Current
Topics in Thermo-fluid Dynamics Research
The aim of the proposed research is to develop computational tools and thereby to investigate the motion of particles in a turbulent flow field. Two complementary approaches will be adopted : Lagrangian and Eulerian. In the Lagrangian approach, individual particles are tracked as they encounter various fluid eddies. The approach is stochastic in nature, and several (a few thousand) particles have to be tracked to calculate meaningful statistical averages. In the Eulerian approach, attention is focused to a fixed region in the flow field. This approach is computationally much faster than Lagrangian stochastic calculations (by several orders of magnitude, especially if the particle size is small), and can easily be integrated with commercially available flow solvers that calculate the basic flow field in the continuum fluid. A new Eulerian theory being developed at Bristol shows much promise. The application of the
results and
methods of analysis are diverse, being relevant for environmental
engineering, mechanical engineering, chemical engineering, and
physiology.
For example, it would be used to study the movement and dispersion of
hazardous
substances in the atmosphere. The computations will be complemented by
experiments.
The mechanisms by which particles migrate to a solid surface through the turbulent boundary layer are complex, and the normally used gradient transport equations such as Fick's law do not provide an adequate mathematical description. Apart from the well known particle transport mechanisms such as 'inertial impaction' and 'turbulent diffusion', there is a tendency of particles to migrate in the direction of decreasing turbulence level ('turbophoresis') and against temperature gradients in the surrounding fluid ('thermophoresis', which is expected to play a role for the deposition on gas turbine blades as they are highly cooled). The Eulerian theory developed at Bristol includes all physical mechanisms of deposition. The project aims at developing a two-dimensional time-marching computational method for calculating deposition on turbine blades. The same methodology is applicable to deposition of water droplets on steam turbine blades and of various particles on gas turbine blades. In recent years, the
combined cycle
has rapidly emerged as the favoured technology for electricity
generation
because of its high efficiency and low pollution characteristics.
Currently,
the most popular fuel is natural gas but in the long term it is likely
to be replaced by coal. Gas turbines burning coal suffer degradation of
the turbine blading by deposition and erosion of solid ash particles
which
are too small to be removed by the filtration system.
Existing numerical methods for predicting two-dimensional, non-equilibrium wet-steam flows in the blade-to-blade plane usually assume the flow to be inviscid. The project aims to introduce the effects of viscosity. Apart from making numerical predictions more realistic, the calculations are expected to throw new light on the nature of nucleation in boundary layers.
Previous works have identified various relaxation processes and their time scales, and their effects on the structure of shock waves in a pure vapour-droplet flow. It is intended to extend these studies by considering a mixture of an inert carrier gas and a condensable vapour (e.g. moist air). A pure vapour implies that the phase change is heat transfer rather than diffusion controlled. The situation may change if an inert carrier gas is present. Another difficulty arises since the size of the liquid droplets (formed through homogeneous nucleation) may sometimes be comparable to or less than the mean free path of the surrounding vapour. Thus a kinteic approach has to be taken for developing a universal theory applicable at any arbitrary Knudsen number.
Heat exchangers are devices in which heat is transferred from one fluid to another through a solid surface. They have a wide range of applications, e.g., in refrigerating plants, air-conditioning units, power industry, process industry and propulsion. It is not practicable to stretch the performance limits of conventional manifolding techniques by using any finer tubes than what are used today. What is needed is a radical departure from the established methods for configuring heat exchanger design. A systematic design philosophy is being developed at Bristol. The new rationale draws parallel from the designs used in natural gas exchangers found in biological systems. The objective is to emulate nature to improve engineering design and, in turn, explain the physical rationale of the ‘design’ of biological systems. The outlook is a new generation of heat exchangers with a few order of magnitudes higher power transfer rate and compactness. Mathematical models are
being developed
in order to optimize the branching rules of the fractal manifold. New
manufacturing
techniques are also required.
Maintained by A. Guha . |
