Contactless Boiling State Monitoring in Mini-Channels Based on Triboelectric Nanogenerators

The thermal performance and stability of mini-channel heat exchangers are mainly affected by the boiling state of the liquid, but the traditional contact monitoring methods have inevitable drawbacks such as flow interference and electrode contamination. In this paper, a contactless boiling state online monitoring sensor based on a liquid–solid/gas–solid interfacing triboelectric nanogenerator is proposed. The liquid–solid/gas–solid-based triboelectric nanogenerator (L-S/G-S TENG) is made of a poly(tetrafluoroethylene) (PTFE) tube covered by a copper electrode, wherein the electrode is located on the outer surface of the tube. As the liquid in the mini-channel boils, the contact form in the L-S/G-S TENG changes from solid–liquid contact to solid–gas contact, and a voltage signal is generated, identifying the boiling states. The voltage change of deionized water when boiling in a channel with an electrode width of 50 mm and an inner and outer diameter of 3 mm × 5 mm will reach 4.68 V. In addition, the effects of electrode width, channel size, and channel thickness, and liquid property on the performance of the L-S/G-S TENG are systematically investigated. It is found that the signal output of the system would increase with the width of the electrode and mini-channel size but decreases with the increase of channel wall thickness and liquid conductivity. This work not only provides an ingenious method for contactless online monitoring of the liquid boiling state in mini-channels but also makes a solid step forward for the microfluidic sensing technology based on triboelectric nanogenerators.