Experimental Study of Nanofluids for Droplet Cooling Applications Using Temperature Microsensors

The use of nano-fluids in droplet cooling (boiling) was explored parametrically in this experimental study. The experimental parameters are: nanofluid composition, superheat, liquid subcooling, and spray parameters (nozzle diameter, injection distance, size of droplets, injection pressure, mass flow rate, etc.). Two test fluids were used in the experiments: de-ionized (DI) water and nanofluid. The nano-fluid consists of silica nano-particles with a nominal diameter of 10nm dissolved in water at 2% concentration by weight. An experimental apparatus was fabricated to measure the surface temperature fluctuations during spray cooling of a heated surface. An array of microthermocouples (Thin Film Thermocouples or "TFT") was micro-fabricated on a heated surface to measure the surface temperature fluctuations during spray cooling. The TFT are capable of measuring temperature fluctuations up to a speed of 100 MHz. In the experiments, the exit of the single droplet spray was set a distance of 10 mm away from the surface and was aligned with the location of the TFT array. The spray was produced by pumping test fluid using a syringe pump into a traversing spray head. Silicon wafers with surface micromachined TFT array were clamped on the top of the heater apparatus for measuring temperature changes on the surface of the heater. Wire bead K-Type thermocouples were embedded in the heater apparatus and were used to measure heat flux. The transient temperature data from the TFT were recorded by a data acquisition system connected to a computer. The nano-fluid spray was found to cause fouling of the heater surface due to precipitation of the constituent nano-particles during boiling. This caused the overall heat flux to decrease drastically when compared to spray cooling using water. The nano-fluid spray was found to enhance heat flux by 300% compared to the base heat flux without the spray.