The fuel viscosity is attributed to two parameters:
cohesion and molecular interchange. While the former decreases with temperature and the latter increases with temperature. So, is it possible for a fuel viscosity to increase with temperature where the temperatures considered are around its flash point temperature?
Regarding 'fuel' an increase in viscosity at raised temperatures is completely plausible:
- 'fuel' denominates a mix of hydrocarbons with quite differing boiling points. See eg. the diagram here: https://de.wikipedia.org/wiki/Leichtbenzin
(Even if you do not understand German, you should be able to interpret the diagram. Otherwise use a translation program.)
The viscosity of 'fuel' is as well the result of mixing hydrocarbons with quite differing individual viscosities.
- When temperature approaches the flash point, particularly components with low boiling points form the vapor. Low boiling point and low viscosity go somehow 'hand in hand'. Thus the viscosity of the remaining fluid will increase due to a reduced percentage of low-boiling components.
- This will continue as well towards even higher temperatures. Until eventually some very high viscosity residue (eg. tar) is left.
The general trend to a reduction in viscosity due to higher temperatures is overcompensated by the boil-off of the low-boiling components.
Regards
The former effect causes a decrease in the shear stress while the latter causes it to increase. The result is that liquids show a reduction in viscosity with increasing temperature
Regarding 'fuel' an increase in viscosity at raised temperatures is completely plausible:
- 'fuel' denominates a mix of hydrocarbons with quite differing boiling points. See eg. the diagram here: https://de.wikipedia.org/wiki/Leichtbenzin
(Even if you do not understand German, you should be able to interpret the diagram. Otherwise use a translation program.)
The viscosity of 'fuel' is as well the result of mixing hydrocarbons with quite differing individual viscosities.
- When temperature approaches the flash point, particularly components with low boiling points form the vapor. Low boiling point and low viscosity go somehow 'hand in hand'. Thus the viscosity of the remaining fluid will increase due to a reduced percentage of low-boiling components.
- This will continue as well towards even higher temperatures. Until eventually some very high viscosity residue (eg. tar) is left.
The general trend to a reduction in viscosity due to higher temperatures is overcompensated by the boil-off of the low-boiling components.
Regards
In addition to U. Dreher's excellent answer, molecular shape has a large influence on viscosity. Some materials form highly folded, equant shapes when cold and elongate as they warm up. Longer, more tangled molecules increase viscosity compared to smaller molecules. This is the principle in most modern engine oils, for example, whose viscosity does not drop as rapidly with increasing temperature as the older oils did.
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