CAMBIO EPOCAL: HACIA UNA ECONOMÍA COMPLEJA

The experimental results are discussed first, followed by a comparison of the analytical solu- tions to the experimental data. The justification for these steam condensation experiments was described in the introduction along with the requirements for uniformity of radial flow and condensation process in the packed bed. The thermal images along the wall of the packed bed as obtained from the IR camera for one of the experiments are shown in Fig- ure 4.2. These images show that there is no angular non-uniformity in the bed. Although

temperature of walls is lower than the temperature of the bed, shown later, as obtained by internal thermocouples. Moreover, X-ray images shown in Figure 4.3 also indicate radially uniform condensing front. These results show that 1-D axial model with wall heat losses is sufficient for thermal behavior prediction.

Figure 4.2: (TOP LEFT) Picture of experimental packed bed vessel. (LINE WISE: LEFT to RIGHT) Time step images from FLIR software for an experimental run at 50 psi for; 5, 10, 15, 20, 30, 35, 40, 45, 50, 60, 70 seconds after steam injection, respectively.

The propagation of the temperature front for each of the different cases, injection rate and HTF type, while penetrating the randomly packed bed was experimentally studied. The differences between the different steam injection rates will be discussed next. The average condensate collection flow rates for the slow and fast cases were measured to be, respectively, 6 cm3/s and 12 cm3/s. The discussion provided in the next subsections will not explicitly mention the flow rates but will use terminology of fast and slow injection.

For each experiment it was found that the uncertainty in the measurement of condensation collection flow rate is within 5% of the numbers sated above.

Figure 4.3: (Top Left) Image of 6” packed bed with Alumina particles. (Top) X-ray images of packed bed and (Bottom) IR images in 10 second intervals during slow steam injection.

Slow steam injection

The characteristic thermal response of the packed bed system at different times upon slow injection of steam is highlighted in this discussion with explanation of results. Upon in- jection of steam into the bed, there are two thermal transport mechanisms- advection and conduction modes at different spatial locations and different time frames. Near the entry port where steam is introduced, in this present experimental set-up from the top, in a very short time interval temperature of the bed and fluid streams become almost equal to the steam inlet temperature or saturation temperature. With the steam supply continuously available, irrespective of injection rate, the bed temperature at the top is always maintained at a constant top temperature i.e. saturation steam temperature. This constant bed temper- ature at the top will conduct heat from the top to bottom of the bed due to non-negligible thermal conductivity of alumina particles and water condensate in the bed i.e. conduction mechanism. Simultaneously, due to steam injection in the bed it is carrying some amount

of energy as it moves in the bed i.e. advection mechanism.

Due to slow injection rate, initial rise in the temperature at axially farther locations will be dominated by the conduction mechanism. As the steam or two phase mixture front, which is at temperature near the saturation temperature, reaches those regions located far away from injection point there is a sudden change in the temperature. This effect can be seen from temperature measurements obtained by thermocouples at different locations and at different times as shown in Figure 4.4. The rate of increase of temperature for different thermocouples positioned at different axial locations is divided into two distinct regimes with two distinct slopes, especially for last four (3-6) locations. The initial regimes, which show lower slope are governed by conduction mechanism and later regimes with higher slope, are governed by advection. These conclusions are substantiated with the observations that conduction effect shows an observable conduction dominate temperature plot for locations at larger distances from injection point. Similar effects were quantitatively predicted and experimentally observed by Woods et al. in liquid-vapor flows around porous beds37;43. In theses previous studies by Woods et al., cold water was injected into hot rocks.

Fast steam injection

Based on the explanations provided in previous subsection, it is expected that advection term will be higher in fast injection experiments as compared to the slow injection experiments. The higher advection term implies that total amount of influx enthalpy carried by the steam or two-phase mixture is much higher and thus, as the fluid stream moves through the bed it is equilibrating the bed to the saturation temperature at almost constant rate at all spatial locations. Due to much higher rate of enthalpy injection in the bed due to advection term, the effects of conduction will not have much impact on the rate of temperature increase in the bed. The results in Figure4.5, for fast injection, confirm this explanation. This phenomenon of smaller temperature dispersion due to conduction can be seen in the experimental results of fast injection experiments.

Compared to the slow injection case, the time taken for the bed to reach peak temperature throughout in the fast injection case is smaller.