查看更多>>摘要：Traditional battery thermal management using phase change materials (PCM) is restricted by PCM's leakage, low thermal conductivity and incompatibility. In this work, a shape memory composite PCM (SCPCM) composed of styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer (SEBS), paraffin (PA), expanded graphite (EG) and Fe3O4-modified graphene oxide (Fe3O4-MGO) was prepared. The thermal properties and shape stability of the composite material were studied. The compatibility between the phase change matrix and the support material was investigated. The results show that reducing the size of EG particles can effectively reduce the thermal conduction "percolation point". The Fe3O4-MGO particles loaded on EG can effectively reduce the interfacial thermal resistance between EG and PA then further strengthen the heat transfer capability of CPCM. SCPCM shows good thermal stability in the range of phase transition temperature higher than PA (49-85 degrees C). The simulation results show that PA and SEBS molecules exhibit good interfacial compatibility, hydrogen bonds are formed between the molecules where there is a strong interaction. Therefore, PA/SEBS show good thermal stability in the macro system. SCPCM exhibits excellent performance in heat dissipation. Under discharge conditions of 3C, the temperature difference in the SCPCM module is kept within 3 degrees C.
查看更多>>摘要：Building envelopes consist of transparent and non-transparent components, and pose distinct opportunities and challenges in energy conservation. Glazing envelopes, which are transparent, are mainly responsible for the lighting and ventilation of buildings, but the acoustic performance may be weakened. Since glazing envelopes suffer from the defects of high solar transmittance, poor thermal insulation, and low thermal inertia, their energy-saving technologies are significantly different from those of non-transparent envelopes. Different energy saving technologies have been studied to improve the optical and thermal performance of glazing envelopes. However, there is a lack of a review study involving the phase transition process and the improvement of photo thermal transmission in glazing envelopes containing phase change materials. The present work provides a comprehensive overview of research advances in optical transmittance, thermal resistance, and thermal inertia along with photo-thermal transmittance in glazing envelopes, with a special focus on the integration of phase change materials. The study reveals that measurement and numerical models are inadequate to study photo thermal transmission. Besides, it is identified that there is a research gap in the acoustic performance of glazing systems incorporating phase change materials, and there is a lack of database on the optical properties of phase change materials containing nanoparticles.
查看更多>>摘要：The heat transfer characteristics of a wall implanted with heat pipes (WIHP), a new passive natural energy utilization technology, significantly differ from that of an ordinary wall. It features effective heat transfer between indoor and outdoor environments due to the pipe's unidirectional thermal conductivity. The thermal performance of the wall is crucial in terms of reducing a building's energy consumption. In this paper, the experiment and theoretical analysis were combined, and the numerical simulation calculation was used to analyze the effects of material thermo-physical parameters on WIHP heat transfer performance. The results show that for the inside surface temperature, density and specific heat capacity are negatively related to it, and thermal conductivity is also negatively related to it before the heat pipe is in optimum working condition, and then the thermal conductivity is positively related to it. For the inside surface heat flux, it is opposite to the variation of inside surface temperature. The inside surface temperature rise rate (ITRR) is proposed to reflect the thermal response of the inside surface temperature to the variation of outdoor air temperature. When the thermal conductivity increases, the difference between the maximum and minimum ITRR is 0.053?/h. When the density increases, the trend of ITRR decreases first and then increases, and the average value is 0.062 ?/h. When the specific heat capacity gradually increases, the difference between the maximum and minimum of ITRR is 0.006?/h.
查看更多>>摘要：Structure improvement and optimal operating parameters of the thermoelectric cooling component will favor obtaining a lower transient supercooling temperature. The contribution proposed a new I-type thermoelectric cooling structure by connecting the P-type arm with the N-type arm by the sandwich arrangement structure. A one-dimensional transient governing equation is constructed based on the physical model, and the finite volume method provides a numerical solving solution. Moreover, an experimental system is built to analyze the supercooling performance of the I-type cooling layout under different operating conditions. The experimental results validated that the proposed numerical simulation method is feasible and accurate for the component modeling and cooling performance analysis. Moreover, we analyzed the influences of single pulse current amplitude and reference-pulse current groups on the cold end temperature of the I-type thermoelectric cooling structure. The cold end temperature will reach the minimum when the pulse current amplitude is 20 A. Meanwhile, the combined reference-pulse current operation effectively reduces the transient subcooling temperature at the cold end. The experiments show that the minimum cold-end temperature reduces by 31.8 K when the input reference-pulse current waveform is 5 A-20A. In summary, the I-type thermoelectric cooling structure achieves better cooling performance under combined reference-pulse current operating conditions.
查看更多>>摘要：Vapor ejector cooling systems have attracted a lot of interest in the last 25 years, given that their electrical consumption is much lower than that of conventional refrigeration systems. However, ejector systems have suffered from a low coefficient of performance. Among ejector design parameters, the nozzle exit position (NXP) has a strong impact on system performance, but so far, limited and contradicting observations in terms of its optimal value have been reported in different literary works. In this work, a large-sized and well instrumented experimental setup was used to determine the impact of the nozzle geometry and operating conditions on the optimal NXP values. The results showed that the optimal NXP values were significantly impacted by the ejector nozzle geometry and the critical entrainment ratio (omega(crit)), however the primary, secondary and outlet pressures had negligible effect. Up to four optimal NXP values were observed over the tested NXP range. This number was found to increase with the critical entrainment ratio, with two optimal NXP values for omega(crit) < 0.25 and four optimal NXP values for omega(crit) > 0.4. Proper adjustment of the NXP value increased the entrainment ratio by > 34%. For the first time, the wall pressure along the constant area section of the ejector was used to study the optimal NXP values. This led to a new finding: while the critical entrainment ratio was going from peaks to valleys along the NXP range, the wall pressure at the inlet of the constant area section varied inversely, this pressure being at a minimal value when NXP is close to an optimal value. It was also observed that the critical compression ratio decreased at higher NXP values. This work is the first to show that the optimal NXP is linked to the critical entrainment ratio, which provides an explanation to the contradictory observations reported in the literature regarding the impact of operating pressures on the optimal NXP values. Finally, this is the first study that shows the importance of adjusting the NXP whenever operating conditions are modified or a different ejector is used.
查看更多>>摘要：The use of geothermal energy has increased significantly (90 time) since 1995. Among these increases, Ground Source Heat Pumps (GSHP) has contributed by 40 times in an effort to reduce the burning of fossil fuels and contribute to the reduction of green house gas emissions. The space requirements and high initial cost of borehole fields hinder the widespread use of GSHPs. The use of foundation piles, used as a ground heat exchanger, has been proposed to overcome these limitations. Research has shown that foundation piles have a lower depth and smaller spacing compared to boreholes. Phase Change Materials (PCMs) have been proposed as a potential solution to increase the storage capacity and decrease the thermal radius of energy piles. In the current study, a 3-D finite element model was developed to study the effects of PCMs on the performance of energy piles against a real building load for a complete year, while integrating an actual heat pump (HP) performance curve into the numerical model. The effects of the PCM location and melting range were also investigated. The study revealed a 5.2% enhancement in the Coefficient of Performance (COP) during the melting of the PCM, and a negative effect of up to 1.8% during the completely solid-state. Locating the PCM cylinders inside of the concrete shell led to better performance compared to when the PCM cylinders were located outside of the concrete shell. The best locations were found to be between the center of he pile and the U-loop. The PCM melting range of (4-6) degrees C was better than the melting range of (1-3) degrees C for the current study load. Lastly, the use of multiple PCM melting temperatures for a given design was investigated. The results revealed that the use of multiple PCM melting temperatures led to a performance enhancement of up to 26%.
查看更多>>摘要：In solar tower receiver, boiler water-wall and various heat exchangers, the uneven distribution of two-phase flow often occurs among parallel heated pipes, which seriously reduces the operating efficiency and may further induce heat transfer deterioration and soaring wall temperature. In this paper, a coupling model for the flow distribution and heat transfer process of steam-water two-phase flow in parallel heated pipes was established based on the discrete methods. In the present model, two main improvements were proposed as below. On one hand, the calculated equations for the pressure drop and the phase distribution of two phase fluid in both the combining T-junction and dividing T-junction were established to consider the coupling effects of the inlet manifold and outlet manifold on the flow distribution characteristics. On the other hand, the calculation module for the heat transfer and wall temperature along the branch pipes was also coupled in the present model with considering of different heat transfer regions (saturated nucleate boiling region, liquid deficient region, and superheated steam region) along the pipe. The present model was then verified and used to investigate the effects of system pressure, inlet fluid flow, inlet steam quality and manifold type on the flow distribution characteristics of high temperature and high pressure steam-water in parallel vertical upward pipes, and the effect of the uneven distribution of the gas-liquid two phases on the system safety was analyzed and evaluated. It was found that gas phase preferentially enters the first branch pipe near the inlet of the dividing manifold affected by buoyancy, and this results that the inlet steam quality of the corresponding branch pipe is always obviously higher than that of the subsequent pipes. As the system pressure decreases, the outlet fluid enthalpy distribution and outlet wall temperature distribution among branch pipes become more uneven. Moreover, the outlet fluid enthalpy and outlet wall temperature of the branch pipes closer to the inlet of the dividing manifold increase sharply with the decrease of system pressure. In the U-type parallel pipe system, the branch pipes away from the inlet of the dividing manifold are more prone to wall overheating at the outlet, while the opposite is true for the Z-type parallel pipe system.
查看更多>>摘要：The non-uniformity of the secondary flow distribution in the twisted oval tube has impeded its further improvement of the heat transfer performance. A novel twisted tube (NTT) improves the distribution of the secondary flow and enlarges the effect of the secondary flow on the main flow, intensifying the radial mixing and equalizing the temperature distribution of the fluid, which strengthens the convective heat delivery in the near wall region and the core zone comprehensively. Heat transfer characteristics and entransy evaluation of water and engine oil in the NTT are investigated in the low Reynolds number (Re) region. The heat delivery performance of the NTT improves with the reduction of the distance ratio (DR) and the twist pitch ratio (PR), and also raises with the increase of Re. The friction factor of the NTT increases with the reduction of DR and PR, and decreases with the rise of Re. Compared with the twisted oval tube, the NTT could elevate the Nusselt number and enlarge the friction factor by 1.49-1.56 times and 1.32-1.41 times, respectively. Compared with the plain tube, the NTT could improve the Nusselt number and increase the friction factor by 2.42-2.76 times and 1.48-1.56 times, respectively. The equivalent thermal resistance of the NTT decreases with the reduction of DR and PR and reduces with the increase of Re. The Case 4 with the minimum DR and the smallest PR shows the supreme heat transfer performance and the lowest equivalent thermal resistance. The NTT could reduce the entransy dissipation remarkably, compared with the plain tube, the NTT could decrease the equivalent thermal resistance by up to 58% for water under the condition of constant wall temperature and could reduce the equivalent thermal resistance by as much as 53% for engine oil under the condition of given heat flux on the tube wall.
查看更多>>摘要：As one of the most widely used types of heat exchangers, the shell-and-tube heat exchanger (STHX) offers the advantages of being low-cost, easy to clean, and highly reliable under high pressure and temperature conditions. Generally, a large number of tube bundles are used in the STHX to increase the heat transfer capacity. The three-dimensional (3D) two-phase flow simulation of the STHX is made exceedingly difficult if the tube bundle region is full-size modeled. Therefore, simplified models and approaches are required to meet the urgent demands of STHX engineering numerical simulation, especially for 3D analyses. Herein, we have developed an OpenFOAM solver "PorousDriftFoam " suitable for the two-phase flow and the boiling heat transfer numerical simulation of the STHX. The porous media model was applied to simplify the tube bundle region and the drift-flux model (DFM) was adopted in the two-phase flow computational fluid dynamics (CFD) simulation for the STHX. The heat transfer tubes are regarded as the solid region of porous media, while the shell side is considered the fluid region. We calculate the coupling heat transfer between the tube and the shell sides and consider the resistance introduced by the heat transfer tubes and support plates in the STHX. Two international benchmarks, the FRIGG test and the MB-2 experiments, were selected to fully validate the solver. For the FRIGG experiment, the predicted void fraction stands in good agreement with the experimental data, with an average absolute error of 0.07 and an average relative error of 13.3%. For the MB-2 experiment, the predicted values of pressure drop and temperature fit well with the experimental data. The developed solver could be applied in the 3D two-phase flow and heat transfer numerical simulation of a typical STHX in the industry.
查看更多>>摘要：The ongoing depletion of shallow resources has led to the mining of progressively deeper deposits, where high temperature thermal stresses pose heat hazards and threaten worker safety. Mitigating geothermal hazards is therefore an essential component of mining operations in deep mines. In this paper, a novel synergetic mining approach is proposed that controls heat hazards at low cost and simultaneously exploits mineral and geothermal energy. Injection and production channels are designed below the ventilation tunnel to synchronously cool the tunnel surrounding rock and exploit mine geothermal energy. A fully coupled numerical model is established that simultaneously simulates heat and mass transfer in a large-scale ventilation network and geological reservoir. The cooling effect on the tunnel and heat recovery capacity of geothermal exploitation are investigated to assess the feasibility of the proposed scheme. Three case studies are presented to identify the impact of the water injection and production channel layout on the heat and mass transfer characteristics in the rock layer. The temperature of the tunnel surrounding rock is cooled over a few years via low-temperature water injection into the mine and heat production in the deep rock layer; this rapidly reduces the air temperature inside the tunnel. The total heat production rate initially rapidly increases and then gradually decreases. In the ninth year, the maximum heat production rate of the production channel reached 6.01 x 10(3) kW. The cooling effect on the roadway and heat production performance are optimal when the water injection channel is arranged under the air intake side of the main ventilation roadway of the mine and the heat production channel is arranged in the deep rock under the return air side of the roadway. After 4 years of water injection, the temperature at the end of the tunnel in Case 1 rapidly decreased to 30.9 & DEG;C; this temperature is 6.6 & DEG;C and 3.4 & DEG;C lower than those in Cases 2 and 3, respectively. Longer injection channel lengths can significantly reduce the injection pressure requirement in the injection channel. Shorter distances between the injection channel and tunnel are associated with faster reductions of the tunnel temperature. This technique significantly improves the thermal comfort inside the tunnel and produces considerable geothermal energy; a single heat production channel can produce 1.68 x 10(15) J over a period of 10 years.
NSTL
Elsevier