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A novel two-dimensional model for the thermocapillary instability of a thin liquid film was developed and validated in the paper. The model incorporates a novel solution for the unperturbed velocity components and the thin film thickness (based on the boundary layer approach and balance equation) subject to evaporation under the boundary conditions of either mechanical interaction between the liquid and vapor phases or stationary vapor above the interphase boundary. A novel model for thermocapillary instability in a thin film was developed in frames of the linear perturbation method, i.e. modified Orr–Sommerfeld equation, taking into account the surface and London–van der Waals forces. The critical Reynolds number was computed by considering the two-dimensional disturbances, which according to the Squire’s theorem are more dangerous than the three-dimensional disturbances. For constant surface tension at the interphase interface, the unperturbed velocity profile is parabolic, and maximum increases, while the critical Reynolds number decreases with the decreasing capillary number. If the surface tension at the interphase interface depends on the temperature, the maximum of the undisturbed velocity profile increases with the decreasing capillary number and increasing modified Marangoni number, which entails more rapid decrease in the critical Reynolds number. It was also shown that the flow is destabilized by the increase in the temperature difference between the wall and the vapor, by the decrease in the absolute pressure and by the increase in the thermal conductivity. To confirm existence of different flow regimes in the nanofluid film, numerical simulations of capillary flow was performed, which exhibit qualitative consistence with the stability theory. [ABSTRACT FROM AUTHOR]

In this paper, a vertical gypsum plate with thermal capacity placed in an open cavity under dynamic temperature decrease is numerically investigated in order to assess the evolution of the thermal and kinetic structure of the boundary layer and the convective heat transfer coefficient h over time. The particularity of this work resides in the fact that the system was made to simulate the case of a plate with thermal capacity under nocturnal conditions where the temperature is constantly decreasing. Thus, a two-dimensional CFD model is developed under the Ansys Fluent 14.5 environment and is validated using experimental data obtained for a plasterboard studied under dynamic temperature changes. This study contributes to give precise results about temperature, flux, convective heat transfer coefficient and velocity in the vicinity of a vertical wall with a thermal conductivity at the building scale. All the phenomena that occur in the vicinity of the heat capacitive vertical plate (gypsum) are analyzed and explicitly discussed. The main results of this study show that after simulating 1 h real time we observe that turbulent flow begins to appear at the top level of the plate and after this moment the three flows (laminar, transitional and turbulent) coexist. Finally, mean convective heat transfer coefficient depends on the thermal storage capacity of the adjacent material. The average value of convective heat transfer coefficient increases over time by obtaining for a plasterboard ( C p = 1100 J/kg K) value of 1.8 W/m 2 K and for a phase change material in a solid state ( C p = 4350 J/kg K) value of 2.4 W/m 2 K at steady state. [ABSTRACT FROM AUTHOR]

Blunt nose body with sharp corner is a kind of common configuration for spaceship design. However, for its complexity, there were few valid theories can solve this problem perfectly.In this paper, compressiblenonsimilar boundary-layer analysis and numericalsimulation of Navier-Stokes equationswere used together to give some satisfactory results. The influence of nose radius and corner radius on heating rate over blunt body was also summarized which is very useful for the optimization of thermal protection system. [ABSTRACT FROM AUTHOR]

Abstract: In 1998 Guo et al. integrated the boundary-layer energy equation along the thermal boundary layer thickness, and noted that at outside boundary the temperature gradient is zero and the convection term is actually the inner production of vector velocity and temperature gradient, they found that for a fixed flow rate and temperature difference, the smaller the intersection angle between velocity and temperature gradient the larger the heat transfer rate. This idea is called field synergy principle (FSP). Later it has been shown that FSP can unify all mechanisms for enhancing single phase heat transfer. In 2007 Guo and his co-workers proposed a new concept: entransy to describe the potential of a body to transfer thermal energy and the entransy dissipation extreme principle (EDEP). It is indicated that for any heat transfer process the entransy of the system is always dissipated, which can be regarded as the indication of the irreversibility of the transport process. For a heat transfer process with given boundary temperature condition the best one has the maximum entransy dissipation, while for that with given boundary heat flux condition the best one has minimum entransy dissipation. The combination of the two cases is called the entransy dissipation extremum principle. The purpose of this paper is to reveal the inherent interrelationship between the ideas of field synergy principle and the entransy extremum principle. Numerical simulations are conducted for five examples of convective heat transfer. All the numerical results demonstrate the inherent consistency between FSP and EDEP. [Copyright &y& Elsevier]

Abstract: This paper presents a numerical investigation for examining the secondary vortex motion and associated heat transfer process in fluid flow through curved passages. Overcoming current modelling limitations, the study formulates an improved simulation model based on 3-dimensional vortex structures for describing secondary flow and its thermal characteristics. For developing laminar fluid flow through curved rectangular ducts, the analysis performs a detailed parametric study involving the contours of helicity and outer duct wall pressure gradient for a range of flow rates, duct aspect ratios, duct flow curvatures and external wall heat fluxes. The flow conditions leading to hydrodynamic instability and Dean vortex generation in curved passages are carefully analysed, identifying the flow and geometrical parametric influences. Active interaction of buoyancy force on fluid motion arising from wall heating is considered where aspects of boundary layer separation is used in recognising thermal enhancement due to secondary flow. The simulated results are validated with the available experimental data. Surpassing precision and reliability of current practise, the study develops two numerical approaches offering improved capabilities for capturing the onset of hydrodynamic instability and resulting Dean vortices. These techniques are compared and assessed for their merits of application in determining the onset of secondary flow hydrodynamic instability with and without external wall heating. [Copyright &y& Elsevier]

• Thickened velocity boundary layer by pyrolysis invalidates regular size micro-rib. • Higher micro-rib is needed to effectively enhance heat transfer of cracking fuel. • Thermal cracking of fuel shortens the recirculation zone range behind micro-ribs. • Effective heat transfer enhancement for cracking fuel demands a smaller pitch. • Micro-ribs present better-enhanced heat transfer in higher fuel conversion region. Micro-rib is an efficient heat transfer enhancement method for supercritical fuel in the regenerative cooling channel of advanced aero-engines. The endothermic hydrocarbon fuel would crack into smaller molecules at high temperatures and significantly change fluid properties, giving more complexity for flow and heat transfer in the micro-ribbed cooling channel. In this paper, a three-dimensional simulation model, embedded with a skeletal 22-step cracking mechanism, was established to conduct parametric analysis on thermal behaviors of cracking n-Decane at supercritical pressures in asymmetry heated channels with micro-ribs. The results reveal that thermal cracking would thicken the velocity boundary layer in the channel. Under this circumstance, regular size micro-ribs (h / H ≈ 10%) might abnormally weaken heat transfer of cracking hydrocarbon fuel and higher micro-ribs are required for effective heat transfer enhancement in the channel. Due to the low-density zone caused by cracking products near the heated wall, heat transfer enhancement brought by micro-ribs becomes less sensitive to the increase of micro-rib height. Besides, thermal cracking shortens the range of recirculation zones behind micro-ribs, thus much smaller micro-rib pitch is required for effective heat transfer enhancement. Furthermore, micro-ribs have better heat transfer enhancement performance in higher fuel conversion region, where the density gradient is smaller. [ABSTRACT FROM AUTHOR]