Flow Boiling Research Articles

Heat Transfer Deterioration by the Copper Oxide Layer on Horizontal Subcooled Flow Boiling

Water–copper is one of the most common combinations of working fluid and heating surface in high-performance cooling systems. Copper is usually selected for its high thermal conductivity and water for its high heat transfer coefficient, especially in the two-phase regime. However, copper tends to suffer oxidation in the presence of water and thus the heat flux performance is affected. In this research, an experimental investigation was conducted using a cooper bare surface as a heating surface under a constant mass flux of 600 kg·m−2·s−1 of deionized water at a subcooled inlet temperature ΔTsub of 70 K under atmospheric pressure conditions on a closed-loop. To confirm the heat transfer deterioration, the experiment was repeated thirteen times. On the flow boiling region after thirteen experiments, the results show an increase in the wall superheat ΔTsat of approximately 26% and a reduction in the heat flux of approximately 200 kW·m−2. On the other hand, the effect of oxidation on the single phase is almost marginal.

Pressure and Thermal Characterisation of Dynamic Instabilities During Flow Boiling in Micro/Mini-channels at Different Azimuth Orientations

• Flow boiling in microchannels at different azimuthal orientation are investigated. • 3 instability types were identified: two-phase mixing, minor and major reverse flow. • Flow instability improves the average heat transfer coefficient by up to 52% • Top-heated channels had the greatest increase in heat transfer coefficient. • Instability frequency increased with mass flux increases and heat flux decreases. Fluid instability was investigated experimentally during flow-boiling of Perfluorohexane (FC-72) in a flat horizontal micro/mini channel with a hydraulic diameter of 909 μ m and an aspect ratio of 10 (5 mm x 0.5 mm). One-sided heating at different azimuth channel orientations ( θ ) in terms of gravity were considered, which ranged from bottom-heating ( θ = 0°) to top-heated ( θ = 180°) in 30° increments. Mass fluxes of 10, 20 and 40 kg/m 2 s were considered at a saturation temperature of 56 °C. Flow instability and the resulting thermal and pressure responses were identified and studied via high-speed video, infrared thermography, and pressure measurements. Various mass flux and heat flux combinations at each channel orientation leading to flow instabilities categorised into two-phase mixing, minor reverse flow and major reverse flow, were studied. Increased mass flux resulted in more frequent and more severe flow instability, irrespective of the channel orientation. Increased heat flux resulted in an increased number of operating conditions that were susceptible to flow instability, but the instabilities occurred at lower frequencies. In general, orientations that had a horizontal heated surface ( θ = 0° and 180°) were most susceptible to flow instability, while the intermediate azimuthal rotations ( θ = 30°, 60° and 90°) exhibited flow instability only at the highest mass flux. It was also found that flow instability often resulted in improved heat transfer performance, particularly in regions of the channel that were occupied by liquid. Where-as no direct relationship between the mass flux and the heat transfer performance improvement could be identified, it was found that better heat transfer improvements were obtained at higher heat fluxes. At the highest mass flux, cases that were horizontal and heated from above ( θ = 180°) exhibited heat transfer coefficient improvements (during instability) of up to 77% and 275% in the single-phase and two-phase regions respectively compared to baseline stable conditions.

STUDY ON HEAT TRANSFER CHARACTERISTICS AND STABILITY OF SUBCOOLED FLOW BOILING IN A NOVEL BOTTOM-DIVERGING MICROCHANNEL WITH EQUAL CROSS-SECTIONAL AREA

The enhancement of boiling heat transfer and its stability have been widely studied in the field of flow boiling in microchannel heat sinks. In this study, a novel diverging microchannel is proposed to enhance the heat transfer performance and boiling stability during subcooled flow boiling in the microchannel by the channel structure of divergent fluid-solid coupling bottom interface and equal cross-sectional area. Two-phase patterns, heat transfer characteristics, and pressure variations of subcooled flow boiling of HFE-7100 in this microchannel, its reversal flow microchannel, uniform microchannel, and diverging microchannel are compared under different heat flux conditions. The results show that the microchannel structure directly affects the nucleation time of bubbles and their behavior. At the same heat flux, the bubble nucleation time in the bottom-diverging microchannel with equal cross-sectional area is the longest, the bubble size is the smallest and the closest to the exit of the microchannel, which is beneficial to rapidly exhaust of the bubbles. The bottom-diverging microchannel with equal cross-sectional area exhibits higher heat transfer coefficient and smaller pressure fluctuation than its reversal flow microchannel, uniform microchannel, and diverging microchannel.

FLOW BOILING HEAT TRANSFER AND HEAT CONDUCTION ANALYSIS OF HIGH-POWER ELECTRONIC DEVICES

A flow boiling experiment in a narrow channel was conducted using subcooled water. Moreover, the experimentally measured heat flux of the channel was applied for the heat conduction analysis of insulated gate bipolar transistors (IGBTs). The relationship between the heat flux and the temperature difference was investigated experimentally, ranging from nonboiling to boiling in the narrow tube. The boiling heat transfer in the fully nucleate boiling region was validated by comparing with the Rohsenow correlation. Moreover, the measured critical heat flux (CHF) agreed with the Hall and Mudawar correlation within ± 15%. The heat conduction analysis of an IGBT module with a heatsink was solved using the measured heat flux in the narrow channel as a boundary condition, applied from the nonboiling region to the film boiling region. The thermal resistance of the material was obtained when the heatsink with narrow channels was modeled. The numerical result indicated that the thermal resistance of the material was influenced by the heat transfer process of the cooling channel.

Fundamentals of Force Balance in Liquid Films and Transition from Macro- to Microchannels in Flow Boiling

Microchannel heat exchangers play a critical role in development of more efficient and environmentally friendly energy systems. Thus, understanding the underlying physics that govern their heat transfer performance is of paramount importance. The most fundamental question is what is considered a microchannel, and what channel size, fluid properties, and flow conditions warrant use of a micro- versus macrochannel model. There have been extensive discussions in the literature regarding the appropriate definition of a microchannel; however, a clear, physics-based distinction of micro- from macrochannels has not emerged. This study analyzes this outstanding scientific question from the perspective of confinement effect on liquid films through investigating the effect of gravity on liquid films thickness. A set of experimental studies are conducted to determine the effect of gravitational force on liquid film asymmetry. The experimental findings are utilized to evaluate the existing criteria for transition from macro- to microchannels. Our analysis suggests that confinement numbers greater than 1.0 distinguish micro- from macrochannels in terms of liquid film symmetry. This analysis is supported by a force balance that shows gravitational force on liquid films exceeds the surface tension force in macrochannels.

Numerical Simulation of Subcooled Flow Boiling in a Vertical Annulus Channel Under Near Atmospheric Pressure Conditions

In the present study, a Eulerian-Eulerian two-fluid model is developed to analyze the flow boiling phenomena under near-atmospheric pressure conditions. The required constitutive correlations for the two-fluid model are provided as flow regime dependent within the algebraic interfacial area density framework. The two-fluid model developed with Rensselaer Polytechnic Institute (RPI) wall heat flux partitioning is used to analyze the subcooled nucleate boiling of water at low pressure in three vertical annulus channels of different heated lengths over a wide range of inlet mass flux, wall heat flux, and inlet subcooling conditions. The subcooled water enters the heated annulus channel from the bottom end and is heated to near-saturation temperature. Upon reaching the saturation temperature, the wall boiling generates dispersed vapor bubbles near the heated wall. Farther along the heated length, larger bubbles can be formed by coalescence and evaporation, and the bubbles move on to the channel core region with increased vapor fraction so the flow regime changes from bubbly to transition regime. Farther along, it may turn to an annular flow regime. The benchmark experimental cases chosen are used to validate the model capability in predicting the bubbly flow and transition flow regime (slug flow regime) characteristics with the proposed methodology. Further, the low-pressure boiling model developed is successfully extended to predict the liquid sodium boiling in flow channels similar to sodium-cooled fast reactor fuel subchannels using suitable interfacial correlations.

Influence of High-Frequency Pulsations of Liquid on Heat and Mass Transfer in Wide Minichannel during Liquid Flow Boiling

Influence of High-Frequency Pulsations of Liquid on Heat and Mass Transfer in Wide Minichannel during Liquid Flow Boiling

Bubble Dynamics and Heat Transfer Characteristics of Flow Boiling in a Single Pentagonal Rib Channel

Abstract Two-phase flow boiling is the preferred method for efficient heat dissipation in electronic equipment, so the bubble dynamics and heat transfer characteristics of flow boiling in a single pentagonal rib channel are numerically analyzed by the volume of fluid (VOF) model. Three typical rib structures are obtained, whose block ratios are 0.4, 0.75, and 0.25, and the corresponding length-diameter ratios are 1.72, 0.59, and 12.70, respectively. The Reynolds number is 14,000 and 28,000. The heat flux is set from 50 to 150 kW/m2. The results show that the starting position of the bubble is at the cone and sides of the rib, and its moving direction is related to the configuration of the rib, the heat flux, and fluid velocity. Additionally, the bubbles are shedding at the corner of the rib, and the short rib does not appear to be the vortex shedding phenomenon. The wall temperature and temperature fluctuation increase with the increase of heat flux and with the decrease of fluid velocity. The heat transfer coefficient decreases with the increase of heat flux and with the decrease of fluid velocity. The short rib channel has a higher heat transfer coefficient, approximately 14% and 50% higher than that of the median and long rib channels, respectively. Compared with the single-phase flow, the main frequency of the lift coefficient increases with the increase of the fluid velocity while decreasing with the increase of heat flux. The main frequency of the median rib without harmonic signal keeps constant, and that for long rib decreases by 38%. The study of the instantaneous characteristics of bubble growth in pentagonal rib channels is instructive to the efficient heat dissipation in electronic equipment.

Numerical Simulation Research of Bubble Characteristics and Bubble Departure Diameter in Subcooled Flow Boiling

Three-dimensional subcooled flow boiling of R134a in a horizontal tube was simulated by a VOF (volume of fluid) model combined with the level set method. Bubble characteristics were explored at heat flux of 0.3 MW/m2, inlet subcooling of 3 K, and inlet velocity of 0.4 m/s. It was observed that five representative bubbles occurred in subcooled flow boiling, including sliding bubble, coalescing bubble, non-departed bubble, bouncing bubble, and continuous-boiling bubble. The results showed that the bubble radial velocity was an important factor of bubble departure after a sliding process. Moreover, the effect of heat flux, inlet velocity, and inlet subcooling on bubble departure diameter were investigated. The departure diameter increased with increasing inlet velocity from 0.2 to 0.4 m/s and heat flux from 0.2 to 0.4 MW/m2, while diameter decreased with inlet subcooling from 3 to 10 K. Finally, based on the influence of heat flux, inlet velocity, and inlet subcooling on average departure diameter of the bubble except the coalescing bubble, a model was proposed to predict the average departure diameter. The deviation of the model was within 5%.

Study of the Effect of the Reduced Pressure on a Mechanistic Heat Transfer Model for Flow Boiling of CO2 in Macroscale and Microscale Tubes

The effect of the reduced pressure on flow boiling of CO2 in macroscale- and microscale-tubes was analyzed in this paper. First, flow boiling heat transfer at high and low reduced pressures were simulated using a general mechanistic heat transfer model for flow boiling CO2 to analyze the heat transfer mechanisms. Then, the simulated heat transfer coefficients were compared to an experimental database at the reduced pressure from 0.1332 to 0.9082 (the corresponding saturation temperature from -40.6 to 26.77 °C), the tube diameter from 0.529 to 9.52 mm, the heat flux from 2 to 72 kW/m2 and the mass flux from 100 to 1500 kg/m2s. The model predicts 81.3% of the data before the dryout inception within ±30% while it only predicts 21% of data in the dryout and mist flow regimes within ±30%. The poor prediction of the heat transfer in these two regimes are because the model does not capture the dryout inception and completion. Furthermore, the heat transfer mechanisms are discussed from the standpoint of the flow regime variations and unstable flow boiling phenomena in macroscale and microscale tubes. It is recommended that the model should be improved by considering the mechanisms at low and high reduced pressures.

Prediction of Critical Heat Flux for Subcooled Flow Boiling in Annulus and Transient Surface Temperature Change at CHF

The ability to predict critical heat flux (CHF) is of considerable interest for high-heat equipment, including nuclear reactors. CHF prediction from a mechanistic model for subcooled flow boiling in rod bundles still remains unsolved. In this paper, we try to predict the CHF in an annulus, which is the most basic flow geometry simplified from a fuel bundle, using a liquid sublayer dryout model. The prediction is validated with both water and R113 data, showing an accuracy within ±30%. After the CHF in an annulus is calculated successfully, a near-wall vapor–liquid structure is proposed on the basis of the liquid sublayer dryout model. Modeling of heat transfer modes over the heating surface at CHF is performed, and predictions of the changes in liquid sublayer thickness and heater surface temperature at the CHF occurrence point are carried out by solving the heat conduction equation in cylindrical coordinates with a convective boundary condition, which changes with the change in flow pattern over the heating surface. Transient changes in the liquid sublayer thickness and surface temperature at the CHF occurrence point are reported.

Numerical Study of the Effect of the Rolling Motion on the Subcooled Flow Boiling in the Subchannel

The marine environment may change the force on the fluid and inevitably influence bubble behavior and the two-phase flow in the reactor core, which are vital to the safety margin of a nuclear reactor. To explore the effect of the marine motion on the flow and heat transfer features of subcooled flow boiling in the reactor core, the volume of fluid (VOF) method is employed to reveal the interaction between the interface structure and two-phase flow in the subchannel under rolling motion. The variations of several physical parameters are obtained, including the transverse flow, the vapor volume fraction, the vapor adhesion ratio, and the phase distribution of boiling two-phase flow with time. Sensitivity analyses of the amplitude and the period of the rolling motion were performed to demonstrate the mechanisms of the influence of the rolling motion. We found that the transverse flow in the subchannel was mainly affected by the Euler force under the rolling motion. In contrast to the two-phase flow in the static state, the vapor volume fraction and vapor adhesion ratio show different characteristics under rolling motion. Additionally, the onset of significant void (OSV) point changes periodically under rolling motion.

A Smart Model for the Prediction of Heat Transfer Coefficient during Flow Boiling of Nanofluids in Horizontal Tube

The goal of this study is to improve the accuracy and the validity of the prediction of the heat transfer coefficient (HTC) throughout flow boiling of different water-based nanofluids in a horizontal tube by developing an artificial neural network model using Ag/water, Cu/water, CuO/water, Al2O3/water, and TiO2/water nanofluids. The multiple layer perceptron (MLP) neural network was designed and trained by 354 experimental data points that were collected from the literature. Thermal conductivity of nanoparticle, mass flux, volumetric concentration, and heat flux were used to serve as input variables of the model. The heat transfer coefficient (HTC) was used as the output variable. Via the method of the trial-and error, MLP with 8 neurons in the hidden layer was attained as the optimal artificial neural network structure. This developed smart model is more accordant with the experimental data than the correlations of the literature. The accuracy of the developed smart model was validated by the value of mean squared error (MSE=0.042) and the value of determination coefficient (R2= 0.9992 ) for all data.

Numerical Simulation Study on the Flow and Heat Transfer Characteristics of Subcooled N-Heptane Flow Boiling in a Vertical Pipe under External Radiation

In the top submerged lance (TSL) smelting process, flow boiling may occur in the lance’s inner pipe due to the heat coming from the furnace when liquid fuel is adopted. In the current study, a numerical simulation was carried out by coupling the Eulerian two-fluid model with the improved RPI wall boiling model to investigate the subcooled n-heptane flow boiling in the inner pipe. The effects of inlet velocity and pipe wall emissivity on two-phase flow and heat transfer are elucidated. The results show that, for pipes with inlet velocity ranging from 0.3 m·s−1 to 1.0 m·s−1, an increase in inlet velocity leads to a lower void fraction near the outlet, as well as a lower average velocity and a lower average temperature of each phase. Meanwhile, the Onset of Nucleate Boiling (ONB) position approaches to the outlet, and the total pressure drop of the entire pipe reduces when the inlet velocity increases. However, the opposite trends appear when increasing the pipe wall emissivity. The maximum wall temperature corresponding to the critical heat flux (CHF) point is slightly affected by inlet velocity but significantly affected by pipe wall emissivity. The non-equilibrium effect and the specific components of pressure drop are also further investigated.

Numerical Investigation of Boiling Flow in Oval Tube with Different Ellipticity

Improving heat transfer has been the continuous quest in thermal engineering field. With higher transfer rate required, many researchers have shifted their attention towards boiling heat transfer that offer superior transfer rate. Despite its potential, study on this heat transfer process have not been as extensive as its single-phase counterpart. Particularly, to the best of our knowledge, no study on the boiling heat transfer in oval tube with different ellipticity. Thus, numerical investigation using RPI wall boiling model and RNG k-ε turbulence model is conducted for different ellipticity of the oval tube. It is found that the heat transfer performance drops as the ellipticity increases with constant perimeter. Decrease of heat flux and increase of mass flow rate have positive impacts toward the heat transfer performance. The vapor generation resulting by the heat flux and mass flow rate is having high influence in the heat transfer coefficient and pressure drop.

Comparison of Intergroup Mass Transfer Coefficient Correlations in Two-Group IATE for Subcooled Boiling Flow

The interfacial area concentration (IAC) is of vital importance in determining the interfacial transfer terms of mass, momentum, and energy between phases in the two-fluid model. The two-group (2-G) interfacial area transport equation (IATE) dynamically models the IAC evolution of large and small bubbles, and the intergroup transfer bookkeeps the mass transferring between two groups. Different intergroup mass transfer coefficient correlations are employed in the 2-G IATE model, which include one old correlation and three new correlations previously developed by the authors. The two-group IATE is benchmarked with subcooled boiling experimental dataset. The group-1 interfacial area concentration results show that the three modified correlations improve the physically incorrect prediction by the old correlation. With the modified correlations, the 2-G IATE is foundationally capable of predicting the group-1 interfacial area concentration in subcooled boiling flow.

Subcooled Flow Boiling in High Power Density Internal Combustion Engines I: Thermal Survey Measurement Campaign

Subcooled Flow Boiling in High Power Density Internal Combustion Engines I: Thermal Survey Measurement Campaign

Correlation between pressure loss and heat transfer coefficient in boiling flows in printed circuit heat exchangers with semicircular and circular mini-channels

• Boiling flow heat transfer coefficient and pressure drop were evaluated. • Printed circuit heat exchangers with semicircular and circular channels were employed. • Nucleate boiling in corners affected the heat transfer and flow resistance. • Heat transfer coefficient in semicircular channel was twice as high at low quality. • Increase in pressure drop was suppressed by effect of corners at a low quality. In this study, we investigated the correlation between the pressure loss and heat transfer coefficient for boiling flows in printed circuit heat exchangers. The effect of channel cross-sectional shapes—semicircle and circle—was evaluated. The hydraulic diameters of the semicircular and circular channels were 1.04 mm and 1.0 mm, respectively. A subcooled liquid with a subcooling temperature of 10 K was heated with hot water, and the saturation temperature was 30 °C. The refrigerant mass flux was set in a range of 100–400 kg/m 2 ·s. Consequently, despite the lower pressure drop, for low outlet quality, the heat transfer coefficient was higher for the semicircular channel than for the circular channel. The heat transfer was enhanced by the promotion of bubble nucleation in the thick liquid film at the corners, and the flow resistance was suppressed by the decrease in the vapor shear stress. However, for high outlet quality, the difference in the heat transfer coefficient decreased with an increase in the outlet quality. The experimental results were compared with the results obtained by correlation equations. The lowest mean absolute error of heat transfer coefficient and pressure drop for the semicircular mini-channel were 15.1 % and 26.7 %, respectively.

A Phenomenological Investigation on Near-Wall Bubble Behavior Close to Chf in Flow Boiling

A Phenomenological Investigation on Near-Wall Bubble Behavior Close to Chf in Flow Boiling

Characterizing Effects of Particle Size on Flow Boiling in Sintered Porous- Microchannels

Characterizing Effects of Particle Size on Flow Boiling in Sintered Porous- Microchannels