Liquid Crystal Thermography Studies in Pool Boiling in Water at Subatmospheric Pressures

An experimental nucleate pool boiling study in water is presented for a range of constant heat flux (q") operated at three different system pressures (P_∞). Experiments were run at 6.2kPa, 7.0kPa and 8.0kPa with a constant heat flux varying from 1.88kW/m² to 4.55kW/m². The growth of vapor bubbles has been analyzed from inception to lift–off from an electrically heated 0.05 mm thick horizontal Fecralloy surface. Liquid crystal thermography, a non-intrusive technique, is used for the determination of surface temperature distributions. This technique offers some advantages, including ease of handling, reasonable accuracy, and spatial resolution of the temperature field. The color image processing is used to obtain quantitative temperature information from the TLC images captured with a reasonable accuracy and resolution. Boiling parameters like bubble size, contact and the frequency are determined. The bubble cycle with a neck formation is observed at the lowest investigated pressures 6.2kPa and 7.0kPa with a constant heat flux of 1.88kW/m². From the corresponding TLC images it is clearly seen that a dry spot caused by evaporation of microlayer forms beneath the vapor bubble during its growth. An increase in either the heat flux or the system pressure results in a decrease in the size of the bubble and an increase in bubble frequency.