Question

when does change in enthalpy =change in internal energy?

Answer 1

at constant pressure
h=u

Answer 2

∆H = ∆U + p∆V

At constant volume, the change in enthalpy will be equal to change in the internal energy.
But why?

At constant volume, the work done on the system is zero and hence the change in enthalpy of a chemical reaction is only by virtue of temperature. Hence, this result.