Convection entry length

The hybrid solution procedure described in the previous section is computationally more demanding than one that does not rely on the CFD package to predict the heat transfer from the exhaust gas. In fact, this simpler approach was adopted in die early stages of the project, the heat transfer process was modelled using a mean heat transfer coefficient estimated from correlations for convective heat transfer in annuli. However, it was soon realized that this method has a high degree of uncertainty when the heat transfer process takes place under unsteady-state conditions and when the thermal entry length spreads over an appreciable extent of the domain. These conditions are always met in the application under study. 

For advanced modeling purposes, the addition of minor loss, flow field switchback, and manifolding effects can be approached analytically. In practice, however, the actual pressure drop in an cell or stack is very difficult to predict with high precision due to the effects of entry length, local turbulence, additional minor losses from switchback, consumption, uptake and other effects. Additionally, in PEFCs and AFCs with a porous diffusion media, there can be unintentional convective flow under the channels, which reduces overall pressure drop, as discussed in Chapter 6. Therefore, a good starting point is to assume the frictional pressure drop dominates (which has been found to be true in certain PFFCs [32]) and calculate an expected loss from Fq. (5.76). For a particular fuel cell, the pressure drop can be correlated as a function of entrance velocity, since this is relatively easy experimental data to obtain. 

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