Nano-structured surfaces for enhancing critical heat flux (CHF), flow boiling and condensation

At the Multi-Phase Flows and Heat Transfer Lab. (MPFHTL) at Texas A and M University (TAMU): we are developing nano technology enabled platforms for enhancing cooling, sensing and energy storage (involving both experimental and computational studies). Coupling of thermal and hydrodynamic features during phase change (boiling, condensation) causes spatio-temporal fluctuations of surface temperature at the micro/nano-scales, which are termed as 'cold-spots' and can transmit over 60-90% of the total heat transfer. Using silicon nanofins-cooling was enhanced by -120%. Using Carbon-Nanotube (CNT)nanocoatings-cooling was enhanced by 60-300% by leveraging cold-spots and the. 'nano-fin' effect ('nFE', which arises due to a combination of enhanced surface area, surface adsorption of solvent molecules, Kapitza resistance/ interfacial thermal resistance and thermal inertia/ capacitance effects). Nano-thermocouples (Thin Film Thermocouples/ 'TFT') and diode temperature nano-sensors (DTS) integrated with the nanocoatings enabled the study of thermal chaos and micro/nano-scale fractal structures in boiling (due to thermal-hydraulic coupling). The reliability of Phase Change Materials (PCM) was demonstrated for 1000 cycles of repeated melting and solidification using additives (nucleation promoters) and in 3D Printed heat exchangers (i.e., using Additive Manufacturing). We developed compact condensers with nanocoatings (enhanced heat flux by 100%). Also, Specific heat capacity was enhanced by 10-120% in solvents and solid matrices doped with minute concentration of nanoparticles (i.e., nanofluids and nanocomposites). Experiments for the flow of nanofluids in microchannels is showed that the nanoparticle precipitates behaved either as nanofins (enhanced surface area at low concentrations) or caused fouling at high concentrations. In addition, evaporation and condensation experiments performed in nanochannels (lab-on-chip apparatus) demonstrated a significant shift in phase transition temperatures (due to 'confined fluid' effect, e.g., in shale rocks for enhanced oil recovery/ EOR).These interfacial interactions were demonstrated to dominate the net heat transfer (termed as the 'lanolin effect/ nFE') for micro/nano-scale transport phenomena. (author)