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In this study,the heat transfer characteristics of nanofluids used in spray cooling systems were examined.Three nanofluids,i.e.,Cu,CuO,and Al2O3,respectively,with volume fractions ranging from 0.1％ to 0.5％,as well as different volume fractions of a surfactant Tween 20,were used.In addition,their contact angles were measured to examine the heat-transfer characteristics.Under the same experimental conditions,with the increase in the volume fraction of the Cu nanoparticles from 0.1％ to 0.5％,the maximum heat flux qmax increased from 3.36 MW/m2 to 3.48 MW/m2 from the impinging central point to r=30 mm (r is the distance from the impingement point),and the corresponding temperature of qmax increased from 400℃ to 420℃.Results revealed that with increasing Tween 20 concentrations,the contact angle decreased because of the decrease in the surface tension of nanofluids and improvement of the wetting ability,and the corresponding qmax increased from 3.48 MW/m2 to 3.94 MW/m2 at the impact central point.