% Fully developed Nusselt number for outer surface of annular duct, with adiabatic inner surface. 
function [Nuo, foReD, Dh] = AnnNuoFD(Wall,Ri,Ro)
  ispan=Ri./Ro;
  ispan(ispan>1)=1./ispan(ispan>1);
  Dh=2*(1-ispan).*max(Ri,Ro);
  foReD=32*(1-ispan).*(2+(1-ispan.^2)./log(ispan))...
           ./(1+ispan.^2+(1-ispan.^2)./log(ispan));
  switch Wall
  case 'T'
    Nuo=3.657+1.809*ispan.^(1/3)-2.808*ispan.^(2/3)+2.203*ispan; % approximate
  case {'H','H1','H2'}
    Nuo=4.364+2.169*ispan.^(1/3)-3.664*ispan.^(2/3)+2.519*ispan; % approximate
  otherwise
      error('Wall= ''T'' or ''H''');
  end
end
