The zigzag channel is the uppermost channel type of an industrial printed circuit heat exchanger (PCHE). The effect of geometric properties on the flow and heat transfer performance of the channel is significant to the PCHE design and optimization. Numerical investigations were conducted on the flow and heat transfer characteristics of supercritical CO₂ (sCO₂) in semicircular zigzag channels by computational fluid dynamics method. The shear stress transfer (SST) k–ω model was used as turbulence model and the National Institute of Standards and Technology (NIST) real gas model with REFPROP database was used to evaluate the thermophysical parameters of sCO₂ in this numerical method. The effectiveness of the simulation method is verified by experimental data. Thermal hydraulic performance for zigzag channels with different pitch lengths, bending angles, and hydraulic diameters are studied comparatively based on this numerical method, with the boundary conditions which cover the pseudocritical point. The comparison results show that reducing the bending angle and pitch length will strengthen the effect of boundary layer separation on the leeward side of the wall and enhance the heat transfer performance, but the pressure drop of the channel will also increase, and the decrease of channel hydraulic diameter is beneficial to the heat transfer enhancement, but it is not as significant as that of the straight channel.

The influence of the cross section shape of microchannels on the thermos-hydraulic performance of the supercritical CO₂ fluid is an important issue in the design of industrial compact heat exchangers, but few studies have been conducted about this issue. In this paper, comparative studies of the flow and heat transfer performance of SCO₂ fluid in horizontal microchannels with circular, semicircular, rectangle, and trapezoidal cross sections were conducted numerically. The comparison is based on the same hydraulic diameter and length for all channel types and is carried out under the same mass flux, outlet pressure, and wall heat flux. The fluid bulk temperature in this analysis ranges from 285K to 375K, which covers the pseudocritical point of SCO₂. The results show that the circular channel has the highest average heat convection coefficient, while the trapezoidal channel has the worst convective heat transfer performance under the same hydraulic diameter and boundary conditions. The results also indicate that the effect of cross section shape on the heat convection coefficient is significantly greater than that on the channel pressure drop, and the existence of the corner region in the cross section, especially the acute angle, will weaken the heat transfer performance.

The excellent thermophysical properties of supercritical CO₂ (sCO₂) close to the pseudocritical point make it possible to replace water as the coolant of microchannels in application of a high heat flux radiator. The computational fluid dynamics (CFD) method verified by experimental data is used to make a comparison of the thermal hydraulic behavior in CO₂-cooled and of water-cooled microchannels. The operation conditions of the CO₂-based cooling cases cover the pseudocritical point (with the inlet temperature range of 306~320 K and the working pressure of 8 MPa), and the water-based cooling case has an inlet temperature of 308 K at the working pressure of 0.1 MPa. The channel types include the straight and zigzag microchannels with 90°, 120°, and 150° bending angles, respectively. The analysis result shows that, only when the state of CO₂ is close to the pseudocritical point, the sCO₂-cooled microchannel is of a higher average heat convection coefficient and a lower average temperature of the heated surface compared to the water-cooled microchannel. The entropy generation rate of the sCO₂-cooled microchannel can reach 0.58~0.69 times that of the entropy generation rate for the water-cooled microchannel. Adopting the zigzag structure can enhance the heat transfer, but it does not improve the comprehensive performance represented by the entropy generation rate in the sCO₂-cooled microchannel.

The sudden closure of the refueling valve during the refueling process will cause the water hammer phenomenon in the pipeline of the large aircraft fuel system and produce pressure waves propagating along the pipeline. In serious cases, it will cause severe damage to the pipeline. The pressure surge in the pipeline can be affected by many design factors. In this study, the numerical modeling method of the large aircraft fuel system is studied using the Flowmaster program, and the accuracy of steady and transient state analysis of the simulation method is verified by experimental data. Based on this numerical method, a complete system-level simulation model of the large aircraft fuel system was established. Using this model, the effects of valve closure speed, diameter of the refueling main pipe, and refueling volume flow rate on pipeline pressure surge during the refueling process are investigated. The results indicate that all three factors have a significant impact on the pipeline pressure surge of the fuel system. The effect of the valve closing speed and the pipe diameter on the pressure surge of the pipeline is nonlinear and gradually weakened with the decrease of valve speed and the increase of pipe diameter, while the effect of refueling volume flow rate on the pipeline pressure surge is approximately linear.