Isothermal cold start

Jiang et al.16 developed a multiphase, three-dimensional model to describe non-isothermal cold start and to study the effect of temperature rise. Due to the temperature rise during cold start, more water was transported into the membrane and less ice formation occurred in the catalyst layer. It was also found that a lumped thermal analysis significantly overestimated the overall thermal requirement for successful self-cold start. In addition, pre-startup conditions such as gas purge had significant impact on cold start that implied the importance of the shutdown process. 

Figure 9.9 illustrates a typical galvanostatic isothermal cold start for an FC initially dried prior to start-up. The maximal survival time at -10°C for a low current density of 0.1 A cm is quite short, around 2.5 min. Two characteristic periods and their corresponding transitions are distinguished ... 

Tajiri, K., Tabuchi, Y. Wang, C.-Y Isothermal cold start of polymer electrolyte fuel-cells. feciroc/iem. Soc. 154 2 (2007), pp. B147-B152. 

FIGURE 9.9 Typical isothermal galvanostatic cold start at -10°C for a FC initially dried prior to start-up. 

FIGURE 9.16 Change in FC current densityfor isothermal potentiostatic cold start at 0.5 V and-10°C for different dried levels obtained at 20°C prior to start-up. (Reprinted from /. Power Sources, 186, Pinton, E. et al.. Experimental and theoretical investigations on a proton exchange membrane fuel cell starting up at subzero temperatures, 80 88, Copyright (2009), with permission from Elsevier.)... 

FIGURE 9.21 Change in FC current density for isothermal potentiostatic cold starts at different imposed temperatures at 0.5 V and at an identical dried level at 20°C prior to start-up. X, corresponds to R2o"c = 0-5 cm. ... 

FIGURE 9.22 Change in FC voltage for isothermal galvanostatic cold starts at different imposed temperatures at 0.1 A cm and at an identical initial dried level. (Reprinted from Electrochimica Acta, 52, Ge, S. and Wang, C.-Y., Characteristics of subzero start up and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell, 4825-4835, Copyright (2007), with permission from Elsevier.)... 

Non-linear optics probe molecules dispersed in PS [59] revealed that below Tg the relaxation time T follows the Arrhenius form with activation energies of 45 - 50 kcal/(mol K), or 188 -209 kJ/(mol K). Similar relaxation has been found in labeled PS [60], Other non-linear optical probe molecules have much longer decay time ( days) [61], Relaxation of poled order in dye doped PS was probed by isothermal and non-isothermal current measurements [62], Neutron scattering revealed that the fast process of phenyl rings starts at 200 K, and main chain starts at 250 K [63], The relaxation of the segmental alignment dichroism of cold rolled PS at 60 °C can be fitted by two single exponential functions of time with RT s of 76 s and 3600 s [39],... 

Sobolev et al. (1971) determined the phase equilibria within an isothermal section of the system Y-Cr-Si at 800°C based on metallographic and X-ray powder analysis of 80 alloy specimens. Samples were prepared by arc melting and subsequent annealing for 500 h at 800°C, and finally quenched in cold water. Starting... 

The cold crystallization starts at about 400 K at a supercooling not affected by modulation and registers as nonreversing. For separation of such nonreversing transitions, several modulation periods must occur across the transition, otherwise the pseudo-isothermal analysis would not develop the proper sinusoidal oscillations about , as can be seen from the modeling in Figs. 4.100-102 (loss of stationarity). 

Figure 14 represents the partial isothermal section of the Y-Al e system at 770 K after Muravyeva et al. (1971). Three ternary compounds were found to exist from X-ray phase analysis of alloys prepared by arc melting the proper amounts of pure components in an argon atmosphere. The resulting buttons were annealed for two weeks at 770 K and finally quenched in ice-cold water. Purity of starting components was greater than 99.9 mass%. 

Figure 95 represents the isothermal section of the Ho-Ni-Ge phase diagram at 870 K, which was studied by Aslan (1990). The isothermal section was constructed by means of X-ray powder and partly microstructural analyses of 158 alloys which were arc melted, subsequently annealed in evacuated silica tubes for 720 h at 870 K, and finally quenched in cold water. The starting materials were Ho 99.9mass%, Ni 99.98 mass%, and Ge 99.99 mass%. 

Chaban and Kuz ma (1971) investigated the phase equilibria of the system Y-Si-B in an isothermal section at 800°C by means of X-ray and metallographic analysis of arc melted and subsequently annealed alloys (480 h in evacuated quartz tubes, quenched in cold water). Starting materials were Y ingots 99.90%, B powder 99.3%... 

The computer code SWEPT [7] is used for this analysis. SWEPT is a system pressure wave propagation analysis code adequate for the analysis of short-term transients in a closed network due to many engineering issues such as sudden valve closure, sodium-water reactions, pump start up. The primary sodium circuit is modeled as a closed piping network with hot pool, cold pool, and PSP forming pipe junctions. Sodium is assumed to be isothermal at the average temperature of 745 K. Pipe rupture at the header end is analyzed here. 

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