| 1. |
- Buschmann, M. H., et al.
(författare)
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On the proper interpretation of nanofluid convective heat transfer
- 2018
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Ingår i: Proceeding: International Heat Transfer Conference 16. - 2377-424X. ; , s. 2855-2862
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Konferensbidrag (refereegranskat)abstract
- Technological developments of the last decades allow the production and the dispersion of particles of sizes ranging between 10 and 100 nm in liquids. In a large number of recent studies the resulting nanofluids have been reported to display anomalously high increase in convective heat transfer. The present study compiles experiments from five independent research teams investigating convective heat transfer in nanofluid flow in pipes (laminar and turbulent), pipe with inserted twisted tape, annular counter flow heat exchanger, and coil and plate heat exchangers. The results of all these experiments unequivocally confirm that Newtonian nanofluid flow can be consistently characterized by employing Nusselt number correlations obtained for single-phase heat transfer liquids such as water when the correct thermophysical properties of the nanofluid are utilized. It is also shown that the heat transfer enhancement provided by nanofluids equals the increase in the thermal conductivity of the nanofluid as compared to the base fluid independent of the nanoparticle concentration or material. These results demonstrate that no anomalous phenomena are involved in thermal conduction and forced convection based heat transfer of water based nanofluids. The experiments are theoretically supported by a fundamental similarity analysis of nanoparticle motion in nanofluid flow.
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| 2. |
- Cao, Zhen, et al.
(författare)
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Enhancement of HFE-7200 pool boiling heat transfer on copper surfaces with nanoparticle coatings
- 2018
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Ingår i: Proceeding: International Heat Transfer Conference 16. - 2377-424X. ; , s. 1341-1347
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Konferensbidrag (refereegranskat)abstract
- Saturated pool boiling heat transfer of HFE-7200 is investigated experimentally on copper surfaces with nanoparticle coatings at atmospheric pressure. The coatings are generated by an electrophoretic deposition method. Two modified surfaces are prepared with Cu-Zinc nanoparticles of 0.3 mg and 0.6 mg, respectively. During the deposition, ethanol works as the solvent while the electrical potential and deposition time are controlled as 9.5 V and 30 min, respectively. The experimental results show heat transfer is considerably enhanced by the nanoparticle coatings. The surface with 0.6 mg nanoparticles (EDS-2) performs better than the surface with 0.3 mg nanoparticles (EDS-1), and a maximum 140% heat transfer enhancement is achieved on the surface EDS-2 compared with the SS. However, the critical heat flux is not enhanced by the coatings but even slightly decreased. A high speed visualization is employed to capture bubble behavior. It is found that bubbles on EDS-1 and EDS-2 have smaller sizes and higher departure frequency than those on the SS before reaching the critical heat flux. However, at critical heat fluxes, a vapor blanket appears on all surfaces.
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| 3. |
- Hussain, Safeer, et al.
(författare)
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Experimental investigation of heat transfer enhancement on the endwall around an obstacle embedded with a vortex generator pair
- 2018
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Ingår i: Proceeding: International Heat Transfer Conference 16. - 2377-424X. ; , s. 4971-4978
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Konferensbidrag (refereegranskat)abstract
- Heat transfer enhancement on the endwall around an obstacle embedded with a pair of vortex generators is investigated. Obstacles with cylindrical and square cross sections are experimentally examined and compared with and without vortex generators. The vortex generators employed is a pair of longitudinal winglet vortex generator. Local Nusselt numbers are calculated for both types of obstacles in presence of vortex generator experimentally whereas, flow features are captured numerically using ANSYS-FLUENT 17.0. A high heat transfer region upstream of the obstacle has increased its width with the installation of vortex generators for both shapes of the obstacle. However, only a small increase in magnitude occurs. Comparison of local Nusselt numbers shows that for the case of the cylindrical obstacle, there is more heat transfer at the center of the obstacle whereas, for the obstacle with square cross section, more heat transfer is observed at the upstream corners of the obstacle both with and without vortex generators. It is also evident from the findings of the results that installation of vortex generators upstream of an obstacle enhances the endwall heat transfer more in the spanwise direction than the streamwise direction. In addition, flow characteristics obtained from the numerical simulations show that presence of a vortex generator pair reinforces the strength of the horseshoe vortex and as a result additional heat transfer enhancement. Thermal performance for both types of obstacles in presence of vortex generators are calculated and compared at various Reynolds number.
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| 4. |
- Liu, Jian, et al.
(författare)
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Investigation of heat transfer and fluid flow in fractal truncated ribbed channels for the internal cooling of turbine blades
- 2018
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Ingår i: Proceeding: International Heat Transfer Conference 16. - 2377-424X. ; , s. 3239-3246
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Konferensbidrag (refereegranskat)abstract
- Ribs are widely employed in internal cooling passages of turbine blades to enhance heat transfer. From the mathematics, a fractal is an abstract object used to describe naturally occurring objects which exhibits similar properties with increased smaller scales. In a ribbed channel, originating from two sides truncated ribs, fractal truncated ribs are built and the evolving process contains three sides truncated ribs, five sides truncated ribs, nine sides truncated ribs and seventeen sides truncated ribs. Steady Liquid Crystal Thermography (LCT) is employed to measure surface temperature and derive heat transfer coefficients over the ribbed surfaces in the tested channels. Turbulent flow details are presented by numerical calculations with an established turbulence model, i.e. the k-ω SST model. From the study, truncated ribs can enhance the heat transfer at the truncated gap regions. With the scale of fractal truncated ribs becoming smaller, the low heat transfer regions downstream the ribs are effectively reduced. With the evolving of the fractal truncated ribs, the shape of high heat transfer region behind single truncated part keeps similar. The truncated ribs change cause enhancement vortices in the spanwise direction and enlarge local heat transfer. The truncated ribs enhance heat transfer by reducing the recirculating flows and the cases with smaller scales have more uniform heat transfer distributions. With the scale of the fractal truncated ribs becoming smaller, the flow related terms, such as, pressure field, turbulent kinetic energy and vorticity magnitude, also become more uniform.
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| 5. |
- Qian, Jin Yuan, et al.
(författare)
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Pressure fluctuations of liquid-liquid slug flow in cross-junction square microchannels
- 2018
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Ingår i: Proceeding: International Heat Transfer Conference 16. - 2377-424X. ; , s. 7231-7241
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Konferensbidrag (refereegranskat)abstract
- Microchannels are widely used for heat transfer enhancement. The pressure characteristics are one of the most important factors affecting the heat transfer performance, and the pressure regulation may also control the heat transfer precisely in turn. As a common flow pattern of two-phase flow, slug flow has obvious advantages for heat and mass transfer, like steady flow rate and large interface area etc. Due to the interface stress and the velocity difference between the continuous phase and the dispersed phase, pressure fluctuations occur in both the flowing direction and the cross section. In oil and natural gas industries, pressure fluctuations of slug flow can be used for the slug size and velocity measurement, and also to analyze the principle of slug generation. However, when it comes to micro scale, pressure fluctuations of slug flow in microchannels is difficult to measure, not only due to the pressure difference within a low pressure range (10 to 103 Pa), but also because of the small size, for which conventional pressure sensors cannot be utilized. In this paper, a numerical method is adopted for the liquid-liquid slug flow (butanol/water) pressure prediction in a cross-junction square microchannel. To begin with, the validation of the numerical method is carried out by comparing the slug size with experiments under the same working conditions. Then, both pressure fluctuations in the flow direction and in the cross section are investigated. With a transient flow model, pressure fluctuations in the cross section at different flowing time are observed. Finally, effects of the dispersed phase (butanol) injection velocity on pressure fluctuations are performed. This work can be used for further study of the slug generation in microchannels.
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| 6. |
- Wu, Xinghui, et al.
(författare)
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Simulation of heat transfer in the contact line region of pure pentane and pentane/hexane mixture in a closed microcavity
- 2018
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Ingår i: Proceeding: International Heat Transfer Conference 16. - 2377-424X. ; , s. 1945-1955
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Konferensbidrag (refereegranskat)abstract
- Understanding the heat transfer characteristics in the three-phase contact line region has great significance in achieving efficient evaporation/condensation. Phase change becomes especially complicated when a fluid mixture is used. The dynamics of phase change heat and mass transfer in the three-phase contact line region of binary fluid is still not clear. Based on the volume of fluid (VOF) model and the continuum surface force (CSF) model, the heat transfer characteristics of pure pentane and a pentane/hexane mixture in the contact line region of a closed microcavity was numerically investigated. The VOF model is used with source terms added by user defined functions (UDF). A thermal equilibrium model assuming the interface temperature is at saturation was adopted. The effects of fluid properties, contact angle and superheat on heat transfer in the contact line region were discussed. By comparing the heat transfer flux under different conditions, the heat transfer characteristics in the contact line region were investigated. The results indicate that the heat transfer capability of pentane/hexane mixture is superior to pure pentane in the contact line region. The contribution by contact line region to heat transfer rate increases substantially as the wettability and superheat of the binary fluid increases. The thin film profile, interfacial temperature, mass flux and integrated heat transfer near the contact line was also presented.
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