Xie Zhihui

Thermal insulation constructal optimization for steel rolling reheating furnace wall based on entransy dissipation extremum principle

Analogizing with the heat conduction process, the entransy dissipation extremum principle for thermal insulation process can be described as: for a fixed boundary heat flux (heat loss) with certain constraints, the thermal insulation process is optimized when the entransy dissipation is maximized (maximum average temperature difference), while for a fixed boundary temperature, the thermal insulation process is optimized when the entransy dissipation is minimized (minimum average heat loss rate). Based on the constructal theory, the constructal optimizations of a single plane and cylindrical insulation layers as well as multi-layer insulation layers of the steel rolling reheating furnace walls are carried out for the fixed boundary temperatures and by taking the minimization of entransy dissipation rate as optimization objective. The optimal constructs of these three kinds of insulation structures with distributed thicknesses are obtained. The results show that compared with the insulation layers with uniform thicknesses and the optimal constructs of the insulation layers obtained by minimum heat loss rate, the optimal constructs of the insulation layers obtained by minimum entransy dissipation rate are obviously different from those of the former two insulation layers; the optimal constructs of the insulation layers obtained by minimum entransy dissipation rate can effectively reduce the average heat loss rates of the insulation layers, and can help to improve their global thermal insulation performances. The entransy dissipation extremum principle is applied to the constructal optimizations of insulation systems, which will help to extend the application range of the entransy dissipation extremum principle.

Constructal optimization for H-shaped multi-scale heat exchanger based on entransy theory

Analogizing with the definition of thermal efficiency of a heat exchanger, the entransy dissipation efficiency of a heat exchanger is defined as the ratio of dimensionless entransy dissipation rate to dimensionless pumping power of the heat exchanger. For the constraints of the total tube volume and total tube surface area of the heat exchanger, the constructal optimization of an H-shaped multi-scale heat exchanger is carried out by taking entransy dissipation efficiency maximization as optimization objective, and the optimal construct of the H-shaped multi-scale heat exchanger with maximum entransy dissipation efficiency is obtained. The results show that for the specified total tube volume of the heat exchanger, the optimal constructs of the first order T-shaped heat exchanger based on the maximizations of the thermal efficiency and entransy dissipation efficiency are obviously different with the lower mass flow rates of the cold and hot fluids. For the H-shaped multi-scale heat exchanger, the entransy dissipation efficiency decreases with the increase in mass flow rate when the heat exchanger order is fixed; for the specified dimensionless mass flow rate M (M<32.9), the entransy dissipation efficiency decreases with the increase in the heat exchanger order. The performance of the multi-scale heat exchanger is obviously improved compared with that of the single-scale heat exchanger. Moreover, the heat exchanger subjected to the total tube surface area constraint is also discussed in the paper. The optimization results obtained in this paper can provide a great compromise between the heat transfer and flow performances of the heat exchanger, provide some guidelines for the optimal designs of heat exchangers, and also enrich the connotation of entransy theory.

Interaction mechanism among material flows, energy flows and environment and generalized thermodynamic optimizations for iron and steel production processes

Combining modern thermodynamic theories, including finite time thermodynamics, constructal theory and entransy theory, with metallurgical process engineering, a generalized thermodynamic optimization theory for iron and steel production processes is proposed. The simulation platform for the energy consumption and emissions of the iron and steel production process is built, and the evaluation method of the material flows, energy flows and environment combining exergy analysis with life cycle assessment is established. On the basis of the new theory, simulation platform and evaluation method, interaction mechanism investigations for the material flows, energy flows and environment of the elemental packages, working procedure modules, functional subsystems and whole process of the iron and steel production processes are conducted, and multi-disciplinary and multi-objective generalized thermodynamic optimizations of them are also implemented. After optimizations, the selections of the processes and technologies, the distributions of the materials and energies as well as the utilizations of the residual energies and heats are more reasonable. The systems of the whole process are integrated, and the material flows, energy flows and environment are synthetically coordinated. Finally, the efficient allocation of the energies and the cascade utilization of the residual energies are realized, and the energy consumption and emissions of the whole system are significantly decreased. This paper can provide theoretical supports for the designs and operations of the energy and environmental protection center of the iron and steel enterprises by exploring the efficient, energy-saving and low emission technologies of the iron and steel production processes. It also can provide research platforms and lay science and technology bases for solving the common efficient energy-saving problems of the general material transformation processes.