How does the heat transfer rate change with an increase in water velocity in tubes?

The heat transfer rate in tubes is significantly influenced by the velocity of the water flowing through them. An increase in water velocity enhances the convective heat transfer coefficient, which is a measure of the rate at which heat is transferred between a solid surface and a fluid flowing over it. As the velocity increases, the fluid layers near the surface of the tube are disturbed more aggressively, leading to a greater degree of mixing. This increased mixing reduces the thermal resistance between the fluid and the tube wall, allowing for more efficient heat transfer.

Additionally, higher velocities help to minimize the formation of a stagnant boundary layer at the tube walls, which can hinder heat transfer. As the flow rate increases, the bulk fluid temperature can more effectively absorb and carry the heat away from the surface, resulting in an overall increase in the heat transfer rate.

In summary, as water velocity increases in tubes, the enhanced convective heat transfer due to improved mixing and reduced boundary layer thickness contributes to an increased heat transfer rate.