Numerical investigation on thermal hydraulic performance of hybrid wavy channels in a supercritical CO2 precooler

Abstract

Heat transfer enhancement within the precooler in supercritical CO2 Brayton cycles is challenging due to the dramatic variations of thermophysical properties. The local flow and heat transfer characteristics of a straight channel PCHE are analyzed firstly in this paper and the idea of thermophysical property-matching partitioned heat transfer enhancement is proposed accordingly. Subsequently, three types of hybrid channel, where sinusoidal wavy structures are combined with straight channel in different ways, are designed to examine the effectiveness of the partitioned heat transfer enhancement idea. A comparative study on the thermal hydraulic performance is performed with different geometrical parameters under different working conditions. The results indicate that the partitioned heat transfer enhancement shows great advantages in reducing the total pressure drop under the given heat flux boundary conditions. Compared to the full-wavy and the other two hybrid channels, the overall thermal hydraulic performance of the type-C channel is the best. A maximum pressure drop reduction of 23% could be observed in the type-C channel (with the wavy section used in the low-temperature region) compared to that in the type-A channel (with the wavy section used in the high-temperature region). Such advantage will become more prominent when the outlet temperature of the precooler is closer to the pseudocritical temperature. Moreover, the averaged increment of local heat transfer coefficient in the wavy section is 1.56 kW/m2⋅K for the type-C channel when compared to the straight channel section. It's also found that the effects of gravity on the local heat transfer coefficient are almost negligible in the high temperature region, while heat transfer in the low temperature region can be affected by changing the placing direction of the precooler. The present study may provide a practical guidance on the development of highly efficient heat exchangers where the thermophysical properties of the working fluid are highly variable.