Heat and Mass Transfer manuscript No. (will be inserted by the editor) Mechanisms of Heat Transfer for Axisymmetric Bubble Impingement and Rebound D.B. Donoghue · A. Albadawi · Y.M.C. Delaur´ e · A.J. Robinson · D.B. Murray Received: date / Accepted: date Abstract Heat transfer enhancement resulting from the impingement and rebound of bubbles in confined geometries can play an important role in heat transfer applications. Limited studies exist on the impact be- haviour of large ellipsoidal bubbles against a horizontal surface, while the associated fluid flow field has received even less recognition. To address this, the current study investigates the dynamics of a single large ellipsoidal bubble impinging on a horizontal heated surface. The bouncing dynamics have been explored by util- ising synchronised high- speed and IR photography. Due to the large bubble size in the present study only a bub- ble with a low release to surface distance was found to have a symmetric bouncing event. The results showed that separated wake structures initially cooled the sur- face before the wake structures become counter produc- tive and convect warm fluid onto the previously cooled surface. Two cooling zones were observed; the inner re- gion due to the bubble and the outer region due to the bubble’s wake. Keywords Rising bubble · Bouncing bubble · Heat transfer enhancement · IR thermography 1 Introduction Two phase flows occur extensively in nature and in technology and are utilized in systems ranging from healthcare to the energy industry. In particular, two D.B. Donoghue Department of Mechanical & Manufacturing Engineering, Trinity College Dublin, Ireland, E-mail: donoghdb@tcd.ie phase flow plays an important role in convective cool- ing and chemical engineering applications ranging from complex cooling systems to mixing in reactors. Research has shown that two phase flows can produce exceedingly high heat transfer coefficients, which have the ability to be an order of magnitude higher than their single phase counterparts. This has motivated numerous investiga- tions over the past century [1–9]. Although the dynam- ics of free rising bubbles have been studied extensively [10–14], research into their effects on heat transfer is limited; even fewer studies have been performed in re- lation to bouncing bubbles. A number of authors have investigated the effect of a bubble bouncing against a solid surface [15–19] under adiabatic conditions, but the corresponding heat transfer processes have received lim- ited attention to date [3,4,20,21]. Qiu & Dhir [5] conducted an experimental study on the flow patterns and heat transfer associated with a vapour bubble sliding on a downward facing heated sur- face. They showed that the sliding bubble wake struc- ture to the rear of the bubble enhanced heat trans- fer by introducing cooler liquid from the bulk to the surface. This was supported by the IR thermography experiments of Donnelly et al. [22] and more recently O’Reilly Meehan et al. [23] for a single air bubble and an in-line bubble pair, respectively. Complex “packets” of surface convective heat transfer enhancement were observed that was consistent with the vortex structures measured by O’Reilly Meehan et al. [24] using particle image velocimetry. Manickam & Dhir [9] performed a further study, with the same experimental apparatus as Qiu & Dhir [5]. Holographic interferometry was used to measure the temperature field surrounding the bub- ble. It was found that the bubble shape changed from a spheroid to an elongated cylinder and to a segment of