Home >> Resources/Information >> Extrusion, Heating
HEAT TRANSFER in the EXTRUSION of Non-Newtonian Materials
HEAT TRANSFER in the EXTRUSION of Non-Newtonian Materials
(part - 2 of Transport Phenomena in Manufacturing Processes, Seminar given during Fall '98 at SMU, Dallas, TX, USA)
------------------------------------------------------------------------
Arunn Narasimhan [ arunn@engr.smu.edu ]
Laboratory for Porous Materials Applications
Mechanical Engineering Department
Southern Methodist University
Dallas, TX - 75275 0337, USA
------------------------------------------------------------------------
Extrusion of Plastics, Polymeric materials, food processing involving single and multi-screw extruders fall under this domain of study.
Feeding, Conveying, Plasticizing, Homogenizing and Pressuring are the regular sub-processes in Extrusion.
Pressure and Temperature of the extruded material increase as it travels towards the die. As most of the Plastics and polymeric materials are non-Newtonian, their viscosity depend on applied strain rate. Also the viscosity depends on the Temperature. In food processing, moisture content in the food material involves an additional constraint. These property variations complicates the analysis by MIXING the Conservation, Continuity equations and equations used for determining the properties of the materials.
The objective of Transport Phenomena study is employed in obtaining information on one or more of the following parameters.
1. Shear and Temperature evolution of the Material
2. Residence time distribution - which simply means, how long the material gets to remain in what zones of the extruder
3. Mixing - of more than one component while extrusion
4. Pressure rise generated
5. Maximum Temperature reached
6. Torque needed - for ‘pushing-in’ the process material through the entire extruder length
7. Transport (local) in the extruded material
8. Die swell - expansion of die due to the Pressure and Temperature of the Material when it enters the die.
9. Phase change possibilities
10. Optimization - like, what Maximum Torque is required for pushing such and such material from a known Temperature and Pressure through the extruder so that it stays below an already set cut-off Pressure and Temperature(determined from the Die Design)
The main aspects related to Heat and Mass transfer are
1. Non-Newtonian Flow - most of the ‘non-traditional’ areas of application of Extrusion involves them
2. Mixing of more than one input material - almost always under Laminar flow
3. Viscosity Effects (author is unsure of Dissipation or Diffusion ! Its just ‘more’ Energy is wasted in overcoming the resistance generated by the Fluid Brake)
4. Conjugate Phenomena (of Conduction and Convection in Heat Transfer) coupled with Mass Transfer
5. Variations in the Thermo-physical Properties along the travel through the Length of the Extruder
6. Phase Change and Chemical Conversion of the material
7. Creeping and Back Pressure Flows
8. Geometrical Aspects of the Extruder passage
9. Unsteady Transport Analysis in 3D
10. Experimental Aspects of Rotating Environments (Temperatures are measured by Cam-operated Thermocouples !)
11. Heat Transfer in flow of powders (although this can now be studied under porous media domain)
Analysis of Screw Extrusion processes involves as a first step, recognition of the extruded material for its Physical and Chemical Characteristics. Common representation of a Generalized Newtonian Fluid are used for Polymeric Materials.
Although the geometry of the Single Screw Extruder is complex, analysis is done using a simplified geometry. To understand : there is a big screw covering which there is a barrel. Flow results through the passage between screw and barrel along the screw-threads. Trick lies in "stretching out" the Screw linearly - like you get to "unwrap" a length of paper wound along the helical groove of a drill bit and what you have as paper is your screw passage - so that the "top" and "bottom" becomes the barrel and the "sides" remain as screws. This "rectangular" passage is all we have to analyze for flow and energy transport.
Modeling is relatively easier in this 2D case - ideal for FDM enthusiasts- in Single Screw Extruder when compared to Twin or multi Screw Extruders(which is not treated in this summary)
For a heated barrel, under the presence of Viscous Dissipation effects the Thermal Convection along the down-channel direction becomes important. Velocities and Temperature change along the downstream position(former because of the later)
For a fully developed flow, Temperature and Velocities are assumed not to vary downstream. Barrel is treated as Isothermal(inlet temperature of the fluid), Convection is neglected but Viscous Dissipation remains. For Developing Flow, the Temperature of the fluid far downstream is seen to increase above the barrel temperature Therefore beyond a certain length of the channel, Heat Transfer occurs from the fluid to the barrel(if it is under fixed Temperature) This effect is attributed to the Viscous Heating.
Although the simplification of the geometry is interesting, the actual screw passage is a cup like arrangement when viewed across the section. This calls for 3D FEM Modeling of the Geometry. Moreover, as the fluid travels in a helix along the length of the extruder channel, there is a small re-circulation velocity acting normal to the direction of flow in the passage results. This results in the "convection" of Heat from the Barrel to the Screw top surface by the fluid ! This is experimentally verified. This results in the improper heating of the Fluid .
Marching procedures outlined earlier assume knowledge of property values at the exit of the extruder channel. But is back flow or adverse pressure gradient prevail from the die, then they do not work. For this, diffusion of energy along the downstream has to be taken into account.
Another variation is the axial formulation in which the screw is viewed parallel to its axis and at each point in the extruder channel the axial and tangential directions are considered.
Tapered Screws and step screws are variations of the above simple cases. In Tapered screws, additional Pressure difference due to the "nozzle like" passage of flow between screw and barrel has to be taken into consideration.
Residence time distribution which tells us about how much time a fluid particle spends in each region of the extruder is determined experimentally(and the experiments are numerically simulated.) It is mainly affected by the Mass flow rate of the fluid and less affected by the barrel temperature.
Mixing Characteristics, Flow in Twin Screw Extruders, Flow in Dies and Combined Heat and Mass Transfer aspects are left out of this summary for brevity
Further work is needed, as supported by the reviews of Yogesh Jaluria(1996, 1997,1998), in the following areas
1. Powder flow and heat transfer - porous media approach may be implemented.
2. Melting of Polymers
3. Food Processing in extruders
4. Partially Full screws
5. Conjugate Transport
6. Complicated screw elements, Grooved Barrels etc.
