Kim Nanoscale Research Letters 2011, 6:415 http://www.nanoscalereslett.com/content/6/1/415 no additive. A comparison of the CHF values for the two different heater geometries reveals that CHF enhancement is greater with thin wires (50 to approxi- mately 200%) than with flat plates (30 to approximately 80%). This difference in the measured CHF values is due to the different CHF mechanisms with thin wires and large flat plates. Nucleate boiling with flat plates proceeds to film boiling via the hydrodynamic CHF mechanism, whereas CHF with thin wires is caused by the local dryout mechanism governed by boiling incipi- ence phenomena, provided that hydrodynamic instabil- ities are absent [24]. From the point of view of understanding the general characteristics of CHF enhancement in nanofluids, the experimental results obtained with flat plates are more reliable than those obtained with thin wires. Thus, to infer the general effect of heater size from previous stu- dies, the maximum CHF enhancements of alumina- water nanofluids on flat-plate heaters exclusively are plotted against the dimensionless heater size L’, where L, r, s, and g are the characteristic heater size, fluid density, surface tension, and gravitational accelera- tion, respectively. The resulting plot is given in Figure 5. L = L σ g ρl − ρg . (1) Page 6 of 18 It is shown that expansion of the heating area in the range of L’ from 4 to 8 diminishes the CHF enhance- ment of nanofluids. Even though all the data are obtained on the flat plate, the values of L’ are still in the range where CHF of pure fluid is strongly dependent upon the size of heating surfaces [25]. Hamamura and Kato [26] explained that an inflow of liquid from the surrounding, instead of the top, increases CHF on a finite flat-plate-type heater and this effect is stronger on a smaller heater. In this range of L’, the impact of nano- fluids on CHF is likely dependent upon different flow characteristics around the heating surfaces. Experiments are needed to confirm this so that the CHF enhance- ment of nanofluids in many high-flux systems with dif- ferent characteristic dimensions could be predicted accurately. Influence of pressure Pressure affects nucleate boiling heat transfer and CHF by influencing physical properties such as the vapor density, latent heat of vaporization, and surface tension of the working fluids. Liu et al. [20,27] investigated the effect of system pressure on the CHF enhancement of nanofluids, including those with alumina nanoparticles and carbon nanotubes. They found that CHF enhance- ment in nanofluids is a strong function of system pres- sure and the enhancement effect is more significant at Figure 5 Relation between characteristic size of flat-plate heater and maximum CHF enhancement in Al2O3-water nanofluids.