燃料电池阴极流道水淹的可视化、各种两相流流动图和液态水润湿面积定量
Visualization and quantification of cathodechannel flooding in PEM fuel cells
Irfan S． HussainiChao－Yang Wang
Abstract
An understanding of two－phase flowmechanisms in micro－channels is critical to water management in fuel cellapplications． In this work， an in situ visualization study of cathode floodingin an operating fuel cell is presented． Gasrelative humidities of 26％， 42％ and 66％， current densities of 0．2， 0．5 and0．8A cm−2 and flow stoichiometries ranging from 2 to 4 are used in this study which represent typical operating conditions for automotiveapplications． Results are presented in the form of a flow map depicting various two－phase flow patterns． The impact offlooding is also presented in terms of measurable parameters like two－phase pressure drop coefficient andvoltage loss． A new parameter called wettedarea ratio is introduced to characterize channel flooding and liquid watercoverage on a gas diffusion layer， and its repeatability with multipletests is demonstrated．
Fig． 2． Images showing liquid water incathode channels at 0．8Acm−2， 66％ RH and stoichiometries of 4 （above） and 2（below） respectively．
Fig． 3． Flooding in cathode channels at0．8Acm−2， 26％ RH and stoichiometries of 2．5 （above） and 2 （below） respectively（no liquid water is observed at ζ≥ 3）．
Fig． 4． Magnified view of flow patterns inchannels and their corresponding line illustrations showing the form anddistribution of liquid water．
Fig． 5． Flow map of cathode channelflooding on hydrophobic GDL surface．
Fig． 6． Two－phase pressure drop coefficienton cathode side．
Two－phase pressure drop coefficient
The term ΔPsingle－phase represents the mean value of pressuredrop measured during an open－circuit period when gases are flowing at their respectivestoichiometries and a single－phase， gaseous flow exists in the cell．
Fig． 7． Loss in cell voltage due to channelflooding．
Fig． 9． Variation of calculated wetted arearatio （Φ1） withcurrent density， stoichiometry and gas RH．
Fig． 12． Comparison ofΦ1 values from two differenttest runs， at identical operating conditions．
化学计量比为2，RH＝26％时，图3表明存在液态水，电流密度更高时，Φ1越小。
如果图2中标识各点的Φ1值就更有价值了。
读完才发现Φ1中的1应该是l（liquid），而不是1。但是拷贝出来进行检索，匹配的都是1。
Conclusions
Visualization study of cathode side liquid waterin straight channels has been carried out for a range of humidification levels， current densities and stoichiometries representingtypical automotive conditions． An attempt has been made to present the findingsin the form of a flow map and also quantitatively in terms of two－phase pressure drop coefficient，voltage loss and wetted area ratio．
Following are the main conclusions drawnfrom this study．
1． Cathode GDL and channel flooding is apersistent phenomenon accompanying fuel cell operation even under low humidityconditions． In most cases， flooding is found to be significant enough to causesubstantial loss of cell performance． Operatingconditions of low current density and low stoichiometry are found to beparticularly susceptible to severe flooding under all humidity conditionsconsidered．
2． Ata given current density， the two－phase front is found to move upstream towardinlet as flow stoichiometry decreases with other factors remaining constant．The location of the front physically correspondsto the point at which cathode gas RH reaches 100％．
3． Observed flow patterns are represented inthe form of a flow map， possibly the first such representation for two－phaseflow in fuel cell channels． The flow map shows the spectrum of flow patterns andconditions leading to transition from one flow regime to another．
4． Atwo－ to five－fold increase in pressure drop has been observed as a result ofchannel flooding． It represents a significant penalty in terms of parasiticpower spent in driving reactant gases through a flooded cell．
5． Occurrenceof liquid water flooding has been shown to cause a voltage drop of up to 80 mVunder low stoichiometry conditions． This represents a huge loss inperformance considering that a ten－fold increase in the reaction currentdensity on electrode surface translates into a voltage increase of only about 66mV．
6． Occurrenceof rapid fluctuations in cell voltage together with a low and constant membraneHFR is characteristic of channel flooding．
7． The GDL surface area covered by liquidwater may be quantified by the parameter called wetted area ratio． Actual distribution of water is found tovary from channel to channel and from experiment to experiment， but the totalamount of liquid water resident in all the channels is found to be fairlyconstant under a given set of operating conditions．
8． Wetted area ratio is found to berepeatable within an experimental margin of uncertainty， thus indicating thatit represents the state of the fuel cell at given test conditions． Itsrepeatability under a number of test cases has been demonstrated．
