Fireworks

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A Scientific Look at Fireworks, Part II

Posted by Brian (Team UV) on November 11, 2014
Posted in: Well Read. Tagged: , , , , , , , , , , . Leave a comment

Photo Credit: Quotery.com

Welcome back to TeamUV.org and let me just start off by saying that we are going to have a busy next couple of days.  We (of course) have this Well Read post today, will have a special Veterans day post at 1300 hours (1 p.m./in 3 hours), a post announcing the start of our fundraising campaign tomorrow (Wednesday 1000 hours/10 a.m.), and then we will be back to our regular schedule with a Presentation post by Andrew on Thursday.  Whew!

Today, we are going to continue off of my last Well Read post with a discussion of the science behind fireworks.  More specifically, today we are going to take a brief look at what determine the actual shape of the burst, whereas last time we took a look at the thrust created during the initial firing of the fireworks.  Without getting too much into the chemistry of this subject, we can basically see the compounds within the firework’s shell as consisting of a mixture of liquids and solids (fuel, oxidizer, color-producing compounds, a binder, and possibly other additives) that are used for both the bursting charge and the aesthetic effects.  When an aerial firework is initially fired, the reactants combust to produce thrust (as discussed last time), launching the actual firework shell into the air.  At the same time as the shell is rocketing up into the air, a fuse (which was lit by the combustion) is burning.  Eventually, that fuse will burn down to the firework’s charge, igniting the charge and thus producing the explosion, and triggering the reaction of all the other compounds within the shell.  Upon explosion of the entire shell, a bunch of particulate jets are formed and are shot out in many directions.  Generally speaking, these jets are what determine the form of the burst that we see when we watch fireworks.  These jets are shown below; it is also interesting to note the Karman Vortex Streets visible in the turbulent wakes of the projected particulate, as caused by flow separation (and subsequent recirculation) along the blunt body of the projectiles.

Screenshot from the video in the last fireworks post showing particulate jets after explosion of the firework's shell. Photo Credit: Discovery Channel

Screenshot from the video in the last fireworks post showing particulate jets after explosion of the firework’s shell.
Photo Credit: Discovery Channel

So what governs the size, shape, and behavior of these jets?  While a lot is unknown about the exact cause of the instabilities associated with the jets, the main factors that are commonly seen as governing the jet structure are: nature of the particles (composition; liquid, solid, etc.), geometry (shape) of the charge, and mass ratio of explosives to particles (amount of explosive vs. other compounds).  These assumptions are reflected in the research video below (Video Credit: D. Frost, Y. Gregoire, S. Goroshin, F. Zhang) which shows different combinations of particle nature, charge geometry, and explosive-particle mass ratio and their effects on jet structure.  A cool note is that you will be able to see the shock wave (a pressure discontinuity, or large change in pressure in a very small distance – across the shock wave) created by each blast in the video.  The main conclusions of the video (and some possible reasons for these observations) are:

1. Wet mixtures produce more jets (possibly due to smaller molecule size), which disperse sooner (maybe due to liquid surface tension effects).
2. Dry mixtures produce less jets (possibly due to larger particulate), which disperse slower (possibly due to higher momentum due to larger particulate mass and more friction effects due to the generally larger particulate vs. the wetted particulate mixture).

So there you have it: a relatively brief look at what governs the shape of the burst of fireworks!  Please remember to check back later today for our salute to the brave men and women who have served this great nation!  I’ll leave you with another image of fireworks and the Statue of Liberty as today is a great day for patriotism!

The Statue of Liberty surrounded by fireworks. Photo Credit: crazywebsite.com

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A Scientific Look at Fireworks, Part I

Posted by Brian (Team UV) on October 7, 2014
Posted in: Well Read. Tagged: , , , , , , , , , , . 3 Comments
Fireworks. Photo Credit: Office.co.uk

Fireworks.
Photo Credit: Office.co.uk

Fireworks seemingly have the capability to fascinate people everywhere, while trouncing any language, cultural, or religious differences.  But the universality associated with the magnificence of these pyrotechnic displays is not the only aspect of their prominence that makes them so vastly interesting; they are scientifically remarkable!

While one could talk for hours (if not days) on end in the aim of exposing all scientific aspects of something as seemingly ephemeral as a firework display, we are going to look at just one consideration as it relates to the world of engineering: the baseline fluid mechanics that describe the interaction of fireworks with the atmosphere around them.  We will begin this by watching a quick video (below) produced by The Discovery Slowdown, as was found through an article on FYFD (you can find the original article that inspired this one through the link; pardon the blog title, the content is excellent).  For the record, I have no idea what the deal is with the baby sloth in the beginning of the video, haha.

http://www.youtube.com/watch?v=b6YbtD4Sc8M

From 0:14 to 0:30 in the video, you can see the uncontrolled combustion of the reactants used in the fireworks.  This gives you a basic understanding of how this combustion takes place in the absence of any kind of container or packaging.  When the reactants “burn” on the table out in the open atmosphere, the gaseous exhaust products (“smoke”) billow(s) upwards in a turbulent manner, with no real direction.

From about 0:31 to 0:58 in the video, the combustion of the reactants is repeated; however, this time the reaction take place in a cylinder with the top end open.  Now you have controlled combustion (with regards to direction) and the result is effectively a jet of exhaust products (and some still-combusting reactants) that fires out the end of the cylinder in a way analogous to the exhaust kicking out the back end of a rocket, providing it with its thrust. 

At 0:41, the video shows a closeup of the top of the tube, where the gases leave the confines of the tube and exit out to the air.  As Nicole Sharp over on FYFD notes, this provides an excellent example of what we term an “under-expanded” nozzle.

Nozzle exit conditions. Photo Credit: Wikipedia.org (modified)

Nozzle exit conditions.
Photo Credit: Wikipedia.org (modified)

An under-expanded nozzle is simply one where the static pressure associated with the flowing fluid at the nozzle exit is greater than that associated with the atmosphere (P>Patm); in essence, the nozzle has not expanded enough to reduce the pressure in the flow to that of the atmospheric pressure, so when the exhaust gases exit the tube, they expand (or fan) outward rapidly in order to equalize their pressure to that of the atmosphere. There’s your first mini-lesson in rocket science!

It also interesting to note how (at about 0:35) the tube jumps upwards as the gases leave.  This is more than likely due to the fact that as this combustion occurs and the exhaust gases blast out of the tube at high speeds, they form a near vacuum in the tube behind them (by taking the contents of the tube and forcing them upwards, thus leaving a region of relatively low pressure behind the escaping gases in the cylinder).  This vacuum, however, is not representative of a natural state; in essence, the air in the cylinder wants to be at atmospheric pressure, but instead it is at a pressure far lower than the atmospheric pressure.  Once the bulk of the exhaust gases have escaped, the low pressure region immediately begins to collapse upon itself (thus increasing the pressure in the tube), but since the contents of the tube are still moving upwards, the cylinder itself is tasked with collapsing the region and thus is pulled upwards to decrease the volume in the tube between the cylinder base and the escaping gases!  And so the cylinder jumps upwards towards the sky before gravity goes to work pulling it back down.

So there you have it, there are some basic fundamentals of the way firework exhaust gases behave in different settings.  During my next Well Read post, I will discuss how we can use science and engineering to analyze the behavior of the actual jets that shoot outwards from a firework and what charge parameters affect their behavior.  Thank you for your support and please check back on Thursday for Andrew’s Presentation post!