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A numerical model for the flow and filtration characteristics of industrial flue gas in granular bed filter (GBF) was established and the local filtration efficiency for different granule layers was investigated. Numerical validation results show that the GBF structure with large size granules at the inlet region and small size granules at the outlet region can effectively improve the filtration performance of GBF and the underlying mechanism was revealed. Then an experimental system was built to validate the suitability of the optimized GBF structure for the filtration of industrial flue gas with coagulative particles. The experimental results show that the optimized GBF structure is also suitable and its superiority is more significant with the increase of filtration time. The results show that the pressure drop and filtration efficiency of the experimental system increase with the increase of dust particles concentration. The existing of coagulative particles is conducive to the growth of smaller size dust particles, the pressure drop and filtration efficiency increase significantly. In addition, the pressure drop and filtration efficiency decrease with the increase of cooling rate. The results of this study are expected to be useful for the design and optimization of industrial flue gas purification and waste heat recovery.

High efficiency purification of dust particles contained in industrial flue gas is urgently needed to mitigate environmental pollution and improve waste heat recovery efficiency. The agglomeration and growth behavior of dust particles has significant effects on filtration efficiency of industrial flue gas. In this paper, in order to reveal the microscopic mechanism of dust particles agglomeration and growth, three kinds of typical components in industrial flue gas, such as steam (H2O), PAHs (naphthalene) and dust particles (SiO2) were selected to establish a molecular dynamics (MD) simulation system. The agglomeration behavior of dust particles and the formation and growth process of dust clusters were characterized by MD simulation. The effects of temperature, channel width, cooling rate and gas composition on the agglomeration behavior of dust particles and the growth of dust clusters were investigated. It is found that the formation and growth of dust clusters are driven by agglomeration of dust particles. The decrease of gas temperature, cooling rate and channel width can accelerate the agglomeration of dust particles and the growth of dust clusters, which is beneficial to dust particles filtration. In addition, the hydrogen bonds formed between the H2O molecules and SiO2 particles can promote the adsorption of SiO2 particles to the H2O molecules, which is also beneficial to the formation and growth of dust clusters. So increasing the content of steam in industrial flue gas is helpful to the growth of dust clusters and the filtration of dust particles.

In this paper, novel parabolic trough solar receiver-reactors (PTSRR) of gradually-varied porosity catalyst beds are proposed for cost-efficient hydrogen production. A three-dimensional comprehensive model was developed for PTSRRs of the methanol-steam reforming reaction (MSRR) in porous Cu/ZnO/Al2O3 catalyst packed beds, by combining the finite volume method (FVM) and the Monte Carlo ray-tracing (MCRT) method with a MSRR comprehensive kinetic model. The validated model was applied to investigate different novel PTSRRs proposed, as well as the effects and mechanisms of different non-uniform porosity distributions, taking the methanol flow rate, the catalyst temperature limitation and the solar flux nonuniformity into account. It is revealed that the catalyst particles packed in the top part of the traditional absorber-reactor may not only have not fully played their roles but also influenced the multicomponent gas mixture fluid flow and heat transfer greatly. The non-uniform porosity catalyst bed gradually-increased from the bottom to the top better matches previously non-uniform temperature distributions and thus makes PTSRRs operated more safely, more efficiently yet lower cost of locally less packed catalyst mass. This comprehensive model and method offers a useful option of high potential for comprehensive analyses of the whole photo-thermal-chemical conversion process for different PTSRRs and realistic conditions.

During the preparation of nanocomposite phase change material (nanoCPCM), more attentions are focused on thermal conductivity. In fact, adding nanomaterial not only affects thermal conductivity, but also changes the other thermophysical properties of PCM and causes complicated effects on latent heat storage performance. In present study, melting behavior of pure phase change material and its four kinds of nanocomposites in a horizontal concentric tube were investigated and a new evaluation index was developed to examine the effects of nanomaterial on heat storage performance of nanoCPCM. The results show that both thermal conductivity and melting temperature have significant effects on nanoCPCM melting behavior and heat storage rate. Sometimes, although the thermal conductivity is enhanced, the heat storage rate is decreased due to its melting temperature increasing caused by nanomaterial. Especially, when the heat source temperature and PCM melting temperature is close, the variation of melting temperature is more significant. For example, nanoCPCM with SWCNT has the higher thermal conductivity, but its heat storage rate is slower than nanoCPCM with MWCNT, because of its higher melting temperature. Therefore, more attention must be paid to the variation of melting temperature caused by nanomaterial during the preparation and application of nanoCPCMs.

In this paper, a three-dimensional comprehensive model is firstly proposed for Parabolic Trough Solar Receiver-Reactors (PTSRR) of the Methanol-Steam Reforming Reaction process (MSRR) for hydrogen production. This PTSRR-MSRR comprehensive model is established by combining a comprehensive kinetic model of MSRR, a validated Monte Carlo Ray-Tracing (MCRT) optical model with a Computational Fluid Dynamics (CFD) model based on the Finite Volume Method (FVM), as well as useful comprehensive evaluation indicators. It is capable of comprehensively simulating and evaluating the whole complex photo-thermal-chemical conversion process of the entire PTSRR system, including the concentration, collection and conversion of photon energy, coupled heat transfers, fluid dynamics, multicomponent transports and methanol-steam reforming reactions. After validation, this comprehensive model was successfully used to determine the comprehensive characteristics and performance of different PTSRRs and realistic conditions. The effects and mechanisms of the solar time, the reflector geometric parameters, the inlet methanol molar flow rate, the reaction temperature limitation and the nonuniformity of the concentrated solar flux density distribution were discussed in detail. It is revealed that the reaction temperature limitation that should be smaller than the sintering temperature of Cu/ZnO/Al2O3 catalyst particles may only appear locally, which is mainly caused by the nonuniformity of the concentrated solar flux density distribution and the corresponding local peak solar flux density. For the mechanism of this kind of temperature limitation, different control strategies and optimizations were examined. Better comprehensive characteristics and performance of PTSRRs can be obtained, by making a reasonable tradeoff between the optical efficiency, the solar flux nonuniformity and the reflector surface curvature characteristics.

Latent heat storage (LHS) is considered as the most promising technique for thermal energy storage, due to its high energy storage density and nearly constant working temperature. However, the lower thermal conductivity of the phase change material (PCM) used in LHS system seriously weakens thermal energy charging and discharging rates. In order to improve the thermal performance of LHS system, a lot of research on performance enhancement have been carried out. This review paper will concern on the development of PCMs and performance enhancement methods for LHS system in the last decade. The available enhancement methods can be classified into three categories: using high thermal conductivity additives and porous media to enhance PCM thermal conductivity, using finned tubes and encapsulated PCMs to extend heat transfer surface, using multistage or cascaded LHS technique and thermodynamic optimization to improving the heat transfer uniformity. The comparative reviews on PCMs, corresponding performance enhancement methods and their characteristics are presented in present paper. That will help in selecting reliable PCMs and matching suitable performance enhancement method to achieve the best thermal performance for PCM based LHS system. In addition, the research gaps in performance enhancement techniques for LHS systems are also discussed and some recommendations for future research are proposed.

An optimization model for a multistage latent heat storage (LHS) unit with unsteady heat transfer fluid (HTF) inlet temperature was proposed. Thermodynamic analysis and optimization were performed based on the entransy theory. The expressions of the optimum phase change material (PCM) melting temperatures (Tm,opt) were derived. The effects of geometric parameters and unsteady HTF inlet temperature on the optimum phase change temperatures were investigated. The results indicate that with the increase of stage number (n), Tm1,opt increases and Tmn,opt decreases, which is beneficial to extend the selection range of PCM. For fixed entransy dissipation condition, increasing n will not change the fluctuation of the HTF outlet temperature; however a nearly uniform HTF outlet temperature can be obtained by increasing unit length (L). The unsteady HTF inlet temperature has great effects on the optimum phase change temperature. For a 3-stage LHS unit, the optimum phase change temperature of each stage increases by 14.9 K, 26.4 K and 38.0 K respectively with respect to the values obtained by steady method, which causes the heat storage capacity decreases by 6.1% and entransy dissipation decreases by 10.6%. The present work can provide guidance for the design of the multistage LHS unit with unsteady HTF inlet temperature.

An experimental system for a fixed granular bed filter (GBF) used to filter dust particles in industrial flue gas was built. The effects of filtration superficial velocity, bed height, and granules size on filtration and fluid flow performance were investigated. The results demonstrate that the GBF has high filtration efficiency and relatively low pressure drop, e.g. the filtration efficiency can reach 99.45% with a pressure drop of 291 Pa. The increase of filtration superficial velocity leads to the decrease of filtration efficiency but the increase of pressure drop. The filtration superficial velocity lower than 0.6 m/s is recommended to achieve higher filtration efficiency, lower pressure drop and acceptable filtration rate in the studied geometric parameter range (granule size: 3–10 mm, bed height: 30–120 mm). The increase of bed height and the decrease of granules size are beneficial to increase filtration efficiency, but detrimental to reduce the pressure drop. In addition, the effects of granules size on pressure drop are more significant than that on filtration efficiency. Based on the experimental results, the structural optimization was performed for the fixed GBF and an optimized structure with different granules sizes was designed. The validation results indicate that the designed GBF structure has excellent filtration and fluid flow performance compared with the traditional structure with single granules size. Under the investigated filtration superficial velocity region, the average filtration efficiency is enhanced 3.23% and the pressure drop is reduced 49.94%.

Particulate fouling on flue gas heat exchanger surfaces reduces heat transfer efficiency and increases the instability of equipment operation. It is necessary to develop a better understanding of how fouling occurs, aiming to find strategies to predict and reduce it. In this paper, a multiple mean cycle method is proposed based on a comprehensive fouling model considering the deposition and removal mechanisms, combined with the discrete phase model and dynamic mesh method, to predict the fouling morphology. Then the effects of transverse pitch, longitudinal pitch, tube shape and arrangement on fly-ash fouling and heat transfer characteristics are examined. It is found that, for aligned arrangement, the fly-ash particles are mainly deposited at the upwind stagnation region at the first row and whole windward side of the rest rows; for staggered arrangement, the fly-ash particles are deposited at the upwind stagnation region. The multiple mean cycle method can predict the fouling morphology efficiently and keep the characteristic of non-uniform fouling distribution. The fouling mass increases with the increase of relative transverse and longitudinal pitches. The elliptical tube bundle with small relative transverse and longitudinal pitches can obviously reduce the fouling mass especially for the staggered elliptical tube. Compared with the aligned circular tube arrangement, the fouling mass m f is decreased by 81.1% at 10 min. In addition, the weaken degree of Nusselt number after fouling Δ Nu for the staggered elliptical tube is only about 49.8% of the aligned circular tube arrangement. Consequently, using the staggered elliptical tube contributes to reduce the fly-ash fouling, decrease the soot-blowing frequency and ensure the efficiency, economy and safety of flue gas heat exchangers.

The low thermal conductivity of phase change material (PCM) seriously weakens the heat charging and discharging rates of latent heat storage (LHS) system. High efficient performance enhancement method is urgently needed. In present study, three-dimensional simulation models with and without natural convection were established for a shell-and-tube LHS unit to investigate the effects of PCM arrangements and natural convection on the charging and discharging performance. The results show that compared ot the commonly used shell-and-tube LHS unit with PCM in shell side, the LHS unit with PCM in tube side can significantly enhance heat storage rate under the same working conditions and overall dimensions. Natural convection has significant effects on charging performance, espeically when PCM is arranged in tube side. When natural convection is neglected, PCM melting time can be reduced by 25.4% and latent heat storage rate can be enhanced by 36.6% with PCM arranged in tube side. When natural convection is considered, PCM melting time can be reduced by 34.4% and latent heat storage rate can be enhanced by 54.2% with PCM arranged in tube side. However, natural convection has little effects on discharging performance even if for the model with PCM in tube side.

In this study, two typical components in industrial flue gas (SiO 2 solid particle and H 2 O vapor) was selected as a solid-gas system, the transport properties were investigated by equilibrium molecular dynamics (EMD) simulation with three different concentrations of solid phase systems established over the actual temperature range (1000–1400 K) of industrial flue gas. The number of hydrogen bonds was counted and the radial distribution function in the system was obtained to analyze the microstructure of the system. The thermal conductivity, viscosity and diffusion coefficient were calculated and the effects of temperature and solid concentration on the transport properties were investigated. The predicted values of transport properties are in good agreement with the available experimental data. The results show that with the diffusion of water molecules, polar water molecules form hydrogen bonds with silica and other water molecules, and as the temperature increases, the average number of hydrogen bonds of water molecules decreases and the stability of the system decreases. Simultaneously, with the increase of temperature, the thermal conductivity and viscosity of the system increase, when the concentration of the solid phase in the system increases, both the thermal conductivity and the viscosity increase at the same temperature. This study improves the understanding of transport characteristics of industrial flue gas from the microscopic point of view, which is significant to the dust purification and waste heat recovery technologies.

Thermodynamic analysis and optimization of a multistage latent heat storage (LHS) unit with unsteady heat transfer fluid (HTF) inlet temperature were performed based on the entransy theory. The expressions of the optimum phase change material (PCM) melting temperatures ( T m,opt ) were derived. The results indicate that with the increase of stage number ( n ), T m1,opt increases and T mn,opt decreases, which extends the selection range of PCM. For fixed entransy dissipation condition, increasing n will not change the fluctuation of the HTF outlet temperature; however, a nearly uniform HTF outlet temperature can be obtained by increasing unit length ( L ). The unsteady HTF inlet temperature has great effects on the optimum phase change temperature. For a 3-stage LHS unit, the optimum phase change temperature of each stage increases by 14.9 K, 26.4 K and 38.0 K respectively with respect to the values obtained by steady method, which causes the heat storage capacity decreases by 6.1% and entransy dissipation decreases by 10.6%. The greater the temperature fluctuation is, the greater the effects will be. The present work can provide guidance for the design of the multistage LHS unit with unsteady HTF inlet temperature.

Molten salts are attractive candidate materials used for effective thermal energy transfer and storage, which can be applied in the concentrating solar power (CSP) system at high temperatures for efficient and continuous solar energy utilization. In this paper, in order to improve the thermal performance of NaCl based molten salt, the NaCl and single walled carbon nanotubes (NaCl-SWCNT) based composite phase change materials (CPCM) were proposed and designed by composition design strategy of materials. The thermal properties and the microstructure of CPCM systems were investigated by means of molecular dynamics (MD) simulation at nanoscale. The thermal properties including density, melting point, self-diffusion coefficient, thermal conductivity, melting enthalpy and specific heat capacity were predicted, the simulation results are in good agreement with the available experimental data, and the mechanism of desirable thermal performance enhancement was revealed from the microscopic point of view. It was found that the addition of SWCNT can effectively reduce the melting point of molten salts, so as to control the working temperature range of molten salts. With the increase of the SWCNT mass fraction, the thermal conductivity and specific heat capacity increase significantly with the maximum enhancement of 38.59% and 5.87%, respectively, but the melting enthalpy decreases by 36.37%. The above phenomena can be attributed to the variation of atomic energy from nanoscale. This study is expected to provide possible guidance on the design and application of molten salts based PCMs for thermal energy storage at high temperatures.

Nanomaterials have been widely used to prepare nanocomposite phase change material (NPCM) and enhance its thermal conductivity. However, most of the studies are mainly focused on the thermal transport properties and the microscopic mechanism is still not clear. In this paper, molecular dynamics (MD) method was adopted to study both the thermal and phonon transport behaviors of NPCM (CuO/paraffin) to deeply reveal the influence mechanism of nanomaterials on PCM. The results show that the nanoparticle can enhance PCM thermal conductivity, and the enhancement effect is strengthened with nanoparticle mass fraction and NPCM temperature increasing. After that, phonon density of states was calculated to investigate the phonon scattering phenomenon in NPCM. The results show that the phonon scattering is very small in nanoparticle; then with the distance to nanoparticle surface increasing, the phonon scattering quickly increases; however, there exists a nanolayer around the nanoparticle, where the phonon scattering almost keeps constant. The weaker phonon scattering, the higher thermal transport performance. So, the existence of nanolayer caused by nanoparticle reduces the phonon scattering and promotes the heat transfer, which contributes to the thermal conductivity enhancement of NPCM

An orthogonal experiment with five factors and four levels was designed and numerical studies were performed to reveal the effects of molten salt PCM thermophysical properties on heat and exergy storage performance of a shell-and-tube latent heat storage (LHS) unit. The range analysis, regression analysis and optimization were conducted. The PCM selection criteria were established. The results show that for short time LHS system, the order of PCM properties effects on heat storage performance is melting temperature, thermal conductivity, specific heat, density and melting enthalpy successively. PCM with lower melting temperature, higher thermal conductivity and specific heat is beneficial to improve the heat storage rate and heat storage quality. For long time heat storage system, the order of PCM properties effects on heat storage performance is density, melting enthalpy, specific heat, melting temperature and thermal conductivity successively. PCM with higher density, melting enthalpy and specific heat can both improve the heat storage rate and heat storage quality. The orthogonal experiment method is reliable to investigate the effects of PCM properties on LHS performance; and the optimization based on regression analysis is more efficient than that based on range analysis.

Lattice Boltzmann method (LBM) was used to investigate the latent heat storage (LHS) performance of metal foams/paraffin composite phase change material (CPCM). The effects of metal foams PPI (number of pores per inch) and porosity on PCM melting rate, heat storage capacity and heat storage density were investigated. The results show that the CPCM heat transfer performance is determined by both heat conduction and natural convection. Increasing PPI can enhance heat conduction, but weaken natural convection. When the porosity is small, increasing PPI can enhance LHS performance. When the porosity is large, decreasing PPI can enhance the performance. With porosity decreasing, CPCM heat storage rate is improved and the maximum heat storage capacity almost keeps constant, but the heat storage density dramatically reduces. An optimum metal foams structure with porosity of 0.94 and PPI of 45 is recommended. Further, a performance enhancement scheme with nonuniform metal foams porosity was proposed and numerically validated. The results show that the proposed scheme can improve the uniformity of the heat transfer process and enhance the heat transfer performance.

The dust particles contained in the industrial flue gas are discharged from many industrial factories, which have seriously polluted the air and endangered the health of not only human beings but also the animals living on the earth, so they are gaining much attention all over the world. In order to provide possible solutions to the above problems, many filtration technologies have been proposed and developed. As a promising filtration method, the granular bed filter (GBF) based filtration technology has been widely used in the filtration of high temperature flue gas with complex components in recent years owing to its advantages of high efficiency, low cost, simplicity, and resistance to high temperature and high pressure. Inspired by the development of numerical simulation method and experimental technology, the performance of GBF has been optimized and improved, much deeper insight of the formation and growth of dust particles has been obtained. This review focus on the basic principle of the GBF and its performance investigated by numerical and experimental methods. The reviews and outlooks of investigations on the influence factors of GBF filtration performance and the development of performance optimization method of the GBF were also presented. In addition, the microscale mechanism of the agglomeration and growth of dust particles contained in the industrial flue gas during the filtration in the GBF by means of molecular dynamics simulation were introduced, the numerical and experimental method of dust particles filtration in GBF were also discussed. Finally, available recommendations for future research of GBF are proposed, which is expected to be helpful for actual industrial engineering applications.

It is urgently needed to improve the thermal properties of molten salt based phase change materials used for effective storage and utilization of solar energy. In this paper, the physical model of NaCl-SiO2 composite phase change materials (CPCM) was established. An effective method based on molecular dynamics (MD) simulation was proposed and validated to predict the thermal properties of CPCM. The structural deformation during phase transition process of CPCM system was observed and the radial distribution function (RDF) was calculated to analyze the local structure. The results indicate that the thermal conductivity of NaCl is enhanced remarkably with a maximum increase of 44.2% by adding 2.4% volume fraction of SiO2 nanoparticles and the mechanism of the thermal conductivity enhancement was discussed at the atomic level. The shear viscosity increases with the increase of the volume fraction of nanoparticles, with a maximum average increase of 23.6%. The relationship between self-diffusion coefficient and temperature is approximate to predict melting point. The force field and simulation methods adopted in this paper are desired to be useful for the prediction of thermal properties and further investigation into molten salts based thermal energy storage systems.

A compound enhancement method was proposed to improve the latent heat storage (LHS) performance of a shell-and-tube LHS unit, which is consisted of internal enhanced tube (ET) and multiple phase change material (PCM). Numerical validations on the presented method were performed based on comparisons of four different LHS cases: case 1 (basic case); case 2 (simple enhancement case); case 3 and 4 (compound enhancement cases). The results show that the simple enhancement case can only enhance PCM melting rate at the first half of the LHS tube; the compound enhancement case can obtain the synergy enhancement effect for the whole LHS tube. Compared with case 2, the PCM melting time is reduced by 37.3% and 17.4%, the total charging time is reduced by 25.6% and 16.9% for case 3 and case 4 respectively. For case 3, although the PCM melting time and charging time are the shortest, the total thermal energy storage (TES) capacity is reduced by 26.9% due to the lower PCM melting enthalpy. For case 4, not only the melting time and charging time can be obviously reduced, but also the total TES capacity is augmented by 6.6%.

Surface active agent (SAA) was used to improve nanomaterial dispersion during preparation process of carbonate salt/nanomaterial composite phase change material (CPCM) by solution evaporation method. In order to investigate the effects of SAA on CPCM thermal performance, three kinds of PCM samples were prepared and their thermal performances were characterized. The results show that nanomaterial dispersion greatly affects CPCM thermal performance. For CPCM without SAA, its thermal performance is weakened instead of enhanced due to nanomaterial aggregation and the weakening phenomenon is more obvious when nanomaterial has larger specific surface area. SAA decomposes during high temperature CPCM working process. And the effect of SAA on CPCM thermal performance has duality: on the positive side, SAA can improve nanomaterial dispersion and enhance CPCM thermal performance; on the negative side, SAA decomposition products may weaken CPCM thermal performance. So, SAA and its mass fraction should be carefully selected. Sodium dodecyl sulfate (SDS) is a better SAA for high temperature nano-CPCM and a high mass ratio of SDS to nanomaterial is recommended. With mass ratio of SAA to nanomaterial 10:1, PCM thermal conductivity can be enhanced up to 58.75% by adding 1 wt.% multi-walled carbon nanotubes.

To enhance the performance of high temperature salt phase change material, four kinds of carbon nanomaterials with different microstructures were mixed into binary carbonate eutectic salts to prepare carbonate salt/nanomaterial composite phase change material. The microstructures of the nanomaterial and composite phase change material were characterized by scanning electron microscope. The thermal properties such as melting point, melting enthalpy, specific heat, thermal conductivity and total thermal energy storage capacity were characterized. The results show that the nanomaterial microstructure has great effects on composite phase change material thermal properties. The sheet structure Graphene is the best additive to enhance specific heat, which could be enhanced up to 18.57%. The single walled carbon nanotube with columnar structure is the best additive to enhance thermal conductivity, which could be enhanced up to 56.98%. Melting point increases but melting enthalpy decreases with nanomaterial specific surface area increase. Although the additives decrease the melting enthalpy of composite phase change material, they also enhance the specific heat. As a combined result, the additives have little effects on thermal energy storage capacity. So, for phase change material performance enhancement, more emphasis should be placed on thermal conductivity enhancement and single walled carbon nanotube is the optimal nanomaterial additive.

Three-dimensional numerical studies were performed for latent heat storage (LHS) process of salt in a horizontal concentric tube to investigate the effects of liquid phase change material (PCM) natural convection on LHS performance. The results show that due to the effects of natural convection, the high temperature molten salt flows upward which enhances the PCM melting rate in upside and weakens the melting rate in downside. So, although the natural convection can enhance the heat transfer performance of liquid PCM, it also causes larger non-uniformity for the solid–liquid interface and temperature distribution during the PCM melting process. Then a local enhanced fin-tube was designed to improve the uniformity of the melting process. The effects of fin geometric parameters on the LHS performance were numerically investigated. The results show that the local fins can improve the uniformity of the LHS process. However, the fin parameters should be appropriately selected because the excessive large fin parameters will break the uniformity again. In present paper, the following fin parameters are recommended: fin number, 7; dimensionless fin thickness, 0.1; dimensionless fin height, 0.8.

In order to investigate the performance of high temperature molten salt latent heat storage (LHS) unit under variable conditions, the effects of heat transfer fluid (HTF) inlet temperature, velocity and tube geometric parameters on melting time, melting fraction, heat storage rate and solid–liquid interface were numerically investigated. The results show that within the studied parameters, the HTF inlet temperature has the largest effect on LHS rate. With the HTF inlet temperature increasing from 1070 K to 1110 K, the melting time reduces 53.0%. However, the larger inlet temperature will result in more non-uniform melting rate and solid–liquid interface distribution. The second important influential factor is the HTF inlet velocity. When inlet velocity increases from 10 m/s to 20 m/s, the melting time reduces 45.4%. And the effect of velocity on the solid–liquid interface distribution is uniform. LHS tube diameter has the lowest effect on performance. With outer tube radius increasing from 24.0 mm to 28.0 mm, the melting time only augments 16.3% and the solid–liquid interface distribution becomes more uniform. In a general conclusion, when the heat load of the heat source is larger, a larger HTF mass flow rate is suitable to maintain a moderate HTF temperature. And then for the LHS unit, a larger tube diameter is recommendable.

In order to enhance the performance of the latent heat storage (LHS) process and provide the criterion for the selection and match of the multistage PCMs, the effects of PCMs melting temperatures on the heat storage rate, entransy dissipation rate and heat storage quality were numerically analyzed based on the entransy theory. For the single stage LHS unit, although decreasing the PCM melting temperature can augment the heat storage rate, the lower melting temperature causes larger entransy dissipation and reduces the heat storage quality. The larger heat storage rate results in the larger entransy dissipation rate, which is accordant with the entransy dissipation extremum theory. For the two-stage LHS unit with reasonably matching the PCMs melting temperature, the heat storage rate can be augmented and the entransy dissipation rate can be reduced. Then the optimization for the match of the two-stage PCMs melting temperatures was performed based on the entransy theory. The results show that there is an optimal match of the two-stage PCMs melting temperatures to achieve the maximum heat transfer rate or the minimum entransy dissipation rate. And the formulas for the optimum two-stage PCMs temperatures were presented, which can provide the criterions for the selection and match of the PCMs.

A highly efficient thermal strategy to manage a high-powered Li-ion battery package within the required safe temperature range is of great demand for electric vehicles (EVs) applications. A sandwiched cooling structure using copper metal foam saturated with phase change materials was designed. The thermal efficiency of the system was experimentally evaluated and compared with two control cases: a cooling mode with pure phase change materials and an air-cooling mode. The results showed that the thermal management with air natural convection cannot fulfill the safety demand of the Li-ion battery. The use of pure PCM can dramatically reduce the surface temperature and maintain the temperature within an allowable range due to the latent heat absorption and the natural convection of the melted PCM during the melting process. The foam-paraffin composite further reduced the battery's surface temperature and improved the uniformity of the temperature distribution caused by the improvement of the effective thermal conductivity. Additionally, the battery surface temperature increased with an increase in the porosity and the pore density of the metal foam.

Three-dimensional external finned tube heat exchanger has been widely applied in waste heat recovery system. However, the current researches mainly focus on its fluid flow and heat transfer characteristics. The condensation of sulfuric acid and water vapors on heat exchanger surface, which can cause severe low temperature corrosion and reduce equipment efficiency, has rarely been reported. In this study, a heat and mass transfer model for flue gas on finned tube surface was developed to investigate the sulfuric acid and water vapor condensation characteristics and their distributions. The effects of flue gas velocity and temperature, acid and water vapor concentrations, and surface temperature were examined. The results show that the distribution of acid solution concentration on finned tube surface mostly depends on the distribution of fin surface temperature. At the windward side (especially near the minimum flow cross section region), both fin surface temperature and condensed acid solution concentration reaches the maximum value. Increasing velocity leads to vapors condensation rate and acid solution concentration increasing. The acid solution concentration decreases with flue gas temperature or tube wall temperature increasing. The present work could provide effective theoretical guidance for anti-corrosion design of finned tube heat exchanger.

In solar dish collector system, the heat transfer process is a typical coupling heat transfer problem, where the heat conduction, radiation heat transfer, convection heat transfer and phase change heat transfer are coexisting. In the present paper, a coupling model for Monte Carlo Ray Trace Method (MCRTM) and Finite Volume Method (FVM) was established to study the coupling heat transfer problem. Firstly, based on MCRTM, the non-uniform 3D heat flux distributions on the solar dish receiver inner surface were obtained. Then the non-uniform heat flux distribution is used as the boundary condition to simulate the phase change and convection heat transfer process inside the heat transfer tube. The effects of the non-uniform heat flux distribution on temperature field in phase change material (PCM) were examined. The results show that the non-uniform heat flux on the tube surface will result in seriously non-uniform temperature distribution in PCM. Then the optimization analyses for the temperature distribution were performed according to the convection heat transfer process and heat conduction process respectively. The results show that in the studied conditions, when heat transfer fluid (HTF) velocity increases from 10 m/s to 25 m/s, the maximum temperature difference in PCM will decrease from 781.2 K to 497.8 K, which reduces about 36.3%. However, it will cause the heat storage capacity and HTF outlet temperature decrease. When the thermal conductivity increases from 3.8 W m−1 K−1 to 19.0 W m−1 K−1, the maximum temperature difference will decrease from 781.2 K to 409.5 K, which reduces about 47.6%. And it will not result in HTF outlet temperature and heat storage capacity decreasing. So, enhancing the PCM thermal conductivity is an efficient method to achieve more uniform temperature distribution in PCM.

A three-dimensional optical model for a pressurized volumetric receiver (PVR) is developed and corresponding solar radiation propagation process within the PVR is simulated by the Monte Carlo Ray Tracing (MCRT) method. In the computation, the complicated photon transmission process in the SiC porous absorber is simplified as the transmission process in the statistically homogeneous and isotropic turbid medium. Meanwhile, the non-uniform cylindrical coordinate grid is applied in the statistics of energy distribution, which could greatly reduce the number of cells in the computational grid and time compared with normal uniform grid. Based on the above model, the energy distribution in the irregular macro scale porous absorber is determined and then the effects of system parameters, including the incidence angle, the shape of absorber and the optical property of absorber, on the local heat flux of the absorber are investigated. The results show that, under the given operating condition, the radiation heat flux is mostly concentrated at the top area of the absorber and the maximum heat flux value is up to 2.73 × 10 9 W m −3 , but it quickly decreases in the sideward locations. The incidence angle and a relative narrow shape of absorber are helpful to reduce the maximum heat flux in the absorber. Furthermore, as the ratio of absorption coefficient/extinction coefficient decreases, the absorbed radiation energy distribution is more uniform and the max heat flux in the absorber decreases greatly.

In this paper, a more detailed three-dimensional computational model of the whole parabolic trough solar collector (PTC) system and corresponding numerical simulations by combining the Finite Volume Method (FVM) and the Monte Carlo Ray-Trace (MCRT) method were presented. Corresponding codes and solving methods were also developed and applied to simulate and analyze the total involuted photo-thermal conversion process of an experimental LS2 PTC system. The numerical results were compared with experimental data and good agreement was obtained, proving that the model and method used in the present study is feasible and reliable. More details of the characteristics of solar concentrating, solar collecting, fluid dynamics, coupled heat transfer and the whole flow and temperature fields in the receiver were also revealed and discussed. Then some typical heat transfer fluid (HTF) types and residual gas conditions were further studied. It was revealed that the properties of these HTFs/conditions and their varying relations of the fluid temperature affected the characteristics of fluid dynamics, coupled heat transfer and the whole temperature distributions in the receiver, thus affected the thermal loss and the collector efficiency synthetically.

Based on enthalpy method, numerical studies were performed for high temperature molten salt phase change thermal energy storage (PCTES) unit used in a dish solar thermal power generation system. Firstly, the effects of the heat transfer fluid (HTF) inlet temperature and velocity on the PCTES performance were examined. The results show that although increasing the HTF inlet velocity or temperature can enhance the melting rate of the phase change material (PCM) and improve the performance of the PCTES unit, the two parameters will restrict each other for the fixed solar collector heat output. Then three enhanced tubes were adopted to improve the PCTES performance, which are dimpled tube, cone-finned tube and helically-finned tube respectively. The effects of the enhanced tubes on the PCM melting rate, solid–liquid interface, TES capacity, TES efficiency and HTF outlet temperature were discussed. The results show that compared with the smooth tube, all of the three enhanced tubes could improve the PCM melting rate. At the same working conditions, the melting time is 437.92 min for the smooth tube, 350.75 min for dimpled tube which is reduced about 19.9% and 320.25 min for cone-finned tube which is reduced about 26.9% and 302.75 min for helically-finned tube reduced about 30.7%. As a conclusion, the thermal performance of PCTES unit can be effectively enhanced by using enhanced tube instead of smooth tube. Although, the HTF pressure drops for the enhanced tubes are also larger than that of the smooth tube, the largest pressure drop (1476.2 Pa) is still very lower compared with the working pressure (MPa magnitude) of the dish solar generation system. So, the pressure drops caused by the enhanced tubes could almost be neglected.

A coupled simulation method based on Monte Carlo Ray Trace (MCRT) and Finite Volume Method (FVM) is established to solve the complex coupled heat transfer problem of radiation, heat conduction and convection in parabolic trough solar collector system. A coupled grid checking method is established to guarantee the consistency between the two methods and the validations to the coupled simulation model were performed. Firstly, the heat flux distribution on the collector tube surface was investigated to validate the MCRT method. The heat flux distribution curve could be divided into 4 parts: shadow effect area, heat flux increasing area, heat flux reducing area and direct radiation area. The heat flux distribution on the outer surface of absorber tube was heterogeneous in circle direction but uniform in axial direction. Then, the heat transfer and fluid flow performance in the LS-2 Solar Collector tube was investigated to validate the coupled simulation model. The outlet temperatures of the absorber tube predicted by the coupled simulation model were compared with the experimental data. The absolute errors are in the range of 1.5–3.7 °C, and the average relative error is less than 2%, which demonstrates the reliability of the coupled method established in this paper. At last, the concentrating characteristics of the parabolic trough collectors (PTCs) were analyzed by the coupled method, the effects of different geometric concentration ratios (GCs) and different rim angles were examined. The results show the two variables affect the heat flux distribution. With GC increasing, the heat flux distributions become gentler, the angle span of reducing area become larger and the shadow effect of absorber tube become weaker. And with the rim angle rising, the maximum value of heat flux become lower, and the curve moves towards the direction φ = 90°. But the temperature rising only augments with GC increasing and the effect of rim angle on heat transfer process could be neglected, when it is larger than 15°. If the rim angle is small, such as θrim = 15°, lots of rays are reflected by glass cover, and the temperature rising is much lower.

A unified two-dimensional numerical model was developed for the coupled heat transfer process in parabolic solar collector tube, which includes nature convection, forced convection, heat conduction and fluid-solid conjugate problem. The effects of Rayleigh number (Ra), tube diameter ratio and thermal conductivity of the tube wall on the heat transfer and fluid flow performance were numerically analyzed. The distributions of flow field, temperature field, local Nu and local temperature gradient were examined. The results show that when Ra is larger than 10{sup 5}, the effects of nature convection must be taken into account. With the increase of tube diameter ratio, the Nusselt number in inner tube (Nu{sub 1}) increases and the Nusselt number in annuli space (Nu{sub 2}) decreases. With the increase of tube wall thermal conductivity, Nu{sub 1} decreases and Nu{sub 2} increases. When thermal conductivity is larger than 200 W/(m K), it would have little effects on Nu and average temperatures. Due to the effect of the nature convection, along the circumferential direction (from top to down), the temperature in the cross-section decreases and the temperature gradient on inner tube surface increases at first. Then, the temperature and temperature gradients would present a converse variation at {theta} nearmore » {pi}. The local Nu on inner tube outer surface increases along circumferential direction until it reaches a maximum value then it decreases again. (author)« less

The solar energy flux distribution on the outer wall of the inner absorber tube of a parabolic solar collector receiver is calculated successfully by adopting the Monte Carlo Ray-Trace Method (MCRT Method). It is revealed that the non-uniformity of the solar energy flux distribution is very large. Three-dimensional numerical simulation of coupled heat transfer characteristics in the receiver tube is calculated and analyzed by combining the MCRT Method and the FLUENT software, in which the heat transfer fluid and physical model are Syltherm 800 liquid oil and LS2 parabolic solar collector from the testing experiment of Dudley et al., respectively. Temperature-dependent properties of the oil and thermal radiation between the inner absorber tube and the outer glass cover tube are also taken into account. Comparing with test results from three typical testing conditions, the average difference is within 2%. And then the mechanism of the coupled heat transfer in the receiver tube is further studied.

An anisotropic porous media model for mesh regenerator used in pulse tube refrigerator (PTR) is established. Formulas for permeability and Forchheimer coefficient are derived which include the effects of regenerator configuration and geometric parameters, oscillating flow, operating frequency, cryogenic temperature. Then, the fluid flow and heat transfer performances of mesh regenerator are numerically investigated under different mesh geometric parameters and material properties. The results indicate that the cooling power of the PTR increases with the increases of specific heat capacity and density of the regenerator mesh material, and decreases with the increases of penetration depth and thermal conductivity ratio ( a ). The cooling power at a = 0.1 is 0.5–2.0 W higher than that at a = 1. Optimizing the filling scale of different mesh configurations (such as 75% #200 twill and 25% #250 twill) and adopting multi segments regenerator with stainless steel meshes at the cold end can enhance the regenerator’s efficiency and achieve better heat transfer performance.

The air-side heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger with delta winglets are investigated numerically. The three-dimensional simulations are performed with renormalization-group (RNG) k − ɛ model to lay the foundation for the design of the high-performance heat exchanger. The wavy fin-and-tube heat exchangers which have three-row round tubes in staggered or in-line arrangements are studied. The numerical results show that each delta winglet generates a downstream main vortex and a corner vortex. For the in-line array, the longitudinal vortices enhance the heat transfer not only on the fin surface in the tube wake region but also on the tube surface downstream of the delta winglet; for the staggered array, longitudinal vortices are disrupted at the first wavy trough downstream from the delta winglet and only develop a short distance along the main-flow direction, and the vortices mainly enhance the heat transfer of the fin surface in the tube wake region. The longitudinal vortices generated by delta winglet cause considerable augmentation of heat transfer performance for wavy fin-and-tube heat exchanger with modest pressure drop penalty. When R e D c = 3000 , compared with the wavy fin, the j and f factors of the wavy fin with delta winglets in staggered and in-line arrays are increased by 13.1%, 7.0% and 15.4%, 10.5%, respectively.

In the present paper, the classical pressure correction method was extended into low Mach number compressible flow regime by integrating equation of state into SIMPLE algorithm. The self-developed code based on this algorithm was applied to predicting the lid-driven cavity flow and shock tube problems, and the results showed good agreement with benchmark solutions and the Mach number can reach the magnitude of as low as 10−5. The attenuation of sound waves in viscous medium was then simulated. The results agree well with the analytical solutions given by theoretical acoustics. This demonstrated that the present method could also be implemented in acoustics field simulation, which is crucial for thermoacoustic simulation.

The objective of this work is to train an artificial neural network (ANN) to predict the performance of gas cooler in carbon dioxide transcritical air-conditioning system. The designed ANN was trained by performance test data under varying conditions. The deviations between the ANN predicted and measured data are basically less than ±5%. The well-trained ANN is then used to predict the effects of the five input parameters individually. The predicted results show that for the heat transfer and CO2 pressure drop the most effective factor is the inlet air velocity, then come the inlet CO2 pressure and temperature. The inlet mass flow rate can enhance heat transfer with a much larger CO2 pressure drop penalty. The most unfavorable factor is the increase in the inlet air temperature, leading to the deterioration of heat transfer and severely increase in CO2 pressure drop.

Three dimensional numerical studies were performed for laminar heat transfer and fluid flow characteristics of wavy fin heat exchangers with elliptic/circular tubes by body-fitted coordinates system. The simulation results of circular tube were compared with the experiment data, then circular and elliptic ( e = b / a = 0.6) arrangements with the same minimum flow cross-sectional area were compared. A max relative heat transfer gain of up to 30% is observed in the elliptic arrangement, and corresponding friction factor only increased by about 10%. The effects of five factors on wavy fin and elliptic tube heat exchangers were examined: Reynolds number (based on the smaller ellipse axis, 500 ∼ 4000), eccentricity ( b / a , 0.6 ∼ 1.0), fin pitch ( F p /2 b , 0.05 ∼ 0.4), fin thickness ( F t /2 b , 0.006 ∼ 0.04) and tube spanwise pitch ( S 1 /2 b , 1.0 ∼ 2.0). The results show that with the increasing of Reynolds number and fin thickness, decreasing of the eccentricity and spanwise tube pitch, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. There is an optimum fin pitch ( F p /2 b = 0.1) for heat transfer, but friction factor always decreases with increase of fin pitch. And when F p /2 b is larger than 0.25, it has little effects on heat transfer and pressure drop. The results were also analyzed from the view point of field synergy principle. It was found that the effects of the five factors on the heat transfer performance can be well described by the field synergy principle.