Huijun Feng

Generalized Thermodynamic Optimization for Iron and Steel Production Processes: Theoretical Exploration and Application Cases

Combining modern thermodynamics theory branches, including finite time thermodynamics or entropy generation minimization, constructal theory and entransy theory, with metallurgical process engineering, this paper provides a new exploration on generalized thermodynamic optimization theory for iron and steel production processes. The theoretical core is to thermodynamically optimize performances of elemental packages, working procedure modules, functional subsystems, and whole process of iron and steel production processes with real finite-resource and/or finite-size constraints with various irreversibilities toward saving energy, decreasing consumption, reducing emission and increasing yield, and to achieve the comprehensive coordination among the material flow, energy flow and environment of the hierarchical process systems. A series of application cases of the theory are reviewed. It can provide a new angle of view for the iron and steel production processes from thermodynamics, and can also provide some guidelines for other process industries.

Exergoeconomic performance optimization for a combined cooling, heating and power generation plant with an endoreversible closed Brayton cycle

Abstract Finite time exergoeconomic performance of a combined cooling, heating and power generation (CCHP) plant composed of one endoreversible closed Brayton cycle and one endoreversible four-heat-reservoir absorption refrigeration cycle is investigated by using finite time thermodynamics. Heat conductance distribution among hot-, cold-, thermal consumer-, generator-, absorber-, condenser- and evaporator-side heat exchangers and compressor pressure ratio are optimized by taking the maximum profit rate as objective. Numerical examples show that there exists a sole group of optimal heat conductance distribution among hot-, cold-, thermal consumer-, generator-, absorber-, condenser- and evaporator-side heat exchangers and an optimal compressor pressure ratio which lead to the maximum profit rate. The effects of design parameters on the optimal performance of the CCHP plant are discussed.

Optimal piston speed ratio analyses for irreversible Carnot refrigerator and heat pump using finite time thermodynamics, finite speed thermodynamics and direct method

AbstractAn irreversible Carnot refrigeration and heat pump cycle model with external irreversibilities caused by finite rate of heat transfer, heat leakage from heat sink to heat source and internal irreversibilities caused by finite piston speed, friction and gas throttling is established by using the combination of finite time thermodynamics, finite speed thermodynamics and direct method. Different from the model of constant speed of the piston on the four branches, the piston speeds on the four branches in this model are assumed to be unequal. Expressions of cooling load and coefficient of performance (COP) of the Carnot refrigeration cycle as well as heating load and COP of the heat pump cycle are derived for a fixed cycle period. Numerical examples show that the curves of COP versus cooling load and COP versus heating load are parabolic like ones, and there exist optimal finite piston speed ratios on the four branches, which lead to the maximum COPs. Moreover, the effects of the heat leakage coeffici...

Exergy optimisation of irreversible closed Brayton cycle combined cooling, heating and power plant

AbstractA combined cooling, heating and power (CCHP) plant model composed of an irreversible closed Brayton cycle and an endoreversible four-heat-reservoir absorption refrigeration cycle is established by using finite time thermodynamics. Equations of exergy output rate and exergy efficiency of the CCHP plant are derived. Numerical examples are provided by using Powell arithmetic, and optimal heat conductance distributions among hot-, cold-, thermal consumer-, generator-, absorber-, condenser- and evaporator-side heat exchangers and compressor optimal pressure ratios are obtained by taking exergy output rate and exergy efficiency as optimisation objectives respectively. The effects of compressor and gas turbine efficiencies on the optimal exergy output rate and exergy efficiency performances are discussed. The maximum exergy output rate and exergy efficiency performances are compared with each other quantitatively. Optimal design region is obtained with considerations of exergy output rate and exergy effi...

Finite-time exergoeconomic performance optimization for a universal steady-flow endoreversible heat pump model

The finite-time exergoeconomic performance of a universal steady flow irreversible heat pump cycle model, which consists of two heat-absorbing branches, two heat-releasing branches and two adiabatic branches with the losses of heat transfer, heat leakage and internal irreversibility, is analysed and optimized by using the theory of finite-time thermodynamics. The analytical formulae for the heating load, coefficient of performance (COP) and profit rate function of the irreversible heat pump cycle model are derived. The optimal heating load, COP and profit rate characteristics are obtained by searching the optimal heat conductance distributions of the hot- and cold-side heat exchangers for a fixed total heat exchanger inventory. Moreover, analysis and optimization of the cycle performance are carried out in order to investigate the effects of heat leakage, internal irreversibility and price ratio on the performance of the cycle by using numerical examples. It is shown that the heat leakage changes the profit rate characteristics qualitatively, and internal irreversibility decreases the profit rate of the cycle quantitatively. The performance characteristics of the steady flow irreversible Diesel, Atkinson, Otto, Brayton, Dual, Miller and Carnot heat pump cycles are included in the universal results obtained herein.

Heating load, COP, exergy loss rate, exergy output rate and ecological analyses and optimisations for irreversible universal steady flow variable temperature heat reservoir heat pump cycle model

AbstractThe performance of an irreversible universal steady flow heat pump cycle model with variable temperature heat reservoirs is analysed and optimised by using finite time thermodynamics. The universal heat pump cycle consists of two heat absorbing branches, two heat releasing branches and two irreversible adiabatic branches, with the losses of heat resistance and internal irreversibility. Expressions of heating load, coefficient of performance (COP), exergy output rate, exergy loss rate and ecological function of Brayton, Otto, Diesel, Atkinson, Dual, Miller and Carnot heat pump cycles are derived. There exists an optimal heat conductance distribution and an optimal thermal capacity rate matching between the working fluid and heat reservoir which leads to maximum heating load, maximum exergy output rate and maximum ecological function respectively. Performance comparisons among heating load, COP, exergy output rate, exergy loss rate and ecological function objectives are carried out. The effects of t...

Optimal ratios of the piston speeds for a finite speed irreversible Carnot heat engine cycle

The performance of an irreversible Carnot heat engine cycle is analysed and optimized by using the theory of finite time thermodynamics based on Agrawals [2009. A finite speed Curzon-Ahlborn engine. European Journal of Physics, 30 (3), 587–592] model of finite piston speed on the four branches and Petrescu et al.’s [2002b. Optimization of the irreversible Carnot cycle engine for maximum efficiency and maximum power through use of finite speed thermodynamic analysis. In: Proceedings of ECOS’2002, 3–5 July, Berlin, Germany, Vol. II, 1361–1368] model of a Carnot cycle engine with the finite rate of heat transfer, heat leakage from heat source to heat sink and irreversibilities caused by finite speed, friction and throttling through the valves. The finite piston speeds on the four branches are further assumed to be different, which is different from the model of constant speed of the piston on the four branches. Expressions of power output and thermal efficiency of the cycle are derived for a fixed cycle period and internal entropy generation rate. Numerical examples show that the curve of power output versus thermal efficiency is loop shaped, and there exist optimal finite piston speeds on the four branches which lead to the maximum power output and maximum thermal efficiency, respectively. The effects of the heat leakage coefficient and internal entropy generation rate on the optimal finite piston speed ratios are discussed.

Effects of unequal finite piston speeds on the optimal performances of endoreversible Carnot refrigeration and heat pump cycles

The performances of endoreversible Carnot refrigeration and heat pump cycles with loss of heat resistance and finite piston speeds are analysed and optimized by using the combination of finite time thermodynamics, finite speed thermodynamics and direct method. The unequal finite piston speed model on four branches is adopted. Expressions of cooling load of endoreversible Carnot refrigeration cycle and of heating load of endoreversible Carnot heat pump cycle are derived with a fixed cycle period and unequal finite piston speeds on the four branches. Numerical examples show that there exist optimal expansion ratios, which lead to maximum cooling load and maximum heating load for the fixed coefficient of performance (COP), respectively. The maximum cooling load, maximum heating load, optimal ratios of finite piston speeds and optimal hot- and cold-side working fluid temperatures versus COP characteristics for the endoreversible Carnot refrigeration and heat pump cycles are obtained. Moreover, the effects of design parameters on the performances of the two cycles are discussed.

Constructal Optimization for Cooling a Non-Uniform Heat Generating Radial-Pattern Disc by Conduction

A heat conduction model in a radial-pattern disc by considering non-uniform heat generation (NUHG) is established in this paper. A series of high conductivity channels (HCCs) are attached on the rim of the disc and extended to its center. Constructal optimizations of the discs with constant and variable cross-sectional HCCs are carried out, respectively, and their maximum temperature differences (MTDs) are minimized based on analytical method and finite element method. Besides, the influences of the NUHG coefficient, HCC number and width coefficient on the optimal results are studied. The results indicate that the deviation of the optimal constructs obtained from the analytical method and finite element method are comparatively slight. When the NUHG coefficient is equal to 10, the minimum MTD of the disc with 25 constant cross-sectional HCCs is specifically reduced by 48.8% compared to that with 10 HCCs. As a result, the heat conduction performance (HCP) of the disc can be efficiently improved by properly increasing the number of HCCs. The minimum MTD of the disc with variable cross-sectional HCC is decreased by 15.0% when the width coefficient is changed from 1 to 4. Therefore, the geometry of variable cross-sectional HCC can be applied in the constructal design of the disc to a better heat transfer performance. The constructal results obtained by investigating the non-uniform heat generating case in this paper can contribute to the design of practical electronic device to a better heat transfer performance.

Heating load and COP optimisations for a universal steady flow endoreversible heat pump model

The heating load and coefficient of performance (COP) of a class of universal steady flow endoreversible heat pump cycle model, which consists of one heating branch, two cooling branches and two adiabatic branches, are optimised using the theory of finite time thermodynamics. The analytical formulae for heating load and COP versus temperature ratio as well as COP versus heating load of the cycle model are derived. Effects of the total heat exchanger inventory on performances of heat pump cycles are shown by detailed numerical examples. The results obtained herein include the optimal performances of endoreversible Otto, Brayton, Atkinson, Diesel, Dual and Carnot heat pump cycles.