Keywords : Pressure coefficient
Engineering and Technology Journal,
2013, Volume 31, Issue 7, Pages 1331-1350
The present work is an experimental and numerical CFD study of air impingement from orifices of different sizes in the range of (10-40) mm with a step increase of 10 mm. The impinging jet strikes the target plate at the stagnation point, converted to a wall jet beyond the stagnation zone in the outward radial direction on the target plate. Measurements of pressure coefficient (Cp) at points in plate centre and other points in the radial direction from stagnation point on the target plate show that pressure coefficients reduces gradually in the outward direction reflecting the same behavior of velocity distribution of the impinging jet ..The calculated average values of the pressure coefficients Cpavg on the total target plate area are reduced as the target plate – orifice distance is increased and its values are in the range of 0.3-0.7 with the higher jet velocity having the greater value of Cpavg for the tested axial distances in the range of 50-120 mm. Average pressure coefficient for different axial distances are calculated and are in the range of 0.4-0.6 with the higher values are noticed to be for the smaller tested orifice sizes reflecting a concentration of pressures for the small sizes of the orifice. This concentration of pressure of small orifices is attributed to the flow condition of the small orifice relative to the big orifice size where the presence of cross flow within the stagnation region of big orifice sizes tends to reduce static pressures and reduce peak heat transfer coefficients, this cross flow is confirmed by the velocity analysis using CFD numerical analysis zone at the stagnation zone. The percentages of the wall jet relative to the impinging jet velocities are discussed, where smaller orifice sizes are shown to have greater velocity percentages especially at the higher tested jet velocities.