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• A comprehensive review on numerical simulations of boiling heat transfer from atomistic to mesoscopic and macroscopic scales is presented. • Critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling are highlighted. • Potential solutions and future research in the field of boiling heat transfer using the MDS, LB and CFD methods are proposed. Boiling is an efficient heat transfer mode with significant potential for thermal management in high-power electronic equipment. However, a comprehensive understanding of the boiling process, which encompasses bubble nucleation, growth, coalescence, slipping, and detachment across various scales, remains challenging. Molecular dynamics simulation, lattice Boltzmann, and computational fluid dynamics methods are popular and powerful tools for investigating boiling heat transfer phenomena at microscopic, mesoscopic, and macroscopic scales. These methods enable researchers to uncover the underlying boiling mechanisms and propose heat transfer enhancement techniques. Therefore, this paper provides a comprehensive review of boiling heat transfer, spanning from atomistic to mesoscopic and macroscopic scales, utilizing these three numerical methods. It addresses critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling, and proposes potential solutions and future researches, supplementing our previous review [Some advances in numerical simulations of multiscale heat transfer problems and particularly for boiling heat transfer, Annu. Rev. Heat Transf., 6 (2022) 217–269]. Besides, by shedding light on the characteristics of these numerical methods in studying boiling heat transfer, this paper aims to foster their development and advance enhanced heat transfer technologies. [ABSTRACT FROM AUTHOR]

• A 3D thermal lattice Boltzmann model for simulating melt flow and heat transfer in CZ single crystal growth has been constructed. • The coordinate system transformation is adopted to extend the application of LBM on 3D cylindrical cavities. • The critical Reynolds numbers are increased by introducing a transverse magnetic field. A three-dimensional (3D) thermal lattice Boltzmann model of crystal growth is presented to study the magneto-hydrodynamics (MHD) mixed convection in the melt of Czochralski silicon single crystal growth in this paper. In order to solve the problem of lattice Boltzmann method's difficulty in solving the flow of 3D cylindrical cavity, the 3D physical model of crystal growth is transformed from the Cartesian coordinate system to the Curvilinear coordinate system. And the irregular grids in the Cartesian coordinate system are transformed into regular grids in the Curvilinear coordinate system to facilitate the establishment of the Lattice Boltzmann model. Under the above operations, two coupled distribution functions are established to solve the governing equations. Two D3Q19 models are adopted to solve the melt velocity and temperature. And the 3D lattice Boltzmann modeling of crystal growth with cylinder boundary is realized by interpolation in Curvilinear coordinate system. The comparison with the experiment and other references proves that the proposed model and boundary treatment scheme is correct and it proposes a new way for studying the 3D crystal growth or other similar 3D cylindrical cavity simulations. Furthermore, the unstable melt flow for two cases are also simulated in this paper and the critical Reynolds number are also obtained. Results show that the introduction of the transverse magnetic field enhances the stability of the crystal growth system and increases the adjustable range of crystal rotation parameter. The obtained conclusion has an important reference for adjusting the process parameters during the CZ crystal growth. [ABSTRACT FROM AUTHOR]

In this paper, fully coupled particle resolved direct numerical simulations are used for heat transfer calculations. For modelling fluid velocity and temperature, the immersed boundary method (IBM) is used. Particle-particle heat transfer is modelled using well-established relations from the literature. Two-way coupling between particles and fluid is obtained using a body force that is a function of the solid volume fraction present in the fluid grid cell. Results are compared with those of spouted bed experiments to validate the simulations, and good agreement is observed. At the end of the paper, several interesting characteristics of spouted bed are obtained using simulations. The speedup of the method used for simulations was tested on multi-core CPU clusters. It is observed that the simulations scales almost linearly with the number of CPUs (tests were done up to 500 CPUs), showing the capability of the particle resolved DNS to be used on multi-CPU architectures. Distribution of particle temperatures by (a) Simulations (b) Experiments at packed bed conditions; Distribution of particle temperatures by (c) Simulations (d) Experiments at spouting condition. [Display omitted] • Heat transfer in spouted bed is modelled using particle resolved DNS. • Simulations results agree well with experiments. • The underlying physics of heat transfer in a spouted bed is studied by simulations. [ABSTRACT FROM AUTHOR]

The packing is the core heat transfer unit in the cooling tower. To improve the heat and mass transfer performance of the counterflow wet cooling tower, a new double S-wave structure packing is proposed in this paper, and its performance in heat-mass exchange is investigated using a combination of numerical simulation and experiment. The flow heat transfer performance of the double S-wave packing with different sheet spacing, tilt angle and inlet area is studied by numerical simulation, and the law of different structural parameters affecting the packing is derived to determine its optimal structure. The thermal and resistance performance of double S-wave packing and conventional double-folded packing under different working conditions were studied in comparison with each other by building an experimental bench. The results show that the cooling efficiency of double S-wave packing is 13.74% higher and the pressure drop is 3.58% lower than that of double-folded packing. The research results can provide important guidance and reference for engineering practice. [ABSTRACT FROM AUTHOR]

Abstract In this paper, numerical simulations are carried out to analyse the influence of geometrical parameters on the heat transfer and flow patterns for six spirally corrugated tubes at turbulent flow (R e = 25 × 10 3 ). Some previous studies are restricted to 2-D analyse or 3-D numerical simulations considering a reduced section length of the tubes as the computational domain in order to reduce the computation cost. The novelty of this paper is to perform a high cost numerical simulation using a computational domain with a length of the tube (L = 2 m) and dimensions similar to those used in commercial applications. In addition, a new non-dimensional parameter, the corrugation shape factor (C S F) is proposed, which takes into account the width of the corrugation, a geometrical dimension obviated in other studies. A 3-D unstructured tetrahedral mesh was created, and a grid independence analysis of the numerical solution was performed. The Realizable k − ε turbulence model with enhanced wall treatment was proposed to model the turbulence. Simulations were validated with experimental data available in the open literature. Regarding the ratio between height and diameter (H / D), the Nusselt number augmentation seems to augment as the H / D increases for low values of H / D (H / D < 0.05), whereas it decreases for values of H / D > 0.05. Considering the cases with the same diameter, in all the analysed cases, the pressure drop and Nusselt number augmentation decrease linearly as the P / D increases as well as both parameters increase linearly as the H / D , S I and C S F increase. The numerical results indicate that the augmentation of the Nusselt number and the friction factor (compared with the smooth tube) increases approximately linearly with the C S F in the range tested in this work (4 < C S F < 18), regardless of the tube diameter. [ABSTRACT FROM AUTHOR]

Highlights • Gas injection increases a maximal velocity within the boundary layer. • Gas suction decreases an overshoot phenomenon till his absolute vanishing. • Separation of thermal boundary layer at gas injection. • Conventional point of thermal boundary layer separation is proposed. Abstract The paper reports on numerical simulation of an accelerated laminar boundary layer over the heated permeable wall. Xenon was used as a working gas. The finite difference method was applied to solve the system of laminar boundary layer equations with variable gas properties. It has been shown that the gas injection in the mentioned conditions increases maximal velocity within the boundary layer and decreases skin-friction coefficient and thermal Stanton number. On the contrary, gas suction decreases an overshoot phenomenon down to its vanishing and increases skin-friction coefficient and thermal Stanton number up to the value at asymptotic suction. The paper considers a separation of thermal boundary layer at gas injection. The proposed conventional point of this separation is situated farther from the leading edge than the separation point of the dynamic boundary layer. [ABSTRACT FROM AUTHOR]

Highlights • Rich physical discussions regarding magnetic effects in the equation of energy using the pipe flow as a test case. • Proposition of a correction parameter on our previous theory. • New numerical simulation results based on Langevin Dynamics to justify the physics explored in our first paper. • Identification of functional dependencies between the correction parameter and the volume fraction of particles using our new numerical simulation results. Abstract This work presents important additional discussions regarding our recent publication: Heat transfer increase for a laminar pipe flow of a magnetic fluid subjected to constant heat flux: an initial theoretical approach. In our first paper we proposed a theoretical law to predict the mean Nusselt number for a laminar pipe flow of a magnetic fluid subjected to a constant heat flux from the walls. We have considered a magnetic fluid flowing inside a small pipe under the action of an uniform magnetic field. From our analysis we have proposed a functional dependency on the mean Nusselt number with respect to magnetic parameters. This increase/decrease in the heat transfer rates inside the pipe arose from a production term in the equation of energy. We have interpreted it as a mechanism related to the deformation of clusters of magnetic particles formed in the microstructure of the fluid. In the present manuscript we present additional physical discussions regarding our previous derivation and propose a correction parameter ζ in this mechanism. This parameter intends to provide a more realistic modeling of this phenomenon, consistent with the restrictive assumptions assumed on our first paper. We also present some new results using Langevin Dynamics to show that this production term spontaneously appear when a non-superparamagnetic fluid is subjected to a shear field under a constant uniform magnetic field. [ABSTRACT FROM AUTHOR]

Highlights • We establish a more reasonable vortex cooling model for gas turbine blade leading edge. • Experiments and numerical simulations are conducted. • Influential mechanisms of temperature ratios are revealed more deeply. • This paper provides a guideline for the gas turbine blade leading edge cooling. Abstract Based on combined Infrared Thermography and numerical simulations, the heat transfer and flow performance in a vortex cooling configuration with 5 nozzles were investigated at various Reynolds numbers and temperature ratios. Validation was conducted between experiment and numerical simulation and results showed that the k - ω turbulence model has the best accuracy to predict the vortex cooling behavior. High Nusselt number regions appear under each nozzle in all cases, but air from downstream nozzles is easily impacted because of the “anti-cross flow” ability. Along the axial direction, targeting surface has a fluctuant heat transfer coefficient distribution, while along the circumferential direction, the heat transfer coefficient decreases linearly. Overall heat transfer coefficient increases with increasing Reynolds number and decreasing temperature ratio. Especially, the influence of temperature ratio under 3 setting methods is compared and discussed in detail, revealing the fundamental impact of temperature ratios. A globally-averaged Nusselt number correlation is presented over 22,000 < Re D < 39,000, 0.88 < TR < 0.94, and results in this paper are compared with other vortex cooling systems. [ABSTRACT FROM AUTHOR]

Abrasive liquid nitrogen (L-nitrogen) jet fracturing is a novel technology, which is expected to be suitable for hot dry rock (HDR) fracturing. In this paper, numerical simulation has been presented to analyze the fluid flow, heat transfer and thermal stresses distribution in HRD fracturing with abrasive L-nitrogen jet. The simulation is carried out with a three-dimensional model in transient state. The low Reynolds number κ-ε model is employed to accurately predict the near wall flow. The heat transfer between L-nitrogen and hot rock on solid-liquid interface is computed by an inverse method of conjugate heat transfer. The thermo-elastic model is used to calculate the thermal stresses distribution in rock. Numerical results indicate that abrasive L-nitrogen jet has a better performance in perforation than abrasive water and supercritical CO 2 jet. Great tensile stress distributes in the region near interface, however, its affected depth is limited. This tensile is expected to be favor for fractures generation and growth along perforation direction. Short perforation length can promote the heat transfer on interface. The initial rock temperature has an important influence on values of thermal stresses. Experiments, in which the hot granite specimens with a small hole in center were impacted by L-nitrogen jet, have been conducted to validate the effect of thermal stresses on fracturing. The fractal method is adopt to quantitatively describe the flow and transport capability in rock masses. Experimental results show that numerous thermal cracks were generated on the interface. Rising rock temperature can significantly increase the number and size of thermal cracks and improve the connectivity of fractured rock, which are benefit to fracture initiation during L-nitrogen jet fracturing. The results in this paper would shed light on L-nitrogen jet fracturing for HDR as well as some high-temperature oil reservoirs. [ABSTRACT FROM AUTHOR]

This study focuses on the heat and moisture transfer behaviours in the Phyllostachys edulis (Moso bamboo) panels at various temperature and relative humidity (RH) conditions. Moso bamboo panels with different lamination methods were prepared by assembling bamboo strips from different layers of the bamboo culm wall. Dynamic coupled heat and moisture transfer experiments were conducted. Unsteady state numerical modelling was conducted by COMSOL Multiphysics™. A rigorous approach was adopted in this paper. A series of parametric studies of numerical simulation are firstly presented in this paper and then validated by the experiments. Both experimental and simulation results appear to be consistent with the results of measurements of the basic hygrothermal parameters, which demonstrates the robustness of the results. The temperature and RH results indicated that although the panel made from layers of the internal part of bamboo culm wall can provide good insulation performance, its ability to resist high RH variation is inferior to the layer from the external part of bamboo culm wall. The parametric study found that density is the most critical parameters to influence the temperature distributions in the transient state. The thermal conductivity dominates the temperature variation in the steady state. The water vapour diffusion resistance factor is the key parameter which influences the RH simulation results. Numerical simulation with moisture transfer shows better consistency than the simulation without moisture in both equilibrium and transient states. The results of this study demonstrated that the external part of the bamboo culm wall can be utilised to minimise the RH variation of the panel while the internal part is suitable for increasing the thermal insulation performance of the panel. [ABSTRACT FROM AUTHOR]

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]

Natural convection is one of the most important modes of fluid flow and heat transfer. This paper presents a review of the research on turbulent natural convection in rectangular cavities using numerical and experimental techniques. In this review we have attempted to summarize the published papers on this topic with some interesting and important results. Numerous configurations of the enclosures with different initial and boundary conditions, heat source locations and radiative properties of medium and walls have been considered under the effects of various parameters such as the Rayleigh and Prandtl numbers, surface emissivity, cavity inclination angle, thermal properties, etc. Finally, some suggestions have been provided for future studies in the considered area. [ABSTRACT FROM AUTHOR]

• A novel circuit gas gap heat switch (GGHS) is proposed for dilution refrigerators. • The GGHS features interleaved cylindrical walls forming a cavity filled with gas. • Heat transfer characteristics are simulated considering 4He actual gas properties. • Effects of operating and geometrical parameters on GGHS performance are studied. • Experimental results validate the proposed simulation model. The gas gap heat switch (GGHS) used for controlling heat transfer between different stages can be an important component for the precooling process of some dilution refrigerators. In this paper, a novel circuit GGHS is a rotationally symmetric heat switch assembly with annular fin arrangements to strengthen the heat-transferring effect. A numerical model considering 4He actual gas properties is proposed to investigate the heat transfer characteristics in the circuit GGHS, in which the effects of charge pressure, cold end temperature, thickness and length of walls on the mean thermal conductance (MTC) are studied. Simulation results show that the MTC increases with the growing cold end temperature, wall thickness, and length, respectively. Given the pressure of 10 kPa, cold end temperature of 4.2 K, wall thickness of 0.97 mm, and wall height of 66 mm, the theoretical MTC is 0.828 W/K. Experimental results indicate that the proposed simulation model is reasonable. Furthermore, the increment of the MTC decreases with the growing temperature. The GGHS used in experiments had a cold end temperature of 4 K, wall thickness of 0.65 mm, and wall height of 33 mm; the measured MTC was 0.219 W/K. Only when the temperature is above 10 K does the charge pressure have a pronounced effect on the MTC. This study provides helpful theoretical guidance for the design and optimization of the circuit GGHS. [ABSTRACT FROM AUTHOR]

The particle sedimentation and phase change in the mushy zone are accompanied by a variety of complex transfer processes involving momentum, heat, and mass. It is very important to investigate the coupling process of particle settling and melting in the mushy zone for revealing the mechanism of the solid-liquid phase change. In this paper, the motion process and heat transfer characteristics of an elliptic particle as well as two particle interactions in the vertical channel were solved by the finite element method with the consideration of coupling of the phase change and motion. The Arbitrary Lagrangian-Euler Method (ALE) was employed to track the interface of the moving particle and fluid. Results show that the elliptical particle settles along the channel's center line vertically under gravity and undergoes phase change simultaneously. Uneven velocity and temperature fields surrounding the particle are generated by sedimentation, resulting in the particle with an irregular shape. When the initial position of the particle deviates from the center line, the particle deflects during the sedimentation and shifts both left and right in the X -direction. The different initial angles of settling also affect the sedimentation velocity and melting rate. When two particles settle simultaneously, they will lead to complex movement and heat transfer due to the interaction between each other. If the arrangement is eccentric, both particles undergo complex deflections during sedimentation, leading to different melting rates for the two particles. [ABSTRACT FROM AUTHOR]

• This paper reports on calculation of dislocation density in 200 mm SiC crystal grown by PVT method. • Numerical study based on a model was carried out for modeling of dislocation density. • Evolution of crystal growth interfaces were captured using a dynamic mesh technique. Physical vapor transport (PVT) is the dominant method for growing 200 mm SiC crystals, and the crystals produced by this method still have dislocations, which affect the performance of the device. In this study, the finite element analysis of 200 mm SiC crystal growth has been conducted to investigate the influencing parameters on the dislocation density. The calculations are based on the model of multiple resistance heating. The transient heat transfer for the crystal growth is first calculated. A dynamic mesh technique is then employed to consider the shape evolution of the crystal during the growth. Finally, the distributions of the internal stress and dislocation density have been calculated based on the Alexander-Haasen model. The comparison among different parameters provides guidance for reducing the thermal stress and dislocation density in the 200 mm SiC crystal growth. [ABSTRACT FROM AUTHOR]

The use of recycled paper in HDPE (High Density PolyEthylene) matrix composites has recently been introduced as an interesting alternative to traditional recycling process for paper. HDPE is also used as double wall greenhouse glazing because panels are easy to install, UV stabilized, and affordable. These type of products must also be strong enough and durable in order to react under tensile loads provided by wind and harsh weather conditions. An interesting idea may be the insertion via injection moulding of chopped basalt and waste paper – i.e. , two natural products – in pure HDPE samples. It completely follows the environmental sustainability concept centred on the triple “ re ”, i.e. , re cycle, re use and re duce. The research presented herein starts with a Differential Scanning Calorimetry (DSC) inspection of pure HDPE and HDPE mixed with 5% by weight of waste paper plus 5% by weight of chopped basalt as fillers in order to obtain an insight related to the temperature at which possible thermal events (endothermic or exothermic) occur. The dog-bone samples were also inspected under UV conditions (380 nm) before and after tensile tests. The latter approach was simulated firstly by Comsol Multiphysics® computer program, and secondly recorded in real time via thermographic inspections. The temperature variation in a region of interest (ROI) selected at the centre of the samples was mapped in the time during the inspection by infrared thermography (IRT) method using a pseudo-static matrix reconstruction algorithm realized in Matlab® environment. Instead, the combined use of thermocouples aimed at emphasizing the knowledge of the heat transfer in the time due to the mechanical stress applied at the borders of the inspected samples. The aim was to understand whether modified HDPE can (or cannot) be a valid competitor of pure HDPE for the production of semi-transparent and robust panels. [ABSTRACT FROM AUTHOR]

Fluids that are infused into the human body must be at a temperature that is compatible with the internal thermal state of the body. Since infusants are typically stored at temperatures that are too low for compatibility, a heating means is required to achieve the appropriate infusion temperature. This paper sets forth a synergistic investigation involving coupled experimentation and numerical simulation of the characteristics of one of the main categories of body-fluid heating means. The methodology developed here serves equally well as a design optimization tool. The paper encompasses two stages: (a) an experimental and numerical evaluation of a generic warming device in common use and (b) a redesign utilizing the same tools to elevate the performance of devices of this category. The numerical simulation dealt with steady and unsteady three-dimensional fluid flow and heat transfer which are endemic to devices of this kind. The two-pronged approach developed here was shown to be capable of coping with an operating feature called stopflow wherein an officiating physician orders an immediate cessation of fluid flow. The thermal events following stopflow are well described by the numerical simulations. [ABSTRACT FROM AUTHOR]

Experimental studies used for evaluation of quality of the fire-fighters protective clothing are based on irradiation of the clothing sample by an external pulse of a given duration. The other side of the clothing sample is in contact with a material imitating the human skin, which has properties different than these of the skin. Therefore the problem considered in this paper was: what conditions should be satisfied to convert the experimental results, carried out on the stand with a skin-imitating material, to the case of the real human skin with different properties for which the experiments are not made? This will allow for better evaluation of the clothing protecting properties and estimation of the possible burns. Due to multilayer structure of the protective clothing and skin as well as complex phenomena involved in energy transfer through them the problem is very complicated. Therefore at the first attempt to solve the problem a simplified two-layer, clothing-skin system irradiated from external heat source was considered in the present paper. Temperature variation in the system was initially determined using the in-house 1D numerical code accounting for heat conduction in the skin and clothing, non-grey thermal radiation absorption, scattering and emission together with the water vapour diffusion and phase transformations of bound water to the gaseous phase in the protective clothing. Then the analytical model for temperature variation in the considered system was proposed and solved using the Laplace transform approach. In the latter model water vapour diffusion with its phase transformations were excluded and radiation emission was neglected. The analytical model allowed for deriving a formula which can be applied for converting the skin surface temperature obtained from the experiments carried out using the skin-imitating material to the material having skin properties. In order to verify the range of validity of the formula the analytical predictions were compared to the results of numerical simulations of temperature in the protective clothing-skin system considering cases in which different phenomena were included. For the majority of the cases studied good matching between analytical and numerical results was obtained for time corresponding to the heating and cooling processes of the protective clothing-skin system. [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]

Thermal energy storage is a key technology for the solar thermal power plants. This paper set up a single spiral heat storage tank using concrete as heat storage material and Cu-water nano-fluid as heat transfer fluid. In this paper, temperature distribution and the effects of the parameters i.e., inlet temperature and inlet velocity, on the charge time of the storage tank have been investigated based on numerical simulation. The results show that the charge time of the storage tank is about 72,000 s before the other factors have been changed. The charge time of the storage tank increases with the decrease of the inlet temperature and inlet velocity. [ABSTRACT FROM AUTHOR]

This paper research downhole temperature distribution in oil production and water injection using reproducing kernel Hilbert space method (RKHSM) for the first time. The aim of this paper is that using the highly accurate RKHSM can solve downhole temperature problems effectively. According to 2-D mathematical models of downhole temperature distribution, the analytical solution was given in a series expansion form and the approximate solution was expressed by n -term summation of reproducing kernel functions which initial and boundary conditions were selected properly. Numerical results of downhole temperature distribution with multiple pay zones, in which different radial distance and different injection–production conditions (such as injection rate, injection temperature, injection time, oil layer thickness), were carried out by mathematical 7.0, and numerical results correspond to general knowledge and show that use RKHSM to research downhole temperature distribution is feasible and effective. [ABSTRACT FROM AUTHOR]

This paper presents a computational heat transfer model of a rotary kiln used for the production of rutile titanium dioxide by the calcination of paste-like hydrous titanium dioxide. The work details the modelling of several chemical reactions occurring in the solid bed region along with turbulent convection of gas, radiation heat exchange among hot gas, refractory wall and the solid surface, and conduction in the refractory wall. Finite-difference techniques are used and the steady state thermal conditions are assumed. The kiln is divided into axial segments of equal length. The solution is of marching type and proceeds from the solid inlet to the solid outlet. The direction of gas flow is opposite to that of the solids. Mass balance of each species in the solid charge, and mass and energy balances of the solid and gas in an axial segment are used to obtain solids and gas temperatures, and species concentration at the exit of that segment. The kiln length predicted by the present model is 45.75 m as compared to 45 m of an actual kiln reported by Ginsberg and Modigell (2011). The steady-state axial gas and solid temperature profiles have been also satisfactorily validated with the numerical results of the aforementioned paper. The output data consist of refractory wall temperature distribution, the axial solids and gas temperature profiles, axial solids composition profile, the length required for drying of the solid charge and the total kiln length required to achieve 98% conversion of anatase TiO 2 to rutile TiO 2 . A detailed parametric study with respect to the controlling parameters such as percent water content (with respect to dry solids), solids flow rate, gas flow rate, kiln inclination angle and kiln rotational speed lent a good physical insight into the rutile-TiO 2 production process in a rotary kiln. [ABSTRACT FROM AUTHOR]

Actual thermal response tests, used to estimate the subsurface thermal conductivity in the geothermal domain, do not provide any estimate on the velocity of the groundwater flow and its orientation. These parameters are important for sizing geothermal borefield, since they influence the heat transfer around a geothermal borehole and the surrounding ground. To correct this shortcoming, a conceptual test has been developed in which heating cable sections inject heat in a borehole. Three temperature probes are strategically located at the borehole edge. This paper applies inverse heat transfer strategies to this thermal response test concept in order to identify the ground thermal conductivity, as well as the groundwater flow velocity and its direction. The suggested thermal response test and parameters estimation methodology are detailed. The influence of initial guessed values for the three unknown parameters was also studied. The work presented in this paper was carried out by numerical simulations. [ABSTRACT FROM AUTHOR]

In this paper, the capacity of a natural circulation loop for transferring heat from a heat source to a heat sink has been analyzed. It is concluded that the capacity of the natural circulation loop depends on the conversion efficiency of the thermal energy from the heat source to the driving force for the circulation of the flow. The low conversion efficiency leading to weak driving force in such loops has been demonstrated analytically and validated through simulation results. This issue has resulted in a low heat transfer capacity in the circulation loop. To increase the heat transfer capacity, one has to improve this efficiency. To meet such a need, a novel efficiency booster has been developed in this paper. The booster essentially increases the flow driving force and hence significantly improves the overall heat transfer capacity. Design and analysis of this booster have been performed in detail. The performance has been examined through extensive computer simulations. It is concluded that the booster can indeed drastically improve the heat transfer capacity of the natural circulation loop. [ABSTRACT FROM AUTHOR]

To optimize the uneven distribution of flow field in the mechanical draft cooling towers, four cases for deflector plates are proposed. Based on the analysis of the velocity and temperature fields in the tower, the effect of the deflector plates to strengthen the local heat transfer is investigated. In addition, the thermal and ventilation performance is investigated with different fan speed and circulating water flow rate. The results show that the arrangement of the deflector plates can guide the ambient air into the inefficient heat transfer region. Of the four cases studied, Case 4 has the optimal performance. Compared with the original tower, the outlet water temperature reduces by 0.22 °C, the cooling efficiency increases by 1.53%, and the Merkel number increases by 6.35%, while the ventilation rate keeps at 699 m3/s. The thermal performance of Case 4 is better than that of the original tower when only the fan speed or the circulating water flow rate are changed, and it is improved with the increase of the fan speed and the decrease of the circulating water flow rate. This paper can guide the optimization of enhanced local heat transfer in mechanical draft cooling towers. • A numerical model of a mechanical draft cooling tower is developed. • The model can be used for local heat transfer enhancement studies. • Deflector channels are created by the action of the deflector plates. • Regions with lower heat transfer efficiency are optimized. • Utilizing deflector plates exhibits the capacity to enhance the thermal efficiency. [ABSTRACT FROM AUTHOR]

• Normal mode stability analysis performed on counjugate heat transfer (CHT) 1D problem. • Stability analysis reveals existence of lower stability bound for coupling coefficient in Robin-Dirichlet interface boundary conditions. • Validity of stability analysis results is verified empirically. • Stability analysis is extended to more general CHT problems. This paper aims to evaluate and validate a mathematical coupling model, based on a normal mode stability analysis, for steady conjugate heat transfer problems. In order to achieve this goal, an aerothermal computational code was designed specifically ex novo to meet the needs of the current study. A large number of numerical simulations were carried out within the framework of the Dirichlet-Robin fluid-solid interface procedure and with conditions strictly identical to those adopted in the mathematical model. These coupled computations have been synthesized in five representative cases. The computed results show that the stability limit and the numerical Biot number derived from the coupling model are very reliable parameters. Many other physical and numerical aspects, such as the splitting of the coupling period are also considered in this work from which some interesting and promising results are obtained. At the end of this paper, we assess the general potential, limits and constraints of the mathematical model and its extension to multidimensional cases is discussed. [ABSTRACT FROM AUTHOR]

In this paper an attempt has been made with experimental as well as theoretical analysis to find the variation in heat transfer coefficient with Reynolds number and numerical simulation attempt has also been done with different turbulence models in forced convection turbulent flows. Experimental analysis was carried out by using forced convection experimental setup at constant power supply to the heater and by varying the different flow rates of air with the help of a blower. The tube heat exchanger is fully insulated. In turbulent flows, the heat transfer coefficient is greatly affected by the turbulence, hence in this paper numerical analysis was carried out with different turbulence models and analysis has been done with both experimental and theoretical values to find the best turbulence model. [ABSTRACT FROM AUTHOR]

In this paper an attempt has been made to find the enhancement of heat transfer with different types of twisted tape inserts (TTWTT), that is inserts of twisted tape over the twisted tape at an angles of 90 and 180 degrees twist. In plain twisted tape at the location of swirl flow the mixing of fluid particle are not satisfactory so that the heat transfer is somewhat less than the turbulent regime so in this paper numerical analysis of thermal and hydraulic characteristics of water flow has been studied by inserting twisted tapes over the main twisted tape at the location of uniform stream lines forming zone. The numerical simulation was performed in order to gain an understanding of physical behaviour of the fluid flow (decaying swirling flow field), fluid temperature and local Nusselt number characteristics of a tube fitted with TTWTT. [ABSTRACT FROM AUTHOR]

In order to understand thermal characteristics of high altitude airships, the numerical simulation of convective-radiative coupled heat transfer performance at a certain time in a day is carried out using Ansys Fluent software. The governing equations of flow are using the compressible Reynolds averaged N-S equations and standard κ-ɛ turbulence model. A non-uniform temperature distribution boundary condition in the complex thermal environment and heat transfer mechanism is applied on the surface of high altitude airships based on user definition function (UDF) programming. The paper then simulates the external convection heat transfer characteristics at various flow conditions at 40 degrees north latitude at the altitude about 20 km at noon in the summer solstice. Finally, the corresponding calculation results of the surface temperature distribution, the flow field distribution and the convective heat flux distribution of airship envelope are presented, which are also analyzed in this paper. [ABSTRACT FROM AUTHOR]

In the numerical simulation of heat transfer and fluid flow problems, residual is often adopted as a convergence criterion. Since residual is susceptible to several factors, such as the property of physical problems, the expression form of discretized equations, the scale of grid and the nondimensionalization of governing equations, the converged value differs greatly in different cases. Thus, it is hard to set a specific residual value in a specific numerical calculation. To give a universal convergence criterion in this paper, a residual based on normalization idea is proposed. Range of specified values of this improved convergence criterion for general heat transfer and fluid flow problems is also given in this paper. Numerical simulation results indicate that the improved convergence criterion based on normalized residual is much more reasonable than the one based on traditional residual. [ABSTRACT FROM AUTHOR]

The advanced numerical simulation of a realistic physical system typically involves multi-physics problem. For example, analysis of a LWR core involves the intricate simulation of neutron production and transport, heat transfer throughout the structures of the system and the flowing, possibly two-phase, coolant. Such analysis involves the dynamic coupling of multiple simulation codes, each one devoted to the solving of one of the coupled physics. Multiple temporal coupling methods exist, yet the accuracy of such coupling is generally driven by the least accurate numerical scheme. The goal of this paper is to review in detail the approaches and numerical methods that can be used for the multi-physics temporal coupling, including a comprehensive discussion of the issues associated with the temporal coupling, and define approaches that can be used to perform multi-physics analysis. The paper is not limited to any particular multi-physics process or situation, but is intended to provide a generic description of multi-physics temporal coupling schemes for any development stage of the individual (single-physics) tools and methods. This includes a wide spectrum of situation, where the individual (single-physics) solvers are based on pre-existing computation codes embedded as individual components, or a new development where the temporal coupling can be developed and implemented as a part of code development. The discussed coupling methods are demonstrated in the framework of LWR core analysis. [ABSTRACT FROM AUTHOR]

Nonplanar solidification front causes defects in directional solidification process. The transverse temperature gradient, which can be divided into radial temperature gradient and circumferential temperature gradient, is one of the reasons causing the nonplanar solidification front. This paper presents a method by varying the wall thickness of the mould to decrease the circumferential temperature gradient and hence flatten the solidification front. The equations for calculating the wall thickness were deduced by heat transfer analysis. The optimized mould contour was obtained by solving the equations numerically. It was proved by simulation that the circumferential temperature gradient can be almost reduced by 40%. Furthermore, the starting time difference of solidification at one cross section along the circumferential direction almost reduces by half. Additionally, this paper also researches the influences of parameters of the mould and the furnace etc. on the shape of the mould and the circumferential temperature gradient. The circumferential temperature gradient decreases as the thermal conductivity and the emissivity of the mould increase. And the temperature of the cooling ring impacts little on the circumferential temperature gradient. According to the development trend of the blade manufacturing, this method may be applied widely. [ABSTRACT FROM AUTHOR]

• A numerical model of the nitrogen condensation is developed to predict the performance at different practical operations. • The fin efficiency η of the tall fins at low temperature (<120 K) can be as high as 0.7. • An improved correlation of condensation heat transfer is proposed. • The 1 kW cooling power require only 1.5 K temperature difference at 77 K. Of importance is the study of the cryogenic condensation as it physically behaves differently owing to the interesting fluid properties of cryogens appearing at extremely low temperature. In the paper, we focus on numerically exploring the nitrogen condensation in mini channels having the high aspect ratio of 12.5 towards the application of the kW-scale cooling power. It is found that the heat transfer rate and coefficient lie in between the Nusselt condensation (NC) and extended surface condensation (ESC) solutions among the selected operational conditions. The numerical results reveal that the longitudinal conduction plays a role. Interestingly, the reversing flow happens as the inlet flow rate is insufficient to satisfy the nitrogen condensation and particularly it becomes dominant at the larger temperature difference. Most importantly, an improved correlation of the cryogenic condensation heat transfer is proposed and strongly suggests that the fin efficiency η can be as high as 0.7, which is different from the expectation that tall fins are ineffective in condensation. In terms of the 1 kW cooling power, the high-aspect-ratio fin exchanger studied just needs the temperature difference of 1.5 K and it can improve 10 times the efficiency of condensation. It is important that the studies can inform the optimization of the cryocoolers. [ABSTRACT FROM AUTHOR]

The Partially-Averaged Navier–Stokes (PANS) method is intended to provide the optimum fidelity on any given numerical grid while varying seamlessly between the Reynolds-Averaged Navier–Stokes (RANS) model and direct numerical simulation (DNS). This method belongs to so called bridging or seamless methods. The PANS method has been more often used in the research community but also in the industry in last years. This is probably due to its simplicity, robustness and recent theoretical extensions as well as due to the detailed validations on the number of complex cases presented in many publications. As for any relatively new modeling approach, the list of validation cases should be further extended. Therefore, an additional test case is included in this paper. The PANS method is validated here for the fluid flow and heat transfer in a matrix of surface-mounted cubes. The ζ-f variant of PANS was used and compared with its RANS counterpart. The calculation procedure, which now includes the energy equation as well, is explained. A brief description of on-going developments and possible next improvements is also given in this paper. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this paper is to define the guidelines for the design of new boiling test sections with a large number of artificial nucleation sites during nucleate boiling for thin substrates horizontally immersed in a saturated liquid with artificial cavities located on the upper surface. The findings of numerical simulations of pool boiling heat transfer for a single bubble and for a large number of nucleation sites based on the analysis of experimental cases were analysed. Dedicated test sections were used in experiments for the study of boiling mechanisms and interactions between active sites so that the numerical models representing the physics of the problem could be improved. The hybrid nature of the code used in this study, combining the complete solution of the three-dimensional time-dependent energy equation in the solid substrate with semi-empirical models representing the physical phenomena occurring in the liquid side, in a simplified way, allows a large number of simulations in a reasonable computational time. The present paper focuses in the first part on the capability of the model to reproduce the experimental results for various conditions, while in the second part, the results for a large number of nucleation sites are analysed. Regarding the single bubble growth, two series of simulations will be presented in this paper: the first one analyses the mechanisms of nucleate boiling on a silicon substrate immersed in the dielectric fluid FC-72. The second series studies the behaviour of bubbles on metallic substrates, platinum and titanium, in saturated water. In the last section, the effect of the position of a site during simulations of a large population of sites (of the order of 100) on the waiting time, growth time, type and occurrence of coalescence and the thermal characteristics is presented. [Copyright &y& Elsevier]

• The numerical simulation of HPHE applied in RHRS is proposed for the first time. • Heat transfer performance of 3-D HPHE with thermal resistance model. • The effects of diameters, Re and arrangements on heat transfer are discussed. The residual heat removal system (RHRS) discharges residual heat from the reactor core and other equipment when the nuclear power plant is shut down. Heat pipe heat exchanger (HPHE) has the characteristics of high reliability, stability, volume heat capacity and modular operation, which is suitable to be used in RHRS. In this paper, a three-dimensional numerical simulation and experimental study of HPHE under steady-state turbulent flow and heat transfer with thermal resistance network are performed. The simulation and experiments are completed under the operating condition of residual heat removal system. The validation of the computational model is performed by comparing the experimental data and empirical correlation of average Nu in condenser and evaporator. The maximum deviation is less than 10%, which means that the simulation is accurate enough. Then investigation compares heat transfer and turbulent flow of HPHE with two bundle arrangements, two types of heat pipe diameters and different Re. When the flow is in the state of vigorous turbulence (Re ≈ 104), the heat transfer power of each heat pipe with diameter of 16 mm exceeds 2.65 kW. For heat pipe with the diameter of 19 mm, heat transfer performance of each is improved by about 3% to 6% than heat pipes with the diameter of 16 mm. The results show that HPHE not only has safety advantage of avoiding leakage of radioactive liquid, but also has good heat transfer performance applied in RHRS, which verified its feasibility. [ABSTRACT FROM AUTHOR]

• Fully resolved simulations of fluid-particle systems. • Surface reaction with significant heat effects. • Temperature-dependent reaction rates determined by the Arrhenius equation. • Simulation parameters adopted from realistic POX reaction. In this paper, an efficient ghost-cell based immersed boundary method (IBM) is used to perform direct numerical simulation (DNS) of reactive fluid-particle systems. With an exothermic first order reaction proceeding at the exterior particle surface, the solid temperature rises and consequently increases the reaction rate via an Arrhenius temperature dependence. In other words, the heat and mass transport is dependently coupled through the particle thermal energy equation and the Arrhenius equation, and they offer dynamic boundary conditions for the fluid phase thermal energy equation and species equation respectively. The fluid-solid coupling is enforced at the exact position of the particle surface by implicit incorporation of the boundary conditions into the discretized momentum, species and thermal energy conservation equations of the fluid phase. Different fluid-particle systems are studied with increasing complexity: a single sphere, three spheres and a dense array consisting of hundreds of randomly generated particles. In these systems the mutual impacts between heat and mass transport processes are investigated. [ABSTRACT FROM AUTHOR]

• Applying strongly-coupled fluid-solid model in thermal field. • Coupling coolant channel, hot channel, film hole together. • Analyzing the temperature distribution near the interface. • Analyzing the global flow structure. This paper applied the state-of-the-art flow simulation method, i.e. Direct Numerical Simulation (DNS), and strongly coupled the DNS with the heat-transfer governing equations to solve the flow and thermal problems in 3-dimensional (3D) film-cooling structures in the aero-engine. The strongly-coupled fluid-solid simulation was conducted to study the heat exchange on the wall surfaces and was compared to the case with the adiabatic heat-transfer boundary condition on the wall surface. The inlet condition was given as the uniform flow. The width of the delivery channel was used as the characteristic length. The Prandtl number was set at Pr = 0.71. The DNS was performed at lower Reynolds number of Re = 500 with the far-field condition being applied in the y and z directions. The dimensionless velocity, turbulence intensity, pressure, and temperature were analysed. The results suggested that the coolant flow was driven by the pressure in the delivery channel with strong vortical interactions and high turbulence shears. In the simulation with the strongly-coupled fluid-solid model, the solutions presented the interactions in the heat transfer near interfaces, the temperature wall boundary layer was observed and presented approximate linear profiles near the wall surfaces. The effects of the delivery channel were concentrated beyond the linear layers. The study discovered two important fluid-solid coupling parameters and pointed out the necessity of applying the strongly-coupled fluid-solid heat-transfer model in the thermal simulation. [ABSTRACT FROM AUTHOR]

Workers exposed to hot environment experiences physiological changes, implicating human health and occupational safety. Phase change material (PCM) cooling vest is effective for personal cooling and thermal comfort. In this paper, the human-PCM-clothing model was constructed by combining the bioheat model and the PCM-clothing model to predict the cooling effect of PCM on thermal response in hot environments. The multi-segmented human bioheat model considered the effects of high temperature on metabolic rate, blood flow rate, and dehydration on the sweating rate. The PCM-clothing model analyzed heat and mass transfer between the PCM and its thermal environment. The model was tested with published data and the relative errors between the simulated and measured results were below 2.2% for mean skin and 0.33% for core temperature. The numerical parametric study was investigated to study the effects of the melting temperature and latent heat of the PCM packages on heat transfer. PCM with high latent heat and appropriate melting temperatures benefits body cooling performance. The equation for the optimum solution corresponding to the latent heat of PCM at different melting temperatures was derived. This study can guide the choice of PCM packages and the human thermal response analysis in a hot environment. • The human-PCM-clothing model was established to predict the cooling effect of PCM. • Multi-segmented bioheat model was improved for workers working in a hot environment. • PCM clothing model for heat and mass transfer between different layers was proposed. • The optimum latent heat for different melting temperatures of PCM was obtained. [ABSTRACT FROM AUTHOR]

Abstract: In the paper, being the first part out of two (the second one is focused on processes of heat transfer), results of the computer simulations of the flow in a circular pipe with a ball inserts turbulising the flow are presented. An influence of the diameter of balls and their longitudinal distance on a pressure drop in the turbulent flow as a function of the Reynolds number (for Re =10,000÷300,000) was analyzed. The investigations were carried out for different diameters of the balls (Db =7, 10, 13, 16 and 19mm) and different distances between them (L =20, 24, 28, 32, 36, 40, 48, 60 and 85mm) at a constant inner diameter of the tube (Dp =26mm). The results indicate that for the whole tested range of ball diameters and their longitudinal arrangements, there is an analytical dependency which allows for expressing a friction factor by the relationship: f =A⋅Re B. The constants A and B were functions of two variables: a dimensionless diameter of the ball (X = Db/Dp) and a dimensionless longitudinal distance between balls (Y = L/Dp). These two constants are closely related. They can be calculated analytically for the entire range of X and Y using a formula of the fourth order surface polynomial (15 coefficients of that polynomial are presented in the paper). The paper presents also experimental investigations carried out using a PIV (Particle Image Velocimetry) apparatus. A comparative analysis of selected inserts and numerical calculations was the goal. The obtained results show a good correlation between the experiment and computer modelling. [Copyright &y& Elsevier]

Highlights • First BeLi 2 F 4 loop to operate in 40 years. • Salt can withstand large temperature gradients. • Flow is stable but contains odd temperature jumps and spikes. • Heat transfer follows normal correlation behavior but is offset. Abstract A new large natural circulation FLiBe loop has operated for the first time in 40 years. Reaching temperatures above 700 °C with stable flow rates around 5 cm/s, the loop exhibits a range of flow phenomena pertinent to high temperature natural circulation safety systems for nuclear reactors. This paper discusses the system and several operating conditions spanning a wide range of behavior to aid the development of other systems, simulation benchmarks, or system-codes. Analysis of heat transport phenomena indicates that standard Nusselt number correlations apply to a first approximation. More accurate salt-freezing models may be necessary for air-cooled salt-systems due to the insulating affect of the low thermal-conductivity frozen salt layer on the cold pipe walls. [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 To overcome the deficiencies of parallel-flow shell and tube heat exchanger with round rod baffles (RRB-STHX), STHX with a new type of anti-vibration baffle (hexagon clamping baffle) and equilateral triangle cross sectioned wire coils (HCBetsw-STHX) is proposed in this paper. Response surface methodology (RSM) is adopted to investigate its thermal-hydraulic performance. Baffle distance, coil pitch, coil diameter and Reynolds number are considered as four design parameters. Analysis is conducted based on CFD to acquire the objective functions (Nu H , Nu H / Nu R , f H , f H / f R and PEC) for different combination of design parameters. The RSM with Central Composite design is used to identify the relationships between the objective functions and the design parameters. Variances of the linear term, quadratic term and interactive term for design parameters in the response variables are analyzed. The mechanism of heat transfer enhancement of HCBetsw-STHX is illustrated from the view of field synergy theory. The results indicate that the heat transfer enhancement of HCBetsw-STHX is quite better than that of RRB-STHX. In the given design space, the Nu H / Nu R is in the range of 1.335–1.720; the f H / f R is in the range of 5.462–12.936. The most significant factor on Nu H is Reynolds number, while the most significant effect factor on Nu H / Nu R , f H , f H / f R and PEC is coil pitch. The sensitivity of three objective functions (Nu H , f H and PEC) to design parameters is explored. In addition, the design parameters optimization for the maximum value of Nu H and PEC and that for the minimum value of f H are also carried out and corresponding flow structures are shown and illustrated. Highlights • A new parallel-flow heat exchanger with hexagon clamping baffles and wire coils is proposed. • Parameter sensitivity of factors on thermal-hydraulic performance is explored. • Design parameters optimization is explored to maximize the Nu and PEC and to minimize the f based on the generated response surface. [ABSTRACT FROM AUTHOR]

Highlights • Six heat transfer regimes identified. • Existence of a turbulent flow but laminar-like heat transfer regime for low Prandtl numbers. • Prediction of Nusselt number in turbulent flow and turbulent heat transfer regime. Abstract This paper investigates heat transfer regimes in fully developed plane channel flows without considering the buoyancy effect. Analyzing the governing thermal energy equation, aided by direct numerical simulation (DNS) data, six heat transfer regimes are identified including (i) laminar flow and laminar heat transfer (LamF-LamH), (ii) transitional flow and laminar-like heat transfer (TraF-LamH), (iii) transitional flow and transitional heat transfer (TraF-TraH), (iv) turbulent flow but laminar-like heat transfer (TurF-LamH), (v) turbulent flow and transitional heat transfer (TurF-TraH), and (vi) turbulent flow and turbulent heat transfer (TurF-TurH). One key result is the clarification of a TurF-LamH regime which exists only for low Prandtl number fluid (Pr ≪ 1). A critical non-dimensional number is determined as Re τ Pr 1 / 2 ≲ 50 where Re τ is the Reynolds number defined by the channel half-height δ and the frictional velocity u τ . In the TurF-LamH regime, the simplified thermal energy equation yields a prediction of Nusselt number as Nu ≈ 6.0. The clarification of the TurF-LamH regime provides valuable insight into the understanding of the Nusselt number data for liquid metals, which have very low Prandtl number. Another major finding is that, in the TurF-TurH regime, the Kader-Yaglom-style equation is shown to be better than the traditional power law correlations at predicting Nusselt number, for either low or high Prandtl numbers. No separate correlations are needed for the prediction of Nusselt number at low Prandtl numbers and high Prandtl numbers. Another advantage of the Kader-Yaglom-style equation is that the equation can be theoretically connected to the log-law for the mean velocity and the mean temperature. More importantly, in the TurF-TurH regime the prediction of Kader-Yaglom-style equation can be reliably extended to ultra high Reynolds numbers, for either low or high Prandtl numbers. [ABSTRACT FROM AUTHOR]

Highlights • Can modification of surfaces enhance heat transfer in a vertical falling film unit? • Experimental and numerical studies (VOF) of hydrodynamics and heat transfer. • Significant bulk mixing is induced by the modified surfaces. • Modified surfaces affect flow structures similarly as backflow in waves. • Modification of surfaces improved heat transfer by a factor of 2.5. Abstract In this paper we investigate experimentally and numerically the potential improvement of heat transfer in falling film units by introducing modifications of the heat transfer surfaces. Two different modified surfaces are investigated and the obtained results are compared to the ones acquired on a smooth surface. In our experiments we work with a broad range of operating conditions with Kapitza and Reynolds numbers in the range of Ka = 167–574 and Re 18–423, respectively. We compare local heat transfer measurements conducted at multiple positions and relate them to the film thickness measurements conducted by a laser triangulation scanner. Our experiments identify both cases where the introduction of modified surfaces significantly enhances the heat transfer and conditions where such a measure does not have a significant impact. The observed differences are compared to numerical simulations providing details of the flow structures in the liquid film in different cases. For that purpose, we directly solve the full Navier-Stokes equations in two dimensions, using the volume of fluid (VOF) numerical framework. Our results reveal that the heat transfer can be enhanced by a factor of up to 2.5 due to appearance of significantly enlarged time-dependent recirculation zones occurring behind the surface modifications, as opposed to such zones present in falling film units with smooth heat-transfer surfaces. [ABSTRACT FROM AUTHOR]

Highlights • A hyperbolic lattice Boltzmann method (HLBM) is proposed based on CV model and classical LBM. • A non-Fourier heat conduction process was simulated by HLBM and classical LBM. • The results of HLBM are closer to the experimental values than that of classical LBM. Abstract Classical lattice Boltzmann method (LBM) can recover Fourier's law. Since Fourier's law results in a parabolic heat conduction equation, the classical LBM is called the parabolic Boltzmann method (PLBM). But the Fourier's law is based on the absurd assumption that the heat transfer rate is infinite, so whether the classical LBM can be used to observe the non-Fourier heat transfer problem remains controversial. In this paper, a hyperbolic lattice Boltzmann method (HLBM), which can be used to analyze the non-Fourier effect, was derived based on the Cattaneo-Vernotte (CV) model. To verify the accuracy of the HLBM, the process of a gold film irradiated by the laser was simulated using the HLBM and the PLBM and the results were compared with the experimental data. Because of the non-equilibrium heat transfer between electron and lattice during the laser irradiation, the two-step HLBM/PLBM models were proposed according to the two-temperature model. The results show that the electron temperatures simulated by the two models are not much different from each other, and both of them coincide with the experimental values. However, the thresholds obtained by the two models are different, and the results of HLBM are closer to the experimental values. [ABSTRACT FROM AUTHOR]