COMPUTATIONAL FLUID DYNAMICS

Computational Fluid Dynamics (CFD) is the study of fluid flow, often combined with simultaneous heat or mass transfer, using computer modelling techniques. It provides a means of solving the Navier Stokes equations, together with conservation equations for energy, species etc. in complex geometries. By subdividing the region of interest into thousands or millions of small volumes (known as cells), the local pressure, velocity, temperature etc. are obtained for each cell. There is now commercial software available that is optimized to run on multi-core computers or clusters allowing simulation of complex phenomena. Our focus is on the extension of commercial software rather the development if new codes. Most of our modelling work uses the CFD solvers ANSYS CFX or ANSYS Fluent, and TranSAT for applications where we use the Level set method to determine gas-liquid interfaces.

Laminar Single Phase Flow

It is possible to obtain significant enhancement to heat transfer with a relatively low increase in the pressure drop by introducing bends in the channel that create Dean’s vortices. We have performed studies to investigate the effect of channel cross-section (circular, semi-circular, rectangular and triangular) and the channel path (serpentine, sinusoidal and trapezoidal) for a wide range of conditions in the laminar regime.

Streamlines showing the mixing at bends.

Temperature plot showing the efect of the Dean votices.

Laminar channel flow and heat transfer - publications

Turbulent Single Phase Flow

In laminar flow the conservation equations are known and can be solved in their native form. However, in turbulent flow it is necessary to introduce a turbulence model because it is impractical to use a computational mesh sufficiently fine to resolve all of the length-scales occurring in turbulent flow. Therefore empirical models are introduced to remove this problem at the cost of including further equations that need to be solved. A key task in the simulation of turbulent flow and heat transfer is ensuring that the turbulence model is suitable for the flow under study, especially when flow separation or heat transfer are important.

Turbulent flow and heat transfer - publications

Two Phase Flow

In many important engineering applications the flow comprises two phases (a gas and a liquid) which can have very different flow regimes depending on the volume fractions of the gas. With increasing gas flow rate for fixed liquid flow rate the flow regime transitions from bubbly flow, to Taylor flow to annular flow. Each regime has different pressure drops and heat and mass transfer rates. As with turbulent flow, additional relations/assumptions are required to close the system of equations. In addition it is necessary to locate the interface between the gas and liquid phases. Many different methods to solve such flows have been developed.

Modelling techniques - publications

Taylor Flow

Amongst the various flow regimes that occur Taylor flow is particularly important because of its properties (gas bubbles separated by liquid slugs with a film of liquid at the wall with little back mixing).

Taylor bubble: nose and tail section

Thus far our work has considered the important numerical considerations in the modelling of such flows and the heat transfer enhancement they give.

Taylor flow simulations - publications

Experimental Studies and Model Validation

A key component of the modelling work performed here is validation of the CFD results against experimental data. The length-scale of interest in our applications is channel diameters of the order of 0.1-2 mm, as these are used in micro-structured heat exchangers and reactors. The experimental facilities include a state-of-the-art microPIV system, which can be used to probe the conditions in the liquid film occurring in Taylor flow. In addition we can use the same high speed cameras to take high quality photographs which yield detailed information on bubble shapes. Instrumented flow rigs to determine pressure drop, heat transfer rates etc. across a wide range of flow regimes have been constructed and are used to collect data for the modeling of the results from CFD simulations.

Model validation - publications

Summary

Currently work is underway to collect more validation data across a number of flow regimes, and simulations of Taylor flow, annular flow and the influence of turbulence are underway. For more information please contact Adjunct Professor David Fletcher.

PUBLICATIONS

Laminar channel flow and heat transfer
N.R. Rosaguti, D.F. Fletcher and B.S. Haynes, Laminar flow and heat transfer in a periodic serpentine channel. Chem. Eng. Technol., 28(3), 353-361, (2005).

N.R. Rosaguti, D.F. Fletcher and B.S. Haynes, Laminar flow and heat transfer in a periodic serpentine channel with semi-circular cross-section. Int. J. Heat Mass Trans., 49(17-18), 2912-2923, (2006).

P.E. Geyer, N.R. Rosaguti, D.F. Fletcher and B.S. Haynes, Thermohydraulics of square-section microchannels following a serpentine path. Microfluid. Nanofluid., 2(3), 195-204, (2006).

P.E. Geyer, N.R. Rosaguti, D.F. Fletcher and B.S. Haynes, Laminar flow and heat transfer in periodic serpentine mini-channels. J. Enhanced Heat Transf., 13(4), 309-320, (2006).

N.R. Rosaguti, D.F. Fletcher and B.S. Haynes, Low Reynolds number heat transfer enhancement in sinusoidal channels. Chem. Eng. Sci., 62(3), 694-702, (2007).

P.E. Geyer, D.F. Fletcher and B.S. Haynes, Laminar flow and heat transfer in a periodic trapezoidal channel with semi-circular cross-section. Int. J. Heat Mass Trans., 50(17-18), 3571-3480, (2007).

N.R. Rosaguti, D.F. Fletcher and B.S. Haynes, A general implementation of the H1 boundary condition in CFD simulations of heat transfer in swept passages. Int. J. Heat Mass Trans., 50(9-10), 1833-1842, (2007).

R. Gupta, P.E. Geyer, D.F. Fletcher and B.S. Haynes, Thermohydraulic performance of a periodic trapezoidal channel with a triangular cross-section. Int. J. Heat Mass Trans., 51(11-12), 2925-2929, (2008).

Turbulent flow and heat transfer
D.F. Fletcher, P.E. Geyer and B.S. Haynes, Assessment of the SST and omega-based Reynolds stress models for the prediction of flow and heat transfer in a square-section U-bend. Comput. Therm. Sci., 1(4), 385-403, (2009).

Modelling techniques
D.F. Fletcher, B.S. Haynes, J. Aubin and C. Xuereb, Modelling of microfluidic devices. In Handbook of Micro Reactors Vol. 1: Fundamentals, Operations and Catalysts, (Eds. V. Hessel, J.C. Schouten, A. Renken and J.-I. Yoshida), Chapter 5, 117-144, Wiley-VCH, (2009).

R. Gupta, D.F. Fletcher and B.S. Haynes, Taylor flow in microchannels: A review of experimental and computational work. J. Comput. Multiphase Flows, 2(1), 1-31, (2010).

Taylor flow simulations
R. Gupta, D.F. Fletcher and B.S. Haynes, On the CFD modelling of Taylor flow in microchannels. Chem. Eng. Sci., 64(12), 2941-2950, (2009).

R. Gupta, D.F. Fletcher and B.S. Haynes, CFD modelling of flow and heat transfer in the Taylor flow regime. Chem. Eng. Sci., 65(6), 2094-2107, (2010).

Experimental studies and model validation
T.S. Fouilland, D.F. Fletcher and B.S. Haynes, Film and slug behaviour in intermittent slug-annular microchannel flows. Chem. Eng. Sci., 65(19), 5344-5355, (2010).

S.S.Y. Leung, Y. Liu, D.F. Fletcher and B.S. Haynes, Heat transfer in well-characterised Taylor flow. Chem. Eng. Sci., 65(24), 6379-6388, (2010).