Vapor Pressure Gradient Research Articles

Performance Comparison of Cross- and Forward-Flow Configurations for Multiple-Effect Vacuum Membrane Distillation.

This work addresses retrofitting the infrastructure of multiple-effect vacuum membrane distillation (V-MEMD) units by using cross-flow configuration (CFC). In this configuration, the feed water is evenly divided and distributed over the effects. In this case, the feed water stream for each effect is kept at a high temperature and low flow rate. This will lead to an increase in the vapor pressure gradient across the hydrophobic membrane and can also maintain the thermal energy of the stream inside the individual effect. It is found that CFC improves internal and global performance indicators of productivity, energy, and exergy. A mathematical model was used to investigate the performance of such a modification as compared to the forward-flow configuration (FFC). The cross-flow configuration led to a clear improvement in the internal performance indicators of the V-MEMD unit, where specifically the mass flux, recovery ratio, gain output ratio, and heat recovery factor were increased by 2 to 3 folds. Moreover, all the global performance indicators were also enhanced by almost 2 folds, except for the performance indicators related to the heat pump, which is used to cool the cold water during the operation of the V-MEMD unit. For the heat pump system, the specific electrical energy consumption, SEEC, and the exergy destruction percentage, Ψdes, under the best-operating conditions, were inferior when the feed water flow was less than 159 L/h. This can be attributed to the fact that the heat rejected from the heat pump system is not fully harnessed.

192 Engineering Strategies and Solutions to Improve Swine Resilience and Alleviate Physiological Stress in a Changing Climate

Abstract Heat stress negatively affects a variety of production parameters in the swine industry including growth, carcass composition, and reproduction. The combination of climate forecasts and improved genetic performance, indicate increasingly negative impacts of heat stress for the future. Therefore, future heat stress mitigation approaches will need engineered solutions that also consider expected economic risk and corresponding uncertainty. Physically modifying the environment is currently the best approach by utilizing the heat dissipation (albeit, poor) mechanisms of the pig, that is, via the 4 modes of heat transfer, conduction, convection, radiation, and evaporation. A temperature gradient for sensible or vapor pressure gradient for latent heat loss are needed to drive heat exchange. Strategies can then be grouped into (a) modifying the thermodynamic properties of the incoming air via the ventilation system; (b) building characteristics or features; (c) animal zone environment modification; and (d) changes in management. Examples of each strategy include (a) evaporative cooling pads, earth-air-heat exchanger; (b) insulation, spatial orientation; (c) fogging, cooling the drinking water, increasing air velocity; and (d) stocking density. Engineering of future controlled swine environments will need enhanced modeling of heat transfer between pigs and their environment, advanced design/analysis methods of environmental control, and creation of innovative technology that improves sensing, controlling, and modifying the environment. Much of the engineering process starts with and relies on established standards that describe key analytical methods for design, data to enable design computations, and criteria/goals for different environments. There is and will continue to be a need for consistent updates to these data to ensure modern intensive housing is able to provide the proper thermoneutral zone, ventilation rates, etc. for prolific pigs in a changing climate.

Accuracy of water vapor permeability of building materials reassessed by measuring cup's inner relative humidity

Cup experiments are the most widely used method to measure the water vapor permeability of porous building materials. For this test, cup assembly is designed to create a vapor pressure gradient across a sample and, thus, to allow vapor diffusion through it. Water vapor permeability is assessed by weighing cup assembly over time. While the external conditions are generally well controlled, appropriate solid desiccants or saturated salt solutions are used to control the conditions inside the cup. However, precise knowledge of these conditions is crucial, as they are used to calculate permeance and hence water vapor permeability. In this study, dry and wet cup experiments were performed with different solid desiccants or saturated salt solutions on nine materials with Sd-values ranging from 0.08 m to more than 1 m. In all the experiments, the relative humidity within the air layer in the cup was measured with wireless sensors. The results showed that relative humidity was rarely the same as the expected value and that the relative humidity of solid desiccant also varied over time, resulting in from 5% to 450% relative errors in the predicted Sd-value. A correction method is proposed to assess the “real” Sd-value from the theoretical Sd-value and differences between dry and wet cup experiments are discussed.

Water Vapour Transport in Biopolymeric Materials: Effects of Thickness and Water Vapour Pressure Gradient on Yeast Biomass-Based Films

Water Vapour Transport in Biopolymeric Materials: Effects of Thickness and Water Vapour Pressure Gradient on Yeast Biomass-Based Films

Membrane Distillation: Recent Configurations, Membrane Surface Engineering, and Applications.

Membrane distillation (MD) is a developing membrane separation technology for water treatment that involves a vapor transport driven by the vapor pressure gradient across the hydrophobic membrane. MD has gained wide attention in the last decade for various separation applications, including the separation of salts, toxic heavy metals, oil, and organic compounds from aqueous solutions. Compared with other conventional separation technologies such as reverse osmosis, nanofiltration, or thermal distillation, MD is very attractive due to mild operating conditions such as low temperature and atmospheric pressure, and 100% theoretical salt rejection. In this review, membrane distillation’s principles, recent MD configurations with their advantages and limitations, membrane materials, fabrication of membranes, and their surface engineering for enhanced hydrophobicity are reviewed. Moreover, different types of membrane fouling and their control methods are discussed. The various applications of standalone MD and hybrid MD configurations reported in the literature are detailed. Furthermore, studies on the MD-based pilot plants installed around the world are covered. The review also highlights challenges in MD performance and future directions.

Influences of Powder Source Porosity on Mass Transport during AlN Crystal Growth Using Physical Vapor Transport Method

We developed a two-dimensional (2D) transport model to investigate mass transport during bulk AlN crystal growth via the physical vapor transport (PVT) process using the finite element method (FEM), taking the powder source porosity, buoyancy, and vapor diffusion into account. The porosity effects of the powder source on mass transport under various growth conditions were investigated in detail. The simulation results show that the porosity of the powder source significantly affects the mass transport process during AlN sublimation growth. When the porosity of the powder source decreases, the growth rate becomes more uniform along the seed deposition surface, although the sublimation rate and crystal growth rate decrease, which can be attributed to the reduced specific surface area of the powder source and the reduced flow rate of Al vapor in the powder source. A flat growth interface can be achieved at a porosity of 0.2 under our specific growth conditions, which in turn facilitate the growth of high-quality AlN crystals and better yield. The decomposition of the powder source and the transport of Al vapor in the growth chamber can be suppressed by increasing the pressure. In addition, the AlN growth rate variation along the deposition surface can be attributed to the Al vapor pressure gradient caused by the temperature difference in the growth chamber.

The effect of minimal differences in the skin-to-air vapor pressure gradient at various dry-bulb temperatures on self-paced exercise performance.

The effects of dry-bulb temperature on self-paced exercise performance, along with thermal, cardiovascular, and perceptual responses, were investigated by minimizing differences in the skin-to-air vapor pressure gradient (Psk,sat - Pa) between four temperatures. Fourteen trained male cyclists performed 30-km time trials in 13°C and 44% relative humidity (RH), 20°C and 70% RH, 28°C and 78% RH, and 36°C and 72% RH. Power output was similar in 13°C (275 ± 31 W; means and SD) and 20°C (272 ± 28 W; P = 1.00), lower in 36°C (228 ± 36 W) than 13°C, 20°C, and 28°C (262 ± 27 W; P < 0.001) and lower in 28°C than at 13°C and 20°C (P < 0.001). Peak rectal temperature was higher in 36°C (39.6 ± 0.4°C) than in all conditions (P < 0.001) and higher in 28°C (39.1 ± 0.4°C) than 13°C (38.7 ± 0.3°C; P < 0.001) and 20°C (38.8 ± 0.3°C; P < 0.01). Mean heart rate was higher in 36°C (163 ± 14 beats·min-1) than all conditions (P < 0.001) and higher in 20°C (156 ± 11 beats·min-1; P = 0.009) and 28°C (159 ± 11 beats·min-1; P < 0.001) than 13°C (153 ± 11 beats·min-1). Mean cardiac output was lower in 36°C (16.8 ± 2.5 L·min-1) than all conditions (P < 0.001) and lower in 28°C (18.6 ± 1.6 L·min-1) than 20°C (19.4 ± 2.0 L·min-1; P = 0.004). Ratings of perceived exertion were higher in 36°C than all conditions (P < 0.001) and higher in 28°C than 20°C (P < 0.04). Self-paced exercise performance was maintained in 13°C and 20°C at a matched evaporative potential, impaired in 28°C, and further compromised in 36°C in association with a moderately lower evaporative potential and marked elevations in thermal, cardiovascular, and perceptual strain.NEW & NOTEWORTHY This is the first study to investigate the effects of dry-bulb temperature (13, 20, 28, and 36°C) on self-paced exercise performance by minimizing differences in the skin-to-air vapor pressure gradient (i.e., evaporative potential) between conditions. Performance was similar in 13°C and 20°C with a matched evaporative potential, whereas it was reduced at 28°C and further impaired at 36°C in association with a large decrease in dry heat loss and moderate reduction in evaporative potential.

Numerical simulation of heat and mass transfer during sublimation drying of porcine aorta

Freeze-drying is an effective method for aortal preservation. However, there are few studies on heat and mass transfer of aortal freeze-drying. In this study, the contactless weighing device was used to monitor freeze-drying process. The radiation view factor of the aortal segment was estimated, and the quasi-steady state model was used to analyze heat and mass transfer, there was a certain amount of heat flux in the frozen layer, which provided sublimation heat for internal sublimation interface. By theoretical calculation, the vapor diffusion coefficient was determined. The unsteady state model was used for numerical simulation. The simulated result of free water removal ratio was consistent with experiment result. Both heat flux density and mass flux density decreased non-linearly in the early stage, but approximately linearly in the late stage. The velocity of inner and outer sublimation interfaces was almost the same. The nodes temperature gradually increased and finally approached the heating temperature, and the temperature gradient of both dried and frozen layers decreased gradually with drying time. The water vapor pressure gradient was smaller near the aortal wall and larger in the middle region.

The overlooked non-rainfall water input sibling of fog and dew: Daily water vapor adsorption on a !Nara hummock in the Namib Sand Sea

The Namib Sand Sea is a hyper-arid coastal fog desert, with limited vegetation scattered across the extensive sand dunes. Minimal and unpredictable rainfall, along with proximity to the ocean, make non-rainfall water inputs (NRWIs), i.e. fog, dew and water vapor adsorption, an important part of the ecohydrology. We studied water accumulation across an elevated hummock formed around a !Nara (Acanthosicyos horridus) thicket in the Namib dunes during February 2020. Continuous measurements of temperature and relative humidity (RH) gradients between the soil and the air were complemented with three nights of intensive NRWI observations using 14 micro-lysimeters. While the overall objective was to study total NRWI accumulation, micro-lysimeter placement was designed to address additional questions about how NRWI accumulation was affected by hummock micro-topography and the position of the !Nara thicket. Water vapor adsorption was the primary form of NRWI, occurring nightly and accounting for at least half of the total water accumulation, even on nights with fog. This was confirmed by the measured water vapor pressure gradients between the air and the soil, which indicated nightly downward vapor fluxes. Actual vapor pressures in the air were similar on nights with and without fog, indicating fog was mostly determined by temperature. Soil RH was used as a proxy for soil water content and was found to be strongly correlated with air RH. Average nighttime air RH was correlated with total nighttime water accumulation with an R2 of 0.69–0.78. Average nighttime air RH was correlated with total nighttime water accumulation with a coefficient of determination (R2) of 0.69–0.78. Even though measurements were limited to the soil, comparison between micro-lysimeters placed on the hummock inside and outside the !Nara thicket indicated consistent, but not significant, lower water contents inside the thicket, suggesting interception of water vapor by the spines and stems of the !Nara. We conclude that water vapor adsorption, though not always appreciated as such, is a large contributor to total NRWI. The fact that water vapor adsorption was the primary NRWI in sand, which has a much lower surface area than most other soil types, and in a place where fog occurs regularly, has implications for deserts worldwide.

Thin film evaporation model for two-phase capillary heat transfer devices: examination of boundary conditions and vapour pressure gradient

Thin film evaporation model has been used by several researchers to study the transport phenomena of two-phase capillary devices, such as heat pipes and capillary pumped loops. The present work is focused on mathematical modeling of the evaporation phenomena in such devices. Liquid-vapour interface in the evaporator is modelled using a thin film model and the lubrication approximation, along with a slip boundary condition at the wall and a shear boundary condition at the interface. Different models are used for the evaporating mass flux and the vapour pressure gradient at the liquid-vapour interface is considered. An attempt is also made to determine the non-evaporating film thickness that satisfies the underlying physics. The film thickness profile and the pressure components are obtained by numerical simulations. It is found that the choice of the evaporating mass flux model has a significant effect on the results, and is very important for heat transfer characterization.

Comparative thermodynamic analysis of membrane-based vacuum air dehumidification systems

Recently, membrane-based vacuum air dehumidification (MVD) systems have been studied as an advanced cooling scheme. In general, the working principle of these systems is based on the separation of water vapor from air caused by a vapor pressure gradient between the feed and permeate side of a membrane when humid air passes over the membrane. After permeating, the vapor is discharged to the atmosphere through a vacuum compressor or pump. Several types of MVD systems with different operation drivers have been introduced recently, but the comparative analyses with the working properties were limited. In this study, three promising MVD systems of the sweep air mixed (S-MVD) type, water vapor discharge (W-MVD) type, and condenser combined (C-MVD) type, are analyzed comparatively with an idealized membrane with infinite selectivity on H2O. From the simulation, the W-MVD system exhibited the best dehumidification performance because of the lowest compression ratio and water vapor flow rate. However, the C-MVD system, which is relatively simple to implement, practically exhibited better performance under limited conditions of a hot and humid climate. After the comparison of the three MVD systems, the applicability of each MVD system for different climates was discussed in detail.

WATER VAPOR TRANSPORT IN PLASTERS CONTAINING SUPERABSORBENT POLYMERS

A proper characterization of material properties represents an important step towards an efficient building design. Considering the present issues in the construction sector, moisture loads pose a risk not only to increased material deterioration but also to the health of building inhabitants. In this paper, modified plaster mixtures with superabsorbent admixture are designed in order to improve passive moderation of finishing layers against varying humidity conditions. The relationship between the amount of applied superabsorbent admixture and resulting water vapor transport properties is identified and the influence of temperature on water vapor transport is analyzed. The steady-state cup method is used for the determination of water vapor transport properties, namely the water vapor diffusion permeability, water vapor diffusion coefficient and water vapor diffusion resistance factor. The obtained data show temperature as a very significant factor affecting water vapor transport in the analyzed plasters. Considering the dry-cup method arrangement, relative humidity probes should be used for monitoring relative humidity under the sealed sample for a sufficiently precise determination of water vapor pressure gradient.

Membrane distillation regeneration of liquid desiccant solution for air-conditioning: Insights into polarisation effects and mass transfer

Membrane distillation (MD) embodies ideal attributes for the regeneration of liquid desiccant solutions used in air-conditioning systems. The MD process has been experimentally proven technically viable for the regeneration of liquid desiccant solutions; however, it suffers severely from temperature and concentration polarisation effects. In this study, for the first time a descriptive mass and heat transfer (DMHT) model is developed to quantitatively describe the mass transfer and the negative impacts of temperature and concentration polarisation during the MD regeneration of the LiCl desiccant solution. The simulation results demonstrate significant reduction in water flux along the membrane due to decreasing mass transfer coefficient (Cm) and transmembrane water vapour pressure gradient (ΔPm). Over the length of the membrane leaf of 0.145 m, water flux reduces by 31% from 11.0 to 7.6 L/m2⋅h. The temperature and concentration polarisation effects cause a substantial decline in the process driving force - ΔPm is only two thirds of the water vapour pressure difference between the bulk feed and distillate (ΔPb). Temperature polarisation is the predominant cause of the reduction in ΔPm compared with ΔPb; however, the negative impact of concentration polarisation is also notable. Finally, amongst the key operating conditions, the inlet feed temperature and concentration exert the most profound influence on the temperature and concentration polarisation during the DCMD regeneration of the hyper saline LiCl solution.

Precipitation extremes influence patterns and partitioning of evapotranspiration and transpiration in a deciduous boreal larch forest

High latitude boreal forests are experiencing dramatic changes in climate and hydrology. It is not clear how boreal forests will adapt to hydrological change or how stable they will be to extreme climate fluctuations and shifts in ecosystem water availability (EWA; residuals between precipitation and evapotranspiration). Although there have been numerous studies in North American and European boreal forests, the Siberian boreal region is underrepresented. Moreover, Siberia is dominated by deciduous conifers (larch) that may have different response to shifting hydrology than boreal evergreens do. We observed evapotranspiration (ET) by eddy covariance technique and transpiration (T) by sap-flow probes on a subsample of trees within the flux-tower footprint through two growing seasons in a larch forest in northernmost China. Ecosystems at the margins of their zone could be amongst the first to experience significant shifts in structure and function. At this site there have already been signs of permafrost degradation and more frequent temperature and precipitation anomalies. The canopy-dominant larch accounted for half the total T fluxes. The remaining 50% was distributed evenly among intermediate and suppressed trees. T is the dominant subcomponent in ET, where overall T/ET varies of 66%–84% depending on precipitation patterns. In dormant and early growing seasons, T still constitutes a majority of ET even though the canopy foliage is not fully developed because cold soil creates a negative soil to air vapor pressure gradient that impedes evaporation. However, in the peak growing season, excess precipitation reduces T while providing sufficient wetness for surface evaporation. ET from standard data product based on MODIS satellite reflectance underestimates tower ET by 17%–29%. Solar-induced chlorophyll fluorescence measured by satellite is well correlated with tower ET (r2 = 0.69–0.73) and could provide a better basis for regional ET extrapolations. A global comparison of data for 2000–2018 period reveals that boreal forests not only have the smallest annual MODIS ET but also the least EWA compared to temperate and tropical forests. Also, even though boreal deciduous and evergreens have comparable annual ET, their T/ET and EWA are distinct. This work highlights how short-term precipitation extremes may shift ecosystem function and structure by changing EWA through exported runoff. Sites along boreal ecotones are critical to observe for signs of shifts in their structure, function, and response to climate anomalies.

Performance comparison of a two-bed solar-driven adsorption chiller with optimal fixed and adaptive cycle times using a silica gel/water working pair

The previously published studies based on the performance prediction of solar-powered adsorption chillers incorporate a fixed cycle time which is either prespecified, or determined as a result of a numerical optimization procedure. However, the cycle time of a solar-powered commercial adsorption chiller cannot be expected to remain constant with the number of cycles owing to the continuously varying intensity of solar radiation from sunrise till sunset. This paper presents the first attempt of comparing the numerically predicted performance of a solar-powered two-bed silica gel/water adsorption chiller based on adaptive and fixed cycle time conditions using a two-stage iterative optimization. The adsorption/desorption (ads/des) stage for the adaptive cycle time condition is terminated as soon as the ads/des uptakes reach their corresponding equilibrium values, while the preheating/precooling (PH/PC) stage is ended as soon as the vapor pressure gradient inside either of the two beds becomes negligibly small. The optimal ads/des as well as the PH/PC times for the adaptive cycle time condition have been used as a baseline for maximizing system performance for the fixed cycle time condition using a two-stage iterative optimization, and the two optimized cycle time conditions have been compared in the context of commercial applications.

Micrometeorological methods to estimate sugarcane evapotranspiration in coastal northeastern region of Brazil

The aim of the present work was to evaluate the performance of Bowen ratio-energy balance method, as well as the energy balance closure by Eddy covariance technique for a sugarcane crop in Brazilian northeastern region. Micrometeorological measurements were carried out between June 7th and November 17th, 2013. Latent and sensible heat fluxes were determined through Eddy covariance technique (EC) and by the Bowen ratio-energy balance method (BREB), considering two approaches. The first, estimated the air temperature and water vapour pressure gradient in two levels above the canopy. The second method measured the air temperature and water vapour pressure at the first level and the surface temperature from radiometric measurements. Latent heat flux was also estimated as energy balance residue from determinations of the sensible heat flux by Eddy covariance. The degree of energy balance closure was dependent on the time of the day considered. Bowen ratio - energy balance estimated from the first approach, showed the best agreement with the eddy covariance measurements to estimate latent heat flux, while in the second case, when the Bowen ratio was estimated using the surface temperature, the linear relationship was the most discrepant. Therefore, the Bowen ratio conventional method is more suitable for estimating latent heat flux in sugarcane.

The Effect of Spacer Orientations on Temperature Polarization in a Direct Contact Membrane Distillation Process Using 3-d CFD Modeling

Membrane distillation is an emerging technology that uses hydrophobic membranes to separate nonvolatile solids from liquids. The vapor pressure gradient between the feed and the permeate sides drives the process. Low-grade thermal energy is used to heat feed water and create a pressure gradient. A large vapor pressure gradient across the membrane surfaces results in high permeation rates. The feed spacer is an important element of the membrane module that forms channels for feed and permeate flow. A good feed spacer design helps improve permeation. In this paper, 3-d CFD simulations are carried out for spacer-filled channels, and the effect of inlet velocity, filament orientation and spacing on heat transfer is studied. Temperature polarization is used as the parameter for heat transfer performance evaluation. Shear stress and temperature polarization index have been calculated for different spacer orientations in a direct contact membrane distillation process. The results show a major influence of the studied parameters on temperature polarization and shear stress. A comparison of 2-d and 3-d analyses reveals that the average shear stress in the two approaches is nearly the same, but the standard deviation of shear stress is lower for the 2-d case. Similarly, the average value and the standard deviation of temperature polarization index are lower than those obtained in the 3-d analysis. The findings also show that for staggered axial filaments, the temperature polarization index distribution is more uniform suggesting that such orientations are more suitable for enhancing heat transfer in a membrane distillation process.

Moisture transfer by turbulent natural convection

We present an experimental and numerical study of natural convection with moist air as convecting fluid. By simplifying the system as two-component convection, an experimental method is proposed for indirectly measuring the moisture transfer rates in buoyancy-driven flows. We verify the results using direct numerical simulations. It is found that the non-dimensionalized transfer rates for both sensible heat ($Nu_{T}$) and water vapour ($Nu_{e}$) are essentially determined by a generalized Grashof number $Gr$ (the ratio of combined buoyancy generated by the imposed temperature and vapour pressure gradients to viscous force), and are only weakly dependent on the buoyancy ratio $\unicode[STIX]{x1D6EC}$ (the ratio of buoyancy induced by temperature variation to that due to vapour pressure variation). Moreover, we show that the full set of control parameters $\{Gr,\unicode[STIX]{x1D6EC},Pr,Sc\}$ is more suitable than other choices for characterizing the two-component system under investigation. As a special case, the Schmidt number dependence for passive scalar transport rates in buoyancy-driven flows is also deduced.

Seasonal near-surface air temperature dependence on elevation and geographical coordinates for Pakistan

This study evaluates the monthly, seasonal, and annual characteristics of near-surface air temperature as a function of both elevation and geographical coordinates over Pakistan, using a climatic data set from 53 observation stations ranging between 5 and 2317 m asl and stepwise multiple regression. Forcing processes for temperature gradient (TG) variation were determined by analyzing its relationship with moisture flux (qv) and gradients of rainfall and saturation vapor pressure (es). The bi-model pattern of TGE (temperature gradient with topographic elevation), which earlier studies (TAAC 113(3):671-682, TAAC 132:1129-1141) from the southern slopes of the Himalayas have observed, is also consistent in this study region. However, the forcing strengths, mechanisms, and processes for the monthly variations in TGs magnitude and their diurnal range are somewhat different. The annual cycle of TGE in this study is inversely (positively) associated with the rainfall (es) gradient. Monsoon and orographic controls for TGE are stronger in summer, especially in the day, while the influences of inversions and mountainous topoclimate are higher during the non-monsoon period, in particular in winter and at night. Mountain barrier effects are higher in summer, which considerably hamper the relationship between temperature and geographical coordinates and rainfall and elevations. Steeper values of mean TGEs in the non-monsoon period are the result of strong dry convectional cooling at higher elevations due to high thermal forcing effects at lower elevations. Sensible heating reduction due to westerly-driven precipitation and cloud cover in daytime in the northern mountainous region (or higher latitudes) further strengthens the TGE value in this period. This effect is weaker in the surrounding regions; therefore, the TGE magnitude in this study is expected to be higher, especially in the pre-monsoon season. Gradients are more shallow in winter (excluding latitude), but values are greater than those in summer, due to distinct variations in moisture and cloud amount, i.e., lower in winter and higher in summer, particularly at lower elevations. Observed distinct differences in TG diurnal range between elevation (high) and latitude (low) in this study are due to a wide variation in temperature, topography, and elevations within the same latitudinal belts. In this study, the lowest gradient values for elevation are observed in August and the highest in May, 1 month later than observed in adjacent mountain regions due to the late arrival of monsoon moisture in summer and intense thermal forcing effects following the wet early pre-monsoon months. Distinct variability of gradient magnitudes between the northern mountainous region and the southern flat terrain is due to differences in topoclimates, moisture amounts, elevations, and geographical coordinates.

Impact of test parameters on the water vapor permeability of textiles

PurposeUnlike other materials, textiles associate with aesthetic and mechanical properties such as flexibility and removability that allow them to be deployed or folded as required and which make them good candidates for clothing and furnishing but also, eventually, for other applications such as building. Actually, the clothing should ensure appropriate heat and mass transfers between the human body and its environment in order to maintain the thermo-physiological comfort. For that, it is important to determine water vapor permeability (WVP) of textile. Several normalized procedures with variants depending on the nature of the tested material exist to measure the WVP. One of the methods used is the “dish method” described by the British Standard (BS 7209). The purpose of this paper is to determine the influence of the test parameters on the WVP measurements.Design/methodology/approachConsequently, WVP of different textiles was measured while varying several parameters like: nature of fabrics, air layer thickness, vapor pressure gradient and air velocity.FindingsA decrease in the WVP values was observed with an increase in the air layer thickness and the number of textile layers. On the other side, an increase in the water pressure gradient induces an increase in the WVP value. Finally, it was also observed that air velocity has an impact on the WVP measurements.Originality/valueIn addition to intrinsic properties of fabrics, i.e., nature of fiber, woven structure, the influence of the several extrinsic properties, i.e., the influence of the air layer thickness, the number of textile layers, the vapor pressure gradient and the air velocity, on the WVP were investigated. Some researchers have already investigated the impact of these parameters on the WVP measurement separately. However, this study presents a difference from other studies that it takes into account the influence of the both intrinsic and extrinsic properties on WVP. In addition to these, this work combine several extrinsic properties which are presented separately during other studies. The first time, in this study the influence of the air velocity on WVP was investigated. Results on both hydrophilic and hydrophobic fabrics showed a great variation in the results when varying the location of the cups inside the climatic chamber. This is the reason why future studies look at studying more deeply the effect of air velocity on the WVP properties on different types of fabrics by connecting WVP values with air velocity values. It is also planned to make tests with the rotation device and by fixing the value of the temperature and RH. The objective will be to obtain reliable values that do not take into account the effect of air velocity.