Spark Ignition of Combustible Vapor in a Plastic Bottle as a Demonstration of Rocket Propulsion: Details Allotted to an On-line Appendix

J.R. Mattox, Department of Chemistry & Physics, Fayetteville State University, Fayetteville, NC, JRMattox@uncfsu.edu

 

This document provides supplemental information and links to video recordings for the article entitled “Spark Ignition of Combustible Vapor in a Plastic Bottle as a Demonstration of Rocket Propulsion”, which is available here:mattox 2017 physics teacher

 

Video Recordings

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A whoosh demonstration with a 3.8-L milk bottle fueled with isopropanol.

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A whoosh demonstration with a 19-L bottle fueled with isopropanol (splint ignition is used after spark ignition failed (at 20.5 C – see below).

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A whoosh demonstration with a 19-L bottle fueled with methanol.

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A 3.8-L milk bottle launched as a rocket fueled with isopropanol.

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A 3.8-L bottle launched as a rocket fueled with methanol.

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A 19-L bottle launched as a rocket fueled with methanol

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More video & photos follow the remainer of the appendix text in the order they are mentioned in the text.

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Dependence of Fuel Choice on Temperature

Isopropanol is preferred for these demonstrations.1 However, an ambient temperature of 21° C or higher is required for the partial pressure of isopropanol to be at least stoichiometric in air (at 0.045 bar). Methanol has a higher partial pressure at any specific temperature, and may be used for this demonstration (with more vigilance) down to 19° C (where it has a stoichiometric partial pressure of 0.123 bar). Ethanol can only be used down to 22° C (where it has a stoichiometric partial pressure of 0.065 bar). I note that these temperatures substantially exceed the respective flash points for these liquids because the flash point test is done with a flame which provides more energy for ignition than the spark used for this demonstration.

 

I have found that to use spark ignition at room temperature, these alcohols must not contain any substantial amount of water (isopropanol is sold for household use with up to 50% water – far too much).

 

Lower temperature demonstrations are possible with a hydrocarbon gas. I have successfully launched 3.8-liter HDPE (milk) bottles with 12% of the air displaced by methane (9.5% is stoichiometric in air), and 6% by propane (4.0% is stoichiometric). The specific volume of gas was added by displacing this volume of water with the bottle inverted in a water filled basin. The bottle was capped underwater as soon as the water had been displaced. Remaining water was quickly drained after allowing time for the gas to mix. These mixtures should ignite outside in any climate. I have not tried the whoosh bottle demonstration with these mixtures, but expect it will also work if the gas mixture is contained prior to ignition as shown in figure 3 of the printed article.1

 

 

Diethyl Ether

I have found that diethyl ether is a very suitable fuel for “whoosh demonstrations”. It is temperature versatile (it can be used down to -36° C, where it has a stoichiometric partial pressure of 0.034 bar). Because of its volatility, it presents more danger for dangerous combustion of accidentally pooled dense vapor than the other 3 alcohols described in this work, but this can be safely managed through the procedures specified.1 Also, its toxicity is less than both methanol and isopropanol.1

 

At room temperature, the fueling procedure specified for the other 3 alcohols1 would result in a vapor mixture far too rich to ignite. Instead, the specific amount of diethyl ether required in solution with air should be added to the demonstration bottle as a liquid, and the bottle rapidly capped. In this case, there is no need to spread the liquid on the interior surface of the bottle, it evaporates completely within a few seconds of its addition. As with other alcohols, air will leave the bottle around the cap to accommodate the evaporation of the liquid fuel. For 3.8-L bottles, 0.7±0.1 ml of diethyl ether is recommended (0.56 ml is the stoichiometric amount); for 19-L bottles, 3.0±0.2 ml is recommended.

 

The evaporation of diethyl ether is rapid at room temperature, but the density will not initially be uniform. Approximately 30 minutes are required at room temperature until diffusion (with a diffusion constant in air of 9×10-6 m2/s) results in a uniform mixture in a 3.8-L bottle. However, mixing can be accomplished rapidly by inverting the bottle repeatedly every ~5 seconds over the course of ~1 minute.

 

I have placed, ~2 ml of diethyl ether in a 3.8-L bottle, resulting in a mixture that is too rich for spark ignition. After demonstrating this, I taped this bottle to another 3.8-liter bottle containing initially only air. I then equalize the mixture between the bottles as described above, and then demonstrated spark ignition of both bottles sequentially.

 

 

Moisture Problems

 

Even small amounts of water can suppress spark ignition using these three specified alcohols1, either in solution in the liquid alcohol or present in the demonstration bottle before adding dry alcohol. This results from the consequent reduction alcohol vapor concentration according to Raoult’s law.

 

I find that, small amounts of water do not inhibit the ignition of diethyl ether because of its high vapor pressure. Even though reduced by water, it is still enough for complete evaporation. Thus, combustion products from this demonstration can be immediately purged by filling a used demonstration bottle with water, draining thoroughly, and immediately refueling with diethyl ether. This is not practical for the other three specified alcohols.1 The combustion of methane and propane described above should also not be inhibited by a small amount of water.

 

 

Occasional Bottle Rupture

 

I have occasionally witnessed both 3.8-L and 19-L bottles rupture during attempted whoosh rocket demonstrations. The rupture of these thin plastic bottles has been observed to be loud, but is not expected to be dangerous. A video of an attempted launch of a 3.8-L bottle fueled with methanol (heated to 28° C – see below) that ruptured is linked here.

 

A video of an attempted launch of a 3.8-L bottle fueled with diethyl ether that ruptured is linked here. A ~8 cm diameter disk that broke away from the bottle along a mold seam in apparent traveling toward the camera. A photograph of a pristine bottle of the same type, the ruptured bottle, and this disk, is linked here.

These ruptures are thought to be caused by pressure resulting from deflagration. Detonation would be very inappropriate for these demonstrations, with a supersonic flame front resulting in a shock that induces ignition through compressional heating. This can produce pressure in excess of 10 atm in a fuel air mixture for a fully developed detonation. Fortunately, detonation will not occur in these volumes using the fuels discussed above with air, especially when ignited with a low power spark2.

 

The loud sound is thought to result from combustion gases that result from the deflagration flowing through a breach in the bottle at supersonic velocities, creating a “booming” sound (similar to a popping balloon).

 

I found that a rupture of a 3.8-L bottle using compressed air produced this same sound. Thus, the sound is clearly not resulting from a detonation of the vapor mixture, rather a deflagration that results in sufficient pressure to cause bottle rupture. The rupture with compressed air was observed to occur at a pressure of less than 0.3 atm gauge. I have produced the same sound by gluing the lid on a 3.8-L HDPE bottle with cyan acrylic cement, placing it on the floor, and jumping on it to induce rupture. Substantial impulse is required to thus rupture these bottle – I suggest that one jumps in such a way as to contact the bottle with both feet, prepared to subsequently land with both feet on the floor following rupture, or to cope with an impulse in a random direction if the bottle doesn’t rupture.

 

The rupture of these bottles when ignited from a distance by spark1 is not expected to be dangerous because the bottles either remain intact, or if a piece breaks away, it does not acquire a substantial velocity. The rupturing 3.8-L HDPE bottles usually open along a mold seam.

 

I have not yet observed 3.8-liter bottle to rupture using either ethanol or isopropanol (at room temperature). I note that the vapors of methanol and diethyl ether in stoichiometric mixture with air have higher laminar flame velocities (0.56 m/s, and 0.47 m/s respectively at room temperature)3 than ethanol and isopropanol (both 0.41 m/s at room temperature)3.

 

I have observed that 3.8-L bottles rupture predominantly when the bottle fits tightly into the launch base and is thus mildly retained upon ignition. I have also observed that if rupture does not occur, a retentive fit in the base produces higher flights. So far, I have not seen isopropanol nor ethanol fueled bottles rupture, even using a retentive base. However, I have observed combustion of isopropanol to cause rupture of a 3.8-L bottle with the bottle opening constricted by ~80% (by clamping the bottle opening over a male garden hose fitting with a 13 cm length of vinyl hose attached with a 1.5 cm internal diameter – allowing insertion of the ignition cable shown in figure 31 through the hose into the bottle).

 

I have observed 19-L bottles rupture on two occasions when fueled by methanol (out of ~100 launches – most fueled by methanol) using the base shown in figure 1 which does not provide any retention. I expect that reported “explosions” that have occurred while attempting to perform the whoosh demonstration with alcohol vapor in 19-L bottles have similarly resulted from bottle rupture during deflagration. I have not yet witnessed bottle rupture during whoosh bottle demonstrations (with unobstructed bottles with the opening pointed upward).

 

 

Use of hydrogen and acetylene for fuel, and substituting oxygen for air

 

Spark ignition (from a substantial distance) enables safe exploration beyond limits previously specified for the whoosh demonstrations. I find that ignition of a stoichiometric mixture of hydrogen and air, although not excessively loud, always ruptures 3.8-L HDPE milk bottles, with multiple fragments resulting (that are not hazardous beyond ~1 m because of the small thickness of these bottles – 0.5 mm). I have also found this to be the case for methanol vapor in oxygen in 3.8-L HDPE milk bottles.

 

I have also ignited by spark a stoichiometric mixture of hydrogen and oxygen in a 3.8-liter bottle – video is linked here; and a stoichiometric mixture of acetylene and oxygen in a balloon with a diameter of 15 cm (volume 1.7 L) – video is linked here. This balloon was simultaneously popped and its contents ignited by spark initiated flame on cloth saturated with liquid methanol – I have found that a low-energy spark on the outside of a balloon will not initiate ignition nor pop the balloon. For both of these stoichiometric gas mixtures, the combustion was very energetic. Accompanying very loud booms may indicate that detonations occurred. These mixtures are clearly inappropriate for class room demonstration.

 

I have also demonstrated the ignition of a balloon filled only with acetylene (by spark initiated flame of cloth saturated with liquid methanol) – video is linked here. A substantial amount of soot results.

 

Hydrogen and acetylene are both expected to be inappropriate for “whoosh demonstrations”. In air, at small volumes, their combustion is expected to be a deflagration, albeit vigorous; but their laminar flame speeds are too fast (3.1 m/s and 1.7 m/s respectively at room temperature)3.

 

 

 

 

Ignition at elevated temperature

 

I have also experimented with heating bottles in an enclosure containing warm water prior to ignition. This heating was done after adding the alcohol to the bottle, and before spreading it on the bottle’s interior surface. I thus was able to spark ignite a 19-liter whoosh bottle using consumer 91% isopropanol (with 9% water) after being in an enclosure with water at 28° C. Video is linked here. It would not spark ignite at the temperature of the room (22° C) prior to warming. After the whoosh, a flickering, multi-colored flame persisted for an extended interval (~3 seconds).

 

Applications of alcohol vapor combustion

 

The video linked here shows the deflagration of 48 cm diameter balloon filled with pure hydrogen. Ignition was obtained by spark initiated combustion of the vapor of diethyl ether (0.5 mL of liquid) in a 2.3 L chamber (at the bottom of the white PVC “potato canon” strapped to the ladder in the video). Rather than a potato (which would be potentially hazardous as it fell), a wad of cloth (~200 cm2) was placed in the barrel above this chamber. A hardwood twig, ~1 cm in diameter and ~10 cm long and pointed at the top, was placed above the wad to pop the balloon just before burning gases reached it. This twig is apparent falling to the ground ~2 seconds after ignition.

 

 

Video recording with higher time and spatial resolution

 

Video recording of some of these demonstrations with higher time and spatial resolution is planned. Edited segments are expected to be posted at http://jmattox.fsufaculty.uncfsu.edu/home/rocket-science-laboratory-study/. Additional demonstrations that may be developed in the future may also be posted at this site.

 

 

Collaboration in video analysis

 

The author is willing to provide video files of these demonstrations for analysis by interested potential collaborators.

 

 

Provision of launch bases for whoosh rocket demonstrations

 

The author is investigating the possible manufacture and marketing of whoosh rocket launch bases for 3.8-L milk bottles, similar to that shown in figure 2 of the printed paper.1 Inquiries by e-mail are welcome.

 

References

 

  1. Mattox, J.R., “Spark Ignition of Combustible Vapor in a Plastic Bottle as a Demonstration of Rocket Propulsion,” The Physics Teacher, pp 30-33, Vol. 55, January 2017
  2. Turns, S.R., “Introduction to Combustion: Concepts & Applications,” 3rd Edition, McGraw Hill Education, India (2012)
  3. “SFPE Handbook of Fire Prevention Engineering,” 5th Edition, Springer (2016)

 

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Video & photos that are mentioned in the text follow in order specified.

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A video of an attempted launch of a 3.8-L bottle fueled with methanol (heated to 28° C)

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A video of an attempted launch of a 3.8-L bottle fueled with diethyl ether that ruptured

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A photograph of a pristine bottle of the same type, the ruptured bottle, and the ejected disk

rupture-110116-postflight

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Ignition of a stoichiometric mixture of hydrogen and oxygen in a 3.8-liter bottle

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Ignition of a stoichiometric mixture of acetylene and oxygen in a balloon with a diameter of 15 cm

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Ignition of a balloon filled only with acetylene

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19-liter whoosh bottle using consumer 91% isopropanol (with 9% water) at 28° C.

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deflagration of 48 cm diameter balloon filled with pure hydrogen. Ignition was obtained by spark initiated combustion of the vapor of diethyl ether (0.5 mL of liquid) in a 2.3 L chamber (at the bottom of the white PVC “potato canon” strapped to the ladder in the video