Experimental evaluation of the effect of an internal heat exchanger on a transcritical CO_2 ejector system

This study presents experimental results focused on a performance comparison of a transcritical CO2 ejector system without an internal heat exchanger(IHX) (EJE-S) to a transcritical CO2 ejector system with an IHX(EJE-IHX-S) . The comparison includes the effects of changes in operating conditions such as cooling water flow rate and inlet temperature. Experiments are conducted to assess the influence of the IHX on the heating coefficient of performance(COPr) ,heating capacity,entrainment ratio,pressure lift,and other parameters. The primary flow rate of the EJE-IHX-S is higher than that of the EJE-S. The pressure lift and actual ejector work recovery are reduced when the IHX is added to the transcritical CO2 ejector system. Using a more practical performance calculation,the compression ratio in the EJE-S is reduced by 10.0%-12.1%,while that of EJE-IHX-S is reduced only by 5.6%-6.7% compared to that of a conventional transcritical CO2 system. Experimental results are used to validate the findings that the IHX weakens the contribution of the ejector to the system performance.

Thermodynamic Evaluation of Transcritical CO_(2) Heat Pump Considering Temperature Matching under the Constraint of Heat Transfer Pinch Point

The non-linear temperature glide in the supercritical CO_(2) cooling process makes the heat transfer pinch point of heat exchanger show multiplicity,like size,location distribution and quantity,which makes the thermodynamic performance of the CO_(2) transcritical cycle more complex and eventually affects the evaluation of the optimal operating state of the system.Based on the second law of thermodynamics and the constraints of heat transfer pinch,a thermodynamic evaluation method of CO_(2) transcritical heat pump system was proposed according to the degree of temperature matching.The influence mechanism of multi-characteristic change of heat transfer pinch point on temperature matching degree and the effect of temperature matching degree on thermodynamic performance of CO_(2) transcritical heat pump system were discussed.The relationship between temperature matching degree,COP and exergy efficiency of the system was analyzed.It is considered that the change of temperature matching index value can clearly characterize the change trends of COP and exergy efficiency.That is,the smaller the temperature matching degree is,the closer the temperature distribution of heat transfer fluids on both sides of the heat exchanger is to Lorenz cycle,and the greater the COP and exergy efficiency are.Furthermore,by monitoring the outlet temperature of the CO_(2) cooler,which has an essential relationship with the temperature matching degree during the heat exchange process,the deviation between actual and optimal working condition can be judged online,which is beneficial to real-time evaluation of the working state of the system.

Combustion Characteristic and Mechanism of a Mixture Working Fluid C_(3)H_(8)/CO_(2)

In the CO_(2)transcritical power cycle,conventional cooling water can hardly condense subcritical CO_(2)because its critical temperature is as low as 30.98°C.In order to avoid this condensing problem,CO_(2)-based mixtures have been proposed as working fluids for transcritical power cycle.They can raise the critical temperature by mixing a little C_(3)H_(8)as the secondary component to CO_(2).However,the flammability of the mixture may limit its application.This article investigated laminar flame speed of C_(3)H_(8)/CO_(2)which represents the mixture’s combustion characteristic by a so-called heat flux method and studied the inhibition mechanism of CO_(2)on the combustion based on the Premixed Laminar Flame-Speed Calculation reactor of Chemkin-Pro.The experimental results showed that the laminar flame speed shows a peak value with changing the equivalence ratio and accelerates with raising the mole fraction of the organic gas.Additionally,a slight upwards trend was observed for the corresponding equivalence ratio of the peaks.The flammable range for the equivalence ratio extended with the mole fraction of C_(3)H_(8)increasing.With the mole fraction of C_(3)H_(8)of 0.15,the maximum laminar flame speed was 12.8 cm/s,31.7%of that of the pure C_(3)H_(8).The flammable range was from 0.41 to 1.33,decreasing by 23.3%compared with that of C_(3)H_(8).A flammable critical mixing ratio was also found as 0.08/0.92 for C_(3)H_(8)/CO_(2)at the normal condition.By simulating,it was found that the most key free radical and elementary reaction which determine the inhibition of CO_(2)on the combustion are OH and H+O_(2)=O+OH,respectively.