FLOW CHARACTERISTICS OF ELLIPTICAL ORIFICE PLATES

ABSTRACT

This paper presents the results of the experimental investigation concerning the clear water flow through elliptical orifice plates in a circular pipe. Experiments were conducted in a pipe line having diameter of 38.1 mm and using elliptical orifice plates with orifice area to pipe area a/A = 0.20, 0.33, 0.50, 0.66 and 0.84. For a comparative study, five circular orifice plates with same a/A ratios were also tested simultaneously. The data collected in the present study alongwith the data collected from literature have been analysed to study the position of vena contracta, and variation of discharge coefficient C_d with relevant flow and geometric parameters. The dimensionless pressure distribution curves were analysed to locate the position of vena contracta with area ratio. The position of vena contracta is a function of area ratio. As area ratio increases, the location of the vena contracta shifts towards the orifice plate.

Similarly, the position of vena contracta L_v/D is a function of ellipticity ‘e’ which is defined as the ratio of minor axis to major axis of the ellipse i.e. 2b_o/2a_o and the displacement angle θ. The angle θ is the angle between the X-axis and major axis (2a_o) of the ellipse. As ellipticity increases the position of vena contracta decreases. Also as displacement increases, the position of vena contracta decreases.

Further, it has been found that for low values of orifice Reynolds number R_o, C_d depends on both a/A ratio and R_o. At higher R_o values (R_o≥3×10⁴), C_d depends only of a/A ratio. For the same area ratio a/A, the elliptical (Horizontal) orifice plate has smaller values of coefficient of discharge at higher R_o as compared to circular orifice plates. For higher R_o values, the relationship between limiting values of discharge coefficient C_dl and a/A for elliptical orifice plates is given by the equation

C_dl = 0.596-0.324(a/A)+1.01(a/A)²

Also at higher R_o values, the relationship between C_dl and a/A for different shapes of orifice plates is given by

C_dl = 0.639—0.288(a/A)+0.817(a/A)²

The limiting C_d values i.e. C_dl increases as ellipticity increases for all a/A ratio. However, there is no significant effect on C_dl with displacement angle θ.

KEY WORDS:

• Elliptical orifice plate
• Discharge coefficient
• Vena contracta
• Pressure distribution
• Ellipticity
• Displacement angle