  Photon trapping is believed to be an important mechanism in super-Eddington
accretion, which greatly reduces the radiative efficiency as photons are
swallowed by the central black hole before they can escape from the accretion
flow. This effect is interpreted as the radial advection of energy in
one-dimensional height-integrated models, such as the slim disc model. However,
when multi-dimensional effects are considered, the conventional understanding
may no longer hold. In this paper, we study the advective energy transport in
super-Eddington accretion, based on a new two-dimensional inflow-outflow
solution with radial self-similarity, in which the advective factor is
calculated self-consistently by incorporating the calculation of radiative
flux, instead of being set as an input parameter. We find that radial advection
is actually a heating mechanism in the inflow due to compression, and the
energy balance in the inflow is maintained by cooling via radiation and
vertical ($\theta$-direction) advection, which transports entropy upwards to be
radiated closer to the surface or carried away by the outflow. As a result,
less photons are advected inwards and more photons are released from the
surface, so that the mean advective factor is smaller and the emergent flux is
larger than those predicted by the slim disc model. The radiative efficiency of
super-Eddington accretion thus should be larger than that of the slim disc
model, which agrees with the results of some recent numerical simulations.
