Ramesh and Prabhu Nanoscale Research Letters 2011, 6:334 http://www.nanoscalereslett.com/content/6/1/334 particle concentration, heater diameter and surface roughness. The results indicate that the nanoparticles have pronounced and significant influence on the boil- ing process deteriorating the boiling characteristics of the fluid. The deterioration in boiling performance was observed to be more drastic at a higher surface rough- ness. It has been observed that the shift of the curve to the right is not proportional to the particle concentra- tion and it is strongly dependent on the tube diameter even for the similar values of surface roughness [78,113]. Zhou observed a reduction in pool boiling heat transfer of nanofluids [114]. Similarly Bang and Chang also observed that the addition of alumina nanoparticles caused a decrease of the pool nucleate boiling heat transfer. The heat transfer coefficient was decreased by increasing the particle concentration. On the other hand, CHF performance has been enhanced to 32 and 13%, respectively, for both horizontal flat surface and vertical flat surface in the pool [115]. You et al. observed the addition of nanoparticles to the water have no sig- nificant effect on nucleate pool boiling heat transfer. However, the measured pool boiling curves of nano- fluids saturated at 60°C have demonstrated that the CHF increases dramatically (approx. 200% increase) compared to pure water [116]. Similarly pool boiling experiment on water-silica nanofluids by Vassallo et al. observed that no improvement in pool boiling heat transfer but the CHF increased by about three times. They observed the formation of a silica coating over the heater surface [117]. Wen and Ding observed a signifi- cant enhancement in the pool boiling heat transfer of alumina nanofluids. The enhancement increases with increasing particle concentration and reaches approxi- mately 40% at a particle loading of 1.25% by weight [118]. Kim et al. showed 200% enhancement of CHF of nanofluids on a bare heater compared to that of pure water by increasing nanoparticle concentration. SEM images of the heater surface taken after pool boiling CHF tests revealed that CHF enhancement of nanofluids was closely related to the surface microstructure and enhanced topography resulting from the deposition of nanoparticles [119]. Kim et al. reported that the forma- tion of the porous nanoparticle layer during the nucleate boiling is a plausible mechanism for enhancement of CHF [95]. The nucleate boiling heat transfer experi- ments of water-CuO nanoparticles by Liu et al. showed that the both boiling heat transfer coefficient and CHF of the nanofluids increase with the increase of the mass concentration. However, when the concentration (opti- mum mass concentration) is over 1 wt%, the CHF is basically close to a constant value, and the heat transfer deteriorates gradually. They also found that the boiling heat transfer of the nanofluid on the smooth surface is almost the same with that of water on the smooth Page 11 of 15 surface at atmospheric pressure whereas boiling heat transfer of the nanofluids on the grooved surface increases remarkably [120]. Kathiravan et al. observed the enhancement of heat transfer coefficient during the pool boiling of water-CNT nanofluids of 0.25, 0.5 and 1.0% concentration by volume of CNT by 1.76, 1.203 and 1.20 times greater than that of heat transfer coeffi- cient of water, respectively, at the critical heat flux. They also observed that there is no fouling over the test-section [121]. Another study by Park et al. shows that the pool boiling heat transfer coefficients of the aqueous solutions with CNTs are lower than those of pure water in the entire nucleate boiling regime but the CHF increased up to 200% as compared to that of pure water. They observed the deposition of a thin film of CNTs on the surface and decrease in the contact angle [122]. So, it is clear that the CHF during pool boiling of nanofluids increased even when the pool boiling heat transfer of nanofluid may decrease or remain unchanged. During quench hardening process, the surface heat transfer conditions between the steel part and the quenchant are the most important factors controlling the microstructural evolution, generation of stresses and distortion [1]. Kobasko showed that very fast and uni- form part cooling within the martensitic range actually reduces the probability of part cracking and distortion, while improving the surface hardness and durability of steel parts [123]. The enhanced CHF of the nanofluids during pool boiling revealed that nanofluids may be sui- table for cooling at high heat flux applications [124]. According to Kim et al. the use of nanofluids can afford a significant acceleration of quenching by means of pre- mature destabilization of film boiling due to nanoparti- cle deposition [125]. The quenching of 304 stainless steel probe into different concentration of nanofluids yielded varying peak heat transfer coefficient (HTC) and Grossmann severity of quenching [126]. Jagannath and Prabhu measured the interfacial peak HTC of water was 1280 W/m2 K and the peak HTC decreased from 1400 to 965 W/m2 K with increases in Al2O3 nanoparticle concentration from 0.01 to 4 wt% when copper is quenched [103]. Similarly Babu and Kumar observed that the peak heat flux during quenching in CNT nano- fluids increases with an increase in the CNT concentra- tion until 0.50 wt.% and starts decreasing with further increase in the CNT concentration [105]. These results suggest that for the same base fluid there is an optimum level of nanoparticle concentration to enhance/decrease the heat transfer characteristics of nanofluids. The enhancement and deterioration of pool boiling heat transfer of nanofluids could be utilized in quenching heat treatment in two ways either to promote or decrease the rate of heat transfer depending upon the