Transport Exam 2

1) A pump is used to take water from a reservoir and push it through a filter. At the desired flow rate, the gauge pressure at the pump outlet is 8 atm, while the filter discharges to the atmosphere. If frictional losses in the pipes (same diameter everywhere) and other fittings can be ignored relative to filter losses, what is the head loss caused by the filter? Use round numbers for your variables and conservations: water density 1000 kg/m3; 1atm = 100,000 Pa; water viscosity 0.001 Pa*s; gravitational acceleration 10 m/s2 2) If the flow rate in the set-up described in the first problem is increased by ramping up the pump power, what will happen to the head loss of the filter

1) point 1 - filter inlet; point 2 - filter outlet no heat added no shaft work no viscous work no change in height no change in velocity (P2-P1)/p + g*hL = 0 (P1-P2)/(p*g) = hL but since P1 = 8atm at gauge pressure... P1/(p*g) = hL --> 800,000/(1000*10) = 80 m 2) increase...higher flow rate, greater friction, higher head loss.

T/F: Water flows through two pipe sections with the same internal diameter and length; one of the pipes is perfectly smooth, the other has rough walls. At all flow rates, the head loss in the smooth pipe will be lower than in the rough pipe.

False. At laminar flow the head losses are the same for both pipes.

T/F: The no slip boundary condition cannot be applied to shear thinning liquids

False All fluids with viscosity must obey no-slip

T/F: Water flowing through an open channel; at the water-air interface the no slip boundary condition can be applied.

False At air water interface, no stress BC is appropriate

T/F: The viscosity for most gases and liquids decreases with increasing temperature

False For gases, when viscosity increases, Temperature increases

T/F: The shear stress in a fluid flowing in a circular pipe under steady fully developed flow conditions is at a maximum at the center of the pipe.

False Max at the wall

T/F: The pressure head (in Pa) generated by a centrifugal pump is independent of the fluid being pump.

False Pump head is m or ft independent of fluid. Pump head in pressure is not (H*p*g)

T/F: The pressure increase generated by a centrifugal pump is the same for all fluids.

False Pump head is the same. deltaP = H*p*g is not

T/F: At a Reynolds number of 1000, the flow in a circular pipe is always laminar

True Re < 2300 is laminar

T/F: The continuity equation applies to both Newtonian and pseudo-plastic fluids

True Continuity equation applies to all fluids

T/F: The quantities of force, mass, and acceleration are dimensionally dependent.

True F = ma

T/F: The continuity equation can be applied to shear-thinning fluids

True It can be applied to all fluids

T/F: Shell momentum balances can be used to find both velocity profiles and average velocities.

True Once v(x,t) is found, the average can be found

T/F: For a pseudo-plastic fluid, the viscosity decreases with increasing shear rate.

True Pseudo-plastic is shear thinning, so if shear increases, viscosity decreases

T/F: The Reynolds number describes the relative importance of inertial forces to viscous forces.

True Re = inertial/viscous

T/F: The friction factor K for an open globe valve is 7.5; it is possible to achieve the same friction factor with a gate valve.

True gate valve that has a K = 4.4 when its half opened

T/F: Velocity, Viscosity, and acceleration are dimensionally independent quanties

True m/s, kg/m-s, m/s2 can not multiply two of them to get another

T/F: A centrifugal pump placed at the top of a drinking water well cannot pump water from the well is the water level is 20 m below the pump.

True minimum pressure pump inlet is Pvap ~ 10 m

T/F: Hooke's law does not apply to Newtonian fluids

True not spring force in fluids

T/F: The quantities of stress, viscosity, and velocity are dimensionally independent

True stress = viscosity * dvx/dy dvx/dy does not have the same dimensions as velocity

T/F: In Hagen-Poiseuille flow the shear stress at the pipe wall is equal to zero.

False deltaP= (32*u*L*v)/(d^2)...used in incompressible newtonian fluids and laminar flow. <-- doesnt matter for this question Stress is maximum at walls

T/F: In a fluidized bed, the void fraction decreases with increasing fluid velocity

False higher velocity, more space between particles, higher e

T/F: The viscosity of Newtonian fluids does not depend on temperature

False viscosity always depends on temperature

T/F: When an open valve is partially closed, K decreases

False, K increases due to more friction

T/F: The superficial velocity in a packed bed is always higher than the actual fluid velocity.

False.

In a packed bed, the transition from laminar to turbulent flow occurs at lower Re values than in straight cylindrical pipes. A. True B. False

A. True

Cavitation at the impeller blades of centrifugal pumps can be explained with Bernoulli's equation. A. True B. False

A. True The impellers creates the difference in velocity between the two points, which causes a change in pressure which allows us to use the Bernoulli's equation. When fluid enters a centrifugal pump, it is accelerated by the impeller blades. As it moves radially outward, it continues to accelerate since the end of the blades have a larger velocity than the interior (v=r*omega). The fluid then exits the housing and enters a diffuser. In the diffuser, the cross sectional area of flow increases, causing velocity to decrease significantly. When you apply Bernoulli here, you can see as velocity decreases the pressure drop increases, which accounts for the increase in pressure from the pump.

Which of the following sets of quantities/variables could NOT be a possible choice of core variables for a dimensional analysis problem (e.g. a Buckingham Pi Analysis) in a fluid mechanics problem? A) pipe diameter, viscosity, density B) pipe diameter, rotational frequency, fluid viscosity C) pressure drop, viscosity, fluid velocity D) viscosity, force, density E) None of above

B) none of them include mass

Packed beds can be operated at arbitrary fluid flow rates. A. True B. False

B. False

Bernoulli's equation can be used to explain the working principle of a positive displacement pump. A. True B. False

B. False Positive displacement pumps work by creating a vacuum to suck up fluid and then forcing it out using a chamber that changes in volume. This mechanism cannot be explained with just Bernoulli.

Positive displacement pumps do not require priming. A. True B. False

B. True Only CP's require a priming to achieve a hf (height of fluid in pipe), so that it is easier for the pump to suck up the water PDP's suck up water by vacuum so it doesnt need priming

You run several flow experiments in a 10 cm diameter pipe with fluids of different viscosities (u) at different mass flow rate (Q), as shown below. Which of these experiments, if any, were conducted under turbulent conditions? A) Q = 10 kg/s, u = 0.1 Pa-s B) Q = 1 kg/s, u = 0.01 Pa-s C) Q = 20 kg/s, u = 0.01 Pa-s D) Not enough information given to determine the answer E) None of the above

C Q = p*v*pi*(D^2) / 4 Q = p*v*D * (pi*D)/4 (4*Q)/(D*pi) = pvD (4*Q)/(D*pi*u) = Re

Consider a viscous Newtonian fluid flowing downward in a long, narrow pipe of diameter D, inclined at angle alpha relative to the horizontal plane. The flow is steady ant incompressible. How do the average velocities vin and vout compare? A) vout > vin B) vout < vin C) vout = vin

C conservation of mass

Two identical cups are filled to the same level with ice water (liquid water at 0C). One of the two cups also has a couple of large ice cubes floating in it, which stick out above the water level. Which cup is heavier? A) The cup with the ice cubes B) The cup without the ice cubes C) Both weigh the same amount

C) Both weigh the same amount weight of floating ice cubes is the same as weight of displaced water; because water levels are the same, weight of the cups are the same

T/F: The superficial velocity in a packed bed is always higher than the actually fluid velocity

False

T/F: The flow rate from a positive displacement pump can be regulated through a gate valve at the pump outlet.

False. Centrifugal pump can be regulated through a gate valve at the pump outlet, not PDP. PDP will keep pumping the same amount of fluid

T/F: A viscous fluid is flowing between two parallel plates in unidirectional, fully developed, pressure-driven flow. The magnitude of the shear stress in the fluid is smallest at the plates.

False. Highest at the plates/walls

T/F: A pipe of constant diameter is used to transport viscous fluid from one large storage tank (I) to another (II). The frictional losses at the entrance of the pipe (from tank I into the pipe) are larger than the frictional losses at the pipe outlet (from the pipe outlet (from the pipe to tank II).

False. Kexp > Kcon

T/F: For fully developed flow of a viscous liquid through a cylindrical pipe, the magnitude of the shear stress is largest at the center of the pipe

False. Largest at the walls

T/F: The term dWu/dt in the macroscopic energy balance represents the shaft work done on the control volume

False. represents the viscous work

T/F: The following equation can never be used for flowing gases: gradient vector * velocity vector = 0

False. If flow in incompressible, the equation is valid

How do you find the core variables? How do you find the number of dimensionless groups?

MLT = 3 core variables. The choose three fro the group that will be repeated in all pi equations. (ones that have L, M, and T available. Number of parameters - number of core variables = number of dimensionless groups Example: P = f(L,A,g,v,p,u) 7 parameters - 3 core variables = 4 dimensionless groups. was given that p and g should not be repeater, so choose L, v, and u as core variables. Choose P, A, g, and p as dimensionless groups.

T/F: In steady-state flow, streamlines and pathlines are the same

True

T/F: In the macroscopic energy balance, the term dQ/dt represents the rate at which heat is added to the control volume.

True

T/F: On a weather map, gradientP is a vector that is oriented perpendicular to the isobars

True

T/F: The Ergun equation applies to both laminar and turbulent flow.

True

T/F: The viscosity of most gases increases if the temperature goes up.

True

T/F: Air is blown across a flat plate with free-stream velocity v, so that a laminar boundary forms. If y is the height above the plate, and x is the distance from the leading ege, it can be said that the velocity gradient dvx/dy in the boundary layer will decrease for increasing values of x.

True x increases, so y increases, so the velocity gradient will decrease

developed head

the energy required to start pumping up the water and its the energy required to make up for the head loss.. is added to the point 1 side of Bernoulli's equation.