Numerical hydrothermal analysis of water-Al2O3 nanofluid forced convection in a narrow annulus filled by porous medium considering variable properties

A study on the flow and heat transfer characteristics of nanofluid forced convection in an annular porous medium, as the main part of a cylindrical heat pipe, is numerically carried out to investigate the effect of nanoparticles on hydrothermal performance of a cylindrical heat pipe. The Al2O3-water mixture is considered as working fluid and a single-phase approach with variable properties adopted to formulate it. The influence of heat load and particle concentration level on thermophysical properties, velocity, pressure, and temperature fields, thermal performance of heat pipe is evaluated. It is found that the use of nanoparticles presents a better wall temperature uniformity. The values of velocity in wick structure decreases as more nanoparticles are added in base fluid. The profiles of pressure drop for low and middle particle concentration levels are nearly similar to each other, while the pressure drop increases significantly as concentration is raised up to 6 %. Both evaporation heat transfer coefficient and overall heat transfer coefficient improve as base fluid is replaced by nanofluid and the influence of the use of nanofluid on heat transfer enhancement becomes more remarkable for higher heat load. Results also revealed that the hydrothermal characteristics of heat pipe are more affected by nanoparticles as porosity and thermal conductivity ratio of porous medium increases and decreases, respectively. In addition, the thermal-hydraulic performance of heat pipe is investigated and the best value is found for the middle particle concentration and highest imposed heat load.