The flow of air into a suction opening
Consider a simple length of ducting attached to the inlet side of a fan and another length of ducting attached to its outlet. Air is accelerated as it approaches the suction opening and, in order to produce this increase in kinetic energy, a negative potential energy has to be set up within the opening. Figure 15.5(a) illustrates that the air, in negotiating the entry to the duct, is compelled to undergo a change of direction (unless it happens to be on the centreline of the duct) and that this involves the setting up within the duct of a pocket of turbulence which reduces the area of entry available to the air. The reduced area is termed the ‘vena-contracta’.
Three immediate conclusions can be drawn from this.
(1) The velocity of airflow through the vena-contracta must be higher than that prevailing in the succeeding downstream length of duct.
(2) The curved paths followed by the air in the eddies within the pocket of turbulence involve the expenditure of energy—in accordance with Newton’s first law of motion.
(3) There is a drop in total pressure as the air flows through the open end of a suction duct, because of conclusion (2) above, regardless of the presence of any grille at the opening. If a grille is present then the loss of total pressure will be greater.
An application of Bernoulli’s theorem (equation (15.18)) permits the changes of total, static and velocity pressure to be determined as air enters the system. Figure 15.5(a) shows a plot of such pressure changes for a suction opening at the end of a duct, and (b) shows airflow into a ‘no-loss’ entry. The end of the duct is constructed in such a way that solid material occupies the space normally filled with turbulence and prevents the formation of a vena-contracta. Virtually no losses occur, and the static suction set up just within the open end of the duct, where it has attained its proper diameter, is numerically equal to the velocity pressure, but opposite in sign.
(No-loss entry pieces of this kind provide a very reliable and accurate method of measuring the rate of airflow through a duct. It is only necessary to take a careful measurement of the static pressure on the section where the taper has just ceased and to convert this to velocity, by means of equation (15.27). Such a method is usually suitable for laboratory uses only.)
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