melting in cavity
 

Please I am trying to study melting in the cavity,
as I know Ra= gb delta(t)/nu alpha
and delta (T)= t(hot) -T(melting)
Stephan number= cp*delta(T)/latent heat of fusion
the cold temperature is included in which expression?
and shall I take the reference temperature as cold temperature.
I could not find these in tutorial
thaks for your help in advance.