No abstract

An experimental investigation was performed with the view to assess the heat transfer characteristics of a water-based nanofluid in a micro heat exchanger employed to quench a high heat flux heater for industrial and microelectronic cooling applications. The experiments were conducted at heat fluxes 10-70 kW/m 2 and for nanofluids at various mass concentrations of 0.1-0.3% and passing flow rates of 0.1-5 l/min. Thermo-physical properties of the nanofluid including thermal conductivity, heat capacity, density and viscosity of nanofluid were experimentally measured at 40 °C close to the temperature of the experiments. Results showed that the heat transfer coefficient and pressure drop were augmented by 40.1% and 67% at wt% = 0.3 compared to the base fluid, respectively. The enhancement in the heat transfer coefficient was associated with the improvement in the thermal conductivity of the base fluid together with the intensification of Brownian motion and thermophoresis effect. The increase in the pressure drop was also attributed to the increase in the viscosity of the working fluid which induces layer-layer frictional forces in the bulk of the coolant in micro heat exchanger.

A range of experiments was conducted to measure the heat transfer characteristics of titanium oxide/deionized water nanofluid (NF) inside a steel-made Pyrex annular system. A set of experiments was designed and performed at inlet temperature (IT) of the NF (333 K-363 K), the applied heat flux (AHF) (4.98 kW/m 2 to 112 kW/m 2 ), 1988 < Re < 13,588 and dispersion concentration of wt.%=0.05 to wt.%=0.15) on the average heat transfer coefficient (HTC) and boiling section's average pressure drop (PD). It was demonstrated that the increase in the volume flow and the AHF can increase the HTC while increasing the weight concentration of the NF, initially increased the HTC such that the maximum enhancement in the HTC was 35.7% at wt.%=0.15 and Re=13500, however, over the time, the HTC of the NF decreased. The reduction in HTC was attributed to the formation of continual sedimentation on the boiling surface after 1000 minutes of the operation. The IT of the NF slightly increased the HTC, which was due to the enhancement in the thermal and physical properties such as thermal conductivity. The maximum enhancement in HTC due to increase of the IT from 333 K to 363 K was 4.2% at wt.%=0.15 and Re=13500. The bubble formation was also found to be a strong function of the applied HF such that with increasing the HF, the rate of the bubble formation increased, which was also the reason behind the augmentation in the HTC at larger AHFs. Also, the PD was augmented due to the increase in the velocity and flow and also weight concentration of NF. The highest value measured for PD was 9 kPa recorded at a weight fraction of 0.15 and Re=13500, which was 28% larger than that of measured for the base fluid. It was also found that a continual fouling layer of nanoparticles (NPs) was formed on the boiling surface, which induced a thermal resistance against the boiling heat transfer. The fouling formation reduced the HTC of the NF such that the maximum reduction in the HTC was 21.6% after 1000 minutes of the operation of the heater.