I've heard that in a high velocity nozzle, any random 'fluid particle' experiences an increase in kinetic energy at the cost of internal energy and pressure energy.And I do understand that for any thing to accelerate a net external force is needed(which, is due to the pressure gradient in this case,yes). But the shouldn't the fluid particle, still be able to change its kinetic energy at the cost of its internal energy? If yes, plz explain.
In Newtonian physics, fluids, like anything else, obey conservation of momentum, or $F=ma$.
The Bernoulli principle is just a re-statement of conservation of momentum.
The only thing that can change a parcel of fluid's speed (i.e. accelerate it) is a force. One kind of force is a pressure gradient. Another kind of force is gravity. If the fluid is conductive, electromagnetism can give still another kind of force. Nothing but a force can accelerate the fluid.