查看更多>>摘要：Pulsating heat pipe (PHP) is a very efficient solution for electronics cooling。 Several strategies can be applied to improve the thermal performance of PHPs。 In this context， the heat transfer enhancement of a flat plate pulsating heat pipe with a channel modification in the evaporator region， resulting in ultra sharp lateral grooves， was investigated experimentally。 Chamfers were machined in the lateral walls of thirteen semi-circular cross section U-turn channels， drilled in flat copper plates。 To form the sharp-grooved circular channels， two plates were faced against each other and diffusion bonded， resulting in a monolithic piece with high quality channels。 The ultra sharp grooves had an angle of 29。1 ± 2。9°。 The lateral grooves work as artificial nucleation sites， helping in the bubble formation， and act as a capillary medium， spreading the liquid over the evaporator region， delaying the dry-out。 Therefore， the device could be less dependent on gravity， enabling it to be considered for applications in microgravity environments。 To ascertain the efficiency of the proposed device， its performance was compared with another similar PHP with the same external geometry and with round ordinary cross-section channels of the same 2。5 mm channel diameter。 Distilled water was selected as the working fluid， which， as predicted by literature models， worked at confinement conditions。 As usual， the thermal behaviors of PHPs were characterized by their temperatures and pressures， depending on the operation conditions。 The best filling ratio for each PHP was experimentally determined， considering heat loads from 20 up to 350 W (from 0。35 up to 6。1 W/cm2)。 The influence of the ultra sharp grooves on the thermal performance of the PHP was investigated for a large range of power inputs， reaching up to 1200 W (20。9 W/cm2)， for the best filling ratio。 The gravity influence in the PHP operation was evaluated by tests in three orientations: gravity-assisted， horizontal and against-gravity。 Both cross-section profile PHPs performed effectively well for heat fluxes up to 20。9 W/ cm2， even in the against-gravity position， showing that the devices are suitable for temperature control of electronics， including those with high heat fluxes。 Besides， the gravity effect could be neglected for heat powers beyond 600 W (10。4 W/ cm2)， which make them adequate for microgravity applications。 The presence of ultra sharp grooves in the evaporator section of the PHP reduced by 2。1 °C the average evaporator temperature， decreased the temperature variations among sections and improved the thermal performance by 12% in the horizontal and gravity-assisted orientation。
查看更多>>摘要：Steam jet ejector chillers (SJECs) can be a suitable cooling technology in buildings where solar heat gains constitute a significant part of the cooling load。 In this study， an SJEC was combined with a radiant chilled ceiling (CC) instead of traditionally used convective cooling terminals in an effort to increase the coefficient of performance (COP) by increasing the supply water temperature to the terminal (Twater，sup) and thus the evaporation temperature。 Experiments were used to verify a mathematical model of SJEC。 The model was combined with Twater，sup obtained from heat transfer calculations in three types of CC performed by a verified software。 The COP of the ejector cooling system was 0。25 and 0。38 for R718 (water) and R1233zd， respectively， at a generation temperature of 130 °C and Twater，sup of 7 °C， assuming a fancoil。 Increasing Twater，sup to 9 °C， which was considered to be the lowest Twater，sup theoretically possible for CC involving pipes underneath the surface， improved COP by up to 14% while providing a maximum cooling capacity of about 80 W per m2 of room area。 Further increasing Twater，sup to 15 °C improved the COP by 50% compared to the fancoil， while covering cooling loads of at least 40 W per m2。 The estimated reduction in generation temperatures due to increased COP was up to 12% (Twater，sup = 9 °C) and 37% (Twater，sup = 15 °C)。 This means a lower energy input needed for the cooling machine， which allows a smaller solar collector area。
查看更多>>摘要：In metal additive manufacturing (AM)， the in-situ measurement of the melt pool characteristic plays a significant role in monitoring the quality of the printed components。 In this work， based on dual-wavelength thermometry， a coaxial melt pool temperature measurement system with a single high-speed camera in the laser powder bed fusion (LPBF) process is developed， including the design of the relay and optical path amplification system， and the beam splitting and chromatic aberration correction system。 Moreover， a dual-waveband image-matching method with sub-pixel accuracy， and an overall parameter calibration and optimization method are proposed to improve the accuracy of the coaxial temperature measurement system。 Besides， the validation experiment measured by a high-temperature blackbody furnace and a standard photoelectric pyrometer indicates that the temperature measuring error of the developed system is less than 1%。 The melt pool characteristics including the temperature distribution， profile， temperature gradient， and cooling rate were measured by the developed coaxial temperature measurement system， and the distribution of average temperature and peak temperature under different linear energy densities during single-line printing was also compared and analyzed。 The single-line printing results of different parameters show that the higher the linear energy density， the higher the average temperature and peak temperature of the melt pool， and the optimized parameters minimize the fluctuation of melt pool temperature and are more favorable to the formation of high-quality parts。 In multi-layer printing mode， the heat accumulation is strong， resulting in the slow cooling rate of the melt pool。
查看更多>>摘要：In this paper， a two-dimensional hybrid thermal lattice Boltzmann (LB) method coupled with the leaky dielectric model was firstly presented to study droplet evaporation on a heated substrate under the influence of an electric field。 Contact angle hysteresis and Marangoni effects are included。 The accuracy of this model is validated by simulation a static droplet evaporation and droplet deformation in the electric field。 Then， the validated model is used to explore the effects of the electrical field on droplet evaporation， by varying the electric field strength and direction。 The electric field is seen to change the droplet morphology， internal flow， heat transfer and thus evaporation time。 The droplet is elongated along the direction of the electric field， and the contact angle is reduced。 A pair of vortices is formed inside the droplet， which is consistent with the Marangoni flow direction， while the pair of vortices outside the droplet is oriented in the opposite direction to the internal one。 For all droplets， the average heat flux to the evaporating is linearly related to the contact line length density。 Additionally， it is discovered that applying a vertical electric field increases the evaporation time of a sessile droplet， while applying a horizontal electric field decreases it。 Our research aids in achieving precise regulation of droplet evaporation and gives a better understanding of the influence of electric fields on droplet evaporation。
查看更多>>摘要：Plate heat exchanger is a kind of widely used efficient energy exchange equipment， which has the advantages of high heat transfer coefficient， compact structure， easy cleaning and maintenance。 However， the accumulation of condensate forming liquid film weakens heat transfer effect of ordinary plate condenser (OPC)。 Effectiveness of liquid-separation condensation on improving heat transfer effect has been confirmed in previous research， but the design of liquid-separation unit may cause insufficient vapor–liquid separation thereby hindering the improvement of heat exchange effect。 In this paper， a novel U-shaped tube liquid-separation plate condenser (LPC) is firstly designed。 Moreover， this research mainly focused on investigating the feasibility of U-shaped liquid-separation unit， the universal applicability of the LPC and structure optimization of the LPC。 Experimental results indicate that the heat transfer capacity of the LPC is increased 1 to 1。5 times than that of the OPC and the heat transfer coefficient of the LPC could be improved up to 31。2%， heat transfer coefficient improving rate of the LPC is in the range of 20% to 36% under different cooling methods。 Furthermore， numerical results on structure optimization of the LPC are the short-to-long axis ratio at 0。6， the 2-stages liquid-separation and the liquid-separation position at 3/4， respectively。 This work has potential for practical application。
查看更多>>摘要：Thermal runaway (TR) is the most critical safety issue of lithium ion battery (LIB)， and more uncertain hazard factors may be introduced under working state。 In this study， TR propagation in LIB modules during charging was investigated firstly。 A shorter TR propagation time is observed with increasing charging rate， and the average TR propagation time at 3C charging rate is only 12。1% of that at 0。5C。 Besides， the TR propagation exhibited an obvious acceleration effect and the TR onset temperature decreased with TR propagation at high charging rates， the lowest TR onset temperature is only 127。4 ℃。 Redistributed current led to rapid heat generation of the remaining cells， and the side reaction heat gradually replaced the heat absorbed from surroundings and became the main source of heat accumulation。 Coupled with the heat generation of charging， the TR propagation accelerated。 In addition， the heat conduction through air accounts for 67% of the total heat transfer， so reducing the thermal conductivity between cells can be considered as a means of mitigating TR propagation。 This study delivers an underlying analysis of TR propagation during charging， and which is expected to contribute references for the safety of LIB application。
查看更多>>摘要：Industrial exhausts release a tremendous amount of latent heat in form of moist flue gases and at the same time a significant amount of energy is consumed by the industrial process involving compressed dry air of low humidity levels。 The open absorption technology can efficiently recover this latent waste heat and is also capable to provide dry air。 This study presents a novel multifunctional open absorption system coupled with a compressed air dryer and is characterized by providing a preliminary flash regenerator driven by the heat recovered from the compressed air which can benefit the further regeneration at low heat source temperatures。 The validated process and thermodynamic models of this novel multifunctional system are developed and a parametric study is carried out to assess its thermal performance with the effect of important operational parameters。 The parametric results indicate that the system is capable to provide compressed dry air for industrial applications at low humidity level of 1。05 g/kg (with dewpoint temperature of ?16。3 °C) and also can provide hot water for 184。45 kW heating capacity by operating at a performance coefficient of 1。945 and heat recovery efficiency of 91。95%。 The system is also capable to recover the water at the rate of 118。8 kg/h with 92。8% recovery efficiency by operating at 120 °C regeneration heat source and 600 kPa pressure in the dryer。 The drying performance of dryer can be enhanced significantly by increasing the operating pressure and by improving the solution inlet parameters。 The outlet humidity of air can be decreased up to 0。27 g/kg (with dewpoint temperature of ?28。34 °C) by increasing the solution concentration up to 61% at 32 °C temperature and operating pressure of 800 kPa。 Moreover， the required heat source temperature in main regenerator can be decreased from 120 °C to 113 °C in effect of the increased air pressure， when the recovered heat from compressed air dryer is capable enough to provide 13。2–42。7% preliminary solution regeneration。
查看更多>>摘要：Passive radiative cooling is currently the frontier technology in renewable-energy research。 In terms of extraterrestrial applications， radiative cooling is a critical component to the thermal management system of a spacecraft， where the extreme environment of space can cause large temperature variations that can break and damage equipment。 For terrestrial applications， nocturnal or daytime radiative cooling is expected to lead to cost-effective passive heat management without the need of inefficient and costly artificial refrigeration technologies。 However， most currently available radiative cooling systems cannot be changed dynamically and radiate a constant static amount of thermal power。 Dynamically tunable adaptive radiative cooling systems will be a critical development to prolong the lifetime of spacecraft or improve the efficiency of terrestrial cooling systems。 Here we propose stretchable radiative cooling designs that can be substantially tuned by using the simple physical mechanism of mechanical strain。 When their structure is stretched， the radiated power is significantly reduced。 We develop a modeling method that can simulate mechanical stretching combined with electromagnetic response to compute the tunable thermal emission of these new adaptive radiative cooling systems。 The presented photonically engineered structures can be used as coatings to achieve efficient adaptive thermal control of various objects in a cost-effective and environmentally friendly way。 The proposed designs are much simpler to be realized than others found in the literature and the best design achieves a high thermal emission power with a tunable range on the order of 132 W/m2。
查看更多>>摘要：The use of phase change materials (PCMs) in energetic systems today is in large parts restricted by dynamic issues， or what could be termed the “rate problem”; i。e。， how long will it take (heat transfer rate) to store， or recover， a given amount of thermal energy in a PCM-based thermal energy storage system (TES) for a specific application? There is no theory that can easily answer this question for a given PCM-TES。 Therefore， an important long-term objective in the latent heat thermal energy storage community is to determine heat exchanger design rules for PCM-TES。 To this end， it is imperative to study a large variety of heat exchange configurations: heat exchanger size and shape， tank size and shape， use of fins or not， various PCMs， charging and discharging conditions， etc。 This paper presents a series of new controlled experimental characterizations of a specific PCM-heat exchanger (HX): a vertical finned tube-and-shell system designed in a way that the number of finned-tubes used in the HX can be varied。 Experimental characterization was performed for systems having 4， 8 and 12 finned-tubes。 The inlet heat transfer fluid (HTF) temperature， the initial system temperature， and the HTF flow rate were also varied consistently for each geometrical arrangement to cover a wide range of operating conditions。 Results are first presented in terms of power curves， followed by two comparative metrics: mean power and normalized power。 Determining and applying these two comparative metrics is the first step forward in the search for PCM-HX design theory and rules。 These two possible metrics， the mean and normalized power， applied to this modular finned tube-and-shell PCM-TES shed light on their merit as comparative metrics as well as the impact of PCM-TES operating conditions。
查看更多>>摘要：In this paper， as for an ejector utilized in a carbon capture system， by using CFD simulation， its three key geometries such as length of constant-pressure mixing chamber (XL3)， length and diameter of constant-area mixing chamber (XL5 and D5) were first optimized individually。 Then， the relative sensitivity of ejector performance to the three geometries was identified。 Next， the effect of six optimization sequences (S1: XL3 → D5 → XL5， S2: XL3 → XL5 → D5， S3: XL5 → D5 → XL3， S4: XL5 → XL3 → D5， S5: D5 → XL3 → XL5， S6: D5 → XL5 → XL3) of the three geometries on entrainment ratio (ER) was evaluated。 And then， the ER under varied working conditions was analyzed。 Finally， four typical working conditions were selected for doing re-optimization to check whether the ejector performance can be significantly improved and how big difference of the optimized geometries with re-optimization can be generated。 The results showed that: (1) the relative sensitivity of ER to D5 is greater than XL3， and that to XL5 is the least; (2) Among six optimization sequences of three geometries， S2 offers the best ejector performance， both S5 and S6 generate the equal and lowest ER; (3) The change of ER with the primary flow pressure is largely related to the back pressure; (4) Re-optimization under four typical operating conditions can improve ejector performance by 4。3% ～ 128。8%， and optimum D5 and XL3 increases evidently as compared to that without re-optimization。 The research results would enhance the application of ejector in carbon capture and relevant chemical fields。
NSTL
Elsevier