Applied Thermal Engineering最新文献

High-pressure storage tanks are widely used for transportation due to their convenience. However, sometimes the tanks themselves or the stored medium may become contaminated with impurities. NaCl is one of the most commonly encountered impurities. The pressure response curves of 1.0 mol/L NaCl solution and pure water within a 17L square stainless steel vertical storage tank are being investigated. The experimental results indicate that the pressure rebound is most pronounced when the medium inside the tank releases gas phase. Unlike pure water, NaCl solution still exhibits pressure rebound during pure liquid phase release. High-speed imaging reveals that the addition of NaCl impurities reduces the nucleation radius of bubbles, sharply increases their quantity, and exacerbates boiling phenomena. As the rupture area increases, the remaining liquid level of 1.0 mol/L NaCl is much lower compared to pure water. At the 70 % leakage position, the pressure recovery ratio of both liquids is the highest. After adding NaCl impurities, under all operating conditions, the pressure recovery ratio and pressure rise ratio exceed those observed under pure water conditions. This indicates that the presence of NaCl exacerbates the boiling of superheated liquids and increases the possibility of tank rupture.

For fully utilizing the low-grade waste heat (LGWH) of the proton exchange membrane fuel cell (PEMFC), a novel coupled system primarily involving a PEMFC and a capacitive salinity/heat engine (CSHE) is proposed. Based on thermodynamic and electrochemical theories, mathematical formulas of the power output, efficiency, and exergy efficiency are deduced accounting for the significant irreversible losses within the coupled system. The superiority is expounded by contrasting the generic performance characteristics of the sub-systems and the coupled system. In addition, numerical results demonstrate that the maximal power output density and its corresponding efficiency and exergy efficiency of the coupled system enhanced by 11.43%, 12.75%, and 9.96%, respectively, over the PEMFC alone under the same operating conditions. Furthermore, the impacts of some parameters, for instance, proton exchange membrane (PEM) thickness, the PEMFC working temperature and working pressure, electric charge density ratio, charging cut-off voltage, and Stern layer distance, on the coupled system performance are meticulously discussed. The results of the study may supply a reference for the design and operation of such a salinity gradient energy recovery system and create a new pathway for recycling LGWH from PEMFC.

Latent heat thermal energy storage (LHTES) offers significant energy-saving benefits, but its application is limited due to the low thermal conductivity of phase change material (PCM). To address this issue, this article studied the combined application of metal foam structures with different porosity gradients and multiple PCMs with different melting points through numerical simulation to accelerate the melting of PCM and enhance system efficiency. The results showed that the application of multiple PCMs improved the heat transfer performance of the system, reducing the complete melting time by 9.18% compared to using a single PCM with uniform metal foam. Based on the multi-PCM storage system with an average porosity of 0.90, this article designed metal foam structures with one-dimensional and two-dimensional porosity gradients and explored their impacts. The results indicated that in the structure with a one-dimensional porosity gradient along the heat flow direction, the positive gradient decreased thermal resistance, further reducing the complete melting time by 6.18%, while the negative gradient increased it by 19.78%. However, the temperature non-uniformity was lowest with the negative gradient and highest with the positive gradient. The optimal two-dimensional porosity gradient multi-PCM storage model not only reduced thermal resistance but also effectively solved the issue of uneven melting, reducing the complete melting time by 17.96% and increasing the energy storage efficiency by 20.16% compared to the single PCM system with uniform porosity. Furthermore, the article conducted a dimensionless analysis of the optimal structure and different gradient structures, establishing formulas for the liquid fraction concerning modified Fourier number, modified Stefan number, and modified Rayleigh number.

Energy storage is an important means of solving the instability of renewable energy sources. As a novel energy-storage technology, thermally integrated pumped thermal electricity storage systems have gained considerable attention owing to their capacity to enhance the power-to-power efficiency of pumped thermal electricity storage systems by integrating low-grade heat sources. However, previous studies have predominantly focused on the integration of low-grade heat sources during the charging phase. In this paper, a novel coldly integrated pumped thermal electricity storage system that integrates liquefied natural gas on the discharge side is proposed. Seawater and R1233zd(E) were utilised as the system’s heat source and working fluid, respectively. The performance of the proposed system was compared with that of a previous thermally integrated pumped thermal electricity storage system that used geothermal energy as the heat source on the charge side. System models were constructed in MATLAB, and thermodynamic and economic comparative analyses were conducted. The Genetic Algorithm was adopted for the multi-objective optimisation of the two systems. The results indicate that, within given temperature ranges for thermal and cold storage temperatures, the proposed system exhibits a higher power-to-power efficiency and has a lower levelised cost of storage compared to the existing system. The optimal power-to-power efficiency and levelised cost of storage solutions obtained via multi-objective optimisation were 1.45, 0.297 $·kWh⁻¹ and 0.807, 0.411 $·kWh⁻¹ for the coldly integrated and thermally integrated pumped thermal electricity storage systems, respectively. The proposed system outperformed the thermally integrated pumped thermal electricity storage system under comparison in terms of thermodynamic and economic performance. The findings of this study shed light on the design and performance of coldly integrated pumped thermal electricity storage systems.

The evaporators of heat pumps extract heat from the environment. To improve the heat pump performance in cold environments, evaporators must operate with small temperature drops of the heat source, which requires large flow rates of the secondary fluid. This paper discusses dimpled plate heat exchangers (DPHEs) with flexible flow section areas. The flow boiling of R410A is measured in six channels. The flow section area ratios of neighboring channels are 0.86–2.83. To analyze outlet superheat, the two-phase and dryout zones are distinguished using infrared measurement. The dryout zone is a function of the superheat and mass flux, which is quantified. The heat transfer coefficients (HTCs) increase with increasing mass fluxes and heat fluxes. Compared with experimental results from a large scope, the present data fall into the combined regimes of convective boiling and nucleate boiling. Smaller hydraulic diameters slightly enhance the heat transfer. The experimental HTCs are accurately predicted by a correlation of chevron plate heat exchangers (CPHEs). The performances of the DPHEs and CPHE are compared for small temperature drops in cold environments. The DPHEs have superior performance by mitigating the unbalance of mass fluxes. The typical overall heat transfer coefficient is improved by 62%. The improvement depends on the flow section area ratios.

Supercritical CO₂/Xe mixture has a potential application prospect in the Brayton cycle system from the perspective of thermodynamic. Supercritical cooler is one of the key components there. However, the cooling heat transfer characteristics and mechanism of supercritical CO₂/Xe mixture is still unclear, which restricts the development and design of supercritical cooler. The heat transfer of supercritical CO₂/Xe mixture crossing T_pc cooled in a horizontal tube with inner diameter 8 mm is numerically explored in this study. The influences of operating parameters on supercritical mixture heat transfer are thoroughly examined. Buoyancy effect is evaluated through the buoyancy criterion Gr/Re². Further, the mechanism of supercritical CO₂/Xe mixture cooling heat transfer is revealed. It is found that heat transfer enhancement may occur during supercritical CO₂/Xe cooled process, and heat transfer is weakened when mass fraction of Xe increases. Considering the bulk specific heat and buoyancy dominating jointly the behavior of supercritical cooling heat transfer, a modified Dittus-Boelter correlation is newly developed to predict heat transfer coefficient of supercritical CO₂ and its mixture with Xe. The correlation matches well with the experimental and numerical data, and the average relative deviations of the new correlation are 18.27 % and 14.14 %, respectively. The investigation provides insight into the characteristics and mechanism of supercritical CO₂/Xe mixture cooling heat transfer. The correlation can provide significant theoretical guidance for accurate design and optimization of supercritical cooler.

This study proposes a multi-objective adaptive energy management strategy for fuel cell hybrid electric vehicles considering fuel cell health state. By integrating rule-based control with multi-objective optimization methods, the strategy aims to improve system efficiency, extend the lifespan of proton exchange membrane fuel cells (PEMFC), and reduce operating costs. Based on a comprehensive model of PEMFC output characteristics and lifetime degradation, this study introduces an optimized point line (OPL) strategy. This strategy dynamically adjusts operating constraints according to the state of health (SOH) of the PEMFC, ensuring optimal vehicle performance throughout its lifecycle. To optimize the OPL strategy parameters, a particle swarm optimization algorithm with compression factor was employed, enhancing the strategy’s optimization efficiency, adaptability, and robustness to better handle various real-world operating conditions. The strategy was evaluated under US06 and WLTC driving cycles and compared with traditional power following (PF) and point line (PL) strategies. Results show that compared to the PL strategy, the OPL strategy achieved a 36.4% and 34.2% reduction in operating costs under US06 and WLTC cycles, respectively. Moreover, PEMFC lifetime degradation decreased by 44.9% and 39.4% in these cycles. In high-power regions, the average operating efficiency of PEMFC improved by 2%. The strategy demonstrated good adaptability to different driving conditions, providing an effective solution for optimizing the performance and durability of fuel cell hybrid electric vehicles.

Highly flexible load requirements and cogeneration economy need to challenge the operation of a multipurpose small modular reactor cogeneration plant with once-through steam generators. Therefore, the present study develops a once-through steam generator dynamic model to analyze the multipurpose small modular reactor operation scheme under cogeneration conditions at different power levels. The once-through steam generator dynamic model is derived based on conversation equations using the moving boundary method. It is verified with RELAP5 results and open literature and the maximum relative error is 3.21 %. Three operation schemes are proposed for multipurpose small modular reactor cogeneration operation: Scheme 1 with constant steam pressure and average coolant temperature, Scheme 2 with constant steam temperature and pressure, and Scheme 3 with constant steam temperature and pure sliding steam pressure. Steady-state and dynamic characteristics with three operation schemes are simulated and investigated at three power levels: 100 %, 70 % and 30 %. The steady-state results show that Scheme 1 is more favorable for the primary loop, while Scheme 2 is beneficial to the secondary loop, and Scheme 3 can improve the thermal efficiency at low power level. The transient findings indicate that disturbances from the reactor side have a significant impact on the once-through steam generator and the minimum settling time is 3.3 s. Consequently, steam temperature control of once-through steam generator is achieved by regulating the control rods for Schemes 2 and 3, while steam pressure is suggested to be controlled by the feedwater valve for Scheme 1. For cogeneration conditions at 70 % power level, Scheme 2 can achieve the highest steam flow of turbine, the highest steam flow of steam extraction and the smallest steam specific volume, which are 87.88 kg·s⁻¹, 28.96 kg·s⁻¹, and 0.0503 m³·kg⁻¹, respectively. Scheme 2 is recommended for high power levels under cogeneration operation. In contrast, Scheme 1 is more suitable for the condensing unit operation for low power levels.

This study presents the development of a multiple-distribution-function lattice Boltzmann model (MDF-LBM) for the accurate simulation of multi-component and multi-phase flow. The model is based on the diffuse interface theory and free energy model, which enable the derivation of hydrodynamic equations for the system. These equations comprise a Cahn-Hilliard (CH) type mass balance equation, which accounts for cross diffusion terms for each species, and a momentum balance equation. By establishing a relationship between the total chemical potential and the general pressure, the momentum balance equation is reformulated in a potential form. This potential form, together with the CH type mass balance equation, is then utilized to construct the MDF-LBM as a coupled convection–diffusion system. Numerical simulations demonstrate that the proposed MDF-LBM accurately captures phase behavior and ensures mass conservation. Additionally, the calculated interface tension exhibits good agreement with experimental data obtained from laboratory studies.

Using nanofluids instead of conventional heat transfer fluids as a passive method is a well-established and widely used technique by researchers to increase the rate and thermal performance of engineering equipment. In this study, the hydrothermal method with a bottom-up approach was used for the synthesis of graphene quantum dots and nitrogen-doped graphene quantum dots. Then, nanofluid samples were prepared in a two-step process, by adding nanoparticles to binary base fluids of deionized water and ethylene glycol in volume concentrations of (50:50) and (60:40), in four concentrations of 100, 200, 500, and 1000 ppm. In order to better understand the boiling heat transfer mechanism and measure its characteristics such as critical heat flux and heat transfer coefficient, an experimental system was designed and built. Nanofluids based on graphene quantum dots have unique features such as compatibility with the environment, economic efficiency, high stability and suitable heat transfer capability. For this reason, their selection in the pool boiling heat transfer process, in addition to saving energy, is introduced as one of the most effective options for improving CHF and HTC. The tests were performed under saturated conditions, atmospheric pressure, and on a vertical flat and polished copper thermal plate. Prepared nanofluids GQDs and N: GQDs based on DI-water maintained their apparent stability for two months. For GQDs nanofluids at an optimal concentration of 500 ppm with a volume ratio of (60:40) DI-water and EG, compared to DI-water, the greatest increase in CHF and HTC is 90.69, 85.011 % and for N: GQDs at a concentration of 500 with a volume ratio (50:50) DI-water and EG, 75.37 and 78.17 % compared to DI-water.