Global heat transfer

S. G. Mueller, R. Eckstein, D. Hofmann, L. Kadinski, P. Kaufmann, M. Koelbl, E. Schmitt. Modelling of the PVT-SiC bulk growth process taking into account global heat transfer, mass transport and heat of crystal-Uzation and results on its experimental verification. Mater Sci Eorum 0 51, 1998. 

It is important to note that the compacityfactor is defined by the ratio of the surface area offered to heat transfer over the volume of the reactive medium. The thermal performances are estimated from the product between this compacity factor and the global heat-transfer coefficient. Consequently, owing to the large value of this factor combined with the conductivity performances of the SiC material, the heat-exchange performances are expected to be very high, which can be noticed from the last column of this table. 

Then, the quantity of heat that could be removed in batch reactors whose volume varies from 11 to 1 m is calculated. In order to compare with experimental results, the temperature gradient is fixed at 45 °C (beyond which water in the utility stream would freeze and another cooling fluid should be used). The maximum global heat-transfer coefficient is estimated at an optimistic value of 500 W m K h The calculated value of the global heat transfer area of each batch reactor. A, is in the same range as the one given by the Schweich relation [35] ... 

The results of all the thermal-capillary models discussed so far have neglected the influence of convection in the melt in transporting heat to the solidification interface. The status of convection calculations that neglect the coupling to global heat transfer and capillary consideration is discussed later. The union of thermal-capillary analysis with detailed convection calculations is discussed in the subsection on melt flow. 

Finally, the global heat-transfer coefficient includes the resistance of the jacket wall ... 

Adiabatic measurements and kinetic parameters. The experimental temperature measured in a semi adiabatic box and the adiabatic temperature rise, obtained from a paste with w/c=0,45, is shown in the Fig. 1. Once the adiabatic temperature is known the global heat transfer coefficient (U) can be determinate with a good correlation coefficient (Fig. 2). 

The global heat transfer coefficient comprises all transfer resistances as shown in Equation 5.8. In general, a minimum volumetric heat transfer coefficient of U, >1 MW m is required for fast and exothermic reactions [16]. 

In this equation, m represents the mass flow rate, is the mean specific heat capacity of the mixture in the considered temperature range, and U is the global heat transfer coefficient. 

With a volumetric global heat transfer coefficient of = 1.06 10 kWm K and a temperature difference of AT = 10 K, the maximum total heat generated can be evacuated. 

Distribute the total DT (increment of temperature) into the number of effects inversely proportional to the global heat transfer coefficient... 

Finally the overall heat transfer coefficient is obtained from equation 8. The global heat transferred for each tube is computed with equation 9. We call A7 / semi logarithmic temperature difference . It is the best compromise between pure logarithmic temperature difference that has no sense here (only one tube) and pure arithmetic temperature difference that does not allow to follow the evolution of water properties along the tube. The heat exchange diagram of the 0TB is presented in figure 3. 

He. 3t trans l rer temperature difference at process start / end required heating / cooling time local / global heat transfer coefficient specific heat capacity and heat conductivity of the product... 

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