Two-dye laser-induced fluorescence (LIF) thermometry for ice-water systems

We present an extension to the two-dye two-color laser-induced fluorescence (LIF) thermometry technique so that it can be applied to ice-water systems for direct heat flux measurements near the ice-water interface. Our chosen dye pair consists of the temperature-sensitive dye rhodamine B (RhB) and the temperature-insensitive dye sulforhodamine 640 (SR640), mixed at the concentration ratio of \(C_\textrm{RhB}/C_\textrm{SR640} = 0.5\) which we found to be optimal in terms of signal strengths and temperature sensitivity when they are excited at 527 nm. Our extension relies on the creation of uniformly dyed ice and this can be achieved by adding 5 g/L of sugar into dyed solutions prior freezing. The dyes remain potent after one freezing and melting cycle and the added sugar brings negligible change to the freezing point of water. The developed technique has been applied to measure the temperature change due to the melting of a horizontal ice block immersed into an initially stagnant body of water in a box. The calculated Nusselt numbers are in good agreement with the established empirical correlation for free horizontal convection. Moreover, an important finding is that in situ calibration of the LIF technique is not needed as long as the concentration ratio, solvent, image magnification, and dye thickness remain the same among experimental setups.