The effective latent heat of aqueous nanofluids

Nanoparticle suspensions, popularly termed 'nanofluids', have been extensively investigated for their thermal and radiative properties (Eastman et al 1996 Mater. Res. Soc. Proc. 457; Keblinski et al 2005 Mater. Today 8 36–44; Barber et al 2011 Nanoscale Res. Lett. 6 1–13; Thomas and Sobhan 2011 Nanoscale Res. Lett. 6 1–21; Taylor et al 2011 Nanoscale Res. Lett. 6 1–11; Fang et al 2013 Nano Lett. 13 1736–42; Otanicar et al 2010 J. Renew. Sustainable Energy 2 03310201–13; Prasher et al 2006 ASME J. Heat Transfer 128 588–95; Shin and Banerjee 2011 ASME J. Heat Transfer 133 1–4; Taylor and Phelan 2009 Int. J. Heat Mass Transfer 52 5339–48; Ameen et al 2010 Int. J. Thermophys. 311131–44; Lee et al 2014 Appl. Phys. Lett. 104 1–4). Such work has generated great controversy, although it is (arguably) generally accepted today that the presence of nanoparticles rarely leads to useful enhancements in either thermal conductivity or convective heat transfer. On the other hand, there are still examples of unanticipated enhancements to some properties, such as the specific heat of molten salt-based nanofluids reported by Shin and Banerjee (2011 ASME J. Heat Transfer 133 1–4) and the critical heat flux mentioned by Taylor and Phelan (2009 Int. J. Heat Mass Transfer 52 5339–48). Another largely overlooked example is the reported effect of nanoparticles on the effective latent heat of vaporization (hfg) of aqueous nanofluids, as reported by Ameen et al (2010 Int. J. Thermophys. 31 1131–44). Through molecular dynamics (MD) modeling supplemented with limited experimental data they found that hfg increases with increasing nanoparticle concentration, for Pt nanoparticles (MD) and Al2O3 nanoparticles (experiments). Here, we extend those exploratory experiments in an effort to determine if hfg of aqueous nanofluids can be manipulated, i.e., increased or decreased by the addition of graphite or silver nanoparticles. Our results to date indicate that, yes, hfg can be substantially impacted, by up to ±30% depending on the type of nanoparticle. Moreover, in this paper, we report further experiments with changing surface area based on volume fraction (0.005 to 2%) and various nanoparticle sizes to investigate the mechanisms for hfg modification in aqueous graphite and silver nanofluids.