INTRODUCTION

User Objectives and Goals:

1. To study properties of black body
2. To determine the emissivity of the test slab

Theory

When heat is supplied at a constant rate to a body, heat loss takes place by conduction, convection and radiation. If two bodies of the same geometry are heated under identical conditions as shown in Fig 1 and Fig 2, the heat loss by conduction and convection can be assumed to be same for both the bodies.

The heat loss by radiation depends on:

• Characteristic of the material
• Geometry of the surface
• Temperature of the surface

Rate of heat loss to surrounding air by radiation is given by:

$Q=σ ϵ_1 A_1(T_1^4-T_2^4)$
$σ$ = Stefan-Boltzmann constant = $-5.67×10^{-8} W/m^2K^4$
$A_1$ = surface area ($m^2$)
$ϵ_1$ = emissivity
$T_1$ = surface temperature of the body $(K)$
$T_2$ = surrounding atmospheric temperature $(K)$

Equations / formulas:

Temperature of the black body $(K)$, $T_b = \frac{T_1 + T_2}{2}+273$

Temperature of the gray body $(K)$, $T_g = \frac{T_3 + T_4}{2}+273$

Chamber air temperature $(K)$, $T_a= T_5+273$

Emissivity of test plate (gray body), $∈ = \frac{T_g^4 - T_a^4}{T_b^4 - T_a^4}+273$