Flow pattern transition instability during flow boiling in a single microchannel
Introduction
Flow instabilities are undesirable in boiling two-phase microchannel systems. Flow oscillations may affect the heat transfer and fluid flow characteristics, possibly resulting in wall temperature and heat flux oscillations, or they may induce dryout. Heat transfer and fluid flow in microscale channels are used in many applications, including microchannel heat sinks and microfluidic devices. In addition, two-phase microchannel heat sinks are one of the strongest candidates for heat removal devices in high heat flux environments by virtue of their large surface area to volume ratio, compact dimensions, and low flow rate requirements. However, two-phase flow instabilities often appear in boiling process systems in which two-phase flow is involved. Yadigaroglu [1] noted that two-phase flow instabilities generated by unstable flow patterns, particularly slug flow, could amplify flow disturbances and lead to dryout. Therefore, pressure and flow rate oscillations in microchannels should be identified and prevented to ensure safe operations. Bergles [2] and Boure et al. [3] noted that sustained flow oscillations may induce forced mechanical vibrations of components or system control problems. Flow oscillations can affect the local heat transfer performance, possibly resulting in oscillatory wall temperatures or inducing a critical-heat flux. Kandlikar [4] also remarked that flow instabilities in mini/microchannel systems are a concern in the design of microchannel evaporators.
Recently, a few published studies have reported pressure-drop oscillations and hydrodynamic instabilities of flow boiling in microchannels. Zhang et al. [5] performed boiling experiments using deionized water in 113 μm silicon microchannels. They observed bubble inceptions and measured the temperature variations of the channel wall using the resistance change of seven boron doped silicon thermometers. They found that the 4–5 Hz resistance fluctuation frequencies measured by the thermometers agreed well with the bubble formation frequencies.
Kandlikar et al. [6] observed the flow patterns of boiling in six 60-mm-long parallel channels with 1-mm hydraulic diameter and reported pressure fluctuations with 1 Hz frequencies that were attributed to the boiling phenomena, specifically the evolution of the vapor. Flow reversals were observed due to growth of slug bubbles in the microchannels. The vapor interface moved in a direction counter to the bulk fluid flow, and a pressure-drop fluctuations with periods of a few seconds were also observed. Recently, Kandlikar [7] noted a similarity between flow boiling in microchannels and nucleate boiling. Thus, flow boiling in microchannels can be treated as a similar process to the bubble ebullition cycle in pool boiling.
Qu and Mudawar [8], [9] conducted experiments on flow boiling of water in 21 parallel 231-μm-wide and 713-μm-high microchannels at mass fluxes ranging from 135 to 400 kg/m2 s with heat fluxes up to 1300 kW/m2. They encountered two types of two-phase hydrodynamic instabilities in parallel microchannels: pressure-drop oscillations and parallel channel instabilities. The pressure-drop oscillations, which had 20-s periods, were eliminated by throttling the upstream flow of the test section, or in other words, by adding system stiffness.
Hetsroni et al. [10] performed experiments on flow boiling of Vertrel XF in 21 parallel silicon triangular microchannels with a base of 250 μm. They observed 1-2-s oscillations of the pressure drop and outlet fluid temperature. Recently, Hetsroni et al. [11] reported an alternating occurrence of single-phase liquid flow and two-phase flow led to pressure-drop fluctuations with a short period. They explained the fluctuation phenomena based on bubble inception and growth behavior.
Wu and Cheng [12] performed experiments on flow boiling of water in 8–15 parallel silicon microchannels with trapezoidal cross-sections having hydraulic diameters of 82.8 and 158.8 μm. They showed that the wall temperature, fluid temperature, fluid pressure, and fluid mass flux varied with large amplitudes and long periods of 31 and 141 s in the small- and large-diameter systems, respectively. The temperatures and pressures fluctuations were in phase, but the pressure and mass flux fluctuations were out of phase. They reasoned that the out of phase fluctuations of pressure and mass flux triggered boiling onset oscillations [13] that sustained the fluctuations. Wu and Cheng [14] performed experiments on flow boiling of water in eight parallel silicon microchannels with trapezoidal cross-sections having a hydraulic diameter of 186 μm. They observed three types of unstable boiling modes. The pressure and mass flux were out of phase for single-phase/two-phase alternating flow and boiling onset oscillations sustained the fluctuations.
Brutin and Tadrist [15] carried out experiments in a single vertical channel with a hydraulic diameter of 889 μm using n-pentane as a working fluid. They derived a stability map drawn using the outlet vapor quality and inlet Reynolds number, and reported that a slight increase of the exit vapor quality at Reynolds numbers lower than 1000 could be used to identify the unstable regime. Recently, Tadrist [16] noted that static two-phase flow instabilities may appear in narrow channels with high confinement numbers (Co > 1).
In this study, we developed novel techniques and a new experimental apparatus to investigate boiling instabilities and related heat transfer mechanisms in a single microchannel. The new apparatus made it possible to observe flow patterns while simultaneously measuring the momentum and thermal transport parameters. The objective of this study was to conduct thorough experimental investigations of flow boiling instabilities, study the fluctuation behavior of flow boiling parameters such as pressure drop and mass flux, and determine the reason for boiling instabilities in a single microchannel.
Section snippets
Experimental apparatus
The experimental apparatus consisted of three major subsystems: a working fluid loop, a flow visualization system, and a data acquisition system. Fig. 1 shows a schematic diagram of the flow loop, which was configured to supply deionized water to the test section. The inlet reservoir served as a deaeration chamber due to the constant-temperature-controlled hot plate that removed any dissolved gas. Deionized water was delivered to the test section by dual operation syringe pumps: one syringe
Results and discussion
The tests were conducted for mass fluxes of 170 and 360 kg/m2 s and heat fluxes of 200–530 kW/m2, as summarized in Table 1. The inlet subcooled liquid was heated to saturation as it passed through the test section for low heat flux conditions. No oscillation or fluctuations were observed for single-phase flow. As the heat flux was further increased, inception and growth of bubbles occurred, and then temporal variation of the fluid pressure and wall temperatures was observed. We will focus on the
Two-phase instabilities in a single microchannel
Many experimental and theoretical investigations of fluctuation or oscillation instabilities in conventional macro two-phase systems have been carried out. Flow instabilities can be classified as static instabilities, such as flow excursion, flow pattern transition instability, and bumping, or as dynamic instabilities, such as acoustic oscillations, density-wave oscillations, thermal oscillations, parallel channel instability, and pressure-drop oscillations [1], [2], [3].
Kandlikar et al. [6],
Conclusions
In this study, experiments were performed to simultaneously measure and visualize flow boiling of water in a single microchannel with a hydraulic diameter of 103.5 μm. Tests were performed at mass fluxes of 170 and 360 kg/m2 s and heat fluxes of 200–530 kW/m2. The key findings of this study are as follows.
- (1)
Flow boiling fluctuations with a very long period and large amplitude were observed. Both the pressure drop and mass flux had these fluctuations, but a phase shift occurred between them that
Acknowledgments
This research was supported by the Program for the Training of Graduate Students in Regional Innovation, which was conducted by the Ministry of Commerce, Industry, and Energy of the Korean government.
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