Nuclear Engineering - Multiphase systems and technologies

Multiple choice test

Exam of Multiphase Systems and Technologies – a.y. 2024 -25 1st call of the W inter session – January 21 st, 202 5 – Test Nuclear Surname Name Person code Correct answers Score Mark the correct answer (s) in the dedicated space Question 1 In a drift -flux model, the distribution parameter is defined as Answer a 1− ������̅ ������̅ ∫ ������������������������ Ω Ω ∫ ������������(1− ������)������Ω Ω b ∫ ������������������ Ω Ω 1 Ω∫ ������������ Ω Ω ∫ ������������ Ω Ω X c ������������ ������������(1− ������̅ ������̅ ) 2������������ ������������ d ������(1− ������̅ ������̅ ) � (������������ ������������) � (������������ ������������) � Question 2 You would adopt the following expression for the two -phase gas -liquid multiplier Answer a In case of homogeneous flow b In case of horizontal flow c In case of confined flow (mini channels) X d In case of laminar flow in both the phases Question 3 The experimental data reported in the following figure likely refer to (Legend: the plot reports the actual gas velocity against the mixture velocity) Answer a A bubbly flow X b A slug flow c A stratified flow d A slip ratio lower than 1 Question 4 The pool boiling model reported below takes into account the following parametric effects: λ, thermal conductivity η, dynamic viscosity Answer a Heater shape, system pressure, buoyancy forces, heat flux b Heater shape, fluid properties, heat flux c Heater size, system pressure, surface finish, heat flux d Heater size, type of fluid, system pressure, heat flux X Question 5 The term departure from nucleate boiling (DNB): Answer a Refers generically to the onset of film boiling X b In flow boiling typically occurs at small qualities or even in subcooled boiling X c Is the same as dryout d Indicates the suppression of nucleate boiling as the thermodynamic quality increases Question 6 You would consider using the following void fraction correlation for horizontal flows because Answer a It accounts for the different shear stress of the phases through a bulk viscosity b It accounts for the slip ratio through the parameter λ c It accounts for the relative velocity of the liquid to the mixture through the parameter λ d It accounts for the gravitational drainage through the Froude number X Question 7 The critical radius of a bubble nucleated at a surface is Answer a The maximum bubble radius b The radius associated with the minimum bubble curvature c The radius associated with the maximum wall superheat required to activate a cavity d The radius associated with the minimum wall superheat required to activate a cavity X Question 8 The dimensionless groups useful to characterize saturated flow boiling in vertical macro pipes are: Answer a The Bond number and the Confinement number b The Bond number and the Convection number c The Boiling number and the Convection number X d The Boiling number and the Confinement number Question 9 At the dryout occurrence: Answer a The liquid on the wall is fully evaporated X b The liquid on the cross -section is fully evaporated c Film boiling sets up d The melting point of the heat transfer surface is achieved Question 10 The term liquid holdup refers to: Answer a The reciprocal of the void fraction b The opposite of the void fraction c The ratio of the cross -section area occupied by the liquid to the total cross -section area X d The capillary force making the liquid wet a surface Question 11 The actual (or photographic) density of a two -phase mixture is Answer a The sum of the reciprocal of the specific volumes of the phases b The holdup weighted average of the phase densities X c The reciprocal of the bulk specific volume d The volume quality weighted average of the phase densities Question 12 The velocity and concentration profiles reported in the figure suggest: jc: mixture velocity at the pipe axis αc: void fraction at the pipe axis Both the profiles range from 0 to 1 Answer a A slip ratio higher than 1 and a distribution parameter lower than 1 b Both slip ratio and distribution parameter higher than 1 X c A slip ratio lower than 1 and a distribution parameter higher than 1 d Both slip ratio and distribution parameter lower than 1 Question 13 What is the surface property relevant to the heat transfer coefficient in film boiling? Answer a The roughness b The emissivity X c The thermal effusivity d The wettability (i.e. the contact angle) Question 14 In CHF models, the so -called Kutateladze ’s constant is found to mainly depend on: Answer a Thickness of the vapour blanket b Interfacial emissivity c Heater emissivity d Heater shape and size X Question 15 To model subcooled boiling, the following heat transfer mechanisms have to be considered: Convection and radiation Answer a Convection and nucleate boiling X b Convection and radiation c Radiation and film boiling d Only convection Question 16 At the onset of nucleate boiling (ONB) Answer a The liquid temperature profile is independent of the bubble equilibrium curve b The bubble radius is higher than the critical radius c The surface temperature must equate the saturation temperature at the system pressure d The liquid temperature profile and the bubble equilibrium curve must meet at least at one point X Question 17 The region of the Nukiyama’s curve between the Critical Heat Flux and Minimum Heat Flux states can be explored: Answer a Only by supplying a constant heat flux, i.e. the heater characteristic curve is horizontal b Only by providing a constant heater temperature, i.e. the heater characteristic curve is vertical c If the heater characteristic curve is sufficiently steep to get only one intersection with the boiling curve X d If the heater characteristic curve has a positive slope Question 18 The following expression of the Lockhart -Martinelli parameter is valid for (1− ������ ������ ) 0.8 (������������ ������������) 0.5 (������������ ������������) 0.1 Answer a Laminar gas flow/Laminar liquid flow b Laminar gas flow/Turbulent liquid flow c Turbulent gas flow/Laminar liquid flow d Turbulent gas flow/Turbulent liquid flow X Question 19 The momentum averaged specific volume is found: Answer a In the expression of the gravitational pressure gradient of a two -phase flow b In the expression of the accelerative pressure gradient of a two -phase flow X c In the expression of the frictional pressure gradient for a homogeneous mixture d In the slip ratio models of the void fraction Question 20 The mixture velocity is: Answer a The sum of the superficial velocities of the phases X b The sum of the actual velocities of the phases c The ratio of the mass flux to the actual density d The ratio of the mass flux to the momentum averaged density