boundary layer degradation 2

[chemproject]

Hello.
I have a liquid flowing through a pipe, which produces laminar flowrate properties. The boundary layer produced by this is considered to be a thin film. The pipe is exposed to high temperatures which degrade the flowing liquid.
If the flowrate is reduced, does the boundary layer increase in size?
will the fuel degrade more rapidly with a larger boundary layer?
Does heat alter the boundary layer of a flowing liquid ?

 

[kmpillai]

Hi,
What do you mean boundary layer?
If I am understood correctly the meaning the boundary layer you mentioned,as per theory any fluid flowing over a solid object will develop a boundary layer, which is an imaginary region from the solid surface extends a certain depth into the flow. Within the boundary layer viscos effect are very significant and velocity gradient is more.
Now let us come to your question, for pipe flow the boundary layer thickness will goes on increasing along the flow direction and after a certain length from the entrance of the pipe the boundary layer will reach a condition called fully developed flow. In that situation the boundary layer thickness is equal to the radius of the pipe.

For laminar flow the following equation can be used for calculate the entrance region (before the fully developed condition)
x/D = 0.05 Re
where, x-is the entrance length
D-pipe inner diameter
Re-Reynolds number based on the pipe inner dia

Regards,
KMPillai

 

[m777182]

If you reduce flowrate your boundary layer will increase.The exchange of material among the layer film and the bulk flow will be smaller so the layer will be more degraded.Heat reduces viskosity of the liquid thus helps you increasing the Re number in the pipe.Try to insulate your pipe, increase the flow rate and try to cool down your liquid before entering the critical pipe. Though higher Re number provides better heat transfer into the liquid which you do not want, it shortens the residence time of the liquid in the pipe and allows for better mixing in the pipe.A precooler will decrease the reaction rate of degradation.Insulation around the pipe will decrease heat transfer. Sometimes it works if you decrease the outer Re number of your pipe-make a stagnant layer of hot liquid or gas around the pipe!It will decrease heat transfer as well.
m777182

 

[chemproject]

Hello,
Thanks for the help!
Much appreciated!

 

[davefitz]

The prandtl number for most oils decreases greatly as the temperature increases, so the convective film heat transfer coeficient will drop as the heat flux increases.

Eventually the fuel oil will "crack", or the oil in the film will deteriorate as the light factions pass out of the film and the residual carbon will remain in the film and coke up the piping.

I am not sure of teh overall configuration of your process, but if you need to heat the fule oil you may wish to use a temperature limited heating medium ( such as condensing steam or dowtherm) to avoid such deterioration. If the heat flux is the define boundary condition ( eg radiant heat transfer from hot gases) then you will likely need to improve the inside heat transfer by using internal ribs or fins or reduce the outside flux by coating the outside of teh tube with ceramic .