136 research outputs found
A review on heat transfer enhancement with nanofluids
No full textAdvances in technology miniaturization with increasing power density call for new technologies for enhancing heat transfer. Enhancement of heat transfer with use of nanofluids has been a hectic topic of research and development since the term “nanofluid” named in 1995, mainly because the thermophysical properties of nanofluids in most reports in the literature showed supremacy or improvement over their base fluids that may not allow the fulfilment of the present cutting-edge technology needs. Significant progress in this field has been made in the past two decades. This review summarizes a variety of the experimentally-measured thermal properties of common nanofluids, the enhancement mechanisms discovered or hypothesised, the models used for properties and heat transfer characteristics, and the applications of nanofluids for enhancing heat transfer. The model of artificial neutral network is particularly emphasized. Applications to cooling technology, renewable energy and energy systems, and building technology are detailed. Challenges and areas for future research are identified.Peer reviewe
Monitor in-situ superconducting temperature via optical whispering-gallery mode sensors
No full textMonitor in-situ superconducting temperature via optical whispering-gallery mode sensorsPeer reviewe
High thermal conductance across c-BN/diamond interface
No full textHigh thermal conductivity electronic components with low interfacial thermal resistance are of technological importance and fundamental interest of research. Diamond, a superhard material with ultrahigh thermal conductivity at room temperature, is desirable for microelectronics thermal management. Cubic polymorph of boron nitride (c-BN) is a promising material due to wide bandgap and diamond like structure and properties. To understand the nature in thermal transport of diamond, c-BN and the most commonly used silicon (Si) semiconductor, ab initio phonon Boltzmann transport equations are employed to investigate lattice vibrational properties of these three materials. At 300 K, the predicted thermal conductivity of Si, diamond and c-BN reached 142, 2112, and 736 W/(m·K), respectively. What's more, heat transport phenomena across the interfaces of Si/diamond, c-BN/diamond and Si/c-BN are unfolded. In comparison, the interfacial thermal conductance of c-BN/diamond is ten-fold of Si/diamond; besides, the thermal conductance across Si/c-BN interface is 20.2% larger than that of Si/diamond at 300 K and 18.9% larger at 340 K. These findings provide us new vision and potential solution to heat dissipation of high-local-power density devices, shedding light on future thermal management of c-BN and diamond related electronics.Peer reviewe
Enhancement of hot spot cooling by capped diamond layer deposition for multifinger AlGaN/GaN HEMTs
No full textThe impact of a capped diamond layer for enhanced cooling of multi-finger AlGaN/GaN high-electron-mobility-transistors (HEMTs) has been investigated under steady-state operating condition. By depositing a capped diamond thin film onto the HEMTs, the temperature distribution around the hot spots tends to be more uniform and the junction temperature can be suppressed significantly. The capped diamond serves as a highly effective heat spreader and its thermal spreading ability depends on the structural design patterns and working conditions. Some key parameters affecting the thermal performance of the capped diamond have been examined, including the heat dissipation power density, gate pitch distance, embedding depth of the heat source, thermal boundary resistance, substrate material as well as the cap thickness. For the twelve-finger model with 20 µm gate pitch distance and gate power density 6 W/mm, a 20 µm layer of capped diamond could reduce the junction temperature by 12.1% for GaN-on-Diamond HEMTs and by 25.3% for GaN-on- SiC HEMTs, respectively. Even with a 1 µm capped diamond layer, the reduction would be 7.6% and 9.9%, respectively. The temperature reduction for GaN-on-Si is more significant.Peer reviewe
Natural convection and radiation heat transfer of an externally-finned tube vertically placed in a chamber
No full textA three-dimensional numerical study was made to investigate effects of fin angle, fin surface emissivity, and tube wall temperature on heat transfer enhancement for a longitudinal externally-finned tube placed vertically in a small chamber. The numerical model was first validated through comparison with experimental measurements and the appropriateness of general boundary conditions was examined. The numerical results show that the mean Nusselt number increases with Rayleigh number for all the fin angles investigated. The maximum heat transfer rate per mass occurs when the fin angle is about 60° for fin surface emissivity between 0.7 and 0.8 and 55° when the surface emissivity increases to 0.9. With increasing tube wall temperature, both the natural convection and radiation heat transfer are enhanced, but the fraction of radiation heat transfer decreases in the temperature range studied. Radiation fraction increases with increasing fin surface emissivity. Both convection and radiation heat transfer modes are important.Peer reviewed
Heat transfer enhancement - a brief review of 2018 literature
No full textThis article is a review of literature on heat transfer enhancement research published in 2018 in the English language. A topic search using “heat transfer” in the Web of Science resulted in about 17,000 articles published in 2018, of which nearly 4600 were relevant to heat transfer augmentation. Thus, some selection is inevitable. The included studies are grouped into the fields identified in the Aims and Scope of the Journal of Enhanced Heat Transfer, which considers a wide range of scholarly articles related to the subject of “enhanced heat and mass transfer” in natural and forced convection, phase-change heat transfer, conduction and radiative heat transfer, and the general topic of “high performance” heat transfer concepts, devices, or systems (e.g., heat exchangers and heat pipes)
Spectral investigation of solar energy absorption and light transmittance in a water-filled prismatic glass louver
No full textWater-filled prismatic glass louver was proposed to save energy consumptions in buildings because such innovative louvers can harvest solar energy as well as improve daylighting quality rather than “block” sunlight like traditional louvers. To enable this technology the effectiveness of ultraviolet (UV) and infrared (IR) energy harvest and visible (VIS) light transmittance was investigated via Monte Carlo simulations in this case study. The 7-band spectral model for glass and water was evaluated and adopted for several cases of solar spectra of different air mass (AM) coefficients with both direct and diffuse irradiation. Absorption and transmittance in different band regimes as well as in water and glass respectively were differentiated and compared. Practical solar data in Phoenix, Flagstaff, and Golden were utilized to demonstrate the performance of the proposed louver under different locations and realistic conditions. Results show that the device facing normally to direct sunlight can harvest around 51-54% of the total solar energy and transmit 74-76% VIS for daylighting in the range of AM1 to AM3. In particular for AM1.5, VIS transmittance reaches 76% for both direct and diffuse irradiation; UV absorption achieves 80% and 85% and IR absorption reaches 64% and 82% for diffuse and collimated irradiation, respectively. In all the three places tested, the device absorbs about 81% IR and 87% UV, and transmits about 76% VIS.Peer reviewe
Prediction of Self-Ignition Fire Propagation and Coal Loss in An Inclined Seam
No full textThe thermo-chemical processes of coal spontaneous combustion in a practical inclined outcrop seam were investigated in order to understand underground mineral self-ignition, fire propagation, and reserves loss. A heat and mass transfer model of porous coal-bearing stratum was employed, combining convection and radiation with transient exothermic source which is coupled with coal oxidation, oxygen supply, and fuel consumption. It is found that spontaneous combustion firstly occurs under lean oxygen condition and fire development controlled by the reaction heat release in the early oxidation process shifts to oxygen restriction after coal self-ignition. The stratum porosity affects significantly the fire propagation. Fire propagation rate slightly increases as the inclined angle decreases. Compared with indirect surface survey, the predicted reverses loss is more reasonable; and thus, the present model could provide a useful reference to loss estimation in coal fire hazards.Peer reviewe
Reduction of Angle Splitting and Computational Time for the Finite Volume Method in Radiative Transfer Analysis via Phase Function Normalization
No full textThe commonly implemented splitting of solid angles to ensure scattered energy conservation in the finite volume method does not exactly conserve phase function asymmetry factor after directional discretiza- tion, leading to significant changes in scattering effect for radiative transfer analysis in highly anisotropic scattering media. In addition, use of a large number of split sub-angles results in drastic increases in com- putational CPU time and computer memory. The phase function normalization approach considered in this study is found to guarantee accurate conservation of both scattered energy and asymmetry factor simultaneously after directional discretization as well as depress solid angle splitting, vastly reducing the computational convergence time with improved accuracy. As a test, radial and axial radiative heat flux profiles in a scattering cylinder generated both with and without the phase function normalization are compared among different levels of angle discretization and splitting as well as with the discrete- ordinates method. The effects of changes in optical thickness, angular resolution, scattering albedo, and phase function approximation are examined.Peer reviewed
Enhanced conduction and pool boiling heat transfer on single-layer graphene-coated substrates
No full textMolecular dynamics simulations were employed to understand the improved thermal conductivity and water boiling heat transfer characteristics of adding single-layer graphene (SLG) to substrates. The 100, 110, and 111 planes of Cu, Ni, Pt, and Si were selected for study based on common heat transfer and graphene-compatible materials. Vibrational density of states data was analyzed in order to view heat flux trends. After equilibration at 300K the temperature was increased to 400K for 3 ns to induce nucleate boiling (~27K wall superheat). It was found that the addition of SLG greatly improved the overall thermal conductivity of the composite substrate, with increases in the 1-2 orders of magnitude range. The temperature gradients for SLG-coated substrates were found to be much lower than bare substrates. Nanoscale boiling curves were produced. The CuG100 case shows a 14% increase in critical heat flux (CHF) (~0.36 GW/m2) over the Cu100 case, and the PtG100 shows a 9% increase (~0.48 GW/m2) over the Pt100 case. The SLG-coated substrates also required less superheat to achieve the CHF condition.Peer reviewe
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