查看更多>>摘要：Electrocaloric cooling is a promising technology that could replace existing vapor-compression technology with lighter and more efficient solid state based devices。 Among those， electrocaloric coolers using electrostatic actuation are rising; therefore harnessing their advantages and understanding their shortcomings is still a work in progress。 In this paper， we pin down two limitations that stem from a commonly used device design， and we propose a model that takes them into account。 The first is due to the displacement of the film， which moves the air around and causes losses。 We estimate the air losses through a Poiseuille flow model。 The second is related to the non-ideal Brayton cycle the device performs and which results in generating a heat flow opposite to cooling。 We estimated the effect of this back-flow on the cold flux using a 2D thermal diffusion model。 Ultimately， our results point at several possible improvements for further designs。 Among them， a slight reduction of the air pressure in the device looks as a promising solution to reduce air losses。
查看更多>>摘要：To be used in the market， thermal and friction characteristics of newly launched heat exchangers must be acknowledged， usually by experimental methods。 Steady-state Kays and London is considered the classical experimental technique。 In this setup， one of the heat exchanger streams is usually vapor， to provide controlled known heat transfer and temperature conditions， while the other stream is subjected to different flow rates， for the determination of the equipment thermal and pressure drop behaviors。 Large expensive industrial boilers are usually used， resulting in difficulties in stabilizing and controlling vapor temperatures。 In the present work， a new experimental setup， based on the above-mentioned technique， is proposed for determining the heat transfer characteristics of compact heat exchangers。 The boiler vapor flow is substituted by the working fluid (water in vapor state) of a two-phase thermosyphon。 Being smaller and much more flexible， this technology allows for easy control of the vapor temperature， while providing uniform temperature distribution along one of the heat exchanger streams， which is difficult to obtain with the classical procedure。 In the proposed apparatus， the “known side” of the heat exchanger takes the role of the condenser of the thermosyphon。 Two well-known heat exchanger cores， composed by circular and square cross section channels， were used to validate the proposed arrangement， considering the ranges: 2200 < Re < 8000 and 850 < Re < 2800， respectively。 The wall temperatures were tested in the range 120 °C to 220 °C with a precision of ± 0。5 °C in steady-state。 In addition， the resulting Nusselt number (Nu) and the Fanning friction factor (f) data were compared with consolidate literature correlations showing an average discrepancy of 15% for both geometries and parameters。 Therefore， the use of thermosyphons results in smaller， simpler and more precise test benches to be used for the evaluation of heat exchangers， as it provides the desired constant wall temperature conditions for the characterization of core geometries， substituting， with advantages， the use of large and expensive steam boilers。
查看更多>>摘要：Due to their enhanced thermophysical properties， nanofluids have great potential for improving heat-transfer efficiency。 Nanofluids are employed in various thermal applications in the automotive industry， heat exchangers， solar power generation and more。 Among the applications of this technology， its use to enhance the heat transfer of solar collectors appears promising。 It is therefore not a surprise that the use of nanofluids in solar collectors has become a popular research area。 Still， there are important obstacles with the use of nanofluids in solar collectors。 Stability is the most evident， in addition to environmental aspects and the need to design suitable large-scale production processes for the application of nanofluids at the required scale for large solar collectors’ fields。 In this literature review， we study nanofluids in solar collectors， and parabolic-trough collectors in particular， at temperatures between 100°C and 300°C。 We present recent advances and research on nanofluids and consider the progress in understanding stability mechanisms， characterization and preparation methods， as well as their thermophysical properties。 We describe the main research gaps and suggest areas of further research。
查看更多>>摘要：Thermal management continues to be a constructive procedure applied to extend the life span in electronic equipment。 In this paper， a novel mini-channel heat sink with tessellated fins is proposed with different arrangements of oblique channels width and slant angle of secondary channels and the heat cooling capacity of the mini-channel heat sink is investigated numerically and experimentally at the Reynolds number (Re) in the range of 573 to 2295。 The results indicated that the new mini-channel heat sink with tessellated fins is more conductive to strengthen the heat transfer effect。 The simulation results show that with the increase of volume flow rate， the temperature difference and pressure drop of the model bottom plate with tessellated fins decrease obviously。 When the oblique channel slant angle is 70°and the inclined flow channel width is 1。8 mm， the heat transfer effect of better quality can be achieved basically。 By processing the single-objective and multi-objective genetic algorithm optimization， optimal designs is predicted and analyzed based on the two variables associated to the geometry of the channels。 It is found that the optimized model promotes better thermal performance and uniform substrate temperature， i。e。， the pressure drop can be reduced by up to 73%， the average Nusselt number can be raised by up to 17%， and the minimal temperature difference of 17 K at Qv = 30 ml/s。 To the end， the error calculation proves that the multi-objective optimization results are in good agreement with the simulation results。
查看更多>>摘要：Without defects inherent in conventional welded finned tubes， nonwelded integrally-molded spiral finned (IMSF) tubes proposed here have “zero” contact thermal resistance and high joint strength out of trapezoidal cross-section of the fin， and thus greatly improve thermal performances and service life of economizers for residual heat recovery。 To guide the applications of this new economizer， a three-dimensional physical-numerical model is established to study airside thermal-hydraulic and fouling performances of the economizer with IMSF-tubes。 We investigate the effects of longitudinal and transverse pitches of finned tube bundles on the airside thermal-hydraulic and fouling performances of IMSF-tubes at various Reynolds numbers。 The results show that a longitudinal pitch more than 80 mm is suggested for economizer design to avoid working instability of boiler systems due to high pressure drop at small longitudinal pitch， and as compromised， a transverse pitch of an economizer may be more than 100 mm since a small transverse pitch improves thermal-hydraulic performance， but worsens fouling and raises pressure drop。 We also put forward new correlations of the Colburn and Fanning friction factors for the IMSF-tube economizer。 Compared with other finned tubes， IMSF-tubes improve not only mechanics but also thermal-hydraulic performances of heat exchangers。 Our research work may contribute to applications of IMSF-tube economizers for residual heat recovery。
查看更多>>摘要：In this study， laminar single-phase fluid flow and conjugate heat transfer for three different shapes of microchannel heat sinks (MCHS) were numerically investigated using Computational Fluid Dynamics (CFD) and optimized with Non-dominated Sorting Genetic Algorithm II (NSGA-II) to reveal geometric design variables for optimum solutions。 A clear comparison of different shapes of MCHS under the same design constraints is necessary to establish an understanding of efficient MCHS designs。 The hydraulic diameters of rectangular， triangular and trapezoidal microchannels were kept constant at 150μm for a fair comparison。 Minimization of power consumption and enhancing heat transfer were determined as objective functions of the optimization represented by pumping power and averaged Nusselt number (Nu)。 The objective functions were expressed in terms of the design variables consisting of Reynolds number (Re)， aspect ratio of rectangular channel， apex angle of triangular channel and junction angle of trapezoidal channel。 For each configuration， parametric CFD analyzes were performed to establish correlations between design variables and objective functions to be used in the optimization process。 For the rectangular MCHS configuration， low aspect ratio was shown to increase heat transfer with a power consumption penalty。 For triangular and trapezoidal microchannels， 50° of apex angle and 60° of junction angle were suggested as optimum geometric parameters， respectively。 Pareto frontal comparison of the three configurations revealed that rectangular microchannel was the most effective configuration in terms of thermal and hydrodynamic performance。 For the same amount of heat transfer， the rectangular microchannel configuration required 17% and 40% less pumping power than the trapezoidal and triangular microchannels， respectively。
查看更多>>摘要：This paper proposes a sizing method to guide the design of water-circulating radiative sky cooling systems and water-based energy storage solutions。 Following this method， the choice of operational flow rate in the radiative sky cooling (RSC) panels and the water storage is based on four indicators: sub-ambient temperature， cooling power density， minimum storage temperature and useful energy stored。 The method is applied to the BaityKool Solar Decathlon Middle East (SDME) prototype in order to design a water-radiative sky cooling system with storage in the climatic conditions of Dubai。 We developed passive strategies for the BaityKool prototype， including a multi-functional innovative exterior wall and a semi-indoor courtyard space， combined with active solutions (in particular a hydraulic radiative sky cooling system)。 The experimental campaign conducted on the RSC system over three successive nights in November (ambient air temperature between 22。7 and 31。4 °C) indicates an average cooling power of 30–45 W m?2 for a maximum sub-ambient temperature drop of 2。8 °C， and shows that great attention to the water pipes and storage insulation can lead to an increase in the thermal performance of radiative sky cooling systems。
查看更多>>摘要：The solar air heater has a low convective heat transfer coefficient because the absorber plate and moving air form a laminar sub-layer。 Adding artificial roughness to the absorber surface is one of the most efficient heat transfer and thermal efficiency augmentation techniques utilised in SAH duct construction。 The absorber plate is given new double arc discrete shaped roughness geometry。 The Nu， f， and ηth characteristics of a rectangular duct were evaluated employing a new double arc discrete roughness element on one side of the SAH wall and insulation on the other three sides。The outdoor investigation has been carried out with roughness parameters are relative roughness pitch (p/e) of 6。67， 8。33，10，11。67， relative roughness height (e/Dh) of 0。027， angle of arc or attack angle (α) of 60°， Reynolds number (Re) in the range of 3000–14000， and as aspect ratio (W/ H) of 8 were selected。 Solar radiation intensity ranged from 860 W/m2 to 895 W/m2 on a bright sunny day was used。 Between 11:00 A。M。 and 2:00P。M。， from morning to afternoon， the outside test was also done。 Optimum results were found at a p/e of 8。33 and α of 60° for Nu， h and f & THP。 Optimum results were found at a p/e of 11。67 and α of 60° for ηth。 The maximum enhancement of Nu & f by 2。87 & 2。47 of the roughened plate respectively was found when compared to the smooth absorber plate under similar flow conditions。 Maximum value of Nu and f were found 121。66 and 0。0342 respectively。 Maximum value of h and ηth were noticed 75。23 W/m2k and 87。95% respectively。 The maximum enhancement of thermo-hydraulic performance (THP) was noticed under-investigated parameters of 2。82 at p/e of 8。33， and Re of 13935。 Heat transfer and friction factor correlations have been developed deviation were found with experimental values within ± 18%。The mathematical model， which is utilised for SAH duct design and performance prediction under various operating conditions， has been validated using experimental data。
查看更多>>摘要：The creative proposal of ducted fuel injection (DFI) in the application of heavy-duty diesel engines is to improve the quality of gas-fuel mixture in the combustion zone。 The present research employed experimental and simulation methods to investigate the impact of the roughness of the inner wall of the duct on DFI spray characteristics。 The study was carried out under injection pressure of 120 MPa and high ambient density of 54。2 kg/m3。 From the perspective of spray tip penetration (STP)， it was found that DFI spray configured with the smooth-walled and rough-walled duct both accelerate the axial diffusion with respect to free spray， and the smooth-walled duct performs best。 Under the duct confinement， the gas entrainment does not occur inside the duct， slowing down the spray momentum decay， resulting in stronger axial diffusion after the spray penetrates through the duct。 However， from the perspectives of spray cone angle (SCA) and spray area (SA)， for the two duct configurations， the smooth-walled duct shows better overall diffusion compared to that of free spray， but the rough-walled duct shows weaker overall diffusion while increasing axial diffusion with respect to free spray， highlighting the strong spray-film interaction of the smooth wall promotes the overall diffusion， while the weak interaction of the rough wall accelerates the attenuation of the overall diffusion momentum with respect to free spray。 Regardless of the smooth-walled or rough-walled duct， the pumping effect caused by the huge pressure difference between the inside and outside of the duct formed by the duct confinement causes strong gas entrainment at the duct inlet， promoting the strong gas-fuel mixing inside the duct， which is beneficial to the formation of leaner combustion。
查看更多>>摘要：One of the concerns about Rankine cycle and most energy generation cycles， is the energy wasted as heat。 Therefore， various methods such as combining cycles have been used to enhance the overall efficiency of Rankine cycle and to decrease the waste heat。 This study has focused on the combination of ejector refrigeration cycle with Rankine cycle。 The present review addresses polygeneration plants including an integration of ejector refrigeration cycle and Rankine cycle to enhance the overall performance of the plant。 The integrations may include a steam Rankine cycle， or an organic Rankine cycle and an ejector refrigeration cycle coupled with other energy sources。 According to the results， combination of the ejector refrigeration cycle and the Rankine cycle have led to an increase of the overall plant efficiency with applications in energy production， cooling， and heating。
NSTL
Elsevier