A numerical assessment of copper oxide and alumina nanoparticles during CHF occurrence Ataollah Rabiee * , Alireza Atf School of Mechanical Engineering, Shiraz University, Shiraz, Iran article info Article history: Received 10 February 2015 Received in revised form 2 June 2015 Accepted 12 June 2015 Available online xxx Keywords: Critical heat flux Nanoparticle Nucleate boiling CFD Dryout abstract Subcooled nucleate boiling in forced convection has been drawing significant attention in many fields due to its good heat transfer efficiency and high heat removal capacity. Such advancement in sub-cooled nucleate boiling is the result of continuing efforts from experimental, theoretical and numerical re- searchers, particularly focusing on critical heat flux (CHF). CHF heat transfer regimes are inefficient and the occurrence of CHF can cause a large temperature gradient in the heated wall leading to physical burnout. One way to increase the level of the CHF is to add certain nanoparticles to the base fluid. The present paper compares the effects of the addition of copper oxide and alumina nanoparticles on CHF phenomenon within the general-purpose computational fluid dynamics (CFD). The governing equations solved are generalized phase continuity, momentum and energy equations. Wall boiling phenomena are modeled using the baseline mechanistic nucleate boiling model developed in Rensselaer Polytechnic Institute (RPI). To simulate the critical heat flux phenomenon, the RPI model is extended to the dry-out phenomenon by partitioning wall heat flux to both liquid and vapor phases considering the existence of thin liquid wall film. It was shown that the presence of copper oxide in comparison with alumina nanoparticles in the base fluid, delays the dryout phenomenon more dramatically and in specific con- centration, CHF threshold would be enhanced and consequently the safety margins of the operation would be improved. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Subcooled flow boiling occurs in many industrial applications and is characterized by large heat transfer coefficients. Therefore, boiling plays an important role in industrial heat transfer processes such as macroscopic heat exchangers in nuclear and fossil power plants. However, for the incorporation of nucleate boiling in most practical applications, it is imperative that CHF is not exceeded. Critical heat flux signifies the highest limit of the nucleate boiling heat transfer in any system; micro or macro. Beyond critical heat flux, there is not only a deterioration of the capability of heat dissipation, but also a potential risk of damage due to burn-out. For decades, researchers have been trying to develop more efficient heat transfer fluids and also to increase the CHF threshold which would improve the thermal efficiency and reduce operational costs. To achieve this purpose, nanofluids have been employed in several experimental facilities. In this study, it is tried to compare the effects of the addition of copper oxide and alumina nanoparticles on boiling flow field accompanied by critical heat flux using computational fluid dy- namics. The activities performed by researchers in this area are mentioned in the following. Velldandla et al. (1995) conducted an experiment to simulate subcooled flow boiling in a vertical annular channel. In this study, wall temperature distribution, fluid temperature and the volume fraction of vapor at various locations along the channel were measured. Hoyer (1998) organized an experimental setup on boiling flow while the dryout phenomenon occurred due to exer- tion of a high heat flux compared with CHF. It was observed that a film of vapor takes place around the channel wall, causing a sudden rise in wall temperature. In addition to experimental researches, Li et al. (2010) investi- gated the two phase boiling flow field in a vertical channel using RPI model developed by Ransselaer Polytechnic Institute, using the available computational code named FLUENT. In the framework of analyzing, the water and vapor phases were considered separately based on EulerianeEulerian approach. Again, Li et al. (2011) simulated the boiling phenomenon as well as the dryout in a * Corresponding author. E-mail address: rabiee@shirazu.ac.ir (A. Rabiee). Contents lists available at ScienceDirect Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene http://dx.doi.org/10.1016/j.pnucene.2015.06.013 0149-1970/© 2015 Elsevier Ltd. All rights reserved. Progress in Nuclear Energy 85 (2015) 121e129