查看更多>>摘要：Heat transfer deterioration (HTD) of supercritical CO2 heated in a tube influences the efficiency and safe operation of the system due to the occurrence of local high temperature. To suppress and delay the HTD, the characteristics and mechanisms of self-excited oscillation pulsating flow on HTD of supercritical CO2 are studied by experiment and simulation at pressure 8 MPa, mass fluxes from 350 to 800 kg/m2.s, heat fluxes from 30 to 200 kW/m2. The Helmholtz oscillator is introduced into the inlet of the vertical tube for generating a pulsating flow of supercritical CO2. The heat transfer performance is compared with that of without Helmholtz oscillator at the inlet of the tube. The results show that the self-excited oscillation pulsating flow improve the heat transfer performance significantly. The heat transfer parameters present oscillations with small amplitude along flow direction before pseudo-critical point (Tpc). The average heat transfer coefficient can be up to 3.4 times and the enhancement takes place mainly at the HTD region which is located at the entrance section of the heated tube. The effect of suppressing HTD is more significant at higher heat flux, and the peak of wall temperature can be reduced by 100 K at a heat flux of 200 kW/m2. Compared to the steady flow, the mechanism analysis of the selfexcited oscillation pulsating flow based on radial distributions of velocity, turbulent kinetic energy (TKE), and density reveals that the velocity distribution of "M- shape" appears later and gentler. The production and diffusion of TKE are improved at log layer (30 < y+ < 0.2r). In addition, the period and the amplitude do not show monotonous trends on heat transfer performance. The effects of pulsating parameters on HTD are optimized that the heat transfer performance with the period of 0.016 s and the amplitude of 100 kg/m2.s is the best in calculated cases.
查看更多>>摘要：High infrastructure expenditure and large operating costs of hydrogen refueling stations reduce the competitiveness of fuel cell vehicles over conventional fuel vehicles. The present study tends to explore the potential of reducing the precooling cost in refueling stations through a composite hydrogen refueling process integrating a turbo-expander. Accordingly, a thermodynamic model of the process following the requirements of SAE J2601 protocol is established to evaluate its feasibility. Based on the model, dynamic simulations are carried out to compare the performances of the proposed refueling process and conventional one. Obtained results reveal that applying the turbo-expander could reduce the precooling energy consumption by 52.6%. Moreover, the infrastructure expenditure of the proposed process is about 210,000$ lower than that of the conventional one. From a feasible investigation of the proposed process, it is concluded that refueling process integrating a turbo-expander has a remarkable potential in hydrogen fueling stations.
查看更多>>摘要：Designing air-water systems for industrial applications requires a fundamental understanding of mass accommodation at the liquid-vapor interface, which depends on many factors, including temperature, vapor concentration, and impurities that vary with time. Hence, understanding how mass accommodation changes over a droplet's lifespan is critical for predicting the performance of applications leveraging evaporation. In this study, experimental data of water droplets on a gold-coated surface evaporating into dry nitrogen is coupled with a computational model to measure the accommodation coefficient at the liquid-vapor interface. We conduct this measurement by combining macroscopic observations with the microscopic kinetic theory of gasses. The experiments utilize a sensitive piezoelectric device to determine the droplet radius with high accuracy and imaging to measure the droplet contact angle. This setup also quantifies the trace amounts of non-volatile impurities in the droplet. For water droplets evaporating in a pure nitrogen stream, the accommodation coefficient directly relates to vapor flux over the droplet's surface and is affected by the presence of impurities. We obtained a surface-averaged accommodation coefficient close to 0.001 across multiple water droplets evaporating close to room temperature. This quantification can aid in conducting a more accurate analysis of evaporation, which can assist in the improved design of evaporation-based applications. We believe the modeling approach presented in this work, which integrates the kinetic theory of gases to the macroscale flow behavior, can provide a basis for predicting evaporation kinetics in the presence of extremely dry non-condensable gas streams.
查看更多>>摘要：In this work, a segmented converging thermoelectric generator is proposed and optimized by designing a variable converging angle in each segment of the heat exchanger. A mathematical model of thermoelectric generator is developed to determine the corresponding converging angles. Also, the effects of air temperature and mass flow rate on the segmented converging structure are explored. Results show that the output power of thermoelectric generator can be apparently improved by using the segmented converging heat exchanger that can make the temperature differences of thermoelectric modules in all segments approximately identical. Compared to the conventional one, the output power can be increased by 12.5% at the air temperature of 500 K and air mass flow rate of 20 g/s, where the converging angles of the segments of heat exchanger are 0 degrees, 2 degrees, 1 degrees, and 1.8 degrees respectively. It seems that the converging angles between the segments change like a sine wave, and the wave range rises with increasing the air temperature and decreasing the air mass flow rate. In addition, the backpressure power loss induced by the segmented converging heat exchanger is lower than the conventional plate-type one due to an apparent decrease of the head loss of outlet cover. This novel optimization structure of the heat exchanger can provide a global performance improvement for the thermoelectric generator system.
Garcia, John Carlo S.Tanaka, HirokiGiannetti, NiccoloSei, Yuichi...
13页
查看更多>>摘要：In this paper, a multiobjective optimization of the structure of a flat-tubed microchannel heat exchanger is performed to reduce its volume and fan power at a specified capacity. Design variables include tube height, tube width, tube length, fin height, and fin pitch. A weight-based, real-coded genetic algorithm is implemented to optimize the design variables within their specified range of dimensions. To further improve the numerical simulations of the microchannel heat exchanger performance, correlations for the air-side Nusselt number, friction factor, and fin efficiency are developed and validated. In the optimization, the Pareto optimal fronts are obtained by varying weights of the two conflicting objectives. A reference microchannel heat exchanger operating at different capacities is optimized. Results show that the volume and fan power of the reference microchannel heat exchanger can be reduced by up to 45% and 51% respectively, depending on the weighting factor selected. The optimization approach of this study provides the optimal solutions at the given domain of geometric parameter dimensions.
查看更多>>摘要：Experimental tests were carried out to investigate the dynamic behaviors of parallel boiling channels under stable and unstable flow conditions with/without forced vertical vibrations. The parallel channels with a length of 1.91 m and diameter of 11.9 mm were attached to a vertical vibration platform. The inlet flow was controlled with average velocity of 0.142-0.277 m/s and subcooling of 6.0-12.6 degrees C, and the parallel channels were loaded with heat flux of 12.5-36.9 kW/m2. Overall 35 flow conditions were tested along with three static and vibration subset conditions (f = 0, 1.06 and 1.62 Hz), which covered the stable and unstable flow conditions in the parallel channels, and the dynamic variations of void fraction, pressure difference and temperature were recorded and analyzed. A non-dimensional void difference was proposed to identify the system stability of parallel channels, and fast Fourier transform (FFT) was utilized to extract the dominant frequencies of flow behaviors. While system is stable under vibrations, the dominant frequencies of flow properties were identical to the vibration frequencies; whereas if the system is unstable, the extracted frequencies were dominated by unstable flow oscillations, which are much lower than the vibration frequencies. In addition, the unstable flow oscillation frequencies can increase with increasing inlet velocity and heat flux, and an empirical correlation was proposed for estimating the oscillation frequencies, which can predict the present database with an averaged accuracy of 8.85%.
查看更多>>摘要：The present paper investigates the influence of using two grooved channel geometries on the hydrothermal performance of a helical microchannel heat sink. The results were compared with those obtained for the plain channel geometry. Based on the results, the highest flow mixing is observed for the staggered grooved configuration. Moreover, the convective heat transfer coefficient enhances by nearly 70% and 17% for the MCHS with the staggered and parallel grooved channels, respectively, as compared to that with the plain channel. The influence of Reynolds number (Re) escalation on the convective heat transfer coefficient is insignificant, while a 21% increase is obtained when NF concentration is increased from 0% to 1%. Furthermore, the increase of phi in the grooved configurations is more effective than that in the plain configuration. In addition, the lowest thermal resistance and temperature uniformity factors were obtained for the staggered grooved configuration. Besides, the parallel grooved configuration represents the highest frictional entropy generation rates among the three studied geometries due to the considerable reduction of the fluid flow cross-section area between the groove ribs. The highest heat transfer coefficient and pressure drop are associated with the staggered grooved configuration. For the plain, parallel grooved, and staggered grooved configurations, the increase of Re from 500 to 2000 escalates the NF pressure drop by 2730%, 4420%, and 4460%, respectively. Therefore, the highest performance evaluation criterion of more than unity was obtained for the MCHS with the staggered grooved channels for the studied range of Re (500-2000). The numerical analysis was also performed to evaluate the influence of groove pitch on the hydrothermal performance of the staggered groove channel for the pitch range of 0.15 mm to 0.75 mm. The results demonstrated that the highest PEC is associated with the groove pitch of 0.75 mm.
查看更多>>摘要：An advantageous solution for thermal energy storage is an air solid packed bed that consists in a tank filled with a solid material through which an air stream passes transferring heat to the filler in charge and collecting it in discharge. Effective tools to predict the thermal behaviour are required to optimize efforts towards the technology consolidation. To contribute to it, an experimental and numerical study on an air solid packed bed is presented here including some novel assumptions. The model is based on two energy balances applied to the solid and air phase separately. It also accounts for the thermal capacity of the tank walls by means of a correction of the solid density. The effective conductivity of the solid was evaluated through two components: one for the conduction through the solid and another for the radiative heat transfer. The last one gains relevance as the system average temperature rises, although it can be neglected below 500 degrees C. Convective heat transfer coefficient was introduced according to published correlations that were adequate for the operation conditions. The model was validated with experimental results from an own facility. The most critical parameter in the system behaviour is the thermal capacity of the filler. This fact points out the need of an accurate measurement of the specific heat. Finally, the possibility of simulating this kind of systems assuming an effective medium and solving an only heat balance was proposed if the convective heat transfer coefficient between solid and air is above a limit established in 100 W/m2K. These conclusions serve as reference to optimize efforts in predicting the thermal behaviour of air solid packed beds systems.
查看更多>>摘要：In medium-frequency heat treatment, it is difficult to thoroughly heat the interior of welds of heavy-wall pipes, and blind zones of heating or overheating zones with latent defects are very readily present. To solve the electromagnetic-heating-induced latent defects, we researched the mode of heat transfer inside the welds with heat sources under motion conditions to determine the temperature field evolution, spatial magnetic field distribution, and spatial matching relationship in cases of dual heat sources interfering with each other, tangential to each other, and not interfering with each other. With the increase in the axial spacing between the internal and external heat sources, the magnetic field intensity at the middle of the thickest part of the welding seam on the steel pipe increased and then decreased. When the heat sources interfered with each other, the temperature rise at the weld center from electromagnetic heating was no longer fast after the Curie point was reached. The induced magnetic field heating concentration positions shifted toward both sides of the weld center. The latent defect area was reduced by 18.7 mm2 when the spacing between the internal and external heat sources was 19 mm. This study explores the dual-heat-source interaction mechanism for the first time in the heat treatment of pipes to minimize the blind zones of heating and overheating zones.
查看更多>>摘要：The temperature of the battery jet is one of the key basic parameters for the design of battery thermal management system (BTMS) for vehicles, with sufficient results under combustion conditions in the presence of air. However, the original temperature distribution of battery jet in an inert atmosphere and its variation with the state of charge are not very clear for prismatic Ni-rich automotive batteries. This is closer to the real inside environment of the battery pack. In this work, a 50 Ah commercial prismatic cell with a Li(Ni0.6Mn0.2Co0.2)O-2 cathode is triggered to thermal runaway using external heating in a sealed chamber with a nitrogen atmosphere to avoid combustion caused by oxygen from the outside. The results show that the farther away from the safety valve, the lower the temperature of the jet. The jet temperature and its rise rate show an increasing trend with the maximum value of 701 degrees C and 173 degrees C/s detected with increasing states of charge. Therefore, the BTMS design needs to take into account the high thermal load and high thermal shock caused by thermal runaway even in the absence of external air to participate in the combustion.
NSTL
Elsevier