Measurement of Heat Transfer Rates under a Liquid Drop During Dropwise Condensation

The quasi-periodic and statistical nature of drop formation, along with very high heat transfer coefficients at low operating temperature difeerentials, makes the experimental determintaion of transfer coefficients quite challenging. We demonstrate the use of high resolution Liquid Crystal Thermography (LCT) coupled with digital videography to measure the spatial distribution of temperature during dropwise condensation in the pendant mode over a polyethylene substrate. Using a one-dimensional heat transfer model, heat flux profiles through individual condensing droplets have been obtained. The measured heat flux as a function of drop diameter matches published data for large drop sizes but fails for small drops (< 0.4 mm) where the thermal resistance of the LCT sheet is a limiting factor. To a first approximation, the present work shows that drop size can be correlated to the local heat flux. It is demonstrated that the average condensation heat flux over a surface can be obtained entirely from the drop-size distribution.