The blending happens in lines 324-327 (R2022a):
% Smooth transition for heat transfer coefficient between liquid, mixture, and vapor
heat_transfer_coeff = simscape.function.blend( ...
simscape.function.blend(Nu_I*k_I/Dh, Nu_mix_I*k_sat_liq_I/Dh, 0, A.transition_range, alpha_I), ...
Nu_I*k_I/Dh, (1 - A.transition_range), 1, unorm_I);
The simscape.function.blend applies a 3rd order polynomial inside. You can select it and right click to open its code (or Ctrl+D).
function y = blend(y1,y2,x1,x2,x)
% y = blend(y1,y2,x1,x2,x)
% Blend between two values y1 and y2 as x varies between x1 and x2. The
% blending function ensures y is continuous and differentiable in the
% range x1 <= x <= x2.
% Copyright 2017-2018 The MathWorks, Inc.
definitions
u = (x-x1)/(x2-x1);
transition = 3*u^2 - 2*u^3;
y = if le(x,x1)
y1;
elseif ge(x,x2)
y2
else
(1-transition)*y1 + transition*y2;
end
end
end
