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

Mechanical Measurements Prof S.P.Venkatesan
Indian Institute of Technology Madras
is in a stable configuration. The thickness of the layer is chosen to be very small
(of the order of a mm) so that heat transfer is conduction dominant. The guard
heat input is so adjusted that there is no temperature difference across the gap
between the main and the guard heaters. It is evident that all the heat input to
the main heater flows downwards through the liquid layer and is removed by the
cooling arrangement. Similarly the heat supplied to the guard heater is removed
by the cooling arrangement at the top.
Radial heat conduction apparatus for liquids and gases:
Another apparatus suitable for the measurement of thermal conductivity of fluids
(liquids and gases) is one that uses radial flow of heat through a very thin layer of
liquid or gas. A cross sectional view of such an apparatus is shown in Figure 4.
Heater is in the form of a cylinder (referred to as bayonet heater or the plug) and
is surrounded by a narrow radial gap that is charged with the liquid or the gas
whose thermal conductivity is to be measured. The outer cylinder is actually a
jacketed cylinder that is cooled by passing cold water. Heat loss from the
bayonet heater except through the annular fluid filled gap is minimized by the use
of proper materials. Thermocouples are arranged to measure the temperature
across the fluid layer. Since the gap (the thickness of the fluid layer) is very
small compared to the diameter of the heater heat conduction across the gap
may be very closely approximated by that across a slab. Hence Equation 2 may
be used in this case also.