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A three-dimensional transient mathematical model (following a fixed-grid enthalpy-based continuum formulation) is used to study the interaction of double-diffusive natural convection and non-equilibrium solidification of a binary mixture in a cubic enclosure cooled from a side. Investigations are carried out for two separate test systems, one corresponding to a typical model “metal–alloy analogue” system and other corresponding to a real metal–alloy system. Due to stronger effects of solutal buoyancy in actual metal–alloy systems than in corresponding analogues, the convective transport mechanisms for the two cases are quite different. However, in both cases, similar elements of three-dimensionality are observed in the curvature and spacing of the projected streamlines. As a result of three-dimensional convective flow patterns, a significant solute macrosegregation is observed across the transverse sections of the cavity, which cannot be captured by two-dimensional simulations. [Copyright &y& Elsevier]

• Stack-based thermal resistance network model developed for micro channel heat exchangers. • Correlations developed with straight, sinusoidal and zigzag channels for sCO 2 recuperator. • Hybrid model demonstrates that 20 rows are sufficient to attain the correct stack performance. • Several combinations of stack dimensions generated for different channel configurations. • Sinusoidal channel offered least pressure drop and stack volume compared to straight and zigzag channels. In this work, a full-scale Thermal Resistance Network (TRN) coupled with a unit-cell-based CFD model identified as the hybrid model is developed and implemented in designing a Mini Channel Heat Exchanger (MCHE) as a recuperator used in sCO 2 Brayton power block. The TRN framework converts the MCHE domains into a thermal circuit using resistances and advection currents that can be used to model the entire stack with varying thermo-physical properties of working fluids. The results from the TRN model with CFD-based heat transfer and pressure drop correlations developed for different channel lengths, are compared with the original CFD data to ascertain the lowest possible channel length for precise correlation development. An appropriate channel length is further used for deriving correlations for sinusoidal and zigzag flow paths. The hybrid model helps determine the upper limit of the side width (fin width) of the channels based on equivalent thermal resistance between fluid and walls as the lower limit is constrained by stress criteria. The present model estimates that at least 20 rows/plates need to be modelled for the correct performance estimation of a full-scale MCHE. The minimum rate of heat loss-based stack optimization approach is implemented in estimating the optimum number of rows and stack dimensions. The channel and stack dimensions are obtained and compared for straight, sinusoidal, and zigzag flow paths for Reynolds numbers ranging from 5 × 103 to 20 × 103, ensuring a temperature pinch of 5 °C at the cold inlet. For cold inlet Reynolds number and channel diameter of 1.5 × 104 and 1.5 mm, the stack volumes for straight, zigzag, and sinusoidal channels are 1.8 m3, 1.21 m3, and 1.15 m3, respectively with sinusoidal and zigzag paths resulting in almost similar pressure drop as ∼110 Pa, 27% lower compared to the straight channel. The article presents a guideline for hybrid model implementation in designing mini/micro channel heat exchangers. [ABSTRACT FROM AUTHOR]

Abstract: The variation in temperature and concentration plays a crucial role in predicting the final microstructure during solidification of a binary alloy. Most of the experimental techniques used to measure concentration and temperature are intrusive in nature and affect the flow field. In this paper, the main focus is laid on in-situ, non-intrusive, transient measurement of concentration and temperature during the solidification of a binary mixture of aqueous ammonium chloride solution (a metal-analog system) in a top cooled cavity using laser based Mach–Zehnder Interferometric technique. It was found from the interferogram, that the angular deviation of fringe pattern and the total number of fringes exhibit significant sensitivity to refractive index and hence are functions of the local temperature and concentration of the NH4Cl solution inside the cavity. Using the fringe characteristics, calibration curves were established for the range of temperature and concentration levels expected during the solidification process. In the actual solidification experiment, two hypoeutectic solutions (5% and 15% NH4Cl) were chosen. The calibration curves were used to determine the temperature and concentration of the solution inside the cavity during solidification of 5% and 15% NH4Cl solution at different instants of time. The measurement was carried out at a fixed point in the cavity, and the concentration variation with time was recorded as the solid–liquid interface approached the measurement point. The measurement exhibited distinct zones of concentration distribution caused by solute rejection and Rayleigh Benard convection. Further studies involving flow visualization with laser scattering confirmed the Rayleigh Benard convection. Computational modeling was also performed, which corroborated the experimental findings. [Copyright &y& Elsevier]

Abstract: A numerical scheme with hybrid explicit and implicit time stepping in solidification problems is presented. An explicit time stepping scheme is used for solving coupled temperature and concentration fields, while an implicit scheme is used for solving equations of motion. The explicit approach results in a local point-by-point coupling scheme for the temperature and concentration fields that uses constitutive model for back diffusion in solid. The present method offers distinct advantages of simplicity and flexibility in incorporation of micro-scale models, as demonstrated by the use of a back diffusion parameter in the microsegregation model. Results from the present method are compared with those in the literature using a fully implicit method, and they show significant improvement in final macrosegregation prediction. [Copyright &y& Elsevier]

Abstract: The effectiveness of thermal conductivity enhancers (TCEs) in improving the overall thermal conductance of phase change materials (PCMs) used in cooling of electronics is investigated numerically. With respect to the distribution of TCE and PCM materials, the heat sink designs are classified into two types. The first type of heat sink has the PCM distributed uniformly in a porous TCE matrix, and the second kind has PCM with fins made of TCE material. A transient finite volume method is used to model the heat transfer; phase change and fluid flow in both cases. A generalized enthalpy based formulation and numerical model are used for simulating phase change processes in the two cases. The performance of heat sinks with various volume fractions of TCE for different configurations is studied with respect to the variation of heat source (or chip) temperature with time; melt fraction and dimensionless temperature difference within the PCM. Results illustrate significant effect of the thermal conductivity enhancer on the performance of heat sinks. [Copyright &y& Elsevier]

A two-dimensional transient fixed-grid enthalpy-based numerical method is developed to analyze the effects of turbulent transport during a binary alloy solidification process. Turbulence effects are introduced through standard k–ϵ equations, where coefficients are appropriately modified to account for phase-change. Microscopically-consistent estimates are made regarding temperature-solute coupling in a non-equilibrium solidification situation. The model is tested against laboratory experiments performed using an NH4Cl–H2O system in a rectangular cavity cooled and solidified from the top. Particular emphasis is laid on studying the interaction between Rayleigh–Benard type convection and directional solidification in the presence of turbulent transport. Numerical predictions are subsequently compared with experimental results regarding flow patterns, interface growth and evolution of the temperature field, and the agreement is found to be good. [Copyright &y& Elsevier]