Reduction In Latent Heat Research Articles

Light-induced latent heat reduction of silver nanofluids: A molecular dynamics simulation

In recent years, some researches have revealed the addition of nanoparticles into base fluids can not only enhance the absorption of solar energy but also have an obvious influence on the latent heat of vaporization (LHV). In present work, a calculation model is developed based on the combination of molecular dynamics (MD) simulation and the electromagnetic theory to study the influence of the localized surface plasmon resonance (LSPR) effect on the LHV of silver nanofluids in the light-induced distillation process. The results show the LHV of silver nanofluids is slightly higher than that of pure water without the LSPR effect, and the water molecules are completely absorbed on the surface of silver nanoparticles due to the attraction force. However, under the effect of LSPR, the LHV of silver nanofluids is decreased with the increasing intensity of the local enhanced electric field on the surface of the silver nanoparticle. When the intensity reaches 3.0 × 1010 V/m, the LHV of silver nanofluids is reduced by 65.84% relative to that of pure water. Both of the kinetic energy and potential energy of water molecules under the local enhanced electric field induced by the LSPR are also increased with the intensity of electric field, which leads to a state conversion of water molecules from an inactive state to an active state. The required energy for water molecules from a liquid state to a vapor state is reduced under the illumination of the resonant light. Hence, the LSPR effect of nanoparticles is a significant influence in the enhanced distillation of the plasmon nanofluids.

Experimental Investigation on Mechanism of Latent Heat Reduction of Sodium Acetate Trihydrate Phase Change Materials.

Sodium acetate trihydrate (SAT) phase change material (PCM) has been well known for thermal energy storage due to its high latent heat and resource abundance. However, SAT suffers from severe latent heat reduction after heating and cooling cycles. Although a few of previous researches showed the reduction could be effectively inhibited by using thickeners, the mechanisms of the reduction process and thickeners’ inhibition have not been deeply explored till now. In this work, SAT modified by 5 wt.% nucleating agent of disodium hydrogen phosphate dodecahydrate (SAT/5 wt.% DSP) was prepared and 200 thermal cycles were carried out. The differential scanning calorimeter, Rheometer, X-ray diffractometry, and scanning electron microscope were used to investigate the extent of latent heat reduction, viscosity, phase composition and microstructure, respectively, and the infrared thermal imaging method was used to evaluate heat storage capacity. It was found that the latent heat of SAT/5 wt.% DSP dropped dramatically and the relative decrease in latent heat was measured to be 22.44%. The lower layer of SAT/5 wt.% DSP contained 24.1 wt.% CH3COONa, which was quantitatively consistent with the reduction extent. Furthermore, the phase change endothermic time of the lower layer was only 44.1% of that of the upper. SAT/5 wt.% DSP was further modified by 3 wt.% thickener of carboxymethyl cellulose (SAT/5 wt.% DSP/3 wt.% CMC) and endured 200 thermal cycles. The extent of the latent heat reduction of SAT/5 wt.% DSP/3 wt.% CMC was only 9.29%, and phase compositions were more homogeneous. The 3 wt.% CMC increased viscosity by 14 times, which effectively prevented the Stokes sedimentation velocity of CH3COONa in melts and inhibited the final macroscopic phase separation.

Novel core/void/shell composite phase change materials for high temperature thermal energy storage

• A novel core (=Al-Si/Bi)/void/shell (=Al 2 O 3 ) composite PCM was successfully prepared. • The Al-Si alloy and Al 2 O 3 shell were in-situ prepared by the displacement reaction. • The void layer was formed by the evaporation of most of the outer Bi in Al/Bi alloy. • The thermal stability of Al-Si/Bi/Al 2 O 3 was obviously improved due to the void layer. • The Al-Si/Bi/Al 2 O 3 possessed an adjustable thermal energy storage performance. Metallic solid-liquid phase change materials (SLPCMs) are crucial for the thermal energy storage technology of various industrial systems. However, the encapsulation of metallic SLPCMs is still technically difficult. In this pursuit, the present research envisaged the development of a novel technology to successfully prepare the core(=Al-Si/Bi)/void/shell(=Al 2 O 3 ) composite SLPCMs by using Al/Bi immiscible alloy powders as starting material and tetraethoxysilane as SiO 2 source. The Al-Si alloy and Al 2 O 3 shell were in-situ synthesized by the displacement reaction between SiO 2 and molten Al. Interestingly, most of the Bi distributed in the shell of Al/Bi immiscible alloy powders could not only improve the activity of alloy powders and promote the formation of precursor shell, but also be recycled by evaporation to form the void layer during the calcination process of composite SLPCMs. The produced void layer provided a space buffer to alleviate the volume expansion of the core SLPCM, and thereby improving the thermal cycling stability of the prepared composite SLPCMs. The thermal cycling test results showed that after 300 thermal cycles, the melting latent heat reduction of the core(=Al-Si/Bi)/void/shell(=Al 2 O 3 ) composite SLPCMs (24.3–31.7 J/g) was much less than that of the core(=Al-Si)/shell(=Al 2 O 3 ) composite SLPCM (58.1 J/g). Moreover, the prepared Al-Si/Bi/Al 2 O 3 exhibited an adjustable melting temperature (571.9 °C to 631.9 °C) and average particle diameter (39.3 μm to 112.6 μm), relatively high thermal conductivity [2.068 W(mK) −1 to 2.966 W(mK) −1 ], and excellent thermal energy storage capacity (209.5 J/g to 278.2 J/g). Thus, the prepared Al-Si/Bi/Al 2 O 3 composite SLPCMs are potential thermal energy storage materials, which can be used to improve the energy efficiency of various industrial systems.

Experimental study on the thermal behavior of RT-35HC paraffin within copper and Iron-Nickel open cell foams: Energy storage for thermal management of electronics

In this paper, experimental investigations are carried out to study the thermal performance of metallic foams impregnated with phase change material (PCM) based heat sinks for thermal management of electronics. Herein, RT-35HC with melting point 34–36 °C is chosen as PCM and copper foam1 (95% porosity), copper foam2 (97% porosity) and Iron-Nickel foam (97% porosity) are used as thermal conductivity enhancer. Various configurations of the heat sink are investigated for 5400 s each for charging and discharging processes under heat flux 0.8–2.4 kW/m2 for PCM volume fractions 0.0, 0.6, 0.7 and 0.8. Results revealed that copper foam-based heat sink showed 5–6 °C less base temperature as compared to that of Iron-Nickel foam. While investigating the effect of foam porosity, copper foam with lower porosity (95%) has shown 11% less base temperature at the end of the charging cycle. It was also noticed that the maximum thermal conductivity enhancement of PCM was found to be 34 times for 95% porosity copper foam with the latent heat reduction of 37%. Copper foam1-PCM composite posed the maximum enhancement in operation time of heat sink 7.9 times more as compared to that of the empty aluminum heat sink. Copper foam -PCM composite with 95% porosity of foam with 0.8 vol fraction of PCM is best recommended configuration for the present experimental study.

Enhancement of heat transfer in paraffin wax PCM using nano graphene composite for industrial helmets

Abstract Humidity and high temperature both adversely affect the productivity of workers. At these atmospheric conditions even a safety helmet won't provide a comfortable situation for the wearer. This paper suggests a cooling system for these safety helmets using NanoGraphene (NG)/paraffin composite Phase Change Material (PCM) which absorbs and stores the heat thus achieving comfort cooling for the wearer. The heat absorbing capacity of a PCM depends on its thermal conductivity, which can be enhanced by adding different concentrations of nanoparticles. Such a technique is employed in this work, in which nano graphene is added to paraffin wax. This composite PCM is employed in an Industrial helmet for providing thermal comfort to the wearer. In principle, wearer's head transfers heat to the PCM mainly by conduction without the use of any kind of external power source. Moreover, the system maintains the wearer head temperature very close to the PCM temperature range of 28–33 °C thereby avoiding any kind of discomfort which would otherwise affect his/her alertness. It is found that the Nano-graphene/paraffin composite PCM integrated helmet provides a faster cooling compared to a normal PCM due to the changes in its thermal properties such as thermal conductivity and latent heat upon adding NG at various concentrations. Among the various tested concentration of nano graphene, the thermal conductivity of NG/paraffin composite PCM showed a maximum of 146% improved thermal conductivity and a maximum of 3% reduction in latent heat at 3% concentration of NG, when compared to those of the ordinary PCM. The designed helmet incorporated with composite PCM can provide thermal comfort to its wearer for more than four hours for all concentrations at an ambient temperature of 35 °C.

Membrane contactors modelled for process intensification post combustion solvent regeneration

Membrane gas-solvent contactors are potentially more efficient at undertaking solvent regeneration in carbon capture applications compared to traditional desorber columns. This potential is investigated here through modelling of two experimentally reported membrane contactors based on Teflon AF1600 and polydimethylsiloxane active layers, by a simple approximate model and a one-dimensional mass transfer model. The respective membrane contactors could be operated at temperatures below that corresponding to the vaporisation of the solvent, due to the separation of the solvent and gas phase by the membrane. This required a steam sweep operating under vacuum conditions. The calculated module length for the Teflon AF1600 membrane varied between 8.9–70.2 m with decreasing regeneration temperature. This increase in required length was due to reductions in overall mass transfer coefficient and mass transfer driving force as temperature is lowered. It was determined that at 110 °C a PDMS contactor of length 1.1 m was required to regenerate the solvent, achievable with commercial modules. A comparison of the equipment volume footprint, known as process intensification, revealed that both the Teflon AF1600 and PDMS membranes required a lower volume than a standard packed column when operating at temperatures above 90 °C. Temperatures higher than 95 °C also designated the transition above which membrane contactors have a lower energy duty than the corresponding solvent column approach. This energy duty is a trade-off between the reduction in latent heat required to produce CO2 and regenerate the solvent at lower temperatures, countered by the work duty of the vacuum pump needed to operate the steam sweep at pressures below atmospheric. The investigation demonstrated that membrane contactors are a viable alternative technology for solvent regeneration.

Paraffin/CuO nanocomposites as phase change materials: Effect of surface modification of CuO

AbstractIn this research, different loadings of copper oxide (CuO) nanoparticles (0.5, 1 and 3 wt%) were utilized to fortify the thermal conductivity of paraffin. In order to improve dispersion and inhibit nanofiller agglomeration, the surface of CuO nanoparticles was treated with trimethoxy octadecyl silane. Surface treatment of CuO nanoparticles was validated by Fourier transform infrared spectroscopy and thermogravimetric analysis. The scanning electron microscopy results revealed that the samples loaded with the surface treated CuO had proper dispersion, even up to 3 wt% of loading. Rheological results showed that CuO inclusion and particularly its treatment increased the viscosity of paraffin. Differential scanning calorimetry results exhibited that latent heat of pure paraffin slightly diminished by inclusion of untreated CuO nanoparticles, while inclusion of the treated nanoparticles led to less reduction in latent heat of paraffin. The thermal conductivity (KD2 test) results revealed that CuO inclusion, particularly the treated CuO, increased the thermal conductivity of paraffin over 2‐fold. All in all, this research study corroborates the potential of the treated CuO nanoparticles to produce paraffin‐based composite phase change materials with improved performance.

A fundamental study of intelligent building envelope systems capable of passive dehumidification and solar heat collection utilizing renewable energy

Experimental studies and simulation analysis were conducted to assess the hygrothermal performance of intelligent roof systems, which perform passive dehumidification in summer and solar heat collection in winter by air circulation through the roof ventilation layer to reduce sensible and latent heat load using solar heat. A high-efficiency air conditioning system separating latent and sensible heat can be developed using fibrous insulation with excellent moisture conditioning properties. The thermodynamic-potential-based fundamental principle of the intelligent skin system, which performs passive dehumidification and solar collection, was explained based on non-equivalent thermodynamics. The performance characteristics of temperature and humidity fluctuations of the intelligent roof were experimentally and numerically demonstrated. This work enables the combined analysis of heat and moisture movement in the envelope system. The experimental and numerical simulation results indicate that significant sensible and latent heat reduction can be achieved by running the system using renewable energy.

Nanofluid PCMs for thermal energy storage: Latent heat reduction mechanisms and a numerical study of effective thermal storage performance

Abstract The latent heat of fusion of paraffin-based nanofluids has been examined to investigate the use of enhanced phase change materials (PCMs) for thermal energy storage (TES) applications. The nanofluid approach has often been exploited to enhance thermal conductivity of PCMs, but the effects of particle addition on other thermal properties affecting TES are relatively ignored. An experimental study of paraffin-based nanofluids containing various particle sizes of multi-walled carbon nanotubes has been conducted to investigate the effect of nanoparticles on latent heat of fusion. Results demonstrated that the magnitude of nanofluid latent heat reduction increases for smaller diameter particles in suspension. Three possible mechanisms – interfacial liquid layering, Brownian motion, and particle clustering – were examined to explain further reduction in latent heat, through the weakening of molecular bond structures. Although additional research is required to explore detailed mechanisms, experimental evidence suggests that interfacial liquid layering and Brownian motion cannot explain the degree of latent heat reduction observed. A finite element model is also presented as a method of quantifying nanofluid PCM energy storage performance. Thermal properties based on modified effective medium theory and an empirical relation for latent heat of fusion were applied as model parameters to determine energy stored and extracted over a given period of time. The model results show that while micro-scale particle inclusions exhibit some performance enhancement, nanoparticles in PCMs provide no significant improvement in TES performance. With smaller particles, the enhancement in thermal conductivity is not significant enough to overcome the reduction in latent heat of fusion, and less energy is stored over the PCM charge period. Therefore, the nanofluid approach may not be justifiable for energy storage applications. However, since the model parameters are dependent on the material properties of the system observed, storage performance may vary for differing nanofluid materials.

Torrential rainfall responses to radiative and microphysical processes of ice clouds during a landfall of severe tropical storm Bilis (2006)

Ice clouds are an important component in precipitation systems. The radiative processes of ice clouds directly impact radiation in heat budget and the microphysical processes of ice clouds directly affect latent heat and net condensation through deposition processes, which may eventually change surface rainfall. Thus, torrential rainfall responses to radiative and microphysical processes of ice clouds during a landfall of severe tropical storm Bilis (2006) are investigated with the analysis of sensitivity experiments. The two-dimensional cloud-resolving model is integrated for 3 days with imposed zonally uniform vertical velocity, zonal wind, horizontal temperature and vapor advection from NCEP/GDAS data. One sensitivity experiment excludes the radiative effects of ice clouds and the other sensitivity experiment excludes ice microphysics and associated radiative and microphysical processes. Model domain mean surface rain rate is barely changed by the exclusion of radiative effects of ice clouds due to the small decrease in net condensation associated with the small reduction in latent heat as a result of the offset between the increase in radiative cooling and the decrease in heat divergence. The exclusion of microphysical effects of ice clouds decreases the mean rain rate simply through the suppression of latent heat as a result of the removal of deposition processes. The total exclusion of ice microphysics decreases the mean rain rate mainly through the exclusion of microphysical effects of ice clouds.

Isotropic-to-Nematic Phase Transition in a Liquid-Crystal Droplet

In this paper, we focus on the isotropic-to-nematic phase transition in a liquid-crystal droplet. We present the results of an experiment to measure the growth of the nematic phase within an isotropic phase liquid-crystal droplet. Experimentally, we observe two primary phase transition regimes. At short time scales, our experimental results (R(t) approximately t0.51) show good agreement with a Stefan-type model of the evolution of the nematic phase within the isotropic phase of a liquid crystal. As time progresses, the growth of the nematic phase is restricted by increased confinement of the droplet boundary. During this stage of growth, the nematic phase grows at a slower rate of R(t) approximately t0.31. The slower growth at later stages might be due to a variety of factors such as confinement-induced latent heat reduction; a change of defect strength during its evolution; or interactions between the defect and the interface between the liquid crystal and oil or between adjacent defects. The presence of two growth regimes is also consistent with the molecular simulations of Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) who identify an early stage domain regime and a late stage confinement regime. For the domain and confinement regimes, Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) obtained growth exponents of 0.49 +/- 0.05 and 0.25 +/- 0.05. These are remarkably close to the values 0.51 and 0.31 observed in our experiments.

Comparison of the NCAR LSM1 land surface model with BOREAS aspen and jack pine tower fluxes

Tower fluxes measured at the Boreal Ecosystem‐Atmosphere Study southern study area old aspen (SSA‐OA), southern study area old jack pine (SSA‐OJP), and northern study area old jack pine (NSA‐OJP) sites during the three 1994 intensive field campaigns (IFCs) (May 24 to June 16 (IFC‐1), July 19 to August 10 (IFC‐2), and August 30 to September 19 (IFC‐3)) were compared to fluxes simulated by a land surface model for the same period. Comparisons were limited to the average diurnal cycle for these periods to mitigate large day‐to‐day variability in the observations and problems with missing data. For consistency with the global implementation of the model, vegetation and soil parameters were not set to site‐specific values but rather were the generic needleleaf evergreen and broadleaf deciduous vegetation and the generic sandy and loamy soil used in the global model. Despite the use of generic vegetation and soil, the model reasonably simulated the diurnal cycle of sensible heat, latent heat, net radiation, and CO2 fluxes for the SSA‐OJP and SSA‐OA sites. The main errors were that the model did not reproduce the midday reduction in latent heat seen at the SSA‐OJP site during IFC‐1 and IFC‐2 and had less photosynthetic CO2 uptake than observed at the SSA‐OA site. Differences in vegetation structure and physiology between the two sites were important to accurately simulate the fluxes. The needleleaf evergreen vegetation resulted in higher net radiation and a higher Bowen ratio than the broadleaf deciduous vegetation. Soil differences were less important. The NSA‐OJP site was not so well simulated: midday latent heat flux was overestimated, and photosynthetic CO2 uptake was underestimated during each IFC. The only difference in the simulated southern and northern jack pine sites was in their atmospheric forcings; vegetation structure and soil types were the same. These results suggest the model is able to reproduce variability between vegetation types but not within vegetation types.