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The aim of this paper is to study the spray cooling heat transfer characteristics using CFD method. The two phase mathematical model is established based on Euler-Lagrange approach. In this model, unsteady-state flow conditions are simulated. Heat and mass transfer between the vapor and the water droplet are computed. The averaged error of numerical results is 10% compared to experimental results in our previous papers. It is concluded that the heat flux and its distribution on heated surface are influenced by the heated surface temperature, mass flux, nozzle to surface distance and the number of nozzles. Furthermore, the heat flux distribution do not change obviously with the increasing of heated surface temperature, and the heat flux increases with the increasing of mass flux. Moreover, there is an optimal nozzle to surface distance while the spray impingement zone circumscribes in heated surface, and more nozzles lead to higher heat flux and better heat flux distribution on heated surface. Considering the disadvantage of manufacture difficulty and cost increase for more nozzles, there is an optimal nozzle number in certain space and heat dissipation requirement. The optimal number of nozzles is 8 in this article. [ABSTRACT FROM AUTHOR]

Efficient cooling system is an essential part of heavy duty extrusion pelleting line which plays an important role in 300 kt/a production engineering of polyolefin. In this paper, a wickless gravity-assisted two-phase cooling system (WGATPCS) used in heavy duty extrusion pelleting line was presented. The volume of the fluid (VOF) model in FLUENT was used to simulate the two-phase flow and heat transfer characteristic during the operation of WGATPCS by solving the equations of continuity, momentum and energy. The vapor velocity and temperature distribution of cooling system were obtained. The operating conditions applied in this paper are the followings: the filling ratio is 0.3, cooling water flow rate is 180 L/h, and heating power is 11 kW. The predicted temperature profile showed a good agreement with experimental data and the maximum relative error was below 0.99%. Therefore, it was concluded that the CFD model successfully reproduced the heat and mass transfer processes in proposed cooling system. The variation of liquid film thickness, the characteristics of liquid film and vapor-liquid interface were also analyzed. The maximum liquid film thickness was 0.017 mm at the bottom of condenser pipe, the velocity of vapor and liquid at vapor-liquid interface were 0.00785 m/s and 0.09 m/s respectively. The average condensation and evaporation heat transfer coefficients were 68083 W/(m 2 K) and 3929 W/(m 2 K), respectively. [ABSTRACT FROM AUTHOR]

Highlights • Analytical transient heat transfer model during bubble ebullition waiting period is presented. • The model covers a large spectrum of geometries. • The new model has been parametrically tested and validated against experimental data. • More work is still needed to extend the current models to very high heat fluxes. Abstract The multi-field two-fluid modeling concept has been successfully used in the past to perform Computational Multiphase Fluid Dynamics (CMFD) simulations of forced-convection boiling. Since such simulations are normally performed at the RANS-level, they do not allow for resolving local phenomena at the bubble-scale level. Instead, closure laws making use of empirical correlations deduced from selected experimental data are adopted. As different data sets of reference frequently correspond to different, and not always overlapping, experimental conditions, their application introduces considerable uncertainties into the predictions of the overall CMFD models. It has been shown before (Podowski et al., 1997) that one way to improve the accuracy of computational models is to apply a multiscale modeling approach, in which single-bubble-nucleation-level models, validated against separate-effect experiments, are used to formulate mechanistic RANS-level closure laws. This paper presents a mathematically rigorous and physically consistent model of the bubble ebullition cycle, which takes into account the coupling of transient heat transfer between solid walls and the surrounding coolant during wall quenching period. The proposed modeling approach covers a large spectrum of the geometries pertaining to both experimental boiling test sections and nuclear reactor fuel assemblies. The new model has been parametrically tested and validated against experimental data. A good agreement between the model predictions and experimental data has been obtained. The overall objective of the current work is to establish a first-principle theoretical framework for the formulation of mechanistic ensemble-averaged closure laws compatible with existing CMFD solvers. [ABSTRACT FROM AUTHOR]

Highlights • CFD simulation of flow blockage accident and its induced heat transfer impairment is performed. • Three dimensional distributions of fluid and solid temperature downstream the blockage are obtained. • The influence from the blockage area and its location on the temperature peaks and inter-channel mixing is analyzed. Abstract Flow blockage accident is a severe accident. When fuel assembly is partially or totally blocked, the heat transfer between coolant and cladding is significant impaired due to a rapid decrease of coolant flow, which leads to a local increase of cladding and coolant temperature and possibly results in a release of fission products. A recirculation region is formed downstream the blockage and a temperature peak can be found in this region. In this paper, CFD modelling and simulation for a LBE-cooled 19-pin wire-wrapped bundle were carried out to consider the influence from the flow blockage on the rod surface temperature and cross flow. Three-dimensional flow and temperature distributions of LBE were obtained using commercial CFD software. The influence from the blockage area and its location was investigated as well as the axial length of blockages. It was found that flow blockages in the center of bundle result in a higher surface temperature while comparing with blockages in the side sub-channels. The cross flow between sub-channels through gaps has been evaluated and their dependency on the blockage area is also evaluated based on CFD simulation results. [ABSTRACT FROM AUTHOR]

Abstract This paper aimed to develop a computational fluid dynamics simulation for an industrial boiler. The industrial boiler is used for the sulfuric acid production. A numerical model is carried out using the commercial code "ANSYS-FLUENT". The simulations are developed for the case of the industrial boiler and a new proposed design. A clear understanding of the combustion phenomenon and the fluid flow within the boiler is provided from the simulations. The local characteristics of the fluid flow and the heat transfer such as the temperature; the velocity magnitude and the density distributions are presented and analyzed. The comparison between the numerical results and the experimental data is performed and a good agreement is found. Highlights • A numerical simulation is developed for an industrial boiler. • The fluid flow and the heat transfer characteristics are analyzed and discussed. • The numerical model is validated by comparison between numerical results and industrial measurements. [ABSTRACT FROM AUTHOR]

Highlights • The mechanistic formation and physically self-consistent depletion of microlayers beneath growing bubbles with heat conduction into the solid is demonstrated. • Validation of the microlayer formation and ∼depletion model with recent measurements. • Predictions are used to identify the circumstances and phenomena that cause microlayers to be generated. • Predictions demonstrate that the importance of microlayers to bubble growth increases with the superheat. • Investigation of the significance of evaporative thermal resistance for microlayer formation and evaporation. Abstract Microlayers, the few-microns-thick layers of liquid that sometimes remain beneath bubbles growing on a heated substrate, are widely observed in experiments, but theoretical understanding of their formation, behaviour and role in bubble growth is limited. In this paper we present detailed interface-tracking simulations of the formation and depletion of such microlayers. Validation of our results is presented to the degree that available measurements of such a rapid and microscopic phenomenon allow. The work extends previous mechanistic hydrodynamic-only CFD simulations of early microlayer formation up to typical bubble departure times. These calculations confirm the understanding that the bubble growth rate and the resulting bubble shape determine the presence and overall extent of microlayers underneath steam bubbles. Their thickness is strongly influenced by viscous effects and surface tension. We then present coupled, physically self-consistent CFD simulations of the formation and evaporative depletion of such microlayers. This modelling suggests strongly that the evaporation process itself constitutes a significant fraction of the small resistance to heat and mass transfer presented by the very thin liquid layer. Inclusion of representations of evaporative thermal resistance, consistent with those suggested in the literature, is seen to promote the prediction of microlayer formation. Identification of the classes of conditions under which microlayers seem likely to be formed is presented, along with an assessment of their relative contributions to bubble growth. Comparisons of the predictions with recent detailed microlayer measurements indicate good agreement. [ABSTRACT FROM AUTHOR]

Wedge-shaped hot-films are a promising alternative to hot-wires for supersonic free flow disturbance measurements if hot-wires cannot be used due to harsh flow conditions. Up to the present, wedge hot-films could not serve as quantitative tools because of an insufficient modelling of the substrate’s influence on the thermal balance. The present paper shows a static calibration of a wedge-shaped hot-film sensor that is based on a combination of an analytical, a numerical and an experimental approach. The substrate’s impact was analysed by the help of CFD simulations and modelled by a newly introduced substrate factor. The obtained sensitivities were discussed and qualitatively explained compared with standard hot-wires. A quantitative comparison of experimentally detected mass flux fluctuations proved the wedge hot-film data to match similar hot-wire results very well. The presented approach has the potential to upgrade hot-films of various shapes to tools for quantitative fluctuation measurements. [ABSTRACT FROM AUTHOR]

The reflow soldering process of large size components was always problematic in microelectronics manufacturing due to the possibility of component displacement failures after soldering; like tombstone formation or skewing, which can be traced back to the different heating of the opposite component sides. During vapour phase soldering, the efficiency of heat transfer highly depends on the thickness of the condensate layer. In this paper, the inhomogeneity of condensate layer formation and its effects were investigated at large size components during vapour phase soldering by numerical simulations. For this purpose, a 3D computational fluid dynamic model was established. According to the condensate layer formation in different cases, the onset differences in the melting of the solder alloy at the opposite leads of the component were calculated. By the results, the risk of the component displacement during reflow soldering was analysed. It was found, that the congestion of the condensate layer around the large size components can cause considerable differences in the onset of the solder alloy melting, which can yield in component displacement failures after soldering. The extent of difference in the onset of melting depends on the location of the component on the board and on the applied soak temperature. Keep-out zones on the board were suggested to reduce the possibility of the component displacement failures during the vapour phase soldering process. [ABSTRACT FROM AUTHOR]

This paper investigates the heat transfer of superheater tubes in one 600 MW boiler combining the 3-D CFD simulation and the 1-D analytical model. To confirm the validity of this method, the simulation results were compared with the on-line measured outlet steam temperature and standard operation data provided by the plant manufacturer. The thickness of oxide scale formed on the steam-side of superheater tubes was measured by the metalloscope, and the influence of oxide thickness on the tube wall temperature over the entire length of superheater tubes was assessed by this model. The maximum outlet steam temperature was found to be 847.5 K, and the highest outlet tube wall temperature was 876.8 K. As the oxide thickness grows up to 152 μm, the tube wall temperature of Tube 7 and Tube 9–11 would exceed the allowable metal temperature (866 K). Therefore, changing the Tube 7 and Tube 9–11 materials to the better heat-resistant steel alloy is suggested to avoid tube overheating. [ABSTRACT FROM AUTHOR]

Pressurization system plays a significant role in aerospace missions, and non-condensable helium is usually used to provide pressurization effect to a cryogenic propellant tank. In the present paper, the helium effect on the pressurization behaviors of cryogenic propellant tanks are concerned and investigated, and an enhanced computational fluid dynamic (CFD) approach, which could simultaneously account for the species transfer as well as its effect on phase change, is introduced to assist the analysis. Two pressurization events, including depressurization after an active helium pressurization and pressurized discharge process, are selected as the research objectives, and the pressure responses, species concentration distribution, as well as mass transfer amount evolutions are obtained and analyzed. The results show that species transfer on pressure behaviors is strongly related to the pressurization time. When a pressurization lasts within several hundred seconds, the multicomponent effect generally has a relatively small influence on the thermodynamic performance. However, when the multispecies system experiences a longer time, the species transfer effect appears. For the depressurization performance after a pre-pressurization operation, the residual helium in the helium-existence case leads to a continuous liquid evaporation in the depressurization, while a pure hydrogen case experiences a vapor condensable process. Moreover, the helium and hydrogen could sufficiently diffuse with each other within the depressurization period. For a typical pressurized discharge process, the species diffusion as well as its effect on mass transfer rate affects the physical field distribution in a certain degree but a weak influence on the general pressurization performance. The specie concentration along radial direction causes a distinguishable radial temperature gradient. Generally, the present study gives a clear exhibition on the pressure and thermodynamic behaviors of pressurization using helium. Through the present CFD model, the multicomponent diffusion as well as its influence on phase change rate could be achieved. [ABSTRACT FROM AUTHOR]

A fully coupled heat transfer model at pore-scale of volumetric solar receiver is established in this paper. The X-ray computed tomography technique is applied to reconstruct the porous structure. By generating the voxel mesh and coupling with the Monte Carlo Ray Tracing method, the energy source due to the solar radiation could be determined and then added to the unstructured CFD mesh. The governing equations are solved by the commercial CFD software FLUENT. The results show that the details of the fluid flow and heat transfer in volumetric solar receiver are successfully captured. The pressure drop correlation corresponds satisfactorily to the previous study. The local convective heat transfer coefficient varies in a small range along the inlet fluid flow direction inside volumetric solar receiver and the average Nusselt number could be correlated to a power function of the Reynolds number. The radiation transfer inside the porous media is visualized and thermal radiation loss is evident at the entrance of the solar receiver. The proportion taken by radiation in the total heat transfer is determined as a function of the average temperature of porous skeleton. [ABSTRACT FROM AUTHOR]

This paper presents the experimental results of laminar flow behavior of water in circular micro pin-fin (C-MPF), square micro pin-fin (S-MPF) and diamond micro pin-fin (D-MPF) heat sinks using micro-PIV flow visualization technology at first. All three micro pin-fin heat sinks have a hydraulic diameter of 200 μm. Second, numerical simulation results of the fluid flow characteristics in these heat sinks with CFD are compared to the experimental results of fluid flow behaviors measured with the micro-PIV flow visualization. The normalized time averaged streamline patterns and instantaneous velocity contours in the three heat sinks were obtained for laminar flow of Reynolds number from 10 to 200. By comparison, the experimental results favorably agree with the simulated results of fluid flow. Of the three types of heat sinks, the vortexes occur the earliest in the D-MPF heat sink, which also present very complicated back flow. The strong vortexes and back flow effectively enhance the mixing of fluid and therefore lead to higher pressure drops in the D-MPF heat sink as compared to the other two types of heat sinks. The vortexes in the D-MPF heat sink are very much easily involved in the main flow than those in the other two types of heat sinks due to the high deceleration and pressurization zone. Finally, numerical simulation results of heat transfer at steady state in the three heat sinks are presented. The initial temperature of the working fluid and the ambient air is maintained at 293 K and a constant heat flux of q w = 400 kW/m 2 is adopted in the central area at the bottom of the heat sink. The Reynolds number ranges from 40 to 300 for the fluid flow and heat transfer simulations. It shows that D-MPF heat sink has better heat transfer performance than the other two type heat sinks. The combined effects of the vortex in the main flow at the front side wall and the strong vortex intensity behind the D-MPF heat sink obtained in both experimental and numerical results may reasonably explain the better heat transfer enhancement behaviors as compared to those in the other two types of heat sinks. Further experiments on the heat transfer performance will be conducted to compare to the simulated results in the follow-up planned research. [ABSTRACT FROM AUTHOR]

This paper examines the application of CFD modelling to simulate the complex multiphase characteristics inside a wickless heat pipe (thermosyphon). Water and refrigerant R134a were selected as working fluids. A novel and comprehensive three-dimensional CFD model of a wickless heat pipe was developed to simulate both the complex multiphase heat and mass transfer characteristics of boiling and condensation and the heat transfer characteristics of the cooling fluid in the condenser – heat exchanger. The CFD simulation has successfully predicted, for the first time, a boiling regime and two phase flow pattern that takes place with water at low power throughput, known as geyser boiling. The effects of the power throughput on the characteristics of the geyser boiling were investigated. The CFD simulation was also successful in modelling and visualising the multiphase flow characteristics, emphasising the difference in pool boiling behaviour between these working fluids. Temperature profiles and visual validation of the resulting 3D CDF findings were conducted using two experimental facilities. [ABSTRACT FROM AUTHOR]

Fast airflow and heat transfer simulation is becoming increasingly more and more important in building design, real-time system control, and emergent environment management. Conventional algorithms such as SIMPLE or PISO need great calculation effort and thus cannot meet the requirement of computing speed. Some developed fast calculation methods such as FFD lack prediction accuracy due to the inherent simplifications. This paper developed a new fast computational method named semi-Lagrangian PISO (SLPISO) that integrates the semi-Lagrangian scheme with a PISO solver. The SLPISO solver can significantly reduce the computational cost compared to SIMPLE and PISO while maintaining the required accuracy. The accuracy and stability of the developed algorithm were analysed. A confined space lid-driven cavity flow case and a mixing convection case in a confined space were used to evaluate and illustrate method performance. Results reveal that SLPISO possesses similar accuracy to PISO but can tolerate much larger time steps in the transient simulation. A larger time step will increase the false diffusion, and thus smooth the gradients of the flow field. A proper diffusion coefficient corresponding with a given time step is, hence, necessary to obtain accurate results. [ABSTRACT FROM AUTHOR]

A numerical model for heat and mass transfer in vacuum membrane distillation (VMD) under laminar flow was developed using Computational Fluid Dynamics (CFD). Three-dimensional (3D) simulations of temperature and concentration polarization in single and multiple fibre VMD modules were used to estimate permeate flux (kg m −2 h −1 ) over a range of temperatures (30–70 °C), vacuum pressures (10–100 mmHg), crossflow velocities (0.0072–0.72 m/s), and feed concentrations (0–0.4 kg/L NaCl). Simulated flux differed by less than 7% from experimental data for a module with comparable dimensions. Simulations indicate that a 56% increase in fibre packing density resulted in a 24% flux decline at high operating temperature (70 °C), and more than 50% flux decline at low crossflow velocity (0.0072 m/s). The effect of vacuum pressure on flux was found to be independent to the module packing density, while the effect of salt concentration was found to be 28% lower than estimates based on Raoult's law, due to lower spatial variations in membrane surface temperatures at higher salt concentrations. The approach developed in this paper may be used to evaluate performance of alternative configurations for VMD models to aid module design, scale-up and process optimization. [ABSTRACT FROM AUTHOR]

Venetian blinds, which were originally designed to provide sun shading and privacy, also have the potential to reduce heat transfer caused by internal and external temperature difference when integrated within the cavity between the two panes of a double glazing unit. In this paper, the thermal performance of a glazing system with and without a Venetian blind with various slat orientation angles under different temperature conditions is investigated through both experiment (undertaken in a large climate chamber) and numerical simulation (obtained via Computational Fluid Dynamic modelling). The thermal resistance of a Venetian blind glazing system varies with the change of slat inclination angle, and it also highly depends on the mean temperature of the glazing and the temperature difference between the indoor and outdoor environment. Inclusion of a Venetian blind modifies both the absolute and relative strengths of convection and radiation. Vertically oriented slats showed the most significant contribution to increasing radiative thermal resistance, which led to the best overall thermal performance. The system achieved up to 28% improvement of U-value when compared with a glazing unit without a Venetian blind. Empirical correlations generated based on simulations could be used for future building energy simulation. [ABSTRACT FROM AUTHOR]

Ambient air vaporizer (AAV) utilizes ambient air which belongs to low grade energy to regasify LNG in gas terminals. Study on the heat transfer performance of AAV is the foundation of its optimal design and operation. This paper presents a CFD-based coupled numerical model to investigate the heat transfer performance of AAV. The heat transfer coupling between the LNG boiling phase change and the natural convection of the air is taken into consideration. The mixture model combined with Lee model was used to calculate the heat and mass transfer in the process of LNG boiling. The coupled numerical model was calculated and validated by the operating data of a real-life AAV. Based on the simulation results of the coupled model, the influence of the air temperature, the inlet flow of LNG and the location in the fin tube bundle on the heat transfer performance of AAV were analyzed. [ABSTRACT FROM AUTHOR]

The heat transfer performance of indirect air-cooling (IDAC) towers in large power stations is sensitive to the ambient wind velocity. To ensure the economic and reliable operation of units under windy conditions, it is important to conduct research on the optimization of different wind-break structures. This paper uses computational fluid dynamics method (CFD) to simulate the heat transfer performance of a 1000 MW IDAC tower power stations with four different wind-break structures namely, cross walls, wind-break walls, cross line-screen, and louvers. The research results show that the order of the effective heat transfer improvement of four wind-break structures is the wind-break, cross wall, line-screen and louvers. The wind-break wall is the most optimal structure, but its performance is strictly influenced by the direction and velocity of the wind, and the cross walls and cross line-screen structure have similar limitation in the practice operation. The louver is installed in each sector, and it is the next best option for increasing the heat transfer performance. It can be flexibly adjusted based on the wind direction and velocity. With the decrease in the louver opening, k , there is a decrease in the heat transfer rate of the windward sectors, and a significant increase in the heat transfer rate of the leeward sectors. Thus the total heat transfer rate of the IDAC tower can be improved tremendously. Based on the analysis of heat transfer and air flow mechanisms, there is an optimal opening, k , which achieves the largest heat transfer performance in an IDAC tower at each wind velocity. This study provides an effective and practical approach to improve the efficiency of power plants in a windy area. [ABSTRACT FROM AUTHOR]

This paper extends the in situ adaptive tabulation (ISAT) algorithm for accelerating the simulation of complex heterogeneous chemical kinetics within transient, three-dimensional, computational fluid dynamics (CFD). The ISAT algorithm, initially developed for homogeneous combustion kinetics, takes advantage of the fact that initial conditions for the chemistry in a particular cell (i.e., temperature and composition) may have been present in this cell or another cell earlier in the simulation. In such cases, the solution can be extracted from a tabulation of prior solutions more efficiently than solving the local kinetics problem. The ISAT algorithm uses efficient tabulation and retrieval algorithms, greatly accelerating the solution process. Illustrative results are based on the simulation of methane reforming in a catalytic microchannel reactor, considering coupled fluid mechanics, catalytic chemistry, and conjugate heat transfer. [ABSTRACT FROM AUTHOR]

This paper presents a computational fluid dynamics (CFD) simulation of a partial combustion lance (PCL) aiming to evaluate the effect of nozzle design on its performance. Four turbulence models, namely, standard k – ɛ (SKE), realisable k – ɛ (RKE), renormalised (RNG) k – ɛ and Reynolds stress model (RSM) were used. The combustion process was modelled using the species transport model, whereas the heat transfer was calculated by considering a combined convection–radiation boundary condition. The best CFD prediction was obtained using the second-order discretisation, standard pressure, unsteady solver and RSM with a 6% deviation from the experimental data. The modified nozzle design shows increases in excess of 45% of the peak combustion temperature in the PCL. The findings from this work may be useful for design retrofits of a PCL. [ABSTRACT FROM AUTHOR]

Today computational fluid dynamics (CFD) is widely used in industrial companies, research institutes and technical safety organizations to supplement the design and analysis of diverse technical components and large systems. Such numerical programs are applied to better understand complex fluid flow and heat transfer phenomena. In the last decades there is an increasing interest in the nuclear community to utilize such advanced programs for the evaluation of different nuclear reactor safety issues, where traditional analysis tools show deficiencies. Within the FP7 European project THINS (Thermal Hydraulics of Innovative Nuclear Systems), CFD and coupled 1D-3D thermal–hydraulic simulations are being carried out. These are dedicated to the analysis of the thermal–hydraulics of gas, liquid metal and supercritical water cooled reactors. Such concepts utilize innovative fluids, which have different properties from the ones used in the current nuclear reactors. In order to improve the thermal–hydraulic predictions of their behavior, CFD development, application and validation activities are performed within THINS. This overview paper highlights some of the CFD related work within the European project. [ABSTRACT FROM AUTHOR]

In the mechanical olive oil extraction process, heating is a common practice to promote the extraction of extra virgin olive oil, especially in a malaxer machine. Nevertheless, the process of heat transfer in the malaxer machine has not been fully explored. Temperature is a dominant factor in various physiochemical changes and in enzymatic reactions that occur during the malaxation process and a bad management of this parameter may affect the quality of the oil irreversibly. To establish a model of the temperature distribution of the whole malaxer machine, computational fluid dynamics (CFD) may be the most effective method. In this study, a mathematical model that analyses the fluid flow and heat transfer of an olive paste malaxing process was developed and solved using a finite element-based software (COMSOL Multiphysics). The objective is to study the olive paste behaviour in order to improve the design of a malaxer machine increasing the effect of the fluid flow on the heat transfer performance. The results of this paper show that particular design parameters influence the flow and temperature fields inside the malaxer machine. The validity of our mathematical model is corroborated by our experimental work on the olive paste heating in an experimental industrial malaxer prototype equipped with specifically designed kneading tools in an industrial continuous olive oil plant. Good agreement exists between the simulated and experimental results. The 3D CFD simulation model has been developed for two different geometry configurations of the machine. The temperature field and the trajectories of single particles determine an intermediate cold area that is wider in traditional malaxer machines (with a single set of blades) compared to the examined malaxer prototype (with a double set of blades). Additionally, a warm area close to the horizontal axis is shown to be smaller and characterised by higher temperatures. The presence of two sets of blades reduces the unevenness of the temperature field during olive paste kneading. [ABSTRACT FROM AUTHOR]

Light pipes transmit daylight into building interiors. Their installation into thermally insulated roofs of low energy buildings can be a problem because of thermal bridges and condensation problems. This article is focused on a CFD simulation thermal analysis that includes four variations of light pipes with a segment of a flat roof. Common light pipes with a hollow light guiding tube were compared to special light pipes containing an additional glass unit located inside the tube. The additional glass units increase thermal resistance and reduce condensation risks of the light guiding systems. The light pipes were compared in two different simulation models run in ANSYS Fluent software and the CalA program. Temperature profiles and air flow patterns of the cross sectional profiles of the light pipes served to determine the total heat transmittance and heat losses of the studied light pipes installed in a segment of a thermally insulated flat roof. The paper compares simplified 2D rotational–symmetrical numerical model based on the thermal diffusion equation with the complex 3D CFD numerical simulation. The results confirm that the simplified 2D numerical model is suitable for the thermal evaluation of the light pipes containing an additional glass unit, too. The additional glass unit with the triple glass improves thermal resistance up to 88% in case of light pipe with diameter 600 mm and reduces optical transmittance to 28%. [ABSTRACT FROM AUTHOR]

This paper presents both preliminary experimental and numerical studies of pressure drop and heat transfer characteristics of Stirling engine regenerators. A test bench is designed and manufactured for testing different regenerators under oscillating flow conditions, while three-dimensional (3-D) numerical simulations are performed to numerically characterize the pressure drop phenomena through a wound woven wire matrix regenerator under different porosity and flow boundary conditions. The test bench operating condition range is initially determined based on the performance of the commercial, well-known Stirling engine called WhisperGen™. This oscillating flow test bench is essentially a symmetrical design, which allows two regenerator samples to be tested simultaneously under the same inflow conditions. The oscillating flow is generated by means of a linear motor which moves a piston in an oscillatory motion. Both the frequency and the stroke of the piston are modified to achieve different test conditions. In the numerical study, use of a FVM (finite volume method) based CFD (computational fluid dynamics) approach for different configurations of small volume matrices leads to a derivation of a two-coefficient based friction factor correlation equation, which could be later implemented in an equivalent porous media with a confidence for future regenerator flow and heat transfer analysis. [ABSTRACT FROM AUTHOR]

• Two different immersed surfaces are studied: a sphere and a vertical cylinder • CPFD properly predicts low heat transfer rates in defluidized regions • TFM approach over predicts heat transfer rate up to 77 % compared to CPFD • Different local heat transfer mechanisms are observed on the heat transfer surface This paper presents a 2-D numerical simulation of a freely bubbling fluidized bed with immersed surfaces, using the Computational Particle Fluid Dynamics (C P F D) model implemented in the Barracuda commercial software. The heat transfer coefficients obtained are compared with an experimental study available in the open literature and numerical simulations based on the two-fluid model approach performed by other authors. Two different immersed surfaces, representing spherical and cylindrical geometries were studied. The simulations results show different heat transfer mechanisms, depending on the angular position in the two immersed surface geometries studied. The time average heat transfer coefficient around the whole heat transfer surface were 25 % and 38 % lower than the experimental study, for the cylindrical and spherical surfaces, respectively. These differences are lower than the results obtained with the two-fluid model approach reported in the open literature. The numerical results indicate that CPFD-Barracuda is able to properly simulate the heat transfer and the dynamics of the bed in defluidized regions, such as on the top of an immersed surface, where the two-fluid model fails and overpredicts the heat transfer rate. [ABSTRACT FROM AUTHOR]

The operating temperature influences both the electrical efficiency and thermal efficiency of a photovoltaic thermal (PVT) module. In this paper, a simplified thermal transfer model is used to search for the optimal structure of a high efficiency PVT module. For an aluminum collector with a given thickness, the optimal ratio between size and spacing and cross sectional geometry of the flow channels has been found with numerical simulations using computational fluid dynamics (CFD). The temperature distribution of the PVT module constructed with an aluminum collector is found to be more uniform than the conventional sheet-tube PVT module, due to using an improved thermal transfer mode based on the surface contact. To evaluate the impact of cross sectional geometry, the operating temperatures, electrical and thermal efficiencies of two PVT modules designed with rectangle and arch geometries are examined under outdoor conditions. Measured experimental intercept values of instantaneous thermal efficiencies are close to simulation results, demonstrating the utility of the approach as a reference for a new generation of PVT modules in future. • An effect of cross sectional geometries on the performance of the PVT module was investigated. • An optimal structure of the PVT module was provided by ratios of Si: Sp. • Good agreement presented in temperatures between numerical and experimental results. [ABSTRACT FROM AUTHOR]