Evaporative Resistance Research Articles

Evaluation of Protective Ensemble Thermal Characteristics Through Sweating Hot Plate, Sweating Thermal Manikin, and Human Tests

The purpose of this study was to evaluate the predictive capability of fabric Total Heat Loss (THL) values on thermal stress that Personal Protective Equipment (PPE) ensemble wearers may encounter while performing work. A series of three tests, consisting of the Sweating Hot Plate (SHP) test on two sample fabrics and the Sweating Thermal Manikin (STM) and human performance tests on two single-layer encapsulating ensembles (fabric/ensemble A = low THL and B = high THL), was conducted to compare THL values between SHP and STM methods along with human thermophysiological responses to wearing the ensembles. In human testing, ten male subjects performed a treadmill exercise at 4.8 km and 3% incline for 60 min in two environmental conditions (mild = 22°C, 50% relative humidity (RH) and hot/humid = 35°C, 65% RH). The thermal and evaporative resistances were significantly higher on a fabric level as measured in the SHP test than on the ensemble level as measured in the STM test. Consequently the THL values were also significantly different for both fabric types (SHP vs. STM: 191.3 vs. 81.5 W/m2 in fabric/ensemble A, and 909.3 vs. 149.9 W/m2 in fabric/ensemble B (p < 0.001). Body temperature and heart rate response between ensembles A and B were consistently different in both environmental conditions (p < 0.001), which is attributed to significantly higher sweat evaporation in ensemble B than in A (p < 0.05), despite a greater sweat production in ensemble A (p < 0.001) in both environmental conditions. Further, elevation of microclimate temperature (p < 0.001) and humidity (p < 0.01) was significantly greater in ensemble A than in B. It was concluded that: (1) SHP test determined THL values are significantly different from the actual THL potential of the PPE ensemble tested on STM, (2) physiological benefits from wearing a more breathable PPE ensemble may not be feasible with incremental THL values (SHP test) less than approximately 150–200 W·m2, and (3) the effects of thermal environments on a level of heat stress in PPE ensemble wearers are greater than ensemble thermal characteristics.

Degradation behavior of a commercial 13Cr ferritic stainless steel (SS405) exposed to an ambient air atmosphere for IT-SOFC interconnect applications

The objective of this work was to verify the feasibility of using low Cr ferritic stainless steels to improve IT-SOFC interconnect properties in terms of electrical conductivity and Cr evaporation resistance by taking advantage that low Cr ferritic stainless steels may form conductive and moisture-stable oxide layers of chromite spinels. In this context, surface degradation of a commercial 13Cr ferritic stainless steel (SS405) was studied at 700 °C in ambient air (containing ca. 3% H2O by volume) for up to nearly 5000 h. The results showed a slow, nearly linear oxidation rate indicating that the oxide scale could not act as a fully protective barrier during a prolonged exposure at 700 °C. Oxidation-induced degradation was mainly due to the effects of Cr depletion in the subscale matrix region and internal oxidation attack. However, analysis of the oxide scale after long-term exposure revealed the presence of an interesting example of all-spinel structure consisting of outer layer of iron-substituted Mn–Cr spinel particles on the top of an inner and continuous Fe–Cr spinel layer. Since SS405 steel contains also Al and Si as minor additions, the distribution of these elements in the spinel oxide scale was studied and found to be rather different. Whereas Si was present exclusively in the inner oxide regions, Al migrated also to the outermost part of the oxide scale. Possible explanations for this unexpected Al surface enrichment are discussed.

Thermo-physiological and comfort properties of Ugandan barkcloth from Ficus natalensis

The United Nations Educational, Scientific and Cultural Organization in 2005 proclaimed that Ugandan barkcloth largely produced from mutuba tree (Ficus natalensis) as a “Masterpiece of the Oral and Intangible Heritage of Humanity”. An exploratory investigation of thermo-physiological and comfort properties of barkcloth, a nonwoven material produced through a series of pummeling processes from mutuba tree in Uganda, is fronted. Barkcloth was extracted from the F. natalensis tree in Nsangwa village, Buyijja parish in Mpigi district, Central Uganda. Thermal conductivity, thermal diffusivity, thermal absorptivity, thermal resistance, fabric thickness, and peak heat flow density were measured using an Alambeta device, whereas a Permetest device was used for the measurement of the moisture vapour permeability and evaporation resistance. The study was carried out under relative humidity of 40% and at a laboratory room temperature of 24°C and the results show that the thermal conductivity is in the range of cotton fabrics rendering barkcloth from F. natalensis, a comfortable fabric. The lower value of thermal absorptivity of barkcloth compared to the value of cotton renders the fabric a warm feeling when in contact with the skin. Barkcloth had a higher moisture vapor permeability compared to cotton and other fabrics, meaning its clothing comfort properties are reasonable.

The effect of metallisation, porosity and thickness on the thermal resistance of two-layer fabric assemblies

In this study, we evaluated the thermal resistance of various combinations of commercially available, metallised and conventional, mid- and shell-layer materials, and additionally investigated the effect of plasma metallisation of the non-metallised materials. The evaporative resistance of the shell layers was measured, and did not increase due to plasma metallisation. Metallised mid- and shell-layers increased the thermal resistance of fabric assemblies, and this increase was greatest when the thickness and optical porosity of the mid-layer were suitably high.

Influence of Wearing Impact Protective Garment on Thermophysiological Comfort of the Wearer

In this study one commercially available hip impact protective garment was evaluated in terms of thermo-physiological wear comfort, namely Dry Thermal Resistance, Evaporative Resistance and Permeability Index. Objective evaluation of thermal characteristic of this hip impact protective garment is carried out by means of a Thermal Manikin. The impact protective garment was evaluated in ensemble with other garments.

Heat stress evaluation of two-layer chemical demilitarization ensembles with a full face negative pressure respirator.

The purpose of this study was to examine the heat stress effects of three protective clothing ensembles: (1) protective apron over cloth coveralls including full face negative pressure respirator (APRON); (2) the apron over cloth coveralls with respirator plus protective pants (APRON+PANTS); and (3) protective coveralls over cloth coveralls with respirator (PROTECTIVE COVERALLS). In addition, there was a no-respirator ensemble (PROTECTIVE COVERALLS-noR), and WORK CLOTHES as a reference ensemble. Four acclimatized male participants completed a full set of five trials, and two of the participants repeated the full set. The progressive heat stress protocol was used to find the critical WBGT (WBGTcrit) and apparent total evaporative resistance (Re,T,a) at the upper limit of thermal equilibrium. The results (WBGTcrit [°C-WBGT] and Re,T,a [kPa m2 W−1]) were WORK CLOTHES (35.5, 0.0115), APRON (31.6, 0.0179), APRON+PANTS (27.7, 0.0244), PROTECTIVE COVERALLS (25.9, 0.0290), and PROTECTIVE COVERALLS-noR (26.2, 0.0296). There were significant differences among the ensembles. Supporting previous studies, there was little evidence to suggest that the respirator contributed to heat stress.

Effect of W and Ti addition on oxidation behavior and area-specific resistance of Fe–22Cr–0.5Mn ferritic stainless steel for SOFCs interconnect

Ferritic stainless steel (FSS) is an appropriate material to be used in solid oxide fuel cells (SOFCs) interconnect. Chromium in FSS evaporates at high temperature and it is deposited on the cathode surface of SOFC and makes the cell poisonous. So the present study, the effect of W and Ti addition on oxidation behavior, electrical property and Cr evaporation resistance were discussed in terms of microstructure property of the oxide scale by using SEM analysis. W and Ti addition reduce the oxidation rate, Cr evaporation rate and area-specific resistance of the FSS. Presence of W in this steel formed chi (χ) phase in scale/alloy interface and prevented the diffusion of cation to the oxide scale. It is also a barrier for the influence of the oxygen anion to the inward of the FSS. The presence of high amount of W and low amount of Ti were effective in improving oxidation and electrical conductivity. However, to enhance the Cr evaporation resistance, high Ti content is required.

Molecular Mechanism of Stabilization of Thin Films for Improved Water Evaporation Protection

All-atom molecular dynamics simulations and experimental characterization have been used to examine the structure and dynamics of novel evaporation-suppressing films where the addition of a water-soluble polymer to an ethylene glycol monooctadecyl ether monolayer leads to improved water evaporation resistance. Simulations and Langmuir trough experiments demonstrate the surface activity of poly(vinyl pyrrolidone) (PVP). Subsequent MD simulations performed on the thin films supported by the PVP sublayer show that, at low surface pressures, the polymer tends to concentrate at the film/water interface. The simulated atomic concentration profiles, hydrogen bonding patterns, and mobility analyses of the water-polymer-monolayer interfaces reveal that the presence of PVP increases the atomic density near the monolayer film, improves the film stability, and reduces the mobility of interfacial waters. These observations explain the molecular basis of the improved efficacy of these monolayer/polymer systems for evaporation protection of water and can be used to guide future development of organic thin films for other applications.

Visualization experiments on the condensation process in heat pipe wicks

Visualization experiments are conducted to explore the condensation process in sintered copper wicks of operating flat-plate heat pipes. Two working fluids, water and methanol, and two wick types, mesh and powder wicks, are investigated. The effect of non-condensable gas has been carefully controlled at an insignificant level. The steady-state condenser resistances Rc and the evaporator resistances Re are measured at incremented heat loads Q. For water, cyclic dropwise condensation is observed on exposed wick tops due to the finite water/copper contact angle. For methanol, the condenser is submerged in the liquid as a result of zero contact angle and ineffective liquid delivery out of the condenser. This leads to quiescent methanol condensation with higher Rcs. For all the tests, Rcs (in Kcm2/W) are several times larger than Res. The main reasons are the thicker liquid layer in the condenser wick and the large thermal resistance associated with the condensed liquid layer, with the latter dominating. Considerable under-prediction of Rc may exist in current theoretical models which simply applies the conduction-limit model without considering the thermal resistance from the condensed-liquid layer.

Study on thermal and evaporative resistances of multilayered fabric ensembles

The present work deals with the study on thermal and evaporative resistance of multilayered fabric ensembles meant for cold weather applications. Three-layered structure is used to study the thermal comfort properties. Knitted fabric and polytetrafluoroethylene coated fabrics were used in inner and outer layer, respectively. Needle punched fabrics produced from polyester fibre were used in middle layer. Fifteen different non-woven fabrics were produced according to Box and Behnken experimental design for three variables and three levels by varying mass per unit area, punch density and depth of needle penetration. The produced fabrics were evaluated for thermal and evaporative resistances with and without inner and outer layer fabrics. Thickness, air permeability, bulk density and porosity of the needle punched fabrics were studied. The properties of the fabrics were analyzed for statistical significance by using ‘Design-Expert’ statistical software. Artificial neural network model was developed to predict the properties of fabrics and validation of model was done with the testing data-set. The performance of prediction was evaluated by mean square error, mean absolute error percentage and correlation coefficient. It was concluded that the predicted properties of fabric correlated well with the experimental results.

Thermophysiological comfort characteristics of plated knitted fabrics

Thermophysiological comfort properties of polyester elastane-plated fabric have been studied by varying the polyester filament shape factor, elastane linear density, and fabric loop length. Thermal and evaporative resistances were measured using sweating guarded hot plate. Absorption and wicking properties were measured using M/K GATS. Moisture management properties were measured by moisture management tester. Multilinear regression equations were calculated for all the properties by using generalized linear model (GLM). The hypothesis tests were performed on estimated model (GLM) and the probability values (p-values) were calculated for each variable. Most of the heat and moisture transmission properties were found to be significantly affected by all three variables (i.e. filament shape factor, elastane linear density, and fabric loop length) at 95% level of significance.

Statistical and ANN analysis of thermal and evaporative resistances of multilayered fabric ensembles

In this paper, thermal and evaporative resistance of multilayered fabrics meant for cold weather conditions have been reported. Polyester hollow fibers of 6 denier and 15 denier were used to produce needle-punched fabrics. Full factorial experimental design was used to produce 30 different needle-punched nonwoven fabrics from two different linear densities of fiber by varying mass per unit area and punch density at three and five different levels, respectively. The needle-punched nonwovens were used as insulative middle layer. Single jersey knitted fabric and polytetrafluoroethylene-coated fabric was used as inner and outer layer, respectively. The multilayered fabric ensembles were evaluated for thermal and evaporative resistance using sweating guarded hot plate (SGHP) system. Regression equations were developed to draw the contour plots and to analyze the effect of different parameters on thermal and evaporative resistance of fabrics. Two independent one-way analysis of variance (ANOVA) were conducted to find the significance of linear density of fiber and effect of inner and outer layers on thermal and evaporative resistance of fabrics. The two-way ANOVA was conducted to analyze the effect of mass per unit area and punch density on thermal properties of fabrics and ‘F’ values were calculated. Mean square values of pure error and lack of fit were studied to analyze the fitness of the developed model for thermal properties of fabrics. An artificial neural network (ANN) model was developed to predict the thermal and evaporative resistance of multilayered fabrics and compared with the experimental values. It has been observed that the ANN model predicts the thermal and evaporative resistance of multilayered fabrics with high degree of accuracy.

Modeling of thermal properties of multilayered fabrics by ANN consisting of polypropylene needle-punched nonwovens

This paper presents the prediction of thermal and evaporative resistances of multilayered fabrics meant for cold weather conditions using artificial neural network (ANN) model. Thermal and evaporative resistances of fabrics were evaluated using sweating guarded hot plate method. The significance and interdependency of thickness on other fabric and process parameters and its effect on prediction performance of ANN model is analyzed in detail. For this purpose, two different network architectures were used to predict the thermal properties of multilayered fabrics. In both the networks, three-layer structure consisting of input, hidden and output layers was used. First, network was constructed with four input parameters, namely linear density of fiber, mass per unit area, punch density, and thickness of nonwoven fabric which predicts thermal and evaporative resistances. Second network was made with three input parameters, namely linear density, mass per unit area, and punch density. The network parameters were optimized to give minimum mean square error (MSE), mean absolute error percentage, and good correlation coefficient. The trend analysis was conducted and influence of various input parameters on the thermal properties of multilayered fabrics was studied. The significance of each input parameter in the prediction of thermal properties was studied by carrying out sensitivity analysis. The mean square error of the test dataset before and after the exclusion of the corresponding input parameter is taken for analysis. The input parameters were ranked based on the MSE ratio of test dataset. The predicted thermal properties of multilayered fabrics are correlated well with the experimental values. It was observed that the ANN model with minimum input parameters, namely linear density of fiber, mass per unit area, and punch density can predict the thermal properties of multilayered fabrics with good accuracy.

The stability performances of epoxy resin-based monolayers on resisting disruption of temperature and wind

In this paper, epoxy resin-based monolayers (ER monolayers) were prepared as water evaporation retardants. The interfacial stability performances of molecular films on resisting disruption of temperature and wind were investigated. The dependence of evaporation resistance on temperature was explicit and supported. Energy barrier theory was performed to explain the retardation mechanism of water evaporation of ER monolayers. The disruption of wind on monolayers was researched through simulation of surface waves over the subphase water. The results showed that ER monolayers presented better stability to resist disturbances of temperature shift and surface waves compared to C16OH monolayer. ER monolayers had a good effect on retardation of water evaporation due to their ability to spread spontaneously and form closely packed films over the water surface.

Visualization experiments for groove-wicked flat-plate heat pipes with various working fluids and powder-groove evaporator

This work experimentally compares the evaporation characteristics of three working fluids: water, methanol and acetone, in groove-wicked flat-plate heat pipes. In addition, the performance enhancement by filling copper powders in the evaporation section is investigated. The parallel, U-shaped grooves have a width of 0.25mm and a depth of 0.16mm. With stepwise increase of heat load, the behavior of the working fluid in the grooves is visualized and the evaporator resistances are measured. For all the three fluids, longitudinal liquid recession with a steep-sloped liquid front can be visualized above a certain heat load. Behind the short liquid front is the accommodation region where the meniscus appears to anchor on the top corners of the groove walls. Under a thermally stable situation, independent longitudinal oscillations of the liquid front exist in different grooves, forming a constantly varying zigzag front line. In response to increasing heat load, the liquid fronts gradually leave the heated zone, accompanied by increasing plate temperatures. Water exhibits the best performance with highest maximum heat loads and lowest evaporator resistances. However, the differences in the maximum heat loads for different fluids are much smaller than the differences in their figures of merit. No boiling is observed in the groove wick for all the three fluids. With the groove-powder evaporator, the evaporation performance is greatly improved for water. However, the improvements for methanol and acetone are not as significant as for water. Condensation modes for the three fluids are also compared. Dropwise condensation is observed for water, and film condensation for methanol and acetone.

Analysis of thermal properties of multilayered fabrics by full factorial and Taguchi method

The present paper reports the thermal and evaporative resistances of multilayered fabric ensembles meant for cold weather conditions. A three-layered structure was used to simulate the multilayered fabric ensemble. Knitted, through air-bonded nonwoven and coated fabrics were used as inner, middle and outer layer fabrics, respectively. Inner and middle layer fabrics were varied in mass per unit area in three different levels. Three different coated fabrics, namely, polyester polymer-coated fabric and two different polytetrafluoroethylene-coated fabrics which vary in pore size and porosity were used in outer layer. Taguchi’s L9 orthogonal array, meant for three variables and three levels were used to form nine different multilayered fabric ensembles. Signal to noise ratio was studied to predict and optimize the thermal properties of multilayered fabric ensembles. Full factorial design was used to study the effectiveness of predicting thermal properties of multilayered fabrics using Taguchi’s approach. Regression equations were developed and contour plots were drawn to analyse the effect of chosen parameters on thermal properties of fabrics. In both the methods, the percentage contributions of each factor for thermal and evaporative resistances of multilayered fabrics were studied using sum of squares method. It was found that the Taguchi’s method reduces the number of experiments significantly and predicts the thermal properties of multilayered fabrics with good prediction performance. Mass per unit area of inner layer was found to have no significant effect on thermal and evaporative resistances of multilayered fabric ensembles. Mass per unit area of through air-bonded nonwoven fabric was found to be the most significant parameter in determining the thermal resistance of multilayered fabrics. Mass per unit area of middle layer and pore size of coated fabrics were found to contribute almost equal level in deciding the evaporative resistance of multilayered fabrics.

Evaluation of thermal and evaporative resistances in cricket helmets using a sweating manikin

The main objective of this study is to establish an approach for measuring the dry and evaporative heat dissipation cricket helmets. A range of cricket helmets has been tested using a sweating manikin within a controlled climatic chamber. The thermal manikin experiments were conducted in two stages, namely the (i) dry test and (ii) wet test. The ambient air temperature for the dry tests was controlled to ∼23 °C, and the mean skin temperatures averaged ∼35 °C. The thermal insulation value measured for the manikin with helmet ensemble ranged from 1.0 to 1.2 clo. The results showed that among the five cricket helmets, the Masuri helmet offered slightly more thermal insulation while the Elite helmet offered the least. However, under the dry laboratory conditions and with minimal air movement (air velocity = 0.08 ± 0.01 ms−1), small differences exist between the thermal resistance values for the tested helmets. The wet tests were conducted in an isothermal condition, with an ambient and skin mean temperatures averaged ∼35 °C, the evaporative resistance, Ret, varied between 36 and 60 m2 Pa W−1. These large variations in evaporative heat dissipation values are due to the presence of a thick layer of comfort lining in certain helmet designs. This finding suggests that the type and design of padding may influence the rate of evaporative heat dissipation from the head and face; hence the type of material and thickness of the padding is critical for the effectiveness of evaporative heat loss and comfort of the wearer. Issues for further investigations in field trials are discussed.

Effect of the Process of Lamination Microporous Nanofiber Membrane on the Evaporative Resistance of the Two-Layer Laminate

This thesis deals with the assessment of the effect of the process of lamination microporous nanofiber membrane on the overall evaporative resistance of the two-layer laminate. For most multi-layer materials, lamination process is due to the deterioration of some properties of the whole laminate, particularly to the increase of the evaporative resistance. The authors of this thesis investigate the effect of the lamination primarily on the evaporative resistance of two-layer laminates. The study deals with the relations of volume porosity with the values of the evaporative resistance of the laminate too. In this thesis are both measured the separated layers of selected materials (upper material and the membrane), then the surface materials with the coated lamination points and subsequently created two-layer laminates.

Vapor chambers with jumping-drop liquid return from superhydrophobic condensers

In closed-loop phase-change systems, self-propelled jumping drops on a superhydrophobic condenser offer a new mechanism to return the working fluid to the evaporator, eliminating the requirement for either external forces or wick structures along the return path. Here, we report the heat transfer performance of a jumping-drop vapor chamber consisting of two parallel plates, a superhydrophobic condenser and a superhydrophilic evaporator. With proper removal of the non-condensable gases including those dissolved in the working fluid, the overall thermal resistance of the jumping-drop chamber is primarily governed by the conduction resistance of the wicked evaporator in series with the interfacial resistances at the phase-change interfaces. The lumped resistance model was verified by systematically varying the parameters of the chamber such as wick thickness, vapor spacing, vapor temperature, and heat flux. As an alternative mechanism to transport the working fluid, the jumping drops can enable novel phase-change heat transfer systems such as planar thermal diodes and heat spreaders, for which this work provides practical design guidelines.

Evaporation rates across a convective air boundary layer are dominated by diffusion

Key Points The contributions of diffusion and advection to evaporative flux are quantified Diffusion dominates evaporation flux from partially wet surfaces Surface water content‐dependent evaporative resistance expression is developed