查看更多>>摘要：Thermal management complexity increases in high-performance chips， where the heat loads vary spatially and temporally， while liquid cooling systems are usually designed for most stringent stationary conditions。 Several works developed heat transfer enhancement techniques to increase the cooling capacity of liquid cooled heat sinks， but pumping power is increased in a permanent way due to the addition of elements within the channels。 Here， a liquid cooling self-adaptive heat sink that can efficiently adapt the distribution of its heat extraction capacity to time dependent and non-uniform heat load scenarios is proposed。 Numerical design of the mesoscale cooling device with bimorph metal/SMA fins， definition of the fabrication and training procedure of the SMA fins to reach the desired behavior and experimental assessment is presented。 The capacity of the self-adaptive fins to locally boost the heat transfer is numerically and experimentally demonstrated。 Results obtained show that the self-adaptive fins can improve the temperature uniformity by 63% with respect to plain channel。 The reduction in thermal resistance using bimorph metal/SMA fins sample allows the surface maximum temperature gradient to remain almost constant although heat flux increases。 Energy savings are maximized in applications where partial load intervals contributes significantly to the overall operating period。
查看更多>>摘要：Although thermal metamaterials have received considerable attention for approximately a decade， previous studies have so far focused on heat conduction or thermal convection in porous media， hampering applications related to moving in media。 Based on transformation heat transfer， here we have analytically designed Venturi-effect concentrators by simultaneously manipulating the dynamic viscosity and the thermal conductivity。 Further based on this theory and integral median theorem， inhomogeneous concentrators metamaterials simplified to homogeneous counterparts are proposed。 Noteworthily， the proposed method can be employed to simplify not only thermal metamaterials， but also other inhomogeneous metamaterials， such as optical metamaterials， electromagnetic metamaterials， acoustic metamaterials， among others。 Numerical simulations demonstrate that these Venturi-effect concentrators can not only amplify both flow velocities and heat fluxes in thermal-flow fields， but also achieve cloaking effects in thermal-flow systems。 Since these Venturi-effect concentrators do not interfere with the thermal-flow field outside the concentrators， they secure advantages over conventional Venturi tubes and will facilitate potential applications related to Venturi effects。
查看更多>>摘要：The heat transfer in gypsum exposed to fire is significantly affected by heat conduction， mass transfer and condensation/evaporation effects of water vapour in the porous structure。 In the past， numerical models to predict the heat transfer in gypsum were mainly limited to heat conduction and mass transfer of water vapour in thin gypsum boards used in wall assemblies。 Thus， in the present study a multi-phase approach is proposed to predict the heat transfer within gypsum under fire exposure including conduction， mass transfer and condensation/evaporation。 The consideration of water vapour transport and its condensation in the porous structure was leading to a good prediction of the heating process of gypsum up to approx。 100 °C。 Furthermore， the calculated temperatures above 100 °C were adequate up to 2 cm from the fire side。 However， at a higher distance from the fire the additional implementation of a thermal radiation model was crucial to improve the heat transfer in gypsum。 Including the thermal radiation， the proposed numerical model is able to calculate the temperatures in the gypsum blocks in close accordance to the measurement。
查看更多>>摘要：Groundwater Heat Pump (GWHP) systems have gained attention for space heating and cooling due to its efficiency and low installation costs。 It is known that thermal transfixion and land subsidence are main geological environmental hazards in the application of GWHP。 But few studies paid attention to groundwater quality。 In this research， a three-dimensional Hydro-Thermal–Mechanical–Chemical numerical model was established based on Biot's consolidation theory， solute and heat transport in porous media theory。 The numerical model could evaluate geological environment from four aspects: hydraulic head， temperature， groundwater quality， and land subsidence。 The coupling effect of building load and extraction–injection groundwater is considered to study land subsidence。 The model is applied to a GWHP in Nantong， China。 The results show that increasing the temperature change and decreasing the circulation ratio of GWHP will alleviate thermal accumulation， thermal transfixion， and freshwater salinization of the confined aquifer III。 But it will aggravate the occurrence of land subsidence。 The geological environment involves four factors。 The development of shallow geothermal energy should be considered comprehensively。 Optimizing operation scheme by changing the groundwater seepage field would alleviate geological environment deterioration。
查看更多>>摘要：Current working fluid selection approaches for the absorption refrigeration cycle are based exclusively on steady-state cycle performance。 In real operating conditions， the cycle is subject to exogenous disturbances， with detrimental effects on their performance， resulting in increased resource utilization or even failure to meet load demands。 In the present work， the problem of working fluid evaluation for the absorption refrigeration cycle is investigated， by considering both the steady state and the dynamic performance of the system。 The evaluation of 13 novel， organic working fluids is performed utilizing a cascaded proportional-integral and a cascaded model predictive controller， implemented on a single-effect absorption refrigeration cycle。 A set of previously validated non-linear， dynamic process models， developed in ASPEN Plus are used to create the linear models required for the model predictive controller。 The closed loop dynamic performance is evaluated based on speed of response， resource utilization and deviations from the desired operating point (setpoint)， while operating under a disturbance scenario involving load demand changes。 Multi-criteria assessment results indicate that the cascaded model predictive control is considerably more consistent than the cascade PI controller。 The novel mixture of acetaldehyde/dimethylformamide exhibits superior performance than the conventional NH3/H2O mixture， by 57% in speed of response and 76% in resource utilization。 The mixture ιs also 25% and 12% better than NH3/H2O in steady-state cost per ton of cooling and coefficient of performance， indicating high economic potential and robustness for single-effect ABR systems。
查看更多>>摘要：Developing portable power sources with high energy density is of vital importance in the present electronic age。 A powerful hybrid micro thermoelectric generator powered by a newly designed ultrahigh capacity miniature combustor is first developed in this work to provide an electric power of over 50 W with a systematic efficiency of 2。92%， filling a research gap – the lack of a concrete method to augment the electric power of combustion-powered micro thermoelectric generators。 Segmented thermoelectric modules (PbTe and Bi2Te3) and convectional thermoelectric modules (Bi2Te3) are first combined to suit the inherent uneven hot-end temperature distribution of a combustion-powered micro thermoelectric generator。 This method substantially promotes the performance of thermoelectric generator because the total electric power (51。0 W) and electric power per thermoelectric module (12。75 W) are considerably larger than those in previous studies。 Temperature distribution， input power between 1229 and 1749 W， equivalent ratio between 0。82 and 1。0， wiring method， cooling intensity， flue gas emissions (CO， NO， and CO2)， and various efficiencies are explored in detail to characterize the proposed micro thermoelectric generator。 Results show that the present micro thermoelectric generator runs with high systematic efficiency under different input powers and equivalent ratios and produces lower pollutant emissions (CO and NO) compared with those in previous studies。 The combination of an ultrahigh capacity miniature combustor with volumetric heat load of 78。9 WM/m3， an all-in-one designed combustor-collector-spreader， and hybrid thermoelectric modules (segmented and convectional thermoelectric modules) is the essential innovation to reach the abovementioned improved performance。 Detailed discussions concerning various efficiencies (systematic， combustion， heat collection， and TE efficiencies)， power density， energy density and pressure drop of hybrid thermoelectric generator are conducted in this work。 The present work presents a concrete method to develop a high capacity combustion-powered micro thermoelectric generator and provides in-depth insights into the utilization of inherent uneven hot-end temperature distribution of a combustion-powered micro thermoelectric generator。
查看更多>>摘要：As electronic devices become smaller， they need modern and efficient heat removal solutions。 One such solution is microchannel arrays， where the flow around the pin-fins improves heat transfer。 However， there is still no consensus on the best array configuration。 This numerical study， therefore， focuses on the effect of pin-fins geometry， spacing， and arrangement on performance。 Pressure drop and flow characteristics were investigated for 20 different inline and staggered layouts with circular or elliptical pin-fins。 The results show that the diameter of the pins has an influence on the strength of the vortices。 Larger diameters lead to flow stabilisation and lower pressure drop， while smaller diameters lead to stronger vortex shedding and higher pressure drop。 In addition， the flow is more stable when the pin-fins are staggered and closely spaced。 This behaviour can be attributed to the greater influence of the cross-sectional area than the meandering of the flow。 Finally， the optimum performance was attained using the S-Dx100-Dy40-Px200-Py120 case。
查看更多>>摘要：The operational flexibility of thermal power plants plays a critical role in accommodating the high penetration of intermittent renewable power。 The load-cycling rate， which is an important feature of operational flexibility， is insufficient for thermal power plants。 The unpredictable fuel side deviations that weaken the control system effect restrict the load-cycling performances of thermal power plants because fuel side parameters， including coal feed rate， air flow rate， and coal quality， cannot be controlled precisely due to the accuracy error of devices and variation in fuel characteristics。 To explore the influence of fuel side deviations on the load-cycling processes of thermal power plants， dynamic simulation models of a coal-fired plant are developed， and the load-cycling processes with continuous deviations in the fuel side are simulated。 The deviations of output power， live steam pressure， and live steam temperature are calculated and used to evaluate the influences。 Results show that coal quality deviation has the most significant effect on load-cycling performance， which makes the maximum and cumulative deviations in output power increase to 1。15 and 1。10 times that without fuel side deviation。 The effect of multiple input deviations lies upon the most significant deviation。 For example， the maximum and cumulative deviations of output power increase to 1。15 and 1。09 times with deviations in the coal feed rate， air flow rate， and coal quality， which are close to those with only coal quality deviation。 Reducing the impact of the dominant factor can effectively enhance the load-cycling performance of the coal-fired unit。
查看更多>>摘要：The inclusion of a thermoelectric subcooler as an alternative to increment the performance of a vapour compression cycle has been proved promising when properly designed and operated for low-medium power units。 In this work， a computational model that simulates the behaviour of a carbon dioxide transcritical vapour compression cycle in conjunction with a thermoelectric subcooler system is presented。 The computational tool is coded in Matlab and uses Refprop V9。1 to calculate the properties of the refrigerant at each point of the refrigeration cycle。 Working conditions， effect of the heat exchangers of the subcooling system， temperature dependent thermoelectric properties， thermal contact resistances and the four thermoelectric effects are taken into account to increment its accuracy。 The model has been validated using experimental data to prove the reliability and accuracy of the results obtained and shows deviations between the ±7% for the most relevant outputs。 Using the validated computational tool a 13。6 % COP improvement is predicted when optimizing the total number of thermoelectric modules of the subcooling system。 The computational experimentally validated tool is properly fit to aid in the design and operation of thermoelectric subcooling systems， being able to predict the optimal configuration and operation settings for the whole refrigeration plant。
查看更多>>摘要：The droplet behaviors on sub-freezing surfaces are crucial for air-precooler frosting prediction。 Experimental investigations are conducted to explore droplet cooling and freezing on femtosecond laser textured surfaces。 Test samples with eight different microstructures are used， the initial contact angles (iCA) of which are in the range of 121。7° ± 0。4°–146。9° ± 2。2°。 The sample surface temperature is ?16。8 °C ± 0。16 °C。 Deionized water droplet cooling and freezing under natural convection are studied。 The water droplet wetting states on various samples are evaluated。 The temporal surface temperature of the droplet is obtained， based on which the cooling and freezing stages are identified。 A temperature-based analysis of the droplet cooling rate， nucleation onset， recalescence crystal production， and internal solidification is conducted。 The effects of the surface structure including the micro-pillar width (a)， inter-pillar distance (b)， and the pillar height (h) are examined。 It is found that the variation of micro-structure only affects the initial cooling and onset of nucleation。 The temperature increment and crystal production at recalescence show no dependency on the micro-pillar geometric parameters。 A calculated 3。34–12。62 wt% solid fraction is found following recalescence。 The droplet ice nucleation is suppressed with a larger b/a ratio and a lower a/h ratio。 A 138% increase of freezing delay time (FDT) is found when b/a increases from 0。5 to 4。0 and a/h decreases from 2 to 0。25。 However， for b/a increases from 0。5 to 4， only an 8。7% and 15。6% increment of the average recalescence and internal freezing duration is achieved respectively。 Increasing the micro-pillar width and the mutual distance are suggested for anti-icing surface design。
NSTL
Elsevier