An experimental investigation into the spread and heat transfer dynamics of a train of two concentric impinging droplets over a heated surface

Abstract

Extensive studies of two concentric droplets consecutively impinging over a thin heated foil surface are carried out to compare the spread and heat transfer dynamics of a single drop, and drop-on-drop configurations using high speed imaging and infrared thermography. Millimeter-sized deionized water droplets (2.80 ± 0.04 mm) are impinged upon a heated Inconel surface (thickness of 25 μm) from a fixed height corresponding to a Weber number (We) of 50 ± 2 and Reynolds number (Re) of 3180 ± 90 with a flow rate of 20 droplets per minute. Surface temperature is chosen as a parameter, and is varied from 22 °C (non-heated) to 175 °C. Temperature and heat flux distributions associated with droplet-surface interactions are obtained, and the outcomes of the process are measured in terms of spread diameter, droplet input heat transfer, dynamic contact angle, and surface mean temperature. A decline in the droplet heat transfer for drop-on-drop impingement is observed for all temperatures investigated in the present work. This is attributed to the surface pre-cooling by the initial droplet and also to the reduced surface area-to-volume ratio i.e., increased spreading film thickness. High heat transfer rates are observed around the three-phase contact line region, especially during the receding phase of the droplet, for both configurations, confirming the significance of contact line evaporation in droplet-hot wall interactions. Theoretical models predicting the maximum spread factor and corresponding input heat transfer into the droplet are identified from the literature, and found to be in good agreement with present experimental observations.