Study of pool boiling heat transfer with FC-72 on open microchannel surfaces

The paper presents investigations into pool boiling heat transfer for open microchannel surfaces. The experiments were carried out with saturated FC-72 at atmospheric pressure. Parallel microchannels fabricated by machining were about 0.2 to 0.4 mm wide and 0.2 to 0.5 mm deep. Analyzed surfaces with microchannels allowed to obtain heat transfer coefficients within the range of 6.1 – 9.8 kW/mK, which in relation to the flat surface gives a 3 – 5 fold increase in HTC. One of the reasons for the increase in the heat transfer coefficient when increasing the heat flux was the growing number of active nucleation sites at the bottom of microchannels and its side surfaces.


Introduction
The use of a refrigerant phase change on a properly designed enhanced surface gives the possibility of obtaining significant values of heat transfer coefficients. The selection of geometric parameters, i.e. the width, height, distance between microchannels, allows to increase the transferred heat flux while the superheat decreases.
The authors in [1] have determined the boiling curves for n-pentane at atmospheric pressure. Enhanced surfaces were formed from copper particles with a diameter of 200 μm and sintered. Forms took the shape of a monolayer i.e. a layer of sintered powder, columnar posts wicks and mushroom posts wick. It was found that the monolayer wicks without and with the mushroom post structure provide 20% and 87% CHF enhancements, respectively, compared to the plain surface. In the work of Udaya Kumar et al. [2] surfaces were made with an electrodeposition technique. Copper wires with diameters of 35 nm, 70 nm, 130 nm and 200 nm were deposited on copper specimens with a smooth surface. The working fluid was Fluorinert FC-72. The percentage increases observed in CHF for the samples with nanowires were 38.37%, 40.16%, 48.48% and 45.57% whereas the percentage increase in the heat transfer coefficient were 86.36%, 95.45%, 184.1% and 131.82% respectively as compared to the bare copper surface. Whereas in the publication [3] boiling curves for FC-72, depending on pressure (100 -300 kPa) and subcooling (0 -72 K) were obtained using a silicon heater coated with microporous layers. Microporous, diamondcoated chips were found to provide an average 1.6 enhancement factor on FC-72 CHF, relative to a bare chip, yielding values that ranged from 19.4 W/cm 2 at 1 atm and 1.6 K of subcooling to 47 W/cm 2 for 3 atm in 71 K of subcooling. In the paper [4] experiments were carried out on these surfaces at atmospheric pressure, using FC-72 as the working fluid. The results showed that in comparison to the smooth surface, pool boiling heat transfer was significantly enhanced by the micro-pin-fin surfaces and the maximum superheat was considerably decreased.

Experimental setup
Thermal and visualization measurements were made at the measurement stand, Fig. 1 [5][6][7][8], which allowed measuring the temperature difference between the working fluid and the heating surface and determining the heat flux. These parameters are necessary to determine the boiling curves. The temperature measurement was carried out using a K-type (NiCr-NiAl) thermocouple with a thickness of 0.5 mm. The measurement data acquisition system was a product of FLUKE Hydra 2635A. Calibration of the thermocouples was carried out using the Altek calibrator.    The specimens with test surfaces were made of copper and had parallel grooves with a constant pitch, made with an end mill of 0.2 -0.4 mm in diameter (CNC machining process) and 0.2 to 0.5 mm deep. The test section consisted of a 32 x 32 mm 2 square copper specimen with a 27 x 27 mm 2 boiling region. Table 1 compiles the surface codes, specifications according to Fig. 3a and summarizes calculation results of constant C and exponent n in dependence q = CT n for FC-72 boiling on eleven MC surfaces and smooth surface.
By Newton's law of cooling, HTC is equal: According to Fourier's law: The temperature gradient was calculated using three point's backward finite difference method as given below, Fig. 3.
Tw was extrapolated by following equation as: where δs is distance between microchannel bottom (base) and thermocouples T3 and T4.
The difference between temperatures of the heated surface and liquid T (superheat) is shown by the following equation: Based on the analysis of errors, estimates for the uncertainty limit on the calculated values were as follows:  at low heat flux (2 kW/m 2 ) relative error of heat flux ±35%, relative error of heat transfer coefficient ±40%,  at high heat flux (130 kW/m 2 ): relative error of heat flux ±27%, relative error ofheat transfer coefficient ±31%.

Results
The investigated surfaces with open microchannels provided higher heat transfer coefficients compared to smooth surfaces. For the tested samples with microchannels, Table 1, a significant intensification of heat flux was observed, with the heat transfer coefficient depending on the depth and width of the microchannels. The best results were obtained for sample MC-0.3-0.5-0.6, where the heat transfer coefficient was α = 9.8 W/m 2 K at superheat ΔT ~ 9.3 K and heat flux about 90 kW/m 2 , Fig. 4a, b.
Analyzed surfaces with microchannels allow to obtain heat transfer coefficients within the range of 6.1 -9.8 kW/m 2 , which in relation to the flat surface gives a 3 -5 -fold increase in HTC. The largest maximum heat flux yielded surfaces with microchannels with depths of 0. 3

Conclusion
The analysis of the graphs shows the following conclusions:

Subscripts
Cu copper, s sample, Sn tin w wall.