查看更多>>摘要：At present, most of existing heat flux sensors (HFSs) have the disadvantages of large volume, complex manufacturing process, poor embedding property and affecting existing thermal field distribution, which seriously hinders the development and practicality of chip thermal management technology. To address the above issues, this paper presents a principle and structure of a novel thermopile-type HFS based on printed circuit board process (PCB) for monitoring heat flux of chips. According to the proposed principle, PCB process based thermopile-type HFS was designed and manufactured and its corresponding sensor equation was established. Experimental setups were built and manufactured HFSs were experimentally tested and also applied to monitoring the heat flux of chip on PCB, which is mimicked by a small electronic heating system. Theoretical and experimental results show that HFS designed and manufactured according to the proposed principle can sense heat flux accurately with the sensitivity, nonlinear error, repeatability error, time response and measurement range of 0.2670 μV/(W/m2), 1.59%, 1.30%, 5 s and 0–20 kW/m2, respectively, and has the potential to be conveniently and flexibly embedded in electronic system to measure heat flux of chips with affecting existing temperature field as little as possible.
查看更多>>摘要：Metal foam heat sinks can be employed for thermal management in high heat flux applications. They are generally produced by attaching metal foam to the metal substrate using heterogeneous epoxy-adhesives. This process is inefficient due to interfacial thermal contact resistance. Reducing the thermal contact resistance and minimizing the airflow resistance inside the foam enhance the thermal efficiency of the heat sink. We develop foam/foam-fin heat sinks to achieve this, wherein foam and substrate are thermally fused bonded. This paper presents an experimental study to investigate the influence of thermal contact resistance, geometrical configuration, and structural parameters such as pore density, foam height, and the number of fins on the thermal performance of novel integral foam/foam-fin heat sinks. The fused bonding method reduces the thermal contact resistance ～19 times compared to the epoxy-gluing process. Consequently, the Nusselt number for the fused-bonded foam-fin heat sink increases by ～ 30% than the epoxy-glued heat sink. Moreover, the heat transfer rate per unit mass of foam-fin heat sink is ～2 times higher than the commercially available fin heat sink due to the higher specific surface area and reduced vortex formation adjacent to the foam-fins. Thus, fused-bonded foam-fin heat sinks are efficient, compact, lightweight, and easily recyclable heat exchangers. These results will be effective in designing efficient foam/foam-fin heat sinks for electronics cooling.
查看更多>>摘要：The critical heat flux (CHF) is an important parameter in pool boiling, establishing the limit in heat flux and temperature in most applications. A higher CHF offers the system a wider operational range or an additional safety factor under normal conditions. In this work, a new vapor management approach is adopted by using a chimney-like vapor shroud during the pool boiling process. The vapor shroud footprint is equal to the area of the heated surface, and it is placed directly above the heated surface with a specified gap between its lower edge and the surface. The CHF increases for most vapor shroud arrangements (shroud length and gap), with a maximum increase of 83% compared to the baseline. The CHF shows a monotonic increase with increasing shroud length and passes through a maximum (unimodal) as the gap is changed. The maximum CHF occurs for gaps ranging from 1.8 mm to 3.1 mm, corresponding to a Bond number of approximately one, based on gap size. The increase in CHF is believed to be caused by improved liquid feeding and vapor release dynamics caused by the shroud. Analysis shows that the hydrostatic pressure and the interfacial stability are important to the nonlinearity in CHF behavior. A simple model based on scale analysis has been provided to predict the shroud effects on the CHF.
查看更多>>摘要：This publication lays the foundation for the description of the Multiscale Boiling Experiment, which was conducted within two measurement campaigns on the International Space Station between 2019 and 2021. The experiment addresses fundamental questions about two-phase heat transfer during boiling processes. For this purpose, single or few subsequential bubbles are selectively ignited on a heated substrate using a short laser pulse. A detailed investigation of the phenomena is possible, as the boiling process is temporally slowed down and spatially enlarged in microgravity. Within the Multiscale Boiling Project, the undisturbed growth of the bubbles, the influence of a shear flow, and the influence of an electric field are investigated within the same test facility using FC-72 as working fluid. Within the project, thirteen research groups from eight countries are collaborating. Over 3000 data sets have been generated over an 11-month measurement period. In the context of this publication, besides the motivation and necessity of such investigations, the basic structure of the experiment, the objectives of the investigations, and the organization are described. Finally, first results of the investigations are presented. Therefore, this publication has the primary aim to serve as a basis for many further planned publications and present the project as a whole.
查看更多>>摘要：This paper presents an experimental evaluation of the thermal and electrical performance of a commercial 26650 cylindrical LiFePO4 battery cell. The thermal management of lithium-ion batteries is a key problem for electric mobility applications, where batteries are subjected to severe operating conditions. Accordingly, this study aims to demonstrate the reliability of infrared thermography in the quantitative analysis of heat generation in battery cells. In our opinion, infrared thermography can be very attractive owing to: (i) the non-contact nature of this technique and (ii) its capability to quickly scan measurement areas. Consequently, infrared thermography and thermocouple probe results were compared, providing evidence of similar behaviour. Moreover, we present an electrical characterisation of our lithium-ion battery. In particular, the cell potential, open circuit potential, and entropic heat coefficient vis-à-vis the state of charge were experimentally measured. The obtained experimental data were used to evaluate a simplified heat generation term that is widely employed in numerical approaches. The different contributions to heat generation were carefully analysed. The results show that the reversible term considerably influences the total thermal power. Moreover, infrared-based heat generation estimation can be considered reliable.
查看更多>>摘要：The parameters required to calculate the energy efficiency of household refrigerating appliances (i.e. refrigerators, freezers and their combinations) are determined by standard measurements. According to regulations, these measurements are carried out when the appliances are new. It is known from previous studies that various technical aging mechanisms can increase electrical energy consumption by up to 36% over a product lifespan of 18 years. In order to determine the time dependence of the energy consumption of household refrigerating appliances, repeated measurements are carried out in this work. Eleven new appliances are examined under standard measurement conditions. After just two years of operation, an additional energy consumption of up to 11% is determined. Furthermore, 21 older appliances that had previously been measured in new condition are tested again after up to 21 years of operation. For these older appliances, an average increase of energy consumption of 28% is found. For individual appliances, the maximum increase is 36%. An aging model is developed on the basis of these measurement results, which may help to predict the aging-related increase of energy consumption of household refrigerating appliances. This model shows an average increase in energy consumption of 27% for an appliance age of 16 years. Supplemental performance tests of eight compressors do not show any significant aging effects related to these devices after two years of operation. Furthermore, measurements of the thermal conductivity of aged polyurethane foam test samples are carried out and an increase of its thermal conductivity of 26% over a period of about three years is determined.
查看更多>>摘要：Worldwide automotive emission regulations for heavy-duty diesel engines are increasingly stringent, especially for nitrogen oxide (NOx) and particulate matter (PM). The development of high-efficiency aftertreatment technologies and the optimization of the urea injection strategies are key factors for the diesel engines to meet the future regulations. In this paper, the effects of temperature and space velocity on close coupled selective catalyst reduction (ccSCR) and SCR are investigated firstly. The highest NOx conversion efficiency of ccSCR and main SCR is observed at 350 °C, and an increase of space velocity leads to a more significant decrease on NOx conversion efficiency of ccSCR than that of SCR. Then three phases of urea injection state for both nozzles are determined according to the catalyst inlet temperature in the Federal Test Procedure (FTP) cycle. Urea injection strategy is optimized by urea injection ratio calibration and urea injection strategy updating based on the characteristic of three phases, as well as NH3 sensor feedback. Finally, the composite tailpipe NOx emission under FTP cycle is reduced below 0.027 g/kW·h, satisfying California air resources board (CARB) ultra-low NOx emission regulation with a penalty of slight increase in CO2 emissions. However, adopting a high reactivity gasoline can improve the efficiency while maintaining the low level of NOx emissions.
查看更多>>摘要：Investigations of phase shifters and power recovery mechanisms are of sustainable interest for developing Stirling pulse tube cryocoolers (SPTC) with more compact design and higher efficiency. The warm displacer type SPTC has been demonstrated with a very promising theoretical efficiency benefiting from its strong phase shifting and work recovery functions. Extending this same phase shifting approach to miniature pulse tube refrigerators may provide advantages over the other alternatives, since inertance tubes are severely limited in their phase shifting capability when the associated acoustic power decreases below 20 W. A comprehensive investigation regarding the micro-pulse tube cryocooler with a displacer phase shifter is carried out in this work, including the design of the major components of the cryocooler, theoretical analysis regarding the phase relationship in the whole displacer type cryocooler system, and 2D simulation work addressing the performance of the cryocooler systems. In the optimum phase angle conditions of the 3 mm pulse tube size cryocooler, the mass flow rate leads the pressure wave by 28.8° at the regenerator warm end, and lags the pressure wave by 27.7° at the regenerator cold end. A cooling power of 1.5 W at 80 K and a COP (Coefficient of Performance) of 20.96% is obtained with an input power of 7.4 W.
查看更多>>摘要：The stringent requirement and rapid increase in heat dissipation for the high heat flux devices have brought great challenges to cooling performance of mini-channel heat sink (MCHS). Longitudinal vortex generator (LVG) is an effective technology for heat transfer enhancement in MCHS. It remains unclear, however, what effects of LVG cross-section on the thermal–hydraulic characteristics of MCHS are. Herein, a new mini-channel with trapezoidal cross-section LVGs is designed, as well as the effects of front width (a), back width (b) and height (h) of trapezoidal cross-section LVG on local and overall thermal–hydraulic characteristics are explored at the Reynolds number (Re) of 347–868 using numerical simulation method. As results, comparing with rectangular cross-section LVG, trapezoidal cross-section LVG mostly brings better thermal performance, and trapezoidal cross-section LVG with a > b generally performs better comprehensive performance. The best comprehensive performance can be achieved as h equivalents to half of channel height. The recommended optimal parameters are a = 0.6 mm, b = 0.3 mm and h = 0.75 mm to obtain the best comprehensive performance, yielding the maximum performance evaluation criterion (PEC) of 1.756 at Re = 543. Furthermore, comparing to previous studies with LVG, the PEC in present study is better at studied range of Re. Overall, the trapezoidal cross-section LVG as a new and effective technology with optimal parameters is conducive to heat transfer enhancement and comprehensive performance improvement in MCHS.
查看更多>>摘要：For actual vapor-compression refrigeration or heat pump systems with variable-temperature heat reservoirs, two-stage evaporation cycle is an efficient way to improve the coefficient of performance (COP). However, its application is limited because the two-stage evaporation cycle requires two individual suction pressures carried out by two cylinders or two compressors. To solve this problem, a two-stage evaporation cycle with a common single-stage compressor is proposed. An ejector is applied to lift the lower evaporation pressure up to the higher evaporation pressure or the compressor suction pressure. A thermodynamic model is developed to analyze its performance. Numerical results indicate that the COP of the novel cycle is 12.1% higher than the ejector cycle, 6.9% higher than the two-stage evaporation cycle and 22.0% higher than the basic refrigeration cycle under rated operating conditions. The exergy analysis indicates that TSEC-E can reduce 49.6% exergy loss of expansion process and 21.5% exergy loss of heat transfer process in evaporators in comparison with the basic refrigeration cycle. Although TSEC-E is applicable with various refrigerants, particularly it is more effective for CO2 transcritical cycle. Moreover, performance improvement of the novel cycle under variable operating conditions is numerically investigated.
NSTL
Elsevier