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• MR-1 hybrid nanofluid droplet gives the highest evaporation rate (up to 370%). • The evaporation rate increases by 240% as V fd /V sd increases from 1 to 20. • The evaporation rate drops moving from MR-2 to MR-5 hybrid nanofluid droplets. • High droplet spreading is achieved in GNP/MR-1 nanofluid/hybrid nanofluid droplets. • The pore size and areal porosity increase with increasing residue size. Droplet evaporation offers high heat rejection rates and is widely used in the form of spray cooling or dropwise cooling of various heat dissipating devices. However, due to the limiting heat flux removal capacity of conventional fluids, such as water, these cannot be used in thermal management of high heat flux devices. In this research, the evaporation of silver (Ag)-graphene (GNP) hybrid nanofluid droplet and its residue effects on the evaporation of following Ag-GNP hybrid nanofluid droplet, due to its synergistic thermal properties, is experimentally investigated for various mixing ratios, from MR-1 (0.1(Ag):0.9(GNP)) to MR-5 (0.9(Ag):0.1(GNP)), and different residue sizes. A theoretical model is also proposed for hybrid nanofluid droplet evaporation and semi-empirical relations are developed to estimate the hybrid nanofluid droplet spreading over its residue surface. The results show a substantial increase in the droplet evaporation rate with increasing residue size and decreasing mixing ratio. MR-1 hybrid nanofluid droplet gives the highest evaporation rate (up to 370%) on its highly wetted residue surface, while the evaporation rate significantly drops moving from MR-2 to MR-5 hybrid nanofluid droplets on their partially wetted residue surfaces. Moreover, the evaporation rate substantially increases (up to 240%) with increasing residue size for MR-1 hybrid nanofluid droplet resting on its residue surface, however, the effect of residue size on droplet evaporation rate considerably diminishes moving from MR-2 to MR-5 hybrid nanofluid droplets resting on their respective residues. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Droplet phase change is the key phenomenon for high heat transfer rates in spray or drop-wise cooling applications. Despite high cooling efficiency of the spray cooling technology, conventional fluids, such as water, cannot be used for thermal management of modern high heat flux devices due to their immense power density, resulting in early device failures. To address this issue, in this research, we experimentally study the evaporation performance for various volumes of the copper-alumina hybrid nanofluid (CAHF) droplet on a plain copper substrate and compare it with water (H 2 O) droplet in sub-boiling and boiling regimes (i.e., for substrate temperatures of 25–170 °C). We also numerically investigate and compare the internal velocity and thermal fields of CAHF and H 2 O droplets on a heated plain copper substrate. Besides the plain copper surface, we examine the phase change behaviour of the subsequent CAHF droplet over a heated residue surface that was obtained from the phase transition of the first CAHF droplet on a heated plain copper substrate. Our results demonstrate that the evaporation rate of CAHF droplets on a plain copper surface is up to 24% and an order of magnitude higher than water droplets in sub-boiling and nucleate boiling regimes, respectively. Moreover, the evaporation rate of the CAHF droplet on a residue surface increases up to 141% and 800% compared to that on a plain copper surface in sub-boiling and nucleate boiling regimes, respectively. Furthermore, the latent heat flux up to 10 times can be achieved using the CAHF droplet compared to H 2 O droplet on a plain copper substrate in the nucleate boiling region, making the CAHF a potential fluid for high heat flux cooling applications. • Evaporation rate of CAHF droplet is up to 24% that of water droplet for T s ≤ 100 °C. • CAHF droplet evaporation rate is up to 1300% that of water droplet for T s > 100 °C. • Latent heat flux of CAHF droplet is up to 1000% that of water droplet at T s > 100 °C. • Evaporation rate of CAHF droplet on its residue is up to 800% than on plain copper. • CAHF droplets exhibit higher thermal Marangoni convection than H 2 O droplets. [ABSTRACT FROM AUTHOR]