28 August 2024, Volume 33 Issue 5
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Journal of Thermal Science. 2024, 33(5): 1607-1617. https://doi.org/10.1007/s11630-024-2019-8Direct pore-scale and volume-averaging numerical simulations are two methods for investigating the performance of porous volumetric solar receivers. To clarify the difference in the prediction of heat transfer processes, a direct comparison between these two methods was conducted at both steady state and transient state. The numerical models were established based on X-ray computed tomography scans and a local thermal non-equilibrium model, respectively. The empirical parameters, which are indispensable to the volume-averaging simulation, were determined by Monte Carlo ray tracing and direct pore-scale numerical simulations. The predicted outlet air temperature of the receiver by the volume-averaging simulation method corresponded satisfactorily to that in the direct pore-scale simulation. The largest discrepancies were observed when the receiver’s working temperature was elevated, with differences of 5.5% and 3.68% for the steady state and transient state simulations, respectively. However, the volume-averaging method is incapable of capturing the local temperature information of the air and porous skeleton. It underestimates the inlet temperature of the receiver, leading to an overestimation of the receiver’s thermal efficiency, with the largest difference being 6.51%. The comparison results show that the volume-averaging model is a good approximation to the pore-scale model when the empirical parameters are carefully selected.
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Journal of Thermal Science. 2024, 33(5): 1618-1629. https://doi.org/10.1007/s11630-024-1963-7In this paper, a novel NH3/CO2 ejector-cascade refrigeration system with regenerator is proposed, which can recycle the waste heat at the outlet of the compressor. After establishing the mathematical model of the system, the theoretical energy and exergy analysis are carried out and compared with the conventional cascade refrigeration system. It is concluded that compared with the conventional cascade refrigeration system, the novel ejector-cascade refrigeration system with regenerator has the advantages of less power consumption of the compressor, less component exergy destruction, high system performance, and is more suitable for working at a lower temperature. Under the working conditions studied in this paper, compared with the conventional cascade refrigeration system, the COP of the novel ejector-cascade refrigeration system with regenerator is increased by 9.58%; the exergy efficiency is increased by 9.50%, and the optimal evaporation temperature is about –45°C.
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Journal of Thermal Science. 2024, 33(5): 1630-1641. https://doi.org/10.1007/s11630-024-1977-1The supercritical CO2 Brayton cycle has potential to be used in electricity generation occasions with its advantages of high efficiency and compact structure. Focusing on a so-called self-condensing CO2 transcritical power cycle, a model was established and four different layouts of heat recuperation process were analyzed, a without-recuperation cycle, a post-recuperation cycle, a pre-recuperation cycle and a re-recuperation cycle. The results showed that the internal normal cycle’s share of the whole cycle increases with increasing the cooling pressure and decreasing the final cooled temperature. Heat load in the supercritical heater decreases with increasing the cooling pressure. From perspective of performance, the re-recuperation cycle and the pre-recuperation cycle have similar thermal efficiency which is much higher than other two layouts. Both thermal efficiency and net power output have a maximum value with the cooling pressure, except in the condition with the final cooled temperature of 31°C. Considering both the complexity and the economy, the pre-recuperation cycle is more applicable than the other options. Under 35°C of the final cooled temperature, the thermal efficiency of the pre-recuperation cycle reaches the peak 0.34 with the cooling pressure of 8.4 MPa and the maximum net power output is 2355.24 kW at 8.2 MPa of the cooling pressure.
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Journal of Thermal Science. 2024, 33(5): 1642-1656. https://doi.org/10.1007/s11630-024-2008-yThermal energy conversion and also storage system is to advance knowledge and develop practical solutions at the intersection of micro and nano-scale engineering, energy conversion, and sustainability. This research addresses the challenge of enhancing these critical aspects to ensure prolonged system performance and durability in the context of evolving energy technologies. This research analyses the anti-oxidation and filtration behaviours of micro and nano-scale structures in the context of electro- and photo-thermal energy conversion and also storage systems. A micro multiscale hierarchical structure strategy is presented to fabricate multi-scale double-layer porous wick evaporators with the electrospun nanofibers made of gelatin-polyamide 6 (GPA6) and Ti3C2Tx MXene/silver nanowire with Cellulose Micro/NanoFibers (CMNF) cryogens by using spark plasma sintering (SPS) based high-pressure hydrothermal treatment model. An excellent anti-oxidation effect was offered by coating the film in thermal conditions and the anti-oxidation properties were further examined from 500°C to 850°C. The results are analysed using Matlab software to improve the efficiency of energy conversion processes by integrating nanostructures into thermal systems, to increase energy output while minimizing losses. The silver nanowire is with a heat transfer coefficient of 78%, a mass remaining rate of 98.7%, and an energy storage efficiency of 23.8%. This study enhances energy density and duration by integrating nanostructures into thermal systems while minimizing energy losses, and it not only exhibits excellent anti-oxidation properties but also possesses superior filtration capabilities for designing and engineering multifunctional nanomaterials.
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Journal of Thermal Science. 2024, 33(5): 1657-1671. https://doi.org/10.1007/s11630-024-1973-5A new solar energy and biomass-based distributed energy system using H2O/CO2 hybrid gasification is proposed, and their complementarity to enhance the system’s energy efficiency is investigated and shown. In the system, concentrated solar energy is used to provide heat for biomass gasification; two gasifying agents (H2O and CO2) are adopted to enhance syngas yields, and the produced solar fuel is further burned for power production in a combined cycle plant. Results show that CO share in gasification products is remarkably increased with the increment of CO2/H2O mole ratio caused by the boudouard reaction with the consumption of fixed carbon, while the H2 share is decreased; the optimal solar-to-fuel efficiency, 27.88%, is achieved when the temperature and CO2/H2O mole ratio are 1050°C and 0.45, respectively. The emission reduction rate of CO2 in the system under design conditions is reduced by 2.31% compared with that using only H2O agent. The annual power production of the system is increased by 1.39%, and the thermodynamic and environmental performances are significantly improved. Moreover, an economic assessment is conducted to forecast the technical feasibility of the hybrid gasification technology. This work provides a promising route to improving the thermochemical utilization efficiency of solar energy and solid fuel.
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Journal of Thermal Science. 2024, 33(5): 1672-1687.The heat transfer efficiency of a thermal energy storage unit (TESU) can be improved by the addition of novel longitudinal fins. A series of TESUs are analyzed using the finite volume method (FVM) to determine the effect of fin angle on the heat transfer performance. As the fin angle increases, the TES rate first increases, then decreases, reaching a maximum rate at 60°, where the melting time is less by 30.9%, 28.58%, 21.99%, 9.02%, and 18.1% than at 0°, 15°, 30°, 45°, and 80°, respectively. In addition, it is found that the melting time of the phase change material is significantly greater at the bottom of the TESU. The time percentage of this stage decreases as the fin angle increases through these percentages by 7%, 14%, 23%, 33%, and 20%, respectively. Further, the response surface methodology (RSM) is applied to optimize the longitudinal fin by minimizing the total melting time. The analysis concludes that a fin angle of 58.68° reduces the complete melting time of the stearic acid by 44% below the time at 0°. These findings fill a gap in knowledge of the effect on melting performance of the design angle of longitudinal fins and provide a reference for the design of horizontally placed longitudinal finned thermal energy storage units.
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Journal of Thermal Science. 2024, 33(5): 1688-1700. https://doi.org/10.1007/s11630-024-2020-2The melting process of solid-liquid phase change materials (PCM) has a significant impact on their energy storage performance. To more effectively apply solid-liquid PCM for energy storage, it is crucial to study the regulation of melting process of solid-liquid PCM, which is numerically investigated based on double multiple relaxation time lattice Boltzmann method (MRT-LBM) in this work. In this work we pay more attention to the effects of different Stefan numbers (Ste) and Rayleigh numbers (Ra) on the melting process. The results indicate that the PCM melting is greatly influenced by the Ste number and Ra number, which can be divided into the heat conduction dominant stage and the convection dominant stage, according to the onset time of convection FoC. In order to describe the contribution of the heat conduction dominant stage to the whole melting process quantitatively, we firstly propose the ratio of the heat conduction dominant stage Rpc, which can be defined as the ratio of FoC to the complete melting time FoM. Rpc gradually decreases as the Ra number increases, and when the Ste number rises: Rpc=90.0% when Ste=1.0 and Ra=1×105, Rpc=39.6% when Ste=0.1 and Ra=1×105, and Rpc=14.0% when Ste=1.0 and Ra=1×107. A regime map about the effects of different Ste numbers and Ra numbers on Rpc has been further summarized. The discovered findings would be helpful in regulating melting process in the energy storage of solid-liquid PCM.
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Journal of Thermal Science. 2024, 33(5): 1701-1711. https://doi.org/10.1007/s11630-024-1982-4Rectangular microchannel heat sinks (MCHS) are widely used to cool high-heat-flux electronic devices. However, previous studies focused mainly on MCHS with uniform channels (UCs). This study considers a microchannel heat sink with non-uniform channels (NUCs). A mathematical model is developed based on energy equations and the Darcy flow principle. Explicit expressions for total thermal resistance and coolant pressure drop are derived using the thermoelectric analogy. Experiments and numerical simulations are performed to verify the mathematical model. As non-uniformity increases, total coolant pressure drop decreases but at the cost of higher thermal resistance. The overall performance of NUCs is better than that of UCs because of their lower ratio of pumping power to cooling power. Heat transfer performance of NUCs changes little for more than 120 channels and depends mainly on channel arrangement. A multi-objective optimization is conducted to minimize the thermal resistance and pumping power of an NUC. An optimal NUC saves 64% pumping power compared with a conventional UC for the total thermal resistance of 0.1°C/W, indicating that the use of non-uniform channels could be very helpful to reduce the flow resistance of MCHS.
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Journal of Thermal Science. 2024, 33(5): 1712-1725. https://doi.org/10.1007/s11630-024-1931-2To unravel the intricacies of two-phase gas-liquid flow characteristics and heat transfer behavior, an array mini-channel gravity plate heat pipe (AMGPHP) is proposed in this work, which allows for observing the internal changes in the state of the working fluids. The flow patterns such as pool flow, columnar flow, and slug flow, are experimentally explored and analyzed in detail. It is found that the optimal volume fill ratio is 20% by utilizing start-up time and thermal resistance as performance evaluation metrics. With this fill ratio, a medium optimization strategy by blending ethanol within R141b is proposed and evaluated. In comparison to pure working fluids, the heat transfer performance of AMGFHP in the binary fluid has been significantly augmented due to temperature and concentration shifts resulting from disparate boiling points. Under the 10% volume fraction ethanol blending condition, the equivalent thermal conductivity of the heat pipe is dramatically elevated, with a value of 3110 W/(m·°C), along with the reduction of the minimum start-up power to 4 W. In general, applying such a medium to heat pipes has considerable potential in practical applications.
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Journal of Thermal Science. 2024, 33(5): 1726-1743. https://doi.org/10.1007/s11630-024-1992-2This research investigates innovative fin-type radiators for automobile engine cooling system. Micro-channel and helical radiators, along with straight type, were analyzed for heat transfer characteristics under various conditions. The uniqueness of this study is evident in the design of microchannel and helical radiators. For helical radiators, the inner rod features 4/8 helical-shaped water galleries, while the outer tube frame with embedded fins remains consistent. In contrast, the microchannel radiators have compact trapezoidal-shaped water galleries with separate fin strips. Furthermore, the novelty of the research is enhanced by the utilization of 3D printing technology in the manufacturing process. In constant fin height analysis at varied water and air flow rate, Microchannel Water Air Radiator with fin height 10.5 mm (MCWAR10.5) depicted a higher heat transfer rate amongst the radiators. In comparison to Straight Water Air Radiator with fin height 9.5 mm (SWAR9.5), the heat transfer rate is 30.3% and 1.3 times higher. However, in constant fin surface area analysis, microchannel radiator (MCWAR3.2) illustrates lower heat dissipation than Helical radiator (HWAR138) but higher than HWAR134 and Straight radiator (SWAR6). The examination of pumping loss indicated that the Micro-channel radiator outperformed helical radiators due to its lower pressure loss. The average pressure loss for Micro-channel radiators was 0.74 kPa, making it 1.2 times higher than that of a straight radiator (0.62 kPa), indicating a better trade-off.
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Journal of Thermal Science. 2024, 33(5): 1744-1756. https://doi.org/10.1007/s11630-024-2007-zThe design of heat exchangers in the advanced supercritical power conversion system cannot be separated from the study of heat transfer issues. Half-side heating mode is often encountered for solar receiver and supercritical boiler. Here, the characteristics of supercritical CO2 (sCO2) convection heat transfer in vertical tubes with circumferentially half-side heating was numerically investigated through the SST k-ω turbulent model which matches well with the experimental data. Then, heat transfer between sCO2 upflow and downflow was compared. Similar to film boiling heat transfer at subcritical pressure, numerical results were processed according to the supercritical pseudo-phase transition hypothesis, with liquid-like phase in the tube core region and vapor-like film in the region near the heated tube wall. The structure of two layers was demarcated by pseudo-critical temperature Tpc. Therefore, sCO2 heat transfer was assessed according to double thermal resistances caused by vapor-like film near the wall and core liquid-like phase. The findings suggest that wall temperature for upflow is higher than that for downflow, which is attributed to larger thermal resistance in the fluid domain for upflow than that for downflow. The difference guarantees the excellent heat transfer performance for downflow than upflow. It is also further concluded that the formation of vapor-like film near the wall due to pseudo-phase transition plays a key role in dominating wall temperature and inducing heat transfer deterioration in half-side heating tubes. The present contribution is significant to the design of supercritical heat exchanger under half-side heating mode.
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Journal of Thermal Science. 2024, 33(5): 1757-1772. https://doi.org/10.1007/s11630-024-2010-4As the promising cooling method for the next generation of data centers, the internal heat transport mechanism and enhancement mechanism of single-phase immersion liquid-cooled (SPILC) systems are not yet well understood. To address this, a steady-state three-dimensional numerical model is constructed herein to analyze flow and thermal transport capacities in servers using SPILC and traditional air-cooling methods. Moreover, this paper emphasizes the influence of component positioning, and underscores the benefits of optimizing coolant flow distribution using baffles. The results indicate that the SPILC system outperforms the traditional air-cooling approach at the same inlet Reynolds number (Re). When Re=10 000, the SPILC method reduces the maximum temperature by up to 70.13%, increases the average convective heat transfer coefficient by 287.5%, and provides better overall thermal uniformity in data center servers. Moreover, placing devices downstream of high-power components creates “thermal barriers” and degrades thermal transport for upstream devices due to increased flow resistance. Excessive spacing between high-power devices can lead to the formation of bypass channels, further deteriorating heat transfer. Additionally, the addition of baffles in the inlet section of SPILC systems effectively enhances heat dissipation performance. To maximize the heat dissipation capacity, minimizing bypass channels and optimizing the flow distribution of coolants are crucial.
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Journal of Thermal Science. 2024, 33(5): 1773-1793. https://doi.org/10.1007/s11630-024-2030-0To enhance the thermo-hydraulic performance of cooling channels, this investigation examines the influence of distinct cross-sectional shapes (i.e., triangular, rectangular, and hexagonal) of twisted pin fins and their arrangements in straight and cross rows. An ambient air cooling test platform was established to numerically and experimentally investigate the flow and heat transfer characteristics of 360° twisted pin fins at Re=15 200–22 800. The findings reveal that straight rows exhibit higher Nu values than cross rows for triangular and rectangular twisted pin fins, and Nu increases with Re. In contrast, for hexagonal twisted pin fins, only straight rows at Re=19 000 exhibit superior overall thermal performance compared to cross rows. Notably, the heat transfer performance of the cooling channel with hexagonal twisted fins surpasses both triangular and rectangular configurations, especially at high Reynolds numbers (Re=22 800). Although the heat transfer coefficient of the cooling channel with hexagonal twisted fins is significantly enhanced by 132.71% compared to the flat channel, it also exhibits the highest thermal resistance and relative friction among the three types of twisted fins, the maximum of which are 2.14 and 16.55. Furthermore, the hydrothermal performance factor (HTPF) of the cooling channels with different types of twisted pin fins depends on the Reynolds number and arrangement modes. At Re=15 200, the highest HTPF achieved for the cross-row hexagonal twisted pin fins is 0.99.
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Journal of Thermal Science. 2024, 33(5): 1794-1808. https://doi.org/10.1007/s11630-024-2013-1Efficient thermal management of lithium-ion battery, working under extremely rapid charging-discharging, is of widespread interest to avoid the battery degradation due to temperature rise, resulting in the enhanced lifespan. Herein, thermal management of lithium-ion battery has been performed via a liquid cooling theoretical model integrated with thermoelectric model of battery packs and single-phase heat transfer. Aiming to alleviate the battery temperature fluctuation by automatically manipulating the flow rate of working fluid, a nominal model-free controller, i.e., fuzzy logic controller is designed. An optimized on-off controller based on pump speed optimization is introduced to serve as the comparative controller. Thermal control simulations are conducted under regular operating and extreme operating conditions, and two controllers are applied to control battery temperature with proper intervals which is conducive to enhance the battery charge-discharge efficiency. The results indicate that, for any operating condition, the fuzzy logic controller shows excellence in terms of the tracking accuracy of set-point of battery temperature. Thanks to the establishment of fuzzy set and fuzzy behavioral rules, the battery temperature has been throughout maintained near the set point, and the temperature fluctuation amplitude is highly reduced, with better temperature control accuracy of ~0.2°C (regular condition) and ~0.5°C (extreme condition) compared with ~1.1°C (regular condition) and ~1.6°C (extreme condition) of optimized on-off controller. While in the case of extreme operating condition, the proposed optimized on-off controller manifests the hysteresis in temperature fluctuation, which is ascribed to the set of dead-band for the feedback temperature. The simulation results cast new light on the utilization and development of model-free temperature controller for the thermal management of lithium-ion battery.
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Journal of Thermal Science. 2024, 33(5): 1809-1825. https://doi.org/10.1007/s11630-024-1999-8Lithium-ion batteries (LIBs) undergo various degradation phenomena such as material decomposition, structural change and uneven lithium ion distribution during long-term cycles, which would affect their performance and safety. In order to improve the performance of the LIBs during their life cycle, preload force is preset when the batteries are assembled. Different preload forces will in turn affect the cycle life and heat generation of the battery. In order to address this issue, this work carries out charge/discharge cycle tests on a NCM811 battery under different preload forces. Isothermal calorimetry tests are performed to investigate the battery heat generation under different states of health (SOHs) and preload forces. Based on the test results, an empirical prediction model for heat generation power as a function of SOH is established. Results show that when the preload force is 5 N·m, the battery capacity decreases in the slowest rate and the average heat generation power is the lowest. Changes in peaks of the incremental capacity curve can be used to characterize the loss of lithium at the electrode, which in turn characterizes the change of heat generation power of the battery. The average heat generation power is mainly affected by the SOH, going through a period of trough with the decrease of the SOH and continuing to increase after crossing the critical point. In general, these findings emphasize the relationship between preload force, SOH and heat generation power, which is helpful for the judgment of optimal preload to improve the efficiency of LIBs.
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Journal of Thermal Science. 2024, 33(5): 1826-1838. https://doi.org/10.1007/s11630-024-1984-2To control the transition process in a laminar separation bubble (LSB) over an ultra-high load compressor blade at a Re of 1.5×105, the effects of wall heat transfer were considered and numerically investigated by large eddy simulations (LES). Compared with the adiabatic wall condition, the local kinematic viscosity of airflow was reduced by wall cooling; thus the effects of turbulent dissipation on the growth of fluctuations were weakened. As such, the transition occurred much earlier, and the size of LSB became smaller. On the cooled surface, the spanwise vortices deformed much more rapidly and the size of hairpin vortex structures was decreased. Furthermore, the rolling-up of 3D hairpin vortices and the ejection and sweeping process very close to the blade surface was weakened. Correspondingly, the aerodynamic losses of the compressor blade were reduced by 18.2% and 38.4% for the two cooled wall conditions. The results demonstrated the feasibility of wall cooling in controlling the transition within an LSB and reducing the aerodynamic loss of an ultra-highly loaded compressor blade.
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Journal of Thermal Science. 2024, 33(5): 1839-1850. https://doi.org/10.1007/s11630-024-1995-zThe paper reports on 3D numerical simulations of unsteady compressible airflow in a blade cascade consisting of flat profiles using a hybrid LES/RANS approach including a transition model. As a first step towards simulation of blade flutter in turbomachinery, various incidence angle offsets of the middle blade were modeled. All simulations were run for the flow regime characterized by outlet isentropic Mach number Mis=0.5 and zero incidence. The results of the LES/RANS simulations (pressure and Mach number distributions) were compared to a baseline RANS model, and to experimental data measured in a high-speed wind tunnel. The numerical results show that both methods overpredict flow separation taking place at the leading edge. In this regard, the hybrid LES/RANS method does not provide superior results compared to the traditional RANS simulations. Nevertheless, the LES/RANS results also capture vortex shedding from the blunt trailing edge. The frequency of the trailing edge vortex shedding in CFD simulations matches perfectly the spectral peak recorded during wind tunnel measurements.
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Journal of Thermal Science. 2024, 33(5): 1851-1866. https://doi.org/10.1007/s11630-024-2018-9In this paper, a numerical simulation method is used to calculate a 1.5-stage axial transonic compressor to explore its unsteady flow mechanism. The performance curve is compared with the experimental data to verify the calculation method with a high numerical accuracy, which shows that the unsteady calculation has good reliability. According to the analysis of the data from the monitoring points under the near-stall condition, the unsteady disturbances originate from the tip region of blade and perform the strongest at the blade pressure surface with a broadband characteristic. Further analysis is conducted by combining with the characteristics of the transient flow field at the tip of blade. The results show that the unsteady pressure fluctuations are caused by the migration of the new vortex cores. These new vortex cores are generated by the breakdown of leakage vortex in the downstream, which is induced by the leakage vortex and shock wave interference. Moreover, the relationship between the unsteady flow characteristics and the working conditions is also studied. The leakage vortex intensity and the shock wave strength gradually increase with the decrease of flow rate. When the combination of the leakage vortex intensity and shock wave strength reaches the first threshold, a single frequency of unsteady disturbances appears at the blade tip. When the combination of the leakage vortex intensity and shock wave strength reaches the second threshold, the frequency of unsteady disturbances changes to a broadband.
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Journal of Thermal Science. 2024, 33(5): 1867-1882. https://doi.org/10.1007/s11630-024-2035-8For complex flows in compressors containing flow separations and adverse pressure gradients, the numerical simulation results based on Reynolds-averaged Navier-Stokes (RANS) models often deviate from experimental measurements more or less. To improve the prediction accuracy and reduce the difference between the RANS prediction results and experimental measurements, an experimental data-driven flow field prediction method based on deep learning and ℓ1 regularization is proposed and applied to a compressor cascade flow field. The inlet boundary conditions and turbulence model parameters are calibrated to obtain the high-fidelity flow fields. The Saplart-Allmaras and SST turbulence models are used independently for mutual validation. The contributions of key modified parameters are also analyzed via sensitivity analysis. The results show that the prediction error can be reduced by nearly 70% based on the proposed algorithm. The flow fields predicted by the two calibrated turbulence models are almost the same and nearly independent of the turbulence models. The corrections of the inlet boundary conditions reduce the error in the first half of the chord. The turbulence model calibrations fix the overprediction of flow separation on the suction surface near the tail edge.
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Journal of Thermal Science. 2024, 33(5): 1883-1896. https://doi.org/10.1007/s11630-024-1983-3Experimental analysis was conducted to study the impact of fuel-air mixing and dilution jet on the temperature distribution in a small gas turbine combustor using various optical diagnostic techniques. The strength and velocity of the swirler at the venturi exit were adjusted to modify the fuel-air mixture, which is presumed to dominate the heat release of the main combustion zone. Additionally, the dilution hole configuration, including the number and size of the holes, was varied to investigate the dilution effect on outlet temperature distribution. Various optical diagnostic techniques, such as particle image velocimetry, planar Mie scattering, and OH* chemiluminescence, were used to measure the flow field, fuel spray distribution, and flame structure, respectively. A reduction in swirling strength led to a decrease in the average flow rate in the throat, which improved the structure and symmetry of the axial vortex system in the sleeve, enhanced the mixing of fuel and gas in the dome swirling air, and ultimately, improved the temperature uniformity of the heat release zone. Compared to larger and sparse dilution jets, smaller and dense dilution jets tended to generate hot spots shifted towards the radial middle area.
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Journal of Thermal Science. 2024, 33(5): 1897-1906. https://doi.org/10.1007/s11630-024-2011-3The flame stability limit and propagation characteristics of a reverse-flow combustor without any flame-stabilized device were experimentally investigated under room temperature and pressure. The results indicate that it is feasible to stabilize the flame in the recirculation zones constructed by the impact jet flow from the primary holes and dilution holes. The flame projected area is mainly distributed in the recirculation zone upstream of the primary holes, whose presence and absence mark the ignition and extinction. During the ignition process, the growth rate and value of the flame projected area first increase and then decrease with the inlet velocity increasing from 9.4 m/s to 42.1 m/s. A rapid reduction followed by a slow reduction of ignition and lean blowout equivalence ratios is achieved by the increased inlet velocity. Then the non-reacting fluid structure in three sections was measured, and detailed velocity profiles were analyzed to improve the understanding of the flame stabilization mechanism. The results are conducive to the design of an ultra-compact combustor.
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Journal of Thermal Science. 2024, 33(5): 1907-1919. https://doi.org/10.1007/s11630-024-1996-yThermochemical recuperation heat recovery is an advanced waste heat utilization technology that can effectively recover exhaust waste heat from oxy-fuel Stirling engines. The novel combustor of a Stirling engine with thermochemical recuperation heat recovery system is expected to utilize both reformed gas and diesel fuels as sources of combustion. In this research, the effects of various factors, including the H2O addition, fuel distribution ratio (FDR), excess oxygen coefficient, and cyclone structure on the temperature distribution in the combustor, combustion emissions, and external combustion system efficiency of the Stirling engine were experimentally investigated. With the increase of steam-to-carbon ratio (S/C), the temperature difference between the upper and lower heating tubes reduces and the circumferential temperature fluctuation decreases, and the combustion of diesel and reformed gas remains close to complete combustion. At S/C=2, the external combustion efficiency is 80.6%, indicating a 1.6% decrease compared to conventional combustion. With the increase of FDR, the temperature uniformity of the heater tube is improved, and the CO and HC emissions decrease. However, the impact of the FDR on the maximum temperature difference and temperature fluctuation across the heater is insignificant. When the FDR rises from 21% to 38%, the external combustion efficiency increases from 87.4% to 92.3%. The excess oxygen coefficient plays a secondary role in influencing temperature uniformity and temperature difference, and the reformed gas and diesel fuel can be burned efficiently at a low excess oxygen coefficient of 1.04. With an increase in the cyclone angle, the heater tube temperature increases, while the maximum temperature difference at the lower part decreases, and the temperature fluctuation increases. Simultaneously, the CO and HC emissions increase, and the external combustion efficiency experiences a decrease. A cyclone angle of 30° is found to be an appropriate value for achieving optimal mixing between reformed gas and diesel fuel. The research findings present valuable new insights that can be utilized to enhance the performance optimization of Stirling engines.
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Journal of Thermal Science. 2024, 33(5): 1920-1934. https://doi.org/10.1007/s11630-024-2004-2The investigation of syngas flame propagation has great benefits for the effective use of gas turbines. This essay sets out to study the flame propagation of premixed oxygen-rich combustion (oxygen enrichment coefficient in volume Ω: 0.21, 0.27, 0.32, 0.37) of syngas (H2:CO=2:8) in half-closed rectangular ducts at elevated temperatures (T: 300 K, 400 K, 500 K) and evaluate the effects of initial temperature and oxygen enrichment coefficient on the LBV from sensitivity analysis and kinetic analysis. This paper presents the effect of the expansion effect on laminar burning velocity for the first time, and separates the effect of the expansion effect on laminar burning velocity by a new method. Research shows that as the initial temperature goes up, the faster the exponential growth phase of the flame front velocity, the slower the slow growth phase. The smaller and earlier the maximum flame front velocity arrives, the slower the average flame speed is. As the oxygen enrichment coefficient goes up, the peak value of the flame front velocity gradually decreases. Oxygen-rich combustion and increasing initial temperature inhibit flame propagation in a half-open tube, but promote laminar burning velocity, which increases the key chemical bond and adiabatic flame temperature. The net reaction rate shows that oxygen-rich combustion mainly promotes the combustion reaction of H2(R2). However, increasing the initial temperature mainly promoted the oxidation of CO(R29). Analysis of the reaction path showed that oxygen-rich combustion and increased initial temperature promoted the reaction of H2 and CO with key chemical bonds, increased OH concentration, and inhibited OH cracking reaction.
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Flame Morphology and Characteristic of Co-Firing Ammonia with Pulverized Coal on a Flat Flame BurnerJournal of Thermal Science. 2024, 33(5): 1935-1945. https://doi.org/10.1007/s11630-024-2001-5Ammonia as a new green carbon free fuel co-combustion with coal can effectively reduce CO2 emission, but the research of flame morphology and characteristics of ammonia-coal co-combustion are not enough. In this work, we studied the co-combustion flame of NH3 and pulverized coal on flat flame burner under different oxygen mole fraction (Xi,O2) and NH3 co-firing energy ratios (ENH3). We initially observed that the introduction of ammonia resulted in stratification within the ammonia-coal co-combustion flame, featuring a transparent flame at the root identified as the ammonia combustion zone. Due to challenges in visually observing the ignition of coal particles in the ammonia-coal co-combustion flame, we utilized Matlab software to analyze flame images across varying ENH3 and Xi,O2. The analysis indicates that, compared to pure coal combustion, the addition of ammonia advances the ignition delay time by 4.21 ms to 5.94 ms. As ENH3 increases, the ignition delay time initially decreases and then increases. Simultaneously, an increase in Xi,O2 results in an earlier ignition delay time. The burn-off time and the flame divergence angle of pulverized coal demonstrated linear decreases and increases, respectively, with the growing ammonia ratio. The addition of ammonia facilitates the release of volatile matter from coal particles. However, in high-ammonia environments, oxygen consumption also impedes the surface reaction of coal particles. Finally, measurements of gas composition in the ammonia-coal flame flow field unveiled that the generated water-rich atmosphere intensified coal particle gasification, resulting in an elevated concentration of CO. Simultaneously, nitrogen-containing substances and coke produced during coal particle gasification underwent reduction reactions with NOx, leading to reduced NOx emissions.
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Journal of Thermal Science. 2024, 33(5): 1946-1960. https://doi.org/10.1007/s11630-024-1993-1In this study, we conduct a thorough evaluation of the STGSA-generated skeletal mechanism for C2H4/air. Two STGSA-reduced mechanisms are taken into account, incorporating basic combustion models such as the homogeneous reactor model, one-dimensional flat premixed flame, and non-premixed counterflow flame. Subsequently, these models are applied to more complex combustion systems, considering factors like flame-flow interaction and flame-wall interaction. These considerations take into account additional physical parameters and processes such as mixing frequency and quenching. The results indicate that the skeletal mechanism adeptly captures the behavior of these complex combustion systems. However, it is suggested to incorporate strain rate considerations in generating the skeletal mechanism, especially when the combustion system operates under high turbulent intensity.
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Journal of Thermal Science. 2024, 33(5): 1961-1973. https://doi.org/10.1007/s11630-024-2006-0Adequate destruction of the aromatic structure in coal is key to further reducing the emission of pollutants. In this research, activation reactions of Shenmu coal powder were carried out in a vertical tube furnace. The study investigated the evolution mechanism of carbon covalent bonds during the activation process by altering the ratio of H2O to CO2 in the activation atmosphere. The theoretical validation was conducted through density functional calculations. The two gas molecules follow different pathways to increase the reactivity of char. CO2 mainly participates in the cross-linking reaction by intensifying branching, while H2O and char have lower adsorption energy barriers and are more likely to generate oxygen-containing functional groups. Gas molecules partially compete for active sites in a mixed gas atmosphere, but there is a synergism between the two effects. The synergism can be attributed to two possibilities. The inclusion of H2O mitigates the generation of five-membered rings to a limited extent, while concurrently enhances the development of oxygen-containing functional groups. Introducing oxygen-containing functional groups can effectively diminish the adsorption energy barrier associated with the interaction between gas molecules and char, consequently leading to a reduction in the energy demand for subsequent bond cleavage.
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Journal of Thermal Science. 2024, 33(5): 1974-1989. https://doi.org/10.1007/s11630-024-2022-0Effective operation strategies in the solid oxide fuel cell (SOFC) can adjust the spatial distribution of temperature gradient favoring the long-term stability. To investigate the effects of different operating conditions on the thermal behavior inside SOFC, a three-dimensional model is developed in this study. The model is verified by comparing it with the experimental data. The heat generation rate and its variation under different operating conditions are analyzed. The combined effects of operating voltage and gas temperature are considered to be the key factor influencing the temperature gradient. Compared to the original case, the temperature of SOFC decreases by 21.4 K when the fuel velocity reaches 5 m/s. But the maximum temperature gradient increases by 21.2%. Meanwhile, higher fuel velocities can eliminate about 32% of the area with higher temperature gradient. And when the oxidant velocity reaches 7.5 m/s, the peak temperature gradient effectively decreases by 16.59%. Simultaneous adjustment of the oxidant and fuel velocities can effectively reduce the peak temperature gradient and increase the safety zone. The effects of operation conditions on the temperature gradient of the cell are clarified in this study, which can be a reference for further research on the reliability of SOFCs.
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Journal of Thermal Science. 2024, 33(5): 1990-2003. https://doi.org/10.1007/s11630-024-1981-5In response to the Kigali Amendment to the Montreal Protocol and global low-carbon emission environmental requirements, the phase-out and decomposition of numerous HFC refrigerants have become urgent, necessitating efficient and mild decomposition methods. This study investigates the thermal decomposition and oxidative thermal decomposition pathways of the typical hydrofluorocarbon refrigerant HFC-134a, employing a combination of experimental and quantum chemical DFT simulation methods. Quantum chemical simulations reveal that the initial reaction bond cleavage serves as the rate-determining step during the thermal decomposition process, with the most easily detectable closed-shell products including CF2=CHF, HF, CH3F, CHF3, CH2F2, and CF4. Reactive oxygen species can significantly reduce the Gibbs free energy barrier for HFC-134a decomposition. To achieve efficient degradation of HFC-134a, appropriate catalysts should be developed and selected to increase the level of reactive oxygen species in the reaction system. Experimental studies further corroborate that HFC-134a may undergo degradation through distinct reaction pathways under varying temperature (240°C to 360°C) and pressure (0.1 MPa to 4.5 MPa) conditions, in agreement with simulation predictions.
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Journal of Thermal Science. 2024, 33(5): 2004. https://doi.org/10.1007/s11630-024-2038-5
