Boiling heat transfer exhibits great ability to transport energy. Researches have shown the surface treatments are able to enhance the boiling heat transfer performance, including lowering the incipient superheat temperature, increasing the boiling heat transfer coefficient, and increasing the critical heat flux. Furtherly, the hybrid-scale structure is expected to achieve better improvement on boiling heat transfer performance. Therefore, the effects of the hybrid-scale structure of WO~3~ nanowires with sintered titanium coating on boiling performance were investigated in this study.

In this study, an apparatus was established, it consisted of a cartridge heater (FIREROD E1A53, Watlow, USA), copper heating block, spring, quartz glass tube, plate sample, and Teflon cases. The cartridge heater was inserted to the heating block. The plate sample would be attached on the top of the heating block, and the heat from the cartridge heater would transfer through the plate sample, then to the *deionized water*. The Teflon cases were for insulation of lateral heat transfer. To observe the boiling phenomena, a highly transparent quartz glass tube was used as the side wall of container of *deionized water*. The spring was compressed, created an axial force under the heating block to confirm the connection to the plate sample.

The plate sample was made of the pure copper substrate, and 15 mm in diameter for the upper part, and 25 mm in diameter for the lower one. Each of the height were 10 and 5 mm respectively. At the upper part, two holes of 1.3 mm in diameter was drilled to the centerline of the sample for positioning the T-type thermocouples. The distance of two holes were 4 mm.

The surface of substrate was polished by sandpaper with grit sizes of 1500. After polishing, the titanium foil would be heat-pressed on the top of the substrate, and then the sintered titanium coating and WO~3~ nanowires would be processed on sequentially.

During the experiments, the voltage of cartridge heater was gradually adjusted to achieve incipience of nucleate boiling for each sample, and then increased 5.0 ACV a step. The *deionized water was heated by the pre-heater, and maintained at 99.5 ^o^C at least by temperature controller. Each sample would be conducted about three trials of boiling experiments.*

*Finally, the heat flux and superheat temperature could be calculated by Fourier’s law with the temperature difference measured from the sample by T-type thermocouples. The boiling curves of different surface treatments could be plotted out.* *From comparing those boiling curves, the change in incipient temperature, aging effect, and the boiling heat transfer coefficient might be calculated and discussed. The wettability could be analyzed from change of contact angle, the hydrophilic surface is preferred.*

*Also, from the images of bubble motion were shot by the high speed camera (TM-6740 GE, PULNIX) and the SEM images. By comparing image from high speed camera frame by frame, the bubble release frequency, the diameter of bubble, departure diameter, and quantity of steam could be calculated.* From the SEM images, the relation between the size of nucleation cavity and the departure bubble might be built up by Hsu’s model. By those analyzes, the effects of the hybrid-scale structure of WO~3~ nanowires with sintered titanium coating on boiling performance could be quantified.

The initial result showed that the contact angle of pure copper and the sintered titanium coating surface was 77.0^o^ and 0.0^o^ respectively. The incipient superheat temperature and *the bubble release frequency* of pure copper surface were 6.7 ^o^C and 0.0265 Hz at 115800 W/m^2^, and the sintered titanium coating was expected to lower the incipient superheat temperature and increase the *bubble release frequency due to the improvement on the wettability.* The hybrid-scale structure of WO~3~ nanowires with sintered titanium coating was expected to improve furtherly on boiling.


The Effects of Hybrid-Scale Structure of WO3 Nanowire With Sintered Titanium Coating on Boiling Heat Transfer

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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