Energy Engineering - Multiphase system and technologies

Multiple choice test

Exam of Multiphase Systems and Technologies – a.y. 2024 -25 Pre -call of the winter session – December 16 th, 2024 – Test A Surname Name Person code Correct answers Score Mark the correct answer in the dedicated space Question 1 The cross -section averaged void fraction in a gas -liquid flow is Answer a The ratio of gas to the liquid volume flow rates b The ratio of gas to the mixture volume flow rate c The ratio of the actual to superficial velocity of the gas d The ratio of the actual velocity of the gas to the superficial velocity of the mixture NONE OF THEM Question 2 With reference to the nucleate boiling region in the following figure Answer a The white dots refer to the smoothest copper surface b The roughness of the surface decreases from right to left c The white squares correspond to the highest values of the heat transfer coefficient at constant heat flux d The white squares refer to the smoothest copper surface X Question 3 The critical radius of a bubble nucleated at a conical cavity is Answer a The radius at the bubble departure b The radius of the mouth of the cavity X c The radius corresponding to the height of the cavity d The radius corresponding to the RMS roughness Question 4 The reversible component s of the pressure gradient in a gas -liquid flow are Answer a Accelerative, gravitational, frictional b Accelerative, gravitational X c Accelerative, frictional d Gravitational, frictional Question 5 The gravitational component of the pressure gradient in a gas -liquid flow Answer a Decreases with the inclination angle b Decreases with increasing the actual density of the mixture c Is maximum for vertical layouts X d Lowers with the system pressure Question 6 The following void fraction correlation is based on Answer a The slip ratio model b The drift -flux model c The K -εH model d A general approach based on dimensional analysis X Question 7 What is the correct definition of the Confinement number for a gas -liquid flow? Answer a (dp/dz) go/(dp/dz) lo b ε/ε H c L/D, where L is the pipe length and D the pipe diameter d L/D, where L is the Laplace length and D the pipe diameter X Question 8 The statement “liquid alone” means that Answer a The liquid flows with its superficial velocity X b The liquid flows with its actual velocity c The liquid flows separately from the gas, like in a stratified flow d The liquid flows with the total volume flow rate of the mixture Question 9 In the following void fraction model, A SM represents Answer a The distribution parameter b The slip ratio X c The drift velocity d A tuning parameter not associated with a specific physical meaning Question 10 The figure below represents the behavior of the two -phase heat transfer coefficient during saturated flow boiling . Which statement is correct? Answer a The lower curve is associated to a higher boiling number b The lower curve is associated to a higher boiling number than the upper one c The lower curve is associated to a smaller boiling number than the upper one X d The liquid deficient region is not represented in any of the two behaviors Question 11 The superficial velocity of a phase is Answer a The velocity at the interface b The velocity that the phase would take if it flowed alone in the pipe X c The velocity that the phase would take if it flowed with the total volume flow rate d The velocity at the pipe surface Question 12 The Nukiyama’s curve relates Answer a The wall heat flux and the wall superheat X b The wall heat flux and the liquid subcooling c The wall superheat and the liquid subcooling d The heat transfer coefficient and the wall superheat Question 13 In the power law relationship between the wall heat flux and the wall superheat, reported below, useful to describe nucleate boiling, the exponent n takes a typical value of ������������"= ������∆������������ Answer a 1/3 b 1 c 1/4 d 3 X Question 14 The heat transfer coefficient for film boiling is typically modeled as a combination of Answer a The conductive and the convective heat transfer coefficients b The convective and the radiative heat transfer coefficients X c The boiling number and the convection number d The boiling number and the Froude number Question 15 The phenomenon called “dryout” Answer a Is typical of pool boiling b Is associated with the onset of nucleate boiling in a subcooled liquid c Is associated with the onset of film boiling in a bubbly flow d Is associated with the complete evaporation of the liquid film in an annular flow X Question 16 The Laplace length, reported below, is a characteristic linear dimension for a system dominated by Answer a Inertia and buoyancy b Inertia and capillarity c Buoyancy and capillarity X d Buoyancy and friction Question 17 In nucleate boiling at constant heat flux, the heat transfer coefficient is increased Answer a By increasing the system pressure X b By decreasing the system pressure c By increasing the wall superheat d By decreasing the saturation temperature Question 18 In a gas -liquid mixture Answer a The volume quality is always larger than the mass quality X b The volume quality is larger than the mass quality only if the slip -ratio is lower than 1 c The volume quality is the same as the void fraction d The volume quality is larger than the void fraction only if the slip -ratio is lower than 1 Question 19 The mass flow rate of a gas -liquid mixture is Answer a The holdup weighted average of the individual mass flow rates of gas and liquid b The product of the actual density and the volume flow rate c The product of the bulk density and the volume flow rate X d The product of the momentum averaged density and the volume flow rate Question 20 The slip -ratio is equal to 1 when Answer a The void fraction is equal to 0.5 b The flow is inviscid c The superficial velocities of the phases are the same d The actual velocities of the phases are the same X