g., eczema, or dermatitis), sensitive skin, or pre-existing medical conditions known to affect the dermal vasculature (e.g., diabetes) will be excluded. Types of intervention(s) This review will consider studies that evaluate bed bath practices performed by healthcare staff, in particular nurses. Intervention The intervention of interest is innovate bed bath practices, including all bag bath interventions, not limited to any specific type or brand. For the purpose of this systematic review, bag bath interventions include bathing patients with pre-packaged disposal washcloths by use of a different cloth to wash each part of the patient’s body. The washcloths typically comprise rayon/polyester cloth pre-moistened with an evaporating no-rinse cleanser and emollients. Comparator The comparator is the traditional bed bath (towel bed bath) intervention, regardless of type and frequency. For the purposes of this systematic review, traditional bed bath refers to a practice using one new washcloth for each body part and the equipment includes a washbasin, water, towel, washcloths and soap. Types of outcomes This review will consider studies that include the following outcome measures: Primary outcome: • Hospital-acquired infections defined as infections that becomes evident 48 hours after admission and are measured through surveillance of infection rates. Secondary outcome: • Skin microbiology measured by pathogen microbial counts on skin. • Skin barrier function and skin integrity measured by skin flakes, skin dryness, water vapour pressure gradient and skin pH.

Solar membrane distillation: desalination for the Navajo Nation

Provision of clean water is among the most serious, long-term challenges in the world. To an ever increasing degree, sustainable water supply depends on the utilization of water of impaired initial quality. This is particularly true in developing nations and in water-stressed areas such as the American Southwest. One clear example is the Navajo Nation. The reservation covers 27,000 square miles, mainly in northeastern Arizona. Low population density coupled with water scarcity and impairment makes provision of clean water particularly challenging. The Navajos rely primarily on ground water, which is often present in deep aquifers or of brackish quality. Commonly, reverse osmosis (RO) is chosen to desalinate brackish ground water, since RO costs are competitive with those of thermal desalination, even for seawater applications. However, both conventional thermal distillation and RO are energy intensive, complex processes that discourage decentralized or rural implementation. In addition, both technologies demand technical experience for operation and maintenance, and are susceptible to scaling and fouling unless extensive feed pretreatment is employed. Membrane distillation (MD), driven by vapor pressure gradients, can potentially overcome many of these drawbacks. MD can operate using low-grade, sub-boiling sources of heat and does not require extensive operational experience. This presentation discusses a project on the Navajo Nation, Arizona (Native American tribal lands) that is designed to investigate and deploy an autonomous (off-grid) system to pump and treat brackish groundwater using solar energy. Βench-scale, hollow fiber MD experiment results showed permeate water fluxes from 21 L/m2·d can be achieved with transmembrane temperature differences between 40 and 80˚C. Tests run with various feed salt concentrations indicate that the permeate flux decreases only about 25% as the concentration increases from 0 to 14% (w/w), which is four times seawater salt concentration. The quality of the permeate water remains constant at about 1 mg/L regardless of the changes in the influent salt concentration. A nine-month MD field trial, using hollow fiber membranes and completely off-the-shelf components demonstrated that a scaled-up solar-driven MD system was practical and economically viable. Based on these results, a pilot scale unit will be constructed and deployed on the tribal lands.

Drying Comparison of Nonhygroscopic and Hygroscopic Materials

This paper investigates and presents the simulation of drying for hygroscopic and nonhygroscopic materials. This present work used a coupled mathematical model of mass, heat and gas transfer that implemented to finite element method in two dimensional and numerically compute using Skyline solver to capture highly nonlinear transient process. Bound water contribution was taken into account in the drying of hygroscopic materials by incorporating constitutive equation of bound water. The results showed drying process can be divided into three periods named constant rate period (CRP), first falling rate period (FRP1) and second falling rate period (FRP2). Capillary action is dominated during CRP before vapour diffusion takes place in FRP1. Bound water movement is generated by vapour pressure gradient exists that represent hygroscopic material.

Phoretic forces on aerosol particles surrounding an evaporating droplet in microgravity conditions

The work presents the results of an experimental campaign performed at the Drop Tower Facility (Bremen) in microgravity conditions, concerning the scavenging process of an evaporating single droplet in stationary conditions. In the experimental conditions the thermo- and diffusiophoretic forces are the only ones that can determine the scavenging of the aerosol.The research is finalized to help solve the open question concerning the contribution of thermo- and diffusiophoretic forces in aerosol scavenging process due to cloud droplets. Although earlier theoretical and experimental papers have addressed this problem, the results are contradictory and inconclusive.As phoretic forces depend on aerosol diameter and water vapour pressure gradient, experiments were performed by changing the aerosol diameter (range 0.4μm–2μm) and the water vapour gradient. The experimental results show a prevalence of the diffusiophoretic over thermophoretic force, for the considered aerosol. The measured values of the particle velocities due to phoretic forces increase with increasing aerosol diameter and vapour pressure gradient.

A simple test method for measuring water vapor resistance of porous polymeric materials

A simple test method is proposed for measuring water vapor resistance of fabrics. A piece of cotton fabric connected to a container filled with distilled water through a plastic tube was used on a hot plate to generate a saturated water vapor condition on one side of the sample. The temperature of the cotton fabric (approximation of human skin covered with sweat) was measured by a thermocouple. The water vapor resistance of the sample was determined based on the water vapor pressure gradient across the sample and the heat flux. Five types of textile fabric laminated to PU/TPU membranes, plus one type of conventional fabric, were tested by using this simple apparatus as well as the sweating guarded hot plate instrument. The results showed that good agreement was observed between these two test methods. In addition, the surface temperature of the cotton ‘skin’ varied with different fabrics. This is in accordance with the actual intended situation, i.e., the skin temperature of the body is related to the ability of clothing materials to transfer water vapor. Therefore, this simple test apparatus better simulates real-life conditions than the sweating guarded hot plate instrument.