Internally heated convection is a fundamental phenomenon, largely governing the dynamics of natural systems such as the atmosphere and Earth’s mantle. It also plays an important role in industrial applications. Here we have investigated the separation of the top thermal boundary layer in order to understand the cell enlargement and the dynamics of the cell pattern formation. To observe the development of the thermal boundary layer non-invasively, the temperature distribution of the vertical plane in a convective cell was visualized by particle image thermometry (PIT). Micro-encapsulated thermo-chromic liquid crystals (TLCs) were seeded in the test fluid and illuminated by a white light sheet, and scattering light was taken by a digital camera. For quantitative temperature measurement, we have calibrated the temperature changes with the variation of the hue color component. The development of the thermal boundary layer with respect to the Rayleigh number has been investigated. The results show the local Rayleigh number determined from the thickness of the thermal boundary layer, which increases towards a critical local Rayleigh number ∼600.

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