Hi all,
Last weekend at the Wicksteed 2010 event, my Steam Jet powered experimental boat had its maiden voyage (see Wicksteed 2010 page 10 for description -
http://www.modelboatmayhem.co.uk/forum/index.php?topic=21652.msg238462#msg238462 ).
Having developed a flash boiler for the Vital Byte experimental boat, I thought the light boiler could be used as the basis of a steam jet engine and last year tried out a couple of unsuccessful ideas and soon discovered that the steam condensed in the air before producing a force.
Last February, I had the idea of expanding the steam through a nozzle into the very hot combustion gases of the boiler much in the same way as an exhauster on a steam loco which uses the cylinder exhaust steam to draw the fire in the furnace.
A prototype engine (boiler? - I call it a Bengine) was built and gave an encouraging performance - the jet exhaust actually set alight a piece of wood! The "Bengine" was about 1200mm long though, quite impractical for a model boat. Then, as I was winding a flash boiler coil on a piece of 22mm copper pipe as a former, I had the bright idea of keeping the copper former as the exhaust pipe through which the steam jet would be aimed through.
This exhaust pipe not only supported the boiler coil, but also passed back through the furnace and eventually was used as part of the butane/propane gas pre-heater. Using this arrangement the length was reduced to 350mm and also the construction was simplified.
I built a “swinging” test rig to test the thrust produced by the Bengine and was confused by how little the swing moved horizontally compared with force exerted against a sheet of aluminium held in the jet exhaust stream. The swing indicated a thrust of less than 1 Newton (1kg held in the hand exerts a downward force of approximately 10 Newtons) – this equated to about 3 ounces of force in old money.
The force on the aluminium sheet was telling me a different story. At a very rough estimate, it was about 0.7 kg , which was about 7 Newtons. Well, according to Newton’s Third Law every action has an equal and opposite reaction – so what was happening here.
I also rigged up a simple Pitot Tube (the thing aircraft use for measuring their air-speed) to measure the exhaust velocity of the Bengine – astonishingly it was calculated to be in excess of 250mph (400km/hr for our Antipodean friends). I used this figure to calculated the mass flows, momentum and kinetic energy of the exhaust and, sure it enough, it confirmed the small amount of thrust being developed, but didn’t explain the force on the aluminium plate.
So, being a Mayhemer, I thought let’s build a boat and get some real world experience to confirm or contradict the theory. Having built the boat in record time off I set for Wicksteed 2010 ( the home test tank being too small for a proper test).
One little side feature was the addition of a jet vectoring device in place of the usual rudder. This device was based on the Coanda effect where a fluid flow follows a curved surface.
The maiden voyage confirmed the lack of thrust being developed, confirmed that the vector thrust steering was practical and, as chance would have it, the equivalent of the aluminium plate sailed by the exhaust in the form a metre long model of a destroyer. The model destroyer was heavily blown over to port by the Bengine exhaust blast – the steam jet boat was no friend to other users on the pond.
There was the dichotomy – the jet could knock over another boat, but could hardly move the boat it was suppose to be propelling.
What was going on with Newton’s Third law????
Ian