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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.

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.

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.

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

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.

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 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.

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.

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.

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.

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.

Three-dimensional numerical simulations were performed for laminar flow of wavy fin-and-tube heat exchangers by using body-fitted coordinates (BFC) method with fin efficiency effect accounted. The prediction results of average Nusselt number, friction factor and fin efficiency were compared with the related experimental correlations [R.C. Xin, H.Z. Li, H.J. Kang, W. Li, W.Q. Tao, An experimental investigation on heat transfer and pressure drop characteristics of triangular wavy fin-and-tube heat exchanger surfaces, J. Xi'an Jiaotong Univ. 28 (2) (1994) 77–83] and Schmidt approximation [T.E. Schmidt, Heat transfer calculations for extended surfaces, Refrigerating Engineering (April 1949) 351–357]. For Reynolds numbers based on the tube outside diameter ranging from 500 to 4000, the mean deviation is 3.3% for Nusselt number, 1.9% for friction factor and 3.6% for fin efficiency. The distributions of local Nusselt number and fin efficiency on fin surface were studied at wavy angle equal to 0° (plain plate fin), 10° and 20° respectively. The local Nusselt number decreases along the air flow direction, but fin efficiency increases in general. The wavy angle can greatly affect the distributions of local Nusselt number and fin efficiency, and make the distributions present fluctuation along the flow direction. The result also shows that the fin efficiency at the inlet region of wavy fin is larger than that of plain plate fin at the same region. With the increase of Reynolds number, the effects of wavy angle on the distributions of local Nusselt number and fin efficiency are more and more significant.

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.

Three-dimensional numerical studies were performed for the performance of wavy fin-and-tube heat exchangers in Body-Fitted Coordinates (BFC) system. Effects of geometric parameters on air-side heat transfer and fluid flow characteristics and fin efficiency were examined. The results showed that with the increase in Reynolds number, wavy angle, fin thickness and the decrease in fin pitch and transverse tube pitch, the heat transfer performance are enhanced; however, pressure drops are also increased. So, in practical applications, the wavy angle had better be located between 10° and 20° and the fin pitch should be located between 1.2 mm and 2.0 mm. The fin efficiency and average fin surface temperature decrease with the increase of Reynolds number, wavy angle, fin pitch and transverse tube pitch. With the increase of fin thickness, the fin efficiency and average temperature on fin surface also increase.