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They're not silly high, but I can't comment as to whether they're correct for your application. That's for you to check as I've not done (and am not doing) the model. 

HTC from fluid to solid in soil, I'd look for a correlation similar to the Ergun equation. Or if there are any papers on packed bed reactors. Check how the solid interacts with the fluid walls (it doesn't) before deciding if you should use it.