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cycle and the characteristics of typical processes. The second part introduces performance characteristics for the whole cycle, and the last part gives a brief overview of cycle modification. 1.2.1 Operating Processes of the Basic Transcritical CO2 Cycle The basic cycle and its corresponding T-s diagram are respectively shown in Figure 1.3a and b. The cycle is composed of four processes as follows: 1–2: Isentropic compression process. CO2 from the evaporator is compressed from saturated vapor to super-heated gas in supercritical state. 2–3: Isobaric cooling process. The supercritical CO2 enters the gas cooler and rejects heat by gas cooling without phase change. 3–4: Adiabatic expansion process. The high-pressure CO2 is expanded adiabatically to low pressure as vapor-liquid mixture through expansion valve. 4–1: Isobaric evaporation process. Two-phase CO2 flows into the evaporator and absorbs heat from outside heat source under a subcritical pressure. It is then evaporated to a saturated vapor state at the outlet of the evaporator and enters the compressor again to repeat the process 1–2. Each process in the cycle shows unique heat transfer and flow characteristics, which attracted researchers so much because of their help with component design and cycle opti- mization. A general view of the two-heat exchange process is summarized below: ● Heat rejection process (2–3) is the most particular process in transcritical cycles because heat is rejected by single phase cooling instead of condensation as in conven- tional subcritical cycles (therefore, the heat exchanger is called a gas cooler instead of a condenser). For this process, special heat transfer and flow characteristics can be observed due to the sharp changes of thermodynamic and transport properties5 of CO2, especially when it works in the region near pseudo-critical temperature (Tpc).6 In the near-critical region, the fluid behaves with high expansion and low thermal diffusion, bringing new features in fluid flow and convective structures, such as a piston effect (a kind of thermal relax- ation process) in micro-channels. These unique mechanisms of fluid flow and heat trans- fer play an important role in the heat rejection process in heat pumps. Research has focused on the unique mechanisms since the 1960s, usually in the in-tube heating/cooling process, like Shitsman [8], Krasnoshchekov et al [9], Shiralkar and Grif- fith [10], Kurganov et al [11], Jiang et al [12], etc. These in-tube researches contributed to the development of correlations, and the design/optimization/control/operation of con- ventional channel/mini-channel/micro-channel7 gas coolers. 5 The thermodynamic properties include density (ρ) and specific heat (Cp), the transport properties include viscosity (μ) and thermal conductivity (k). 6 The pseudo-critical temperature (Tpc) is defined as the temperature at which the specific heat (Cp) reaches a peak under isobar condition. 7 As the classification of channels given by Kandlikar [13] and Kandlikar and Grande [14], the hydraulic diameters (Dh) of conventional channels, mini-channels, and micro-channels are respectively in the range of >3, 0.2–3 and 0.01–0.2 mm. 1.2 Fundamentals 7PDF Image | What is a heat pump
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CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
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