Document Type : Research Article
department of mechanical engineering, yazd university
Department of Mechanical Engineering, Yazd University
Department of Mechanical Engineering, Isfahan University of Technology
Pool boiling heat transfer and critical heat flux (CHF) were experimentally studied in subcooled temperatures ranging 0 to 20 under transient power conditions. The heating rate in the test section was increased linearly depending on time by applying voltage control for 1 to 1000s. The increase in heat flux starts from the free convection regime to the film-boiling regime. The transient boiling heat transfer coefficient (TBHTC), transient wire superheat temperature, transient heat flux and transient CHF were also obtained. The results showed that in the case of all subcooled temperatures and periods, the TBHTC increased in the nucleate boiling region because of the growth, separation, motion and turbulence of the bubbles. The TBHTC also decreased in the transition from nucleate boiling to film boiling because some part of the wire covered by temporary thin vapor film. The wire isolated from the body of working fluid and the bubble separations decreased. The TBHTC again increased in film boiling due to the increment of radiation heat transfer. The TBHTC decreased in the second part of the film boiling due to the heat flux and the vapor film thickness around the wire had increased. Relative to the saturation condition, the timely average of the wire superheat temperature for subcooled temperatures of 10 and 20, respectively, decreased by 9.23 and 9.29% in the nucleate boiling region and in a time period of 1000s. A high-speed camera was used to show bubble and vapor film behavior around the heating wire for the subcooled temperatures.