1. A heat exchanger, comprising:a chassis enclosing an accommodating space and having a mounting opening at a bottom of the accommodating space, and an outer outlet at a lateral of the accommodating space;
a cooling core installed in the accommodating space and disposed above the mounting opening;
an external circulation fan module installed into the accommodating space from the mounting opening and disposed between the outer outlet and the mounting opening; and
a cover plate having a plurality of holes and configured to cover the mounting opening and combine with the chassis;
outside air enters from the holes into the accommodating space and flows through the cooling core, and then exchanges heat with the cooling core, and flows out from the outer outlet,
the external circulation fan module includes an axial fan and a fan frame, the external circulation fan module is fixed within the chassis by the fan frame, and the cover plate is locked onto the fan frame to integrate with the chassis, and
the fan frame has at least one flap, and the flap perpendicularly extends from the fan frame toward the cover plate and has a locking hole.