Effect of nanofluids on thermal performance of closed loop pulsating heat pipe

The paper describes the effect of nanofluids on the closed loop pulsating heat pipe (CLPHP) performance using copper and silver colloidal nanofluids. An experimental program has been carried out on the thermal performance of the CLPHP with different working fluids (DI water, copper and silver colloidal nanofluids). Copper nanoparticles are characterized by the X-ray diffraction and Scanning Electron Microscope. Copper nanofluid is prepared by a two step method and distribution of nanoparticles is justified by using Dynamic Light Scattering technique. Another nanofluid i.e., silver colloidal nanofluid solution is synthesized using chemical reduction method and characterized using UV-vis Absorption Spectroscopy and Dynamic Light Scattering techniques. The thermal performance of the device has been investigated with varying heat power in the range of 50-240 W for different working fluids in the vertical orientation. The overall thermal resistance and effective thermal conductivity are evaluated to predict the thermal performance of the device. Experimental results show that the nanofluid charged CLPHPs enhance the heat transfer limit by 33.3% and have lower evaporator wall temperature compared to that of DI water. The intensifying nucleation sites are found in the inner tube of the evaporator section due to usage of nanofluid in the CLPHP. The device with nanofluids achieves better thermal performance as compared to that of DI water. Furthermore, a preliminary visual study is carried out to understand the device operation with copper nanofluid in the glass tube CLPHP. Several important features of CLPHP operation are delineated by the visualization.