Heat Transfer through Textiles

HEAT TRANSFER THROUGH TEXTILESSHAHRUKH SHAHBAZ

ContentsHEAT MODES OF HEAT TRANSFEREQUATIONS FOR CONDUCTIONTHERMAL CONDUCTIVITYTHERMAL CONDUCTIVITY OF TEXTILE FABRICSTHE EFFECT OF PROPERTIES

HEATHeat is defined as total energy of molecular motion in a substance.It is denoted by Q and its measuring unit in SI system is Joule.

HEATJames Clerk Maxwell defined heat in a comprehensive way in his Theory of Heat. TransferableMeasurableCannot be treated as a substanceForm of the energy

WHY DOES HEAT FLOW?When a substance has different level of thermal energy from its surroundings, phenomenon of heat transfer takes place and is referred to the variance in thermal energy level indicated by the difference of temperature.

MODES OF HEAT TRANSFER:There are three modes of heat transferConductionConvection Radiation

CONDUCTIONIn conduction transfer of heat takes place between neighboring molecules due to temperature gradient and it is always form a higher temperature to a lower temperature till there is equilibrium.

CONVECTIONConvection is the concerted, collective movement of groups or aggregates of molecules within fluids (e.g., liquids, gases) either through advection or through diffusion or as a combination of both of them.

RADIATIONHeat can be transmitted though empty space by thermal radiation often called infrared radiation. This is a type electromagnetic radiation.

THERMAL CONDUCTIVITY:Heat transfer by conduction depends on the materials heat conductivity, The main characteristics of heat conductivity is:Conductivity factor [W/(mC)]

SPECIFIC HEAT RESISTANCE

The specific heat resistance, r [(mC) /W] is a characteristic inverse to the heat transfer factor , as in the following equation:

THERMAL RESISTANCEThe thermal resistance, Rth, of textile fabrics is a function of the actual thickness of the material and the thermal conductivity k. This function is given by the following relationship:

HEAT FLOWThe heat flow, Q, through the textile fabric is given as the following

Where A is the surface area exposed to the hot air, T1 is the initial air temperature and T2 is the transient air temperature.

EQUATIONS GOVERNING CONDUCTION

Qx Q x+ x + E gen, element = E element / T

Rate of heatenergy conductionat xRate of heatenergy conductionat x + xRate of heat generation inside elementRate of total heat generation inside element-+=

THERMAL CONDUCTIVITY OF MATERIALSSr #MaterialThermal Conductivity at 25 oC ( W/m K)1Copper4832Silver4503Gold3454Air0.0265Water0.616Polymer0.357Textile0.1 0.035

THERMAL CONDUCTIVITY OF TEXTILE FABRICSThermal conductivity of a textile fabric involves a disperse system consisting of textile substance and air. Heterogeneous system

THERMAL CONDUCTIVITY OF TEXTILE FABRICSThe thermal conductivity of jute/cotton blended knitted fabric is determined by using following formula:

THERMAL CONDUCTIVITY OF TEXTILE FABRICS

K = Thermal conductivity of sample (10-3WM-1K-1)M = Mass of brass disc (C) in kgsS = Specific heat of the material of the disc (370 JKg-1K-1)R = Rate of fall of temperature (dT/dt)H = Thickness of brass disc in mmR = Radius of the brass disc in mmD = Thickness of the specimen in mmA = Area of cross section of the specimen in mm2

THE EFFECT OF PROPERTIES

EFFECT OF FABRIC PROPERTIES VERSUS THERMAL CONDUCTIVITYFabric Tightness:

EFFECT OF FABRIC PROPERTIES VERSUS THERMAL CONDUCTIVITYFabric Thickness:

CONCLUSIONThe thermal conductivity of the textile fabric is an important feature which defines its specific area of usage. The wearables are required to act as an insulation for the outer environment and preferred if they have good conductance in certain cases. The variation in the physical and chemical properties plays an important role in defining the textile fabric for particular usage.

REFERENCESE. David Mooney, Introduction to Thermodynamics and Heat Transfer, Prentice-Hall, USA, (1955).A. K. Haghi, Heat and Mass Transfer in Textiles: Theory and Applications, WSEAS Press, Canada, 2007.R. Siegel and J. Howell, Thermal Radiation Heat Transfer, Taylor and Francis, 2002.Anon, Available online: (http://searchcio-midmarket.techtarget.com), [Accessed 18 January, 2015].Q. Kern. Donald, Process Heat Transfer, McGraw-Hill Book Company, 1950.F.P. Incropera and D.P. Dewitt, Fundamentals of Heat and Mass Transfer, John Wiley & Sons, 3rd edition, 1990.Anon, Avaiable online: http://www.hk-phy.org/contextual/heat/hea/condu02_e.html, [Accessed 15 January, 2015].W. Malalasekera, Heat Transfer and Fluid Flow Notes, Provided by Textile dept of Technical University Liberec, 2009. J. T. Williams, Textile for cold weather apparel, Wood head publishing limited, USA, 2009.K. Greenwood, W. H. Rees and J. Lord, Studies in Modern Fabrics, The Textile Institute, Manchester, pp. 197, 1970.