Increasing Boiling Heat Transfer Using Low Conductivity Materials

We report the counterintuitive mechanism of increasing boiling heat transfer by incorporating low-conductivity materials at the interface between the surface and fluid. By embedding an array of non-conductive lines into a high-conductivity substrate, in-plane variations in the local surface temperature are created. During boiling the surface temperature varies spatially across the substrate, alternating between high and low values, and promotes the organization of distinct liquid and vapor flows. By systematically tuning the peak-to-peak wavelength of this spatial temperature variation, a resonance-like effect is seen at a value equal to the capillary length of the fluid. Replacing 18% of the surface with a non-conductive epoxy results in a greater than 5x increase in heat transfer rate at a given superheat temperature. This drastic and counterintuitive increase is shown to be due to optimized bubble dynamics, where ordered pathways allow for efficient removal of vapor and the return of replenishing liquid. The use of engineered thermal gradients represents a potentially disruptive approach to create high-efficiency and high-heat-flux boiling surfaces which are naturally insensitive to fouling and degradation as compared to other approaches.Bi-conductive surfaces were fabricated by embedding lines of a low-conductivity epoxy into a copper substrate. Copper sheets (1 mm thickness) were cut to size and grooves were machined into them using Wire Electrical Discharge Machining (EDM). The EDM wire thickness was 0.254 mm with a reported minimum spark gap of 0.381 mm  0.127 mm. The copper was then treated with an alkaline solution to produce an oxide layer with nano-scale surface roughness to promote adhesion between the copper and epoxy. The surfaces were then coated with a non-conductive high-temperature two-part epoxy (Aremco 526N) filling all of the grooves. The epoxy was cured at 93 C for 2 hours, followed by 163 C for 12 hours to achieve a maximum strength bond. After curing, the surfaces were manually sanded with 200 grit sand paper until the bare copper between each epoxy division was exposed. The bare copper surfaces (with no epoxy divisions) were also sanded using the same method. The surfaces were finally cleaned with solvents and dried with N.Surfaces were characterized using a custom-built test set-up as previously reported by Rahman .. The setup consists of a copper heater block with PTFE insulation embedded with two cartridge heaters allowing for a maximum power of 1,000 W. Five T-type thermocouples were inserted into the copper block equally spaced 6 mm apart with the top most thermocouple located directly beneath the sample. The temperature measurements were recorded using NI DAQ system, where the average heat flux in the copper block was calculated using Fourier's conduction law. The sample surface temperature was calculated by considering all of the relevant thermal resistances between the surface and the top most thermocouple, as described. A polycarbonate chamber was used as the water bath; with an immersion heater and thermocouple probe maintaining saturated conditions and atmospheric pressure. Degassed, deionized water was used as the working fluid, and all tests were carried out up to CHF. Visualization of the boiling process was conducted at low heat fluxes using a Phantom V210 high-speed camera (Vision Research) recorded at 3,100 fps. The surfaces were initially maintained at saturated condition for 1 hour after which a small increment of heat flux (2 W/cm to 5 W/cm) was applied to the surfaces until the nucleation was observed. Heat flux was further increased up to 20 W/cm until the visibility of bubble dynamics becomes difficult. The chamber heater was turned off to minimize the bulk fluid motion while capturing the movies, with all movies being recorded within one minute to maintain saturation conditions, as described by Mukherjee and Dhir and Son ..