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To explore the wetting behavior of nanofluid under high voltage, a contact angle measurement system under electric field is designed and set up. The effects of mass concentration, the type of nanoparticles and the temperature of dielectric layer are considered. The experimental results manifest that the contact angle reduction rate of SiO2-water nanofluid is gradually increased with the increase of nanofluid concentrations from 0 to 0.05 wt%. While, it is decreased when the concentration is varied from 0.05 to 0.25 wt%. On the other hand, the contact angle reduction rate of Al2O3-water nanofluid is generally greater than SiO2-water nanofluid with the same volume concentration. In addition, the reduction rate of the contact angle of the SiO2-water nanofluid would be gradually increased with the increase of the surface temperature of the dielectric layer. Moreover, the experimental values are greatly deviated from the results calculated by Young–Lippmann equation and its modified form of nanofluid. Hence, the present study proposes a dimensionless surface tension correct factor to obtain the modified equation which is based on the Young–Lippmann equation. The influence of electric charge, electric field force, drag force and Brownian force on nanoparticles under high voltage are considered in the modified equation. The results show that the modified equation can predict the trend of the nanofluid contact angle under higher voltage. [ABSTRACT FROM AUTHOR]

The dynamic model of nanoparticle in the basefluid under the electric field is proposed to simulate the motion and track zone characteristics of nanoparticle in the basefluid under the electric field. The effects of electric field, temperature, zeta potential and radius of nanoparticle are discussed in the present study. Besides, the heat transfer enhancement mechanism of nanofluid under the electric field is analyzed. The results show that: the magnitude velocity of nanoparticle in the basefluid is enhanced little by the electric field. While, the track zone range of nanoparticle is enhanced greatly. Specifically, the track zone of nanoparticle under the electric field is increased by increasing the electric field intensity, temperature and zeta potential. However, that is decreased by increasing the radius of nanoparticle. By changing the direction of the electric field in a certain time, the track zone of nanoparticle can be controlled to limit in a certain range. The energy transport and thermal conductivity enhancement mechanism of nanofluid under the electric field is also analyzed in the present study. [ABSTRACT FROM AUTHOR]

The coupled effect of nanofluid and electric field contributes better heat transfer on the microgroove heat sink that has considerable prospects for addressing the heat dissipation of mobile devices. However, the wettability and the electric field enhanced heat transfer limit (EHL) of microgroove at high heat flux are still unknown. Thus, the wettability and EHL of Al 2 O 3 -water nanofluid in the vertical microgroove under electric field and the capillary wetting length prediction model were experimentally studied and developed. Accordingly, the maximum enhancement of electric field on nanofluids with optimal concentration would be reached at high heat flux, which is 1.6 times than that on pure water and it presents the wall temperature difference of 20.1 °C. Moreover, drying up phenomenon occurs at high nanofluid concentrations can be retarded by increasing the voltage. The increase of electric field reduces the clogging of nanoparticles to avoid the drying up phenomenon in microgrooves, which has greatly promoted heat transfer and reduced the wall temperature. The increase of electric field reduces the clogging of nanoparticles, and the replenished working fluid can retard the drying up. Besides, the prediction model of capillary wetting length agrees well with the experimental value. [ABSTRACT FROM AUTHOR]

Compared with conventional surface absorption collectors, nanofluid-based direct absorption solar collectors (DASC) have superior optical absorption performance and photothermal conversion efficiency. However, problems such as nanoparticles deposition and large temperature difference inside the collector still limit the development of DASC systems. To solve these problems, this paper firstly proposed to apply the electric field to manipulate the motion of nanoparticles to make the upper and lower nanofluids to exchange heat in the DASC system. The Al 2 O 3 -thermal oil nanofluids with the concentration of 0.01–0.3% are prepared, and the transmittance of the nanofluids and photothermal conversion as well as mechanism are performed and analyzed. The results show that the electric field can reduce the temperature difference inside the collector and improve the photothermal conversion efficiency of the DASC system. The temperature rise and photothermal conversion efficiency of 0.2 vol% Al 2 O 3 nanofluid at voltage of 10 kV are 62.11 °C and 87.85%, which are 19.24% and 14.17% larger than those without electric field respectively. Under the action of electric field, the resuspension of deposited nanoparticles and heat transfer between upper and lower nanoparticles work together to improve the photothermal conversion efficiency of DASC system. [ABSTRACT FROM AUTHOR]

The droplet evaporation phenomenon is very important for the development of droplet microfluidic technology. The existing studies have shown that high frequency and high electrical field strength have great effects on droplet evaporation and wetting characteristics respectively. However, the effects of high frequency together with high voltage on nanofluid droplet evaporation and wetting characteristics have not been studied and the mechanism is not clear. Therefore, the evaporation characteristics of nanofluid droplets at high voltage and high frequency were experimentally investigated to improve its evaporation rate and reveal its evaporation mechanism. The results show that the evaporation rates increase first and then decrease with the increase of voltage frequency (0–2 kHz), and the frequency at the high voltage obtained the maximum evaporation rate increases (90 Hz) when compared with the low voltage condition. Besides, the effect of electrical field on nanoparticles using the deposition surface was also explored, and the results show that the motion of the particles manipulated by the electrical field forces together with the oscillatory fluctuations of the droplets result in the enhancement of the evaporation rate of the nanofluid. [Display omitted] • Frequency obtained maximum evaporation rate increases when compared to low voltage. • Evaporation rate of nanofluids increases with improvement of concentration and voltage. • Oscillatory fluctuations combined nanoparticle motion result in evaporation enhancement. [ABSTRACT FROM AUTHOR]

Nanofluids with excellent thermal properties can be used for a coolant and the heat exchange. Actually, the heat transfer between the nanoparticles and working medium benefits from the nanoparticles movement, which could be controlled actively by the electrical field with periodically changed direction (PCD electrical field). Therefore, the coupled effect of nanofluid and switching period and the prediction model were investigated. Concretely, the boiling heat transfer coefficient (BHTC) and the critical heat flux (CHF) with various switching periods of the electrical field were compared. Meanwhile, the mechanism of heat transfer enhancement was discussed. Besides, the predicted model was developed. The present study finds that the BHTC and the CHF will be further strengthened due to the coupled effect of the PCD electrical field and nanofluid. However, this enhancement has an optimal concentration. Moreover, the heat transfer performance enhancement is attributed to the continuous heat transfer by nanoparticle and the further decrease of surface tension. The prediction curves mostly coincide with experimental data, and the prediction models are proposed by considering the influence of the switching period, the nanoparticles movement, the gas-liquid interface surface tension and the dynamic thermal conductivity. [ABSTRACT FROM AUTHOR]

In the pool boiling, the deposition of nanoparticles leads to the instability of boiling heat transfer of nanofluids. While, electric field can actively control the movement of nanoparticles. Therefore, the nanoparticle resuspension behavior, boiling heat transfer of resuspended nanofluid and prediction models for boiling heat transfer of resuspended nanofluid under the electric field were studied in the present study. Concretely, the heat transfer performance under the coupling effect between electric field and resuspended nanofluid was discussed. Besides, pool boiling and critical heat flux (CHF) prediction model were proposed to research the heat transfer variation caused by the movement of nanoparticles under electric field. It is found that the nanoparticles can be resuspended by electric field, and the boiling heat transfer of resuspended nanofluid under electric field is significantly enhanced when comparing to the pure base fluid. However, there exists an optimal concentration for the best enhanced effect. Besides, the enhancement of heat transfer results from the heat exchange by nanoparticles, the reduced surface tension of gas-liquid interface and the thermal conductivity enhancement effect. The predicted curves of model agree well with the experimental data. Meanwhile, the models comprehensively consider the effect of heat exchange by nanoparticles, surface tension and thermal conductivity [ABSTRACT FROM AUTHOR]

Nanofluids with excellent thermal properties have significant advantages in the heat transfer enhancement. While, nanoparticles are easily deposited on the heat transfer interface or electrode after long-term running, which causes serious heat transfer instability. Therefore, the resuspension and natural convective heat transfer of nanofluids affected by electrical field with periodically changed direction were studied in the present study. Specifically, the variation of deposited nanoparticle layer on heat transfer surface was observed. Meanwhile, the resuspended nanofluid heat transfer characteristics were analyzed. Furthermore, mechanism of nanoparticle resuspension and heat transfer performance of resuspended nanofluid were also discussed. It is found that nanoparticles could be resuspended stably by electrical field with periodically changed direction. Meanwhile, the heat transfer performance of resuspended nanofluid under electrical field is significantly enhanced when compared to the pure base fluid, which is increased with the improvement of voltage, concentration, and direction switching time. The mechanism analysis shows that the deposited nanoparticles are resuspended by the driven of upward electric forces and drag of basefluid, and limit in a certain range by the periodically changed electrical field. Besides, A dimensionless heat transfer prediction model of nanofluid affected by electrical field with periodically changed direction was proposed, which has an acceptable reliability. [ABSTRACT FROM AUTHOR]