Search

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: A one-dimensional mathematical model of the R744 two-phase ejector for expansion work recovery is presented in this paper. Governing equations were formulated for all passages of the ejector based on the differential equations for mass, momentum, and energy balance as well as a differential representation for the equation of state. For two-flow sections (mixer and diffuser) closing equations for mass, momentum and energy transfer between the primary and secondary flow were introduced. This model utilises the Delayed Equilibrium Model along with the Homogeneous Nucleation Theory for the purpose of the metastable state analysis for a transcritical flow with delayed flashing over the motive nozzle. The thermal properties model was based on a real fluid approach, where the REFPROP 8.0 database was used. Based on the results of experimental tests performed at SINTEF Energi Laboratory, the model was validated for a typical range of operating conditions. The range of available simulation output allowed for the creation of 1D profiles of local values for the flow variables and the computation of the overall indicators, such as pressure lift and expansion work recovery efficiency. [Copyright &y& Elsevier]

Abstract: A nine-step phenomenological soot model has been implemented into the KIVA-3V code for predicting soot formation and oxidation processes in diesel engines. The model involves nine generic steps, i.e., fuel pyrolysis, precursor species (including acetylene) formation and oxidation, soot particle inception, particle coagulation, surface growth and oxidation. The fuel pyrolysis process leads to acetylene formation and it is described by a single-step reaction. The particle inception occurs via a generic gas-phase precursor species, and the precursor is the product of an irreversible reaction from acetylene. The acetylene addition reaction contributes to soot surface growth. The particle coagulation affects both particle size and number density. The oxidation of soot particles includes two mechanisms—Nagle and Strickland-Constable''s O2 oxidation mechanism and Neoh et al.''s OH oxidation mechanism. The quasi-steady state assumption is applied to an H2–O2–CO system for calculating OH concentration. Both acetylene and precursor species have their own consumption paths, each of which is described by a single-step oxidation reaction. Validations of the model have been conducted over a wide range of engine conditions from conventional to PCCI-like combustion. Two engine examples (a heavy-duty diesel engine and a light-duty diesel engine) are presented in this paper. The predictions are compared against measurements, and the applicability of the model to multi-dimensional diesel simulations is assessed. The model''s capability of predicting the soot distribution structure in a conventional diesel flame is included in discussion as well. The work reveals that the nine-step model is not only computationally efficient but also fundamentally sound. The model can be applied to diesel engine combustion analysis and, after calibration, is suitable to be integrated with genetic algorithms for system optimization over a controllable range of operations. [Copyright &y& Elsevier]

Abstract: An improved understanding of the heat transfer in materials consisting of two layers (splat and substrate) is essential for many industrial applications. We are interested in the deposition, rapid cooling and solidification of metal droplets (known as splats) brought into contact with a cold substrate. We therefore need to understand the temperature history in both the splat and the substrate, including phase change phenomena. A new model of the thermal contact resistance based on a random distribution of contact points, rather than the uniform distribution commonly used, is presented in this paper. Phase change has been also considered, using the enthalpy-porosity formulation. Simulations have been conducted with the commercial package CFX-4. The computational results for the cooling rate of the splat obtained using the random contact distribution model are in good agreement with available experimental results. In addition, results obtained from the random model provide information on the inhomogeneity affecting the temperature at the interface between the splat and the substrate. [Copyright &y& Elsevier]

Abstract: Understanding the heat and mass transfer phenomena in a coal gasifier is very useful for the assessment of gasifier performance and optimization of the design and operating parameters. In this paper, performance of an entrained flow air blown laboratory scale gasifier is numerically simulated with Fluent software. In the model, the continuous phase conservation equations are solved in an Eulerian frame, while those of particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The dispersion of the particles due to turbulence is predicted using a stochastic tracking model, in conjunction with the k–ε equations for the gas phase. The coal gasification model adopted includes devolatilization, combustion of volatiles, char combustion and gasification. The gasification performance inside the gasifier has been predicted for different air ratios as well as for different air and steam inlet temperatures. The overall temperature inside the gasifier is found to increase when the degree of air/steam pre-heating is increased, resulting in acceleration of the different reaction steps in the gasifier. The overall gasification performance indices such as carbon conversion, heating value of the exit gas and cold gas efficiency have been predicted. The predicted results show good agreement with available experimental data in literature. [Copyright &y& Elsevier]