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Abstract: Along with the developments of lattice-Boltzmann method in simulation of fluid flow, its thermal features should be also improved. An important problem which is encountered in many cases is the conjugate heat transfer or heat transfer at the fluid–solid interface. In this case, a strict technique is required to calculate the heat exchange between fluid and solid. Only a few works have been published regarding this phenomenon. In this paper, we have presented a model for analysis of the conjugate heat transfer. The model has been validated by two benchmarks: (I) Fluid flow and heat transfer in a backward-facing step channel with heated thick wall, and (II) Fluid flow and heat transfer in a channel with infinite number of heated obstacles mounted on the wall. The numerical tips concerned in the simulation of these problems by lattice-Boltzmann method have been also represented in the paper. [Copyright &y& Elsevier]

Abstract: As we know, the mean temperature difference approach perfectly solves the problem of thermal design of a two-fluid heat exchanger. However, so far, the equivalent method has not been found for that of three-fluid heat exchanger. In this paper, a mean temperature difference method called IMTD (integral-mean temperature difference) is developed in detail for a parallel stream three-fluid heat exchanger with two communications. It includes the derivation of IMTD formulae, the illustration of design and simulation procedures of IMTD thermal model, the corresponding exemplary calculations and discussion, and the validation of IMTD thermal model. The research shows that the derived IMTD formulae and accelerated method proposed in the present IMTD model significantly improve the design process in the past analytical models. It realizes a fast convergence which is usually within 10 steps for all parallel stream flow arrangements. Moreover, the present model has the same accuracy as past exact analytical models and simultaneously can be comparable to past approximate analytical models in simplicity. [Copyright &y& Elsevier]

Abstract: A computational model is developed to describe the thermo-fluid-dynamic behaviour of a helically coiled steam generator device working with water and widely adopted in the nuclear industry. The discretized governing equations are coupled using an implicit step by step method. The mathematical model includes: a subcooled liquid region, a two-phase flow region, and a superheated vapour region (according to the once-through nature of the heat exchanger). All the flow variables (enthalpies, temperatures, pressures, vapour qualities, velocities, heat fluxes, etc.), together with the thermo-physical properties, are evaluated at each point of the grid in which the domain is discretized. A full-scale experimental investigation carried out at SIET thermal-hydraulics labs in Piacenza (Italy), and aimed at characterizing the fluid-dynamic behaviour of two-phase flows in helically coiled tubes, is referenced in the present paper. Two-phase pressure drops data reduction allowed optimizing a suitable form of the friction factor multiplier required by momentum balance equation. Comparisons of the numerical simulations with a wide range of two-phase pressure drops measurements (experiments conducted both in diabatic and adiabatic conditions) are shown in order to validate the proposed model. [ABSTRACT FROM AUTHOR]

Abstract: This article analyzes parallel flow microchannel heat exchangers with non-adiabatic thermal condition imposed on the wall separating the fluids and the ambient. For purposes of analysis, a thermal model comprising of two governing equations, one for each of the fluids, is developed and analytically solved to obtain equations for calculating the axial temperature and thereby the effectiveness of both fluids. Provision is provided in these equations for the temperature of the ambient interacting with the hot fluid to be different from that associated with the cold fluid. Analytical equations for determining the heat transfer between the individual fluids and the respective ambient as well as that between the fluids are also developed in this paper. The temperatures/effectiveness of the fluids depend on parameters such as NTU, fluid heat capacities, thermal resistance between the individual fluid and the respective ambient and ambient temperatures. In addition, under certain operating conditions the phenomenon of temperature-cross is observed. Depending on the ambient temperatures, increase in NTU will continuously improve the effectiveness of one the fluids; the effectiveness of the other fluid over the same NTU range increases before peaking and subsequent decrease. Equations for determining the NTU at which this peak value in effectiveness occurs are also provided. An unbalanced flow microchannel heat exchanger with the hot fluid having the lowest heat capacity has better effectiveness than when the cold fluid has the lowest heat capacity. [ABSTRACT FROM AUTHOR]

Abstract: In the present paper, a numerical model of spot pulsed current GTA welding for partially and fully penetrated weld pools is presented. Heat transfer and fluid flow in the weld pool driven by the combination of electromagnetic force, buoyancy force, surface tension gradient and latent heat are included in our model. A new formulation of the electromagnetic problem is introduced to take into account eddy current in the weld pool. The shape of the free deformable surface under the action of pulsed arc force is also handled after the magneto-hydrodynamic calculation. The numerical model was applied to 304 stainless steel welding. We compare the influence of various pulsed welding parameters such as pulse frequency and current ratio on the weld quality. Experimental study is conducted to compare our numerical prediction with welding macrographies. It shows a good agreement of the model. [Copyright &y& Elsevier]

Abstract: This work is focussed on the development of a numerical simulation model that predicts the thermal and fluid-dynamic behaviour of the two-phase flow distribution in systems with multiple branching tubes like manifolds. The geometry of a simulated branching system is represented as a set of tubes connected together by means of junctions. On one side, the in-tube evaporation/condensation phenomena are simulated by means of a one-dimensional two-phase flow model, and on the other side, the splitting/converging flow phenomena occurring at junctions are predicted with appropriate junction models obtained from the technical literature. The global flow distribution is calculated using a semi-implicit pressure based method (SIMPLE-like algorithm) where the continuity and momentum equations of the whole domain are solved and linked with both the in-tube two-phase flow model and the junction models. In the present paper, the flow distribution model is described and its most significant aspects are detailed. Furthermore, the model is validated against experimental and numerical data found in the open literature. The numerical predictions are compared against an adiabatic single-phase flow manifold system working with water and also against a two-phase flow upwardly oriented manifold system working with carbon dioxide. In addition to this, a numerical comparison of a manifold system with two different orientations is carried out. Concluding remarks about the possibilities that this kind of model offers are presented in the last section. [Copyright &y& Elsevier]

Abstract: In this paper, we develop a collision model for particle–particle and particle–wall interaction inside fluid medium based on stereomechanical impact model. One of the important parameters identified in literature that characterizes the interaction is the Stokes number (St), which is the ratio of the particles inertia and fluid viscosity. In dry air collisions, the coefficient of restitution (e) and coefficient of friction (f) are used to describe post-collision velocities on the basis of pre-collision velocities. In the case of collisions in fluid medium, we use the experimental results that provide the co-relation between the values of e and f as a function of the Stokes number. The collision models developed here are used in our direct numerical simulation of particulate flows to verify the model with some of the well-known behavior of particle interaction in fluids. [Copyright &y& Elsevier]