1. Thermo-physical properties 2. Radiation properties - nptel - Indian ...

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Mechanical Measurements Prof S.P.Venkatesan <strong>Indian</strong> Institute of Technology Madras the heat transfer across any one of the samples is half that supplied to the main heater and the area is the face area of one of the samples. Typically the sample, in the case of low conductivity materials, is 25 mm thick and the area occupied by the main heater is 200×200 mm. The heat input is adjusted such that the temperature drop across the sample is of the order of 5°C. In order to improve the contact between heater surface and the sample surface a film of high conductivity material may be applied between the two. Many a time an axial force is also applied using a suitable arrangement so that the contact between surfaces is thermally good.
Mechanical Measurements Prof S.P.Venkatesan <strong>Indian</strong> Institute of Technology Madras Example 1 A guarded hot plate apparatus is used to measure the thermal conductivity of an insulating material. The specimen thickness is 25 ± 0.5 mm. The heat flux is measured within 1% and is nominally 80 W/m 2 . The temperature drop across the specimen under the steady state is 5 ± 0.2°C. Determine the thermal conductivity of the sample along with its uncertainty. The given data is written down as (all are nominal values) 2 q = 80 W / m , Δ T = 5 ° C, δ = 25 mm= 0.025 m Using Equation 2, the nominal value of the thermal conductivity is qδ 80× 0.025 k = = = 0.4 W / m° C ΔT 5 The uncertainties in the measured quantities specified in the problem are q 80 2 δq =± =± =± 0.8 W / m , δ ( Δ T) =± 0.2 ° C, 100 100 δδ =± 0.5 mm =± 0.0005 m Logarithmic differentiation is possible and the error in the thermal conductivity estimate may be written down as
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