Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts Dropwise evaporative cooling of hot water: A novel methodology to enhance heat transfer rate at very high surface temperatures A.R. Pati a , A. Panda a , Lily a , B. Munshi a , A. Kumar a , A. Sahoo a , S. Ghosh b , S.S. Mohapatra a,* a Department of Chemical Engineering, National Institute of Technology Rourkela, Rourkela 769008, India b Department of Mechanical Engineering, National Institute of Technology Rourkela, Rourkela 769008, India ARTICLE INFO Keywords: Fast cooling Quenching Evaporative cooling Recoiling Latent heat extraction ABSTRACT Newly developed cooling techniques including spray cooling and air-atomized cooling are oriented on one basic phenomenon, i.e., dropwise evaporative cooling. The dropwise evaporative cooling is mainly controlled by the physical and thermal properties of the coolant and the droplet dynamics during evaporation. Different re- searchers have tried to enhance dropwise evaporation process by enhancing any one of the aforesaid char- acteristics and as consequence the achieved heat transfer rate is lower than the quenching rate required for the fast cooling operation. In addition to the above, the open literature does not disclose any methodologies, which consider simultaneously all the above-mentioned properties in the enhancement of heat transfer. Therefore, in the current research, an attempt has been made to augment the heat transfer rate in case of dropwise evaporative cooling process by altering simultaneously the thermal properties, physical properties and the flow dynamics of the droplet. The current proposed methodology to obtain fast evaporation is by altering thermal, physical and flow properties and this is achieved by increasing the water temperature. The experimental investigation con- siders water temperature and the substrate temperature as the independent variables. The heat transfer analysis depicts that the increment in initial plate temperature and water temperature have significant effects on eva- poration time. On increasing water temperature from 10 to 60 °C, the evaporation time is reduced by ∼200% due to the chances of reduction of recoiling characteristics after impingement, creation of high heat transfer area and decrement of sensible heat extraction period. With the increasing substrate temperature, the evaporation time decreases due to the increment of the thermal conductivity of the coolant. In addition to the above, the mechanism for the aforesaid enhancement process is tried to reveal by developing the mathematical models. In addition to the above, the enhancement capability of the hot water is compared with different potential coolants. From the comparison, it is concluded that the heat removal capacity of hot water is significant and it can also replace the considered coolants without depicting the disadvantages of the considered coolants in the literature. For the verification, experimental results are compared with the numerical results. The comparison discloses that the developed model is quite accurate and shows insignificant variation from the experimental results. A suitable model and vapour film thickness are also determined from the numerical investigations. 1. Introduction The heat transfer rate in case of dropwise evaporation is controlled by the physical and thermal properties and the flow dynamics of coolant during evaporation. The physical properties such as surface tension, viscosity and density are related to heat transfer. On decreasing surface tension of the coolant, the contact angle of the coolant droplet decreases [1–8]. This process creates high heat transfer. Furthermore, the decrement of viscosity and density lead to finer atomization and this is favourable for heat transfer. The thermal properties such as specific heat and thermal conductivity are the controlling parameter in case of dropwise eva- poration. The decrement of specific heat and the increment of thermal conductivity enhance the dropwise evaporation rate by decreasing the sensible heat extraction period and increasing the conductive heat transfer rate. In addition to the above, the flow dynamics of the droplet such as recoiling behaviour and vapour bubble coalescence also reg- ulates the heat transfer rate. The increment in the recoiling character- istic of droplet after impingement on the hot substrate and the en- hancement of vapour bubble coalescence during evaporation are detrimental for heat transfer. Bernardin et al. [9] studied the heat transfer mechanism in case of dropwise evaporation at various https://doi.org/10.1016/j.ijthermalsci.2018.01.028 Received 11 April 2017; Received in revised form 15 September 2017; Accepted 24 January 2018 * Corresponding author. E-mail address: mohapatras@nitrkl.ac.in (S.S. Mohapatra). International Journal of Thermal Sciences 127 (2018) 335–350 1290-0729/ © 2018 Elsevier Masson SAS. All rights reserved. T