David F. Fletcher

Flow boiling heat transfer of Freon R11 and HCFC123 in narrow passages

Flow boiling heat transfer coefficients for Freon R11 and HCFC123 in a smooth copper tube with an inner diameter of 1.95 mm have been experimentally investigated. The parameter ranges examined are: heat fluxes from 5 to 200 kW m−2; mass fluxes from 50 to 1800 kg m−2 s−1; vapour quality from 0 to 0.9; system pressures from 200 to 500 kPa; and experimental heat transfer coefficients from 1 to 18 kW m−2 K−1. It was found that the heat transfer coefficients are a strong function of the heat flux and the system pressure, while the effects of mass flux and vapour quality are very small in the range examined. This suggests that the heat transfer is mainly via nucleate boiling. The present experimental data were compared with some existing correlations and recommendations on their use is made.

Flow and mixing fields of turbulent bluff-body jets and flames

The mean structure of turbulent bluff-body jets and flames is presented. Measurements of the flow and mixing fields are compared with predictions made using standard turbulence models. It is found that two vortices exist in the recirculation zone; an outer vortex close to the air coflow and an inner vortex between the outer vortex and the jet. The inner vortex is found to shift downstream with increasing jet momentum flux relative to the coflow momentum flux and gradually loses its circulation pattern. The momentum flux ratio of the jet to the coflow in isothermal flows is found to be the only scaling parameter for the flow field structure. Three mixing layers are identified in the recirculation zone. Numerical simulations using the standard k-ϵ and Reynolds stress turbulence models underpredict the length of the recirculation zone. A simple modification to the C1 constant in the dissipation transport equation fixes this deficiency and gives better predictions of the flow and mixing fields. The mixed-is-b...

Techniques for computational fluid dynamics modelling of flow in membrane channels

Abstract Accurate modelling of the flow and concentration polarisation in pressure driven membrane processes is inhibited by the complex couplings in the flow equations along with any added effects of variable solution properties. A generic computational fluid dynamics (CFD) model has been developed which incorporates these effects and describes the flow across the membrane wall. The results have been validated against classical solutions available in the literature. Extended work indicates that overly simplified expressions for the dependence of viscosity and diffusivity on concentration produce velocity and concentration profiles that may grossly misrepresent reality.

A CFD based combustion model of an entrained flow biomass gasifier

This paper contains the description of a detailed Computational Fluid Dynamics (CFD) model developed to simulate the flow and reaction in an entrained flow biomass gasifier. The model is based on the CFX package and represents a powerful tool which can be used in gasifier design and analysis. Biomass particulate is modelled via a Lagrangian approach as it enters the gasifier, releases its volatiles and finally undergoes gasification. Transport equations are solved for the concentration of CH4, H2, CO, CO2, H2O and O2 and heterogeneous reactions between fixed carbon and O2, CO2 and H2O are modelled. The model provides detailed information on the gas composition and temperature at the outlet and allows different operating scenarios to be examined in an efficient manner.

Physical and numerical modelling of thunderstorm downbursts

Abstract In this study, a static continuous impinging jet has been used to simulate a thunderstorm downburst. Velocity characteristics have been investigated at various positions in the flow over flat terrain, and over simple topographic features, thereby yielding topographical speed-up factors. A generic empirical equation describing the development of the velocity profile over a flat board has been developed. Comparison with previous works with impinging jets and conventional boundary layer flow speed-up factors has been completed. Speed-up factors were similar at the crest of the embankment, but dropped off rapidly behind the crest compared with boundary layer flow. Preliminary studies using computational fluid dynamics to predict the flow regime over topographical features has been shown to give reasonable agreement.

A new volume of fluid advection algorithm: the defined donating region scheme

This paper presents a new volume of fluid (VOF) advection algorithm, termed the defined donating region (DDR) scheme. The algorithm uses a linear piecewise method of free surface reconstruction, coupled to a fully multi-dimensional method of cell boundary flux integration. The performance of the new scheme has been compared with the performance of a number of alternative schemes using translation, rotation and shear advection tests. The DDR scheme is shown to be generally more accurate than linear constant and flux limited schemes, and comparable with an alternative linear piecewise scheme. The DDR scheme conserves fluid volume rigorously without local redistribution algorithms, and generates no fluid ‘flotsam’ or other debris, making it ideal in applications where stability of the free surface interface is paramount. Copyright

A CFD study of unsteady flow in narrow spacer-filled channels for spiral-wound membrane modules

In spiral-wound membrane modules, spacers are used to enhance wall shear stress and to promote eddy mixing, thereby reducing wall concentration and fouling. Insights into the effect of spacer filaments on flow patterns in narrow channels were obtained using a computational fluid dynamics (CFD) code. The flow patterns were visualized for different filament configurations incorporating variations in mesh length, filament diameter and for channel Reynolds numbers up to 1000. The simulated flow patterns revealed the dependence of the formation of recirculation regions on the filament configuration, mesh length, filament diameter and the Reynolds number. When the channel Reynolds number is increased above 300, the flow becomes super-critical showing time-dependent movements for a filament located in the center of a narrow channel; and when the channel Reynolds number is increased above 500, the flow becomes super-critical for a filament adjacent to the membrane wall. For multiple filament configurations, flow transition can occur at channel Reynolds numbers as low as 80 for the submerged spacer at a very small mesh length (lm/hch = 1) and at a slightly larger Reynolds number at a larger mesh length (lm/hch = 4). The transition occurs above Rech of 300 for the cavity spacer (lm/hch = 4) and above Rech of 400 for the zigzag spacer (lm/hch = 4).

Effect of Axial Agitator Configuration (Up-Pumping, Down-Pumping, Reverse Rotation) on Flow Patterns Generated in Stirred Vessels

Single phase turbulent flow in a tank stirred with two different axial impellers, a pitched blade turbine (PBT) and a Mixel TT (MTT), has been studied using Laser Doppler Velocimetry. The effect of the agitator configuration, i.e. up-pumping, down-pumping and reverse rotation, on the turbulent flow field, as well as power, circulation and pumping numbers has been investigated. An agitation index for each configuration was also determined. In the down-pumping mode, the impellers induced one circulation loop and the upper part of the tank was poorly mixed. When up-pumping, two circulation loops are formed, the second in the upper vessel. The PBT pumping upwards was observed to have a lower flow number and to consume more power than down-pumping, however, the agitation index and circulation efficiencies were notably higher. The MTT has been shown to circulate liquid more efficiently in the up-pumping configuration than in the other two modes. Only small effects of the MTT configuration on the power number, flow number and pumping effectiveness have been observed.

Computational fluid dynamics modelling of flow and permeation for pressure-driven membrane processes☆

While models of membrane systems with varying degrees of complexity have been developed since the early 1960s, reports of the use of computational fluid dynamics to provide in-depth insights into the separation phenomenon have only recently appeared in the literature. This paper describes the validation and application of a computational fluid dynamics model of pressure-driven membrane processes involving selective removal of components in the feed channel and their transfer to the permeate channel. The effects of changes in rejection, wall permeation rates and solution properties on velocity and concentration profiles are presented for empty channels and channels with eddy promoters. For high polarisation applications typically encountered in conventional ultrafiltration applications, the results demonstrate the need for very fine meshes near the membrane wall, together with high order numerical schemes and accurate modelling of rejection and physical property variations in order to obtain accurate and reliable predictions of the polarisation and flow phenomenon.

An integral model for the transient pyrolysis of solid materials

The modelling of the spread of fire and its extinguishment still represents a significant challenge. As part of a combined experimental and computational study of fires we have developed a general Computational Fluid Dynamics (CFD) model of fire spread and extinguishment. The primary objective was to produce a flexible computational tool which can be used by engineers and scientists for design or research purposes. The present paper deals with the description and validation of a solid pyrolysis model which has been applied, as a sub-model, in this general computer fire code. The pyrolysis model has been formulated using the heat-balance integral method. The model can be applied to slabs of char forming solids, such as wood, as well as non-charring thermoplastic materials, such as PMMA. Results are compared with analytical solutions, numerical simulations and experimental data. In all cases the integral model performs well. ( 1997 by John Wiley & Sons, Ltd.