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Moisture Effects in Heat Transfer Through test method

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Moisture Effects in Heat Transfer Through test method

Author: Date :2018-11-14 Views: order
The purpose of this research was to examine the effects of moisture on heat transfer through materials comprising clothing systems worn by wildland firefighters, with particular attention given to source and location of moisture in the system. Concern for individuals working in high-risk environments, especially those with the potential of high heat exposures, has risen in recent years. The ultimate goal is to reduce severe burn injury experienced by individuals in this and similar occupations by gaining a better understanding of the mechanisms underlying heat transfer when moisture is a factor.
 
Extensive research has been conducted to better understand heat transfer mechanisms and to improve protective clothing design and performance, leading to a decrease in thermal injuries experienced by individuals in high-risk environments. However, Stull [1] and M?kinen et al. [2] reported that even when wearing improved garments, a substantial number of burn injuries occur.
 
The performance of thermal protective clothing systems is affected by many variables including environmental conditions (temperature, humidity, wind speed, etc.), the nature of the textile used (weave structure, fiber mass and thickness, fiber type, etc.), the mechanisms of heat transfer (convection,conduction, thermal radiation), and the presence of moisture. Moisture in a clothing system can originate from internal or external sources. For wildland firefighters, internal moisture normally comprises perspiration produced by the wearer, while external moisture consists of rain or dew, water spray from hoses, and/or swamp or lake water through which the firefighter must walk. The effect of moisture on heat transfer through a clothing system may depend on the degree of moisture sorption, location of moisture in the system, where it is located on the body, its source (internal or external), the timing of moisture application (before, during, or after exposure to thermal energy), and duration of the heat application.
 
The effects of moisture on heat transfer through clothing systems at lower temperatures such as 21–35 C have been evaluated during comfort assessment by several authors [3, 4, 5, 6, 7, and others].Evaluation of comfort and heat stress has accounted for moisture absorption into, and transport through, clothing systems in interaction with environmental conditions. Some mechanisms outlined in comfort theory can apply to moisture-heat interaction at high heat fluxes, but due to significantly higher exposure temperatures, different mechanisms also occur. Understanding mechanisms by which moisture in textiles affects heat transfer through clothing systems at higher temperatures could lead to improvements in design of thermal protective clothing.
 
At higher heat fluxes and using common measures of heat transfer such as the heat transfer index, thermal protective performance (TPP) and radiative protective performance (RPP), several authors [1, 8, 9, 10, 11, 12, 13] have found that moisture in a clothing system may either increase or decrease thermal insulation. The effects of both internal and external moisture on heat transfer from convective and radiant sources through two-layered protective clothing systems have not been studied, however; nor has the effect of moisture applied during or after heat exposure. In this paper, the effects of both internal and external moisture on the heat transfer through specimens simulating clothing systems typically worn by wildland firefighters will be discussed.
 
METHODS:
This study comprised an experiment in which fabric systems were exposed to both a flame heat source and a purely radiant heat source under five different moisture treatments. Heat flux and transferred energy were plotted against time during and after exposure.
 
1、Materials
Four fabric systems typical of those worn by wildland firefighters and comprising combinations of two different thermal protective outerwear materials and two different underwear materials were evaluated.The two outerwear fabrics were a plain weavefire resistant (FR) cotton (337.5 g/m2) and aplain weave aramid (211.5 g/m2). The twounderwear fabrics were a 100% cotton jerseyknit (176.5 g/m2) and an aramid rib knit(164.0 g/m2).
 
2、Moisture Application
The five moisture treatments were as follows:
1. Both layers oven-dried at 105C for 1 hr and placed in a desiccator for a maximum of 4 hrs prior to testing. Specimens were tested within 40 s of removal from the desiccator;
2. Both layers conditioned in a standard atmosphere (65% RH, 21C) for at least 8 hrs prior to testing to allow specimens to reach moisture equilibrium according to CAN/CGSB-4.2 No.2-M88 [14]. Specimens were placed in a sealed plastic bag to prevent moisture loss when removed from the standard atmosphere and were tested within 40 s of removal from the plastic bag;
3. Outer layer saturated and inner layer conditioned in the standard atmosphere;
4. Underwear layer saturated and outer layer conditioned in the standard atmosphere; and
5. Both outer and underwear layer saturated.
 
For conditions 3 to 5, specimens were conditioned in the standard atmosphere for 24 hrs prior to moisture application and testing. Appropriate layers were saturated with moisture following ASTM D-461: Standard Test Method for Felts, Section 17 [15]. Specimens were immersed in water for a minimum of 5 min, removed from the water, placed between sheets of commercial blotting paper, and rolled over with a 2,000-g metal roller to remove excess moisture. Saturated moisture content of each of the materials was as follows: FR cotton outerwear—35%;aramid outerwear—40%; 100% cotton underwear—50%; and aramid underwear—45%.
 
3、Flame Exposure (FE)
After appropriate moisture application, specimens were tested following CAN/ CGSB-4.2 No. 78.1, with a 6.4-mm spacer, and a calibrated heat flux of 83 kW/m2 [16]. In this method, an open-flame single Mekker gas burner with a heat flux of 84  2 kW/m2 is placed horizontally beneath a specimen. The open flame is a combination of approximately 30% radiative and 70% convective heat flux.
A copper calorimeter sensor is placed behind the specimen. In the standard test, the rate at which the specimen allows heat to pass through to the sensor is determined until the second-degree burn criterion, a function of time-to-burn using the Stoll curve, is reached [1, 16]. Specimens are 100 mm by 100 mm square and are held in place by pins on the specimen holder. The pins in the CGSB method are in place to prevent excessive shrinkage of the test specimen.
 
The standard procedure and data acquisition program were modified in order to measure the heat flux and energy transferred through the fabric systems as a function of exposure time.The flame was not removed from the specimen when the second-degree burn criterion was reached as is done in standard testing. Rather,the flame remained under the specimen for 10 0.5 s in order to drive off excess moisture. Heat flux and transferred energy were measured for 60 s. Moisture loss was not determined quantitatively. However, moisture presence on the copper calorimeter sensor after the test exposure was noted and defined as specimen moisture loss and moisture condensation.
 
4、Radiant Exposure (RE)
After appropriate moisture application, specimens were tested using equipment for the NFPA 1977 Test, but using a 6.4-mm spacer and a calibrated heat flux of 10 kW/m2 [17]. In this method, a bank of quartz tubes, oriented vertically, provide the necessary heat flux. The heat flux is controlled through the use of a power controller. The specimen is mounted in the holder, and the holder is held in place on the lamp source by magnets. A shutter between the specimen and the lamps is removed, initiating the test. A copper calorimeter sensor is placed on the interior of the specimen in order to measure the rate of heat transfer through the specimen. As with TPP, the skin threshold level to reach second-degree burn criterion is measured using a Stoll curve.
 
The procedure and data acquisition program were modified in order to measure the heat flux and energy transferred through the fabric system as a function of exposure time. The quartz tubes were not turned off and the specimen was not removed from the test apparatus when the second-degree burn criterion was reached. Rather, the specimen remained exposed to the heat flux for a total of 100 s. Heat flux and transferred energy were measured during this time. Moisture loss was not determined quantitatively. However,moisture presence on the copper calorimeter sensor after the test exposure was noted and defined as specimen moisture loss and moisture condensation.
 
5、Measurement and Calculation of
Dependent Variables During and after exposure to both radiant and flame heat sources, data for four different dependent variables were collected and calculated: (a) peak heat flux through the fabric systems, (b) time to reach peak heat flux, (c) energy transferred through the fabric systems, and (d) time to reach 0.1 kJ of transferred energy. To accurately determine the total heat flux and total energy received by the copper calorimeter for both FE and RE tests, heat losses during exposure were calculated1. Such losses result from (a) heat transferring via conduction to the ceramic block in which the calorimeter is embedded, (b) heat transferring via convection to the cavity at the back of the calorimeter, and (c) heat re-radiating off the calorimeter.

 

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