GB2531632A

GB2531632A - A mechanical device to suppress contrail formation

Info

Publication number: GB2531632A
Application number: GB1514047.8A
Authority: GB
Inventors: Latif Qureshi Masood
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-08-10
Filing date: 2015-08-10
Publication date: 2016-04-27
Anticipated expiration: 2035-08-10
Also published as: GB2531632B; GB201514047D0

Abstract

A mechanical device C, with a spinning vessel VS, that can be externally attached to the exhaust port of a combustion engine, specifically an aircraft gas turbine engine, to suppress contrail formation. The combusted gases are inputted into the device and contact turbine blades A3, which causes the vessel, and consequently the gases, to rotate in order to centrifugally separate moisture from the gases. Stator vanes A1, A2 may be provided at the entrance and exit of the device. A central shaft S of the device may be coaxial with a low-pressure shaft of the engine. Water in the gases may condense on soot particles, the soot particles acting as nucleation centres for further precipitation. A surface P of the device may be coated with a condensation inducing metal, e.g. copper or silver. Water condensed from the gases may be collected in a drain D.

Description

Descriptions

Introduction and background to the invention

[0001]. This invention aims to reduce the environmental impact of contrails that are formed in the atmosphere by the aircraft exhaust gas emissions. Contrails is a word coined from condensation trails. The combustion products of any hydrocarbon fuel are carbon dioxide and water. When H2O and CO2 are created in the combustor, they are in the gaseous form. When the exhaust exits into the cold atmosphere at altitudes, the H2O component cools and condenses to form vapour particles. In the case of flying aircraft, the atmospheric conditions at certain temperature, pressure and altitude could become favourable to further the condensation of moisture. At some distance behind the aircraft, the vapour trail then condenses sufficiently to become visible to the naked eye. This is the condensation trail. Natural breeze then disperses this contrail to form cirrus clouds. The cirrus clouds reflect heat back to Earth and, thus, contribute to global warming. Considering the ever increasing number of aircraft flying, the magnitude of these cirrus clouds is having an adverse effect on the atmosphere, especially above high density airports. The cirrus clouds reflect heat back to Earth, thus contributing to global warming. The scope of this invention is to reduce contrail formation.

[0002]. Contrail suppression is a field that has had a renewed interest due to environmental awareness. The prior art shows attempts to suppress contrails using one of the five approaches, namely: a) Chemical Additives b) Electromagnetic radiation c) Heat exchangers d) Ultrasound e) Ice nucleation None of the patents claimed so far, nor the available literature employs the concept of the centrifugal effect to rotate the exhaust gases in order to selectively cool and separate the moisture out of the exhaust gases.

[0003]. In the invention device C, (as the device is applied to a typical engine, shown in Figure 2) the exhaust gases from engine E, are forced into a spinning vessel through a very low pressure turbine (VLPT) that causes compression of the airflow in order to initiate condensation of H2O from the molecular state to the vapour state. When the turbine VLPT rotates, the hollow vessel coupled to it spins and the mixture of its gaseous contents all rotate at the same angular velocity, thus preventing boundary layer separation of the gases. The spinning of a mixture of gases around a common axis creates the centrifugal effect, wherein the heavier particles are thrown outwards to traverse a larger radius, and the lighter particles are squeezed inwards to traverse a smaller radius. The outer periphery of the exhaust gases in the centrifugal device contains moisture, and under the pressure generated by the centrifugal effect, this moisture condenses into water. The water droplets are thrown outwards at the walls of the rotating vessel VS where they are removed through a collection and drainage system.

[004] Boundary Layer: The exhaust gas is made to rotate in a vessel by providing it a whirl through the stator SV I.This rotation creates a centrifugal force within the vessel VS which subsequently causes compression of the exhaust gases F6. This compression induces a phase change in water. Rotation of the vessel containing the rotating gas prevents the boundary layer separation on the internal surface P of the vessel VS. A boundary layer would tend to separate the gas particles moving along the surface P thus retarding the internal rainfall on P. The spinning of the vessel also adds to the velocity of the gas in order to obtain a net greater velocity. The condensed water thus produced rotates in a circular ring and collects in the drain D. [0005] Seals: High temperature, low friction, seals Se are needed to prevent the leakage of water between the static part of the structural support 113, holding the drain D region of the vessel rotating in it, in order to prevent the leakage of water at high pressure. This water is then drained out under pressure of the centrifugal force and can be utilized, stored or disposed off.

Description of Drawings:

[0006]. The present invention will be described by the accompanying drawings to illustrate the different aspects of design and workings.

Figure 1: Description of centrifugal forces shows a mixture of gases of different densities F5 rotating in a cylinder. The densest fluid F6 spins out towards the periphery, while, the rarest fluid F8 spins and converges in the centre. The centrifugal force also creates the maximum pressure on the peripheral wall.

Figure 2: Shows the invention of the centrifugal device C fitted to a typical high bypass turbine engine E. A feature of this invention is that no internal modification of any existing engine needs to be done, and the device is an external attachment C. The exhaust airflow F2 is utilized by the invention device C. Figure 3: Description of a block diagram of a typical state of the art, high bypass 3-shaft turbine engine, with intercooler IC, recuperator REC, combustion chamber CC and Nozzle N. The shafts H PS, IPS and LPS are coaxial.

Figure 4: Description of a block diagram of a typical turbine engine as in figure 3, the Nozzle N replaced by the invention, co-axial to the shafts, shown here is the placement of the invention device C on the above turbine engine, where the invention containing a Very Low Pressure Turbine VLPT, Shell Vessel VS and Core Vessel VC.

Figure 5: Description of the phase diagram of water. Region of interest is for the water state to move from the critical point C to the liquid phase in order to reach a design point within this region. (www.chemguide.co.uk) Figure 6: The invention: Block diagram of the working of the device and the heat dissipation from H2O to the other exhaust gases CO2 and N2 and partly to cold airflow El.

Figure 7: The invention workings: Conceptual example of the path of a volume of flow F2 input to the device, its linear energy deflected into flow F3, the energy causing further deflection into rotational flow FS, its split into circular flows F6 and F8. The flow F6 condenses into a moisture laden flow F7 collecting into a rotating water ring and is drained out, while the residual rotational flow F8 is straightened out to exit as linear flow F9 with some residual thrust.

Figure 8: The invention: A simplified example of a cut-away drawing of the invention device showing the conceptually important parts. The assembly shown is supported by the rigid structural parts of the pylon Y1 and Y2 holding the stators SV1 and SV2 with aerofoils Al and A2, the hub containing the bearings B1 and B2 respectively, to hold and freely allow rotation of the shaft S, the said shaft holding the rotor VLPT and the vessels VC and VS. The rotating drain D, part of VS, is placed within the rigid structure Y3, seals Se containing the rotating ring of water.

Description of a turbine engine:

[0007]. Illustrations in Figure 3 describes the workings of a turbofan or a turboshaft engine: A simple turbofan engine comprises of a fan enclosed in a nacelle, this splits into a main bypass flow and a small part of the flow is fed into a Low Pressure Compressor LPC, an intermediate Pressure Compressor IPC, a High Pressure Compressor HPC, a combustor or Combustion Chamber CC, a High Pressure Turbine HPT, an Intermediate Pressure Turbine IPT and a Low Pressure Turbine LPT. The exhaust F2 exits through a nozzle N. [0008]. Such turbine engines 100051 are also called three shaft engines. The LP compressor is driven by the LP turbine through the LP shaft (LPS). The IP compressor is driven by the IP turbine through the IP shaft (IPS), and the HP compressor is driven by the HP turbine through the HP shaft (HPS). The bypass fan (shown as FAN in Figure 2) is geared to the LPS. All three shafts are co-axial.

[0009]. Some turbine engines are two shaft engines, with one less compressor, shaft and turbine. That is, the IPC, IPT and IPS shaft are not included. This invention can be attached to both the two shaft and three shaft turbine engines.

[00010]. Some turbine engines, in addition to the above [00061 also can possibly insert an intercooler IC between the LP compressor and the HP compressor. This is in order to cool the compressed gas so as to allow further compression. The location of IC is shown in the flowchart in Figure 4.

[0011]. In addition to 100081 above, some engines can also possibly insert a recuperator (shown as REC) in Figure 4 between the HPC compressor and the combustor CC. This is done in order to preheat the compressed gas before the gas is combusted. The source of this heat is usually from the LP turbine's exhaust F2.

Description of Combustion Chemistry:

[0012]. In the description of combustion chemistry, the role of nitrogen is simplified here. Nitrogen enters the engine cold, and leaves the engine at the exhaust temperature. The percentage that forms NOx is negligible, though its corrosive effects are considerable. However, in the aspects of this invention, the chemical reactions shown are for the combustible reactants only.

[0013]. Combustion: the hydrocarbon fuel reacts with oxygen in the following stoichiometric ratio if complete combustion takes place as shown: C.H2n+2 + ((3 n+ 1)/2)(02±4N2) = nCO2 + (n+ 1)H20 + 2(3 n+ 1)N2 (Equation 1) This basic equation is only descriptive and not exhaustive, and does not include all the contaminants, nor does it include the larger molecules of fuel.

Now specifically for kerosene, where n=12, we have: Ci2H26 + 18.5(02+4N2) -12CO2 + 131120 + 74N2 (Equation 2) As seen in Equation 2 above, for every Ci2H26 molecule burnt, there are 13 molecules of H2O produced. That is, the molar ratio of kerosene to water is 1:13.

Volume of Kerosene = Molar Mass of Kerosene / Density of Kerosene = (12 x 12 + 26) / 0.82 = 207 cm3 Volume of Water (13 as in Eq.2), Volume Ratio = Molar Mass of Water/Density of Water = 13 x (2+16) / 1 = 234 cm3 = Volume of Kerosene: Volume of Water (Equation 3) = 207: 234 = 1: 1.13 [0014]. The Equation 3 approximation shows that the volume of water in the kerosene engine's exhaust relates 1.13 times the volume of fuel carried by the aircraft. That is, the engine produces more water than the kerosene it burns.

[0015]. In some turbine engines, some water is injected into the combustor in order to lower the flame temperature in order to prevent the formation of NOx. This injected water also adds to the water content of the exhaust.

[0016]. Thus the water at the exhaust consists of the combustion by-product L00121 and the additional water injected in the combustor "131. This additionally injected water also exists in the exhaust in the form of vapour at high temperature and pressure.

[0017]. The present invention intends to trap the water in the exhaust as in "141.

Description of the Centrifugal Effect:

[0018]. The centrifugal effect works on the basic principle of the dynamics of fluids. When mixtures of two fluids of dissimilar densities, contained in a vessel, are rotated on a common axis, the fluids tend to be thrown outwards due to the centrifugal force. Of the two fluids, the denser fluid is thrown outwards the farthest (Figure D. As a consequence, a void is created in the centre, and the rarer fluid is thus squeezed centre-wards to fill the void. The lighter fluid and heavier fluid can then be extracted separately.

[0019]. However, in the aspect of this invention, we are interested in the extraction of the denser fluid. The denser fluid is supposed to be water in the mixture of H2O, CO2 and N2.

[0020]. It should be appreciated that when both H2O, CO2 and N2 are in the gaseous state, the heavier CO2 would spin at the farthest radius, hence, in order to prevent this, a certain amount of condensation is required to convert H2O from the gaseous state to the nucleation state. The mechanical energy spent to turn the turbine VLPT reduces the temperature of the gas and thus aids nucleation. The pressure tends to condense the molecular H2O to coagulate, and form vapour particles that are several molecules heavy. This pre-condensation would make the water vapour denser than the carbon dioxide gas.

[0021]. According to the phase diagram of CO2 and N2, from available literature, it can be inferred that CO2 and N2 will continue to exist in the gaseous state at an exhaust temperature of 400 ° C and a pressure of 0.5 atm. Once the water has been removed, the Density of CO2 at the exhaust temperature and Pressure can be calculated according to the equation below: P = pRT (Equation 4) Where, for CO2: P CO2 = mole fraction of CO2 x exhaust pressure = 0.121 x 50000 Pa R = Universal gas constant * = exhaust temperature P CO2 = 0.121 x 50000 / 673 x 287 Similarly, for N2 P N2 = mole fraction of N2 x exhaust pressure R = Universal gas constant = exhaust temperature P N2 = 0.747 x 50000 / 673 x 287 = 287J / kgK = 673 °K = 0.031 kg/m3 = 0.747 x 50000 Pa = 287J / kgK = 673 °K = 0.1934 kg/m3 [0022]. At the given temperature and pressure CO2 and N2 exist beyond their critical points and will always exist in the gaseous form irrespective of the change in temperature and pressure within the operating range of the invention device.

[0023] Water on the other hand lies below its critical point and can therefore condense into liquid water due a decrease in temperature or due to an increase in pressure. This is illustrated in the Phase diagram of water in Figure 5.

Description of Water Physics:

[0024]. The exhaust mixture of H2O and CO2 follows the Dalton's Law of partial pressure. The partial pressure exerted by water is proportional to the number of moles of water in the mixture of the exhaust gases.

This can be worked out from Equation 2 where the reaction products are: 12CO2 + 13H20 +74N2 Molar ratio of Water = 13 / (12+13+74) = 0.131 Molar ratio of CO2 = 12 / (12+13+74) = 0.121 Molar ratio of N2 = 74 / (12+13+74) = 0.747

Description of Contrails

[0025]. If the pressure of the exhaust mixture of H2O, CO2 and N2 is greater than the saturated vapour pressure at that temperature, then the water condenses into liquid water. The particle size of the condensate is determined by the Kelvin equation.

In (P/Po) = 27V in Ir RT (Equation 5) where: P = actual vapour pressure Po = saturated vapour pressure = surface tension of water Vin = molar volume = radius of droplet = gas constant = temperature.

[0026]. The H2O molecule in the gaseous state is of atomic size, however, as the molecules coagulate to achieve a minimum size of the order of 0.02 microns, then these particles can behave as condensation nuclei. Further coagulations until the particles achieve a minimum size of the order of 20 microns, then these particles behave as water vapour and further coagulations to about 1000 microns and larger, the particles are proper raindrops.

Claims

Claims 1 A mechanical Device C with a spinning vessel VS that can be externally attached to the exhaust port of a combustion engine wherein the combusted gases F2 are input into the said device, the thrust energy of the said gases forcing a set of turbines to create rotational force in spinning the attached vessel VS containing the said gases within, thus rotating the said gases in order to centrifugally generate pressure conducive to the condensation of water and using the same centrifugal effect to separate moisture from the combusted gases F2, allowing the other combustion products to exit the device without thermal dissipation.
2. The Device as claimed in claim 1 wherein rigid supports Y1 and Y2, holding co-axial stator turbines SV1 and SV2, containing aerofoils Al and A2, the stator turbines centers fitted with bearings B1 and B2 respectively, whereas the Shaft S rotates between the said bearings, the vessels VS, VC and turbine VLPT fixed on S to thus rotate with the said shaft.
3. The Device as claimed in claim I wherein the shaft S of the centrifugal mechanism is coaxial with the Low Pressure Shaft of the said engine E, operates when the said engine is also operating, by the engine E forcing the exhaust gases F2 into the stator turbine SV1 containing the aero-foils Al of the said device deflecting and training the airflow F2 such as to turn the turbine VLPT, the airflow emerging out of VLPT as airflow F3.
4. The Device as claimed in claim 1 and claim 3 wherein the Very Low Pressure Turbine VLPT aero-foils A3 co-axial to the vessel VS spins the said vessel thus rotating the said combusted gases contained within the vessel VS creating the radial air-flow F5 on the rotational Z-axis.
5. The Device as claimed in claim 1 and claim 4 wherein the radial air-flow F5, rotating with almost the same angular velocity as the turbine VLPT, the said airflow splits into a dense, moisture laden, air-flow F6 rotating circumferentially and the rarer moisture free airflow F8, rotating closer to the centre of the rotational axis.
6. The Device as claimed in claim 1 wherein the exhaust gas F2 from the core of the engine E exits the engine at a temperature T1, considered here as also the temperatures of the exhaust gases consisting mainly of H20, CO2 and N2, in the molecular state, in variations of some engines could be higher than the critical point for water vapour to form, possibly still contain a minor amount of H2O below the critical point of water, the temperature would drop to T2 when a part of mechanical energy of the exhaust F2 is utilized to drive the turbine VLPT, the pressure and temperature preferably in, or close to, the liquid phase.
7. The Device as claimed in claim 1 and claim 6 wherein the temperature and pressure of a small percentage of the molecular H2O in air-flow F6 initially drops to a liquid phase, condenses on to the soot particles, the soot particles acting as nucleation centers for further precipitation.
8. The Device as claimed in claim 1 wherein the bypass airflow Fl is utilized, without let or hindrance, to absorb some thermal energy from the external surface of the spinning vessel VS.
9. The Device as claimed in claim 1 wherein the material of vessel VS is a thermally conducting material, however, the inner surface P is coated with a condensation initiating metal, one example being copper or silver coating.
10. The Device as claimed in claim 1 wherein the spinning vessel containing the rotating exhaust gases, both having approximately similar angular velocities, prevent the boundary layer separation of the gas from the surface P, thus maintaining a close contact of the water with the said surface.
11. The Device as claimed in claim 1 and claim 7 wherein the outer surface of the centrifugal vessel V is subject to the cold bypass-airflow El from the engine E, cooling the inner surface P thus selectively dissipating thermal energy from the airflow F6 in close contact with P.
12. The Device as claimed in claim 1 and claim 10 wherein the H2O component in the portion of the air-flow F6 in direct contact with the inner periphery P of the spinning vessel V is cooled sufficiently to condense into additional water droplets.
13. The Device as claimed in claim 1 and claim 12 wherein the centrifugal forces on the flow further forces the flow F6 against the cold inner surface P of the vessel VS, the pressure thus generated would tend to move the phase point on the water-phase plot of figure 5 to enter the liquid phase as flow F7, as a rotating conical sheet of water at temperature T3.
14. The Device as claimed in claim 1 and claim 12 wherein due to compression at the periphery of the vessel VS, the vapour would further experience coagulation, the Phase Point on the graph in Figure 7 would move down toward the liquid region furthering precipitation of water nto a liquid state F7 collecting in the drain D as a rotating ring of water.
15. The Device as claimed in claim 1 wherein seals Se between the rigid support Y3 and the spinning vessel VS prevent the liquid in D from leaking out and thus loosing pressure.16 The Device as claimed in claim 1 wherein the diameter of the centrifugal vessel V is enlarged to enhance the centrifugal effect created by the rotor turbine VLPT, finally converging back into the exit stator SV2 of reduced diameter creating back-pressure.17. The device as claimed in claim 1 and in claim 16 wherein the stator turbine aero-foils A2 straighten the radial flow F8 to exit the device C as linear air-flow F9 at temperature T4 into the atmosphere, the aperture of stator turbine SV2 acting as a nozzle preferably increasing exit speed.18. The Device as claimed in claim 1 and claim 15 wherein the residual heat in the drained out flow F7 is partially recovered by the requirements of thermal energy, possibly by the utilities in the aircraft cabin, the leading edges, the engine, before the water F7 is disposed off 19. The Device as claimed in claim 1, and claim 16 wherein due to the condensation of water, the latent heat released by water is absorbed by the residual airflow F8 consisting mainly of CO2 and N2 such that the gases now contain the additional energy released by the water.20. The Device as claimed in claim 1 and all the above claims wherein the air-flow F9 released into the atmosphere does not contain sufficient moisture to create contrails.Amendments to the claims have been filed as follows: Claims 1. A mechanical Device C with a spinning vessel VS that can be externally attached to the exhaust port of an aircraft turbine engine wherein the combusted gases F2 are input into the said device, the thrust energy of the said gases forcing a set of turbine blades to create rotational force in spinning the attached vessel VS containing the said gases within, thus rotating the said gases in order to centrifugally generate pressure conducive to the condensation of water and using the same centrifugal effect to separate moisture from the combusted gases F2, allowing the other combustion products to exit the device without thermal dissipation.2. The Device as claimed in claim I wherein rigid supports Y1 and Y2, holding co-axial stator turbines SV1 and SV2, containing aerofoils Al and A2, the stator turbines centers fitted with bearings B1 and B2 respectively, whereas the Shaft S rotates between the said bearings, the vessels VS, VC and turbine VLPT fixed on S to thus rotate with the said shaft.3. The Device as claimed in claim I wherein the shaft S of the centrifugal mechanism is (r) coaxial with the Low Pressure Shaft of the said engine E, operates when the said engine is also operating, by the engine E forcing the exhaust gases F2 into the stator turbine SV1 C\I containing the aero-foils Al. of the said device deflecting and training the exhaust linear airflow F2 such as to turn the turbine VLPT, the airflow emerging out of VLPT as helical airflow F3. (3)4. The Device as claimed in claim 1 and claim 3 wherein the Very Low Pressure Turbine VLPT aero-foils A3 co-axial to the vessel VS spins the said vessel thus rotating the said combusted gases contained within the vessel VS creating the radial air-flow F5 on the rotational Z-axis.5. The Device as claimed in claim I and claim 4 wherein the radial air-flow F5, rotating with almost the same angular velocity as the turbine VLPT, the said airflow splits into a dense, moisture laden, circular air-flow F6 rotating circumferentially and the rarer moisture free helical airflow F8, rotating closer to the centre of the rotational axis.6. The Device as claimed in claim I wherein the linear exhaust gas F2 from the core of the engine E exits the engine at a temperature T1, considered here as also the temperatures of the exhaust gases consisting mainly of H20, CO2 and N2, in the molecular state, in variations of some engines could be higher than the critical point for water vapour to form, possibly still contain a minor amount of H2O below the critical point of water, the temperature would drop to T2 when a part of mechanical energy of the linear exhaust F2 is utilized to drive the turbine VLPT, the pressure and temperature preferably in, or close to, the liquid phase.7. The Device as claimed in claim 1 and claim 6 wherein the temperature and pressure of a small percentage of the molecular H2O in air-flow F6 initially drops to a liquid phase, condenses on to the soot particles, the soot particles acting as nucleation centers for further precipitation.8. The Device as claimed in claim 1 wherein the bypass airflow El is utilized, without let or hindrance, to absorb some thermal energy from the external surface of the spinning vessel VS.9. The Device as claimed in claim I wherein the material of vessel VS is a thermally conducting material, however, the inner surface P is coated with any condensation initiating metal.10. The Device as claimed in claim 1 wherein the spinning vessel containing the rotating exhaust gases, both having approximately similar angular velocities, prevent the boundary layer separation of the gas from the surface P, thus maintaining a close contact of the water with the said surface.11. The Device as claimed in claim 1 and claim 7 wherein the outer surface of the centrifugal (3) vessel V is subject to the cold bypass-airflow El from the engine E, cooling the inner surface P thus selectively dissipating thermal energy from the airflow F6 in close contact with P. 12. The Device as claimed in claim 1 and claim 10 wherein the H2O component in the portion of the air-flow F6 in direct contact with the inner periphery P of the spinning vessel V is cooled sufficiently to condense into additional water droplets.13. The Device as claimed in claim I and claim 12 wherein the centrifugal forces on the flow further forces the flow F6 against the cold inner surface P of the vessel VS, the pressure thus generated would tend to move the phase point on the water-phase plot of figure 5 to enter the liquid phase as flow F7, as a rotating conical sheet of water at temperature T3.14. The Device as claimed in claim 1 and claim 12 wherein due to compression at the periphery of the vessel VS, the vapour would further experience coagulation, the Phase Point on the graph in Figure 7 would move down toward the liquid region furthering precipitation of water into a liquid state F7 collecting in the drain D as a rotating ring of water.15. The Device as claimed in claim I wherein seals Se between the rigid support Y3 and the spinning vessel VS prevent the liquid in drain D from leaking out and thus loosing pressure.
16. The Device as claimed in claim 1 wherein the diameter of the centrifugal vessel V is enlarged to enhance the centrifugal effect created by the rotor turbine VLPT, finally converging back into the exit stator SV2 of reduced diameter creating back-pressure.
17. The device as claimed in claim 1 and in claim 16 wherein the stator turbine aero-foils A2 straighten the radial airflow F8 to exit the device C as linear air-flow F9 at temperature T4 into the atmosphere, the aperture of stator turbine SV2 acting as a nozzle preferably increasing exit speed.
18. The Device as claimed in claim 1 and claim 15 wherein the residual heat in the drained out water-flow F7 is partially recovered by the requirements of thermal energy, possibly by the utilities in the aircraft cabin, the leading edges, the engine, before the water-flow F7 is disposed off.(r)
19. The Device as claimed in claim I, and claim 16 wherein due to the condensation of water, the latent heat released by water is absorbed by the residual airflow F8 consisting mainly of CO2 and N2 such that the gases now contain the additional energy released by the water.
20. The Device as claimed in claim 1 and all the above claims wherein the exhaust air-flow F9 (3) released into the atmosphere does not contain sufficient moisture to create contrails.