Heat Transfer Deterioration Research Articles

Flow and heat transfer performance of bionic heat transfer structures with hybrid triply periodic minimal surfaces

Efficient energy conversion technologies are essential for mitigating the environmental impact of industrial activities. The development and application of high-efficiency heat exchangers are key components in this endeavor. Heat exchangers based on triply periodic minimal surfaces (TPMS) are widely acknowledged for their exceptional thermal-hydraulic performance. However, current research mainly focuses on the design and optimization of heat exchangers with standard TPMS structures. In this study, a hybrid process is utilized to construct ten novel heat exchangers, followed by comprehensive investigations of their internal flow fields using three-dimensional numerical simulations. In order to establish a baseline, five standard TPMS structures are also introduced as reference cases. The results reveal that complex secondary flow distributions and velocity variations are critical factors influencing the thermal-hydraulic performance. Moreover, the influence of different standard TPMS cells on the flow and heat transfer characteristics varies during the hybridization process. Specifically, the inclusion of the Schwarz cell generally leads to an improvement in heat transfer, while the presence of the Lindinoid cell may result in a deterioration of heat transfer. The Diamond and Lindinoid cells have an adverse impact on the flow characteristics, while the Gyroid and Neovius cells enhance the flow characteristics. To evaluate the overall thermal-hydraulic performance of different channels, a comprehensive performance assessment is introduced. The Diamond channel and the Gyroid-Neovius channel show superior overall performance in their respective Reynolds number regions, with the former exhibiting better flow characteristics and the latter has outstanding heat transfer performance.

Numerical study on thermal–hydraulic performance of hydrocarbon fuel under trans-critical states and supercritical CO2 in a zigzag printed circuit heat exchanger

The supercritical CO2 closed-Brayton-cycle (CBC) is a highly promising integrated scheme for cooling and power generation in hypersonic vehicles. However, the CO2-fuel printed circuit heat exchanger (PCHE) in the CBC faces extreme heat loads and fuel consumption constraints, leading to complex heat transfer phenomena including trans-critical processes and buoyancy effects. In this study, the coupled heat transfer process between the large-molecule hydrocarbon fuel and supercritical CO2 inside the zigzag-type PCHE is investigated using a numerical simulation, based on the practical constraints of the onboard CBC system. The results demonstrate that the distributions of eddy viscosity and specific heat within the boundary layer are significantly improved by the turbulence-enhancing effect of the zigzag channel, thus avoiding local heat transfer deterioration caused by trans-critical processes. Additionally, numerous small-scale vortex structures are found to increase the source term of turbulent kinetic energy generation, effectively suppressing the weakening effect of buoyancy on turbulence. Finally, an optimization study is conducted on the resistance of the zigzag channel starting from the power generation performance of the CBC system. The findings indicate a 71% reduction in pressure loss coupled with only a 7% reduction in volume heat transfer rate compared to the original configuration.

Flow and heat transfer of supercritical hydrogen in a regenerative cooling channel with the arc ribs of a rocket engine

This paper reports on the heat transfer characteristics and flow performance of supercritical hydrogen in a regenerative cooling channel with arc-shaped ribs. A 3-D numerical model of hydrogen flowing inside the cooling channel has been established and the data are compared with experimental results. The effect of rib parameters such as the number of ribs n, rib angle θ, rib size (hr × e), and rib orientation (convex or concave) on the cooling channel performance has been explored. The results indicate that arc ribs effectively mitigate the heat transfer deterioration and thermal stratification phenomena by reducing the wall temperature; nevertheless, the pressure loss due to friction would considerably increase. In studied cases, the wall temperature significantly decreases when the rib angle θ increases from 0° to 90°. The results have also shown that increasing the size of square arc ribs while maintaining the same rib spacing p/hr and length of the ribbed section Lr is beneficial for the overall heat transfer performance. Compared to smooth channel, a channel with large 90° arc ribs oriented either convex or concave to the flow direction enhanced the thermohydraulic performance η by up to 45%. This enhancement in performance of channel due to arc ribs is 1.95 and 1.384 times higher than the previously known designs; V-ribs (23%) and cylindrical ribs (32.5%), respectively. The flow field has shown that the evolution and morphology of rib-induced streamwise vortex considerably improve the wall temperature reduction by intense fluid mixing. In addition, it has been shown that the direct impact of the coolant stream on the ribs, the reattachment of the separated shear layer, and the intermittent growth of the boundary layer helps to enhance heat transfer.

Failure analysis on outer tube breaks of a double-sleeve rapid cooling heat exchanger

Failure analysis of a double-sleeve rapid cooling heat exchanger used for ethylene cracking process is presented in this study. The outer tubes of the heat exchanger break four times in succession. The inner tube and the outer tube materials of the heat exchanger are 13CrMo4-5 and SA106Gr.B, respectively. Based on the analysis of fracture appearance, chemical composition, mechanical properties, oxidation products and metallographic microstructure, it is found that short-time local overheating resulted in the four outer tube breaks. The observed pearlite spheroidization of SA106Gr.B indicate the temperature at the break positions exceed the phase transition critical temperature AC1（735℃）. Based on the analysis of the heat transfer process and boiler water circulation, it is found that the heat transfer deterioration of in a single set of double-sleeve, i.e., water is completely converted into superheated saturated steam, is the root cause of short-time local overheating. In view of the above failure mechanism, several countermeasures were proposed including the verification of the circulation ratio to ensure that the heat absorption per unit volume of cooling water is within the threshold value should be conducted in the design process, considering the uneven distribution of cooling water and cracking gas at the inlet and the influence of shock vibration caused by water and cracked gas, an appropriate safety factor should be considered for the design of gap / water volume during operation, as well as promoting the monitoring of vital parameters (e.g., the temperature during operation).

S-CO2 flow in vertical tubes of large-diameter: Experimental evaluation and numerical exploration for heat transfer deterioration and prevention

Heat transfer deterioration (HTD) problems can undermine the thermal safety of heating tubes for supercritical carbon dioxide (S-CO2) flows. Relevant experimental evidence is highly desired, but most cases were investigated for small tube diameters (below 10 mm) and under limited operation parameters, far from the industry-scale applications. In the present work, large tubes (24 mm) are investigated experimentally with pressures of 7.5–15 MPa, mass flow rates of 100–1200 kg·m−2·s−1 and heat fluxes of 30–350 kW·m−2. The seriousness of HTD in large tubes was experimentally confirmed, and the mechanism is explored via a carefully-designed comprehensive comparison between present numerical simulations and experimental tests. Detailed analysis of vapor-like film development inspires us to mitigate the problem using structured-inner-surface, from the viewpoint of “supercritical pseudo-boiling”. This inspiration is further evaluated numerically: five types of enhancement structures are proposed to interfere with the development of vapor-like film. It was found that the proposed methods can fully prevent HTD problems, and especially the conical strips can achieve a high heat transfer performance (with performance evaluation criteria PEC of 1.21–1.35) and are thus recommended for HTD prevention in vertical large-diameter tubes under near-critical conditions. Overall, the experimental tests and numerical explorations can deliver more evidence on the theory and applications of supercritical fluid flow and heat transfer.

Thermalhydraulic assessment and design optimization of incorporating flow obstructors in a supercritical minichannel heat sink

Modulation of the thermalhydraulic characteristics of the miniaturized heat sink by geometric alteration is a less explored topic, particularly in conjunction with the supercritical fluids, which can experience both enhancement and deterioration in performance based on the conditions. The present study appraises the response of a square minichannel by incorporating rectangular obstructions, with the primary aim of identifying an optimized orientation and design. A performance evaluation criterion, defined as a combined change in both heat transfer coefficient and pressure drop because of geometric modification, is selected as the primary variable. Installation of the baffles substantially augments the turbulent mixing among the fluid layers, realizing a lower wall-to-fluid temperature differential and enhanced heat transfer coefficient. A relatively lower level of temperature is maintained within the baffled section, thereby delaying the appearance of heat transfer deterioration. Obstructions, however, considerably increases the pressure losses, necessitating careful selection of the dimensions. The use of three pairs of baffles has been identified to be the most feasible option. Desirable values of the height, thickness, and inclination angle for individual plates in this orientation have also been identified, along with a detailed thermalhydraulic assessment about the role of the operating variables with the optimum design.

Experimental study on the influence of volume concentration on natural convection heat transfer with Al2O3-MWCNT/water hybrid nanofluids

Recent research has focused on combining multiple types of nanoparticles with a base fluid, a process known as the hybridisation of nanofluids. Studies showed that hybrid nanofluids demonstrated improved rheological and thermal properties compared to mono nanoparticle nanofluids. In this study, alumina–multiwalled carbon nanotube/water hybrid nanofluid was prepared using a two-step procedure at a percentage weight ratio of 10:90 Al2O3: MWNCT at various volume concentrations of 0.00, 0.05, 0.10, 0.15, and 0.20 vol%. The natural convection of the formulated hybrid nanofluid was experimentally studied inside a square cavity with an aspect ratio of 1. An exhaustive investigation on the influence of volume concentration of hybrid nanofluid on natural convection heat transfer was performed. Results showed that an increase in volume concentration improved the heat transfer to an optimum value of 49.27% enhancement, at 0.10% volume concentration in comparison to the base fluid. However, a further increment of volume concentration led to a deterioration of natural convection heat transfer. There is evidence from this study that hybrid nanofluids offer an edge over traditional heat transfer fluids. It is concluded that the application of Al2O3-MWCNT/water hybrid nanofluid in a square cavity demonstrated enhanced natural convection heat transfer.

Improvement of Heat Transfer Model Considering Heat Transfer Deterioration of Supercritical Fluid in Cooling Channel of Scramjet Propulsion System

Improvement of Heat Transfer Model Considering Heat Transfer Deterioration of Supercritical Fluid in Cooling Channel of Scramjet Propulsion System

Toward Improved Correlations for Mixed Convection in the Downcomer of Molten Salt Reactors

Developing heat transfer correlations for buoyancy-driven flows and mixed convection is challenging, especially if the fluid’s Prandtl (Pr) number is not close to 1. For advanced nuclear reactor (Generation IV) designs, the downcomer plays a crucial role in normal operation and loss-of-power scenarios. The fluid-flow behavior in the downcomer can involve forced, mixed, or natural convection. Characterizing the heat transfer for these changing regimes is a serious challenge, especially in the heat transfer deterioration region. In this paper, the downcomer is simplified to heated parallel plates. The high–Pr number fluid FLiBe (a mixture of lithium fluoride and beryllium fluoride) is considered for all simulations. Direct numerical simulations using the graphics processing unit–based spectral element code NekRS are performed for a wide range of the Richardson number, from 0 to 400, at two different FLiBe Pr numbers (12 and 24). This results in an unprecedented 74 cases in total. Each case’s Nusselt number is calculated to evaluate existing heat transfer correlations. Moreover, we propose several new modifications for cases without satisfactory choice. As a result, several novel mixed-convection heat transfer correlations have been built for high–Pr number fluids. The correlations are expressed as a function of the buoyancy number, covering several mixed-convection regimes. The Pr number effect on the Nusselt number behavior is also analyzed in detail. We also propose a novel method to evaluate the heat transfer deterioration region. Modified Reynolds-Gnielinski forced-convection correlations are defined for the laminarization region, and a free-convection correlation is used for the natural-convection-dominated region. These correlations can describe well the trend in the heat transfer–deficient region.

Development of a New Heat Transfer Correlation for Flow of Supercritical Steam in Spiral Tubes Under Deteriorated Heat Transfer Regime

Abstract Heat transfer to supercritical steam flowing inside spiral tube under deteriorated heat transfer regime is investigated in this work using three-dimensional numerical simulations. The numerical solver is first validated with experimental results and subsequently multiple simulation cases are formulated with spiral tube by varying the flow and thermal conditions. The wall temperature and heat transfer coefficient of spiral tubes are compared with the same of straight tubes. Over the parameters range considered, the heat transfer coefficient is improved by 88% when straight tube is replaced with spiral tube, for similar flow and thermal conditions. Most significantly, the deteriorated heat transfer regime, which is otherwise witnessed in straight tubes is completely avoided by using spiral tubes, for the similar flow conditions. Tangential velocity, which arise due to centrifugal force is the primary reason of enhanced heat transfer in spiral tubes. Three standard heat transfer correlations are tested for spiral geometry under deteriorated heat transfer regime and it is found that none of those can predict the heat transfer coefficient accurately. Using the simulation results along with Buckingham π-theorem, a new correlation for heat transfer coefficient is proposed for spiral tubes carrying supercritical steam and operating under deteriorated heat transfer regime. The new correlation can predict the wall temperature and heat transfer coefficient accurately. The correlation has an error band of ±25%.

The effect of water on coking and heat transfer inhomogeneity of supercritical aviation kerosene in a curved mini-channel

Water addition to aviation kerosene draws increasing attention in increasing heat sink and suppressing coke formation for regenerative cooling of scramjet engines. However, the effect of water on flow, reaction and heat transfer, particularly the thermal inhomogeneity in curved cooling channel is rarely reported. In the present study, the effect of water on coking and heat transfer inhomogeneity characteristics of China aviation kerosene, RP-3 studied in a uniformly heated curved mini-channel. A transient three-dimensional numerical simulation model with hybrid pyrolysis and steam reforming reactions of RP-3 is established and validated. The results show that there exists serious mass and thermal stratification in the curved mini-channel due to secondary flow and species diffusion. Specifically, RP-3 and water are diffused to concentrate at the bottom heating wall surface while coke deposition and light gaseous species, e.g., H2, CH4, CO are concentrated at the top heating wall surface. The thermal difference between the top and bottom heating walls is significant. Temperature and coke content of the top heating wall are much higher than those of the bottom heating wall. The gas film and coke deposition formed at the top heating wall surface lead to serious heat transfer deterioration. Therefore, the heat transfer coefficient at the bottom wall of the curved channel is much higher than that of the top wall. Water addition enhances secondary flow and thus exacerbates the coking and heat transfer inhomogeneity in the curved cooling channel. With water content increasing from 0 to 15 wt%, the secondary flow velocity increases by 288.2%, the conversion of RP-3 increases by 114.8% and the average coke content on the heating walls decreases by 40%. Meanwhile, the average heat transfer coefficient increases by 33.9% at the bottom heating wall but decreases by 18.0% at the top heating wall with the increase of water content.

Thermalhydraulic characterization and feasibility assessment of double-cooled annular channel under supercritical heat transfer

Appearance of buoyancy-induced heat transfer deterioration is a common concern during supercritical heat transfer through vertical channels. A novel double-cooled annular configuration is proposed and numerically analyzed here in an attempt to eliminate such deterioration. Supercritical CO2 is allowed to flow in parallel through both inner circular duct and outer annular space, separated by an external volumetric heater. Axisymmetric conservation equations are solved, coupled with suitable turbulence model and property relations. Substantial improvement in heat transfer coefficient is observed for both the streams in comparison with an equivalent plain annular channel, along with considerable reduction in wall temperature. Axial gradient of bulk enthalpy and intensity of turbulence-inspired mixing are earmarked as the primary factors facilitating enhanced heat transfer. Increase is supply flow rate promotes turbulence, and results in significant gain in both peak and average heat transfer coefficient. Heat transfer performance degrades at higher power because of impaired mixing, leading to greater wall-to-bulk temperature differential, whereas exit pressure has only limited influence. Considerable difference can be observed in the mass flux, as well as in velocity and turbulence levels, across both the streams, which is augmented further during downward motion. No deterioration has been observed over the entire parametric ranges explored here. Proposed configuration successfully suppresses the impact of both buoyancy and acceleration on heat transfer, substantiating itself as a viable technical option.

Performance evaluation with turbulent flow and heat transfer characteristics in rectangular cooling channels with various novel hierarchical rib schemes

Turbulators, such as ribs, dimples, and pin-fins, play a vital role in the internal cooling efficiency of turbine blades. As a typical turbulator, various rib configurations using a uniform arrangement scheme have indicated high heat transfer enhancement but the friction loss is simultaneously subject to a great increase. In this work, a novel hierarchical arrangement scheme of ribs is developed aiming to improve the cooling efficiency. Adopting the uniform scheme as a baseline, the hierarchical scheme is implemented for six representative rib configurations (including transverse ribs, angled ribs, V-shaped ribs, inverted V-shaped ribs, M-shaped ribs, and inverted M-shaped ribs) and evaluated for its feasibility and generality. For different cooling designs, turbulent flow and heat transfer of the ribbed cooling channel are studied by three-dimensional numerical simulations based on the finite volume method with a constructed turbulence model. It is found that for all rib configurations, the hierarchical scheme can remarkably reduce the friction loss as desired, especially for the inverted V-shaped rib with a reduction of up to 50%. Due to the occurrence of flow separation, secondary flows offered by transverse ribs are characterized by a two-dimensional recirculation vortex behind the rib. For other rib configurations, secondary flows present a typical three-dimensional characteristic including the downwash flows along the inclined rib leg and the longitudinal vortices. The usage of the hierarchical scheme with small ribs strongly suppresses these secondary flows, which contributes to the significant decrease in form drag loss. Meanwhile, using the hierarchical scheme produces a slight heat transfer deterioration commonly, which is because the constrained secondary vortices weaken the turbulent mixing and convection heat transfer. Significantly, for the two W-shaped ribs, the limited secondary vortices but fully developed under the hierarchical scheme achieve a higher heat transfer enhancement. Finally, for all considered ribs, the hierarchical scheme can improve the overall performance factor of (Nu/Nu0)/(f/f0)1/3 by more than 10%, and up to 21.15% for the V-shaped rib. Adjusting design variables, including the decreasing ratio of the rib size and the initial rib size, the hierarchical scheme still provides even higher performance enhancement.

Thermal performance analysis and burning questions of refrigerant direct cooling for electric vehicle battery

Refrigerant direct cooling technology has been gaining attention due to its advantages of simple system composition and high cooling efficiency, but its temperature control performance does not have advantages over mature liquid cooling in industrial applications, reasons for which should be discovered. This paper proposes a novel discrete model based on an electro-thermal coupling method and thermal resistance network to analyze the local temperature control performance of refrigerant direct cooling and liquid cooling systems under real operating scenarios with acceptable computing time and accuracy. The results show that the phase change of the refrigerant in the refrigerant direct cooling plate significantly affects the temperature control performance. The vapour-liquid two-phase zone has a high heat transfer capability, which can dissipate heat well and maintain a small temperature difference with the maximum temperature of the battery reduced by 28.3% compared with the liquid cooling system. There is a serious heat transfer deterioration in the superheated gaseous zone, causing a significant increase in the local battery temperature, and the maximum temperature difference is much higher than that in the liquid cooling system. When the refrigerant switches from two-phase to single-phase under 2C charging condition, the heat transfer coefficient reduces by roughly 73.6%, which causes the accompanying battery unit temperature to rise by about 12 ∼ 14 °C. Besides, the low refrigerant temperature will cause a large temperature difference in the vertical direction of the battery, which restricts the application of the refrigerant direct cooling system in practice. When the evaporation pressure increases from 300 kPa to 500 kPa, the temperature difference will decrease from 14.1 °C to 8.8 °C which is a 37.6% reduction. Through an in-depth analysis of the local temperature distribution of battery units, two burning questions were identified which deteriorate the temperature control performance of the refrigerant direct cooling system and results can provide directions for performance improvement.

Experimental determination of the role of roughness and wettability on pool-boiling heat transfer of refrigerant

In order to investigate pool boiling heat transfer characteristics of refrigerants on different surfaces, an experimental rig is established and verified in this study. The pool boiling heat transfer experiment is conducted with R134a at three saturation temperatures. Six surfaces with Ra roughness ranging from 0.133 to 5.742 μm and two types of wettability are tested in the study. The heat flux varies from 40 to 280 kW/m2. Experimental results indicate that higher roughness is favorable to boiling heat transfer. Shieldex coating on copper surfaces can deteriorate heat transfer by 20% approximately. Visualization analysis was carried out with a high-speed camera to study bubble behavior. A new-found mode of bubble departing was observed at lower heat flux. Besides, the data were correlated in the form of Rohsenow correlation, and comparison and supplement are concluded based on data obtained in this study.

Heat Transfer and Flow Structure Characteristics of Regenerative Cooling in a Rectangular Channel Using Supercritical CO2

At an extremely high Mach number, the regenerative cooling of traditional kerosene cannot meet the requirement of the heat sink caused by aerodynamic heating and internal combustion in a scramjet propulsion system. As a supplement of traditional regenerative cooling, supercritical CO2 is regarded as an effective coolant in severe heating environments due to its excellent properties of heat and mass transportation. In this paper, the heat transfer and flow structure characteristics of regenerative cooling in a rectangular channel using supercritical CO2 are analyzed numerically using a validated model. The effect of heat flux magnitude, nonuniform heat flux, acceleration and buoyancy and flow pattern are considered to reveal the regenerative cooling mechanism of supercritical CO2 in the engine condition of a scramjet. The results indicate that the heat transfer deterioration phenomenon becomes obvious in the cooling channel loaded with relatively high heat flux. Compared with the cooling channels loaded with increased heat flux distribution, the maximum temperature increased for the channel loaded with decreased heat flux distributions. When larger acceleration is applied, a relatively lower wall temperature distribution and higher heat transfer coefficients are obtained. The wall temperature distribution becomes more uniform and the high-temperature region is weakened when the coolants in adjacent channels are arranged as a reversed flow pattern. Overall, the paper provides some references for the utilization of supercritical CO2 in regenerative cooling at an extremely high Mach number in a scramjet.

Investigation of the heat transfer enhancement and deterioration induced by vortex generators in low Prandtl number sodium-potassium alloy liquid

Liquid metals, being characterized by low molecular viscosity and high molecular thermal conductivity, appear to be a stupendous choice for heat transfer fluids in deep-space nuclear reactors. Nevertheless, because of space nuclear reactor weight and volume constraints, the heat transfer capability of the heat exchanger must be substantially improved for a deeper space exploration mission. In this study, the influence of the vortex generator applied to the liquid metal was investigated to further promote the heat transfer capability of the liquid metal. And the thermo-fluid behaviors of the sodium-potassium alloy (NaK-78) are compared among the channel with the Body-Centered Cubic (BCC) lattice array, the rectangular ring rib array, and the Kagome lattice array under identical porosity. The numerical results indicate that the heat transfer deterioration phenomena occurred in the case of rectangular ring rib channel, which is a relatively rare occurrence in conventional fluids with the local Nu number even being lower than that of the smooth channel at the same location. Therefore, the rectangular ring rib should be carefully considered and evaluated when it is applied to heat transfer enhancement in NaK alloy fluid. The Kagome structure and the BCC structure could effectively promote the heat transfer performance of the NaK alloy fluid by means of the induced composite vortex system. In comparison to the smooth channel, the average Nu of the cases with the Kagome array and the BCC array yield 17.26–37.32% and 51.98–78.85% higher, respectively.

Experimental investigation of combustion and ash deposition characteristics of coal-NH3 cofiring in a 200 kW furnace

The coal-NH3 co-firing experiment was conducted in a 200 kW one-dimensional furnace to study the flue gas emission characteristics and ash deposition characteristics under different ammonia co-firing ratios. As ammonia is a carbon-free fuel, CO2 shows a decreasing trend, while H2O gradually increases with the increase of ammonia co-firing ratio. With the increase of ammonia co-firing ratio, the proportion of unburned carbon increases slightly. Compared with the ammonia in flue gas, the ammonia attached to ash is the main reason for ammonia slip. The total ammonia slip rate of the three co-firing ratio conditions is 0.26% (5% NH3), 0.29% (10% NH3) and 0.67% (20% NH3), respectively. The NOx emission reaches the maximum of 349.05 ppm when the ammonia co-firing ratio is 10%, then decreases as the ratio increase to 20%. Ammonia co-firing does not change the composition of ash particles but has a certain deterioration of heat transfer of heat exchange surface in high temperature zone of furnace. This is related to the adhesion of ammonia on the surface of fly ash and the reduction of the size of fly ash.

Enhancement of flow boiling in the microchannel with a bionic gradient wetting surface

Flow boiling in microchannels is promising to enhance electronic heat dissipation (urgently required). However, vigorous boiling in a narrow space can trigger serious flow reversal and heat transfer deterioration, restraining further performance enhancement. In this work, a bionic gradient wetting surface was developed to overcome the challenge. The upstream hydrophobicity and surface variation induced flowing bubble expansion and frequent interface fluctuation, benefiting the backflow suppression and churn flow appearance. While the downstream hydrophilicity improved liquid supply and vapor venting, preventing partial dry-out and pressure drop increase. The flow regimes on the bionic surface were demonstrated by the quantitative analysis of two-phase flow, exhibiting the shortest backflow duration time and the second-high frequency of churn flow appearance. Moreover, the gradient wettability showed negligible oscillations in average wall temperature and pressure drop and achieved the maximum enhancement rate of 50.1% and 63.2% for the heat transfer coefficient and performance evaluation criterion, respectively. The surface combination scheme incorporated the merits of different wetting surfaces and significantly enhanced the integrated performance, showing immense potential in high-efficient thermal management applications.

Direct numerical simulations of cooling flow and heat transfer in supercritical CO2 Brayton cycle coupled with solar energy

Direct numerical simulation was used to study supercritical carbon dioxide’s flow and heat transfer mechanisms under cooling conditions in the supercritical CO2 Brayton Cycle coupled with solar energy. This paper aims to investigate the effects of property variation, buoyancy, and thermal deceleration under different flow conditions. Momentum balance analysis is used to quantify changes caused by different mechanisms and to describe the flow and turbulence evolution characteristics. In forced convection, due to the dominant thermal deceleration effect, the pressure gradient decreases across the entire cross-section, resulting in the deceleration of the fluid. In upward flow, buoyancy plays a dominant role and acts as a resistance, causing the pressure gradient to increase significantly. As a result, when the core fluid accelerates, the velocity of the near-wall fluid slows significantly, enhancing heat transfer, and the Nusselt number increases by up to 33.21% compared to forced convection. In downward flow, buoyancy is strong, the pressure gradient is negative, and the flow is entirely driven by buoyancy. External and structural buoyancy effects contribute to flow laminarization, leading to heat transfer deterioration. With increasing heat flux, Nu decreases more significantly, with a maximum reduction of 62.94%, and begins to recover after reaching its lowest point. Importantly, in the recovery stage, the structural buoyancy effect triggers turbulence recovery. The recovery of shear production of turbulent kinetic energy is later than that of buoyancy production and can only occur with a strong M−shaped velocity distortion. The FIK identity decomposition shows that the heat transfer deterioration is related to flow laminarization and inhomogeneous effects. Heat transfer recovery is purely due to the increase in turbulence components.