Enhanced printed-circuit heat exchanger for supercritical CO2 Brayton cycle pre-coolers with innovative convergent-divergent mini-channel design

Abstract

Supercritical CO₂ (sCO₂) operates under extreme pressure and temperature conditions, with its thermophysical properties changing rapidly near the critical point. These conditions exceed the capabilities of common welded heat exchangers to handle the substance in sCO₂ Brayton Cycle power plants. Among the most suitable heat exchangers for this application is the Printed Circuit Heat Exchanger (PCHE), which has been extensively adopted in sCO₂ power systems. This unique heat exchanger offers the thermal effectiveness of a plate-type heat exchanger combined with the temperature/pressure capability of a shell-and-tube heat exchanger. Additionally, PCHEs are 75 %–85 % smaller and lighter than the shell-and-tube heat exchangers required in the mentioned cycle. The only drawback of PCHEs is the complexity and cost of the manufacturing process. That is why any proposed thermal improvement techniques for PCHEs are expected not to increase the cost or complexity of the manufacturing or tooling process. As shown in the graphical abstract, precise parallel mini channels are created on thin metal sheets using the photochemical etching process and then joined together with the diffusion bonding technique, making it a single solid-state bond (free from joints, gaskets, brazing, and welding). This research proposes an innovative modification for the mentioned mini-channels in which the diameter of the semi-cylindrical channels is gradually increased or decreased along the sheets, creating convergent-divergent fluid flow behavior to boost its thermal performance without affecting the production cost or complexity of the process. Various convergent/divergent arrangement scenarios are possible, all of which are examined under various area ratios (A_r), inlet temperatures, mass flux (G), and operating pressures from thermal, frictional, and exergetic viewpoints. According to the validated 3D numerical simulation results, this modification is found to be so promising that, in some cases, the improved heat transfer coefficient is over two times higher than that in the base model due to enhanced turbulence and fluid mixing from the convergent-divergent scenario. All other detailed results are presented and discussed in this paper.