查看更多>>摘要：The mechanism of the condensation heat transfer enhancement on the microstructure surface has not been deeply explored, which restricts the full play of microstructure strengthening ability. In this study, a three-dimensional numerical simulation of the condensation on the micro fin surface revealed the liquid film flow behavior and the heat transfer performance, and the model is in good agreement with the experimental and theoretical results. The internal flow field on the curved surface shows that the liquid film is redistributed, and it proves that the surface tension plays a leading role in promoting the transverse movement of the liquid film. The micro fin surface area exhibits different functionalities, the excellent mass transfer region is located at the fin top, and its local heat transfer coefficient can reach 25 kW.m(-2).K-1, which is one order of magnitude higher than Nusselt's theoretical value. The fin bottom is the condensate collection area, where the condensate discharge velocity is increased by 2-3 times. The simulation results of the different micro fins indicate that the curvature radius of the micro fin could affect the condensate flow process, and the aspect ratio of the micro fin determines the local heat transfer enhancement degree and the strengthening area ratio. This study finally pointed out that the high-efficiency micro structure should have a large area of the fin top for enhancing heat transfer, as well as a large volume of the fin bottom for the liquid drainage.
查看更多>>摘要：In view of the nonequilibrium characteristics of water vapor in the condensation process, various models have different degrees of deviation in the prediction of the condensation process. Water droplet surface tension appears in the exponential term of the nucleation rate in the form of a third power, which has a significant effect on the distribution of parameters such as condensation location, water droplet number and wetness. In order to improve the accuracy of numerical simulation, the correction coefficient is used to modify the plane surface tension calculation model. The nonequilibrium condensation process in the Moses-Stein nozzle is simulated. By comparison with the experimental data, the influence of the surface tension correction coefficient on the calculation accuracy of the model is analysed. The functional relationship between the superheat at the steam inlet and the optimal surface tension correction coefficient is obtained. After correction, the error between the simulation results and the experimental values of each working condition of the nozzle is reduced to less than 2%. In addition, this conclusion is verified by the nonequilibrium condensation flow in the turbine cascades. The deviations between the three simulation results and the experimental values are 1.86%, 2.10% and 1.88% respectively, and the simulation accuracy is significantly improved. The results show that with the increase in the surface tension correction coefficient, condensation will be inhibited under the same working conditions. There is an optimal value of the correction coefficient. Under different working conditions, there is a significant positive correlation between the optimal surface tension correction coefficient and the inlet superheat. It can significantly improve the simulation accuracy by using the corrected surface tension value. The results can provide a reference for the calculation of wet steam condensation flow.
查看更多>>摘要：Additive manufacturing has created a paradigm shift in materials design and innovation, providing avenues and opportunities for geometric design freedom and customizations. Here, we report a microarchitected gyroid lattice liquid-liquid compact heat exchanger realized via stereolithography additive manufacturing as a single ready-to-use unit. This lightweight (~240 kg/m(3)) compact heat exchanger (with conjoined headers), with an engineered porosity of 80% and a separating wall thickness of 300 mu m, has a surface to volume ratio of 670 m(2)/ m(3). X-ray computed tomography imaging confirms a defect-free 3D printed heat exchanger. The thermohydraulic characteristics were experimentally measured using water as the working fluid. The measurements indicate that the heat exchanger evinces an overall heat transfer coefficient of 120-160W/m(2)K for hot fluid Reynolds number Reh in the range of 10(-40). Additionally, finite element analysis was conducted to evaluate the thermo-hydraulic characteristics of the gyroid lattice heat exchanger. The experimental results show-a 55% increase in exchanger effectiveness for the additively manufactured gyroid lattice heat exchanger in comparison to a thermodynamically equivalent, most-efficient, counter-flow heat exchanger at one tenth of its size. The superiority of our architected heat exchanger to extant work is also demonstrated.
查看更多>>摘要：Applying serial water loops in multiple screw chillers has the potential to improve the COP of screw chillers above 7.0. However, the maximum potential of this method hasn't been identified and the ultra-efficient target hasn't been achieved until now. Therefore, an energy and exergy analysis is performed on the water-cooled multiple chillers with serial water loops in this paper to recognize their maximum potential. Analysis results show that the water-cooled multiple screw chillers with serial water loops have the maximum potential to increase COP by 26.02% and exergy efficiency by approximately 22% relative to the single screw chiller under the normal operating condition. Adding one chiller based on the single chiller system could increase the COP by 11.38%, and it is not suggested to design multiple chillers with more than four chillers owing to the reduced additional improvement in COP with the increasing number of chillers. Moreover, both condensers and both evaporators should be designed with the equal area of heat exchange, respectively, aiming to set the equal temperature difference through both evaporators and both condensers. On basis of analysis results, the ultra efficient dual screw chillers were developed and tested. The COP and IPLV of the chillers reached 7.22 and 9.50, which were higher than the minimum allowable value of the first-order energy efficiency for water chillers by 20.3% and 26.67%.
查看更多>>摘要：The Printed Circuit Heat Exchanger (PCHE) is one of the potential precooler candidates to be employed in the supercritical carbon dioxide (S-CO2) Brayton cycle with its high compactness, high efficiency and high endurance to extreme condition. Since the S-CO2 precooler operates in the condition close to the pseudo-critical region, it is of importance understanding the performance differences among different types of PCHE with the dramatical change of thermal-physical properties. Thus, in this study the effect of three channel configurations with zigzag, wavy and airfoil fin channel on heat transfer and flow resistance under precooler condition are investigated with numerical method. Based on the local analysis, it is found that the zigzag channel PCHE presents the better heat transfer performance compared to wavy and airfoil fin channel, and differences are increased as the temperature approaches the pseudo-critical region. Meanwhile, the airfoil fin channel shows the lower friction factor with only 6.12% and 41.31% of the zigzag and wavy channel, respectively. Furthermore, comprehensive performances are compared through the methods of entropy number, JF factor (ratio of Colburn j to friction factor) and Q/(m.delta P) (ratio of heat exchange to pressure drop and mass flow). Based on the results, it is recommended for the airfoil channel to be adopted near the pseudo-critical region due to its larger surface area and smoother structural design.
Leong, Kin YuenKazi, Salim NewazChew, Bee TengHuq, Tahsinul...
31页
查看更多>>摘要：Nanofluids of graphene nanoplatelets (GNP) have superior thermal performance characteristics and good sta-bility, are relatively affordable, and can easily be prepared by the two-step method. This review performs an in-depth analysis of the preparation, stability, and thermophysical properties of GNP nanofluids, and their appli-cations in heat exchangers, solar thermal collectors, and heat pipes. This study analyses in detail the performance improvements achieved with pristine, covalent functionalised, and non-covalent functionalised GNP nanofluids compared to water. Covalent functionalisation was found to be superior to non-covalent functionalisation in terms of stability and heat transfer coefficients. Functionalisation by electrophilic addition and free-radical grafting were found to be more environmentally friendly compared to acid treatment. In terms of convective heat transfer coefficient, pristine GNP outperformed functionalised GNP, but both types showed large im-provements compared to water. It was found that stability and heat transfer performance improved as particle size was decreased, while thermal conduction and convection coefficients increased with nanofluid concentra-tion and temperature. Thermal conductivity improvements of over 30% were found for both pristine and covalently functionalised GNP nanofluids at 0.1 wt% concentration. A maximum convection heat transfer co-efficient increase of 200% was achieved using 0.1 wt% pristine GNP nanofluid. By comparison, a maximum improvement of 119% was achieved using covalently functionalised GNP. The convective heat transfer enhancement seemed to increase with decreasing tube diameter. In flat plate solar collector applications, effi-ciency improvements over 20% were obtained for covalently functionalised GNP nanofluids at 0.1 wt%, while an efficiency improvement of over 65% was obtained using pristine GNP in an evacuated tube solar collector. Applications in cooling, heat pipes, and direct absorption solar collectors were also reviewed. From this study, it could be inferred that GNP nanofluids are a viable alternative working fluid. Further research is needed to optimise their performance.
Varela, Richard JaysonGiannetti, NiccoloSaito, KiyoshiWang, Xinming...
11页
查看更多>>摘要：The experimental performance of a new three-fluid fin-tube contactor using a novel ionic liquid desiccant solution is investigated and compared with those of an adiabatic packed bed contactor with LiCl solution for liquid desiccant air conditioning system applications. Owing to the compatibility of the ionic liquid solution with aluminum, a fin-tube contactor is manufactured to exploit internal cooling from a third fluid. The results indicate the superior dehumidification performance of the fin-tube contactor, which yielded an outlet air humidity ratio of 10.5 g(w).kg(da)(-1) at a solution mass flux of 3.06 kg.m(-2).s(-1), as compared to a similar humidity ratio at a solution mass flux of 4.98 kg.m(-2).s(-1) achieved by the packed bed contactor. In terms of cooling performance, the outlet air temperature of the fin-tube contactor is 21.9 degrees C, which is lower than that of the packed bed contactor even at its highest solution mass flux. This indicates that the cooling performance of the fin-tube contactor has minimal dependence on the solution mass flux in achieving cooling effect, owing to the cooling medium flowing inside the tubes. Relatively lower solution pumping power than that of the packed bed contactor can be expected from the fin-tube contactor, in addition to the ability to achieve comfortable and hygienic indoor air at a cooling medium temperature of 17 degrees C or higher, the ionic liquid desiccant and three-fluid fin-tube contactor pair for liquid desiccant air conditioning system is elevated a step higher as an alternative to vapor compression air conditioning system.
查看更多>>摘要：With the development of space technology, the requirements of thermal control on spacecraft are increasing. In space missions, due to the improvement of the detection accuracy, the diameter of deep space telescope is becoming larger and larger. These detection instruments need efficient and reliable thermal management. Thus, the application of the large-scale cryogenic cooling technology, which can provide heat dissipation for the continuous and long life of the detector, has become a difficult problem. In the present study, a large-area cryogenic loop heat pipe is designed for the optical telescope. It is a good combination of multi heat source and large-area cryogenic loop heat pipe. The main evaporator is cylindrical due to fit the lens barrel. The heat exchange area can reach 1 m(2), and the average temperature performance is less than 5 K. The condenser adopts the form of parallel pipeline to further reduce the system resistance and ensure the smooth operation of loop heat pipe. Its operation stability and temperature uniformity of the main evaporator under different total loads are experimentally studied and analyzed. It is proved that the loop heat pipe is feasible for large area multi-heat source cryogenic loop heat pipe in engineering. It can also start up and run normally under anti-gravity conditions. The experimental results show that the large area multi-source heat collection cryogenic loop heat pipe can be started and run normally in the temperature range of 80-120 K.
查看更多>>摘要：A campaign of tests is performed for saturated boiling heat transfer characteristics of water in narrow rectangular channel. The test matrix covers a relative wide range of conditional parameters and 3142 test data are acquired, screened and selected for study. Meanwhile, visualization test has been conducted with three typical flow patterns, namely isolated bubbly, confined bubbly and irregular annular flows, being observed and identified, each pattern corresponding to a prevailing or a transitional combined boiling heat transfer mode. Study of the data trend indicates that two boiling heat transfer mechanisms, namely nucleate boiling and forced convective evaporation, are either one dominant or two combined transitional during different phases of saturated flow boiling within the channel. With the basic view obtained from the test campaign, and with related parametric effect and sensitivity study, as well as the existing correlation assessment with the test data, a new correlation for the saturated boiling heat transfer is proposed and regressed, which consists a modification factor composing the two basic boiling mechanisms and the transition function between. A preliminary assessment is performed for its statistical and predictive accuracy, from which a relatively satisfactory agreement is drawn. The new correlation applies to a heat flux range of 100 -2300kW/m(2), a mass flux range of 500 2000kg/m(2).s, a system pressure range of 0.2 5.5MPa and the equilibrium quality range of 0 0.72. It might be used, in conjunction with the flow pattern and the related heat transfer mechanism, to predict the saturated boiling heat transfer coefficient over a wide range of heat flux, mass flux and pressures with better accuracy than previous correlations.
NSTL
Elsevier