Investigation On Heat Transfer And Flow Characteristics Of Polymer In The Extrusion Processing

Theoretical analysis, numerical calculation and experiments have been performed to investigate the heat transfer and flow characteristics of polymer in the extrusion processing. The material used in the tests is rubber-mixture and the experimental apparatus is a single-screw extruder. The temperature distribution and the velocity profile of the flowing rubber-mixture in extruder are presented by means of the quesa three-dimensional model, under consideration of temperature-velocity- viscosity correlations. The convective heat transfer coefficients between rubber-mixture and the inner surface of the extruder-cylinder are given out both numerically and experimentally. The last, some enhance measures have been brought forward to improve the heat transfer capability of temperature control set, using the results of experiments and the new elements for heat transfer enhancement. In the part of theoretical analysis, the hydrodynamic, heat transfer and rheology essential theories have been used. The volume concerned in the analysis is the molten rubber-mixture in metering part of a single-screw extruder. The physics characteristic, flow and heat transfer characteristic have been analyzed thoroughly. The math. and physical models have been established, and also the corresponding control equations of the model have been given. The distinct characteristics are as follows: a) the rubber-mixture has high viscosity and is a typical non-Newtonian fluid, much more the viscosity is not a fixed value but a function of shear rate and temperature T, viz. , b) the momentum and energy equations are coupled , they must be calculated synchronously, c) the specific heat and thermal conductivity are not fix value but the function of temperature, d) viscosity dissipation become the main heat source, can not be neglected. All these characters increase the difficulty of computer calculation. In the part of computer calculation, the finite difference method(FDM) has been used to different the control equations and the computer calculate results of velocity profile, temperature distribution and the viscosity are gained. Because the temperature-velocity- viscosity correlation, and specific heat and thermal conductivity are the function of temperature, the computer calculate are iterative repetition.In the part of experiments, 6 kinds of experiments have been performed respectively: rheology characteristic test, the specific heat test, the thermal conductivity test, the temperature distribution measure in the extruder, the convective heat transfer coefficients between rubber-mixture and the inner surface of the extruder-cylinder, and the convective heat transfer coefficients between cooling medium and the outer surface of the extruder-cylinder. The first three experiments are physical property tests. Because the studies on the physical character are so few that we can not obtain the valid reference literature about the function of physical character with the changes of the temperature and shear rate, but all these physical character are indispensable to the theoretical analysis and computer calculation. Firstly, the extensional viscosity of rubber-mixture have been obtained on the XLY-2 capillary rheometer, the result indicate the extensional viscosity exhibit power-law relationship. , viscosity constant K and power-law index n are function of temperature, the relation formulas is K=A+BT ,n=C+DT .The specific heat of 6 kinds of rubber mixtures are measured with a Differential Scanning Calorimeter (DSC) ,NETZSCH DSC204 ,and the relations with temperature are linear and the formula is ,where c0 and b can be gained from experimental data. The heat conductivity λ of the rubber mixtures are also measured experimentally. The results show that λ changes with increase of temperature also linearly, that is: λ=λo+bt. The temperature distribution of the rubber mixture in a running pin finned single-screw-extruder is measured by means of the specially designed pin-fin- thermocouples to verify the results of theoretical analysis and t...