Why draft tube is used in Francis turbine

A draft tube is a gradually diverging tube connecting the runner exit of a Francis (reaction) turbine to the tailrace, with its outlet submerged below tailwater. It is used in a Francis turbine for the following reasons:

1. Recovery of exit kinetic energy: in a Francis turbine water leaves the runner with a considerable velocity (about 5–10 m/s). Without a draft tube this kinetic energy ($V^2/2g$) would be wasted in the tailrace. The diverging tube reduces the velocity and converts most of this energy into pressure energy, increasing the useful head.

2. Creating suction (negative head) below the runner: the column of water in the draft tube produces a pressure below atmospheric at the runner exit:

$$\frac{p_1}{\rho g} = \frac{p_a}{\rho g} - H_s - \left(\frac{V_1^2 - V_2^2}{2g} - h_f\right)$$

So the head between the runner and the tailwater ($H_s$) is not lost; the full head is utilised.

3. Setting above tailwater: the turbine can be placed above the tailwater level, making the powerhouse dry, easy to inspect and maintain, while still using the full available head.

4. Higher efficiency and output: overall efficiency improves by several per cent, which matters greatly for large Francis units.

5. Safe discharge of water to the tailrace at low velocity.

   spiral casing + runner
       ===[  ]===
          \  /   <- inlet: high V, p < p_atm
           ||    Hs
 ~~~~~~~~~ || ~~~~~~~~ TWL
          /  \   diverging tube
         /____\  <- outlet: low V

The height $H_s$ is limited by cavitation (Thoma's criterion), since the inlet pressure must stay above vapour pressure.