Temperature measurements inside freezing drops in Poiseuille flow*

Water is cooled down close to its melting point and is introduced through a syringe in the form of drops in an oil-continuous pipe flow. The oil has a temperature below that of the melting point of water and flows at a Reynolds number in the order of 10³. The heat transfer and solidification of the water drops is investigated downstream of the test section with planar laser-induced fluorescence (PLIF). A YLF laser sheet is emitted along the cross-stream direction of the flow and is used to harmonically excite a pair of fluorescent dyes with different emission spectra that are introduced in the water drop. Two cameras are connected to a beam-splitter and the fluorescence signal of each dye is captured in each camera individually. A ratiometric approach is used to accurately measure the temperature field inside the drop and account for reflection effects at the drop interface. The temperature gradients inside the drop and close to the thermal boundary layer are recorded for different Reynolds numbers and drop sizes, but also for different water and oil temperatures.