Condensation Of Zeotropic Mixtures Research Articles

Condensation of ternary low GWP zeotropic mixtures inside channels

The condensation heat transfer coefficient and two-phase frictional pressure drop of ternary zeotropic mixtures R455A (R32, R1234yf and R744 at 21.5/75.5/3.0% by mass composition) and R452B (R32, R1234yf and R125 at 67.0/26.0/7.0% by mass composition) have been measured inside a minichannel (0.96 mm diameter) and a conventional tube (8.0 mm diameter). The effects of tube diameter, mass velocity, vapor quality and saturation-to-wall temperature difference have been investigated. A comparison between the experimental data of the tested mixtures and the condensation heat transfer of the pure components R32 and R1234yf has been performed. The two-phase pressure drop and condensation heat transfer data are compared to predictions of correlations available in the literature. In the prediction of the heat transfer coefficient, the models are adapted to account for the additional mass transfer resistance occurring during condensation of zeotropic mixtures.

Performance investigation of a novel zeotropic organic Rankine cycle coupling liquid separation condensation and multi-pressure evaporation

The decrease in fossil energy reserves and the increase in energy costs have resulted in a strong interest in power generation using renewable heat sources or waste heat. Organic Rankine cycle (ORC) is a promising heat-to-power conversion technology. Although the ORC using zeotropic mixture is superior to ORC using pure fluid in thermodynamic performance due to low irreversibility during the evaporation and condensation of zeotropic mixture, the improvement in thermodynamic performance is usually achieved at the cost of poor economic performance. The studies on improving the thermo-economic performance are limited. In the present study, a novel zeotropic ORC coupling liquid separation condensation and multi-pressure evaporation is proposed. Multi-pressure evaporation is presented to improve heat match between the heat source and working fluid. Liquid separation condensation is applied to control the composition of the mixtures that enter the evaporation processes. Thermodynamic analysis and optimization model of the novel ORC is developed. The superiority of the proposed novel ORC over the traditional simple zeotropic ORC and traditional multi-pressure evaporation zeotropic ORC is elaborated for different mixtures. The contribution of mixture composition adjustment and condensation enhancement through liquid-vapor separation on the cycle performance improvement is investigated. Sensitivity analysis of heat-sink inlet temperature, heat source inlet temperature, vapor quality, heat source specific heat capacity, and heat-sink temperature rise on the cycle performance are conducted. The case results show that net power output of the novel ORC is 13.05–26.18% higher than that of the simple zeotropic ORC. The contribution of mixture composition adjustment on improving the net power output can be up to 3.57% compared with traditional multi-pressure evaporation ORC for mixture R245fa–R365mfc. When the heat transfer enhancement through liquid separation is incorporated into the thermodynamic optimization, the net power output of the novel zeotropic ORC can be increased by 8.22% compared with the traditional multi-pressure evaporation ORC under the same economic constraint.

Condensation of Zeotropic Mixtures in Horizontal Tubes: New Simplified Heat Transfer Model Based on Flow Regimes

The need for optimal design of heat exchangers with in-tube condensation of zeotropic refrigerant mixtures has pushed the research of predictive methods in the last years. Some procedures have been developed, based on the Colburn and Drew (1937, Trans. AIChemE 33, pp. 197–215) analysis, that require significant numerical effort and the diffusivity properties of the mixture to calculate the mass transfer resistance in the process and, hence, are rarely used for heat exchanger design. Proposing a modified version of the well-known simplified approach of Silver (1947, Trans. Inst. Chem. Eng. 25, pp. 30–42) and Bell and Ghaly (1973, AIChE Symp. Ser. 69, pp. 72–79) to include the effects of interfacial roughness and nonequilibrium effects, the present study extends the flow-pattern-based model of Thome, El Hajal, and Cavallini (2003, Int. J. Heat Mass Transfer 46, pp. 3365–3387) for condensation of pure fluids and azeotropic mixtures to zeotropic mixtures. By implementing this within the above flow-pattern-based heat transfer model, it leads to an improved method for accurately predicting local mixture heat transfer coefficients, maintaining a clear relationship between flow regime and heat transfer, and achieving both the goals of higher prediction accuracy and low calculation effort. The method has been verified for refrigerant mixtures (both halogenated and hydrocarbon) having temperature glides of 3.5–22°C, that is temperature differences between the dew point and bubble point temperatures (at a fixed pressure and bulk composition).