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In order to effectually improve the performance of parabolic trough solar collector system and homogenize the temperature distribution on the absorber tube, an innovative convergent-divergent tube was designed as the absorber tube of PTR. The finite volume method (FVM) and the Monte Carlo ray tracing method (MCRT) are combined to simulate the heat transfer process in parabolic trough collector. The average relative error between the numerical results and the experimental results conducted in the Spanish DISS test facility is 1.103%, which confirms the reliability of the simulation results in this paper. The heat transfer characteristics and flow characteristics of PTR and convergent-divergent PTR are compared in the inlet velocity range of 0.05-0.75m/s, and the effect of the number of zoom sections(N) for CD-PTR performance is also studied. The simulation results show that the parabolic trough collector with convergent-divergent tube has significantly enhanced heat transfer capability. The average Nu of CD-PTR increases as the number of zoom sections increases, and is always higher than that of PTR. When Re=86400 and N=25, the average Nu increased by 66%. [ABSTRACT FROM AUTHOR]

Metal foam heat exchangers have attracted a great deal of interest in numerous engineering fields due to their superior thermal capabilities. In the present study, the heat transfer characteristics of a double-pipe heat exchanger with metal foam insert are numerically investigated. The Forchheimer-extended Darcy equation and the local thermal non-equilibrium (LTNE) model are used to predict the fluid and energy transports, respectively. Thermal resistance of the interface solid wall is considered, while the porous-solid boundary follows the continuity principles. The commercial software FLUENT with specific user defined functions (UDFs) is adopted to implement the simulation. Configurations with uniform foam structure are firstly used to analyze the effects of flow arrangement, foam structural parameters (porosity and pore density) and thermal conductivity on the heat exchanger effectiveness and total pressure drop. Then, graded foam structure along the radius is proposed to further make use of the heat transfer potential of metal foam. The overall thermal performance with increasing and decreasing arrangements of porosity and pore density is assessed. The results indicate that the counter flow shows good performance, with 37.5% higher than the parallel flow in effectiveness. The effectiveness and total pressure drop present monotonic variation with the foam structural parameters for the uniform designs, while maximum performance factor occurs at 15 PPI. The effectiveness has a reduction after gradual increase to a peak 0.89 with the increasing of thermal conductivity of foam matrix. For the designs of graded foam structure, using lower porosity and small pore density at both side of the inner pipe wall shows better overall performance with the performance factors of 4.41 and 4.54. [ABSTRACT FROM AUTHOR]

The heat transfer and thermochemical reaction are numerically investigated for the solar driven methane dry reforming process, coupling with the solar radiation transport from concentrator to the interior of volumetric reactor. Using a Monte Carlo ray tracing method, the solar radiation concentration with a multi-dish system and absorption by the foam structure in the reforming reactor are simulated. The solar power density distribution in reforming reactor and the optical efficiency of system are determined and compared at different foam structural parameters. Then, an integrated model of transport phenomena is developed to analyze the thermal and chemical performances under different operating conditions. The results indicate that solar radiation absorption and distribution in the reactor are greatly affected by the foam structural parameters, while the optical efficiency has no significant variation. Assumption of the concentrated solar radiation as collimated leads to a temperature deviation of approximately 10%. Besides, the methane conversion increases with increasing porosity and cell size, and decreases markedly with increasing inlet velocity and CH 4 /CO 2 ratio. [ABSTRACT FROM AUTHOR]

Tube receiver with pin fin arrays inserting was introduced as the absorber tube of parabolic trough receiver to increase the overall heat transfer performance of tube receiver for parabolic trough solar collector system. The Monte Carlo ray tracing method (MCRT) coupled with Finite Volume Method (FVM) was adopted to investigate the heat transfer performance and flow characteristics of tube receiver for parabolic trough solar collector system. To validate the feasibility of the developed MCRT and FVM combined method, the numerical results have been compared with experimental results conducted in the DISS test facility in Spain and the max relative error is less than 5%. The numerical results indicated that the introduction of absorber tube with pin fin arrays inserting design for the absorber tube of the parabolic trough receiver can effectively enhance the heat transfer performance. The average Nusselt number can be increased up to 9.0% and the overall heat transfer performance factor can be increased up to 12.0% when the tube receiver with pin fin arrays inserting was used. [ABSTRACT FROM AUTHOR]

There are two commonly used methods for irradiative transfer problems in porous media: P1 approximation and Rosseland approximation. In this study, the effects of irradiative transfer models on heat transfer performance of porous media solar receiver under concentrated heat flux distribution are numerically investigated. By combining the Monte Carlo Ray Tracing (MCRT) method and FLUENT software with user defined functions (UDFs), the local non-equilibrium thermal equation (LNTE) model with both the Rosseland approximation and modified P1 approximation are established. The numerical results indicate that the maximum temperature difference of porous media solar receiver between the modified P1 approximation and Rosseland approximation is small. The thickness of thermal non-equilibrium region is smaller for the Rosseland approximation condition than that for the modified P1 approximation condition. [ABSTRACT FROM AUTHOR]

Highlights: [•] Using local thermal non-equilibrium model to solve heat transfer of porous media. [•] CH4/H2O mixture is adopted as feeding gas of porous media receiver. [•] Radiative transfer equation between porous strut is solved by Rosseland approximation. [•] Transport and thermophysical models not included in Fluent are programmed by UDFs. [•] Variations of model on thermal performance of porous media receiver are studied. [Copyright &y& Elsevier]

Abstract: In this paper, Monte Carlo Ray Tracing (MCRT) and Finite Volume Method (FVM) coupling method is adopted to solve the radiation, conduction and convection coupled heat transfer problems of porous media receiver with multi-dish collector. The MCRT method is used to obtain the concentrated heat flux distribution on the fluid inlet surface of porous media receiver. The local thermal non-equilibrium (LTNE) model with concentrated solar irradiation on the fluid inlet surface is used for energy equations. FVM software FLUENT with User Defined Functions (UDFs) is used to solve the fluid phase and solid phase heat transfer problems. The effects of solar irradiance, air inlet velocity, average particle diameter, receiver radius and air properties on the temperature distribution are investigated. [Copyright &y& Elsevier]

Abstract: In the parabolic trough concentrator with tube receiver system, the heat transfer fluid flowing through the tube receiver can induce high thermal stress and deflection. In this study, the eccentric tube receiver is introduced with the aim to reduce the thermal stresses of tube receiver. The ray–thermal–structural sequential coupled numerical analyses are adopted to obtain the concentrated heat flux distributions, temperature distributions and thermal stress fields of both the eccentric and concentric tube receivers. During the sequential coupled numerical analyses, the concentrated heat flux distribution on the bottom half periphery of tube receiver is obtained by Monte-Carlo ray tracing method, and the fitting function method is introduced for the calculated heat flux distribution transformation from the Monte-Carlo ray tracing model to the CFD analysis model. The temperature distributions and thermal stress fields are obtained by the CFD and FEA analyses, respectively. The effects of eccentricity and oriented angle variation on the thermal stresses of eccentric tube receiver are also investigated. It is recommended to adopt the eccentric tube receiver with optimum eccentricity and 90° oriented angle as tube receiver for the parabolic trough concentrator system to reduce the thermal stresses. [ABSTRACT FROM AUTHOR]

• A fin-stone hybrid structure enhances heat transfer of phase change materials. • Charging and discharging times are saved by 67% and 73%, respectively. • Satisfactory temperature stability and more yearly supplied energy are achieved. This work proposes a fin-stone hybrid structure integrating fins (popular thermal enhancers) and natural stones (widely used sensible heat storage media) to enhance the heat transfer of phase change materials for on-site thermal energy storage applications, with advantages of low cost, environmental friendliness, and easy accessibility. 3D numerical models of charging and discharging in shell-and-tube heat storage units with various configurations, including fins, the fin-stone hybrid structure, stones, and no heat transfer enhancement, were constructed, and the performance evaluation and comparison were carried out. Compared to fins, fin-stone hybrid structures with 20 mm-, 30 mm-, and 40 mm-sized stones shorten the charging time by 67%, 54%, and 56%, and the discharging time by 73%, 60%, and 46%, respectively. Small stones have better heat transfer enhancement, which is attributed to the small volume, large surface area, and contact with the tube and fins. The advantage of the fin-stone hybrid structure, i.e. the shortening of phase change time, is more significant in charging than in discharging, in comparison with stones, as both heat conduction and natural convection are enhanced. Moreover, the hybrid structure exhibits satisfactory temperature stability with a 48.9 °C temperature change in charging and 37.2 °C in discharging, each lower than the fins, which is beneficial to stabilise the heat transfer fluid outlet temperature. The yearly supplied energy of the hybrid structure with 20 mm-sized stones is 121% and 72% more than that of fins and stones, respectively. [ABSTRACT FROM AUTHOR]

Highlights • High-temperature heat transfer in foam filled double-pipe heat exchanger is studied. • Coupling effect between two fluid sides and the wall thickness effect are considered. • Decreasing porosity and increasing pore density improve the exchanger effectiveness. • Performance improvement is slight while dimensionless exchanger length exceeds 35. Abstract A numerical investigation is performed to analyze the high-temperature heat transfer behavior in a double-pipe heat exchanger filled with open-cell porous foam. The Forchheimer-extended Darcy equation and the local thermal non-equilibrium model are utilized to simulate the flow and thermal transport inside the foam regions, considering the coupling effects between the inner and annular spaces. The effect of solid wall thickness is incorporated in the modelling process, following the continuity conditions of temperature and heat flux at the porous-solid interface. Simulations are conducted for a counter-flow heat exchanger while the thermal radiation transfer is solved using the P1 approximation. The cold fluid flows in the inner pipe, whereas the hot fluid in the annular gap. The temperature distribution, pressure drop, heat exchanger effectiveness and the overall performance are predicted. Effects of thermal radiation, foam structural parameters and the heat exchanger size are examined. The results indicate that thermal radiation promotes the thermal exchange between the two fluid sides. The heat exchanger effectiveness is improved by decreasing the porosity or increasing the pore density, exchanger length and annulus dimension, however, the potential increase in the total pressure drop should be seriously considered. Besides, the overall performance is slightly improved by further increasing the exchanger length when the dimensionless length is greater than 35. [ABSTRACT FROM AUTHOR]