查看更多>>摘要：Ultra-thin loop heat pipe (UT-LHP) is a high-performance heat transfer component that satisfies the requirement of thermal management problems in miniaturized electronic devices. In this study, a novel UT-LHP with capillary wick structures is developed for mobile electronics cooling. By using a novel print wick structuring process on copper substrates, the total thickness of the UT-LHP can be decreased to only 0.3 mm. The effects of the filling ratio and wick structure distribution have been investigated to optimize the vapor-liquid circulation in the UTLHP. The heat transfer performance of the UT-LHP has been evaluated under different heat loads and configurations. The minimal thermal resistance of the UT-LHP is 1.03 degrees C /W at a heat load of 3 W which corresponds to a heat flux of 3 W/cm(2). It is confirmed that the proposed UT-LHP can transfer a heat load of up to 3 W with low sensitivity to gravity. The proposed novel UT-LHP is a promising thermal solution for high-performance mobile electronic devices.
查看更多>>摘要：In the past, fin-and tube heat exchanger (FTHE) tube pattern ratios have been largely based on ad-hoc design principles. Here, we investigate the optimal tube arrangements for a FTHE with plain fins in marine environments represented by two different air types; one for unfiltered air with high condensation rate and one for clean dry filtered air conditions. The thermal-hydraulic efficiency of the FTHE design is measured by comparing a modified ratio of Colburn j-factor and Fanning friction factor. The regression model generated from the CFD data is then used to identify the maximum efficiency for two design specific fin pitches separately. We identified two optimal tube patterns: one for a large fin pitch for unfiltered air, and another for a small fin pitch for filtered air. Manufacturing restrictions were found to significantly limit the maximum achievable efficiency of a tube pattern. By neglecting the related manufacturing restrictions, 4% higher efficiency for a fin pitch of 1.5 mm and 23% higher efficiency for a fin pitch of 3.5 mm is achieved. Without any application specific limitations or manufacturing restrictions the fin pitch 1.5 mm can have a 36% increased efficiency than fin pitch 3.5 mm. These novel results show that development in manufacturing have potential for significant improvements in thermal-hydraulic efficiency.
查看更多>>摘要：For the aim of increasing the heat transfer enhancement, a hybrid method in which active and passive heat recovery techniques have been used together. The usage of nanofluid, MHD and dimpled fins tube have not been utilized together so far. Regarding this issue, this study is the first numerical study to determine effect of usage of three effects together comprehensively. In this study, thermo-hydraulic performance of Fe3O4/H2O nanofluid (ferro-nanofluid) flow inside dimpled tube under magnetic field effect has been examined numerically. The main purpose of the study is to obtain numerical data for turbulent flow in the spherical dimpled tubes providing some aid to design a highly efficient thermal energy storage devices. Dimple geometry with nondimensional pitch ratio (P/d = 3.75, 7.50 and 11.25), Hartmann number (Ha = 75, 150, 225) and nanoparticle volume fraction (phi = 0.5, 1.0 and 2.5 vol%) are the parameters investigated in this study. The numerical analyses have been carried out Reynolds number ranging from 10,000 to 50,000 at a constant heat flux at 20 kW/m2. The simulations have been built up by Realizable k-epsilon turbulence model and single-phase approach. Also, "MagnetoHydroDynamic" (MHD) module has also been activated for defining magnetic field effect. The results showed that Nusselt number increases with increasing Reynolds number and decreasing pitch ratio. The dimple geometry type of P/d = 7.50 has been determined as the most efficient dimple geometry type. In the case of highest magnetic field intensity, the highest Nusselt number increment (72.48%) has been obtained for phi = 2.5 vol% compared to the base fluid of distilled water using as the working fluid for smooth tube. The highest PEC value was also obtained as 1.126 for the case of P/d = 7.5, phi = 2.5 vol% and Ha = 75. In addition, the effect of magnetic field intensity on velocity and temperature distributions has been presented with contour graphs.
查看更多>>摘要：Low-grade thermal energy can be used for superheating the working fluid in the steam ejector. The purpose of the present study is to investigate the relationship of two-phase heat transfer with the ejector performance in the condensing flow regime under the condition of primary steam superheating. The condensation and evaporation phenomena occur in two-phase ejectors. The heat and mass transfer between the liquid and vapor phases change the flow pattern inside the ejector. The results show that the wet steam model has better fitting with experimental data than the ideal gas model. The primary steam superheating influences on the intensity of shock-wave patterns of supersonic flow, liquid mass fraction and two-phase heat transfer in the ejector. By increasing the superheating level, the two-phase heat transfer and exergy destruction are decreased, entrainment ratio in the wet steam model gets closer to entrainment ratio in ideal gas model, critical and limiting pressure are decreased and on-design and off-design regions become smaller. Also the effects of primary steam superheating in the ejector are investigated on the performance of a renewable refrigeration cycle. Superheating the working fluid reduces the generator energy consumption and increases exergy destruction in a refrigeration cycle. At degree of superheating 100 K, two-phase latent heat, energy consumption, and ejector exergy destruction decrease by 40%, 3.9%, and 11.7%, respectively, and entrainment ratio and total exergy destruction increase by 10% and 50%, respectively.
查看更多>>摘要：Based on the existing waste heat recovery technology and seawater desalination technology, a water-heat combined supply system based on waste heat from a costal nuclear power plant is proposed. The proposed system can improve the heating capacity of district heating systems and reduce the energy consumption of watergenerating seawater reverse osmosis systems by recovering the waste heat from exhaust steam in a nuclear power plant. Through the simultaneous delivery of water and heat, the long-distance conveyance cost is reduced. The thermodynamic characteristics, energy-saving properties, environmental benefits, and economic efficiency of the proposed system are discussed and analysed. The results show that the heating capacity of the proposed system is increased by 55.5% compared to the original extraction steam heating load. The exergy efficiency is 16.07% higher and the heating cost is 12.32 yen /GJ lower than those of traditional cogeneration district heating systems. The water supply cost of the proposed system is 2.95 yen /m3 lower than that of the conventional desalination water supply system. The current results of this study will be helpful in solving problems related to heat and water resource shortages and the high costs of long-distance heat and water conveyance in coastal areas.
查看更多>>摘要：Soil thermal imbalance has become a common problem for ground source heat pump (GSHP) system, leading to the fluctuation of ground temperature and decline of heat transfer efficiency. However, compared with vertical ground heat exchanger (GHE), there is a lack of research and evaluation on the soil thermal imbalance for horizontal GHE. Therefore, in this paper, to investigate the extent of soil thermal imbalance and its effects on performance of GSHP system, a sandbox experiment embedded with horizontal stainless-steel GHE was implemented. Various operation conditions were designed to explore effects of intermittent time, water content (rainfall) and buried depth of pipes on soil temperature distribution and restoration performance. The experimental data was validated by a numerical simulation. The results indicated that 5-day intermittent operation of GSHP leads to an increase of 6.41 celcius in soil temperature. After 2-day natural recovery, soil temperature recovery rate can only reach 0.72. Increasing soil moisture content and intermittent control are effective solutions to mitigate thermal accumulation and improve thermal performance. The heat transfer performance of GHE with an intermittent ratio of 6:18 which GSHP system runs for 6 h and stops for 18 h during a day is 33% higher than that with the intermittent ratio of 14:10 and 10:14. The higher the water content, the lower the soil temperature fluctuation and the stronger the soil temperature recovery capacity. This can be attributed to the increase of soil thermal conductivity and capacity, while the decrease of thermal diffusivity.
查看更多>>摘要：The application of the dynamic integrated method, implemented in the form of a linear regression based on averages over a certain period for the evaluation of the thermal performance of the building envelope, is presented and discussed. Previous researches and results on this method are corroborated and extended. A data set obtained from two well insulated twin houses, located in Germany, measured during the winter period, was analysed. Both dwellings were unoccupied but the heating and ventilation systems were operated to simulate the behaviour of a family with two children. The analysis was focused on obtaining accurate heat transfer coefficient (HTC) and solar aperture (gA) estimates characterising the behaviour of the building envelope regarding heat loses and solar gains respectively. To do so, different models for the main heat exchange phenomena were evaluated, finding precise models for the solar gains and the ventilation system. Different options were considered for the dependent variable in the regression equation and the most suitable was determined. The optimum average period was also identified. Finally, the selected model was tested to find the minimum number of days required to obtain accurate results. The consistency of the results obtained from different data series is considered as key validation criteria. With a test duration of 18 days, HTC and gA estimates were within +/- 5% and +/- 8% from the mean, respectively, in both houses.
查看更多>>摘要：Supercritical CO2 Brayton cycle (SCBC) is widely used in high-temperature waste heat recovery combined systems with sequential or cascade configurations due to its compact structure and high efficiency. However, since the heat source conditions of these systems are not clearly defined, there is no practicable design approach to comprehensively adapt the waste heat with different temperatures, especially for the ships. This paper aims to propose an effective thermodynamic configuration method to facilitate the design and application of recuperative SCBC combined systems for the high-temperature waste heat recovery of marine engines. To this end, the factors that may affect the system configuration are investigated, and it reveals that the recuperator effectiveness can not only influence its own performance but also determine the heat source conditions, which will finally affect the performance of the entire system. Therefore, a new system configuration method is proposed based on the recuperative effects, and a novel combined system is designed as a case study for further illustration and multi-objective optimization. The results indicate that an optimal value of the recuperator effectiveness exists in the preliminarily determined range, 0.56-0.8. Under the system optimal operating conditions, the total energy output and electricity production cost are 538.97 kW and 5.34 cent/kWh, respectively, and the corresponding thermal and exergy efficiencies research up to 33.17 % and 61.93 %, respectively. It proves that the configuration method proposed in this paper can realize an efficient design of recuperative SCBC combined system, and provide a reference for other relevant systems.
查看更多>>摘要：Flexible thermoelectric generators have attracted a tremendous amount of attention as they enable waste heat recovery in a wide range of applications ranging from flexible displays to biomedical devices. However, design limitations still exist in flexible micro thermoelectric generators, in particular for applications with a heat source array such as in flexible micro-light-emitting diodes. Hence, this paper proposes a thin film flexible annular thermoelectric generator design that specifically aims for heat recycling in flexible micro-light-emitting diodes for the first time. In order to evaluate the generator performance, a three-dimensional steady-state model is constructed and a systematic study on design parameters is carried out based on the finite element method. The model first takes into account the heat transfer from a heat source array to the thermocouples. Seebeck, Peltier, as well as Joule heating effects, are then considered to evaluate the specific power density of the generator. Additionally, critical design parameters including thermocouple structural dimensions, thermocouple numbers, and substrate thickness have been thoroughly investigated. After model validation using experimental data from the literature, the results show that the relationship between the thermocouple design parameters and the output power is nonmonotonic and for each parameter, a certain optimum range can be found, in which the obtained output power does not vary significantly (within 95% of the maximum output power Pmax). The thickness of the flexible substrate is inversely correlated to the output power. Moreover, the metallic thermocouples serve as a heat sink that helps to lower the temperature of the active region by - 3 degrees C. Overall, the model predicts an impressive specific power density of - 40 mu W/cm2.K2, thus demonstrating the potential applicability of using flexible thermoelectric generators to recycle heat from micro-light-emitting diodes while providing a path to further improve the performance of flexible thermoelectric generators.
查看更多>>摘要：The present work focuses mainly on the effects of heat input, filling ratio, inclination angle, tube diameter and coolant temperature on the thermal performance of a wraparound heat pipe charged with R134a. Results show that thermal resistance decreases with the increase of heat input when the filling ratio is larger than 40%. An optimal filling ratio for the heat pipe with the best performance exists between 50% and 60%. The pressure of working fluid in the heat pipe exceeds 1.6 MPa in the 70% and 80% filling ratios experiments. For larger inclination angles (theta > 10 degrees), the thermal resistance decreases with increasing the heat input and finally tends to a stable value. For heat loads of 420 W and greater, the values of thermal resistance are 0.056, 0.07, 0.034 and 0.027 K/W for outer diameters of 8, 10, 12 and 16 mm, respectively. No significant difference in thermal resistance at different coolant temperatures is observed for heat inputs greater than 300 W. In all experiments, for a 22 degrees inclination angle, an outer diameter of 16 mm, and a filling ratio of 50%, the best performance of heat pipe is observed and the lowest value of thermal resistance is 0.027 K/W.
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Elsevier