Thermodynamic Modeling for Open Combined Regenerative Brayton and Inverse Brayton Cycles with Regeneration before the Inverse Cycle

Lingen Chen,Zelong Zhang,Fengrui Sun

doi:10.3390/e14010058

Abstract

A thermodynamic model of an open combined regenerative Brayton and inverse Brayton cycles with regeneration before the inverse cycle is established in this paper by using thermodynamic optimization theory. The flow processes of the working fluid with the pressure drops and the size constraint of the real power plant are modeled. There are 13 flow resistances encountered by the working fluid stream for the cycle model. Four of these, the friction through the blades and vanes of the compressors and the turbines, are related to the isentropic efficiencies. The remaining nine flow resistances are always present because of the changes in flow cross-section at the compressor inlet of the top cycle, regenerator inlet and outlet, combustion chamber inlet and outlet, turbine outlet of the top cycle, turbine outlet of the bottom cycle, heat exchanger inlet, and compressor inlet of the bottom cycle. These resistances associated with the flow through various cross-sectional areas are derived as functions of the compressor inlet relative pressure drop of the top cycle, and control the air flow rate, the net power output and the thermal efficiency. The analytical formulae about the power output, efficiency and other coefficients are derived with 13 pressure drop losses. It is found that the combined cycle with regenerator can reach higher thermal efficiency but smaller power output than those of the base combined cycle at small compressor inlet relative pressure drop of the top cycle.

Highlights

IntroductionThermodynamic optimization theory [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24] is a powerful tool for the performance analysis and optimization of various thermodynamic processes and cycles
They derived the function relations about the compressor power input, the heat released rate produced by the burning fuel, the turbine power output, the rate of heat released by the exhaust, the cycle power output, the cycle thermal efficiency and the pressure loss of the components due to the flow irreversibility of the working fluid versus the compressor inlet relative pressure drop
A thermodynamic model for combined regenerative Brayton and inverse Brayton cycles is established in this paper by considering the pressure drops of the working fluid along the flow processes using thermodynamic optimization theory based on the first law analysis of [42]

Summary

Introduction

Thermodynamic optimization theory [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24] is a powerful tool for the performance analysis and optimization of various thermodynamic processes and cycles. A thermodynamic model of an open simple Brayton cycle with pressure drop irreversibility was established by Radcenco et al [32] They derived the function relations about the compressor power input, the heat released rate produced by the burning fuel, the turbine power output, the rate of heat released by the exhaust, the cycle power output, the cycle thermal efficiency and the pressure loss of the components due to the flow irreversibility of the working fluid versus the compressor inlet relative pressure drop. The power and the thermal efficiency were optimized by adjusting the bottom cycle pressure ratio and the mass flow rate They studied the performance of the combined Brayton and two parallel inverse Brayton cycles [45,46]. A further step of this paper beyond [37,42,44,45,46] is to analyze and optimize the performance of the combined regenerative Brayton and inverse Brayton cycles proposed in [42] with consideration of the pressure drops and the size constraints by using similar principles and methods as used in [25,26,27,28,29,30,31]

Physical Model

Cycle Analysis

Numerical Examples

Conclusion