Flow And Heat Transfer Of Nanofluids And Pulsating Flows In A Smooth Pipe

In order to improve the heat exchange efficiency of heat transfer equipment,heat transfer medium with higher efficiency is usually used.As new heat transfer medias,the nanofluids and pulsating flows have higher heat transfer effetiveness than traditional fluids.This is due to the addition of nanoparticles in nanofluids which will improve the thermal conductivity of the base fluid.The pulsating flow can significantly inhance the heat transfer since the transient flow will increases the turbulence of the fluid,and enhanced heat transfer consequently.The flow and heat transfer of nanofluids and pulsating flows are studied experimentally and numerically in this paper.For nanofluids,the two-step method is used to make CuO and Al₂O₃water-based nanofluids with the mass fraction of 0.05%,0.1%,0.2%,0.3%and0.4%for CuO nanoparticles.The experimental platform has been built to measure the flow and heat transfer results.The thermal efficiency and flow loss are analyzed under different Reynolds number and particle mass fraction for two kinds of nanofluids.Experimental results show that at Re=2×10⁴?5×10⁴,with the increase of Reynolds number,the heat transfer of the CuO nanofluids and Al₂O₃nanofluid increases gradually.The thermal efficiency for the CuO nanofluids with mass fraction of 0.1%is the highest.At low Reynolds number,the heat transfer of the Al₂O₃nanofluid with mass fraction of 0.05%is better.At higher Reynolds number,the heat transfer of Al₂O₃nanofluid with mass fraction of 0.2%is better.With comparison of the heat transfer and pressure loss between the two nanofluids,it can be concluded that the heat transfer of Al₂O₃nanofluids is higher than that of CuO nanofluids with the same mass fraction.And the flow loss is lower than that of CuO nanofluids.Therefore,the Al₂O₃nanofluids is a more promising coolant.For the pulsating flow,the influence of pulsating amplitude and frequency on the heat transfer are experimentally studied firstly.Then numerical study is performed by using the commercial software ANSYS CFX.The calculated results are compared with the experimental data and show good agreement with each other.Experiment results show that the heat transfer of pulsating flow is enhanced with the increase of pulsating amplitude and frequency.The Nu of pulsating Al₂O₃nanofluid with a mass fraction of 0.2%increases by 0.05%compared with the steady state Al₂O₃nanofluid,and increased by 12%compared with the deionized water pulsating flow.The calculated results show that the Nu number of the pulsating flow changes sinusoidally with tim.The Nu for the pulsating flow with three different pulsating frequencies is almost the same,indicating that the pulsating frequency has little influence on the heat transfer results within the range of 0.0083Hz<f<0.025Hz.With the increase of pulsation amplitude,the peak value of Nu ratio increases gradually,while the trough value decreases gradually.Moreover,at?t=?/2??,the trough value of Nu ratio decreases more obvious.In the range of0.2m³/h<A<0.6m³/h,the pulsation amplitude has great influence on heat transfer results.