Performance of printed circuit heat exchangers in SCO₂ Brayton/Organic Rankine combined cycle: Assessment of simplified boiling temperature and heat flux type of boundary conditions

In this study, a simplified boiling temperature type of boundary condition (BTBC) is developed and used to study the performance of printed circuit heat exchanger (PCHE) with semicircular channels. CFD is used to simulate both fluid and heat flows in PCHE semicircular channels are used as a precooler in a combined supercritical CO₂ Brayton/Organic Rankine cycle. Based on local heat transfer and flow characteristics of SCO₂, the results of BTBC are compared to heat flux type of boundary condition (HFBC). The classical (Gnielinski) correlation and the modified heat transfer correlation (modified Jackson) which are frequently used to capture the heat transfer behavior of SCO₂ are quantitatively evaluated for the two types of boundary conditions. The results show that the junction of different temperature sections of PCHE has the greatest influence on heat transfer coefficients, and the flow from the overheated section into the evaporation section accelerates the increase of heat transfer coefficients and enables them to reach their peak values faster. It is also found that the buoyance influence along the flow direction is significant and different for the two tested boundary conditions. In addition, results show that BTBC have bigger effects on the heat transfer coefficient at the different junctions of the precooler, compared to HFBC. Also, compared to the CFD results using BTBc, more than 90% of the data points predicted by the modified Jackson correlation are predicted within + 35% and −15% error margin, which is a better matching compared to the predictions of Gnielinski correlation. This type of boundary conditions can have a great influence on heat transfer of SCO₂, and thus it is necessary to adopt a more accurate boiling model to capture well the coupled heat transfer between SCO₂ and the organic working medium.