Abstract
48 4.2. Summary and Conclusion The thermal dynamic-model of the electrically heated oven system has been formulated, programmed for computer solution. In the model, all significant parameters that affect heater operation, wall and oven air dynamics, and insulation characteristics can be varied. In respect of these parameters, the problem, the physical model and the mathematical model have been formulated. Then, the model was tested to make sure if it functions correctly, by simulating the system behaviors. The model includes the effect of temperature on electrical resistance, walls, insulation, and oven air, and, accurately simulates the convection and radiation heat transfer processes. Results predicted by the simulation include the temperatures versus time history of the heaters and walls, and of the oven air for either heating and cooling. The steady-state temperatures reached by each oven element, and the contributions of radiation and convection to the total heat transfer rate have been investigated. Multiple runs were made to observe the system behaviors with or without the insulation or thermostat. Besides, the flow type of fluid was checked out whether it is laminar or turbulent. This is essential, since the convection heat transfer rates during the process might change, therefore the simulation results can not be accurate. The simulation results show clearly that the most important independent variable is surface heat flux. The increasing heater surface flux raised the steady-state heater surface temperature and caused it to be reached more quickly. The fraction of energy transferred by radiation increased from 71-82 percent of total heat transfer between surface heat fluxes of 42000-87000 W/m^. Therefore, the surface heat flux of the heater must be considered at the design stage, since it affects both heater surface temperature and the amount of thermal radiation from the heater. Once a specific heater is chosen, the surface heat flux can be controlled by choice of operating strategy. For further research and developments, the blower possessed ovens and the thermostat control systems for the electrically heated ovens may be investigated as it was mentioned in Section 4.1. Moreover, each wall can be divided into regions to observe the temperature distribution over the walls as more accurately. The oven air also can be considered as it possesses different temperatures in the middle and in the comers of the volume. This would be very helpful to predict the thermostat-around temperature more accurately, and consequently, the intervention temperature of thermostat. Furthermore, the trays can be incorporated into the oven model, and the divided volumes with different temperatures can be investigated separately.
