Experimental study of the time period of continued heating rate on the pool boiling characteristics of saturated water. (July 2019) - Ayoobi, Ahmadreza
Highlights: Transient heat input effects on pool boiling heat transfer and critical heat flux. The heat flux is a second-order function of time. The increase in heat flux starts from the free convection to the film boiling regime. The results include: heat transfer coefficient, superheat temperature and heat flux. The heat transfer coefficient decreased in the second part of the film boiling. Abstract: Pool boiling heat transfer and critical heat flux (CHF) in saturated water were experimentally studied under transient power conditions. A chrome-aluminum-iron alloy wire supported horizontally in a pool of water was used as the heating element. The heating rate in the test section was increased linearly depending on time by applying voltage control for 1 s to 5000 s. The heat flux was obtained by a second-order function of time. In this study, 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 with increasing the time period, the TBHTC in the nucleate boiling increased, decreased in transition from the nucleate boiling to the film boiling, and again increased in the film boiling and the incipient boiling point decreased which may be due to a decreased number of nucleation sites and the explosive nature of the initial stage of nucleate boiling. The TBHTC decreased in the second part of the film boiling as both the heat flux and the vapor film thickness around the wire had increased. In addition, increasing the time period decreased the CHF and superheating temperatures in the CHF by up to a period of 100 s. After that, its trend approached the steady state boiling. The transient CHF reduction in the time period of 100 s is 40.5% relative to steady state condition which is the minimum transient CHF. The maximum increase in the transient CHF is in the time period of 1 s and equals to 46.7%. A high-speed camera was used to show bubble and vapor film behavior around the heating wire.