Experimental investigation of thermophysical properties of R718 based nanofluids at low temperatures

Abstract

An extensive experimental investigation on the thermophysical properties of two different types of R718 (water) based nanofluid was performed for particle volume fractions (ϕ) ranging from 0.25–2.5%. R718 (water) is used as a secondary refrigerant in refrigeration systems; therefore, it is important to measure the variation in the thermophysical properties of R718 due to the addition of nanoparticles. The major highlight of the current work is the measurement of the thermophysical properties at a temperature as low as 278 K. Most of the studies available in the open literature are based on the thermophysical property measurement of nanofluids at elevated temperatures (above 293 K). In the present work, the thermophysical properties such as thermal conductivity, viscosity and density of the nanofluids were measured at a temperature varying from 5 to 25 °C (278-298 K) and were later compared with the existing theoretical models and previous experimental results. The specific heat capacity of Al2O3 and CuO based nanofluids was predicted and analysed using a theoretical model. The thermal conductivity of the nanofluid was measured by the Transient hot-wire method and the viscosity of the nanofluid was measured using a Cannon-Fenske viscometer. Copper oxide (CuO) and aluminium oxide (Al2O3) nanoparticles were chosen for the present investigation. The average particle diameter of CuO and Al2O3 nanoparticles was 30 nm. It was observed that the enhancement in thermal conductivity of nanofluids at low temperature (~278 K) is considerably lower than the enhancements at elevated temperatures. The viscosity measurements also revealed that the classical models can be successfully applied for predicting the viscosity of Al2O3 –R718 nanofluids at low particle volume fractions (ϕ < 0.5%).