Numerical Study on Local Flow and Heat Transfer Characteristics of Supercritical CO2 in PCHE with Sinusoidal Channels

Local heat transfer and flow characteristics, which is crucial to the overall performance of supercritical CO2 recuperators, is rarely examined in details in reported studies. In this paper, the local heat transfer and flow characteristics of supercritical CO2 in sinusoidal channel printed circuit heat exchangers are numerically investigated under the working conditions of recuperators. Based on the simplified physical model constituted by 10 pitches, the variations of Re, heat flux, Nu, secondary flow and other relevant parameters along the flow direction are analyzed firstly. Comparison is further made between the high and low temperature recuperators (HTR and LTR). It's observed that the local heat transfer and flow characteristics vary greatly from the inlet to the outlet, especially in the LTR. Differences of 127.5% and 61.7% can be observed on the cold side of the LTR for Re and heat transfer coefficient h, respectively. Results indicate that the temperature difference and heat transfer coefficient h should not be regarded as constant and the distribution of h should be carefully considered in the design of the LTR. The complicated interactions among the varying thermophysical properties, buoyancy and the periodically changed centrifugal force are believed to be the key that shapes the flow fields. Furthermore, the changes of the wavy angle theta are found to have greater influence than the changes of mass flow rate in reshaping the flow field when theta>25 degrees. Though gravity direction strongly affects the local heat transfer and flow characteristics, it's also found that the effects on the overall thermal-hydraulic performance are relatively minor. Yet the installation direction that yields alpha(y)=g should better be avoided for the sinusoidal channel printed circuit heat exchanger.