Measurement of heat transfer coefficient

Heat transfer experiments using LCT are carried out either at steady-state or during a transient phase.

Steady State Approach

For the steady-state technique, the surface is maintained at constant heat flux and sufficient time is allowed to elapse. With the time invariant temperature contours are established, the surface temperature distribution   can be obtained using liquid crystal thermography. For a given free stream temperature , the convective heat transfer coefficient is calculated subsequently as

With the steady state approach, the temperature range covered is limited by the bandwidth of the LCT sheet. Thus, multiple experiments are to be performed with varying heat flux settings. A wider range of experiments can be performed, however, with a single broadband liquid crystal sheet. Clearly, such experiments are at the expense of loss of temperature resolution. The transient method determines the local heat transfer coefficient from the time sequence of images of the surface temperature when the surface heat flux is prescribed (or zero). Alternatively, the surface may be insulated and the flowing medium might be at a temperature higher than the initial surface temperature. The narrow - and the wide-band technique each may be employed to establish the heat transfer coefficient distribution over the surface. In such an approach, the heat transfer coefficient is taken to be time-independent.

Disadvantages

The steady state approach has a major disadvantage in that the heat flux measurement is in itself a challenge. In most instances, measuring losses either to the back-up substrate or by radiation can have a great deal of uncertainty. When the working fluid is air, the losses may be as large as the energy transferred to the fluid phase.