Experimental and Numerical Analysis of the Influence ofMicrochannel Size and Structure on Boiling Heat Transfer

Computational fluid dynamics was used and a numerical simulation analysis of boiling heat transfer in microchannels with three depths and three cross-sectional profiles was conducted.The heat transfer coefficient and bubble generation process of three microchannel structures with a width of 80μm and a depth of 40,60,and 80μm were compared during the boiling process,and the factors influencing bubble generation were studied.A visual test bench was built,and test substrates of different sizes were prepared using a micro-nano laser.During the test,the behavior characteristics of the bubbles on the boiling surface and the temperature change of the heated wall were collected with a high-speed camera and a temperature sensor.It was found that the microchannel with a depth of 80μm had the largest heat transfer coefficient and shortest bubble growth period,the rectangular channel had a larger peak heat transfer coefficient and a lower frequency of bubble occurrence,while the V-shaped channel had the shortest growth period,i.e.,the highest frequency of bubble occurrence,but its heat transfer coefficient was smaller than that of the rectangular channel.

Numerical Simulation on Subcooled Boiling Heat Transfer Characteristics of Water-Cooled W/Cu Divertors

In order to realize safe and stable operation of a water-cooled W/Cu divertor under high heating condition,the exact knowledge of its subcooled boiling heat transfer characteristics under different design parameters is crucial.In this paper,subcooled boiling heat transfer in a water-cooled W/Cu divertor was numerically investigated based on computational fluid dynamic（CFD）.The boiling heat transfer was simulated based on the Euler homogeneous phase model,and local differences of liquid physical properties were considered under one-sided high heating conditions.The calculated wall temperature was in good agreement with experimental results,with the maximum error of 5%only.On this basis,the void fraction distribution,flow field and heat transfer coefficient（HTC）distribution were obtained.The effects of heat flux,inlet velocity and inlet temperature on temperature distribution and pressure drop of a water-cooled W/Cu divertor were also investigated.These results provide a valuable reference for the thermal-hydraulic design of a water-cooled W/Cu divertor.

Application of Superhydrophobic Surface on Boiling Heat Transfer Characteristics of Nanofluids

Boiling heat transfer is a mode using the phase change of working medium to strengthen the heat exchange due to its good heat exchange capability,and it is widely used in heat exchange engineering.Nanofluids have been used in the direction of enhanced heat transfer for their superior thermophysical property.The wetting,spreading and ripple phenomena of superhydrophobic surfaces widely exist in nature and daily life.It has great application value for engineering technology.In this article,the boiling heat exchange characteristics of nanofluids on superhydrophobic surface are numerically studied.It was found that with the increase of superheating degree,the steam volume ratio of unmodified heated surface increases to saturation,while the steam volume and evaporation ratio of modified superhydrophobic surface increase firstly and then decrease.At the same time,bubbles are generated and accumulated more fully on superhydrophobic surface.It was also found that nanofluids with low viscosity are more affected by superhydrophobic surface characteristics,and the increase is more significant with high superheating degree,and the superhydrophobic surface is beneficial to enhancing boiling heat exchange.Compared with the simulation results,it could be concluded that the boiling heat exchange performance of CuO-water nano-fluids on the modified superhydrophobic surface is better than that of CuO-ethylene glycol nanofluids under high superheating degree.

Accurate prediction of the critical heat flux for pool boiling on the heater substrate

While the influence of liquid qualities,surface morphology,and operating circumstances on critical heat flux(CHF)in pool boiling has been extensively studied,the effect of the heater substrate has not.Based on the force balance analysis,a theoretical model has been developed to accurately predict the CHF in pool boiling on a heater substrate.An analytical expression for the CHF of a heater substrate is obtained in terms of the surface thermophysical property.It is indicated that the ratio of thermal conductivity(k)to the product of density(ρ)and specific heat(cp)is an essential substrate property that influences the CHF.By modifying the well-known force-balance-based CHF model(Kandlikar model),the thermal characteristics of the substrate are taken into consideration.The bias of predicted CHF values are within 5%compared with the experimental results.

Enhancement in boiling heat transfer performance using reduced graphene oxide coating with controllable components and porous structures

Enhancement in boiling heat transfer performance is significant for addressing thermal management bottlenecks of advanced electronic systems.Reduced graphene oxides(rGO)are regarded as promising candidates for thermal management due to their excellent thermal properties,chemical stability and adjustable wettability.In this study,rGO coatings with micron pores and controllable oxygen contents are prepared on Al substrate via cathodic electrophoretic deposition and subsequent thermal annealing,leading to enhanced pool boiling performance.The heat transfer coefficient for Al/rGO450is 37.2 kW m-2K-1,which is increased by 112.6%compared with bare Al,also outperformed previously reported Al based substrates.It is assumed that the hydrophilic and aerophobic r GO coatings effectively promote the liquid infiltration and bubble departure during pool boiling process.Importantly,repeatability tests indicate the durable stability of vertically oriented rGO nanosheets.Reverse nonequilibrium molecular dynamics simulation indicates that the interfacial transmission coefficients of Al/rGO increase after thermal annealing,indicative of the enhanced heat transfer performance of heterogeneous interface.Our study opens a new avenue for endowing metal substrates with high pool boiling performance using porous carbon coating nanoengineering strategy with controllable morphology and components.

Numerical study on two-phase boiling heat transfer performance of interrupted microchannel heat sinks

The conventional straight microchannel heat sinks have been reported to inadequately remove the increasing power density of electronics.In recent years,an effective heat transfer enhancement method,flow disruptions have attracted the attention of researchers,where interrupted structures are arranged in the microchannel to enhance flow mixing and heat transfer.However,previous numerical studies of interrupted microchannel heat sinks(I MCHS)mainly focus on single-phase flow condition,and the characteristics of the boiling heat transfer of I MCHS in two-phase flow condition have been rarely explored.Thus,the flow and heat transfer characteristics of two I MCHS based on rectangular microchannel heat sink(R MCHS)are investigated by modeling both single-phase and two-phase flow conditions.These two interrupts consist of a combination of cavities and ribs,namely elliptical cavities and elliptical side ribs(EC-ESR),and elliptical cavities and elliptical central ribs(EC-ECR).The results show that for single-phase flow condition,the maximum Nusselt number is increased by 187%in the EC-ESR design and150%in the EC-ECR design compared with the R MCHS.For subcooled boiling(i.e.,two-phase flow)condition,the EC-ECR design is a promising structure to enhance boiling heat transfer with 6.7 K reduction of average wall temperature and 29%increment of local heat transfer coefficient when compared with those of R MCHS.However,the local heat transfer coefficient in the EC-ESR design is decreased by 22%compared with the R MCHS due to the formation of a rare flow pattern(i.e.,inverted annular flow with vapor film separation)in the microchannel.This flow pattern can induce departure from nucleate boiling(DNB),thereby deteriorating the heat transfer on the channel walls.

Visualization of Boiling Heat Transfer on Copper Surface with Different Wettability

In this work, a pool boiling visualization experiment platform was built to study the influence of wettability on the nucleate boiling heat transfer. The copper surface was prepared by chemical etching method, and three types of copper surfaces(hydrophilic, hydrophobic, and smooth) were obtained. The boiling heat transfer characteristics of these three surfaces were discussed based on the obtained heat transfer curves and the heat transfer coefficient(HTC) curves. It was found that the HTC still increases when the boiling heat transfer closes the critical heat flux(CHF). To better evaluate the boiling heat transfer efficiency, the concept of heat flux growth rate per unit superheat is introduced. The dynamic behaviors of single and multiple bubbles were also recorded to study the mechanism of the boiling heat transfer. The bubble behavior characteristics show good consistent with the proposed heat flux growth rate. By comparing the heat transfer curves and the bubble dynamic behavior, the following characters of boiling heat exchange can be found: At low heat flux, the heat exchange is dominated by the bubble coverage on the surface, and the hydrophobic surface has better boiling heat transfer efficiency than the hydrophilic surface. As the heat flux increases, the vapor channel connecting a big bubble and the wall surface plays an important role in enhancing the heat transfer. The polymerization of bubbles on the hydrophobic surface forms bubble films and breaks the vapor channel, leading to a significantly smaller HTC and a lower CHF than that of the hydrophilic surface. The HTC of hydrophilic surface in this work can reach up to 54.85 kW/(m^(2)·K), which is 2.3 times the hydrophobic surface.

Numerical Investigation on Pool Boiling Mechanism of Hybrid Structures with Metal Foam and Square Column by LBM

In the present study, pool boiling heat transfer performance and bubble behaviors of hybrid structures with metal foam and square column are investigated by lattice Boltzmann method. By using the vapor-liquid phase change model of Gong-Cheng and Peng-Robinson equation of state, the effects of structural parameters, including metal foam thickness, porosity, column height and ratio of column width(W) to gap spacing(D) are investigated in details. The results show that hybrid structure performs better than pure columnar structure in pool boiling heat transfer. The hybrid structure accelerates bubble growth by fluid disturbance while metal skeletons prevent the bubble escaping. The optimum ratio of column width to gap spacing decreases with the increase of heat flux and HTC(heat transfer coefficient) can achieve an increase up to 25% when W/D change from 5/3 to 1/3. The increase of column height enhances heat transfer by expanding surface area and providing space for bubble motion. The metal foam thickness and porosity have a little influence on pool boiling heat transfer performance, but they have an important effect on bubble motion in the regime.

Numerical simulation of single bubble boiling behavior

The phenomena of a single bubble boiling process are studied with numerical modeling.The mass,momentum,energy and level set equations are solved using COMSOL and temperature field in time are analyzed,and reasonable results are obtained.The numeral model is validated by the empirical equation of Fritz and could be used for various applications.

Superlyophilic Interfaces Assisted Thermal Management

Thermal management has become a critical issue owing to the increasing need for various devices including heat dissipation and adsorption.Recently,the rapid growth of scientific reports is seen to improve thermal management efficiency by developing materials with high transfer coefficient and surface improvement to enhance heat transfer rate.Inspired by nature,constructing superlyophilic interfaces has been proved to be an effective way for thermal management and applied in industry and daily life.Herein,state-of-the-art developments of superlyophilic interfaces assisted thermal management are reported mainly from four perspectives around boiling,evaporation,radiation,and condensation.In particular,we discussed the unique role of superlyophilic interfaces during the heat transfer process,such as increasing bubble detachment rate,superspreading assisted efficient evaporation,directional liquid transfer in textiles during radiative cooling,and so forth.Finally,challenges of thermal management assisted by superlyophilic interfaces toward future applications are presented.

Bubble Interaction in Thermal Boundary Layers

The boiling process is characterized by strong nonlinear interactions between bubbles in the thermal boundary layer. The present paper uses a heat balance approach to derive the interaction length between bubbles based on boiling heat transfer mechanisms and bubble dynamics. The effects of some important parameters on the interaction length are then analyzed. Interestingly, the whole range of fluxes from low heat flux nucleate boiling to high heat flux boiling, departure from nucleate boiling (DNB), critical heat flux (CHF), and transition boiling, can be well understood by the concept of interaction length. The present paper also uses the concept of interaction length to present a unified description of DNB and CHF phenomena through a better understanding of nucleate boiling.