Byron W. Jones

Capabilities and limitations of thermal models for use in thermal comfort standards

Thermal models of the human body and its interactions with the surrounding thermal environment are often proposed, and to some extent are used, as the basis for thermal comfort standards. These models range from simple, one-dimensional, steady-state simulations to complex, transient, finite element codes with thousands of nodes. The models are potentially very useful in that they provide a straightforward means to incorporate the numerous physical variables that affect comfort. Some models can be applied to complex situations which would be difficult, if not impossible, to reflect in simple charts or equations. Whether simple or complex, all of these models have limitations for use in standards. These limitations include the accuracy of the physical simulation and the accuracy of the inputs to the model. Perhaps, the biggest limitation is the accuracy with which comfort perceptions can be related to the physiological variables simulated in the thermal models.

EXPERIMENTAL TECHNIQUES FOR MEASURING PARAMETERS DESCRIBING WETTING AND WICKING IN FABRICS

Once capillary pressure and permeability are determined for saturations ranging from near zero to 100%, liquid transport related to both wicking and wetting behavior can be described by Darcys equation. The purpose of the work reported here is to assess and develop experimental techniques that allow capillary pressure and per meability to be measured over a wide range of saturations. Cotton and polypropylene fabrics are the test materials. Capillary pressure head is measured as a function of saturation for cotton and polypropylene fabric samples using the column test, and permeability is measured as a function of saturation using the siphon test. The siphon test works for cotton but not for polypropylene. A new method using a transient measurement technique is developed to determine the permeability of both samples as a function of saturation; it works well for both samples.

Modeling of heat and moisture transport by periodic ventilation of thin cotton fibrous media

Abstract In walking conditions, the air spacing between the fabric layer of a porous clothing system and the human skin changes with the walking frequency. This change will cause air penetration in and out of the clothing system depending on the fabric air permeability. The air passing through the fabric can considerably reduce the heat and moisture transfer resistance of the clothing system and its suitability for a given thermal environment. In this work, the coupled convection heat and moisture exchange within the clothing system subject to sinusoidal air layer thickness variation about a fixed mean is experimentally investigated and theoretically modeled to predict the periodic fabric regain, the fabric temperature and the transient conditions of the air layer located between the fabric and the skin. Experiments were conducted in environmental chambers under controlled conditions using a sweating hot plate at 35 °C that represents the human skin and a gear motor to generate the oscillating fabric motion. The first set of experiments was done using a dry isothermal hot plate to measure the sensible heat transfer. The second set of experiments was conducted with an isothermal sweating hot plate and the total heat (sensible and latent) transport from the plate was recorded. A mathematical model was developed for the heat and mass transport through the air spacing layer and the fiber clothing system. In the fabric, a three-node adsorption model was used to describe the effect of fabric motion (ventilation) on the sensible and latent heat flows from the human skin under different environmental conditions. The fiber model was linked to the transport model of the oscillating air spacing layer that falls between the fiber and the fixed boundary (human skin). The transport equations were solved numerically. The sensible and latent heat transport quantities at the moist solid boundary were calculated. A reasonable agreement was observed between the model predictions of heat loss or gain from the hot plate and the experimentally measured results.

Experimental and Numerical Investigation of the Effect of Phase Change Materials on Clothing During Periodic Ventilation

A numerical and experimental investigation is conducted of periodic ventilation pro cesses in fabric containing microcapsules of phase change materials (PCM). When PCMS are added to textiles, they release heat as the liquid changes to a solid state and absorb heat as the solid returns to a liquid state. In this work, PCMS are incorporated in a numerical three-node fabric ventilation model to study their transient effect on body heat loss during exercise when subjected to sudden changes in environmental conditions from warm indoor air to cold outdoor air. The results indicate that the heating effect lasts approxi mately 12.5 minutes depending on PCM percentage and cold outdoor conditions. Heat released by PCMS decreases the clothed-body heat loss by an average of 40-55 W/m2 for a one-layer suit depending on the frequency of oscillation and crystallization temperature of the PCM. The experimental results reveal that under steady-state environmental condi tions, the oscillating PCM fabric has no effect on dry resistance, even though the measured sensible heat loss increases with decreasing air temperature of the chamber. When a sudden change in ambient conditions occurs, the PCM fabric delays the transient response and decreases body heat loss.

Comparison of Large Eddy Simulation Predictions with Particle Image Velocimetry Data for the Airflow in a Generic Cabin Model

A comparison of computational fluid dynamics (CFD) predictions and experimental data for the airflow in a generic cabin model is presented in this paper. The CFD predictions were generated using the large eddy simulation (LES) model, while the particle image velocimetry (PIV) technique was used to obtain the experimental data. A brief summary of the test setup and the experimental data are described herein. The main focus of this study is to analyze the temporal variation of the experimental data. The time series of the PIV measured velocity components with sampling frequencies of 0.1 and 5 Hz were compared with the CFD predictions, sampling at 20 Hz. Energy spectral analysis, through fast Fourier transformation (FFT) on the PIV data and their CFD counterparts, was performed and is presented in this study. By using direct comparisons of the velocity data, good agreement on the range of the velocity components was obtained at all monitoring locations, hence validating the LES predictions. A similar conclusion can be reached from the results of the energy spectral analysis. The energy-spectrum function calculated from the LES predicted velocity magnitudes has excellent correlation with the Kolmogorov spectrum law in the universal equilibrium range. For the smaller wave numbers, the PIV data taken at 0.1 Hz clearly reveal the characteristic motion of the largest eddy in the flow domain, and it correlates very well with the CFD predictions.

Empirical Evaluation of Convective Heat and Moisture Transport Coefficients in Porous Cotton Medium

The air penetration within a porous clothing system on a moving human being is an important physical process that considerably affects the heat and moisture resistance of the textile material. This effect of the coupled convection heat and mass exchange within the clothing system is experimentally investigated and theoretically modeled to determine the heat and mass transfer coefficients between the air penetrating the void space and the solid fiber as a function of the velocity of penetrating air. Experiments were conducted inside environmentally controlled chambers to measure the transient moisture uptake of untreated cotton fabric samples as well as the outer fabric temperature using an infrared pyrometer. The moisture uptake was conducted at three different volumetric flow rates of 0.0067, 0.018 and 0.045 m 3 /sec/m 2 of fabric area to represent airflow penetrations that could result from slow, medium, and vigorous walking, respectively. The theoretical analysis is based on a two-node adsorption model of the fibrous medium. A set of four coupled differential equations were derived describing time-dependent convective heat and mass transfer between the penetrating air and the solid fiber in terms of relevant unknown transport coefficients

Modeling heat and mass transfer in fabrics

Abstract A numerical model was developed for simulating the heat and mass transfer in fabrics during the wicking process. The model was applied to two different fabrics, cotton and polypropylene. The model shows that, as the water is wicked through the fabric specimens, two temperature zones are formed. Within each region the temperature gradient is small, but between the regions it is more significant. Unlike the temperature, the variation of the fractional saturation is continuous along the specimens. Experiments were also conducted to obtain the temperature distributions during wicking for the two fabrics and to validate the numerical model.

Transient response of the human-clothing system

Abstract 1. 1. A transient clothing model which considers the effects of adsorption and thermal capacitance on the dynamic thermal response of clothing was developed. 2. 2. Moisture adsorption and desorption by the fabric are the major factors that affect the transient response of clothing. 3. 3. This moisture can come from evaporated sweat or from the environment. 4. 4. The clothing model was combined with a modified version of the two-node thermal model of the human body. 5. 5. The combined model shows that, during transients, the mix of latent and sensible heat flow from the skin may differ considerably from the corresponding heat flows from the clothing surface to the environment. 6. 6. The alteration of the heat flows can have a significant impact on the thermal response of the body by changing the sweat rate required to achieve the heat loss necessary to maintain thermal balance.

Bacterial communities in commercial aircraft high-efficiency particulate air (HEPA) filters assessed by PhyloChip analysis

Abstract Air travel can rapidly transport infectious diseases globally. To facilitate the design of biosensors for infectious organisms in commercial aircraft, we characterized bacterial diversity in aircraft air. Samples from 61 aircraft high‐efficiency particulate air (HEPA) filters were analyzed with a custom microarray of 16S rRNA gene sequences (PhyloChip), representing bacterial lineages. A total of 606 subfamilies from 41 phyla were detected. The most abundant bacterial subfamilies included bacteria associated with humans, especially skin, gastrointestinal and respiratory tracts, and with water and soil habitats. Operational taxonomic units that contain important human pathogens as well as their close, more benign relatives were detected. When compared to 43 samples of urban outdoor air, aircraft samples differed in composition, with higher relative abundance of Firmicutes and Gammaproteobacteria lineages in aircraft samples, and higher relative abundance of Actinobacteria and Betaproteobacteria lineages in outdoor air samples. In addition, aircraft and outdoor air samples differed in the incidence of taxa containing human pathogens. Overall, these results demonstrate that HEPA filter samples can be used to deeply characterize bacterial diversity in aircraft air and suggest that the presence of close relatives of certain pathogens must be taken into account in probe design for aircraft biosensors. Practical Implications A biosensor that could be deployed in commercial aircraft would be required to function at an extremely low false alarm rate, making an understanding of microbial background important. This study reveals a diverse bacterial background present on aircraft, including bacteria closely related to pathogens of public health concern. Furthermore, this aircraft background is different from outdoor air, suggesting different probes may be needed to detect airborne contaminants to achieve minimal false alarm rates. This study also indicates that aircraft HEPA filters could be used with other molecular techniques to further characterize background bacteria and in investigations in the wake of a disease outbreak.

Characterization of the frequency and nature of bleed air contamination events in commercial aircraft

Contamination of the bleed air used to pressurize and ventilate aircraft cabins is of concern due to the potential health and safety hazards for passengers and crew. Databases from the Federal Aviation Administration, NASA, and other sources were examined in detail to determine the frequency of bleed air contamination incidents. The frequency was examined on an aircraft model basis with the intent of identifying aircraft make and models with elevated frequencies of contamination events. The reported results herein may help investigators to focus future studies of bleed air contamination incidents on smaller number of aircrafts. Incident frequency was normalized by the number of aircraft, number of flights, and flight hours for each model to account for the large variations in the number of aircraft of different models. The focus of the study was on aircraft models that are currently in service and are used by major airlines in the United States. Incidents examined in this study include those related to smoke, oil odors, fumes, and any symptom that might be related to exposure to such contamination, reported by crew members, between 2007 and 2012, for US-based carriers for domestic flights and all international flights that either originated or terminated in the US. In addition to the reported frequency of incidents for different aircraft models, the analysis attempted to identify propulsion engines and auxiliary power units associated with aircrafts that had higher frequencies of incidents. While substantial variations were found in frequency of incidents, it was found that the contamination events were widely distributed across nearly all common models of aircraft.