Pramod Warrier

Effect of particle size on the thermal conductivity of nanofluids containing metallic nanoparticles

A one-parameter model is presented for the thermal conductivity of nanofluids containing dispersed metallic nanoparticles. The model takes into account the decrease in thermal conductivity of metal nanoparticles with decreasing size. Although literature data could be correlated well using the model, the effect of the size of the particles on the effective thermal conductivity of the nanofluid could not be elucidated from these data. Therefore, new thermal conductivity measurements are reported for six nanofluids containing silver nanoparticles of different sizes and volume fractions. The results provide strong evidence that the decrease in the thermal conductivity of the solid with particle size must be considered when developing models for the thermal conductivity of nanofluids.

The limiting behavior of the thermal conductivity of nanoparticles and nanofluids

We present experimental evidence of negative thermal conductivity enhancement in nanofluids consisting of 2 nm titania nanoparticles dispersed in 50% (w/w) water+ethylene glycol. This behavior is unlike that of other nanofluids, which have been shown to exhibit positive thermal conductivity enhancements. Our results for titania nanofluids suggest that the thermal conductivity of 2 nm titania nanoparticles is smaller than the thermal conductivity of the base fluid at the same temperature, indicating a dramatic decrease in the thermal conductivity of titania particles as the particle size becomes of the same order as the phonon mean free path. Although such a decrease has been predicted for semiconductor nanoparticles by theory and simulation, experimental evidence has hitherto been lacking. Our results provide indirect experimental evidence for this decrease in metal oxide particles, and validate our previous work on alumina nanofluids that showed an exponential decrease in the thermal conductivity of alum...

The thermal conductivity of aqueous nanofluids containing ceria nanoparticles

We present data for the thermal conductivity of nanofluids consisting of two sizes of ceria nanoparticles dispersed in water at 298 K. We also demonstrate that the effective thermal conductivity of these heterogeneous nanofluids can be described by a simple “mixing” rule that incorporates the size dependence of the thermal conductivity of the nanoparticles themselves. Our data follow the same trends with particle size as shown for alumina nanofluids in our previous work, and provide additional validation for that data.

Pool Boiling of HFE 7200–C4H4F6O Mixture on Hybrid Micro-Nanostructured Surface

Steadily increasing heat dissipation in electronic devices has generated renewed interest in direct immersion cooling. The ideal heat transfer fluid for direct immersion cooling applications should be chemically and thermally stable, and compatible with the electronic components. These constraints have led to the use of Novec fluids and fluroinerts as coolants. Although these fluids are chemically stable and have low dielectric constants, they are plagued by poor thermal properties like low thermal conductivity (about twice that of air) and low specific heat (same as that of air). These factors necessitate the development of new heat transfer fluids with improved heat transfer properties and applicability. C 4 H 4 F 6 O is a new heat transfer fluid which has been identified using computer-aided molecular design (CAMD) and knowledge-based approaches. A mixture of Novec fluid (HFE 7200) with C 4 H 4 F 6 O is evaluated in this study. Pool boiling experiments are performed at saturated condition on a 10 mm x 10 mm silicon test chip with CuO nanostructures on a microgrooved surface, to investigate the thermal performance of this new fluid mixture. The mixture increased the critical heat flux moderately by 8.4% over pure HFE 7200. Additional investigation is necessary before C 4 H 4 F 6 O can be considered for immersion cooling applications.

Screening and Evaluation of Mixture Formulations for Electronics Thermal Management Using Pool Boiling

Computer-aided molecular design and figure of merit analysis were used to screen mixture formulations that enhance the pool boiling heat transfer performance of Novec fluid HFE 7200. Mixtures of HFE 7200 with methanol and ethoxybutane were identified as promising candidates for further study, and their thermophysical and dielectric properties were measured. The pool boiling performance of the two mixtures was investigated on a 1 cm × 1 cm silicon substrate with copper nanowire arrays. The addition of both methanol and ethoxybutane to HFE 7200 resulted in a substantial increase in the critical heat flux. However, the addition of methanol had a detrimental effect on incipience superheat and heat transfer coefficient, whereas these properties only changed marginally upon the addition of ethoxybutane to HFE 7200. This suggests that HFE 7200 + ethoxybutane mixtures show promise as candidates for direct immersion cooling of electronics.