Question

Water at 60°F flows from a large tank as shown in the figure below. Atmospheric pressure is indicated by the mercury barometer, where H is 30”. If viscous effects are neglected, at what height, h, of water in the tank will cavitation begin? Dz = 4 in. 1 De = 1 in. 1 De='2 in. Figure 3.83 John Way & Sons, Inc. All these

Answer 1

cavitation will begin when pressure of liquid is below the vapour pressure for given condition
solution given values in question H=30" & water at 60°F viscous effect is neglected find – h when cavitation starts Point to Remember cavitation occurs only when the pressure of en liquid is less than the vapour pressure - so first find the minimum pressure of water in tube , should be greater than vapour pressure to avoid the cavitation for given condition. [Pmin <Pv] for cavitation occurance Parm = Sm gH - 13600 9:81x30x2.54 x lo? pa.101662.99 Pa density of mercury (Sm) = 13600 kg/m3 9 = 9.81 m/s2 H: 30" = 30x2.54cm - D=4" R Di = linch 89 29 D2 2inen apply bernoulli's equation between point A&E PA + V + ZA = P + 1 + 2 PA Parm + Sugh , VA=0 no velocity of water in tank ZA=ZE. by figure Par contre de noga Pantsoh - Batman VE = 12gh

A in tube diameter decreases, velocity of water increases then pressure of water decrease Q = I D x V = constant Dhu p Pt I by bernaullis equ. + + 2 = constant p velocity increases, pressure decreases so minimum pressure exist where diameter is minimum i.e.point Q = 7 du XVE = A x D, ²x Vc (2) ² x 22 hh = (1) ² Ve e = 4829h - V² = 32gh apply bernaultes equation between A and c PA + Vat + z 2 hotel ot ze 49 29 39 29 Patm + high - 329h - 1 Pes Patm t f g h - 185gh Pe: 101662.99 fa +15 Sw g h = Pmin to start cavitation Pe = min < Pv vapour pressure of water at oor = 1705.6 Pa 101662.99 -15 Swgh < 1705.6 gggs 7.39 < 158wgh 0.6792 <h cavitation will start at h= 0.6792m. and cavitation will take place below the 0.6792m. { sw = 100 kg
. pressure is minimum where diameter of tube is minimum.

ans =if height of water column in tank is less than or equal to 0.6792m cavitation will occur