[Nek5000-users] what exactly is thermal flux and how to set it on different sides?

[nek5000-users at lists.mcs.anl.gov]

1.
if n is a normal direction,
is it that
flux = dTEMP/dn (what I supposed)
or is it a  real thermal flux
flux = k dTEMP/dn with k as conductivity?

A am asking because I dont understand the source of instability in solution when I simulate a stratified flow
between two boundaries with  
TEMP=1-z as initial condition
When I set flux on upper and lower boundaries to 0  the program works,
but when I set it to -1 the program blows up in few steps.

2. The next question is how to set differen user defined BC on different boundaries

if you set in .rea files lowcase "f" 
for two boundaries it means that the BC is determined by "flux" in userbc subroutine, 
but I have different expressions for fluxes for different sides. 
I have not found this question described in docs.

3. And sorry for another qiestion:
In benard example there are files like ray_nn.rea ray_dn.rea ray_dd.rea for different combinations of BCs.
But only one .usr file - ray_cr.usr.
I suppose that these BCs must be defined in userbc subroutine of .usr file.
So how does it work?

Van