1. Introduction
The supercritical carbon dioxide (sCO2) Brayton cycle has the advantages of high thermoelectric efficiency, small equipment size, low compression power consumption, and compact system structure. It is very suitable for combining with the new generation of nuclear energy, fossil energy, solar energy, etc. Due to the high compactness requirement of the system equipment, it is necessary to design and manufacture a compact heat exchanger with a high specific surface area as the regenerator or precooler of the cycle. The print circuit heat exchanger (PCHE) is a preferred device because of its high heat transfer efficiency, compactness, and robustness. The flow microchannels formed by chemical etching and the heat exchange core obtained by diffusion welding can withstand the high temperature and pressure conditions of the sCO2 Brayton cycle.
Many researchers have studied the influence of different internal flow microchannel structures on the thermal-hydraulic performance by manufacturing prototypes and constructing test platforms, including straight, zigzag, S-shaped fin, and airfoil fin channels. The straight channel has received much attention for the simple etching process. Mylavarapu et al. [
1] fabricated two straight-channel PCHEs and connected them in series to a high-temperature helium test facility (HTHF). The heat transfer and friction characteristics were analyzed based on the experimental data under the conditions corresponding to the laminar to turbulent transition region. Li et al. [
2] improved the heat transfer correlations using the probability density function (PDF) method and compared it with the numerical and other scholars’ results. Chen et al. [
3] developed a numerical dynamic model and successfully predicted the steady-state and transient behaviors of a straight PCHE by comparing with the experimental results. Shin et al. [
4] tested the pressure drop and analyzed the flow instability in the Korea Advanced Institute of Science and Technology (KAIST) test facility using He and water as the medium. Chu et al. [
5] studied the PCHE thermo-hydraulic performance on the sCO
2-water experiment platform at the transcritical and supercritical states, indicating that the comprehensive performance reduced by about 17.6% at the transcritical state. Xu et al. [
6] also studied a straight PCHE between sCO
2 and water. The heat transfer and pressure drop of the two media were studied separately under the fixed conditions of one side. Park et al. [
7] tested a straight PCHE precooler under the trans-critical, near-critical, and far-critical conditions and proposed a discretization method to design a precooler at the near-critical point. Arslan et al. [
8] applied the sub-heat exchanger model to design a PCHE recuperator and performed the experiments using sCO
2 and water, which indicated high accuracy between the numerical model and the experimental results.
The zigzag structure can significantly improve the heat transfer area and coefficient, accompanied by the disadvantage of increased pressure loss. Nikitin et al. [
9] investigated the heat transfer performance and pressure drop of a zigzag PCHE through experiments and numerical simulations. The overall heat transfer coefficient ranged from 300 to 650 W/(m
2·K) with a compactness of approximately 1050 m
2/m
3 and a maximum power density of 4.4 MW/m
3. Kim et al. [
10,
11,
12] carried out a detailed study on the zigzag-structure PCHE using He, CO
2, and water as working fluids. The correlations of Nusselt numbers and Fanning friction factors were fitted, and the effects of the channel geometric parameters were analyzed. Baik et al. [
13] designed a zigzag PCHE precooler and replaced the shell and tube on the KAIST experimental facility. The effectiveness and pressure loss results were compared with both the PCHE design code KAIST-HXD and the shell and tube heat exchanger. Dai et al. [
14] studied the steady and transient behavior of a hydraulic-fluid PCHE under laminar flow conditions. Bae et al. [
15] studied the CO
2 condensation heat transfer and two-phase flow when the PCHE precooler was close to the critical point. Existing correlations for the CO
2 single-phase and two-phase were compared with the experimental data, and a new set of correlations was suggested. Zhou et al. [
16] designed and manufactured a 100 kW class zigzag PCHE prototype as a recuperator and tested using sCO
2 on both sides. The effectiveness was over 95% and the pressure drop was less than 50 kPa on both sides. Cheng et al. [
17] tested a zigzag PCHE as a precooler on the same platform, and the effects on effectiveness and pressure drop were analyzed with the inlet Reynolds number ranging from 31,157 to 52,806 on the CO
2 side and from 1084 to 1947 on the water side. Further, exergy analysis of the PCHE recuperator based on the experimental results was carried out and new correlations on the Nusselt number and friction factor were developed [
18]. Zhang et al. [
19] studied the global and local performance of an 80 kW zigzag PCHE precooler using a combination method of experiments and numerical simulations. New correlations were developed considering the impact of Prandtl number.
There is little experimental research on the S-shaped-fin and the airfoil-fin PCHEs. Ngo et al. [
20,
21] developed a new S-shaped-fin PCHE and compared its thermal-hydraulic performance with that of zigzag fins. The empirical correlations of Nusselt numbers and pressure-drop factors were proposed, which proved that the pressure drop factor of the S-shaped microchannels was 4–5 times less than the zigzag one through a 24–34% reduction in the Nusselt numbers. Pidaparti et al. [
22] investigated two kinds of discontinuous PCHEs with an offset rectangular and NACA0020 airfoil fin. Empirical correlations for the friction factor and the Nusselt number were proposed, which could match the experimental results.
From the perspective of channel structure, most of the above experimental research focused on the straight and zigzag structures. However, the flow and heat transfer performance still need to be improved by designing new configurations. Aneesh et al. [
23] numerically compared the heat transfer performance of zigzag, S-shaped, and trapezoidal structures, and it was found that the trapezoidal channel has the highest heat transfer performance but maximum pressure loss. From the perspective of function, the existing PCHE prototypes are mostly used as precoolers, but the data and design method as regenerators are still insufficient. From the perspective of conditions, most of the studies are near the CO
2 critical point, and there are few studies on the working temperature and pressure range of the regenerator in the sCO
2 Brayton cycle.
In this work, a new trapezoidal channel PCHE prototype is designed and manufactured to reach a higher heat transfer performance, and it is tested as a regenerator using sCO2. The heat transfer coefficient and heat recovery efficiency are calculated and analyzed under different thermal parameters. Correlations for Nusselt numbers on both trapezoidal channels are proposed with respect to Reynolds numbers and Prandtl numbers, and the pressure drop in the flow channels is evaluated by the peeling method through a single-plate test prototype. In addition, the numerical simulation results verify and expand the experimental conclusions. This work provides new trapezoidal channel experimental results and heat transfer correlations for an advanced PCHE regenerator design in the sCO2 Brayton cycle.
5. Conclusions
In this work, a lab-scale trapezoidal PCHE prototype with two hot plates and one cold plate is experimentally studied as a regenerator of the sCO2 test loop. The thermal–hydraulic performance is analyzed with respect to the inlet temperature on both sides, the working pressure, and the mass flow rate, and the pressure drop of the trapezoidal channels is divided using a single-plate straight prototype.
The overall heat transfer coefficient defined by the cold side exceeds 1.10 kW/(m2·K) and reaches a maximum of 2.53 kW/(m2·K) under the test conditions. The average heat transfer coefficient on either side is calculated by defining a new parameter , and the results are verified by the thermal resistance analysis with a deviation within 5%. The correlations of the Nusselt numbers are proposed on both sides, with the Reynolds numbers ranging from 10,000 to 30,000 and 4800 to 14,000, and the Prandtl numbers ranging from 0.91 to 1.61 and 0.77 to 0.98 on the cold side and hot side, respectively. All experimental results are within a 15% deviation with the correlations, and 92% and 86% values are within a 10% deviation on the cold and the hot sides, respectively.
The pressure drop of the heat transfer core is separated from the measured pressure drop with the help of a single-plate prototype. The pressure drop on the hot and the cold side is less than 7 kPa and 15 kPa under test conditions, respectively. The heat recovery efficiency is defined to evaluate the performance of the PCHE as a regenerator. It decreases with the increase in mass flow rate, and basically remains unchanged with the increase in inlet temperature and working pressure.
A simulation model is established for verification and expansion. It has been proven to reflect the experimental results well with a maximum temperature deviation of 2.80% and 7.92% on the cold and hot side, respectively. Extended simulations of low-Reynolds-number conditions are studied based on the model; new Nusselt number correlations are obtained with the Reynolds numbers ranging from 3796 to 30,000 and 1821 to 14,000, on the cold side and hot side, respectively; the extended correlations are within a 16% deviation with all numerical and experimental results.