For a horizontal injector, the merging among mist drops radially will be severe than that of vertical injector. Beacuse the move of mist drops under the gravity is almost the same with its radial direction.

True, albeit there is the Coulomb equation for (an additional) internal pressure caused by surface friction ona sphere's surface. This means that for some specific values of the surface tension coefficient and small enough droplet radiai they do not merge anymore. In some of my experiments untrasmall droplets started behaving like ping-pong balls, so high was the force holding them together. However, in general I agree with Zhihoua's answer. One more comment, the distance between point of measurement from the exit of the port matters as well.

Thank you for your replies.