This paper reviews the current status of boiling heat transfer modelling, discusses the need for its improvement due to unresolved intriguing experimental findings and emergence of novel technical applications and outlines the directions for an advanced modelling approach. The state-of-the-art of computational boiling heat transfer studies is given for: macro-scale boiling models applied in two-fluid liquid-vapour interpenetrating media approach, micro-, meso-scale boiling computations by interface capturing methods, and nano-scale boiling simulations by molecular dynamics tools. Advantages, limitations and shortcomings of each approach, which originate from its grounding formulations, are discussed and illustrated on results obtained by the boiling model developed in our research group. Based on these issues, we stress the importance of adaptation of a multi-scale approach for development of an advanced boiling predictive methodology. A general road-map is outlined for achieving this challenging goal, which should include: improvement of existing methods for computation of boiling on different scales and development of conceptually new algorithms for linking of individual scale methods. As dramatically different time steps of integration for different boiling scales hinder the application of full multi-scale methodology on boiling problems of practical significance, we emphasise the importance of development of another algorithm for the determination of sub-domains within a macro-scale boiling region, which are relevant for conductance of small-scale simulations.
Steam accumulators are applied as buffers between steam generators and consumers in cases of different steam production and consumption rates. The application of the steam accumulator saves energy, reduces pressure fluctuations, and prevents aging of tubes and pressurized vessels in steam generators. In this paper, modes of the steam accumulator operation are analyzed and the general design of the steam accumulator control system is defined. Equilibrium and nonequilibrium thermodynamic models of the steam accumulator are presented with the aim of predicting the steam accumulator capacity and as support to the design of the control system. The equilibrium model is based on the mass and energy balance equations of the total water and steam content in the accumulator, while the nonequilibrium model is based on the mass and energy balance equations for each phase and closure laws of nonequilibrium evaporation and condensation rates. The steam accumulator pressure transients are simulated for constant steam charging and discharging flow rates, and the influence of the nonequilibrium condensation and evaporation rates on the steam accumulator capacity is shown. It is concluded that the commonly used equilibrium thermodynamic approach to the steam accumulator design does not provide accurate results in cases of rapid charging and discharging transients; therefore, there is a need for the application of the nonequilibrium approach.
Multi-dimensional numerical simulation of the atmospheric saturated pool boiling is performed. The applied modelling and numerical methods enable a full representation of the liquid and vapour two-phase mixture behaviour on the heated surface, with included prediction of the swell level and heated wall temperature field. In this way the integral behaviour of nucleate pool boiling is simulated. The micro conditions of bubble generation at the heated wall surface are modelled by the bubble nucleation site density, the liquid wetting contact angle and the bubble grow time. The bubble nucleation sites are randomly located within zones of equal size, where the number of zones equals the nucleation site density. The conjugate heat transfer from the heated wall to the liquid is taken into account in wetted heated wall areas around bubble nucleation sites. The boiling curve relation between the heat flux and the heated wall surface temperature in excess of the saturation temperature is predicted for the pool boiling conditions reported in the literature and a good agreement is achieved with experimentally measured data. The influence of the nucleation site density on the boiling curve characteristic is confirmed. In addition, the influence of the heat flux intensity on the spatial effects of vapour generation and two-phase flow are shown, such as the increase of the swell level position and the reduced wetting of the heated wall surface by the heat flux increase.
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