Validity of the Semi-infinite Solid Model

The transient technique for the determination of the heat transfer coefficient is mainly based on the model of heat conduction in a semi-infinite solid. Schlutz and Jones (1973) have provided criteria for the validity of the semi-infinite solid assumption. Specifically, they require that the minimum thickness of the material should be greater than where is the thermal diffusivity, and t, the total time. The maximum duration of our experiment is 80 seconds and the selection of the specimen material (Bakelite) and thickness do satisfy the criteria. A simple calculation with the thermophysical properties of Bakelite will reveal that the maximum permissible penetration time of the 25 mm thick Bakelite plate is around 340 seconds, much larger than the total duration of transient run.

The temperature variation of the top and bottom surfaces of the Bakelite plate have been recorded by means of the data acquisition card for one of the transient experiments (Figure 6.7).

Figure 6.7 Temperature variation in the Bakelite plate during the transient run.

It is seen that the lower surface remains at practically a constant temperature during the cooling of the test surface. Thus, the temperature front is well-contained within the Bakelite sheet of finite thickness, confirming the validity of the semi-infinite solid approximation.