pressure measurement in the test system

[AgeOfEmpire]

Hi Friends,

Could you give some information about this problem. I work on polymer processing and I have a simple test system. This system consist of heated reservoir and a piston. Polymer is filled in reservoir and heated when is melted. then polymer flows through small hole. In this system, pressure value is read a constant point in the reservoir. My problem is that pressure value is continuously drop at this point. I know that pressure should drop in reservoir but I dont understand why pressure value is different at the constant point using a constant piston speed.

If you give suggestion I will be pleased.

 

[BigInch]

I suspect you are seeing the effects of a non-Newtonian fluid. Not all fluids have a well defined linear relationship between the shear stress between moving layers and the velocity and distance between the layers. Fluids with a linear relationship of shear stress vs distance beteen layers (as for example, from a pipe wall towards the center) are known as Newtonian fluids and their flow as Newtonian flow. The commonly known fluid mechanics equations are derived assuming Newtonian fluids, such as water, gasoline and most motor oils.

Many other fluid and flow classifications describing various shear stress vs distance between layers characteristic curves exist such as Bingham Plastics, (toothpaste, clay, asphalts, tomato ketchup, or is it catsup or catchsup?), pseudoplastics (grease, soap, paint, latex, paper pulps), dilatant (quicksand) and thixotroic fluids (paints, greases, asphalts, inks). Non-Newtonian fluids can have shear rate values that change with their rate of movement or even over time alone. Many crude oils can have curve values that can change with only how long they have been sitting in a tank before pumping. Could this be it?

For more info, see page 19 of this Power Point Presentation,

http://www.bakker.org/dartmouth06/engs150/04-clsfn.ppt#19

http://virtualpipeline.spaces.msn.com

 

[AgeOfEmpire]

Thank you BigInch but my problem is a bit different. I know non-newtonian flows viscosity change with shear rate values but in this test system piston speed is constant and pressure transducer read continuosly different pressure value at same point not different point.

 

[BigInch]

Humm. If velocity was constant, could it be that the thicknesses of the moving layers was changing? Can you assure that there was constant velocity at each point across the entire flow area at all times?

http://virtualpipeline.spaces.msn.com

 

[AgeOfEmpire]

I am sure that velocity of piston don't change during the test because piston is mounted a tensile/compression machine. Only force values are changing in the machine screen.