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In this paper, the stagnation line heat transfer characteristics and critical heat flux (CHF) of small slot jet impingement boiling were experimentally investigated. Water with the Reynolds number range 34,013–136,054, the size of slot jet nozzle of 12 mm × 1 mm, and the non-dimensional nozzle to target spacing of 0.2 were used. Besides, heat transfer surface sizes were 5 mm × 1 mm and 1.5 mm × 1 mm. It was found that the stagnation line heat transfer characteristics of the jet impingement nucleate boiling in the fully developed region are irrelevant to jet velocity, subcooling and heater size for the slot jet impingement in the present experimental range. The heat transfer correlation of nickel foil heater for the jet impingement nucleate boiling in the fully developed region was obtained. Meanwhile, the CHF of the jet impingement boiling on the stagnation line was also studied. Using the experimental data to modify the semi-empirical and semi-theoretical correlation proposed previously for predicting the CHF, its modified form with heater size effect was also obtained. Through increasing jet velocity and subcooling while decreasing heater size, extremely critical heat fluxes (>10 8 W/m 2 ) were achieved in the experiment, and its maximum value was 1.14 × 10 8 W/m 2 . In addition to decreasing heater size to a moderate value, it is also necessary to change heater’s surface features including CA to achieve a higher heat flux. [ABSTRACT FROM AUTHOR]

Laminar and turbulent forced convection heat transfer and flow characteristics of nanofluids in small smooth tubes are numerically simulated using two kinds of multiphase-flow models. The simulated results are compared with the experimental results from the published papers and the traditional predicting correlations to investigate the applicability of these models for nanofluids. The multiphase-flow models including mixture model and Eulerian model, and both of them belong to the well-known Euler–Euler model. The effects of various parameters such as Reynolds number and nanoparticles concentration on the heat transfer and flow characteristics are investigated and discussed in each model. The study results show that little deviation exists between the simulated results and the traditional predicting correlations for low concentration nanofluid, which indicates that low concentration nanofluid has no meaningful nano-effect on forced convection heat transfer. While, non-traditional fluid characteristics occur and increase with increasing the nanoparticles concentration, and the simulated results using special models of multiphase flow are closer to the experimental data than that of the traditional correlations, which means the multiphase flow models are more accurate than traditional correlations for high concentration nanofluid. Moreover, the numerical results also indicate that the drag coefficients of simulated results have only little difference less than 0.4% with that of experimental results for nanofluids in the laminar flow region. However, the drag coefficients of simulated results have a increase by about 60–22% compared with the experimental results in the turbulent flow region. As conclusion, the present study indicates that the two-phase models, including mixture model and Eulerian model, can predict the forced convection heat transfer and flow characteristics of nanofluid well, and have important implications for the application of nanofluid. [ABSTRACT FROM AUTHOR]

Abstract: The jet impingement boiling is the most effective heat transfer method among all the steady-state heat transfer forms. However, various researches on the jet impingement boiling still lack experimental data for heat transfer characteristics under high impact velocity and special heated surface topography. Due to this reason, the present experimental study investigates the heat transfer characteristics of high-velocity circular jet impingement boiling on the nano-characteristic surface of the stagnation zone with different surface wettability. The wettability of the copper surface is varied to hydrophilic or hydrophobic nano-characteristic surface by modifying surface topography and chemistry. The effects of impact velocity, subcooling and solid–liquid contact angle (CA) on the onset of nucleate boiling (ONB), nucleate boiling heat transfer coefficient (HTC) and critical heat flux (CHF) are studied experimentally. It is found that the effects of impact velocity, subcooling and CA on the ONB are respectively independent. Comprehensive effects of impact velocity and CA on the nucleate boiling HTC are also obtained. The proportion of nucleate boiling HTC after the ONB point gets more significant with increasing wall superheat. The HTC will increase with increasing impact velocity and CA. Some empirical correlations are proposed for predicting the effects of impact velocity, subcooling and CA on the jet boiling heat transfer characteristics. What’s more, the method for predicting the wall superheat corresponding to the CHF is discussed. The comparison results indicate the predicted value agrees well with the experimental data. [Copyright &y& Elsevier]

Abstract: An experimental study was conducted to investigate the thermal performances of a new type of copper wire-bonded flat heat pipe using water, ethanol and nanofluids as working fluids. The influences of the working fluid, the operating temperature or operating pressure, the wire diameter and the space gap between two wires on the thermal performances of the heat pipe were investigated. Experimental results show that the wire diameter has an important effect on the heat transfer coefficient but has hardly impact on the maximum heat flux. The space gap between two wires not only has an important effect on the heat transfer coefficient but also has a significant impact on the maximum heat flux. The maximum heat power increases apparently with increasing the operating temperature. Using nanofluid as working fluid can improve the heat transfer performance of the heat pipe; the best heat transfer performance of heat pipe is achieved at the concentration of 1.0wt.% under different saturation temperature conditions with different nanofluid; nanofluid is a kind of new working fluid to the copper wire-bonded flat heat pipe. [Copyright &y& Elsevier]

Abstract: A new water-based SiO2 functionalized nanofluid was used in a loop thermosyphon as the working fluid. Functionalized nanofluid has unique dispersing ability and has no deposition on heated surfaces. The flow evaporating heat transfer characteristics of functionalized nanofluid in a loop thermosyphon were investigated to understand the sole effect of thermal properties of nanofluid on the heat transfer characteristics of the loop thermosyphon, regardless of the effects of heated surface characteristics. Results show that: functionalized nanofluid deteriorates both the evaporating heat transfer coefficient and the maximum heat flux of the loop thermosyphon. The changes in the thermal properties of functionalized nanofluid mainly result in the heat transfer deterioration. No especial nano-scale effect is found in the present study. [Copyright &y& Elsevier]

Highlights • The natural heat transfer of nanofluids was investigated under electric field. • The electric field forces can reduce a strong electric convection. • In the absence of gravity, the EHD effect on heat transfer is significant. • In the existence of gravity, the EHD effect on heat transfer is limited. Abstract Natural convective heat transfer of nanofluid consisted of transformer oil and Al 2 O 3 nanoparticles in a cavity under a non-uniform high electric field intensity was experimental investigated. The main objective is looking for a coupling enhanced method by nanofluid and electric field. It is found that when the effect of gravity is ignored, the electric field forces applied on the working fluids induce an electric convection and thence induce significant heat transfer enhancement. This enhancement effect increases rapidly with the increase of electric field intensity, the nanoparticles mass concentration of nanofluid and the heat flux. When the effect of gravity exists, a coupled convection is caused by both gravity and electric field force together. For pure transformer oil, the influences of electric field force and gravity on the heat transfer almost offset each other. However, for nanofluid, the impact of the electric filed force is dominant. The study results revealed the enhanced mechanism of natural convection heat transfer of nanofluid under electric field and provided some useful technical support for practical application. [ABSTRACT FROM AUTHOR]

Nanofluid has attracted the attention of many scholars for the excellent heat transfer enhancement capacity. In addition, electric field can further strengthen the nanofluid’s ability of heat transfer. The thermal conductivity enhancement rate of nanofluid under electric field was studied using transient hot wire method under different electric field intensities, temperatures and particle concentrations. The influences of those parameters have been discussed. Besides, based on the static thermal conductivity and the dynamic heat conduction coefficient, a novel prediction model was established, which had a good agreement with experiment data. The mechanism of thermal conductivity enhancement under electric field was analyzed according to the prediction model. [ABSTRACT FROM AUTHOR]

The heat transfer characteristics of transformer oil natural convection in a cavity with a linear electrode were investigated by experiment and numerical simulation. The coupled physical and mathematical models of electric field, flow field and temperature field were established by the lattice Boltzmann method. The experimental results are in good agreement with the numerical simulation. In the absence of gravity, the enhancement effect of electric field on heat transfer is significant, and increases with the increase of voltage and heat flux. As to coupled convection caused by gravity and electric field force together, the electric field force destructs the natural convection caused by gravity, so applying an electric field at a low voltage will degrade heat transfer performance; as the voltage increases, heat transfer gradually becomes intensive, but the effect is limited. The study results revealed the enhancing mechanism of natural convection heat transfer under electric field and provided useful technical support for practical application. [ABSTRACT FROM AUTHOR]