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• An adaptive strategy for active thermal convective cloak via heat gain and loss is proposed. • Adaptive heat sources have been demonstrated to be feasible for heat convection. • An inverse process is built to calculate the heat source via conjugate gradient method. • Thermal cloak is experimentally validated using a heat source matrix. Thermal metamaterials, known for their effective and directional heat control and management strategy, have garnered significant attention leading to the investigation of various functional thermal conductive metadevices. Nevertheless, conventional thermal convective metadevices resort to artificial structures in the flow field, which face the challenge of the coupling of heat and mass transfer and unstable flow field. Herein, we report an adaptive strategy for fabricating an active thermal convective cloak via tunable heat gain and loss. Adaptive body and surface heat sources, rather than graded structures inside functional regions, have been demonstrated to be feasible for heat convection. Accompanied by the active manipulation scheme, an inverse design process was constructed to calculate the heat source distributions based on the knowledge of the target temperature fields. The proof-of-concept heat source matrix, using well-laid out resistance heaters and thermoelectric components, experimentally validated the thermal cloaking pattern in the flow field. The active scheme overcomes the limitations of the complex engineered structures required for conventional thermal convective metadevices. Thus, in this study, we establish a new paradigm for thermal convective camouflage and pave the way for active thermal management in heat convection. [ABSTRACT FROM AUTHOR]

• Similarity principle analysis on nanochannel-based i-TE system is constructed. • Fifteen dimensionless variables are derived to reflect dimension unification. • Performance optimization and sample expansion are realized for applications. • Parameters dominating the nanochannel-based i-TE performance are presented. Ionic thermoelectric (i-TE) systems convert low-grade heat into electricity based on ion migration driven by the Soret effect under a temperature gradient. Current research conducts tentative experiments for superior i-TE materials and performance, whereas universal physical quantities reflecting the unified essence are still needed to guide experiments. In this study, the unified mechanism of i-TE energy conversion is analyzed using similarity principle by establishing nanochannel-based i-TE models, considering the Soret effect and heat transfer. Fifteen dimensionless variables are derived to represent the i-TE energy conversion process. Dimension unification is achieved by normalizing various dimensional phenomena with the deviation of 48.3% into one dimensionless situation with the deviation of 0.011%. These dimensionless variables can be further utilized to practical applications. Specifically, upon equal dimensionless variables, sample expansion is achieved with relative errors within 0.018% and the total time acceleration ratio of 7.33, alleviating experimental burden. Besides, the i-TE output power can be improved by 100% upon higher characteristic output power and equal dimensionless variables. Orthogonal tests are further implemented to clarify the parameter dominance, showing that the cationic reduced Soret coefficient is dominant on performance with contribution rates more than 25%. This work provides unified insight into coupling relations of multi-physical parameters in i-TE energy conversion to guide experimental designs. [ABSTRACT FROM AUTHOR]

Energy efficiency evaluation of natural draft wet cooling towers (NDWCTs) is essential to the energy saving process. There is a lack of comprehensive methods that can evaluate and grade the NDWCTs energy efficiency quantitatively. In this study, a comprehensive energy efficiency assessment indicator (CEEAI) including thermal performance, water conservation, and electric saving aspects is proposed. The CEEAI is based on the analysis of NDWCTs mass and heat transfer mechanism. Total 48 actual test reports of the NDWCTs whose water flow rate changes from thousands to hundred thousand tons per hour for test period spans of 16 years, are collected as the database. The CEEAI is validated through statistical analysis and thermodynamics law. Based on the CEEAI and database, an energy-efficiency grading criteria is proposed, the NDWCTs are graded into high, middle and low energy efficiency. Therefore, the energy efficiency of one NDWCT can be clearly observed, which is beneficial for the common researchers to understand the energy efficiency level among all the NDWCTs. In addition, through comparing the CEEAI of different NDWCTs, the energy efficiency gap can be obtained, which provides the optimization guidelines and theoretical support for the improvement of the overall energy efficiency level for the NDWCTs. • A comprehensive energy evaluation assessment indicator for NDWCT was developed. • 48 NDWCTs actual test reports with various size and years was collected as database. • The proposed indicator was verified by statistical and thermodynamic analysis. • An energy-efficiency grading criteria was proposed based on indicator and database. [ABSTRACT FROM AUTHOR]

Three-dimensional numerical studies were performed for the performance of wavy fin-and-tube heat exchangers in Body-Fitted Coordinates (BFC) system. Effects of geometric parameters on air-side heat transfer and fluid flow characteristics and fin efficiency were examined. The results showed that with the increase in Reynolds number, wavy angle, fin thickness and the decrease in fin pitch and transverse tube pitch, the heat transfer performance are enhanced; however, pressure drops are also increased. So, in practical applications, the wavy angle had better be located between 10° and 20° and the fin pitch should be located between 1.2 mm and 2.0 mm. The fin efficiency and average fin surface temperature decrease with the increase of Reynolds number, wavy angle, fin pitch and transverse tube pitch. With the increase of fin thickness, the fin efficiency and average temperature on fin surface also increase. [ABSTRACT FROM AUTHOR]

Gas catalytic reaction in porous materials is integral to the chemical industry. A novel reactive direct simulation Monte Carlo model is proposed to investigate the hydrogen gas reaction in three dimensional porous fibrous materials with flow state across from rarefied to continuum regimes. Complicated processes involving mass, gas flow, heat transfer, radiation effect, and evolving process of the solid skeleton within chemical reactions can be considered simultaneously. The effects of catalyst coverage and gas ratio on heat and mass transfer in the evolved porous fibrous materials are investigated. Results show that the porous fibrous skeleton evolution results in an increase in temperature compared to the scenarios neglecting skeleton evolution across rarefied to continuum regimes. The temperature increases with an increase in catalyst coverage and inlet hydrogen content. Both chemical reaction heat induced by gas molecules and skeleton evolution rate are contributed to the temperature rise within porous fibrous materials. A formula is derived to accurately and efficiently predict the effect of various factors on the temperature in porous fibrous materials. Above findings can improve the understanding of the real chemical reaction process in porous materials. • Reactive DSMC model under full regimes in evolved porous media is developed. • Skeleton evolution results in an increase in temperature with Kn decreasing. • Chemical reaction heat and skeleton evolution both increase temperature. • Prediction formula is constructed to predict factor effect on temperature. [ABSTRACT FROM AUTHOR]

To quickly optimize the cooling performance of three-dimensional coolant channels in a proton exchange membrane fuel (PEMFC) cell, a generative adversarial network (GAN) model that adopts small-scale sample data to predict the coolant channel topology structure is constructed. The coordinate information of the topological pseudo-density field is directionally input into the neural network to improve prediction accuracy. The results show that more microtopologies in the coolant channels can be predicted with the GAN model than with traditional thermal–fluid–structural topology optimization. The topological configuration with a maximum temperature of 0.15 K, lower than the training dataset, is achieved. The well-trained GAN model can generate topological structures for the optimization objective, that is, thermal performance superior to the training set. The proposed model can also generate topological structures with better fluid and structural performance, which is 20.44% and 9.25% superior to that of the training dataset, respectively. The computation time required by the GAN model is <1 s, while the computation time for the same case is 30.2 h using the traditional thermal–fluid–structural topology optimization. These findings can aid in designing coolant channels in PEMFCs. • Deep-learning accelerating topology optimization of 3D coolant channel is proposed. • Micro-topologies in coolant channels is well predicted with GAN model. • Topologies with better fluid and structural performance are obtained using GAN model. • Computation time is obviously reduced compared to traditional topology optimization. [ABSTRACT FROM AUTHOR]

• Novel reversely-variable circuitry for finned-tube heat exchanger (FTHX) proposed. • Refrigerant flowpath of FTHX alters flexibly between evaporator and condenser modes. • Circuits are fewer for condenser but more for evaporator to match respective needs. • Fewer circuits enhance overall heat transfer coefficient for condenser. • Overall energy performance is improved for air source heat pump (ASHP). The finned-tube heat exchanger (FTHX) is preferred to contain more circuits as an evaporator but fewer as a condenser, so as to obtain the desired performance in both heating and cooling operation of the air source heat pump (ASHP). However, the conventional FTHX has two-way fixed circuitry with the same refrigerant flowpath in the opposite direction. In this article, the reversely-variable circuitry is proposed for the FTHX to exhibit different flowpath in evaporator and condenser roles flexibly. The original 4-branch distributor in the outdoor FTHX of a nominal 3500-W ASHP is replaced with a 2-branch one and a 3-branch one successively to maintain 4 circuits as an evaporator. Two check valves, however, are added between the two distributors and inside the gas header to change the flowpath to two circuits merging into one as a condenser. Compared with the two-way fixed FTHX, the reversely-variable one yields 5.8% larger cooling capacity and 7.2% higher EER for the ASHP with similar heating performance under nominal conditions. Further simulation shows that fewer circuits of the reversely-variable FTHX increase the overall heat transfer coefficient as a condenser, dominating the increase in the capacity of the FTHX and the overall energy performance of the ASHP. This novel design provides new thoughts to the FTHXs and can be popularized to those with different circuits. [ABSTRACT FROM AUTHOR]

• DSMC coupling FDM is proposed to study heat transfer in moved jet impingement. • Factors influencing moved jet heat transfer are analyzed. • Positions of the highest and lowest temperatures move along with the substrate. • Structure parameters satisfying with temperature control requirement are recommended. Temperature control systems in low-pressure environments are an important component of precision instruments. In this study, a comprehensive model coupling the direct simulation Monte Carlo (DSMC) method with the finite difference method is proposed to investigate the heat transfer due to the impact of a moving rarefied hydrogen jet. The cooling heat flow calculated using the DSMC method and radiant heat flow calculated using the boundary integral method are considered as the boundary conditions of the substrate, and the temperature distribution of the substrate is solved using the finite difference method. The influences of different inlet pressures, jet apertures, and impact distances on the jet heat transfer intensity are analyzed. The results indicate that the average convective heat transfer coefficient of jet impingement is positively correlated with the inlet pressure and jet aperture, whereas it is inversely correlated with the impingement distance. The structural parameters that meet the temperature control requirements in the substrate are determined using the proposed comprehensive model. These results can be used as a guide for the design of high-precision temperature control devices. [ABSTRACT FROM AUTHOR]