123MEAN thermal properties KATEDRA MATERIÁLOVÉHO INŽENÝRSTVÍ A CHEMIE

123MEAN thermal properties KATEDRA MATERIÁLOVÉHO INŽENÝRSTVÍ A CHEMIE

Heat transport in substances: conduction transfer of kinetic energy on the bases of disorded movement of molecules. Own heat transfer occurs from the place having higher temperature to place with lower. Conduction solid materials, liquids and in gases. convection heating of fluids (liquid or gaseous matters) causes simultaneous increase of volume and decrease of density. Lighter part of fluid rises and displaces a heavier part of liquid. radiation - heat is transported by electromagnetic waves. This type of transport can be significant for building materials containing big air cavities.

Heat transport by conduction: Heat transfer rate is defined as (Q - heat, τ- time) I q dq d The areal density of heat flow J q (q heat flow density vector, T - temperature) diq Jq dsn The main parameter of heat transport is temperature gradient as mentioned Fourier s relation: q gradt q is heat flow density vector, (W m -1 K -1 ) represents thermal conductivity ability of material to distribute heat within its mass. Thermal conductivity value is not constant for any materiál. This depends on its structure, total porosity, temperature, humidity content, etc.

Heat transport by conduction: High thermal conductivity possess metals mercury 402 W m -1 K -1 Lower values are detected for liquids water 0,56 W m -1 K -1 The worse substances for heat transport are gases good thermal insulators dry air 0,0258 W m -1 K -1 Thermal diffusivity a (m 2 s -1 ) is defined by using thermal conductivity λ, specific heat capacity c and bulk density ρ of tested material. a c where c is specific heat capacity (J kg -1 K -1 ).

Thermal conductivity of chosen materials: Material λ [W.m -1.K -1 ] silver 418 copper 395 aluminum 229 iron 73 granite 2,9-4,0 concrete 1,5 water 1,0 brick 0,28-1,2 glass 0,60-1,0 lime plaster 0,88 lightweight concrete 0,70 linoleum 0,19 cellular concrete 0,15 polystyrene 0,05 glass wool 0,04 extruded polystyrene 0,035 air 0,0258

Thermal conductivity of water: 0.56 0.68 W/mK with increasing temperature increase thermal conductivity values.

Thermal conductivity of dry air and water vapor: increases with increasing temperature.

Thermal conductivity measurement: - Stationary methods thermal conductivity is determined in steady temperature field. Those methods are precise but very time consuming (hours, days) suitable for laboratory measurements, e.g. Bock device. - Nonstationary methods measurement is realized in unsteady temperature field. Those procedures are less precise but relatively quick (a few minutes, tens of minutes) useful for field and for quick laboratory measurements, e.g. Hot-wire, hot-ball or hot-disk method.

Stationary method Bock device: - two plates one heated, the other cooled - After temperature stabilization (hours, days) is determined thermal conductivity as:

Nonstationary method Hot-wire: - heated wire and recording thermocouple, q [W/m] - slope [ln (K/s)] = [K] Temperature field

Nonstationary method Hot-disk: - heated wire and recording thermocouple, q [W/m]

Isomet 2104 Lab device (hot-disk) serving to direct measurement of thermal conductivity, thermal diffusivity and volumetric heat capacity. Device range: Thermal conductivity [W m -1 K -1 ] 0,015-6 Volumetric heat capacity [J m -3 K -1 ] 4,0.10 4 4.10 6 Temperature [ C] -20 - +70

Thermophysical Tester RTB Transient method Generated heat pulse inside the specimen caused dynamic temperature field. From the parameters of the temperature response (the time t m and the magnitude of the temperature response T m ) to the heat pulse, the specific heat capacity, thermal diffusivity and thermal conductivity can be calculated. planar source thermocouple current pulse h T temperature response I t 0 I II III T m t m t specimen

Specific heat capacity: Represents the amount of heat required to heat1 kg of substance of 1 C. This is accumulation property of 1 kg of given material. - Water in comparison with other substances is able to accumulate a high amount of heat.

Calculation of specific heat capacity of moisture containing material: c ( c 0 cwu)/(1 u) u [kg/kg] - weight content of water in the material = water weight/sample weight C 0 [J kg -1 K -1 ] - specific heat capacity of dry material C w [J kg -1 K -1 ]- specific heat capacity of water 4184 J kg -1 K -1 at 20 C