These complications are circumvented when the transient approach is used.

Transient Approach

Here, the knowledge of heat flux is not required, though the thermophysical properties of the base that carries the LCT sheet must be available. An elaborate approach where the transient temperature measurements are combined with the physical laws to provide the local heat transfer coefficient is given in the linked publication here.

The main assumption of the transient approach for heat transfer coefficient evaluation is the small penetration depth of the thermal pulse into the substrate. Analytical solutions to the semi-infinite solid heat conduction problem can be used to relate the transient surface temperature to the heat transfer coefficient. When the substrate has a low thermal diffusivity, a one dimensional model for the substrate conduction is often a good approximation, since the surface temperature is limited to a thin layer near the surface and lateral conduction can be shown to be small (Dunne, 1983; Metzger, 1986). Valencia et al. (1995) numerically assessed the influence of lateral heat conduction on heat transfer experiments and concluded that transient tests were less prone to error than those conducted at steady state.

The transient method has a long history and numerous reviews are available (Baughn, 1995; Ireland, 1999; Ekkad, 2000; Ireland, 2000). In the transient method, the difference in temperature between the model and the surrounding fluid is followed in time, starting with a step change at time t=0 . There are many approaches utilized to accomplish the change in fluid temperature relative to the surface. Clifford et al. (1983), Ireland and Jones (1985, 1986), Metzger and Larson (1986), and Metzger et al. (1991) used a model where the temperature of the fluid was raised suddenly using switching valves. The basic principles and data reduction procedure are described by Ireland and Jones (1985, 1986).