Numerical study on novel airfoil fins for printed circuit heat exchanger using supercritical CO2
Introduction
With the development of Generation IV (Gen IV) nuclear reactors, the interest in the alternatively high thermal efficiency and environmentally friendly power conversion system is increasing. The supercritical carbon dioxide (SCO2) Brayton power cycle is regarded as one of the promising alternatives in the mild turbine inlet temperature region for its high efficiency, stability, safety, compactness, as well as the less influence on the environment [1].
As one of the key components in the SCO2 Brayton cycle, heat exchanger has a significant effect on the efficiency and compactness of the whole system. The Printed Circuit Heat Exchanger (PCHE) is regarded as one of the most promising candidates for SCO2 Brayton cycle [2]. PCHE is manufactured by diffusion bonding the photo-chemically etched plates. It has strong core to ensure the safety and stability, as well as the high heat transfer efficiency, and its unit could up to 85% smaller and lighter than the traditional shell-and-tube heat exchangers.
The continuous zigzag channel is one of the most widely used channels in PCHE. Ishizuka et al. [3] investigated the performance of PCHE with zigzag channel in an experimental SCO2 loop with various pressures, temperatures and mass flow rates. Nikitin et al. [4] proposed empirical correlations for heat transfer coefficient and pressure drop factor based on the experimental performance of zigzag PCHE using SCO2 as working fluid. Kim et al. [5] investigated the thermal–hydraulic performance of zigzag channel using helium test loop and three-dimensional numerical simulation, and found that the local pitch-averaged Nusselt number correlation developed from CFD simulations is more appropriate than the global Nusselt number correlation developed from experimental data. Kim et al. [6] optimized the PCHE design for intermediate heat exchangers (IHX) through the cost analysis. Lee and Kim [7], [8], [9] investigated numerous channel cross-sectional shapes and channel configurations to optimize the zigzag channel, and they found that rectangular channel has the best thermal performance and the worst hydraulic performance while the circular has the worst thermal performance. Ma et al. [10] studied the thermal–hydraulic performance of zigzag-type PCHE using helium as the working fluid at the typical temperature of 900 °C in the very high temperature reactor. Meshram et al. [11] studied and evaluated the performance of PCHE with straight and zigzag channels using FLUENT software, and developed correlations for Nusselt number and friction factor.
The zigzag channel suffers the drawback of a significant pressure drop, so some novel channels were proposed to avoid the significant pressure drop for PCHE. Ngo et al. [12] proposed S-shaped fins for PCHE, whose pressure drop is far less than the zigzag channel with the penalty of a slight decrease of heat transfer. Ngo et al. [13] investigated the thermal and hydraulic performance of Sshape fins and zigzag fins numerically and experimentally, and proposed the Nusselt number and pressure-drop factor correlations for S-shape and zigzag fins. Tsuzuki et al. [14] numerically investigated the flow of CO2 in the S-shape fin PCHE and found that the S-shape fin PCHE has one-fifth pressure drop of the zigzag PCHE. Tsuzuki et al. [15], [16] studied the effect of fin shape on thermal–hydraulic performance of PCHE with S-shape fins numerically, and proposed the relevant Nusselt number correlations.
Kim et al. [17] conducted a numerical investigation on the airfoil fin PCHE, and the results showed that airfoil shape fin PCHE has almost the same heat transfer ability but one-twentieth pressure drop of zigzag channel PCHE. Kim et al. [18] numerically analyzed the performance of airfoil fin PCHE in SCO2 power cycle, and concluded that the fully staggered arrangement has the best performance. Xu et al. [19] explored the effect of airfoil fin arrangements on heat transfer and flow resistance, and found the staggered arrangement has the best thermal–hydraulic performance, and they proposed a new fin structure similar to swordfish. Yoon et al. [20] proposed Fanning factor and Nusselt number correlations for airfoil PCHE based on the simulation results, and they compared the cost of straight, zigzag, S-shape and airfoil PCHEs for intermediate heat exchangers (IHXs) in the high temperature gas-cooled reactors (HTGRs) and the sodium-cooled fast reactors (SFRs). Ma et al. [21] performed the photochemical etching experiment to manufacture the airfoil PCHE plate and found the airfoil fin is not an ideal airfoil profile and it has a fin-endwall fillet, then they numerically investigated the effect of fin-endwall fillet on the thermal–hydraulic performance of airfoil PCHE. Kwon et al. [22] performed CFD analysis for various airfoil fin configurations and made correlations for Nusselt number and Fanning friction factor. Besides, they used cost based objective function to evaluate and optimize the configuration of airfoil fin PCHE. Chu et al. [23] studied the thermal–hydraulic performance of printed circuit heat transfer surface with distributed NACA 0025 airfoil fins and proposed the correlations of j and f factors. Chen et al. [24] compared the comprehensive performance of airfoil fin PCHEs with NACA 00XX series airfoil fins, and found the pressure drop of NACA 0020 airfoil fin PCHE reduces remarkably in comparison with the zigzag PCHE, and the comprehensive performance of NACA 00XX airfoil fin PCHE degrades as airfoil thickness increases.
Guo et al. [25] proposed the field synergy principle to evaluate and optimize the single phase convective heat transfer, in which the heat transfer performance depends not only on the velocity vector and the temperature gradient, but also on their synergy. Guo et al. [26] investigated the heat transfer characteristics of helically coiled tube numerically in terms of field synergy principle, and found the heat transfer enhancement mechanism of the tube could be well described by the field synergy principle. Zhai et al. [27] analyzed the hydraulic and thermal performance of double-layered microchannel with cavities and ribs in terms of the intensity of secondary flow and field synergy principle.
Guo et al. [28] proposed a physical quantity of entransy to describe the heat transfer ability of an object, which was defined as half of the product of internal energy and the thermal temperature. It has been proved that the total entransy always dissipates in an isolated system, and the entransy dissipation can be used as a figure of merit for irreversibility in heat transfer processes. Until now, the entransy and entransy dissipation have been widely applied to the optimization and evaluation of heat exchanger [29], [30], [31], [32], heat exchanger networks [33], [34], chemical heat pump [35], [36], heat storage system [37], [38], [39], and variable thermal physical properties [39], [40], etc. The thermal physical properties of CO2 vary sharply near critical and pseudo-critical temperature under supercritical pressure conditions, which challenges the conventional heat exchanger design and optimization theory seriously [41]. Therefore, the special characteristics in heat transfer and flow of SCO2 have to be taken into account when the channel structure is investigated and optimized.
In the present work, two novel structures of fins are proposed based on the NACA 0020 airfoil fin. The local and overall thermal–hydraulic performance of the novel fins used in PCHE are numerically investigated, and the heat transfer enhancement mechanism is also discussed and analyzed. The present work may be helpful for the optimization and design of airfoil fin PCHE.
Section snippets
Basic physical model
In the previous studies [18], [23], [24], the symmetric NACA 0020 and NACA 0025 airfoil fins have been investigated as fins used in PCHE. NACA 0020 is a kind of symmetric airfoil where the ‘00’ means it has no camber and the ‘20’ means the airfoil has a 20% thickness to the chord length ratio.
In the present work, NACA 0020 airfoil fin is adopted as one representative type of fins used in PCHE. Fig. 1 shows an ideal model of NACA 0020 airfoil fin PCHE. This simplified model contains two hot
Governing equations
The commercial software ANSYS CFX 15.0 is employed for the calculations. The governing equations are listed as follows.
Continuity equation:
Momentum equation:where μeff is effective viscosity coefficient which equals to the sum of dynamic viscosity μ and turbulence viscosity μt.
Energy equation:where Ф is the energy dissipation due to viscosity.
The Shear-Stress Transport (SST) turbulence model [42] is adopted
Theoretical analysis
In this subsection, the PCHE models with the three types of fins at cold side have been simulated using different inlet mass flow rates of SCO2, which are 0.4728 g/s, 0.9456 g/s, 1.4184 g/s and 1.8912 g/s. Meanwhile, all the cases in this part have the same inlet temperatures of 381.05 K at cold side and 553.05 K at hot side, the constant outlet pressures of 8.28 MPa at cold side and 2.52 MPa at hot side, and the mass flow rate is fixed as 0.4335 g/s in per hot fluid channel. The performance of
Conclusions
In the present work, two novel structures of fin (the Fin-II and Fin-III) are proposed to optimize NACA 0020 airfoil fin and improve the performance of PCHE, and the thermal–hydraulic performance of these fins is numerically investigated using SCO2 as the working fluid. The heat transfer enhancement mechanism of the novel fins is also discussed.
The comprehensive evaluation criterion of (j/j0)/(f/f0)1/3 is employed to evaluate the comprehensive performance of the airfoil fin PCHE, and the
Acknowledgements
This work is supported by the National Key Research and Development Program-China (2016YFB0601700), National Natural Science Foundation of China (51676185), and the Youth Innovation Promotion Association, CAS (2014128).
Conflict of interest
None declared.
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