Steve Eckels

Determining temperature ratings for children's cold weather clothing.

This study examined the physical and physiological differences between children and adults that affect body heat generation and losses and then developed a heat loss model for determining the temperature ratings of cold weather clothing designed for use by children of various ages. The thermal insulation values of selected jackets were measured using a heated manikin dressed in two base ensembles, and the temperature ratings were calculated using the model. The results indicated that the type of garments used in the base ensemble had a major effect on jacket ensemble insulation and the predicted comfort temperature. For a given level of insulation, the temperature rating decreased as the wearers age and activity level increased. This is probably because children have a higher surface area per unit mass ratio than adults, and they lose heat faster. However, this effect is partially offset by their higher metabolic rates.

An objective method for screening and selecting personal cooling systems based on cooling properties.

A method is proposed for evaluation and selection of a personal cooling system (PCS) incorporating PCS, subject, and equipment weights; PCS run time; user task time; PCS cooling power; and average metabolic rate. The cooling effectiveness method presented is derived from first principles and allows those who select PCSs for specific applications to compare systems based on their projected use. This can lower testing costs by screening for the most applicable system. Methods to predict cooling power of PCSs are presented and are compared to data taken through standard manikin testing. The cooling effectiveness ranking is presented and validated against human subject test data. The proposed method provides significant insight into the application of PCS on humans. However, the interaction a humans with a PCS is complex, especially considering the range of clothing ensembles, physiological issues, and end use scenarios, and requires additional analysis.

An improved thermal model of the human body

The goal of this study is to develop a more realistic human thermal model. Previous models have been based on simple cylindrical geometries. The current study uses shaped and refined body segments to simulate heat and mass transfer in the human body during a transient process. The body segments and blood vessels (or respiratory tract) were discretized into 3-D and 1-D elements, respectively. Criteria were developed to simulate the circulatory system, respiratory system, and human thermal responses. The finite element method was employed to solve the mass and energy equations which were written for each element. The body model was compared against actual data available in literatures: cold, neutral, and warm conditions, with ambient temperatures of 15°C (59°F), 25°C (77°F), and 35°C (95°F). For steady state simulation, the results showed that the skin temperatures of head, trunk, and limbs matched the experimental data very well for all three conditions, while the neck and limb extremities (hand and foot) showed some difference, especially for the cold condition. In transient process, our simulation gives good predictions for warm and neutral conditions, but 1–2°C difference in skin for cold condition. The comparison of cylindrical-based model, our current model, and experimental data shows that our model is able to give more accurate prediction of human body temperatures than previous models.

Experimental Measurements in Near-Wall Regions by Particle Image Velocimetry (PIV)

The current study investigates the flow field near a surface with a micro-PIV system using a square tube to enhance optical access. Measurements of velocity fields and eddy structures near the wall of tubes are important to the design of in-tube surface geometries. In experimental fluid mechanics, particle image velocimetry (PIV) is now a common way to measure velocity. However, PIV measurements near walls require efforts to deal with low particle density, high shear gradient and wall reflection. The current paper discusses a PIV measurement technique utilized to observe flow dynamics in near-wall regions. PIV uncertainty analysis is discussed in this study. The experimental results are compared with previous results for validation.Copyright

Contribution of wetted clothing to body energy exchange and heat stress

Quantifying the impact of clothing thermal and evaporation resistance is essential to providing representative boundary conditions for physiological modeling. In many models, sweat is assumed to drip off the skin surface to the environment and is not captured in clothing. In high metabolic rate and high temperature and humidity conditions the sweat produced by the body has the potential to saturate semipermeable clothing ensembles, changing the assumptions of the model. Workers, athletes and soldiers commonly wear encapsulating versions of such clothing to protect against environmental hazards. A saturated clothing model is proposed based on the ASHRAE two-node model using a saturated spot element in parallel with the existing method to account for sweat absorbed in the clothing. The work uses fundamental heat and mass transfer principles, modifying the existing formula using clothing measurements and basic assumptions. The effectiveness of the model is demonstrated by comparing the predictions of the original and proposed models, to the results of 21 soldiers exercising. The soldiers wore combat pants and shirt, helmet, gloves, shoes, socks, and underwear, and walked in a thermal chamber for 2 h at 42.2 °C dry bulb temperature, 54.4 °C wet bulb temperature, 20% relative humidity, and airspeed of 2 m/s. Core temperature, seven skin temperatures, heart rate, and total sweat loss were measured. The original model provides an average core temperature difference compared with the human subject results of 1.31 °C (SD = 0.557 °C) while the modified model improves the final prediction of core temperature to within an average of 0.15 °C (SD = 0.383 °C). The new model shows an improvement in the prediction of human core temperature under the tested conditions where dripping sweat will saturate clothing. The format can be used in multi-segmented thermal models and can continue to be developed and improved as more information on wetted clothing properties become available.

A refractive-index and position-independent single-particle detector for large, nonabsorbing, spherical particles

Abstract We show that for spherical particles with real refractive index and diameters greater than ca. 10 microns, the differential scattering cross-section is only independent of the refractive index at angles near 37 ± 5°. We built a device with a modified Gaussian incident beam profile so that the beam transit time of a particle passing through the beam can determine the true incident intensity for the scattering of the particle. By combining the modified Gaussian incident beam profile with detection of scattered light near 37 ± 5°, we demonstrate a refractive-index independent measurement of single spherical particles as they pass through the beam. Copyright

Experimental Analysis of an Air-to-Air Heat Exchanger for Use in a Refrigeration Brayton Cycle

The refrigeration Brayton cycle, which has been used extensively in various industries, has an excellent potential for use in automotive air conditioning applications. However, the air-cycle system has a couple of drawbacks including fog generation and low cycle efficiency. In this research project, an air-to-air heat exchanger called a ‘mixer’ is designed and used at the outlet of a refrigeration Brayton cycle. The primary function of the mixer is to remove moisture from the secondary warm airflow into the system. Successful moisture removal from the secondary airflow results in achieving the second function of fog dissipation from the primary cold airflow. In order for the system to perform appropriately, the moisture removal rate must be kept at the highest possible rate. The experimental results from this research project reveal that to enhance moisture removal rate, one may either increase the primary cold airflow rate, decrease the secondary warm airflow rate, or the combination of the above airflow adjustments. Furthermore, based on experimental results, one may speculate that there is an optimum point in decreasing the secondary airflow rate. However, in increasing the primary airflow rate, one must be aware of the pressure drop through the cold side of the mixer as the higher pressure drop results in higher power consumption for the Brayton cycle. It is important to point out that appropriate levels of the primary and secondary airflows impacts the mixer effectiveness, and that for a constant cold airflow rate, decreasing the warm airflow rate below the cold airflow rate results in higher effectiveness.Copyright