Effect of inclusion of nanoparticles on unsteady heat transfer

New geometry with rectangular inner cylinder containing cold flow has been simulated in this article. Freezing phenomenon has been simulated and NEPCM was mixture of CuO and water. Time-dependent solid fraction term was added to energy equation. Software based on FEM with adaptive grid was implemented for modeling the problem. Diameter of nanomaterial and amplitude of outer wall were assumed as variable. To reach the reliability of assumption of neglecting buoyancy term, comparison with experimental data was illustrated. Providing greater value of A makes the freezing time to reduce about 5.82% which is associated with existence of more NEPCM near the rectangular cylinder when A = 0.3. Influence of A on Tave has no sensible impact for t   320 s. Increasing diameter of nano-powder can augment the conductivity but experimental observation shows that there is optimum value for this factor. As dp augments from 30 to 50 nm and 40 nm, the time of solidification alters from 186.57 s to 222.77 s and 149.37 s. With rise of dp, at first, the time declines about 19.98% then time augments about 49.16%. When A = 0.3 and dp = 40 nm, the quickest process takes place and it takes 149.37 s to reach full freezing.