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Microchannels offer unique advantages on heat transfer performance. In this paper, microchannels are applied to the compound parabolic concentrator (CPC) system. A multi-physical field coupling model based on Finite Element Method is proposed to investigate the homogenization effect of the microchannel heat absorber on the CPC non-uniform concentration. The energy conversion process from optics to heat is simulated using TracePro software, and the heat transfer processes in the microchannel are computed by Fluent using user defined functions (UDF). It is found that the microchannels behave well on weakening the influence of the nonuniformity solar heat flux on the performance of the CPC. The temperature nonuniformity of the outlet section is less than 10−3 in the direction of fluid flow caused by the microchannel, although the maximum surface heat flux inhomogeneity of the microchannel reaches 2.3. The peak value of the heat flux on the surface of the absorber changes from double peak to single peak, and moves to the edge, resulting in more uneven heat flux distribution with the increase of the incident angle within the acceptance semi-angle of the CPC. The result of TracePro clearly shows that when the concentration ratio is less than 5, the heat flux nonuniformity on the surface of the absorber decreases with the increase in concentration ratio. It was interestingly found that the temperature distribution of the heat transfer fluid has weak sensitivity to the changes of truncation ratio. This work provides a way to design a CPC solar collector. [ABSTRACT FROM AUTHOR]

Adsorption separation technology has great potential to separate H2O/CO2 mixture with 1–5% H2O to achieve high CO2 purity for industrial applications. However, there are few studies analyzing the concentration and velocity fields in an adsorption column and the characteristics of H2O/CO2 adsorption separation. A heat and mass transfer model of H2O/CO2 adsorption on aluminum activated F-200 was developed and validated experimentally to predict the breakthrough curves and the H2O/CO2 distributions in the gas and solid phases. The effects of mixture inlet velocity, inlet temperature, steam mole fraction and total pressure on the separation performance have been investigated numerically. As velocity increases, the breakthrough and equilibrium times decrease. The peak temperature gradually increases with increasing H2O concentration, temperature and pressure. Along the height of the adsorption column, the adsorption ratio, the ratio of the amount adsorbed to the equilibrium amount adsorbed first decreases and then increases with increasing H2O mole fraction, but first increases and then decreases with increasing pressure. The heat and mass transfer characteristics of the H2O/CO2 adsorption separation can provide a basis for high purity CO2 separation. [ABSTRACT FROM AUTHOR]

Graphite nanoplatelets (GNPs) are outstanding thermal conductive fillers due to their unique crystal structure which consists of layered graphene formed by sp2-hybridized carbon atoms, but their preparation process is time-consuming. Herein, a high-efficiency and low-cost method is developed to prepare GNPs with controlled size without destroying their crystal structure. GNPs with an average size around 20 μm are obtained by a facile nitric acid treatment within 0.5 h, and 1–5 μm GNPs are produced by controlling ultrasonic time within 6 h in cheap commercial detergent. In addition, thermal network is fabricated by combining ~ 20 μm GNPs with 1–5 μm GNPs in epoxy. The existing researches have qualitatively demonstrated that hybrid carbon fillers with obvious size differences can enhance the thermal conductivity of epoxy through size synergistic effects, but rare quantitative studies pay attention to optimize size synergistic effects. This work demonstrates that different degrees of size synergistic effects can be achieved by tuning the size of small-sized GNPs, the proportion of different sized GNPs and the content of hybrid fillers. The results show that when the hybrid filler loading is 20 wt% and the ratio of ~ 20 μm GNPs to 1.82 μm GNPs is 17:3, the composite produces the optimal size synergistic effect and thermal conductivity reaches 1.33 W/m K, increased by 739% over neat epoxy. This composite has promising application in the thermal management areas due to its high thermal conductivity and economic competitiveness. [ABSTRACT FROM AUTHOR]