Investigation on the cycle performance and the combustion characteristic of two CO2-based binary mixtures for the transcritical power cycle

Highlights: • The paper studies on the performance and the flammability of two CO₂-based mixtures. • The efficiency and the specific power show a peak with changing the mixing ratio. • Optimization is performed for the thermal efficiency and the specific power. • The flame combustion rate are tested under several mixing ratios. • The flammable critical mixing ratios of both mixtures are obtained. -- Abstract: It is difficult to condense subcritical CO₂ in the CO₂ transcritical power cycle by the conventional cooling, because of CO₂'s low critical temperature. With the aim to solve this problem, a new transcritical power cycle is proposed, using a CO₂-based binary mixture as working fluid. Two mixtures are considered, namely, n-butane/CO₂ and isobutane/CO₂. Because n-butane and isobutane are flammable, the flammability of the mixtures are worth of note as well as their cycle performance. A laminar flame combustion rate experimental platform is established to investigate the combustion characteristics of both mixtures under different mixing ratio. The results show that their critical temperature increases with increasing the organic fraction and their critical pressure shows a peak value in considered conditions. The critical temperature can reach 40 °C with the organic fraction of 0.0711 for n-butane/CO₂ and 0.0806 for isobutane/CO₂. Under these mixing ratios, the transcritical power cycle can run by the conventional water cooling. The thermal efficiency reaches the highest value of 12.78% under the mole ratio of 0.28/0.72 for n-butane/CO₂. Isobutane/CO₂ gives the best condition under the mole ratio of 0.32/0.68, with the thermal efficiency reaching 12.97%. The flammable critical mole ratios for n-butane/CO₂ and isobutane/CO₂ are 0.04/0.96 and 0.09/0.91, respectively.