An experimental investigation has been performed to study the nucleate boiling characteristics and the critical heat flux(CHF) of water and water based CuO nanoparticle suspensions(nanofluids) in vertical small tubes with closed bottom.The experimental results indicated that the CHF and the particle concentration of the nanofluid in the tubes had hdlany change with the heating times in the boiling process.The boiling heat transfer of the nanofluids is somewhat poor compared with that of the base fluids

since the decrease of the active nucleate cavities results in the decrease of the solid-liquid contact angle on the heated surface with a very thin nanoparticle sorption layer.The boiling critical heat fluxes for the nanofluids increase significantly than that of the base fluids.An empirical correlation is proposed for predicting the CHF of nanofluids boiling in the vertical tubes with closed bottom.