The measurement of heat transfer from hot surfaces to non-wetting droplets D. Chatzikyriakou a,⇑ , S.P. Walker a , C.P. Hale b , G.F. Hewitt b a Department of Mechanical Engineering, Exhibition Road, South Kensington, Imperial College London, London SW7 2AZ, UK b Department of Chemical Engineering and Chemical Technology, Prince Consort Road, South Kensington, Imperial College London, London SW7 2BY, UK article info Article history: Received 4 May 2010 Accepted 29 November 2010 Available online 29 December 2010 Keywords: Infrared Non-wetting droplets Cooling abstract An experimental method to measure the heat transfer between a hot surface and a non-wetting droplet is reported in this paper. By means of transient, high resolution, infrared microscopy, surface temperature measurements with spatial and temporal resolutions of 100 lm and 4 ms, respectively, are obtained, by observing a thin metallic layer from beneath through an infrared-transparent substrate. Data from the infrared camera is generated at each time-step in the form of a set of temperatures, at closely-spaced locations on the surface of the infrared transparent plate. Subsequent solution of the transient thermal conduction equation within the substrate permits all thermal quantities (heat flux, energy, etc.) to be determined. As a typical result, the heat transferred by a 1.5 mm droplet is measured to be 0.19 J, with the heat flux peaking at 3.5 MW/m 2 during the 10 ms it spends in the vicinity of the surface, and with a peak transient surface temperature reduction of 47 °C. Error analysis indicates that the uncertainty in this measurement of heat transfer is about 15%. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction We consider here the flow of a gas with entrained liquid drop- lets past a hot surface, and address the question of how much of the heat transfer from the surface is attributable to the droplets. For high enough surface temperatures, entrained droplets which approach the surface do not wet it, but rather recoil from the sur- face, bouncing off the layer of vapour generated by evaporation from the portion of the droplet that faces the hot surface. The sur- face temperature at which this occurs depends on material proper- ties and flow conditions and is generally referred to as the Leidenfrost temperature [1]. For example, for water droplets at atmospheric pressure and modest approach velocities, the temper- ature above which wetting does not occur is approximately 220 o C. This phenomenon is of importance in a variety of industrial applications, of which one of the most important, and the one which motivates the present study, is the cooling of fuel in a pres- surized water nuclear reactor following a large loss of coolant accident. In a ‘‘design basis’’ accident in a PWR, a double ended break in a coolant duct is postulated to occur, which would be followed by essentially total loss of coolant from the core over a period of order of tens to hundreds of seconds. Once the pressure in the reactor vessel has fallen to more or less atmospheric pressure, cold water is introduced to the base of the vessel and a liquid front rises around the now severely overheated fuel rods. In the mid to upper part of the core, where fuel is hottest, the conditions are character- ised by a flow of superheated vapour, with a dense population of small entrained liquid drops. It is believed that their serving as a heat sink, via their evaporation into the superheated vapour, is the main role played by these drops. The droplets will also extract some heat from the fuel rods by direct impingement onto the hot metal surfaces. As noted above, such impingement does not, under these conditions, result in wet- ting, but rather the droplets bounce from the layer of vapour gen- erated as they approach the surface. The aim behind this study is to attempt to estimate the contri- bution that this direct cooling by droplets plays in the removal of heat from the fuel, by the measurement of the heat extracted by the close approach to a hot surface of a single, non-wetting droplet. In this present paper we describe the experimental techniques developed for this purpose and present the results of initial measurements. Assessment of the contribution of these droplet-wall collisions to the cooling of the fuel requires a combination of the ‘‘Joules per droplet’’, the measurement of which is the subject of this pres- ent paper, with some knowledge of the ‘‘droplets per unit time per unit area’’. This latter is a difficult area of experimentation: one of the few attempts to measure this is reported by Hewitt and Govan [2]. The main experimental difficulties stem from the small size of the droplets, the brevity of their stay near the wall and the corre- spondingly small amount of heat they extract during this period. The droplet sizes of interest to us lie in the range 0.1–2 mm in 0017-9310/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2010.11.051 ⇑ Corresponding author. Present address: Rue des Bollandistes 9, Etterbeek, Brussels 1040, Belgium. Tel.: +32 472557352. E-mail address: d.chatzikyriakou05@imperial.ac.uk (D. Chatzikyriakou). International Journal of Heat and Mass Transfer 54 (2011) 1432–1440 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt