Mathematical Simulation of Heat and Sweat Transfer
In Primary School Uniform
Using
Cotton and Blended cotton/polyester (50/50) Fabrics
A. Tahan .PhD
Faculty of Applied Arts
Ready Made Garments Technology Dep't.
MansouraUniversity, Damietta,Egypt
1
Abstract-Waste heat from toddler of primary school body must be dissipated all the time and whenever necessary, aided by evaporation of sweat from the skin. Clothing has, as one of its main functions, the control of this heat and moisture transfer from the body to the environment. The physical properties of the fabric’smaterial and construction (structure & design) as well as the physical activities of the toddler have been considered for the way affecting the thermal properties of the fabrics. In this aspect, a mathematical model has been developed to describe the dynamic heat and moisture transport behavior of the school uniform. Considering heat convection, radiation and vapor transfer diffusity through the toddler‘s garment. The fabric materials and construction comfort is now becoming understood especially when two compromised types of fabrics as such cotton and blended cotton /polyester have been investigated. The blended 50/50Cotton/polyester fabric construction has shown a very high performance and specifications for heat and moisture transfer .It was found that the cotton/polyester fabric could transfer 150% heat transfer than cotton at the same condition.
Keywords: physical properties of the fabric, physical activities of the toddler sweat and sweat transfer, Porosity and Permeability Properties,wickability, wettability
Introduction
The most important property of any apparel is comfort. Comfort is an experience that is caused by integration of impulses passed up the nerves from a variety of peripheral receptors smell, smoothness, consistency and color...etc in the brain .Comfort is a qualitative term and it is one of the most important aspects of clothing. The clothing comfort can be divided into three groups, i.e. psychological, tactile and thermal comfort [52] .Psychologicalcomfort is mainly related to the latest fashion trend and acceptability in the society and bears little attention to the properties of fabrics. The tactile comfort has relationship with fabric surface as well as texture and mechanical properties. The thermal comfort is related to the ability of fabric to maintain the temperature of skin through transfer of heat and perspiration generated within the human body. Saville[53] distinguished two aspects of wear comfort of clothing ; (1) “thermo-physiological wear comfort which concerns the heat and moisture transport from skin due to the properties of clothing and the way that clothing helps to maintain the heat balance of the body during various levels of activity”, and (2) Skin sensational wear comfort which concerns the mechanical concept of the fabric with the skin, its softness and pliability in movement and its lack of pricking, irritation and cling when damp”. There is a general agreement that the transmission of air, heat and water vapor from the skin through a garment are probably the most important factors in clothing comfort. Comfort, as felt by the user, is a complex factor depending on the above attributes. Some early study3-6 reported various aspects of comfort related properties of fabrics.
The comfort characteristics of fabrics mainly depend on the structure and types of yarn used.
Based on the previous investigations [1, 2] it was postulated a theoretical analysis according to the physical properties of the fabric and the physical activities of the toddler considering the sweat and sweat transfer through the toddler clothing. What is really meant by clothing as a covering over the body needs to be considered. Different types of clothing items are worn on head, neck, and hands, on feet other than on the torso. Clothing often is rather similar on torso, arms and legs where the dimensions of these cylinders are different. Development in clothing has tended to be about 80% of the body area, called (body clothing). The suspended clothes from the head in much the same way that a poncho or cap is suspended from the shoulders. If the Torso protected by a draped cylinder or shell suspended from the head, and not at close contact with the neck, there would be wonderful opportunities for controllable ventilation even without body motion by chimney effect. Every point of close contact of clothing with body whether at collar, on the shoulders, by belt at the waist, or by snug cuffs at elbows and ankles, reduces the possibility of convection or chimney flow. Hong &Hollies [3] reported that at inner fabric surface, the sweat by skin built up low moisture vapor concentration with cotton fabric, but cotton/polyester conducted a highest rat of concentration. Moreover garment belted at the waist and a skirt permits more airflow than trousers. Air by body motions will increase the internal circulation as well as reducing the external airflow. Neck protection can come from collar turned up from the shoulders (which is looser and larger would approach the idea of hood) or by separate tippet or muffler.
1.Experimental Method and Materials
The fabrics used in this study were provided by Misr spinning & weaving Mahala Elkupra Co-Egypt. The fiber, yarn and fabric structure and design characteristics were designated to be essentially equivalent. . The set of fabrics has been made with equivalent yarn and fabric geometric variables except for fiber type. Three cotton fabric structures with weft and warp count of 16s single yarn Giza (83), twist factor 3.8 Z. The three structures consist of (16x16/87x30) &(16x16/60x60) and (16x16/87x54). The other fourth fabric structures,with the same structure of the cotton Blended spun yarn 50/50 Cotton/ polyester at twist factor 3.8.In addition to the fourth structure is:
As 100% polyester weft
yarns and 100% cotton warp yarn .According to the fabric setting ,the four structures have designated as model (141), model (120) & (117) and model (100) respectively. These types of yarns were chosen intentionally to have a good permeability, a high utility function and economic, in addition too good physical properties. These two types of fabric structure were woven in four designs such as plain (1/1), twill (2/2z), twill (1/3z) and warp satin (4). The need for this qualification arises from the fact that because these types of designs have the same repeat for four picks and four ends. In addition, these weave is rather loose and its usefulness to improve either the permeability, the thermal conductivity of the fabric or firmness of the weave or all of them. The combination of the structure and design are designated as construction.
1.1.The physical properties testing
1.1.1Weight per Unit Area:
Testing, Sampling, marking out, cutting, accuracy of weight and moisture content must be all considered. After cutting by a standard template, the samples with the seven models constructions (3 cotton and 4 cotton/ploy) with the four designs have been weight according to the standard atmosphere according to ASTM [4]. With aid of an electronic balance, with accuracy 0.001g the mean values of weight have been obtained for each of the fabric where considered as moisture fabric.
1.1.2.Thickness of Clothing Materials
Gibson [26] have recognized the fabric thickness as a prime factor in determining the level of effective comfort properties such as insulation, water vapor transmission and water maximum holding capacity or moisture content. The measurement of thickness has been carried out according to the A.S.T.M [4,5]. For the determination of the thickness of a compressible material such as textile fabric, it is essentially that the test consists of precise measurement of the distance of the two plan parallel plats of the device when the cloth separates them. A known arbitrary pressure between the plates being applied and maintained. At the standard condition the mean obtained values are reported to the nearest 1% accuracy .The values showed that the fabric thickness of the blended Fabric is thicker than the cotton fabric.
Lyman [6] and several investigators for the variation of the fabric insulation with thickness indicated that the insulation values are from 1 to 1.6 Clo/cm. Two major reasons for these results are: Firstly, thickness of fabric is measured under a pressure higher than pressure is applied at the fabric is being worn. Secondly, the air spaces in clothing between the worn layers could reach a thickness of 5mm. This result could indicate the insulation of clothing fabric and is due to the air trapped in between Hearl [7] stated that the resistance (conductivity) of the textile materials varies considerably with their moisture content .In generally, the increase in the amount of sodium and potassium salts (sweat) present in cotton clothes was associated with resistance reduction or (conductivity).
1.2.Porosity and Permeability Properties:
Permeability refers to the accessibility of void space to the flow of air or liquid. Schneurell & Spivak [9] have defined porosity as normally as the ratio of void space to the total volume encompasses by the boundaries of the material. Fluid flow in a porous medium had been of continuous interest for the past five decades. This interest stems from the complicated phenomena associated with the flow process in porous media and its very wide engineering and technological applications.
1.2.1.Air Permeability Measurement:
In order to point out fabric characteristic from which thermal insulation may be estimated most accurate, thickness and volume of air per unit area of fabric have been considered. Mary [12] has found that a linear relationship exists between thermal insulation and thickness for both single and multiple yarns. Anne & Mary [12,13] have established that thermal resistance of apparel textiles of dry fabric or one containing very small amount of water depends on its thickness, whilst a lesser extent on fabric construction and fiber conductivity.
Although Speakman [15] has pointed out that an increase in thickness leads to increase in thermal insulation, but Morris [16] has reported that the thermal insulation relationship with thickness holds only for thickness below about 0.1397-cm.
Mary& Morise [11] have found a highly correlated relationship with the thermal insulation when the air permeability (m3 / sec .m2) was measured with a different way .On the other hand the Take-Uchi theory and George[17,18] might explain the mechanism of heat transfer where it depends on the physical fabric characteristic as permeability and thickness. To study the factors involved in clothed of the uniform heat transfer, it was necessary, therefore, to measure the air permeability of each construction. The samples have been tested under the Standard ASTM test [4]. By the device of VACUM-ARQO-Model 9025, the measured values are reported.
1.2.2.Permeability To Water and Moisture Regains Test:
The term wettability in general, one could say that the time factor is involved. Farnsworth & RodwellRebourn and Ashrae [10,19,49] as many textile researchers have attempted to find the material factors that explain subjective sensation of comfort and discomfort. Measurements of fabric permeability properties that are related to actual wearing condition have not been established. Most studies have been concerned with liquid water transport, such as wicking, rather than water vapor transfer. Moisture regain and equilibrium vapor diffusion through fabrics do not predict comfort rating by human subject, where still other parameters such air permeability, fabric weight, structure and design… etc [42]do not include the conformability subject.
Hollies [20,21] indicated that collection of moisture at skin-fabric interface during wear is a key factor related to human confronts. Schneurell& Spivak [9,10] have used a successful method for observing dynamic moisture movement on the fabric surfaces next to skin by using a chemical moisture indicator.
For testing fabric absorption, the equipment and methodology in this study are described in reference [46].The measurements are reported. Before testing, it is normally involving fabrics pre-conditioned to a standard atmosphere to equilibrate specimens to an atmosphere of 20˚C and 65% R.H. The spacemen were immersed on a dish of water for five minutes [22] at 20˚C and be put freely on an inclined surface for the same time. By the way, the mean values of fabric weight have been obtained for each of the dry fabric, moist fabric and fully wet fabric (the maximum holding water). The relation between the thickness and fabric weight However could be correlated as a function of the specific volume
R= m3 /kg.
Figure (1) shows the conducted relationship between the fabric construction and the specific volume R at the three phases: the weight at dry, moist and wet (maximum holding water)
Apparently from the figure that the specific volume of the dry cotton/polyester is highest value than the moist cotton and even the cotton maximum holding water, in addition to the cotton/polyester maximum holding water behavior is higher than the pure cotton maximum holding water. This behavior indicates that the filling spaces in between fiber-to-fiber and yarn-to-yarn in the fabric, construction is decreasing by water filling up. Obviously, the specific volume is significantly influenced by the water content.
Hong [23] investigated cotton, polyester and a blend in plain-woven fabric to show their influence on dynamic surface wetness and moisture transfer through textile. A simulated sweating skin was used with the aid of hygrometer to measure the dynamic surface wetness. The polyester fabric showed the highest rate of change in moisture vapor concentration, whilst the cotton fabrics behave with a slower. In addition, Kim [25] claimed that cottons construction show the lowest rate of inner vapor pressure accumulation with the highest inner surface temperature rise. Also, a mixed fabric layers of cotton and polyester reveal moderate inner surface temperature and vapor pressure changes.
Gibson [26] has used the relationship between the air permeability (F) and thickness (t) represented by. , But this relation did not show the resistance of the fabric when the fabric contain moisture vapor or sweat. Later Gibson [26] found that fiber swell at high humidity could have a large influence on the measured air permeability for hygroscopic fibers such as cotton, wool. However It could be represented therefore by different way as: K= m3/sec .kg, what so called TERV (Thermal Effective Resistance Volume). It could be postulated that the physical meaning of this relationship is that, as water accumulates through void spaces of yarn-to-yarn and the fabric what so ever by hydrophilic or hydrophilicphobic groups or even wicking, there is absolutely changes in the fabric dry thermal insulation. This relationship is in agreement with Niwa [27] observation.
From the ratio of(TERV), a relationship between the fabric construction and the TERV for each of cotton and cotton/polyester fabrics has been estimated. Fig (2) Representscotton fabric and Fig (3) representscotton/poly fabric.
The most interested behavior in Fig (2) that whenever the cotton fabric phase transfer from dry to moist up to wet as well as the fabric transfers from satin to plain design ETSV decreases. This behavior indicates that the thermal resistance decreases with the fabric phase transfer as water content changes even by the structure and design. It might be postulated that the thermal insulation not only as a factor affecting sweat transfer ,but also a subjective comfort .Furthermore ,fabric construction in addition to the moisture concentration gradient next to skin[28] govern the contact area between the skin and clothing fabric . Fig (3) represent the same relationship but for cotton/polyester. The figure is showing a surprised behavior, the model 120and 141 is showing that, whenever the fabric at maximum holding water, the TESV rapidly increases, whilstthe model 100and 117 is not significant. Never the less it could be said that the thermal insulation decreases whenever the fabric transfer from satin to plain design.
Fig (4) explains the difference behavior between the cotton and cotton/polyester at maximum holding water. Obviously model 117shows that the plain design contributes a difference almost about 66%, whilst atsatindesign reached about 300%. More over, whenever the structure is going compact by model 120 the difference for satin is about 90% but the plain did not show a significant difference. Model 141 is not showing much difference.
Depend upon fabric material, the fabric moisture content and the structure. The general feature of the behavior of the cotton and cotton/poly at maximum holding water is showing that TESV is much higher for the blended cotton than the pure cotton.
The forgoing discussion has served to show that there are several ways in which heat and moisture can passthroughfabric in clothing environment.For example,sorption,diffusion,evaporation,condensation and wicking of water through fiber assemblies, all have their own associated heat transport mechanism .On the other hand,the specific fiber arrangement in clothing fabric govern the liberation of the sweat and heat transfer from the fabric in the presence of actual water evaporation as will as diffusion. However Ashrae [19] have reported that heat is produced principally by metabolism Qm where the human body’s average temperature (TS) varies with balance between net heat produced by the body and that exchange with the environment (humidity,fabric and airflow).
Generally, heat exchange between the body (back of hand, forearm, upper arm, cheek, lower back, chest, abdomen, thigh and calf) whilst the environment temperature regulation falls into two categories: