Investigation of Nano-Fin Effect (NFE) for Pool Boiling on Nanostructured Surfaces Using Surface Micromachined Temperature Nanosensors

In the past decade several experiments were reported on the measurement of anomalous enhancement in heat flux during pool boiling on horizontal heaters with nanostructured surfaces. Results from several studies demonstrated that heater surfaces involving nanostructures composed of materials with lower thermal conductivities resulted in higher values of heat flux. The Banerjee research group pioneered the thermo-physical model called “nano-Fin Effect (nFE)” for resolving the conundrums associated with the anomalous enhancement in nano-scale heat transfer during pool boiling on nanostructured heaters. nFE models can also be used for resolving and predicting the anomalous enhancement in the thermo-physical properties of nanofluids (as well as resolving the controversies associated with both enhancement and degradation in convection heat transfer involving nanofluids under similar conditions). Due to surface adsorption of the fluid molecules on a solid (e.g. on nanoparticles and nanofins) a semi-solid phase of fluid molecules with higher density (also called as “compressed phase”) is expected to form on the surface of the nanostructures. Due to nFE, three different mechanisms arise from the complex interactions during energy transfer. The first is the interfacial thermal resistance, as previously discussed, the second is thermal capacitance from the fluid molecules forming a compressed layer, and the third is the bias in heat transfer values (or diode effect) based on direction of the temperature drop. This bias is driven from the concentration gradient of the fluid molecules to the bulk fluid from the solid nanofin surface. Pool boiling studies were performed for different heater configurations: Copper, Silicon, and Anodic Aluminum Oxide (AAO) substrates. The pool boiling experiments were performed for subcooled and saturated conditions. The experimental measurements involved automated recording of temperatures (from temperature nano-sensors or Thin Film Thermocouples/TFT and wire-bead thermocouples) using a digital data acquisition (DAQ) system. High speed digital image acquisition apparatus was used for flow visualization of the bubble nucleation and departure process on these heater surfaces. The experiments were performed using PF-5060 as the test fluid at atmospheric pressure. High speed videos of bubbles departing from the heater surface were acquired using the flow visualization apparatus at 500–1000 frames per second (fps). The acquired images were used to analyze the bubble departure diameter and bubble departure frequency values. Boiling curves were obtained for each of the surfaces to enable comparison for the performance of these test surfaces for pool boiling. While critical heat flux (CHF) was around the same range for both silicon wafer and AAO substrates, Copper heater demonstrated significantly higher values of CHF. However, AAO surfaces demonstrated significantly higher values of heat flux at boiling incipience. Results obtained from the flow visualization experiments show that the bubble departure diameters were reduced for AAO substrates compared to that of Silicon wafer and Copper heaters. The bubble departure frequency was enhanced for AAO substrates compared to that of the plain heaters. This implies that the bubble nucleation site density was enhanced for AAO compared to that of the plain surfaces. In addition, the departure diameter values were smaller at higher values of subcooling (for all of the surfaces).