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Applied thermal engineering
Elservier Science Ltd.
Applied thermal engineering

Elservier Science Ltd.

1359-4311

Applied thermal engineering/Journal Applied thermal engineeringISTPSCIEI
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    Experimental study on the thermal performance of an ammonia loop heat pipe using a rectangular evaporator with longitudinal replenishment

    Yang Z.Zhang Y.Bai L.Zhang H....
    10页
    查看更多>>摘要：In this work, a stainless steel-ammonia loop heat pipe (LHP) using a rectangular evaporator with longitudinal replenishment was designed and fabricated. Extensive experiments have been implemented to investigate its startup characteristics, dynamic operation, heat transfer limit and thermal resistance. In particular, the effect of heat load on the startup performance was evaluated, and the effects of heater area and position on the heat transfer limit were studied. Experimental results showed that the LHP exhibited excellent startup performance. It could realize the startup successfully at a small heat load as low as 2 W. In most cases, it started up in only one situation. The temperature overshoot decreased with the increase of the heat load in the startup. In the power step change process, the LHP responded quickly to the changes of the heat load, and achieved a smooth transition. The LHP could reach a heat transfer limit up to 110 W for all the three heaters, restricted by the capillary limit. The heater area and position played an important role in the heat transfer limit, which became larger when the heater was placed closer to the compensation chamber (CC), due to reduced flow resistance of liquid replenishment in the capillary wick. It was found that the closer the heater was located to the CC, the smaller the thermal resistance of the LHP system. This work contributes to a comprehensive understanding of the thermal performance of the LHP using a rectangular evaporator, providing good guidance and reference for its future applications.
      原文链接:
    • NSTL
    • Elsevier

    Flow behavior and mass transfer of humid air across fiber membrane bundles

    Liu Y.Zhang S.Cui X.Yan W....
    13页
    查看更多>>摘要：In the present work, staggered and inline arranged membrane components are modeled to numerically investigate the flow behavior and mass transfer of humid air across the fiber membrane bundles. For the dehumidification purpose, the humid air flows over the outer surface of the composite membrane fibers, and the lumen side of the membrane maintains a negative pressure where the permeated water vapor is removed from the suction port. The study of flow behavior showed that the velocity contours are denser near the membrane outer wall for both configurations, while a fluid stagnation zone is observed in the staggered arrangement. Through the analysis of the flow and concentration fields in the two configurations, it was found that for the fiber membranes with small radius, the inline arrangement possesses more advantages in terms of dehumidification capacity and energy efficiency. Consequently, the effects of the dimension parameters of inline arrangement on the dehumidification performance of membrane bundles were further explored. The results indicated that the increase of tube pitch has a negative effect on the dehumidification rate. In addition, thickening the thickness of the membrane support layer and decreasing the fiber radius are conducive to the dehumidification performance.
      原文链接:
    • NSTL
    • Elsevier

    Nanofluids: Key parameters to enhance thermal conductivity and its applications

    Younes H.Hong H.Mao M.Lou D....
    26页
    查看更多>>摘要：Recent advances in the customization of nanofluids to obtain specific thermophysical properties have led to an increase in the need for a better understanding of the various factors that impact thermal conductivity. The term nanofluid refers to the combination of any nanoscale material with a base fluid. Although investigations of the development and application of nanofluids have expanded dramatically over the past decade, the number of commercialized applications of nanofluids is somewhat limited, with the majority of the research focused on investigations of the theory and fundamental science. This review discusses the various applications of nanofluids briefly. It focuses on a discussion of the various parameters that have been found to impact the thermal behavior in general significantly and the thermal conductivity of these nanofluids, including parameters such as particle size and shape, the pH of the fluids, surfactant, solvent type, hydrogen bonding, temperature, base fluids, and particular interest here, the alignment of the nanoparticles utilized (carbon nanotubes, Graphene, and metal oxides nanoparticles). These parameters have been found to affect the thermal conductivity of the nanofluids directly and can either increase or decrease the thermal conductivity. In contrast, other parameters, such as the viscosity, have an “indirect” effect on the thermal conductivity. While there is broad agreement that these parameters govern the thermal conductivity and hence, the heat transfer capability of the nanofluids, there is a lack of a clear consensus about the relative importance and impact of these thermophysical properties and, in some cases, actually conflicting data in the literature. This lack of clarity in the effect of these various parameters has resulted in the relatively slow adoption, implementation, and application of these nanofluids in commercial applications. The stability of the nanofluids and the nanoparticle suspension duration are other issues that limit commercialization efforts. The information presented here helps to clarify these issues and also explores the effects of factors such as nanoparticle size and type, which may lead to additional opportunities for commercial applications.
      原文链接:
    • NSTL
    • Elsevier

    Experimental study of operating parameters on the performance of a novel U-type radiant tube with dual flue gas self-circulation structures

    Li W.Fan H.Feng J.Bai W....
    11页
    查看更多>>摘要：Faced with the challenges of gas-fired radiant tube performance improvement and increasingly stringent environmental requirements, we proposed a novel U-type radiant tube (N-URT) with dual flue gas self-circulation structures for heat treatment furnaces. A 1:1 scale experimental system was established to evaluate heating characteristics of the N-URT under different operating conditions. The effects of input power (P) and excess air coefficient (α) on temperature uniformity, NOx emission and thermal efficiency were analyzed and discussed in-depth. It was found that the N-URT can significantly improve the heating performance compared to conventional U-type radiant tube. As P increased from 35 to 50 kW, the maximum wall temperature difference decreased from 86.9 to 63.2 °C, the thermal efficiency dropped from 92.7% to 89.1%, and the outlet NOx emission increased from 89.6 to 103.6 mg·m?3 @8% O2. As α increased, the temperature uniformity increased, but the heat recovery and NOx reduction decreased. In addition, the effect of P on NOx emission and maximum wall temperature difference was more significant than α, indicating that the air supply strategy for a given P should be carefully considered. Accordingly, the experimental results can provide a theoretical guidance for the development and application of the radiant tube.
      原文链接:
    • NSTL
    • Elsevier

    Separate factors’ effects on CHF for flow boiling in an inclined rectangular channel under low mass flux

    Gong S.Dong S.Mei Y.Zhang B....
    13页
    查看更多>>摘要：Flow boiling under natural circulation is a reliable method of safety strategy for a thermal dissipation system, especially in the nuclear power system, however, the CHF is the safety limit. Thus the CHF prediction is paramount important for such systems. In order to well understand CHF mechanism and develop the theoretical models, separate factors’ (heater surface orientation, mass flux and inlet quality) effects on CHF for flow boiling under low mass flux and low pressure are investigated both experimentally and theoretically in this study. A rectangular channel with the cross section of 17 mm × 10 mm is used as the flow channel and the main heater with stainless steel surface has the size of 17 mm × 17 mm. The flow boiling experiments were tested for deionized water under the atmosphere pressure at the outlet. The CHF values for flow boiling were obtained under various surface orientation of 15–90°(0° is completely downward), mass flux of 110–288 kg/m2s and inlet quality of 0.003–0.036. The separate factors’ effects on the CHF were analyzed. It is concluded that the CHF increases with the increase of the mass flux, inlet quality and heater surface orientation, however it is not linear. A CHF model based on the sublayer dry out model is proposed. The developed model well predicts the CHF variation trend with each separate parameters (orientation, mass flux and inlet quality). And more than 90% predicted results have the error within ±15%.
      原文链接:
    • NSTL
    • Elsevier

    Pinch point characteristics and performance evaluation of CO2 heat pump water heater under variable working conditions

    Cui Q.Wang C.Gao E.Zhang X....
    15页
    查看更多>>摘要：The pinch point in the gas cooler significantly impacts the optimal discharge pressure and system performance of the CO2 heat pump water heater, however the pinch point characteristics are still not fully revealed. Given this, a comprehensive thermodynamic investigation is conducted. The thermodynamic model is developed and validated, and a faster and effective method for seeking the optimal discharge pressure is proposed and proven accurate. The pinch point variation pattern with working conditions and its influence on the system performance are investigated. Besides, a comparative study on the CO2 heat pump water heater with R134a is conducted under variable working conditions. The results show that there are four pinch point variation patterns. The pinch points may appear at the hot end, the interior, and the cold end of the gas cooler. At low inlet water temperature and large temperature lift conditions, two pinch points appear exactly at the optimal discharge pressure and are at the interior and the cold end. The appearance of the two pinch points enables the optimal thermal matching of CO2 and water, resulting in a maximum coefficient of performance (COP). The two pinch points are still possible at high inlet water temperatures; however, the thermal matching and COP are not closely related anymore. The CO2 heat pump water heater is more competitive than R134a for low inlet water temperature and large temperature lift conditions, but not vice versa. The results obtained may be helpful for the optimization and efficient control of the CO2 heat pump.
      原文链接:
    • NSTL
    • Elsevier

    Effect of cut-off and compression ratio on the isentropic efficiency during off-design and part-load operations of a Wankel rotary steam expander used for small scale cogeneration

    Gopal V.V.Seshadri S.
    10页
    查看更多>>摘要：Expanding steam from boiler pressure to process pressure using an expansion device instead of a pressure reducing valve is recommended for improving energy efficiency and reducing operational costs. However, there are challenges in realizing this potential with existing technologies (such as turbines) due to wetness of process steam and mass flow rate variations caused by load changes. Positive displacement expanders, such as screw and reciprocating engines, can handle wet steam but, they are designed to handle variable mass flow only by throttling leading to exergy destruction and poor economic performance. In this paper we present an improved Wankel steam expander by analysing the impact of mass flow variations and translating it into key design elements: cut-off and compression ratio of the expander. Theoretical analysis in this study resolves conflicting definitions found in literature on compression ratio and cut-off used for designing a positive displacement expander (such as Wankel) and its impact on isentropic efficiency prediction. Experimental analysis of the Wankel expander reveals that a peak isentropic efficiency of over 85% can be achieved for high cut-off. The range of measured isentropic efficiencies were from 60 to 88% for different pressure ratios and at 40% and 62% cut-off. As the pressure ratio increased, the peak efficiency shifted to a lower cut-off condition. When compared to the Wankel, the peak efficiency of a reciprocating expander was only in the range of 50–55%. Thus, the Wankel expander developed in this study presents an opportunity to improve the exergetic efficiency of process steam.
      原文链接:
    • NSTL
    • Elsevier

    A seasonal biomass-driven multi-generation system integrated with PV/T and GSHP: Adjustable performance assessment

    Zhang X.Zhu R.Zhao B.Yan R....
    13页
    查看更多>>摘要：Introducing solar utilization technology into multi-generation system is an appropriate way to further replace the role of fossil fuel for carbon neutral. However, the intermittency of solar energy is an urgent problem for researchers to solve. Thus, a novel seasonal biomass-driven multi-generation system integrated with photovoltaic/thermal collectors and ground source heat pump is developed. The integrated system is composed of four main parts: photovoltaic/thermal system, Brayton cycle, biomass gasification and ground source heat pump. In addition, photovoltaic/thermal system can improve performance of ground source heat pump and increase mass flowrate of domestic hot water in daytime. To investigate seasonal performance of integrated system, thermodynamic and economic analyses are carried out in this study. At designated point, energetic and exergetic efficiencies of proposed system are 86.44% and 17.12%, respectively. Compared to the performance variations on summer and winter typical day, the results indicate that thermodynamic performances show reverse trend in daytime. In addition, the levelized cost of productions in spring, summer, autumn and winter are 0.189, 0.204, 0.188 and 0.183 $/kWh, respectively. As a result, the proposed system provides an alternative way to make full use of multiple renewable energy and achieve more suitable operation modes.
      原文链接:
    • NSTL
    • Elsevier

    Observation of gunpowder-like thermochemical responses of a thermal energy storage system based on KNO3/NaNO3/Graphite exposed to a heat transfer fluid

    Oh J.Jung H.Yoh J.J.
    15页
    查看更多>>摘要：An application of latent-type thermal energy storage (TES) system and its issues associated with the safe operation for TES composite materials (i.e., solar salts (KNO3/NaNO3) and expanded graphite) are investigated. This work is aimed to present the compatibility of various compositions of TES materials with a heat transfer fluid (HTF) to provide a guideline for safe and reliable system usage. The thermochemical characteristics and chemical kinetic mechanism for HTF-diluted TES samples were measured using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) with additional numerical analysis for validation of the constructed reaction kinetics. Amongst the range of percent HTF dilutions, 20 wt% HTF dilution in comparison to no HTF dilution showed the strongest exothermic runaway reaction. For instance, with a decrease in the onset temperature, the time to runaway was shortened, while the change in the heat of reaction was not significant. The numerical simulations with chemical kinetics obtained using thermochemical analysis revealed that the reaction runway characteristics of HTF-diluted TES materials show a similar detonative behavior as of gunpowder. Moreover, the function of HTF in TES system appeared to be identical to that of sulfur in gunpower as its specific percent composition was responsible for accelerating an exothermic chemical reaction.
      原文链接:
    • NSTL
    • Elsevier

    A parametric study on the performance of vapor chamber in association with pillar distribution

    Muneeshwaran M.Lin Y.-F.Wang C.-C.Lin L....
    9页
    查看更多>>摘要：The performance of vapor chamber is significantly affected by the wick structure, height of the vapor cavity, pillar structure, working fluid, filling ratio, and heat source area. Besides, it can also be influenced by the pillar distribution, which has not been extensively studied in prior literature. Therefore, the present study investigates the influence of pillar distribution on the vapor chamber performance. Totally 11 different vapor chamber models are fabricated and tested, and water is selected as a working fluid for all the samples. The examined geometric features include different mesh layers, mesh sizes, wick structure in condenser surface, pillar distributions, and vapor core space. The results showed that the evaporator with a single-layered mesh provided the thermal resistance of 0.19 K W?1, while triple-layered mesh yielded a thermal resistance of 0.24 K W?1. Moreover, the 2 × 200 mesh offered a maximum heating load of 161 W with a thermal resistance of 0.18 K W?1 against the maximum heating load of 120 W and 75.6 W by the 2 × 150 and 2 × 100 mesh, respectively. The experimental results showed that increasing the pillar numbers alone may not always reduce the thermal resistance, a better pillar distribution can help to direct the vapor flow and improve the performance of the vapor chamber. The proposed vapor chamber with a novel X-wing patterned pillar distribution can offer a thermal resistance of 0.19 K W?1 at a heating load of 146.6 W.
      原文链接:
    • NSTL
    • Elsevier