Jet aircraft emission
PHOTO: MOHAMMED YOUSUF
Why do we see a gap between the jet aircraft and the linear stream of fumes it emits?
N. SADASIVAN PILLAI
Guntakal, Andhra Pradesh
Sometimes the exhaust from a jet engine becomes visible due to the water vapour condensation. These “condensation trails” are called “contrails”.
In a jet engine the air taken in at the intake is compressed and consequently its temperature increases. The temperature goes up further due to burning of petroleum fuel. This hot air at around 1,500 {+o} C, is allowed to expand through a turbine and then through a nozzle.
The jet exhaust let out is at a temperature of around 500 {+o} C and it contains water vapour due to combustion. The exhaust slows down due to air friction, increases in diameter, mixes with the surrounding air and cools further.
If there is sufficient water vapour in the jet exhaust to reach saturation when cooled, it condenses as droplets and further as tiny ice crystals.
These crystals may grow in size due to water vapour condensation from the ambient air or may diminish in size. We see the white streams mainly due to the ice crystals
The black smoke in the jet exhaust seen briefly during take off or landing has nothing to do with the contrails — which are found only at high altitudes from 8 km to 12 km where the temperature is - 40 {+o} C and lower.
Apart from the contrails, vortices at the wing tips may also become visible due to water vapour condensation due to low pressure and temperature in the vortex core if the humidity is high. Such vapour trails are formed only at low altitudes and are short lived.
In a jet exhaust it takes a little while for the air to cool and the ice crystals to precipitate. Hence the contrails are seen only after a gap. Their subsequent length and life span depend on ambient temperature, humidity and wind conditions.
MOHAN D. DESHPANDE
M.S. Ramaiah School of Advanced Studies
Bangalore
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