Visualization of Two-Phase Flows in Nanofluid Oscillating Heat Pipes

Abstract

Two-phase flows in an oscillating heat pipe (OHP) charged with deionized (DI) water and a nanofluid (0.268% v/v) were experimentally investigated. The OHP was made of quartz glass tube (with an inner diameter of 3.53 mm and an outer diameter of 5.38 mm) and coated with a transparent heating film in its evaporating section. The internal two-phase flows at different heat loads were recorded by a charge-coupled device (CCD) camera. Only column flow was observed in the DI water OHP while in the nanofluid OHP the flow first was column, then slug and annular flows as the heat load was steadily increased. Heat transfer in the OHP was strongly related to the two-phase regime. The flow regime transitions effectively increased the operating allowable heat loads in the nanofluid OHP two- to threefold relative to the DI water OHP. The nanofluid OHP had a much lower thermal resistance than the DI water OHP with the most effective heat transfer in the nanofluid OHP occurring in the slug flow regime.