Heat transfer from the electric heater to the ambient from the rear side of the test cell is estimated as follows. A thin copper strip of 25 mm height is firmly held against the Bakelite sheet and its temperature is monitored by an array of thermocouples. The energy lost to the ambient outside the test cell is obtained by applying vertical flat plate correlations of natural convection to the copper strip. The ambient energy loss is found to be about 10% of the electrical input. Radiation losses are found to be negligible for the smaller of the copper blocks since its area is small and the surface is polished. For the larger block, radiation accounts for up to 5% of the energy input. It has been accounted for through detailed calculations including shape factors. Energy transferred in a direction parallel to the gravity vector to the Bakelite sheet supporting the copper block is not considered as a loss since it is recovered by the fluid ahead and beyond the heater.

Figure 4.36: Thinned interferometric fringe patterns around a protruding heater on a vertical surface.

The thermal field in the vicinity of the heater has been studies using Mach- Zehnder interferometer. The path of the light beam is arranged to be parallel to the heater length. The interferograms are collected in the infinite fringe setting and hence the fringes are isotherms. Skeletonized fringes alone have been presented here. The fringe density near the chip is high and is corrupted by refraction errors. Hence the near-wall fringes have been removed using image processing operations. The outer most fringes representing the thermal boundary layer and all the fringes in the wake have been preserved. For the experiments reported here the temperature drop per fringe shift is 3.5 K.