Jul 4, 1997

How to Build the Perfect Drop






Science on Flight Day 5 of MSL-1

July 5, 1997 12:30 p.m. CDT


DCE burn on MSL-1/MET 03/02:34
Squeezing out a drop of fuel and lighting it off in microgravity (photo at right) so you can study how it burns is a complex task that has taken almost 40 years to figure out and demonstrate. Even getting long periods of weightlessness aboard Spacelab was not enough to ensure that the experiments could be done. Special gear was needed to form and ignite the droplet, then scientists had to figure out how best to observe the fireball so they could extract meaningful data that could be applied here on Earth.

Fires in space have been a popular item among news writers covering the MSL- mission, but few have explored how the fires are set up in facilities such as the Droplet Combustion Experiment (DCE).

"The idea of space-based droplet combustion has been around for about 15 years," said Dr. Anthony Marchese of Rowan University in Glassboro, N.J.

The work started with Dr. S. Kumagai of the University of Tokyo in the 1950s. In a prelude to drop towers now used by NASA for various experiments, Kumagai built an apparatus that could form a droplet that would burn during a 2-second fall. But it rarely worked because of the difficulty of timing the release, and because the droplet often was disturbed by the act of releasing it from the injection needle.

Why bother?

So much effort is being expended in studying droplet combustion because society is placing much greater demands on itself in transportation. More people means more planes, trains, and automobiles... and more trucks, motorcycles, and ships - in short, more vehicles of all kinds. With the exception of electrically driven vehicles, each of those vehicles turns a hydrocarbon - gasoline or diesel fuel - into a mist and ignites it to expand and push a piston or turbine and drive the wheels. More vehicles means more fuel is needed, and more emissions are produced.

And that demands higher efficiency as well as reduced emissions from each vehicle.

In the 1940s, it was easier to squeeze a 4 percent improvement out of automobile combustion, explained Dr. Fred Dryer, a DCE co-investigator at Princeton University. Extra efficiency is harder to squeeze out of today's advanced engines, and the designer must also cope with laws that limit or ban emissions of nitrogen oxides, hydrocarbons, particles, and other exhaust products.

"How do I design to that?" Dryer asked. "I have hundreds of engine parameters - turbulence, chemical reactions, droplet trajectories, vaporization - that I have to change in engine testing which is very labor intensive and expensive.... The complexity of that problem is just incredible. Therefore, we need sophisticated computer tools to help guide engineering development."

Princeton University has been looking into the problem since the 1950s when it was involved in gas turbine design, then helped solve combustion instability problems that plagued the engines on the Saturn 5 moon rockets. That work drew Dryer to Princeton 30 years ago. While Princeton is out of the rocket business, their research into the basics of combustion led them back into space.

Isolating the problem

So with DCE, scientists are looking at one small set of parameters, the basic mechanics of a single burning drop to study the complex phenomena which control combustion and emissions.

Droplets burned in DCE will not lead directly to the 100 mile-per-gallon car.

"This is a hard part to get across to the public or to the designers of an engine," Dryer continued. "If I look at the spray in an engine, it's made of droplets 4 to 5 microns across [about 1/5,000th of an inch]. On DCE, we're working with droplets are as large as 5,000 microns [5 mm or about 1/5th of an inch]. That bears no direct relationship to those in a spray so dense you can't see through it, as in an engine. What we will do for engine efficiency is a circuitous but important connection."

Measurements that are accurate and precise - meaning both correct and detailed - will allow scientists to improve the various models for chemistry, diffusion, and other phenomena that are parts of complex engine models. And that, in turn, will lead to better design predictions and cleaner, more efficient engines.

But first, they have to make the drop stand still while it burns.

Fire and the single droplet

"The whole idea was to find a way to make single drops, to make them float and ignite them with little or no motion," explained Dr. John Haggard, the DCE project manager with NASA's Lewis Research Center. Dr. Forman A. Williams, the DCE principal investigator, Dryer, and other scientists built on Kumagai's experiments with their own drop tower experiments in 1976-83. Starting in 1983, Haggard said, it was developed as a space flight experiment. Drop towers did not offer a long enough time for proper tests - some DCE droplets burn for 12 seconds or more - nor did longer times in weightless aircraft provide a smooth enough ride - DCE is among the most sensitive of microgravity experiments.

Working with TRW Co., the group that became the DCE science team developed the technique that viewers can sometimes catch on TV, or by checking the Liftoff project web site. You can also view some mpeg movies of DCE performed in the last 36 hours, available on the Science In Action page (part of this site).

Two small injection needles swing into position, like tiny robot arms, until they almost touch in the middle of the test chamber. A syringe pushes a little fuel through the needles to form a liquid bridge between the tips.

At the same time, two small metal loops are positioned near the bridge. These are nickel chromium wires that glow like a light bulb filament when an electrical current passes through.

Flame on

After the fuel droplet is deployed, the wires are heated. They don't ignite the droplet, but instead ignite the vapor cloud that has formed around the drop as the liquid evaporates.

An instant before the igniter wires fire, the needles pop away to leave the droplet motionless in the middle of the chamber. The wire loops are heated, igniting the vapor and the flame heads inward, from all sides, to light the surface of the entire drop at once.

This complex approach solves the problems that faced Kumagai as he did pioneering work in the field.

As viewers can see on TV, sometimes the drop does not ignite, sometimes it forms a beautiful blue fireball that rivals the best Hollywood special effects departments. Although it looks artistic on TV, science requires hard numbers which can be put into equations.

Those come from two sets of measurements, Dryer explained.

First, a special film camera photographs the droplet at 70 frames per second on 35 mm film. With the droplet illuminated from behind, the large film frames let scientists precisely measure the diameter of the drop as it burns. When it stops shrinking, it stops burning.

Second a special electronic camera, using a modified version of the charge-coupled devices used in camcorders, measures the position of the flame. Marchese, a former student of Dryer, explained that this cannot be done with an electronic temperature sensor (a thermocouple) because that would absorb part of the heat and cause the droplet to move. Neither can it be done by looking at the video image of the fireball, which has too broad a shape when graphed.

The best and brightest

Instead, the scientists look at the intensity of light emissions of hydroxyl, one molecule comprising one atom each of hydrogen and oxygen, which is produced at the flame front. It gives off a distinctive glow at 305 nanometers (nm) wavelength (just barely past human vision and into the ultraviolet part of the spectrum). By simulating the formation and emission of hydroxyl on a computer, it is possible to determine how far the flame stands out from the fuel drop.

From these, Dryer, Williams, and other members of the DCE team hope to gain a better understanding of how a drops burn and use that to improve our understanding of the internal combustion engines we have taken for granted for the last century.

With the limited tests run on the first MSL- flight in April, the DCE team probably will wind up with almost three weeks worth of tests. It won't be enough. The dozens of tests cover a very narrow portion of the hundreds of possible conditions for engines and furnaces. And they don't even touch the burning of solids, an area where Princeton is active with studies of coal and metals.

"These experiments have developed techniques that I wager will be used on the International Space Station for decades," Dryer predicted.


For the next 12 days, you can follow along and learn about the science being performed on the mission through activities on this WWW site, as well as the "Liftoff" Mission Home Page, and the Shuttle Web Site. Check out our daily image and video highlights on the "Science In Action" page!!

  • Check out the twice daily Mission Status Reports prepared by Marshall's Public Affairs Office.
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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack