Temperature Adaptability Analysis of Closed Cycle using CO2-Based Mixture as the Working Fluid for Underwater Platforms
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摘要: 超临界CO2布雷顿循环系统是水下平台动力技术的重要发展方向, 但由于深海低温较远离CO2临界温度致使循环系统存在温度适应性问题。文中提出了利用CO2基混合工质改善循环温度适应性并进一步优化循环性能的方案, 建立了简单回热循环热力学模型, 分析了CO2基混合工质临界参数随加入气体种类、质量分数的变化, 阐明了压缩机入口状态参数对CO2基混合工质闭式循环热力学性能的影响规律, 探讨了混合工质拟临界点位置对回热器夹点、热惯性等的影响。结果表明: 低临界参数混合工质循环可进一步扩大循环温度范围、压比以改善循环热力学性能, 但仅扩大温度范围而降低压比可能会对其造成不利于影响; 综合考虑循环热效率、比功、回热器内部夹点及热惯性, CO2/Xe(0.5/0.5)跨临界朗肯循环、CO2/SF6(0.9/0.1)跨临界液体布雷顿循环以及CO2/SF6(0.5/0.5)跨临界朗肯循环较超临界CO2布雷顿循环热效率最大可提高3.79个百分点、比功最大可提升31.6%, 回热器夹点位于冷端并未加剧其热惯性, 不会减缓系统响应速度。Abstract: The supercritical CO2 Brayton cycle system is an important development direction of underwater platform power technology. However, due to the low temperature in the deep sea which is far away from the critical temperature of CO2, the cycle system has temperature adaptability problems. This paper proposes the plan to use CO2-based mixed working fluid to improve cycle temperature adaptability and further optimize cycle performance. A simple recuperative closed cycle thermodynamic model is established. And we analyze the changes in critical parameters of CO2-based mixed working fluid with the type and mass fraction of added gas, clarify the influence of the compressor inlet state parameters on the thermodynamic properties of the closed cycle of CO2-based mixed working fluid. Besides, the influence of the pseudo-critical point position of the mixed working fluid on the pinch point and thermal inertia of the regenerator is discussed. The results show that the low critical parameter mixed working fluid cycle can further expand the cycle temperature range and pressure ratio to improve the cycle thermodynamic performance. But only expanding the temperature range and reducing the pressure ratio may have an adverse impact on it. Comprehensive consideration of cycle thermal efficiency, specific power, regenerator pinch point and thermal inertia, the thermal efficiency of CO2/Xe (0.5/0.5) transcritical Rankine cycle, CO2/SF6 (0.9/0.1) transcritical liquid Brayton cycle, CO2/SF6 (0.5/0.5) transcritical Rankine cycle is 0.5%-3.8% higher than that of the pure CO2 closed cycle, and the specific power can be increased by 1.2%-31.6%. The pinch point of the regenerator is located at the cold end, which does not increase its thermal inertia and does not slow down the system response speed.
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图 1 超临界CO2简单回热循环构型图
Figure 1. Configuration diagram of supercritical CO2 simple recuperation cycle
图 4 不同混合工质、不同循环的热效率对比: (a) CO2/Kr, SBC, TLBC; (b) CO2/Kr, TGBC, TRC; (c) CO2/Xe, SBC, TLBC; (d) CO2/Xe, TGBC, TRC; (e) CO2/SF6, SBC, TLBC; (f) CO2/SF6, TGBC, TRC; (g) CO2/He, SBC, TLBC; (h) CO2/Ne, SBC, TLBC
Figure 4. Comparison of thermal efficiency with different mixed working fluids and different cycles
图 5 纯质CO2与混合工质闭式循环的比功对比
Figure 5. Comparison of specific power between pure CO2 and mixed working fluid closed cycle
图 6 不同循环中工质cp峰值在热交换器中的分布: (a) CO2/Xe(0.5/0.5)跨临界TRC,(b)CO2/SF6(0.9/0.1)跨临界TLBC循环, (c)CO2/SF6(0.5/0.5)跨临界TRC
Figure 6. Distribution of cp peak value of working fluid in heat exchanger
图 7 不同循环中回热器热冷侧温度及温差分布
Figure 7. Temperature distribution and temperature difference between hot and cold sides of regenerator
表 1 工质性质参数
Table 1. Table of working fluid property parameters
Tcr (℃) Pcr (MPa) ρcr (kg/m3) λ (W/m·K) ν (μPa·s) M (g/mol) Ar −122.46 4.86 535.60 0.0 190 24.20 39.9 Ne −228.75 2.66 486.31 0.0 517 33.46 20.2 He −267.95 0.23 69.58 0.1 643 20.97 4.0 Kr −63.67 5.53 909.21 0.0 102 27.24 83.8 Xe 16.58 5.84 1102.90 0.0 060 24.99 131.3 N2 −146.96 3.40 313.30 0.0 276 18.95 28.0 O2 −118.57 5.04 436.14 0.0 283 21.93 32.0 SF6 45.57 3.76 742.30 0.1 708 42.37 146.0 CO2 30.98 7.38 467.60 0.0 190 16.12 44.0 
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