ANS:A - At high fluid velocity

The heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids when the fluid velocity is high. Here's why:

1. High Fluid Velocity: When the fluid velocity is high, the frictional forces between the fluid particles and between the fluid and the walls of the conduit or pipe increase. These frictional forces result in heat generation through viscous dissipation or fluid friction. At high velocities, this heat generation can become significant compared to the heat transferred between the fluids in the heat exchanger.
2. Low Fluid Velocity: At low fluid velocities, the frictional forces are relatively low, leading to less heat generation through fluid friction. In this case, the heat transferred between the fluids typically dominates over the heat generated by fluid friction.
3. Smooth Surface: The roughness or smoothness of the surface over which the fluid flows affects the magnitude of fluid friction. However, it does not directly determine when the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids. This is primarily determined by the fluid velocity.