Kim Nanoscale Research Letters 2011, 6:415 http://www.nanoscalereslett.com/content/6/1/415 various boiling parameters in terms of the maximum CHF value. Influence of nanoparticle material and size Material and size are important properties influencing the characteristics of nanoparticles. The choice of nano- particles to be suspended in a base fluid is expected to have an essential influence on the maximum possible increase in CHF. Figure 2 shows the increase in CHF for different nanofluids from selected studies in Table 1, all having water without additive as the base fluid, and all tested with flat-plate heaters. Even for the same nanoparticle material, considerable data scatter was observed, presumably due to variations in the dispersion status of the particles and the geometry of the heaters used in the tests. Moreno et al. [22] examined the size dependence of alumina-water nanofluid CHF using gravimetrically separated nanofluids with average particle diameters of 69, 139, 224, and 346 nm. They found that the magni- tude of CHF enhancement was nearly identical for each nanofluid sample under saturated pool-boiling condi- tions at a concentration of 0.025 g/l (see Figure 3). Recently, Jo et al. [23] investigated the size effect using silver nanoparticles with mean particle diameter ranging Page 4 of 18 from 3 to 250 nm. In contrast to Moreno et al. [22]’s results, the greatest increase (approximately 31%) in CHF occurred for the nanofluid with 3-nm particles, and the enhancement decreased with increasing particle size. In summary, it is not possible to draw any conclu- sions on the effects of nanoparticle material and size from an analysis of the existing data. More systematic studies must be carried out to clarify the effects of nanoparticle material and size on CHF enhancement in nanofluids. Influence of heater geometry Nucleate boiling experiments for studying the CHF of nanofluids are normally conducted with thin wires or flat plates. Many previous studies used thin wires as a boiling surface to confirm an intriguing feature of nano- fluids during nucleate boiling: significant CHF increase compared with a reference value for pure water. Thin wires were used to simplify the measurement of average heat flux and surface temperature and the post-inspec- tion of the heater surface. However, the measured CHF values might be different from those obtained with the flat plates used in general applications. Figure 4 sum- marizes the experimental results for both flat plates and thin wires, all under atmospheric conditions and with Figure 2 The CHF increase in nanofluids with different nanoparticles on flat plates.