Saturday, September 8, 2012

Scaling up the putt-putt

How about a steam boat with no moving parts? 

It's called a putt-putt, or pop-pop boat.

The principle is very simple:  a pipe, open at the stern of the boat has a closed end inside the boat that is kept very hot.  Water enters the pipe, and when it gets to the hot end it flashes to steam pushing the water out the pipe and propelling the boat forward.  The pipe is left with only steam in it, which cools and condenses, drawing water into the pipe again, and the process repeats.  Because water is drawn in from all directions at the end of the pipe, but ejected in only one direction, there is a net forward thrust from this cycle.

This cycle is not unique to the putt-putt.  It is essentially the same as that which takes place vertically in a geyser, and it is analogous to the operation of a valveless pulse jet where the pulse energy comes from burning fuel instead of flashing water to steam.

Putt-putt boats have been around for over a century, originating with a design patented in 1891 by a British inventor, Thomas Piot.  They became popular as toys in the early 1900's.  I bought one of these when I was a child by sending in a dollar along with a box top to a promoter for breakfast cereal.  Here is a video of one just like the one I had...

You can buy one for $3.98 at...

Many factors conspire to make the putt-putt principle only work well as a toy.  However I believe that each of the challenges can be met with creative engineering and I intend to build a putt-putt boat big enough to ride in.  The essential characteristic I aim to preserve is that water flashes to steam directly forcing water out of a nozzle to propel the boat, whether it uses valves or not.  My first priority is to make a water pulse jet that can carry me across a pond.

I've been having a lot of fun with various designs, dealing with such issues as pipe size (if the pipe is large enough, the water will lie on the bottom and not behave like a solid slug), energy lost to cooling of steam if the cycle is slower (it will be), and how quickly one can flash enough water to produce a man-propelling pulse.

I wrote a pretty long blog entry detailing how various pipe sizes might produce various amounts of thrust under various pressures, and therefore how much water needed to be vaporized at each cycle and how much heat would be required.  Without going through all the details, it appears that 1-2kW of heat (easy for a propane burner) should be able to produce about 5-10 lbs of thrust, and that should be enough for a leisurely canoe ride.  If it seems to slow, we can burn more propane or drink more wine.

Ordinarily I would delight in presenting those calculations, but there is one significant problem in scaling up the putt-putt cycle that has nothing to do with pipes, valves, calories or thrust equivalents.  The problem is how to flash something like one cc of water to steam in less than a tenth of a second.  In the case of the putt-putts, only a tiny amount of water gets vaporized and, to judge from the sound of it, the flashing takes only 10-20 milliseconds.

If you have ever dropped water on a hot skillet, you know that it bounces.  While it's a lot of fun to watch, it prevents the droplets from turning to steam quickly;  they simply ride around on a cushion of steam made from the first contact.  In the little putt-putts, that first contact produces enough steam for the cycle, but we need to produce 100-1000 times more steam in not much more time.

So I am now meditating on various boiler designs that might work.  Here is a picture of my first serious prototype.  The note on it is because I knew it would invite TSA inspection in my suitcase...

Interestingly, this "Leidenfrost effect" (thanks to emddudley for the ref) is the same problem that is involved in making the hot-head steam engine practical.  So I would be interested to hear any suggestions or actual experience in how to make this work.  Hopefully my first design will let me try out a lot of things such as surface texture, cavity shape, temperature ranges, etc.  So future posts on both the hothead steam engine and the man-sized putt-putt await a solution to the Leidenfrost effect.

Wave Sculpture Elements

Just as a graphical scene can be built up from primitive graphical elements and other structural elements such as transformations, superpositions, etc., so it is also possible to build up a wave sculpture by combining various primitive and compositional elements.

An Example Wave Sculpture
This can all be understood fairly intuitively by considering a sculpture such as that shown below...

Figure 1.  An Example Wave Sculpture 15m x 30m

The basic shape is a promontory of approximately the same extent as the typical distance between wave crests at the beach.  The choice of scale is such that different features of the sculpture are engaged in succession as the wave approaches the shore.  We believe that an ideal choice in this regard is one where the promontory is somewhat less than the distance between incoming crests.  This would afford a period of rest between each cycle of activity on the sculpture.

Next note that the promontory is wider at the base so that the sides are somewhat oblique to the incoming waves.  This is to minimize the effect of one cavity "shadowing" another behind it.  Each cavity, or cell, in the structure should intercept a relatively fresh and energetic section of the incoming wave.

Now note the location of the cavities:  their placement dictates a certain rhythm of activation.  For this design, cells a and b would act together as the wave first engages the promontory,  These cells might be blow-hole cells to give the effect of smoke coming from a dragon's nose.  After this cells, c, d, e, f, g, and h would act in sequence on alternate sides of the sculpture.  An effective choreography would be for these cells to be jets, designed to arch over to the cell on the opposite side that is about to shoot.  The overall sequence would a back-and-forth jumping motion with the arching trajectories creating a sort of arbor way as the wave comes in.

Finally, cells i and j will act together, and cells k and l at the end.  i and j might produce large forward- or outward-directed jets, and k and l might be a pair of ram tubes feeding a jet aimed straight into the air.  Alternatively, i and j might be scoops that fill the top level of a cascade that drains for a while until the next wave comes.

Composite Shapes
The example of figure 1 illustrates relative positioning of cells for timing, juxtaposition of cells for symmetry or sequence, and the use of different cell types to support an overall theme or dramatic effect.

Note that changing tides, and especially storm surges, can be used to effect a complete change in artistic effect.  Some cells might be buried and thus inactive at high tide and others might be out of the water and thus inactive at low tide.  Careful planning and simulation can take advantage of this effect to great advantage.
Figure 2.  A Composite Wave Sculpture

Figure 2 illustrates another composite shape consisting of two promontories similar to that of Figure 1.  These could be matched for symmetry or differentiated for variety.  It may at first appear that this configuration is simply 
two separate wave sculptures, but in fact it is three.  The space between the two promontories has been shaped to be a large converging channel designed to produce a variety of effects or one huge central effect.  For instance the central effect might be a scoop that fills a descending fountain as in figure 3, or it might be a spectacular jet that is only active during high tides and severe weather.
Figure 3.  A scoop feeding a trough with cascading falls

Some Basic Cells
The vocabulary of interesting wave-shaping cells is infinite, but many interesting effects can be obtained from four basic shapes and their simple variants.
Figure 4.  Four basic wave-shaping cell types

Jet - fig. 4a
The jet is designed to convert the forward momentum of a breaking wave into pulse of high pressure that shoots out a jet of water at high velocity.

Scoop - fig. 4b
The scoop is a non-turbulent effect.  In its simplest form, it is designed to shape a wave to run up a slope without ever breaking over.  The water just runs up the slope and back down.  However this shape can be combined with a converging outer channel to deliver a high volume of water to an elevated feature such as the top pool of a cascading fountain as in fig. 3a.

Blow hole - fig. 4c
The blow hole is similar to the jet in that its best effect is produced with high impact pressure, but in this case the configuration is designed to trap air that is then expelled in a swirling blast of air and spray.

Cannon - fig 4d
The cannon is similar to the jet, except that it is designed to shoot a small slug of water at an even higher velocity.  One can think of the jet as a gun, with the entire gun barrel full of water.  While a high impact pressure may shoot the water quite far, the mass in the barrel is large, and therefore resistant to acceleration.  The cannon is shaped so that, when idle, there is pool of water in the exit barrel, but the chamber is mostly full of air.  When a wave impacts the chamber, its first effect is to compress the air and shoot the pooled water out the barrel.

Almost all interesting wave shapes derive their impressive effects from some degree of convergence or focus of the incoming wave.  Focus increases velocity, which increases momentum and impact pressure.  Almost every cell can benefit from a converging shape at its entrance, but huge converging shapes are also possible as illustrated in the overall design of Figure 2.  Here the two promontories, in addition to being active wave sculptures themselves, serve to focus the entire wave volume between them on a narrow space at the shore, affording a great concentration of wave volume and momentum.

A ram constrains moving water to impact a barrier generating high instantaneous pressures.  The simplest version of this feature is a pipe leading to an active cell.  Because the pipe is closed, the momentum of all the water in the pipe will act in creating a pulse of high pressure at the end.  A ram pipe may also be somewhat convergent to increase velocity toward the end.

Help Wanted
Artists:  I'm in the process of building up a credible presentation to fund a real wave sculpture.  I would love to work with an artist to produce a few motivating images of an actual wave sculpture in action.  Can you see my sketches as real life?

Computational Fluid Dynamicists:  At the same time, I would like to work with people who are capable of simulating some of these effects so that we can understand how to improve them and combine them to best effect.  I don't think we can expect any patron to support a wave sculpture installation without a credible simulation of its action.

Environmental Artists:  There are all sorts of challenges involved in funding and permitting environmental art.  If you know people with experience in this area, please put me in touch.