Guan Heng Yeoh

On numerical modelling of low-pressure subcooled boiling flows

Although models of subcooled flow boiling at high pressure have been studied extensively, there are few equivalent studies for numerical modelling at low pressure. Recent experimental and numerical studies on subcooled boiling flow at low pressure have indicated that empirical models developed, and verified, for high-pressure situations are not valid at low pressures. A study has been conducted to extend a two-fluid model, previously used for predicting subcooled boiling flow at high pressures into being applicable for low-pressure conditions. This study demonstrates that the following closure relationships or parameters are important for an accurate prediction of void fraction distributions at low pressures: (i) partition of the wall heat flux; (ii) bubble size distribution and interfacial area concentration; and (iii) bubble departure diameter and its relationship with bubble frequency. Different existing correlations for all these are tested and some new correlations are proposed. Predictions of the proposed model agree closely with a number of published experimental data.

Gas-liquid flows

Gas–liquid flows appear in natural and industrial processes in various forms and often feature complex inter-phase mass, momentum, and energy transfers. One example of naturally occurring gas–liquid flow is the dispersion of marine droplets. Gas–liquid flows are also found in abundance in industrial processes. One significant industrial application is venting of mixture vapors to liquid pools in chemical reactors. This chapter is concerned with gas–liquid flows. Within this flow system, the two phases that coexist simultaneously in the fluid flow often exhibit relative motion among the phases and heat and mass exchanges across the interface boundary. Owing to the complexities of interfaces and resultant discontinuities in fluid properties as well as from physical scaling issues, it is rather customary to apply a statistical, averaged approach in the form of a two-fluid model to resolve such a flow system. Separate transport equations governing the conservation of mass, momentum, and energy are solved for each phase and exchanges that take place at the interfaces between the two phases are explicitly accounted for in which the dynamics of the interaction between the two phases can be effectively described via suitable models of the inter-phase mass, momentum, and energy exchanges. Normally, the coupling between the two phases is very tight, which demands special numerical strategies and solution algorithms to be adopted. This particular flow system is also complicated considerably by the prevalence of particle–particle collisions. A number of population balance methods, along with suitable coalescence and break-up mechanisms, are discussed. In the context of computational fluid dynamics, the application of population balance models to describe the coalescence and break-up dynamics of these gas particles can be coupled with the two-fluid model to predict the wide range of particle sizes within the two-phase flow.

A novel artificial neural network fire model for prediction of thermal interface location in single compartment fire

Thermal interface is the boundary between the hot and cold gases layers in a compartment fire. The height of the interface depends predominantly on the mass of air entrained into the fire plume. However, the analytical determination of the air mass flow rate is complicated since it is highly nonlinear in nature. Currently, computer models including zone models and field models can be applied to predict fire phenomena effectively. In the zone model computation, the compartment on fire is commonly divided into two layers to which conservation equations are applied to evaluate the fire behaviour. However, the locations of the fire bed and the openings are ignored in the computation. Computational fluid dynamics techniques may be employed, but a major shortcoming is the requirement for extensive computational resources and lengthy computational time. A unique, new and novel artificial neural network (ANN) model, denoted as GRNNFA, is developed for predicting parameters in compartment fires and is an extremely fast alternative approach. The GRNNFA model is capable of capturing the nonlinear system behaviour by training the network using relevant historical data. Since noise is usually embedded in most of the collected fire data, traditional ANN models (e.g. feed-forward multi-layer-perceptron, general regression neural network, radial basis function, etc.) are unable to separate the embedded noise from the genuine characteristics of the system during the course of network training. The GRNNFA has been developed particularly for processing noisy fire data. The model was applied to predict the location of the thermal interface in a single compartment fire and compared with the experiments conducted by Steckler et al. (Flow induced by fire in a compartment, NBSIR 82-2520, National Bureau of Standards, Washington, DC, 1982). The results show that the GRNNFA fire model can predict the location of the thermal interface with up to 94.5% accuracy and minimum computational times and resources. The trained GRNNFA model was also applied to rapidly determine the height of the thermal interface with different locations of fire on the compartment floor and different widths of the opening against field model predictions. Among the five test cases, four of them were predicted well within the minimum error range of the experiment results. It also demonstrated that the prediction accuracy is related to the amount of knowledge provided for network training.

PREDICTION AND MEASUREMENT OF LOCAL TWO-PHASE FLOW PARAMETERS IN A BOILING FLOW CHANNEL

A two-fluid model to predict subcooled boiling flow at low pressure is presented. Although considerable success has been achieved in good axial predictions, this study focuses on the capability of the model to predict local two-phase flow parameters within an annulus channel. Comparison of model predictions is made against local measurements carried out by our Korean collaborators. Although reasonable agreement of local profiles of the void fraction, interfacial area concentration, and bubble frequency were achieved, significant weakness of the model was evidenced in the prediction of the mean Sauter diameter, liquid, and vapor velocities. The formulation of a transport equation to account for the dynamically changing interfacial area concentration is proposed. Further modeling work is in progress to incorporate the bubble coalescence behaviour seen during experiments into the transport equation.

On numerical comparison of enclosure fire in a multi-compartment building

This paper reports a validation study of a CFD simulation for an enclosure fire in a single level multi-room building. Model predictions are compared against measured data of Luo and Beck (Fire Safety J 23 (1994) 413). The CFD-based fire model focuses on the use of laminar flamelet approach to account for the combustion of fire. Global radiation in the multi-compartment building is evaluated though the discrete ordinates method (Combust. Sci. Technol. 59 (1988) 321). Soot model proposed by Syed et al. (Proceedings of the 23rd Symposium on Combustion, The Combustion Institute, 1990. p. 1533) that accounts for the essential physical processes of nucleation, coagulation, surface growth and oxidation is utilised to predict the soot formation and burnout. The presence of soot augmenting the global radiative heat exchange was considered. Overall, our results are in good agreement with the experimental data of Luo and Becks and also consistent with their numerical results.

On modelling combustion, radiation and soot processes in compartment fires

Abstract A Reynolds-Averaging-Navier–Stokes Computational-Fluid-Dynamics-based fire model is developed to solve a turbulent buoyant fire in a single-, two- and multi-compartment structure. The model is evaluated as part of a complete prediction procedure involving the modelling of the simultaneously occurring flow, convection, combustion, soot generation and burnout and radiation phenomena. Computational results are compared against available experimental data. Proper handling of the fire chemistry through combustion models such as eddy break-up and laminar flamelet is important to modelling compartment fires. Thermal radiation plays a significant role too. Soot radiation has shown to significantly improve the accuracy of the model predictions.

A numerical and experimental study of natural convection and interface shape in crystal growth

Abstract A numerical and experimental study has been conducted on the crystal growth of succinonitrile in a horizontal Bridgman apparatus. The shape of the solid—liquid interface was significantly influenced by three-dimensional natural convection in the liquid adjacent to the interface. The interface profile observed during experiments was compared with predictions from a two-dimensional (2D) finite element analysis and a three-dimensional (3D) finite difference approach. Good agreement was achieved between the experimental and predicted results. The computed velocities in the vicinity of the interface were found also to be in good agreement with the measured experimental velocities.

Stationary bathtub vortices and a critical regime of liquid discharge

A modified Lundgren model is applied for the description of stationary bathtub vortices in a viscous liquid with a free surface. Laminar liquid flow through the circular bottom orifice is considered in the horizontally unbounded domain. The liquid is assumed to be undisturbed at infinity and its depth is taken to be constant. Three different drainage regimes are studied: (i) subcritical, where whirlpool dents are less than the fluid depth; (ii) critical, where the whirlpool tips touch the outlet orifice; and (iii) supercritical, where surface vortices entrain air into the intake pipe. Particular attention is paid to critical vortices; the condition for their existence is determined and analysed. The influence of surface tension on subcritical whirlpools is investigated. Comparison of results with known experimental data is discussed.

Flickering Behavior of Turbulent Buoyant Fires Using Large-Eddy Simulation

A numerical study investigating the flickering behavior of a turbulent buoyant fire is conducted using large-eddy simulation to examine coupled turbulence, combustion, soot chemistry, and radiation effects. The three-dimensional, Favre-filtered, compressible mass, momentum, energy, and mixture fraction and its scalar variance conservation equations are closed using the Smagorinsky subgrid-scale (SGS) turbulence model. A two-stage predictor-corrector methodology for low-Mach-number compressible flows is adopted. Formation of large-scale vortical structures is well captured, with the predicted puffing frequency agreeing closely with experimentally determined frequencies. Comparisons of instantaneous, mean, and root-mean-square quantities also show qualitative agreement against other experimental data.

COMBUSTION AND HEAT TRANSFER IN COMPARTMENT FIRES

Field modeling that incorporates increasingly complex representations of the physical and chemical processes for compartment fires warrants a detailed evaluation. Detailed quantitative comparisons of our predicted velocity and temperature fields against established the computed results of Lewis, Moss, and Rubini and the experimental data of Steckler, Quintiere, and Rinkinen of a single compartment fire are in good agreement. The prospect of using a flamelet-based combustion model is encouraging. Here, more detailed chemistry can be incorporated, especially that used to predict toxic CO concentrations. This, together with the discrete ordinates radiation method, offers potential in building fire prediction.