Search

Proton exchange membrane (PEM) fuel cell liquid water management remains a topic of research as these technologies continue to penetrate commercial markets. Two-phase flow pressure drop may be considered an in-situ diagnostic tool that can reveal the amount of liquid water accumulating in a reactant flow channel. While the topic of two-phase flow has been studied extensively, having a reliable two-phase flow pressure drop model that can accurately predict the pressure drop in PEM fuel cell flow channels is still an issue. In this study, in-situ and ex-situ two-phase flow pressure drops in PEM fuel cell flow channels are measured and compared with existing models. The objectives of the current study are to compare experimentally measured two-phase flow pressure drops with existing models as well as to improve the two-phase flow pressure drop model for an accurate pressure drop prediction for the application of PEM fuel cell. In-situ and ex-situ experiments were conducted and analyzed for this purpose. The models focused on in this work are all based on experimentally determined two-phase flow pressure drop for channel hydraulic diameters ranging from 0.15 mm to 12 mm. These models were developed for various applications and fluid/gas combinations. For this study’s ex-situ experiments, liquid-gas two-phase flow pressure drops were measured over 20 cm of a 2 mm x 1 mm flow channel machined from an aluminum plate. Water was injected through a Toray (TGP-060) gas diffusion layer (GDL) and emerged on its surface within the flow channel supplied with air flow. Fig. 1 compares 108 experimentally measured ex-situ two-phase flow pressure drops with nine two-phase flow pressure drop models. Figures a-g are based on the separated flow model and figures h-i are based on the homogeneous equilibrium model. The general trend observed in Fig. 1 suggests that although separated flow models outperform homogeneous equilibrium models, they mostly under-predict the two-phase flow pressure drop, especially at lower pressure drops. The seven separated flow model shown in Fig. 1a-g are further compared with each other by defining the mean absolute error (MAE), shown by λ in Fig. 1, (insert Equation 1 from JPG file) In addition, ω, θ, and ε are defined as the percentage of data points predicted within ±10%, ±30%, and ±50%, respectively. It can be observed from Fig. 1 that the model proposed by Mishima and Hibiki [2], shown in Fig. 1c, reflects the best prediction capability with the lowest MAE value. The MAE was further studied at different mass flow qualities and superficial air velocities as shown in Fig. 2. The numbers shown in parentheses indicate the number of experiment runs considered to calculate the MAE at each mass flow quality or superficial air velocity. Fig 2a shows the MAE at different mass flow qualities for each of the separated flow models. Other than mass flow qualities x = 0.87 and x = 0.97, the model proposed by Mishima and Hibiki [2] resulted in the lowest MAE compared to the other six models. However, the model proposed by Saisorn and Wongwises [1] showed a smaller MAE in these two mass flow qualities. Fig. 2b shows the MAE calculated at different superficial air velocities and air flow rates in the flow channel. For superficial air velocities of less than 2 m/s, the model proposed by Saisorn showed a superior performance. However, for superficial air velocities greater than 2 m/s, Mishima and Hibiki's model [2] outperformed. In addition to ex-situ results, experimental in-situ liquid-gas two-phase flow pressure drops are also being measured in an operating PEM fuel cell as shown in Fig. 3. The PEM fuel cell used in this study has seven parallel flow channels, each with 1 mm x 1 mm cross section, machined through stainless steel bipolar plates backed with clear polycarbonate sheets to visualize water. Two high-precision pressure transducers measure pressure drop along 80 mm of two flow channels. In addition to pressure drop measurement, liquid water accumulation and transport with the flow channels is also recorded with a CCD camera mounted directly above the cathode. A combination of ex-situ and in-situ results will cover a large range of liquid/gas flow rates which will be useful for model validation. References: S. Saisorn, S. Wongwises, The effects of channel diameter on ow pattern, void fraction and pressure drop of two-phase air-water flow in circular micro-channels, Experimental Thermal and Fluid Science 34 (4) (2010) 454-462. K. Mishima, T. Hibiki, Some characteristics of air-water two-phase ow in small diameter vertical tubes, International Journal of Multiphase Flow 22 (4) (1996) 703-712. Figure 1

Nine analogues of mecamylamine (2) which differ in the number and substitution pattern of methyl groups, were prepared. In four of these analogues the amine functionality is in an endo orientation. Enantiomers of 2-endo- and 2-exo-N-methylfenchylamine (25 and 26, respectively) were also prepared. The hydrochloride salts of these compounds were tested for nicotinic antagonism relative to mecamylamine in vivo and none was found to be as potent as mecamylamine, although a broad range of activity was observed. In general, methyl substituents at the C1, C2, and C7 positions of the mecamylamine structure do not appear to be significant for antagonistic activity. Methyl substituents at C3, however, appear to be very important for activity. Three sets of enantiomers of N-methylfenchylamine analogues, 28-30, possessing structural features of mecamylamine and nicotine were also prepared. These compounds were inactive as antagonists. Only a small degree of stereoselectivity was elicited in this series, less than that seen with enantiomers of nicotine. Antagonists with the exo N-methylamine functionality are slightly more active than the endo isomers. The extent to which structural modification might change lipophilicities was estimated through calculated partition coefficients; such changes alone appeared insufficient to explain differences in activities of the analogues. Lastly, a tolerance for a tertiary (dimethyl) amine functionality was demonstrated in addition to the lack of tolerance for bulkier substituents at C3 or on the nitrogen.

Title compound, (8), has been synthesized from isoquinolinone (1) (an improved preparation for which is presented) and separated into its antipodes with D- and L-di-p-toluoyltartaric acids. These antipodes and the racemic precursor have been evaluated (and found active) in two in vivo systems for their effects. The most potent of the three, (+)-8, has an ED 50 of 7.13 μmol/kg for inhibition of spontaneous activity and 7.45 μmol/kg for antinociception compared to 4.44 and 4.81 μmol/kg, respectively, for (S)-(-)-nicotine. Compounds (-)-(8) and desmethyl- compound (7) are about one-fourth as potent. Isomer (+)-8 has the 3aR,9bS configuration, the latter corresponding to (S)-(-)-nicotine as determined by X-ray crystallography. However, (+)-(8) failed to compete for [ 3 H]-nicotine binding, and its pharmacological effects were not blocked by mecamylamine. These bridged nocotine analogs either are binding to an as-yet-unidentified nicotinic receptor or they represent a novel class of non-nicotinic analgesics

The nature of mecamylamine's and pempidine's antagonism of nicotine in the central nervous system has not been defined clearly. Although these compounds are thought to be noncompetitive antagonists in the brain due to the fact that they do not compete effectively for agonist binding to brain tissue in vitro, pharmacological evidence is lacking. The alteration of nicotine's dose-response curves for depression of spontaneous activity and antinociception was determined in the presence of increasing concentrations of pempidine. Pempidine was found to increase the ED50 of nicotine (0.73 mg/kg) for depression of spontaneous activity in a dose-related manner. At a dose of 3 mg/kg, pempidine increased nicotine's ED50 4.7-fold. The maximum effect of nicotine was achieved in the presence of the highest dose of pempidine, suggesting competitive antagonism. However, pempidine did decrease the maximum effect of nicotine in producing antinociception at doses that increased the ED50 13.7-fold which suggests a noncompetitive action. The structural requirements for mecamylamine's antagonism of these nicotine effects was also determined in order to address the question of whether the antagonists are interacting at a receptor site. The structure-activity relationships of the mecamylamine analogs revealed that the N-, 2- and 3-methyl groups were important for optimal potency. Optical isomerism was found to have little effect on potency. Addition of pyridinyl groups to the nitrogen abolished the activity of these compounds. The structural requirements for the agonists and antagonists therefore appear to be quite different. The alterations produced similar results for antagonism of both effects of nicotine. Mecamylamine and pempidine therefore appear to exhibit both competitive and noncompetitive properties in antagonizing the central effects of nicotine.