Could anybody help me how to consider the force applied to the pipe bend due to the flow? (flow momentum change) I think the force is critical when design the pipe supports when pipe is filled with water. Thanks.

Not sure if it is slug flow, or transient / waterhammer effects you are considering, but there are many posts to be found on these subjects using the Search feature.

In steady state flow centrifugal force due to flow can be neglected.