Seed bubbles trigger boiling heat transfer in silicon microchannels |
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Authors: | Guohua Liu Jinliang Xu Yongping Yang |
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Affiliation: | (1) Micro Energy System Laboratory, Guangzhou Institute of Energy Conversion, Chinese Academy of Science, 510640 Guangzhou, People’s Republic of China;(2) Beijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, 102206 Beijing, People’s Republic of China; |
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Abstract: | The smooth channel surface of microsystems delays boiling incipience in heated microchannels. In this paper, we use seed bubbles
to trigger boiling heat transfer and control thermal non-equilibrium of liquid and vapor phases in parallel microchannels.
The test section consisted of a top glass cover and a silicon substrate. Microheater array was integrated at the top glass
cover surface and driven by a pulse voltage signal to generate seed bubbles in time sequence. Each microheater corresponds
to a specific microchannel and is located in the microchannel upstream. Five triangular microchannels with a hydraulic diameter
of 100 μm and a length of 12.0 mm were etched in the silicon substrate. A thin platinum film was deposited at the back surface
of silicon chip with an effective heating area of 4,500 × 1,366 μm, acting as the main heater for the heat transfer system.
Acetone liquid was used. With the data range reported here, boiling incipience was not initiated if wall superheats are smaller
than 15°C without seed bubbles assisted. Injection seed bubbles triggers boiling incipience and controls thermal non-equilibrium
between liquid and vapor phases successfully. Four modes of flow and heat transfer are identified. Modes 1, 2, and 4 are the
stable ones without apparent oscillations of pressure drops and heating surface temperatures, and mode 3 displays flow instabilities
with apparent amplitudes and long periods of these parameters. The four modes are divided based on the four types of flow
patterns observed in microchannels. Seed bubble frequency is a key factor to influence the heat transfer. The higher the seed
bubble frequency, the more decreased non-equilibrium between two phases and heating surface temperatures are. The seed bubble
frequency can reach a saturation value, at which heat transfer enhancement attains the maximum degree, inferring that a complete
thermal equilibrium of two phases is approached. The saturation frequency is about a couple of thousand Hertz in this study. |
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