Abstract
Supercritical CO2 is used as a work fluid in both heat pump and power cycles. As a fluid at supercritical pressure is heated or cooled, it may undergo a smooth transition from a liquid-like state to a gas-like state or vice versa. This transition, during which the thermophysical properties vary sharply with temperature, can be referred to as pseudo- boiling or condensation. Using both analytical and numerical methods, it is shown that pseudoboiling theory helps to understand how the unique heat transfer characteristics of a supercritical fluid affect heat exchanger performance and design, in particular a gas chiller. Due to pseudo-condensation, classical approaches such as the ε−NTU and LMTD methods fail when rating or designing a sCO2 gas chiller. Using the heat of pseudo-condensation, the heat exchanger can be regarded to consist of a pre-cooler, condenser and a super-cooler. By further dividing the pre-cooler and super-cooler into two parts and subsequently applying the ε−NTU method per part yields very good results with respect to both the prediction of required size and entropy generation for various operating parameters. The influence of pseudo-condensation is reduced at higher pressures and is negligible when the structural energy required for the transition from liquid-like to a gas-like state is smaller than the required thermal energy required. It is shown that the local effectiveness of the condenser part is reduced (more so than the other parts) when the heat capacity ratio RC is varied from unity to less than unity, leading to enhanced irreversibility due to pseudo-condensation. Furthermore, the enhanced and deteriorated heat transfer regime (such as when a sCO2 downward flow is cooled) lead to significantly different required heat exchanger sizes. Finally, through the use of Monte Carlo simulations, it shown that the uncertainty of a Nusselt correlation complicates designing heat exchangers in which pseudo-condensation occurs. The simulations show that heat exchangers should be 50% larger than the size that is predicted using a Nusselt correlation if the design performance is to be ensured.
Highlights
Since 1990, global warming has prompted the steady cessation of refrigerants with high global warming potential in refrigeration & heat pump cycles
These results show that using the ε − NTU method can be applied reliably if the heat exchanger is divided in three partitions that are defined using T − and T + and by subsequently dividing the partitions in which sCO2 has gas-like properties and liquid-like properties into two separate partitions
By combining the classical ε − NTU method with pseudoboiling theory, the gas chiller can be regarded as the combination of a pre-cooler, a condenser and a super-cooler
Summary
Since 1990, global warming has prompted the steady cessation of refrigerants with high global warming potential in refrigeration & heat pump cycles. Even though the previously mentioned research clearly indicates that variable thermophysical properties affect design and performance, not all researchers include the effects of variable properties in novel Nusselt number correlations for heat exchangers; see for instance Nikitin et al [41] and Kwon et al [30]. The effectivenes can be readily determined if the number of transfer units NTU – a measure of non-dimensional heat transfer area – is known, or vice versa (see Hesselgreaves et al [16], Roetzel et al [48], Zhang [63]) This method is combined with pseudoboiling theory (Banuti [1], Maxim et al [38]) to gain better insight into the effect of pseudo- condensation.
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