% Fully developed Nusselt number for inner surface of annular duct, with adiabatic outer surface.  
function [Nui, fiReD, Dh] = AnnNuiInf(Wall,Ri,Ro)
  ispan=Ri./Ro;
  ispan(ispan>1)=1./ispan(ispan>1);
  Dh=2*(1-ispan).*max(Ri,Ro);
  fiReD=32*(1-ispan).*(-2*ispan-(1-ispan.^2)./ispan./log(ispan))...
                  ./(1+ispan.^2+(1-ispan.^2)./log(ispan));
  switch Wall
  case 'T'
    Nui=4.224-1.492./ispan.^(1/3)+1.972./ispan.^(2/3)+0.137./ispan;  % approximate
  case {'H','H1','H2'}
    Nui=5.272-2.239./ispan.^(1/3)+2.218./ispan.^(2/3)+0.1133./ispan; % approximate
  otherwise
      error('Wall= ''T'' or ''H''');
  end
end
