查看更多>>摘要：Whilst R290 is currently used to a limited extent in room air conditioners there is a desire for wider application due to its excellent performance and negligible global warming potential. The product standard IEC 60335–2–40 specifies requirements to limit the allowable refrigerant charge in such a way that it obstructs the wider use of R290. Airflow of an indoor unit can be used to dilute a refrigerant leak, enabling substantially greater charge quantities to be used. A numerical model based on entrainment theory was developed and supported by analysing the behaviour of experimentally simulated releases under various conditions with indoor unit airflow. The work determines the minimum airflow rate necessary to prevent formation of a flammable concentration within the room. Further, the work includes determination of appropriate response time and leak rate to initiate airflow once a leak has begun. The developed methodology can equally be applied to commercial refrigeration units and other flammable refrigerants.
查看更多>>摘要：A two-phase thermosyphon loop is an ideal heat transfer device to achieve compact and efficient cooling with low energy consumption. In this paper, a two-phase thermosyphon loop with a horizontal parallel tube evaporator for multiple heat source cooling is proposed. The effect of the filling ratio on the transient response of temperature and pressure, heat transfer characteristics, instability issues, as well as flow regimes under different heat loads are investigated experimentally and visually. The filling ratios are divided into three groups according to their unique heat transfer and instability characteristics. The research results show that a too low or too high filling ratio is not conducive to the safe and stable operation of the system. At medium and high filling ratios, temperature and pressure overshoot are prone to occur, threatening the safe operation of the system; geyser boiling is another unstable phenomenon that can be inhibited as the heat load increases. Gravity will lead to uneven distribution of working fluid in the evaporator and further affect the temperature uniformity and heat transfer capability. From the perspective of temperature control performance and heat transfer capability, 30% is the optimal filling ratio in this study, with a minimum thermal resistance of 0.12 ℃ ?W-1 and a maximum heat transfer coefficient of 1345.36 W?m?2?℃?1. This paper provides a basis for designing and applying a two-phase thermosyphon loop for multiple heat source cooling in confined spaces.
查看更多>>摘要：Vapor compression systems are a popular thermal management solution for high heat load applications because they benefit from the high heat transfer rate of two-phase fluid flow. However, model-based design and control of these systems proves challenging because of the complex and coupled hydro-thermal dynamics as the working fluid changes phase in the heat exchangers. When modeling these heat exchangers, there exists a strong tradeoff between model accuracy and computational complexity. While different techniques have aimed to reduce computational complexity of the model, they come at the cost of a decrease in accuracy. On the other hand, high fidelity models are computationally expensive to run. To bridge the gap between computational efficiency and accuracy, this work develops a novel multi-state graph-based dynamic modeling approach. The multi-state graph modeling approach is used to develop a dynamic model of a two-phase heat exchanger. The heat exchanger model is integrated into a vapor compression system model and the model behavior is verified against other models present in the literature. The results demonstrate that the multi-state graph model can accurately capture system dynamics within 1% of the current state of the art modeling approaches while providing valuable modularity for system level modeling. Additionally, the computational load can be made comparable to less accurate approaches.
查看更多>>摘要：F-gas phase-down schedules will remove most of the HFCs (hydrofluorocarbons) used in refrigeration systems in the short term. Besides refrigerant substitution, configuration modifications can be considered in existing installations to increase energy efficiency. Basic cycle, direct injection, economiser, parallel compression, and cascade configurations with internal heat exchanger have been proposed to increase the energy efficiency of a supermarket refrigeration system when replacing R-404A with R-449A at low and medium temperatures. Both systems are based on indirect expansion and are equipped with a subcooler. Their regular operation was recorded using R-404A and R-449A during a representative period. Then, a semi-empirical approach is followed to determine R-449A energy performance with each proposed configuration. Only parallel compression (PC) and basic cycle with internal heat exchanger (IHX) benefit energy performance, highlighting the medium temperature (MT) system. However, this benefit is not extended to the environmental analysis because the R-449A charge in PC significantly increases. Moreover, additional PC components extend the expected payback period, and a drop-in replacement is financially more interesting. Therefore, using R-449A with minor modifications in the MT and LT R-404A refrigeration system decreases 52% and 60% carbon footprint, respectively. The payback period of this action is below one year in both circuits. This study provides a semi-empirical methodology for existing systems to predict alternative refrigerants' energy performance.
NSTL
Elsevier