Microbubble Emission Boiling Research Articles

Acoustic state detection of microbubble emission boiling using a deep neural network based on cepstrum analysis

Microbubble emission boiling (MEB) is a cooling technology in which the heat flux can potentially exceed the critical heat flux (CHF). Reliable predictions of the occurrence of MEB are necessary to achieve stable MEB and to induce it under actual environment conditions. In this study, we developed a method based on deep learning with boiling sound to predict the boiling state of the interval before the low-heat-flux level reaches MEB. The boiling sound was acquired by a hydrophone, and the sound was adopted to machine learning algorithms, which were subsequently applied to classification and regression models. The feature extraction algorithms for the boiling sounds were spectrum or cepstrum methods. Both methods were comparatively investigated in terms of the machine learning accuracy. As a result, in the case of the cepstrum method as the feature extraction, the accuracy was improved. In particular, we found that the regression model demonstrated substantially better accuracy than the classification model. In addition, accurate predictions were possible even when the degree of subcooling was changed.

Bubble-induced oscillating flow in microbubble emission boiling under highly subcooled conditions

Microbubble emission boiling (MEB), a phenomenon that occurs under highly subcooled boiling conditions, can achieve a high heat flux beyond critical heat flux. MEB has been observed to differ from nucleate boiling and is always accompanied by the emission of microbubbles from an oscillating bubble. The heat transfer mechanism of MEB differs from that of nucleate boiling and remains elusive. In this study, we measure the behaviour of a vapour bubble and its induced liquid flow simultaneously during subcooled boiling using the high-speed two-phase particle image velocimetry method. Different from nucleate boiling, we observe a rapid oscillating flow outside the bubble that constantly attaches on the heating surface in MEB. The spatially and temporally averaged velocity magnitude values under this oscillating flow, and their dependency on liquid subcooling and heat flux, are quantified. Then we derive a scaling law for the heat transfer of MEB as a function of Péclet and Jacob numbers. With such correlations, we suggest that the oscillating flow induced by bubble oscillations is important to MEB, thus elucidating a different heat transport process from nucleate boiling.

Investigation on interaction between an oscillating vapor film and its surrounding liquid in microbubble emission boiling

Microbubble emission boiling (MEB) is a boiling phenomenon occurred under highly subcooled condition, which can realize a heat flux higher than critical heat flux. Owing to the complex bubble dynamics in MEB, its heat transfer mechanism remains elusive. In this study, the convection near an oscillating vapor film and its contribution to MEB are investigated experimentally and numerically. Visualization results reveal that the convection near the vapor film is enhanced with its thermal affected area being enlarged over 4-fold within 32 ms after the occurrence of MEB. Different from nucleate boiling, the vapor film attached on the heating surface oscillates rapidly in its volume, width and height in MEB, which may be one of the reasons for the formation of the strong convection. On this basis, simulations of the fluid flow and heat transfer near a stable or an oscillating vapor film were performed. Numerical results show that the oscillation of the vapor film enhances its surrounding convection obviously and improves the heat dissipated from the vapor into cold bulk over 10 times compared with that for a stable film. The above findings suggest that the convection enhanced by the vapor film oscillation plays an important role inMEB.

Effects of surface properties on wall superheat at the onset of microbubble emission boiling

Boiling heat transfer (BHT) is a promising technology for cooling the high heat flux emitted from next generation electronic devices. Pool boiling represents an eco-friendly cooling technology because its pumps do not require an external electronic power source. To increase the maximum heat flux of pool boiling beyond the critical heat flux, microbubble emission boiling (MEB) can be implemented with highly subcooled water. However, conventional MEB experiments use a copper heating surface, which can easily be oxidized in air. Thus, copper heating surfaces do not retain constant surface properties, especially when the BHT boiling pool is open to air. Applying a coating to the copper surface can enable further investigations of the MEB phenomenon and improve the reliability of MEB-based cooling devices. However, it is unclear which surfaces are suitable for promoting the onset of MEB. Herein, we experimentally investigated a stable hydrophilic black chrome plating on copper by performing subcooled pool boiling with water at atmospheric pressure. The results confirmed that the black chrome plating did not interrupt the onset of MEB, but rather, it enabled constant wall superheat during repeated experiments spanning the transition from nucleate boiling to MEB. Moreover, the threshold of liquid subcooling for the onset of MEB with the black chrome plating (20 K) was similar to that with a copper heat transfer surface.

On homogeneity of vapor bubbles’ oscillation and corresponding heat transfer characteristics and boiling sound in microbubble emission boiling (MEB)

We investigate so-called “microbubble emission boiling (MEB)” on a horizontal circular heat transfer surface in a pool via experimental approach. We indicate the correlation among the temporal and spatial behaviors of the vapor bubbles, the averaged heat transfer characteristics, and the accompanying boiling sound. We show that the vapor bubbles oscillate in a non-uniform manner in the radial direction immediately after the MEB transition. As the MEB develops by increasing the power input to the heaters, the vapor bubbles exhibit synchronous oscillation on the heat transfer surface as if they oscillate homogeneously. The homogeneity of the vapor bubble oscillation in the radial direction is quantified by the cross-correlation between the local oscillations. We illustrate significant variations of the heat transfer characteristics and the boiling sound driven by the homogeneous vapor bubble oscillation after the transition to the fully developed MEB.

Hysteresis in microbubble emission boiling (MEB) under highly subcooled conditions

Hysteresis in microbubble emission boiling (MEB) under highly subcooled conditions

Spatial–temporal thermal‐fluid behaviors of microbubble emission boiling (MEB)

AbstractWe investigate non‐uniform behaviors in space of the microbubble emission boiling (MEB). Special attention is paid to radial temperature difference on the heated surface before/after the onset of the MEB. Correlation among the regimes of the MEBs, the radial temperature difference in the heated surface and the vapor bubble behaviors is indicated experimentally. We find that the surface temperature near the center becomes higher in the transition to the MEB, and then it becomes lower in the developed MEB. It is found that a linear correlation between the power input to the heaters and the resultant heat flux on the heated surface breaks in the transition of the MEB. The difference of the MEB regimes, S‐MEB and C‐MEB, is apparently distinguishable by monitoring the radial temperature difference of the heated surface.

Enhancement of subcooled boiling in confined space using ultrasonic waves

Effects of ultrasonic waves on heat transfer and bubble behavior spanning from nucleate boiling to film boiling or microbubble emission boiling (MEB) in a confined space were investigated. Experimental results show that critical heat flux in the confined space is increased by four times in the ultrasonic field. In transition boiling regime, MEB occurs at subcooling over 20 K in the unconfined space, however it cannot be observed in the confined space even for a subcooling of 60 K. Visualizations show that a large coalesced bubble traps in the confined space, which inhibits the supply of liquid onto the heating surface. Applying the ultrasound will reintroduce MEB at subcooling over 25 K. A comparison of MEB in the confined space with and without the ultrasound reveals that the strong bulk convection introduced by the violent oscillation and collapse of vapor film plays an important role in heat transfer of MEB.

Simultaneous audio-visual-thermal characterization of transition boiling regime

Safety concerns due to the highly unstable nature of boiling during the transition from the nucleate to the film boiling regime has limited investigations on this topic. Here we perform simultaneous characterization of the thermal, acoustic, and the bubble visualization data during the transition boiling regime. We show that the key features missed due to the high latency in the thermal data are captured in the audio (0-2kHz). We correlate the acoustic emissions with the high-speed visualization to elucidate the presence of an explosive boiling sub-regime, also referred to as the microbubble emission boiling (MEB). This regime is typically observed after the critical heat flux (CHF) and is characterized by numerous small sized bubbles (≈300±50μm) emanating from an unstable vapor film on the heater surface. We correlate this change in bubble behavior with the corresponding increase in the intensity and peak frequency of acoustic emissions up until the onset of a completely silent and stable film boiling regime. Conversely, MEB features are also observed during the transition from film boiling to the nucleate boiling regime in quenching experiments. We modify the heater surface and the boiling fluid, and vary the test conditions, to confirm the existence of MEB in all experiments over a wide range of subcoolings ranging from 2°C to 25°C. Our experiments suggest that the extent of the MEB increases with, (i) the increase in subcooling, (ii) the decrease in the surface tension of the boiling fluid, and (iii) the increase in surface roughness/wettability of the heater surface. We believe these unique acoustic features of the transition boiling regime can be used as a feedback to design measures to avoid catastrophic failures in boiling based energy and thermal management systems.

Effects of ultrasonic waves on subcooled pool boiling on a small plain heating surface

Experiments were conducted to investigate the effects of ultrasonic waves on subcooled pool boiling on a small plain heating surface. Ultrasonic waves with different amplitudes (0–10 μm) were applied for both nucleate boiling and transition boiling modes with the liquid subcooling and heat flux covering 15–60 K and 0.15–8 MW/m2, respectively. The results show that the acoustic streaming induced by high amplitude ultrasound has an obvious enhancement effect on nucleate boiling at low heat flux, but the augmentation will be restricted with increase in heat flux and decrease in ultrasonic amplitude. In transition boiling region, the ultrasound makes different impact. At liquid subcooling of 15 K under which microbubble emission boiling (MEB) would not occur normally, the strong acoustic streaming triggers MEB, thereby heat transfer is significantly enhanced. At subcooling of 21–40 K, the transition boiling is in the mode of stable MEB, the ultrasound barely affects the heat transfer and bubble behaviors. With the subcooling increasing to 50–60 K, the heat transfer of MEB is enhanced by the 10 μm amplitude ultrasound but deteriorated by the 5 μm amplitude ultrasound. The opposite results depend on the different effects arising from the ultrasound on the collapse and oscillation of the vapor film on the heating surface.

Experimental investigation of transition process from nucleate boiling to microbubble emission boiling under transient heating modes

AbstractThis study investigated the transition from nucleate boiling to microbubble emission boiling (MEB) on a small heating surface under transient heating modes. During the short unstable boiling process after critical heat flux with a rapid increase in wall temperature, capillary waves were observed over the vapor film at subcooling of 30–40 K. The visualization results in this process demonstrated that the partial collapse of vapor film induced strong jet toward the heating surface and therefore avoided complete dryout thereon. This mechanism certainly plays an important role in the transition to stable MEB. Further comparison between the wall temperature and boiling states indicates that the wall temperature at onset point of stable MEB is determined by the predominance of MEB mode over film boiling in the transition process. Effects of liquid subcooling and surface oxidation on the transition process were also investigated at the same time.

Experimental study of sound emission in subcooled pool boiling on a small heating surface

In this study, sound emission in subcooled pool boiling with a small plate heater was investigated experimentally. Acoustic signals were recorded using a hydrophone and analyzed using sound pressure level (SPL), fast Fourier transform, and discrete wavelet transform (DWT). The results show that the SPL combined with an analysis of the sound signal by DWT can be used to identify boiling modes and heat transfer intensification. In contrast to those for a large plate heater or long wire, the plot of the SPL against wall superheat in nucleate boiling region exhibits an ‘N” shape. Visualization results indicate that this difference mainly originates from the effects of heater size on bubble growth and departure. Compared with the nucleate boiling, in the region of microbubble emission boiling (MEB), the SPL exhibits a step increase of 10–20 dB, and continues to increase with the further increase of the heat flux. A comparison of the bubble frequency and features of the acoustic signals shows that the loud boiling sound in the MEB could be mainly attributed to the vapor film collapse and vapor volume oscillations, which also significantly affect the heat transfer process.

Advanced Boiling Cooling Technology Using a Compact Vessel with a Low Water Level

There is a strong demand for compact cooling devices with high performance that is used for such as inverters of electric vehicles. Aiming for improving the performance of cooling devices with downsizing it, boiling heat transfer attracts a lot of attention because of its high heat transfer coefficient. However, there is few reports related to subcooled pool boiling using a compact vessel. In this study, to achieve a higher heat flux removing using a compact vessel, microbubble emission boiling (MEB) characteristics was investigated with a low water level, as well as nucleate boiling regime of subcooled pool boiling.

Subcooled water quenching on a super-hydrophilic surface under atmospheric pressure

The goal of this work is (i) to evaluate the cooling rate on a super-hydrophilic surface asa function of the subcooled degree ΔTsub of the liquid coolant, (ii) to analyze the contact heat transfer q″c of the liquid-solid contact, and (iii) to investigate the mechanism of microbubble emission boiling (MEB). We fabricated a super-hydrophilic surface by anodic oxidation of a zirconium vertical rod, so called completely wettable surface (CWS), which had surface microstructures with super-hydrophilicity. The CWS results in a decrease of the cooling time tcool as compared with the Bare Zirconium surface (BZS) results under small ΔTsub (tcool∼50% decrease for ΔTsub=0, 15, and 40K, respectively). However, its surface effect is limited in the case of large ΔTsub (tcool∼within 5% for ΔTsub=60 and 75K). The fast quench on the CWS under ΔTsub, explained by the increase in minimum film-boiling temperature TMFB and rewetting velocity U, is due to the liquid-solid contact. We evaluate the contact area Ac and volumetric absorption rate of the liquid dV/dt by conducting liquid absorption experiments. The increase in Ac and dV/dt contribute to an increase in q″c, by forming the liquid film at the liquid-solid contact spot. The orders of the time scale between capillary-wicking and liquid-solid contact are comparable. Destabilization of the large vapor bubble is caused by an increase in q″c, which is a major reason for MEB generation, and this mechanism enables the q″ to be significantly high on the CWS under subcooled quenching.

An experimental study on Microbubble Emission Boiling in a subcooled pool: Heat transfer characteristics and visualized presentation

In view of its super high ability of heat dissipation, Microbubble Emission Boiling (MEB) was investigated under atmospheric pressure and liquid subcooling of 0–60K with the aid of a high-speed video camera. An experimental apparatus was designed and fabricated to achieve stable MEB, by which a more detailed bubble behaviors and heat transfer characteristics during MEB were obtained. It is reaffirmed that liquid subcooling of 20K is the minimum subcooling triggering MEB for water. The increase in liquid subcooling would raise the wall superheat and vapor generation rate for forming MEB. The heat transfer coefficient in the beginning of MEB is about 20–70kW/m2K, less than that at CHF (Critical Heat Flux) under the same condition. Three typical interfacial phenomena were observed in MEB, which are partial collapse on the periphery of vapor film, partial collapse on the top of vapor film and violent surface wave process without collapse. The average collapse frequency in MEB is much higher than the bubble departure frequency in nucleate boiling, greatly enhancing the heat transfer process. Finally, a dimensionless heat transfer correlation for MEB is proposed with considering the effect of the vapor film collapse, condensation, Marangoni convection and vapor generation rate.

Transition process leading to microbubble emission boiling on horizontal circular heated surface in subcooled pool

Microbubble emission boiling (MEB) produces a higher heat flux than critical heat flux (CHF) and therefore has been investigated in terms of its heat transfer characteristics as well as the conditions under which MEB occurs. Its physical mechanism, however, is not yet clearly understood. We carried out a series of experiments to examine boiling on horizontal circular heated surfaces of 5mm and of 10mm in diameter, in a subcooled pool, paying close attention to the transition process to MEB. High-speed observation results show that, in the MEB regime, the growth, condensation, and collapse of the vapor bubbles occur within a very short time. In addition, a number of fine bubbles are emitted from the collapse of the vapor bubbles. By tracking these tiny bubbles, we clearly visualize that the collapse of the vapor bubbles drives the liquid near the bubbles towards the heated surface, such that the convection field around the vapor bubbles under MEB significantly differs from that under nucleate boiling. Moreover, the axial temperature gradient in a heated block (quasi-heat flux) indicates a clear difference between nucleate boiling and MEB. A combination of quasi-heat flux and the measurement of the behavior of the vapor bubbles allows us to discuss the transition to MEB.

疎水性コーティング剤を用いたサブクールプール沸騰に関する研究

A next-generation electric vehicle requires a high-performance IC inverter with the thermal emission of more than 500 W/cm2 . To remove such a high heat emission, boiling heat transfer with microbubble emission boiling (MEB) is expected to be an advanced cooling technology because MEB can improve the maximum heat flux. On the other hand, the wall super heat in MEB is higher than that in CHF. In this study, we investigated the characteristics of MEB with and without a hydrophobic coating. As a result, the wall super heat with the hydrophobic coating in MEB was lower than that without the coating.

ポーラスマイクロチャンネルを用いた高熱流束除熱

Experiments were carried out to investigate the characteristics of high-heat-flux heat removal using a porous-micro-channel (a small rectangular channel filled with a porous-metal plate). The working fluid was water and the main experimental parameters were the inlet liquid subcooling, mass flux, and channel size. It was shown that nucleate boiling heat transfer is enhanced in the porous-micro-channel in comparison with the normal rectangular channel. Another important finding was that as in the normal channel, the micro-bubble emission boiling (MEB) occurs in the porous-micro-channel if the mass flux and liquid subcooling are high enough. In addition, the flow oscillation and noise production that often accompany with the MEB in the normal channel were mitigated significantly in the porous-micro-channel. This indicates that the porous-micro-channel enables the stable and high-heat-flux heat removal by the MEB.

気泡微細化沸騰における流れ場のPIV計測

Liquid inlet behavior to a heat surface in micro-bubble emission boiling (MEB) was investigated by PIV measurement. Subcooled pool boiling experiments were performed using a heat surface with a diameter of 10mm. An upper end of a heater block was used as the heat surface. The subcooling was varied from 40 K to 70 K. The flow visualization around the heat surface was carried out using a high speed video camera and a light sheet. The micro-bubbles generated in the MEB were used as tracer particles and the velocity field was obtained by PIV analysis of the acquired image sequence. The distribution characteristics and fluctuation of the velocity were studied.

A study visualizing the collapse of vapor bubbles in a subcooled pool

The condensation processes of vapor bubbles in a subcooled pool were recorded and analyzed, which could help to illustrate the mechanism of collapse of vapor bubbles and emission of microbubbles during Microbubble Emission Boiling (MEB). Vapor bubbles were injected into the subcooled pool through an orifice with the inner diameter of 4mm at different vapor volumetric flow rates. Visualized results showed that the bubble surface became rougher and more unstable with the increase in liquid subcooling and volumetric flow rate, during which four condensation features were confirmed and defined as smooth bubble regime, shape oscillation regime, transition regime and capillary wave regime, respectively. Fine surface wave formed after a while of condensation would enlarge the effective area of condensation, resulting in the acceleration of bubble condensation. Furthermore, the fine surface wave which might also lead to bubble collapse only appeared on bubble surface at liquid subcooling over 25K and vapor volumetric flow rate higher than 0.74m3/h, which corresponded to the occurrence conditions of MEB. After the sudden collapse of the vapor bubble with capillary wave, a great many microbubbles formed, with a greater velocity and much smaller radius with subcooling and vapor injection rate increasing. In the region of MEB, a strong surface wave usually appeared on vapor film before the film collapse and the emission of microbubbles. Nevertheless, the film surface formed in film boiling was relatively smooth compared with that formed in MEB. All of the experiments results and analysis illustrated that the strong surface wave introduced by condensation may be one of the reasons resulting in the formation of MEB.