Since the equipment is in air and surface temperature is generally not high, conduction and radiation heat loss is insignificant. Also in space there is no gravity. Thus buoyancy and therefore free convection is not at all possible in the compartment air. Hence the only suitable mode for cooling is forced convection.
The thermal conductivity of a gaseous medium is dominantly based on diffusion rate, which depends on root mean square velocity.The root mean square velocity for molecules is given by
where M is the molecular mass of the gas & T is the temperature of the gas and ̅ is universal gas constant. Thus diffusion rate and therefore thermal conductivity is lower for gases with higher molecular masses because
Therefore H₂ gas has more thermal conductivity than O₂.
In microwave ovens, the radiation emitted is a result of excitation of crystals like magnetrons and klystrons and in microwave wavelength band which is not in the range of thermal radiation. Also during nuclear reactions like those inside sun, -rays are produced which is actually not considered as thermal radiation. Once these -rays get absorbed by the matter in the sun then temperature rises and thermal radiation is emitted. Thus nuclear reaction does not directly emit thermal radiation.
But during lighting of bulb energy is emitted mainly in form of light and IR due to temperature of tungsten filament. Thus this event is termed as thermal radiation.
In a furnace heat transfer is a result of thermal radiation due to high temperature values and contribution of conduction and convection to the phenomenon is insignificant. Hence optical pyrometers are best suited devices to measure temperature inside the furnace. These devices detect wavelength corresponding to maximum monochromatic emissive power in the infrared radiation and using Wien’s law, i... moret predicts the temperature.