velocity of the entering fluid should be in the direction of tangent to diffuser blade at its entrance. It should be noted that the number of diffuser vanes is kept less than that of moving vanes to make the flow uniform along peripheral direction. Therefore, a diffuser passage is fed by more than one moving vane and the streamlines a the entrance and curved. We know from fluid mechanics that such flows are associated with secondary flow, which cause loss of head. Therefore, initial portion of the passage is designed to straighten the streamlines. This process is completed within a short distance during which the diffuser passage is curved and fluid angles go on changing. The section where streamlines are almost straight is called throat and after this section the diffuser passage is a straight diverging section so that the area of flow goes on increasing. Normally a rectangular section is used with constant depth and varying width. Angle of divergence is around 8 to 10 to avoid separation of fluid from the wall of the passage