• Energy Research

Besides compressor rotary speed and parameters of water flowing through gas cooler and evaporator, refrigerant charge amount and electronic expansion valve (EEV) opening are two important parameters that have significant effects on the performance of a transcritical CO2 heat pump system. In this study, the effects of refrigerant charge amount on the performance of a transcritical CO2 heat pump water heater were investigated experimentally at different EEV openings. An optimal coefficient of performance (COP) was found that corresponded to a specific refrigerant charge and a specific EEV opening. Based on the experiment, the COP peaked at charge of 1.8 kg when EEV opening was 40% of full opening. The heating capacity and the COP increased at first, reached peaks and then decreased with increase of charge amount. The COP decreased 14.95% as the CO2 charge amount was reduced by 22.2% from the optimal charge at 50% EEV opening. As EEV opening varied from 40% to 60% at the same charge amount, the heating capacity decreased more than 30%.

Besides compressor rotary speed and parameters of water flowing through gas cooler and evaporator, refrigerant charge amount and electronic expansion valve (EEV) opening are two important parameters that have significant effects on the performance of a transcritical CO2 heat pump system. In this study, the effects of refrigerant charge amount on the performance of a transcritical CO2 heat pump water heater were investigated experimentally at different EEV openings. An optimal coefficient of performance (COP) was found that corresponded to a specific refrigerant charge and a specific EEV opening. Based on the experiment, the COP peaked at charge of 1.8 kg when EEV opening was 40% of full opening. The heating capacity and the COP increased at first, reached peaks and then decreased with increase of charge amount. The COP decreased 14.95% as the CO2 charge amount was reduced by 22.2% from the optimal charge at 50% EEV opening. As EEV opening varied from 40% to 60% at the same charge amount, the heating capacity decreased more than 30%.

Abstract The thermodynamic irreversible loss by condensation can have an important influence on the flow characteristics and thermal efficiency in air or nitrogen cryogenic turbo-expander involving spontaneous condensation flow. However, the design of wet type turbo-expander for cryogenic liquid plants has been constrained due to the complexity of nucleation theory and the difficulty of data measurement in cryogenic environments. This paper presents numerical simulations for prediction of nitrogen spontaneous condensation flow in cryogenic nozzles. The non-equilibrium simulations were performed using three nucleation theories with the help of ANSYS CFX solver. The standard Redlich-Kwong gas state equation and Eulerian–Eulerian governing equations were used in simulations. Comparison with the equilibrium condensation model the non-equilibrium condensation model achieves a better prediction of the flow characteristics for spontaneous condensation flow in cryogenic environments. The nucleation theory which is based on classical nucleation theory (CNT) and improved by Kantrowitz for non-isothermal effects shows a better prediction of pressure drop, location of condensation onset and supercooling compared with experimental data. The influence of varying nucleation theories on the calculation of nucleation rate, the supercooling distribution and the liquid mass fraction distribution were also analyzed.

Abstract The thermodynamic irreversible loss by condensation can have an important influence on the flow characteristics and thermal efficiency in air or nitrogen cryogenic turbo-expander involving spontaneous condensation flow. However, the design of wet type turbo-expander for cryogenic liquid plants has been constrained due to the complexity of nucleation theory and the difficulty of data measurement in cryogenic environments. This paper presents numerical simulations for prediction of nitrogen spontaneous condensation flow in cryogenic nozzles. The non-equilibrium simulations were performed using three nucleation theories with the help of ANSYS CFX solver. The standard Redlich-Kwong gas state equation and Eulerian–Eulerian governing equations were used in simulations. Comparison with the equilibrium condensation model the non-equilibrium condensation model achieves a better prediction of the flow characteristics for spontaneous condensation flow in cryogenic environments. The nucleation theory which is based on classical nucleation theory (CNT) and improved by Kantrowitz for non-isothermal effects shows a better prediction of pressure drop, location of condensation onset and supercooling compared with experimental data. The influence of varying nucleation theories on the calculation of nucleation rate, the supercooling distribution and the liquid mass fraction distribution were also analyzed.

Abstract Liquid fraction measurement in cryogenic two-phase flow is a complex issue, especially for an industrial cryogenic system. In this paper, a simple thermal method is proposed for measuring the liquid fraction in cryogenic two-phase turbo-expander by an electric heating unit in experimental study. The liquid fraction of the cryogenic two-phase flow is determined through the heat balance built at the outlet of the turbo-expander (inlet of heating unit) and the outlet of the heating unit. Liquid fractions from 1.16% to 5.02% are obtained from five two-phase expansion cases. Under the same turbo-expander inlet pressure and rotating speed, five superheated expansion cases are tested to evaluate the wetness loss in two-phase expansion. The results show that the proposed method is successful in measuring the liquid fraction of cryogenic two-phase expansion for turbo-expander in an industrial air separation plant. The experimental isentropic efficiency ratio and the tested Baumann factor decrease with the increasing mean wetness. Based on prediction of Baumann rule, the cryogenic turbo-expander with low liquid fraction in two-phase expansion cases suffers from more severe wetness loss than that with the higher liquid fraction.

Abstract Liquid fraction measurement in cryogenic two-phase flow is a complex issue, especially for an industrial cryogenic system. In this paper, a simple thermal method is proposed for measuring the liquid fraction in cryogenic two-phase turbo-expander by an electric heating unit in experimental study. The liquid fraction of the cryogenic two-phase flow is determined through the heat balance built at the outlet of the turbo-expander (inlet of heating unit) and the outlet of the heating unit. Liquid fractions from 1.16% to 5.02% are obtained from five two-phase expansion cases. Under the same turbo-expander inlet pressure and rotating speed, five superheated expansion cases are tested to evaluate the wetness loss in two-phase expansion. The results show that the proposed method is successful in measuring the liquid fraction of cryogenic two-phase expansion for turbo-expander in an industrial air separation plant. The experimental isentropic efficiency ratio and the tested Baumann factor decrease with the increasing mean wetness. Based on prediction of Baumann rule, the cryogenic turbo-expander with low liquid fraction in two-phase expansion cases suffers from more severe wetness loss than that with the higher liquid fraction.

Abstract Electronic expansion valves (EEV) are employed increasingly in transcritical CO 2 systems. In order to investigate the mass flowrate characteristics of supercritical CO 2 flowing through an EEV, the effects of EEV outlet pressure on the mass flowrate of CO 2 is studied under constant EEV inlet pressure and inlet temperature. The two-phase flow patterns of supercritical CO 2 flowing through an EEV are revealed, and the conventional choked flow judgment formulas is proved effective for supercritical CO 2 . Based on Bernoulli Equation with expansion factor, the mass flowrate correlation of supercritical CO 2 flowing through an EEV is presented. The influence of the expansion coefficient on the mass flowrate is considered in the correlation in which the correction coefficient is only affected by the EEV opening, the changing rate of the density and the pressure difference in the entrance region. The mass flowrate predicted by the correlation in this paper shows a good agreement with experimental data. The maximum relative errors are within 10%. The average and standard deviations are 0.76% and 5.9%, respectively.