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Guo J.,CAS Institute of Engineering Thermophysics
Applied Energy | Year: 2016

The segmental design method is employed to accurately capture the drastic variations of properties in the supercritical carbon dioxide (S-CO2) recuperator. The local heat capacity flow rates of both fluids have drastic changes in sub-heat exchangers even the mass flow rates of both fluids remain unchanged. When the heat duty is given, the local heat conductance, local temperature difference, local effectiveness and local entransy dissipation number have extremums, which appear in the vicinity of the heat capacity rate ratio is one. The heat transfer performance of recuperator improves at the expense of heat conductance. When the total heat conductance is fixed, there exist the maximum local heat flux rates, and the local effectiveness tends to be constant after the local heat capacity rate ratio reaches one, the local entransy dissipation number has the relatively coherent performance behavior in the sub-heat exchangers. The local heat capacity rate ratio has crucial influences on the heat transfer performance of recuperator, and the design parameters must be considered carefully in the design of S-CO2 recuperator. © 2015 Elsevier Ltd.

Ren Q.,CAS Institute of Engineering Thermophysics
Journal of Thermal Analysis and Calorimetry | Year: 2013

Biomass is an effective substitute for fossil fuels and has a substantial impact on CO2 reduction. Meanwhile, alkali metal-related problem is a major barrier in biomass combustion. Sludge is a waste with low heating value, high water, sulfur, and nitrogen contents, as well as high phosphorus content, which makes the sludge a potential choice to inhibit the alkali metal-related problems during biomass combustion. The nitrogen behavior and the emissions of NOx and N2O are one of the key problems when sewage sludge is co-fired with biomass. Thus, nitrogen transformation during co-pyrolysis of cotton stalk and sludge at different heating rates were studied. The results show that HCN and HNCO are the major nitrogen-containing species for petrochemical sludge. The addition of petrochemical sludge changes the path of the conversion of fuel nitrogen and the presence of cotton stalk in the mixture promotes NH3 and HCN formation. © 2012 Akadémiai Kiadó, Budapest, Hungary.

Hu M.,ETH Zurich | Zhang X.,CAS Institute of Engineering Thermophysics | Poulikakos D.,ETH Zurich
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

Silicene - the silicon counterpart of graphene - has a two-dimensional structure that leads to a host of interesting physical and chemical properties of significant utility. We report here an investigation with nonequilibrium molecular dynamics simulations of thermal transport in a single-layer silicene sheet under uniaxial stretching. We discovered that, contrary to its counterpart of graphene and despite the similarity of their honeycomb lattice structure, silicene exhibits an anomalous thermal response to tensile strain: The thermal conductivity of silicene and silicene nanoribbons first increases significantly with applied tensile strain rather than decreasing and then fluctuates at an elevated plateau. By quantifying the relative contribution from different phonon polarizations, we show first that the phonon transport in silicene is dominated by the out-of-plane flexural modes, similar to graphene. We attribute subsequently the unexpected and markedly different behavior of silicene to the interplay between two competing mechanisms governing heat conduction in a stretched silicene sheet, namely, (1) uniaxial stretching modulation in the longitudinal direction significantly depressing the phonon group velocities of longitudinal and transverse modes (phonon softening) and hindering heat conduction, and (2) phonon stiffening in the flexural modes counteracting the phonon softening effect and facilitating thermal transport. The abnormal behavior of the silicene sheet is further correlated to the unique deformation characteristics of its hexagonal lattice. Our study offers perspectives of modulating the thermal properties of low-dimensional structures for applications such as thermoelectric, photovoltaic, and optoelectronic devices. © 2013 American Physical Society.

Guo J.,CAS Institute of Engineering Thermophysics | Xu M.,Shandong University of Science and Technology
Applied Thermal Engineering | Year: 2012

The optimization of heat exchanger design is investigated by applying the entransy dissipation theory and genetic algorithm. It is found that the role played by the fluid friction is not fully taken into account when the working fluid of heat exchanger is liquid in single-objective optimization approach. In order to circumvent this problem, a multi-objective optimization approach to heat exchanger design is established. © 2011 Published by Elsevier Ltd. All rights reserved.

Ren Q.,CAS Institute of Engineering Thermophysics
Journal of Thermal Analysis and Calorimetry | Year: 2014

Chlorine content in agricultural straw is high, and HCl formation during straw combustion is a challenging problem. The relationship between HCl and the formation of NOx and N2O is important and unclear. Effect of HCl in atmosphere on nitrogen transfer during wheat straw and cotton stalk pyrolysis was performed using a thermogravimetric analyzer coupled with a Fourier transform infrared spectrometer. Pyrolysis of polyvinyl chloride supplies HCl. The pathway of nitrogen transfer in the presence of HCl was studied. The results show that in the presence of HCl, the temperature corresponding to NH3 starting release during wheat straw pyrolysis increases, and those of HCN and HNCO reduce. HCl inhibits the conversion of straw-N into NH3, however, favors the transformation of straw nitrogen into HCN and HNCO. © 2013 Akadémiai Kiadó, Budapest, Hungary.

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