We focus on condensation and collapse processes of vapor bubble(s) in a subcooled pool. We generate the vapor in the vapor generator and inject it/them to form vapor bubble(s) at a designated temperature into the liquid at a designated degree of subcooling. In order to evaluate the effect of induced flow around the condensing/collapsing vapor bubble, two different boundary conditions are employed; that is, the vapor is injected through the orifice and the tube. We also focus on interaction between/among the condensing/collapsing vapor bubbles laterally injected to the pool. Through this system we try to simulate an interaction between the vapor bubble and the subcooled bulk in a complex boiling phenomenon, especially that known as MEB (microbubble emission boiling) in which a higher heat flux than critical heat flux (CHF) accompanying with emission of micrometer-scale bubbles from the heated surface against the gravity is realized under a rather high subcooled condition.

Laminar mixed convection Al 2 O 3 -Water nanofluid flow in elliptic ducts with constant heat flux boundary condition has been simulated employing two phase mixture model. Three-dimensional Navier-Stokes, energy and volume fraction equations have been discretized using the Finite Volume Method (FVM). The Brownian motions of nanoparticles have been considered to determine the thermal conductivity and dynamics viscosity of Al 2 O 3 -Water nanofluid, which vary with temperature. Simulation effects of solid volume fraction and nanoparticles mean diameter on thermal and hydraulics behaviors of nanofluid flow in elliptic ducts have been presented and discussed. The calculated results show good agreement with the previous numerical data. Results show that in a given Reynolds number (Re) and Richardson number (Ri), increasing solid nanoparticles volume fraction increases the Nusselt number (Nu) while the skin friction factor decreases. Increasing nanoparticles mean diameter augments the local skin friction factor whereas it causes the Nusselt number to reduce. But these effects are significant for nanoparticles diameter equal to 13nm especially.

The accuracy with which the solidification and cooling of a continuously cast billet is investigated depends on the setting of the boundary conditions of the numerical model of the temperature field. An in-house numerical model of the 3D temperature field of a concast billet had been used. This model enables the analysis of the temperature field of the actual blank as it passes through the zero-, primary-, secondary- and tertiary-cooling zones, i.e. through the entire caster. This paper deals with the derivation of transfer phenomena under the cooling nozzles of the secondary zone. These phenomena are expressed by the values of the heat transfer coefficients (HTCs). The dependences of these coefficients on surface temperature and other operational parameters must also be given. The HTCs beneath the nozzles are given by the sum of the forced convection coefficient and the so-called reduced convection coefficient corresponding to heat transfer by radiation. The definition of the boundary conditions is the most difficult part of the numerical and experimental investigation of the thermokinetics of this process. Regarding the fact that on a real caster, where there are many types of nozzles (with various settings) positioned inside a closed cage, it is practically impossible to conduct measurement of the real boundary conditions. Therefore, an experimental laboratory device was introduced in order to measure the cooling characteristics of the nozzles. It simulates not only the movement, but also the surface of a blank and for the necessary range of water flow in the operation and the casting speeds. The transfer phenomena beneath the water cooling nozzles are presented on a simulated temperature field for a real 150×150 mm steel billet under different operational conditions. This is ensured by the correct process procedure: real process → input data → numerical analysis → optimization → correction of process. The presented model is a valuable computational tool and accurate simulator for investigating transient phenomena in caster operations, and for developing control methods, the choice of an optimum cooling strategy to meet all quality requirements, and an assessment of the heat-energy content required for direct rolling.

Conductive heat transfer has an important role to play in the cooling of, for instance, electronic equipment. Due to its passive nature in relaying heat, internal conductive cooling may have advantages above internal convective heat transfer at small length scales. It does, however, require optimization of the distribution of high heat-conducting material. In this two-dimensional numerical study, the optimum distribution of high conductive material within rectangular heat-generating domains with different aspect ratios is investigated by using a topology optimization algorithm. The volume fraction of the high-conductive material is set at 10% of the total volume. Of interest are the influences of boundary conditions, thermal conductivity and optimization penalization levels on the resulting cooling material distribution. The obtained conducting trees are compared and discussed.

The mechanism of isolated bubble pool nucleate boiling of water is studied by a novel approach method using the developed MEMS thermal sensor. The local temperature variation beneath isolated bubble was measured using the MEMS sensor at different six wall superheats. Evaporation and dry-out of the microlayer and the rewetting of the dry-out area were obviously observed in the measured temperature variation. Wall heat transfer was numerically calculated by transient heat conduction simulation with the measured temperature as a surface boundary condition. The results showed that the microlayer evaporation transfers high heat flux of a few MW/m 2 , and dominantly contributes to the heat transport from the heating wall during the bubble growth phase. The ratio of the heat transferred from the wall to the latent heat in the bubble at the departure decreased with increasing wall superheat. In other words, the contribution of the heat transfer from the superheated liquid layer surrounding the bubble becomes important with increasing wall superheat. Moreover, the microlayer thickness was calculated by integrating the local heat flux. The derived initial thickness of the microlayer was independent from the wall superheat and became thick as distance from the nucleation site increases.

Direct Chill (DC) casting is a semi-continuous casting technique which is used to produce aluminum rolling ingots and extrusion billets. The knowledge of temperature field is highly essential for the prediction of displacement field and hot tears. Modeling the thermal field of DC casting is a challenging task due to the liquid-solid phase transition, time-dependent domain and boundary conditions, inverse nature of secondary boundary conditions, etc. Therefore, an attempt is made to model the thermal field of DC casting using a finite element method. A temperature-based finite element model is used to capture the effect of latent heat release. A temperature-dependent heat transfer coefficient is employed to incorporate the bottom block and mold boundaries. The influence of casting speed is studied in detail. Through the proper ramping procedures, it is proved that the start-up phase sump depth and mushy length can be lowered. However, it is found that the steady-state sump parameters are independent of ramping. Further, the influences of secondary cooling profile, and melt superheat are investigated. AA1201 alloy is considered for the study.

In the present study, an immersed-boundary method is adopted to simulate natural and forced convection within a domain with complex geometry. The method is based on the direct momentum and energy forcing on a Cartesian grid and issues involving implementation of both the thermal (Dirichlet and Neumann) and dynamic (stationary and nonstationary) boundary conditions are addressed. The second order accuracy of the present method was validated based on natural convection in an annulus between horizontal concentric cylinders. Simulations of flow over a stationary cylinder with heat convection were further conducted to validate the capability of present technique for both temperature boundary conditions. Finally, the influence of the lock-on phenomenon in heat transfer is investigated for flow over a transversely oscillating cylinder. All computed results are in generally good agreement with previous experimental measurements and numerical simulations.

A generalized form of the Ballistic-Diffusive Equations (BDE) for approximate solution of the Boltzmann Transport Equation (BTE) for phonons is formulated. The formulation presented here is new and general in the sense that, unlike previously published formulations of the BDE, it does not require a priori knowledge of the specific heat capacity of the material. Furthermore, it does not introduce artifacts such as media and ballistic temperatures. As a consequence, the boundary conditions have clear physical meaning. In formulating the BDE, the phonon intensity is split into two components: ballistic and diffusive. The ballistic component is traditionally determined using a viewfactor formulation, while the diffusive component is solved by invoking spherical harmonics expansions. Use of the viewfactor approach for the ballistic component is prohibitive for complex large-scale geometries. Instead, in this work, the ballistic equation is solved using two different established methods that are appropriate for use in complex geometries, namely the discrete ordinates method (DOM), and the control angle discrete ordinates method (CADOM). Results of each method for solving the BDE are compared against benchmark Monte Carlo results, as well as solutions of the BTE using standalone DOM and CADOM for a two-dimensional transient heat conduction problem at various Knudsen numbers. It is found that standalone CADOM (for BTE) and hybrid CADOM-P 1 (for BDE) yield the best accuracy. The hybrid CADOM-P 1 is found to be the best method in terms of computational efficiency.

This article investigates thermophysical property measurement of femtogram-level polymeric samples by using the 3ω method on a heated microcantilever probe. A localized thermal scooping method was employed to acquire 449 fg of polyethylene terephthalate (PET) sample, measured gravimetrically, directly onto the heater of the cantilever. It is shown that the sample case has a 3ω signal that is smaller in magnitude than the bare case, suggesting that sample properties could be determined using the processes discussed here. A finite element analysis (FEA) model was also developed to compute the steady periodic behavior of the cantilever in the frequency domain. In order to drastically reduce the computational cost and consider the transient effect of the surrounding air, the FEA model implements the complex thermal conductance of the air as the boundary condition rather than modeling the air as a separate domain. The comparison of the modified model with the model that includes the air in the system reveals that the running time has improved by one order of magnitude while showing excellent agreement. The obtained results will expand the characterization and functionality of microcantilevers leading to advancements in localized thermal analysis.

Numerical modeling of methane-steam reforming is performed in a microchannel with heat input through Palladium-deposited channel walls corresponding to the experimental setup of Eilers [1]. The low-Mach number, variable density Navier-Stokes equations together with multicomponent reactions are solved using a parallel numerical framework. Methane-steam reforming is modeled by three reduced-order reactions occurring on the reactor walls. The surface reactions in the presence of Palladium catalyst are modeled as Neumann boundary conditions to the governing equations. Use of microchannels with deposited layer of Palladium catalyst gives rise to a non-uniform distribution of active reaction sites. The surface reaction rates, based on Arrhenius type model and obtained from literature on packed-bed reactors, are modified by a correction factor to account for these effects. The reaction-rate correction factor is obtained by making use of the experimental data for specific flow conditions. The modified reaction rates are then used to predict hydrogen production in a microchannel configuration at different flow rates and results are validated to show good agreement. It is found that the endothermic reactions occurring on the catalyst surface dominate the exothermic water-gas-shift reaction. It is also observed that the methane-to-steam conversion occurs rapidly in the first half of the mircochannel. A simple one-dimensional model solving steady state species mass fraction, energy, and overall conservation of mass equations is developed and verified against the full DNS study to show good agreement.

This paper presents a study of flushing non-condensable (NC) gases out of a chamber by a saturated steam flow. During the flushing and mixing process, significant heat transfer occurs among the NC gases, steam and the chamber wall, with a coupled steam condensation. The flushing effectiveness hence is strongly dependent upon the mixing, condensation characteristics, the steam feeding rate as well as the thermal capacity of wall. The objective of this study is to explore modeling approaches on such a process which would be applied to assist the optimization of the process design and operation. An experimental system has been developed to provide a set of data for model validations. A simple mechanistic model based has also been developed to show the “equilibrium-based” flushing characteristics. However, to account for finite rate of heat and mass transfer and non-uniform mixing, a more complicated full-field computational fluid dynamics modeling and simulation must be involved. The typical boundary conditions in most commercial CFD codes (such as FLUENT) cannot be directly applied to the flushing processes due to the coupled surface condensation. Hence, in this paper, we have also proposed the condensation-based boundary conditions for the CFD simulations. Full-field CFD simulations with those boundary conditions are being investigated.

Depressurization can be realized by condensing saturated vapor of a pure substance inside a confined chamber. The depressurization rate depends directly upon the effectiveness of cooling to the condensing vapor. The objective of this study is to develop modeling approaches for cooling-controlled depressurization, which will assist the optimization of process design and operation. To this end, an experimental system is set up to provide sets of data for model validations. A simple mechanistic model based on assumption of thermodynamic-equilibrium inside the system has been developed to show the limiting depressurization characteristics with instant heat balance. This simplified model has a merit of quick evaluation on comparisons among various parametric effects. Yet the model is inadequate for real-time quantification in depressurization. The gaps between the measurements and model predictions indicate the importance of local non-uniform heat transfer and condensation. To close the gaps, a complicated full-field computational fluid dynamics modeling and simulation (CFD) is needed, in which the local condensations (especially surface condensation) must be fully account for. The difficulty in CFD approach is the unavailability of condensation-coupled boundary conditions in most commercial CFD codes. Hence, in this paper, we have also proposed the modeling of condensation-based boundary conditions that will be used for CFD simulations.

Using a scale analysis approach, we model phase change (melting) for pure materials which generate internal heat for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. We show the time scales in which conduction and convection heat transfer dominate.

Heat transfer and pressure drop measurements for horizontal macro-tubes under uniform wall heat flux boundary condition have been conducted by various researchers in recent years. From their studies, it was shown that good agreements were observed in the laminar and turbulent regions. However, for the transition region, the heat transfer and pressure drop characteristics depended on various factors, such as inlet configuration, buoyancy effect, and surface roughness. In a recent study by Tam et al. (2010), they measured the heat transfer and pressure drop simultaneously for a horizontal macro-tube with and without internally micro-fins and concluded that under the heating condition, the transition Reynolds number range for heat transfer and pressure drop were completely different. The transition Reynolds number range was documented in their research in great detail. However, for horizontal micro-tubes, there is no information in the literature on the simultaneous behavior of the heat transfer and pressure drop, especially in the transition region. In order to fill in this gap, an experimental setup was built to measure the heat transfer and pressure drop simultaneously for a horizontal micro-tube under uniform wall heat flux boundary condition. Water was used as the test fluid and the test section was a stainless steel micro-tube with 1000μm diameter. For heat transfer, the results indicated that the micro-tube had an earlier start and end of transition compared to the macro-tube and, in the turbulent region, an increase in heat transfer due to the surface roughness was observed. For friction factor under isothermal condition, the micro-tube had a narrower transition range due to the roughness compared to the macro-tube. For friction factor under heating condition, the laminar data and the start of transition were different from the isothermal case, and the effect of heating was not seen on the end of transition.