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 processes 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 approximately 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/m^sup 2^ 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 conditions, 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.
The technology of using phase change materials (PCM) in clothing was developed and patented in 1987 for the purpose of improving the thermal insulation of textile materials during changes in environmental temperature conditions . These phase change materials improve the thermal performance of clothing by absorbing or releasing heat when subjected to heating or cooling during a phase change. The conversion of temperature swings in the environment into oscillations of the melting-freezing interface allows significant abatement of the transient period effect on a human body’s heat loss. The technology is based on the concept of incorporating the phase change materials into the fabric’s structure by enclosing the PCM in a protective wrapping or microcapsule, which is few microns in diameter. Microcapsules prevent leakage of the phase change material during its liquid phase. They are incorporated into the fibers of the fabric by a wet spinning process or coated on the surface of the fabric substrate.
Phase change materials are combinations of different kinds of paraffin (octadecane, nonadecane, hexadecane, etc., . . .), each with different melting and crystallization points. Changing the proportionate amounts of each paraffin type can yield the desired physical properties (melting and crystallization). By careful selection of the phase change temperature, a PCM fabric can act as a transient thermal barrier by protecting the wearer of this fabric from the effects of cold or hot environments. When a PCM fabric is subjected to heating from the sun or a hot environment, it will absorb this transient heat as it changes phase from solid to liquid, and it will prevent the temperature of the fabric from rising by keeping it constant at the melting point temperature of the PCM. Once the PCM has completely melted, its transient effect will cease and the temperature of the fabric will rise. In a similar manner, when a PCM fabric is subjected to a cold environment where the temperature is below its crystallization point, it will interrupt the cooling effect of the fabric structure by changing phase from liquid to solid, and the temperature of fabric will be kept constant at the crystallization point. Once all the PCM has crystallized, the fabric temperature will drop, and the PCM will have no effect on the fabric’s thermal performance. Thus, the thermal performance of a PCM depends on the phase change temperature, the amount of phase change material that is encapsulated, and the amount of energy it absorbs or releases during a phase change.
Previous studies on quantifying the effect of PCMS in clothing on heat flow from the body during sensible temperature transients was conducted by Shim et al. . They measured the effect of one and two layers of PCM clothing materials on reducing the heat loss or gain from a thermal manikin as it moved from a warm chamber to a cold chamber and back again. Their results indicated that the heating and cooling effects lasted approximately 15 minutes, and that the heat release by the PCM in a cold environment decreased the heat loss by 6.5 W for the one-layer PCM clothing and 13.5 W for the two-layer PCM clothing compared to non-PCM suits. Also, Shim and McCullough  studied experimentally the effect of PCM-ski ensembles on the comfort of human subjects during exercise, and they found no appreciable effect of PCM material on comfort compared to non-PCM ski clothing. Shim’s study  of the effect of PCM-ski ensembles on exercise was done after conditioning the human subjects inside cold environmental chambers.
The transport processes of heat and moisture from the human body are enhanced by the ventilating motion of air through the fabric initiated by the relative motion of the human with respect to the environment. Periodic renewal of the air adjacent to the skin by air coming from the environment has a significant effect on heat loss from the body and comfort sensations. When sudden changes in environmental air take place, it is desirable to delay the adjacent air temperature swings to reduce sudden heat loss or gain from the body. The periodic ventilation effect of fabric with no PCM, according to our earlier work (Ghali et al. ), causes a temperature change of about 2.5 to 3°C in the enclosed air layer and about 6 to 8°C in the air void of the fabric during one period of oscillation in a steady environment at 25°C. For the past two decades, the so-called pumping or bellows effect has been studied, and its importance on the heat and mass transfer of the human body has often been discussed [2, 7-8, 11]. In another work (Ghali et al. ), we studied the effect of ventilation on heat and mass transport through fibrous materials to predict the transfer coefficients in a cotton fibrous medium. We further developed and experimentally verified our model to predict temporal variations in temperature and moisture of the air within the fibers in a multilayer three-node model. In realistic applications, ventilation of the clothing system during human motion occurs by periodic motion of air in and out of the air space as the fabric moves outward or inward toward the skin. We  reported original experimental data on sensible and latent heat transport initiated by sinusoidal motion of a fabric plane about a fixed mean air space above a sweating isothermal hot plate placed in a controlled environment. We also presented a mathematical and numerical three-node fabric model in the same work  to predict heat loss, which agreed fairly well with the experimentals. In later work (Ghaddar et al. ), we coupled our three-node fabric model with Gagge’s two-node human thermoregulatory model  to predict the transient thermal response of a walking human at variable levels of activity and ventilation frequencies. This coupled human-fabric model  used an empirically derived correlation of the evaporative and dry convective heat transport coefficients from the skin to the lumped air layer as a function of ventilation frequency. We validated our correlation by comparison to published experimental data of Lotens  and Danielsson .
In this work, we extend our previous fabric ventilation three-node-mode]  to study the transient effect of a phase change material on exercise and how long that effect lasts without conditioning the human subject when going from warm indoor conditions to cold conditions. Our numerical modeling and experimental work, using a dry hotplate inside environmental chambers, will investigate as well the possibility of a PCM fabric regenerating itself during ventilation (exercise) at steady-state environmental condition.
The PCM fabric came from Outlast Company; its thickness e^sub f^ was 2.4 mm and its weight was 319 g/m^sup 2^. The chemical formula and the physical properties (crystallization temperature) of the encapsulated material in this manufactured fabric were not provided by the maker. Figure 1 shows a front view of the experimental setup, which is composed of two square wooden frames hinged to each other by four piano hinges. Both frames have an inner open area of 0.508 × 0.508 m and an outer area of 0.554 × 0.554 m for the upper frame and an area of 0.585 × 0.585 m for the lower one. The upper frame is connected to a rotating shaft through a four-bar linkage mechanism, and the shaft is connected to a gear motor. When the shaft rotates, the upper frame moves sinusoidally in a vertical path away and toward the lower frame in a stroke of 12.7 mm. The piano hinges were necessary to insure that the upper frame moved in a horizontal plane without tilting. The PCM-cotton fabric was taped to the upper frame with aluminum tape, which also covered the exposed surface of both wooden frames. To insure a planar movement of the fabric (no fluttering), the sample was placed between two metallic screens made from 12.7 mm open squares. The movement of the fabric attached to the upper frame caused air to move back and forth across the fabric. To reduce the possibility of air escaping through the hinges or through the lower frame, the hinges were covered with plastic wrap, and plastic foam was taped to the outer rim of the lower frame, as shown in Figure 1. A minimal layer of plastic wrap was used to minimize possible horizontal movement of the air layer between the lower and upper frame.
The PCM-cotton fabric frame was placed on top of a Dynatech sweating guarded hot plate apparatus, model TCB-TX, for measuring the sensible and latent heat transport. The apparatus simulates the function of human skin, and the plate consists of three temperature-controlled heaters-main, guard, and bucking. The measuring section of the main heater is a 25.4-cm square plate surrounded by a guard section, which increases the total size of the plate to 50.8 × 50.8 cm. The function of the guard heater is to eliminate lateral heat flow from the main heater. The third bucking heater located beneath the main heater eliminates the heat flow in the axial direction below the main heater. These two heaters (guard and bucking) thus force all of the heat generated in the main heater to flow in the direction away from the oscillating fabric.
The coupled mass and heat transport equations of the outer and inner nodes of the fabric, the air void, and the air layer are integrated numerically using a first-order Euler-Forward scheme with a time step size of 0.01 second over a total integration period of 1200 seconds. The vapor pressure of the flowing air in the air layer or in the fabric voids is related to the air’s relative humidity (RH) and temperature, and is calculated using the psychrometric formulas of Hyland and Wexler  to predict the saturation water-vapor pressure and hence the vapor pressure at the specified relative humidity. The regain of the cotton material has a definite relation to the relative humidity of the water vapor through a property curve of regain versus relative humidity. The graphic relation of R as a function of relative humidity is interpolated with third-order polynomials for ten relative humidity intervals from zero to 100%. The interpolation functions are used in the simulation to calculate the inner and outer nodes’ relative humidities corresponding to the values of the inner and outer regains, respectively. At every time step, the air mass flow rate and the total regain are updated, and the inner and outer nodes temperatures are compared with the melting temperature of the PCM paraffin to determine the time when phase changes takes place to the time when the paraffin completely solidifies or melts.
It is important to use the appropriate initial temperature and regain conditions of the fabric, since the interest here is the initial transient effect of the PCM on exercise when a step change in the environmental conditions take place, as well as when a steady-state effect is reached. The initial conditions are taken from the steady-state simulation of oscillating fabric inside indoor conditions (26°C, 50% RH) to simulate the condition of a human subject warming up indoors before going outside to exercise in a cold environment, i.e., the climate change takes place at time = 0 in the simulation calculations. The PCM percentage is assumed to be 20%, although simulations are done at different PCM percentages to study that effect. The textile industry does not recommend increasing the PCM percentage because it will increase the cost of the fabric as well as its weight, so 20% is representative of commonly used values by industrial manufacturers. The crystallization point is 30°C and the outdoor cold environmental conditions are 2°C and 50% relative humidity.
Results and Discussion
We have studied the effect of PCMS on transient heat loss during step changes in environmental temperature, followed by a parametric study to investigate the effect of ventilation frequency, PCM percentage, and atmospheric conditions on fabric thermal performance.
Figure 4 shows the time-dependent outer node fabric temperature for the fabric with a PCM ([alpha] = 20%) and without a PCM ([alpha] = 0) at a ventilation frequency of f = 25 rpm, outdoor environmental temperature and humidity of T^sub [infinity]^ = 2°C and RH^sub [infinity]^ = 50%, respectively, and initial outer node temperature T^sub o^ of 32°C due to periodic ventilation in an indoor environment at 26°C, 50% RH. The maximum airflow rate through the fabric at 25 rpm is 0.0244 m^sup 3^/s/m^sup 2^, corresponding to 146.4 L/min/m^sup 2^. The drop in outer node temperature is interrupted by the PCM, and the temperature of the outer node is kept constant at the crystallization temperature of 30°C for 8 minutes during the phase change process. The duration of the PCM effect is about half the duration reported by Shim et al.  of 15 minutes during transient diffusion in a single layer of a PCM fabric 4 mm thick and 455.2 g/m^sup 2^ weight. We expected this result since ventilation will speed up the process of heating or cooling by convection and since the simulated fabric has a smaller weight, 319 g/m^sup 2^, compared to 455 g/m^sup 2^ of Shim et al. . The drop in outer node temperature with no PCM present is about 7°C in the first minute. Figures 5a and b present the variations in temperature of the air layer and the air in the fabric voids for the fabric with PCM and the fabric without PCM, respectively, during a period when phase change takes place. The air renewal in the fabric shows a significant change in air void temperature during steady periodic ventilation with and without PCM. When the PCM is present, the air void temperature exceeds the air layer temperature due to the release of heat from the PCM to the air void. This takes place only during the downward motion of the fabric, when the air coming from the air layer replaces the air void. However, the air layer temperature does not change significantly in the presence of the PCM compared to the case when no PCM is present. The average air layer temperature difference is 0.663°C for the two cases. Although the outer node temperature of the PCM fabric is high, the convective coefficient between the void air and the outer node is not sufficiently high to cause an appreciable temperature change in the outer node. The effect of PCM presence on the sensible heat loss in watts during the phase change from a skin area of 0.064516 m^sup 2^ is shown in Figure 6 at a ventilation frequency of [function of] = 25 rpm, T^sub [infinity]^ = 2°C, RH^sub [infinity]^ = 50%, and T^sub 0^ (t = 0) of 32°C due to periodic ventilation in an indoor environment at 26°C and 50% RH. The sensible heat loss from the skin decreases when a PCM is incorporated in the fabric, and this decrease is 41.249 W/m^sup 2^ on average during the phase change process. This drop in sensible heat loss from the skin is attributed mainly to the decrease in radiation heat loss from the hot plate (skin) because of the higher outer node temperature of the PCM fabric compared to the non-PCM fabric.
When the oscillating PCM fabric attains periodic steady state at a temperature lower than 30°C, the encapsulated PCM in the outer node will be all solid phase and the PCM will have no effect on the thermal performance of the fabric. The reason is that, despite the fact that the void temperature is undergoing a big change in temperature during steady-state conditions while the fabric is moving back and forth from the hot plate, the outer node temperatures undergoes a minor change of 0.1°C (see Figure 4). This change is not enough to cause a phase change in the encapsulated PCM. This is consistent with the experimental findings presented in this work where there is no effect of PCM at steady conditions.
Simulations at different ventilation frequencies of 25, 35, and 45 rpm correspond to low, medium, and vigorous exercise activity for a PCM fabric with maximum predicted volumetric airflow rates through the fabric of 146.4, 193.6, and 263.77 L/min/m^sup 2^, respectively. Figure 7 shows the transient outer node temperature for ventilation frequencies of 25, 35, and 45 rpm at a = 20%, T^sub [infinity]^ = 2°C, RH^sub [infinity]^ = 50%, and T^sub 0^ (t = 0) of 32°C due to periodic ventilation in an indoor environment at 26°C and 50% RH. The duration of the phase change effect decreases from 8 minutes at [function of] = 25 rpm to 7.68 minutes at [function of] = 35 rpm to 7.35 minutes at [function of] = 45 rpm. As the frequency increases, the duration of the phase change process decreases because of the increased convective heat loss from the outer node to the environment. Increasing the frequency of oscillation increases the outer transfer coefficients, the induced mass flow rate of atmospheric air, and the convective heat and mass transfer coefficients from the skin to the trapped air layer. Hence, the void and air layer temperatures are expected to decrease with increasing frequency when the mean steady periodic values for the air layer temperature are 23.6, 20.73, and 18.65°C and when fabric air void temperatures are 15.4, 14.1, and 12.65°C at [function of] = 25, 35, and 45 rpm, respectively. The corresponding sensible and latent heat losses in watts, based on a skin area of 0.064516 m^sup 2^, increase with increased ventilation frequency as illustrated in Figures 8a and b. The saving in sensible heat loss increases with the frequency increase when a PCM is present and when it is not. The average decreases in sensible heat loss during a phase change at [alpha] = 20% are 41.249 and 46.695 W/m^sup 2^ at [function of] = 25 and 45 rpm, respectively, when compared with [alpha] = 0 (no phase change) at the respective frequencies.
We have also studied the effect of outdoor environmental conditions on the duration of the PCM fabric phase change process. Figure 10 shows the outer node temperature variation and the sensible heat loss during phase change in watts for a skin area of 0.064516 m^sup 2^ at two outdoor environmental conditions of T^sub [infinity]^ = 2°C, RH^sub [infinity]^ = 50% and T^sub [infinity]^ = 10°C, RH^sub [infinity]^ = 50%, while fixing the other parameters at [function of] = 25, [alpha] = 20%, and an initial outer node temperature T^sub 0^ (t = 0) of 32°C due to periodic ventilation in an indoor environment at 26°C and 50% RH. Increasing the atmospheric temperature increases the duration of the phase change process due to decreased radiation heat loss to the surrounding environment. The void temperature increases with increasing atmospheric conditions, since the atmospheric air in the fabric void and air layer is at a higher temperature. As a result, the mean air void temperature during a phase change increases from 15.9 to 20.05°C when T^sub [infinity]^ increases from 2 to 10°C, with all other parameters fixed. The resulting sensible heat loss is expected to decrease, as is clear from Figure 1Ob.
Our work illustrates that PCM garments can improve the comfort of people as they go through environmental step changes when their work requires that they go to and from a cold storage facility and a warm environment on an intermittent basis. In other situations, PCMS may improve the comfort of people as they go through step changes from a very active state (high metabolic production) to an inactive state on an intermittent basis in a cold environment. This is a common situation in outdoor sports.
We have incorporated PCMS into our three-node ventilation model . The presence of PCMS in fabric causes a temporary heating effect when subjected to a sudden change from a warm environment to a cold environment. This effect is translated into a decrease in the sensible heat loss during the phase change process compared to a fabric without a PCM. The duration interval of the phase change process decreases with increased frequency of ventilation, while the duration increases with an increased percentage of PCM in the fabric and with an increased outdoor environmental temperature.
After the initial transient when the PCM is in the solid phase and steady periodic conditions are reached, we have verified by simulation and by experiment that the PCM has no effect on the thermal performance of the fabric.
We wish to thank the Institute of Environmental Research at Kansas State University for allowing the use of their facilities. We also greatly acknowledge the support of the University Research Board of the American University of Beirut, grant DCU-148860-73117.
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Manuscript received February 27, 2003; accepted May 5, 2003.
Beirut Arab University, Beirut, Lebanon
N. GHADDAR1 AND J. HARATHANI
American University of Beirut, Beirut 1107-2020, Lebanon
Kansas State University, Manhattan, Kansas 66506, U.S.A.
1 Corresponding author: Nesreen Ghaddar, Professor and Chairperson of the Department of Mechanical Engineering, Faculty of Engineering and Architecture, American University of Beirut, P.O. Box 11-236, Riad El SoIh, Beirut 1107 2020, Lebanon, tel: + +961-1-3500000 ext 3594/3590, fax: + +961-1-744462, email: email@example.com, web address: http://webfaculty.aub.edu.lb/~farah/.
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