Flow boiling in vertical narrow microchannels of different surface wettability characteristics

Abstract

An experimental investigation of saturated flow boiling in a high-aspect-ratio, one-sided heating rectangular microchannel was conducted with deionized water as the working fluid. The bare silicon wafer bottom surface of the microchannel was hydrophilic with a contact angle of 65° ± 3°, compared with the super-hydrophilic surface deposited by a thin film of 100-nm-thickness silicon dioxide through PECVD with a contact angle less than 5°. In experimental runs the mass fluxes were in the range of 120 kg/m 2 s-360 kg/m 2 s, the wall heat fluxes were spanned from 4 W/cm 2 to 20 W/cm 2 and the inlet vapor qualities were varied from 0.03 to 0.1. Parametric study and flow visualization on pressure drop, local heat transfer coefficient, and flow pattern for surfaces of different surface wettability characteristics were carried out. Measured total pressure drops in single phase and two phase flow experiments agreed well with predicted values. The experimental data points were almost all located in the annular flow regime, and the local heat transfer coefficients approached a constant value and then increased towards the exit along the flow direction. According to flow visualization, the local dryout phenomenon occurred on the untreated hydrophilic surface at high heat fluxes for low mass fluxes, accompanied with deteriorative heat transfer performance, while it was not observed on the super-hydrophilic surface at the identical condition. Meanwhile severe heat transfer deterioration was obtained on the hydrophilic surface with increased inlet vapor quality, while the heat transfer coefficient of the super-hydrophilic surface was relatively constant which outperformed the untreated silicon wafer surface without increased pressure drop penalty.

Experimental Study and Mechanism Analysis of the Flow Boiling and Heat Transfer Characteristics in Microchannels with Different Surface Wettability

“…It is speculated that the super-hydrophilic surface has better re-wettability, there is enough liquid for replenishment, the liquid film distribution is uniform, and the heat transfer is enhanced, so at high heat flux, the heat exchange mechanism is speculated to be thin liquid film evaporation. In this experiment, it is further confirmed that in the flow boiling in microchannels at high heat flux, the super-hydrophilic surface could delay or even avoid the occurrence of local dryout, thereby increases the critical heat flux, which is consistent with the results of Zhou et al [ 12 ]. This advantage of super-hydrophilic surface is expected to play a vital role in the practical application of high heat flux density microchannel two-phase heat dissipation system.…”

“…How to solve the problem of frequent dryout and improve the heat exchange performance of the microchannels in flow boiling was particularly critical [ 8 , 9 , 10 , 11 ]. Zhou et al [ 12 ] examined the impact of surface wettability with contact angles of 65° and 0° on deionized water flow boiling in microchannels. Flow visualization indicated local dryout occurring on the untreated surface at high heat flux and low mass flux, attached to deterioration in heat transfer performance.…”

Research on the Manufacturing Process and Heat Transfer Performance of Ultra-Thin Heat Pipes: A Review

“…Thus, the state of boiling heat transfer was changed [ 64 ]. The hydrophilic surface can effectively improve the surface wettability and facilitate the flow of liquid to nucleation sites [ 65 ], while the heat transfer contact area and the number of nucleation sites increase [ 66 ]. In addition, the hydrophilic surface lead to the turbulence intensity and heat transfer coefficient of the working fluid in the pipe increasing [ 67 ], and it can successfully eliminate the inhibitory effect of bubble separation due to the network of connected micropores on the surface of the wick [ 68 ].…”

Local heat transfer in subcooled flow boiling in a vertical mini-gap channel

“…Uncertainty of measured parameters and calculated parameters are shown in Table 2. Detailed calculation is in reference [17].…”