Experimental investigation of transient pool boiling characteristics of Fe3O4 ferrofluid in comparison with deionized water

Pool boiling heat transfer and critical heat flux (CHF) in ferrofluid were experimentally studied under transient power conditions. The heat flux increment was begun from the free convection regime to the film boiling regime. Transient boiling heat transfer coefficient (TBHTC), transient CHF, scanning electronic microscopy (SEM) images of surface wire, and high-speed camera images were obtained. The following results were obtained for ferrofluid: as the time period increased, (1) the heat transfer coefficient of the nucleate boiling increased because of bubble production and its movement, then decreased in the transition from the nucleate to the film boiling because of an unstable layer of vapor around the wire heater and finally increased again in the film boiling because of the high temperature of the wire surface and radiation heat transfer; (2) the CHF and its wire superheat temperatures at CHF point decreased by up to a period of 100 s; (3) the incipient boiling point decreased. In the second part of the film boiling, the heat transfer coefficient was decreased because of the high heat flux and the more vapor film thickness around the wire. A comparison between two working fluids show that by increasing the time period, the nucleate boiling range of ferrofluid boiling was increased more than the deionized (DI) water boiling. Furthermore, the wire superheat temperature of the ferrofluid in all its regimes was decreased and the CHF of ferrofluid boiling were increased more than the DI water boiling. The ferrofluid CHF at the time period of 1 s decreased by 11.3% relative to DI water, and at the time period 5000 s increased by 160%.