EUREKA NOVEMBER 2003 COVER FEATURE STORY

Harnessing the tornado



A surprisingly large number of fluid based processes can be enhanced by inducing vortices. Tom Shelley reports

By deliberately inducing substantial swirls in fluid flows, it is possible to extract useful amounts of energy from low wind speeds, produce heat and cold at the same time, improve internal combustion engine efficiency, obtain drinking water from desert air, improve industrial washing and keep particles in pumped slurries in suspension.

Vortex based developments are taking place in every industrialised country, including the UK, but a particularly large proportion originate in Russia, where Soviet era governments targeted vortices as an especially promising field of research.

The most striking example of the power of a vortex, if impractical to harness as a useful source of energy, is the tornado. A less powerful example is the small whirlpool that forms in the bath tub, sink or toilet when water is draining out. In each case, there is lower pressure and temperature in the centre, than towards the outside.

The vortex starts when particles are pulled towards the central suction. Because there is less room in the centre than outside, and because the particles already have some momentum, some of them have to end up off to one side of the point of suction when they are arrive. This deflection results in particles spiralling in. Once the spiral starts, it tends to influence all the other particles as they arrive, establishing the vortex.

It is also possible to establish a vortex deliberately, by introducing gas or fluid tangentially into a round chamber, and removing gas or fluid from one or both ends. In the case of a gas, it expands as its enters the chamber, converting internal heat energy into kinetic energy. However, gas near the centre moves at a lower circumferential speed than at the periphery and has a lower kinetic energy. Conservation of angular momentum should make the gas in the centre rotate much faster than at the periphery, but this tendency is countered by friction between the gas streams. Angular momentum is thus transferred from the gas in the centre of the chamber to gas at the periphery. The net result is that gas removed from the centre is cooler than gas removed from the periphery. This is the principle behind the vortex tube originally invented in 1928 by George Ranque, a French physics student, and rediscovered by Rudolph Hilsch, who published a widely read scientific paper about it in 1945. Vortex tubes powered by compressed air and producing cold air at temperatures down to -40 deg C are available commercially in the UK from Meech Technology in Oxfordshire.

Russian researchers, however, have taken both principles and applied them to a whole raft of novel devices. Some of these have been developed to the point of commercial exploitation by a small company based in Zhukovsky, Moscow: NVF Noteka.

The first of these is what the firm calls a WhirlWind Energetic Installation (WWEI), which is designed to take low velocity winds, and convert these into an upward moving, spiralling air jet, which can be used to drive a rotor. By this means, the developers claim to be able to recover useful energy from lower speed winds, as well as greatly reducing engineering costs relative to conventional wind turbines.

Even better performance can be achieved by combining the effect of the wind with that of rising hot air produced in a greenhouse, or factory operation. The heated air is fed into an air duct, with a WWEI unit at its top end, provided this is more than 50m above the input to the duct.

By pumping water through a vortex tube, the company has developed a water heater, which it claims, is 98% efficient, as opposed to converting water flow to electrical energy in a turbine and then using this as a source of heat.

By using a special patented probe in a car exhaust pipe to induce swirling, it is possible to increase suction and reduce pressure fluctuations. The company says that test rig and on the road tests on a 1.5 litre engine fitted with the probe showed an increase in effective engine power or 10 to 12%. Fuel consumption was reduced by 10% at 2,000 rpm and by 20% at 4,000 rpm. Because of more complete combustion in the engine, carbon monoxide was reduced by 10 to 15% and hydrocarbons by 15% to 20%.

Using a variant of the WWEI type of device, fitted with condensation grids, it is possible to condense drinking water from the atmosphere. When the vortex jet emerges from the device, it is allowed to expand, bringing about a drop in temperature below the dew point.

An interestingly novel approach is the use of vortex to enhance washing actions applied to flat and slightly curved surfaces. A 'Vortex washing vacuum cleaner' has a circular washing head with a brush round its periphery and tangential nozzles. The centre of the head is connected by way of a flexible hose to the vacuum cleaner. Suction presses the head against the surface to be washed and the combination of the tangential water jets and suction induces the vortex, which causes an inner disk, equipped with two linear brushes to rotate. The developers say that the machine functions as well as the best imported products and is much cheaper to make. A machine with a 0.8kW to 1.2kW motor weighs 15 to 30kg and can wash 2 to 4 m2/min.

It should at this point be mentioned that vortex technology is also researched in the UK. A team led by Professor Nick Miles at the School of Chemical, Environmental and Mining Engineering at the University of Nottingham has patented the induction of a vortex within a pipe in order to improve the transport of slurries. Their "Swirl in pipes" technology lifts solid particles so they cannot settle, reducing wear, particularly on the insides of bends.

The technique induces swirl gradually. The optimised geometry used to attain this is crucial. There are no sudden changes so pressure drop is reduced and energy consumption is minimised.

Meech Vortex Tubes
Email NVF Noteka
Susan Huxtable at Nottingham University

Pointers

* By encouraging the formation of vortices, it possible to harvest energy from relatively slow moving flows
* Vortex technology can be used to cool and heat without using moving parts
* Inducing swirl looks to be advantageous to apply on the exhaust side of internal combustion engines as well as input side.
* Vortices can also be used to enhance vacuum cleaner based washers and improve the transport of slurries through pipes

For more technical developments see www.eurekamagazine.co.uk

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