I run a endothermic reaction (steam reforming of iso-octane) in a tubular reactor (d=1.3 mm, length=5 cm). it is a wall reaction, laminar flow and tube thickness was set as 1 mm. when I look at the surface heat transfer coeff. contour, at the entrance of the tube its magnitude is positive then it changes to negative value and finally it is getting zero value.. do you now anything about why this kind of changing happens?

you can see the image following this link http://www.flickr.com/photos/32138568@N06

thank you in advance

Ty to see the air/iso-octane temperature in the section of tube (doing a plane surface). The heat conduction can give a air/iso-octane more cold than the tube in a side of tube and heater in the other side.

Thank you Gregorio..

Please could you give me more elaborative information?

could you check the following links.the first one is surface heat transfer coeff. contour on a plane surface and the second one is temperature profile on the same surface..

http://www.flickr.com/photos/32138568@N06/3016926110/ http://www.flickr.com/photos/32138568@N06/3016925930/

I think iso-octane is heating the tube at the beginning. Here you have the positive values of coefficient. When you have negative values is because heat conduction in the tube gives more heat than isoctane. So the tube is heating the iso-octane.

got it thanks a lot