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| Heat Transfer Modes-Step Change in Surface Temperature |
| Published:2009-8-3 21:30:50 |
Step Change in Surface Temperature
Consider contact or trapped sheet forming．When the sheet is placed against the isothermal hot plate．its surface temperature immediately increases to the plate temperature ．As time increases，energy is conducted to the interior of the sheet．If the sheet is held against the hot surface long enough,the sheet temperature will eventually equal the hot surface temperature everywhere throughout the sheet．If the sheet is heated on both sides as shown ,the temperature profile through the sheet will be symmetric about the centerline．
Constant Energy Input to the Sheet Surface
The amount of energy the heaters interchange with the sheet surface per unit area is called heat flux．If the energy input is constant，the time-de．pendent temperature profile of Fig.3.5is obtained．This case illUStl4ates some basic concepts in radiation heat transfer to plastic sheet．If the energy input is equal on both sides of the sheet．the time-dependent temperature profile through the sheet is symmetric about the centerline．The temperature profile is again symmetric about the centerline when the sheet iS heated on both sides．Unlike the previous case．the sheet surface temperature continues to increase with time．Unlike the previous case，the sheet temperature never reaches a constant value．
It is apparent from these two cases that there is an interrelationship or coupling between the energy input to the sheet surface and energy conducted to interior of the sheet．Consider this coupling in concept．
An extreme example of very low heat flux to the sheet surface coupled with very high thermal conductivity or diffusivity or very thin sheet (or both)．The temperature profile through the sheet is essentially flat and the centerline temperature essentially equals the surface temperature．
If the energy input to the sheet surface is very high and the polymer thermal conductivity or diffusivity is very low or the sheet is very thick．the sheet surface temperature will appear to reach a fixed temperature nearly instantaneously. A more typical coupling between sheet surface and internal temperatures． |
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