Alla förståelsefrågor (ish) från boken + tentor (ENG + SV) Flashcards
1
Q
Definiera interna, externa och öppna kanalflöden
A
Externt - Ett ej avgränsad flöde, alltså fritt flöde över en platta eller dylikt
Internt - Ett avgränsat flöde, t.ex. genom ett rör eller dylikt
Öppet - Både internt och externt. Ett flöde från ett rör som rinner ut till en flod exempelvis
2
Q
Definiera kompressibel och inkompressibel vätska
A
Kompressibel - Densiteten varierar
Inkompressibel - Densiteten förblir ungefär konstant.
Dock kan t.ex. luft approximeras som inkompressibel (trots att den är kompressibel) beroende på tryck- och temperaturförändringar etc.
3
Q
Vad innebär no-slip?
A
Ett antagande man kan använda sig av som innebär att en fluid förblir stilla relativt en fast yta för de fluidpaket som har kontakt med väggen. Man kan säga att fluidpaketen fastnar mot ytan pga. av vätskans viskositet.
4
Q
Skillnad på klassisk och statistisk approach?
A
Klassisk - Makroskopisk, baseras på experiment och analyser av vätskans uppförande
Statistisk - Mikroskopisk, baseras på det genomsnittliga uppförandet av en större grupp molekyler
5
Q
Definiera stationär process?
A
En process som förblir oförändrad över tid inom systemet. Ex. Ett jämt flöde genom ett rör
Ostationär: En ballong som blåses upp, eller att vi förbränner något
6
Q
Definiera system, omgivning, avgränsning (boundary).
A
System - Det område vi valt att studera
Omgivning - Området utanför systemet
Avgränsning - Den verkliga/imaginära avgränsningen mellan system och omgivning
7
Q
Hur definieras “mach-numret” för ett flöde?
A
Mach nummer: Ma = V /c = speed of flow/speed of sound, där c = 346 m/s
Ma=1: sonisk (“ljudhastighet”), Ma<1: subsonisk, Ma>1: supersonisk (“överljudshastighet”),
Ma»1 hypersonisk
8
Q
Hur förändras den dynamiska viskositeten för vätskor och gaser vid varierande temperatur?
A
Vätskor: Minskar vid ökande temperatur
Gaser: Ökar vid ökande temperatur
Påminnelse: Tänk bilmotor på vintern. Svår att starta pga. att oljan har högre viskositet vid låg temperatur.
9
Q
Hur förändras den kinematiska viskositeten för vätskor och gaser vid varierande temperatur?
A
Vätskor: Minskar vid ökande temperatur
Gaser: Ökar vid ökande temperatur
10
Q
Skillnad mellan mätar-tryck och absolut-tryck?
A
Mätar-tryck: Trycket relativt atmosfärstrycket
Absolut-trycket: Trycket relativt vacuumtryck
11
Q
Två identiska fläktar, en vid havsnivå och en på toppen av ett berg. Jämför volymflöde och massflöde.
A
Luftens densitet är högre vid havsnivå, vilket inte kommer påverka volymflödet, men massflödet kommer vara högre vid havsnivån.
12
Q
Definiera resulterande hydrostatiskt kraft som utövas på en nedsänkt yta, samt tryckets centrum.
A
Den resulterande hydrostatiska kraften som utövas på en nedsänkt yta är resultanten av alla tryckkrafter som utövas på ytan. Den resulterande kraftens angreppspunkt kallas tryckets centrum och den ligger generellt inte i mitten på kroppen pga. det hydrostatiska tryckets variation.
13
Q
Varför är dammar tjockare i botten?
A
Pga. att tryckkrafterna ökar med djupet.
14
Q
Vad innebär flytkraft?
A
Den uppåtriktade kraft som en fluid utövar på en nedsänkt kropp kallas flytkraft. Den beror på kommer från tryckökningen som bildas vid större djup.
F B= p fg V
Om F B> W, så flyter kroppen.
15
Q
Påverkas flytkraften av djupet och kroppens densitet?
A
Oberoende av djupet och densitet (material spelar ingen roll), endast volymen påverkar.
16
Q
Anta stationärt adiabatiskt flöde av en inkompressibel vätska. Om temperaturen är konstant under flödet, är det rätt att säga att friktionen är försumbar?
A
Ja, eftersom irreversibiliteter, som friktion, skulle öka entropin och temperaturen.
17
Q
Vad är kinetisk energi korrektionsfaktor?
A
Den kompenserar för felet som uppstår då vi räknar med medelhastighet. Är ofta försumbar vid turbulenta flöden, men kan ha påverkan vid laminära flöden.
Rek. anv. α = 1, 05 vid fullt turbulent och α = 2.0 .
Kan göra stor skillnad om hastigheterna är
höga, annars kan den ofta försummas vid turbulenta flöden åtminstone.
18
Q
Why are liquids usually transported in circular pipes?
A
Liquids are usually transported in circular pipes because pipes with a circular cross section can withstand large pressure differences between the inside and the outside without undergoing any significant distortion.
19
Q
What is the physical significance of the Reynolds number?
A
Reynolds number is the ratio of the inertial forces to viscous forces, and it serves as a criterion for determining the flow regime. At large Reynolds numbers, for example, the flow is turbulent since the inertia forces are large relative to the viscous forces, and thus the viscous forces cannot prevent the random and rapid fluctuations of the
fluid.
20
Q
How is the hydrodynamic entry length defined for flow in a pipe? Is the entry length longer in laminar or turbulent flow?
A
The region from the tube inlet to the point at which the boundary layer merges at the centerline is called the hydrodynamic entrance region, and the length of this region is called hydrodynamic entry length. The entry length is much longer in laminar flow than it is in turbulent flow. But at very low Reynolds numbers, L h is very small (e.g., L h = 1.2D at Re = 20).
21
Q
Consider laminar flow in a circular pipe. Will the wall shear stress t w be higher near the inlet of the pipe or near the exit? Why? What would your response be if the flow were turbulent?
A
The wall shear stress τ w is highest at the tube inlet where the thickness of the boundary layer is nearly zero, and decreases gradually to the fully developed value. The same is true for turbulent flow.
22
Q
What is the physical mechanism that causes the friction factor to be higher in turbulent flow?
A
In turbulent flow, it is the turbulent eddies due to enhanced mixing that cause the friction factor to be larger. This turbulent mixing leads to a much larger wall shear stress, which translates into larger friction factor.
23
Q
Consider laminar flow of air in a circular pipe with perfectly smooth surfaces. Do you think the friction factor for this flow will be zero? Explain.
A
During laminar flow of air in a circular pipe with perfectly smooth surfaces, the friction factor is not zero because of the no-slip boundary condition, which must hold even for perfectly smooth surfaces.
24
Q
Explain why the friction factor is independent of the Reynolds number at very large Reynolds numbers.
A
At very large Reynolds numbers, the flow is fully rough and the friction factor is independent of the Reynolds number. This is because the thickness of viscous sublayer decreases with increasing Reynolds number, and it be comes so thin that the surface roughness protrudes into the flow. The viscous effects in this case are produced in the main flow primarily by the protruding roughness elements, and the contribution of the viscous sublayer is negligible.
25
Define equivalent length for minor loss in pipe flow. How is it related to the minor loss coefficient?
Equivalent length is the length of a straight pipe which would give the same head loss as the minor loss component.
26
Which has a greater minor loss coefficient during pipe flow: gradual expansion or gradual contraction? Why?
A gradual expansion, in general, has a greater minor loss coefficient than a gradual contraction in pipe flow. This is due to the adverse pressure gradient in the boundary layer, which may lead to flow separation.
27
A piping system involves sharp turns, and thus large minor head losses. One way of reducing the head loss is to replace the sharp turns by circular elbows. What is another way?
Another way of reducing the head loss associated with turns is to install turning vanes inside the elbows. There are many other possible answers, such as: reduce the inside wall roughness of the pipe, use a larger diameter pipe, shorten the length of pipe as much as possible, etc.
28
Explain when an external flow is two-dimensional, three-dimensional, and axisymmetric. What type of flow is the flow of air over a car?
The flow over a body is said to be two-dimensional when the body is very long and of constant cross-section, and the flow is normal to the body (such as the wind blowing over a long pipe perpendicular to its axis). There is no significant flow along the axis of the body. The flow along a body that possesses symmetry along an axis in the flow direction is said to be axisymmetric (such as a bullet piercing through air). Flow over a body that cannot be modeled as two-dimensional or axisymmetric is three-dimensional. The flow over a car is three-dimensional.
29
What is the difference between streamlined and blunt bodies? Is a tennis ball a streamlined or blunt body?
A body is said to be streamlined if a conscious effort is made to align its shape with the anticipated streamlines in the flow. Otherwise, a body tends to block the flow, and is said to be blunt. A tennis ball is a blunt body (unless the velocity is very low and we have “creeping flow”).
30
What is drag? What causes it? Why do we usually try to minimize it?
The force a flowing fluid exerts on a body in the flow direction is called drag. Drag is caused by friction between the fluid and the solid surface, and the pressure difference between the front and back of the body. We try to minimize drag in order to reduce fuel consumption in vehicles, improve safety and durability of structures subjected to high winds, and to reduce noise and vibration.
31
What is lift? What causes it? Does wall shear contribute to the lift?
The force a flowing fluid exerts on a body in the normal direction to flow that tends to move the body in that direction is called lift. It is caused by the components of the pressure and wall shear forces in the direction normal to the flow. The wall shear contributes to lift (unless the body is very slim), but its contribution is usually small.
32
What is terminal velocity? How is it determined?
The maximum velocity a free falling body can attain is called the terminal velocity. It is determined by setting the weight of the body equal to the drag and buoyancy forces, W = F D + F B.
33
What is the difference between skin friction drag and pressure drag? Which is usually more significant for slender bodies such as airfoils?
The part of drag that is due directly to wall shear stress τw is called the skin friction drag F D, friction since it is caused by frictional effects, and the part that is due directly to pressure P and depends strongly on the shape of the body is called the pressure drag F D, pressure. For slender bodies such as airfoils, the friction drag is usually more significant.
34
What is flow separation? What causes it? What is the effect of flow separation on the drag coefficient?
At sufficiently high velocities, the fluid stream detaches itself from the surface of the body. This is called separation. It is caused by a fluid flowing over a curved surface at a high velocity (or technically, by adverse pressure gradient). Separation increases the drag coefficient drastically.
35
What is drafting? How does it affect the drag coefficient of the drafted body?
Drafting is when a moving body follows another moving body by staying close behind in order to reduce drag. It reduces the pressure drag and thus the drag coefficient for the drafted body by taking advantage of the low pressure wake region of the moving body in front.
36
What is the more common term for an energyproducing turbomachine? How about an energy-absorbing turbomachine? Explain this terminology. In particular, from which frame of reference are these terms defined—that of the fluid or that of the surroundings?
A more common term for an energy producing turbomachine is a turbine. Turbines extract energy from the moving fluid, and convert that energy into useful mechanical energy in the surroundings, usually in the form of a rotating shaft. Thus, the phrase “energy producing” is from a frame of reference of the fluid – the fluid loses energy as it drives the turbine, producing energy to the surroundings. On the other hand, a more common term for an energy absorbing turbomachine is a pump. Pumps absorb mechanical energy from the surroundings, usually in the form of a rotating shaft, and increase the energy of the moving fluid. Thus, the phrase “energy absorbing” is from a frame of reference of the fluid – the fluid gains or absorbs energy as it flows through the pump.
37
What are the primary differences between fans,
| blowers, and compressors? Discuss in terms of pressure rise and volume flow rate.
A fan is a gas pump with relatively low pressure rise and high flow rate. A blower is a gas pump with relatively moderate to high pressure rise and moderate to high flow rate. A compressor is a gas pump designed to deliver a very high pressure rise, typically at low to moderate flow rates.
38
Explain why it is usually not wise to arrange two (or more) dissimilar pumps in series or in parallel.
Arranging dissimilar pumps in series can create problems because the volume flow rate through each pump must be the same, but the overall pressure rise is equal to the pressure rise of one pump plus that of the other. If the pumps have widely different performance curves, the smaller pump may be forced to operate beyond its free delivery flow rate, whereupon it acts like a head loss, reducing the total volume flow rate. Arranging dissimilar pumps in parallel can create
problems because the overall pressure rise must be the same, but the net volume flow rate is the sum of that through each branch. If the pumps are not sized properly, the smaller pump may not be able to handle the large head imposed on it, and the flow in its branch could actually be reversed; this would inadvertently reduce the overall pressure rise. In either case, the power supplied to the smaller pump would be wasted.
39
Consider a typical centrifugal liquid pump. For each statement, choose whether the statement is true or false, and discuss your answer briefly.
(a) V flow at the pump’s free delivery is greater than V flow
at its best efficiency point.
(b) At the pump’s shutoff head, the pump efficiency is zero.
(c) At the pump’s best efficiency point, its net head is at its
maximum value.
(d) At the pump’s free delivery, the pump efficiency is zero.
(a) True: The maximum volume flow rate occurs when the net head is zero, and this “free delivery” flow rate is typically
much higher than that at the BEP.
(b) True: By definition, there is no flow rate at the shutoff head. Thus the pump is not doing any useful work, and the
efficiency must be zero.
(c) False: Actually, the net head is typically greatest near the shutoff head, at zero volume flow rate, not near the BEP.
(d) True: By definition, there is no head at the pump’s free delivery. Thus, the pump is working against no “resistance”,
and is therefore not doing any useful work, and the efficiency must be zero.
40
What are the mechanisms of heat transfer? How are they distinguished from each other?
The mechanisms of heat transfer are conduction, convection and radiation. Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas which is in motion, and it involves combined effects of conduction and fluid motion. Radiation is energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.
41
What is the physical mechanism of heat conduction in a solid, a liquid, and a gas?
In solids, conduction is due to the combination of the vibrations of the molecules in a lattice and the energy transport by free electrons. In gases and liquids, it is due to the collisions of the molecules during their random motion.
42
Consider heat transfer through a windowless wall of a house in a winter day. Discuss the parameters that affect the rate of heat conduction through the wall.
The parameters that effect the rate of heat conduction through a windowless wall are the geometry and surface area of wall, its thickness, the material of the wall, and the temperature difference across the wall.
43
Define emissivity and absorptivity. What is Kirch- hoff’s law of radiation?
Emissivity is the ratio of the radiation emitted by a surface to the radiation emitted by a blackbody at the same temperature. Absorptivity is the fraction of radiation incident on a surface that is absorbed by the surface. The Kirchhoff's law of radiation states that the emissivity and the absorptivity of a surface are equal at the same temperature and wavelength.
44
What is a blackbody? How do real bodies differ from blackbodies?
A blackbody is an idealized body which emits the maximum amount of radiation at a given temperature and which absorbs all the radiation incident on it. Real bodies emit and absorb less radiation than a blackbody at the same temperature.
45
How do the thermal conductivity of gases and liquids vary with temperature?
The thermal conductivity of gases is proportional to the square root of absolute temperature. The thermal conductivity of most liquids, however, decreases with increasing temperature, with water being a notable exception.
46
Consider heat conduction through a plane wall. Does the energy content of the wall change during steady heat conduction? How about during transient conduction? Explain.
In steady heat conduction, the rate of heat transfer into the wall is equal to the rate of heat transfer out of it. Also, the temperature at any point in the wall remains constant. Therefore, the energy content of the wall does not change during steady heat conduction. However, the temperature along the wall and thus the energy content of the wall will change during transient conduction.
47
Consider heat conduction through a wall of thickness L and area A. Under what conditions will the temperature distri- butions in the wall be a straight line?
The temperature distribution in a plane wall will be a straight line during steady and one dimensional heat transfer with constant wall thermal conductivity.
48
What does the thermal resistance of a medium represent?
The thermal resistance of a medium represents the resistance of that medium against heat transfer.
49
Consider a window glass consisting of two 4-mmthick glass sheets pressed tightly against each other. Compare the heat transfer rate through this window with that of one con- sisting of a single 8-mm-thick glass sheet under identical conditions.
The window glass which consists of two 4 mm thick glass sheets pressed tightly against each other will probably have thermal contact resistance which serves as an additional thermal resistance to heat transfer through window, and thus the heat transfer rate will be smaller relative to the one which consists of a single 8 mm thick glass sheet.
50
Consider steady heat transfer through the wall of a room in winter. The convection heat transfer coefficient at the outer surface of the wall is three times that of the inner surface as a result of the winds. On which surface of the wall do you think the temperature will be closer to the surrounding air tem- perature? Explain.
Convection heat transfer through the wall is expressed as Q = hAs ( Ts − T∞ ) . In steady heat transfer, heat transfer rate to the wall and from the wall are equal. Therefore at the outer surface which has convection heat transfer coefficient three times that of the inner surface will experience three times smaller temperature drop compared to the inner surface. Therefore, at the outer surface, the temperature will be closer to the surrounding air temperature.
51
Consider two cold canned drinks, one wrapped in a blanket and the other placed on a table in the same room. Which drink will warm up faster?
The blanket will introduce additional resistance to heat transfer and slow down the heat gain of the drink wrapped in a blanket. Therefore, the drink left on a table will warm up faster.
52
What is thermal contact resistance? How is it related to thermal contact conductance?
The resistance that an interface offers to heat transfer per unit interface area is called thermal contact resistance, Rc . The inverse of thermal contact resistance is called the thermal contact conductance.
53
Will the thermal contact resistance be greater for smooth or rough plain surfaces?
The thermal contact resistance will be greater for rough surfaces because an interface with rough surfaces will contain more air gaps whose thermal conductivity is low.
54
A wall consists of two layers of insulation pressed against each other. Do we need to be concerned about the ther- mal contact resistance at the interface in a heat transfer analy- sis or can we just ignore it?
An interface acts like a very thin layer of insulation, and thus the thermal contact resistance has significance only for highly conducting materials like metals. Therefore, the thermal contact resistance can be ignored for two layers of insulation pressed against each other.
55
Consider two surfaces pressed against each other. Now the air at the interface is evacuated. Will the thermal con- tact resistance at the interface increase or decrease as a result?
Heat transfer through the voids at an interface is by conduction and radiation. Evacuating the interface eliminates heat transfer by conduction, and thus increases the thermal contact resistance.
56
Explain how the thermal contact resistance can be minimized.
Thermal contact resistance can be minimized by (1) applying a thermally conducting liquid on the surfaces before they are pressed against each other, (2) by replacing the air at the interface by a better conducting gas such as helium or hydrogen, (3) by increasing the interface pressure, and (4) by inserting a soft metallic foil such as tin, silver, copper, nickel, or aluminum between the two surfaces.
57
When plotting the thermal resistance network associated with a heat transfer problem, explain when two resistances are in series and when they are in parallel.
Parallel resistances indicate simultaneous heat transfer (such as convection and radiation on a surface). Series resistances indicate sequential heat transfer (such as two homogeneous layers of a wall).
58
What are the two approaches used in the develop- ment of the thermal resistance network for two-dimensional problems?
Two approaches used in development of the thermal resistance network in the x-direction for multi- dimensional problems are (1) to assume any plane wall normal to the x-axis to be isothermal and (2) to assume any plane parallel to the x-axis to be adiabatic.
59
Can the thermal resistance concept be used for a solid cylinder or sphere in steady operation? Explain.
No. In steady-operation the temperature of a solid cylinder or sphere does not change in radial direction (unless there is heat generation).
60
In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural convection or forced convection? Why?
The convection heat transfer coefficient will usually be higher in forced convection since heat transfer coefficient depends on the fluid velocity, and forced convection involves higher fluid velocities.
61
Consider a hot baked potato. Will the potato cool faster or slower when we blow the warm air coming from our lungs on it instead of letting it cool naturally in the cooler air in the room? Explain.
The potato will normally cool faster by blowing warm air to it despite the smaller temperature difference in this case since the fluid motion caused by blowing enhances the heat transfer coefficient considerably.
62
What is the physical significance of the Nusselt number? How is it defined?
Nusselt number is the dimensionless convection heat transfer coefficient, and it represents the enhancement of heat transfer through a fluid layer as a result of convection relative to conduction across the same fluid layer. It is defined as Nu = hLc where Lc is the characteristic length of the surface and k is k the thermal conductivity of the fluid.
63
When is heat transfer through a fluid conduction and when is it convection? For what case is the rate of heat transfer higher? How does the convection heat transfer coefficient dif- fer from the thermal conductivity of a fluid?
Heat transfer through a fluid is conduction in the absence of bulk fluid motion, and convection in the presence of it. The rate of heat transfer is higher in convection because of fluid motion. The value of the convection heat transfer coefficient depends on the fluid motion as well as the fluid properties. Thermal conductivity is a fluid property, and its value does not depend on the flow.
64
What is viscosity? What causes viscosity in liquids and in gases? Is dynamic viscosity typically higher for a liquid or for a gas?
Viscosity is a measure of the “stickiness” or “resistance to deformation” of a fluid. It is due to the internal frictional force that develops between different layers of fluids as they are forced to move relative to each other. Viscosity is caused by the cohesive forces between the molecules in liquids, and by the molecular collisions in gases. Liquids have higher dynamic viscosities than gases.
65
What fluid property is responsible for the development of the velocity boundary layer? For what kind of fluids will there be no velocity boundary layer on a flat plate?
The fluid viscosity is responsible for the development of the velocity boundary layer. For the idealized inviscid fluids (fluids with zero viscosity), there will be no velocity boundary layer.
66
What is the physical significance of the Prandtl number? Does the value of the Prandtl number depend on the type of flow or the flow geometry? Does the Prandtl number of air change with pressure? Does it change with temperature?
The Prandtl number Pr = ν / α is a measure of the relative magnitudes of the diffusivity of momentum (and thus the development of the velocity boundary layer) and the diffusivity of heat (and thus the development of the thermal boundary layer). The Pr is a fluid property, and thus its value is independent of the type of flow and flow geometry. The Pr changes with temperature, but not pressure.
67
Will a thermal boundary layer develop in flow over a surface even if both the fluid and the surface are at the same temperature?
A thermal boundary layer will not develop in flow over a surface if both the fluid and the surface are at the same temperature since there will be no heat transfer in that case.
68
How does turbulent flow differ from laminar flow? For which flow is the heat transfer coefficient higher?
A fluid motion is laminar when it involves smooth streamlines and highly ordered motion of molecules, and turbulent when it involves velocity fluctuations and highly disordered motion. The heat transfer coefficient is higher in turbulent flow.
69
What is the physical significance of the Reynolds number? How is it defined for external flow over a plate of length L?
Reynolds number is the ratio of the inertial forces to viscous forces, and it serves as a criterion for determining the flow regime. For flow over a plate of length L it is defined as Re = VL/ν where V is flow velocity and ν is the kinematic viscosity of the fluid.
70
What does the friction coefficient represent in flow over a flat plate? How is it related to the drag force acting on the plate?
The friction coefficient represents the resistance to fluid flow over a flat plate. It is proportional to the drag force acting on the plate. The drag coefficient for a flat surface is equivalent to the mean friction coefficient.
71
What is the effect of streamlining on ( a) friction drag and ( b) pressure drag? Does the total
As a result of streamlining, ( a) friction drag increases, ( b) pressure drag decreases, and ( c) total drag decreases at high Reynolds numbers (the general case), but increases at very low Reynolds numbers since the friction drag dominates at low Reynolds numbers.
72
In flow over blunt bodies such as a cylinder, how does the pressure drag differ from the friction drag?
Friction drag is due to the shear stress at the surface whereas the pressure drag is due to the pressure differential between the front and back sides of the body when a wake is formed in the rear.
73
Consider laminar forced convection in a circular tube. Will the heat flux be higher near the inlet of the tube or near the exit? Why?
The heat flux will be higher near the inlet because the heat transfer coefficient is highest at the tube inlet where the thickness of thermal boundary layer is zero, and decreases gradually to the fully developed value. Same for turbulent flow.
74
In the fully developed region of flow in a circular tube, will the velocity profile change in the flow direction? How about the temperature profile?
In the fully developed region of flow in a circular tube, the velocity profile will not change in the flow direction but the temperature profile may.
75
Someone claims that the shear stress at the center of a circular pipe during fully developed laminar flow is zero. Do you agree with this claim? Explain.
The shear stress at the center of a circular tube during fully developed laminar flow is zero since the shear stress is proportional to the velocity gradient, which is zero at the tube center.
76
Someone claims that in fully developed turbulent flow in a tube, the shear stress is a maximum at the tube sur- face. Do you agree with this claim? Explain.
Yes, the shear stress at the surface of a tube during fully developed turbulent flow is maximum since the shear stress is proportional to the velocity gradient, which is maximum at the tube surface.
77
When will the hull of a ship sink in water deeper: when the ship is sailing in fresh water or in sea water? Why?
The buoyancy force is proportional to the density of the medium, and thus is larger in sea water than it is in fresh water. Therefore, the hull of a ship will sink deeper in fresh water because of the smaller buoyancy force acting upwards.
78
Consider two fluids, one with a large coefficient of volume expansion and the other with a small one. In what fluid will a hot surface initiate stronger natural convection currents? Why? Assume the viscosity of the fluids to be the same.
The greater the volume expansion coefficient, the greater the change in density with temperature, the greater the buoyancy force, and thus the greater the natural convection currents.
79
Consider a fluid whose volume does not change with temperature at constant pressure. What can you say about natural convection heat transfer in this medium?
There cannot be any natural convection heat transfer in a medium that experiences no change in volume with temperature.
80
Will a hot horizontal plate whose back side is insulated cool faster or slower when its hot surface is facing down instead of up?
No, a hot surface will cool slower when facing down since the warmer air in this position cannot rise and escape easily.
81
Consider laminar natural convection from a vertical hot plate. Will the heat flux be higher at the top or at the bottom of the plate? Why?
The heat flux will be higher at the bottom of the plate since the thickness of the boundary layer which is a measure of thermal resistance is the lowest there.
82
Classify heat exchangers according to flow type and explain the characteristics of each type.
Heat exchangers are classified according to the flow type as parallel flow, counter flow, and cross- flow arrangement. In parallel flow, both the hot and cold fluids enter the heat exchanger at the same end and move in the same direction. In counter-flow, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite direction. In cross-flow, the hot and cold fluid streams move perpendicular to each other.
83
Classify heat exchangers according to construction type and explain the characteristics of each type.
In terms of construction type, heat exchangers are classified as compact, shell and tube and regenerative heat exchangers. Compact heat exchangers are specifically designed to obtain large heat transfer surface areas per unit volume. The large surface area in compact heat exchangers is obtained by attaching closely spaced thin plate or corrugated fins to the walls separating the two fluids. Shell and tube heat exchangers
contain a large number of tubes packed in a shell with their axes parallel to that of the shell. Regenerative heat exchangers involve the alternate passage of the hot and cold fluid streams through the same flow area. In compact heat exchangers, the two fluids usually move perpendicular to each other.
84
What is the role of the baffles in a shell-and-tube heat exchanger? How does the presence of baffles affect the heat transfer and the pumping power requirements? Explain.
In the shell and tube exchangers, baffles are commonly placed in the shell to force the shell side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing between the tubes. Baffles disrupt the flow of fluid, and an increased pumping power will be needed to maintain flow. On the other hand, baffles eliminate dead spots and increase heat transfer rate.
85
What is a regenerative heat exchanger? How does a static type of regenerative heat exchanger differ from a dynamic type?
Regenerative heat exchanger involves the alternate passage of the hot and cold fluid streams through the same flow area. The static type regenerative heat exchanger is basically a porous mass which has a large heat storage capacity, such as a ceramic wire mash. Hot and cold fluids flow through this porous mass alternately. Heat is transferred from the hot fluid to the matrix of the regenerator during the flow of the hot fluid and from the matrix to the cold fluid. Thus the matrix serves as a temporary heat storage medium. The dynamic type regenerator involves a rotating drum and continuous flow of the hot and cold fluid through different portions of the drum so that any portion of the drum passes periodically through the hot stream, storing heat and then through the cold stream, rejecting this stored heat. Again the drum serves as the medium to transport the heat from the hot to the cold fluid stream.
86
Under what conditions is the thermal resistance of the tube in a heat exchanger negligible?
When the wall thickness of the tube is small and the thermal conductivity of the tube material is high, which is usually the case, the thermal resistance of the tube is negligible.
87
Vad betyder no-slip villkoret?
Ett antagande man kan använda sig av som innebär att en fluid förblir stilla relativt en fast yta för de fluidpaket som har kontakt med väggen. Man kan säga att fluidpaketen fastnar mot ytan pga. av vätskans viskositet.
88
Definiera stationär process?
En process som förblir oförändrad över tid inom systemet. Ex. Ett jämt flöde genom ett rör Ostationär: En ballong som blåses upp, eller att vi förbränner något
89
Vad är kännetecknande för en Newtonsk fluid?
En Newtonsk fluid är en fluid som har en skjuvspänning som är linjärt proportionell mot hastighetsgradienten (deformationen).
90
Varför ökar viskositeten med temperaturen för gaser?
Temperaturen ökar pga. att partiklarnas kinetiska energi ökar, vilket leder till att partiklarna interagerar mer, dvs. de krockar och hakar i varandra och det bildas friktion mellan dem.
91
Varför minskar viskositeten med temperaturen för vätskor?
Strukturerade molekylers bindningar frigörs då de inte klarar av att hålla kvar atomerna pga. den ökade kinetiska energin.
92
Vad är en fluid?
Ett ämne som befinner sig i vätskefas eller gasfas
93
Vad innebär hydrostatik?
Läran om vätskor i vila
94
Vad innebär viskositet?
Motstånd till flöde. Kan ses som ett mått på hur trögflytande en fluid är.
Hög viskositet - fler bindningar, ex. honung
Låg viskositet - färre bindningar, ex. vatten
95
Vad innebär Pascals princip?
Tryck som utövas i någon del av en vätska i vila överförs utan förlust till alla delar av vätskan. Det är även så att trycket förblir konstant i horisontell riktning för en fluid. Ett verkligt exempel på pascals princip är en hydraulisk domkraft.
96
Vad är eller vad beskriver Reynolds-tal?
Beskriver förhållandet mellan tröghetskrafter och viskösa friktionskrafter. Kan därigenom avgöra om fluiden strömmar laminärt eller turbulent.
Cirkulärt rör gäller:
Re<2100: Laminärt
Re>4000: Turbulent
97
Vilka parametrar påverkar friktionstryckfallets storlek?
Viskositet, hastighet, rören svängs, längd, areaändring, friktion från ytan (glatt eller ej)
98
Vilka krafter representeras av strömningsmotståndet?
Dragkraft:
Den kraft en fluid utövar på en kropp i flödesriktningen kallas dragkraft. Beror på friktion mellan fluiden och den fasta ytan och tryckskillnaden mellan fram och bak på ytan.
Dragkraften kan delas upp i två krafter:
Friktionskraft (skin friction drag): Beror på väggens skjuvspänning
Tryckkraft (Pressure drag): Trycket som beror på kroppens form.
Lyftkraft:
Den kraft en fluid utövar på en kropp i normalvektors riktning och tenderar att flytta kroppen i den riktningen kallas lyftkraft.
99
Vad är engångsförluster?
Beror på geometrin hos systemet, ex. böjar, ingångar, utgångar, ventiler. Finns tabulerat. K L är förlustkoefficienten.
100
Vad betyder följande begrepp? Free delivery:
Maximalt volymflöde, då uppfordringshöjden, H=0. Då pumpen inte utsätts för någon belastning. Effektiviten är också noll eftersom pumpen inte uträttar något arbete.
101
Vad betyder följande begrepp? Shut of head:
Maximal uppfordringshöjd, då är volymflödet och effektiviten noll eftersom inget nyttigt arbete uträttas.
102
Vad betyder följande begrepp? BEP
Maximal verkningsgrad (H*, V*, bhp*)
103
Vad betyder följande begrepp? Kavitation:
Uppkommer om trycket just innan vätskan kommer in i pumpen blir lägre än ångbildningstrycket. (Varmt vatten => större risk)
Ångblåsor => Imploderar då fluiden når regioner med högre tryck
=> Vibrationer, sänkt verkningsgrad, skador
104
Vad betyder följande begrepp? Deplacementpump:
Flödet bestäms av varvtal alternativt slagfrekvens och i princip inte alls av mottrycket. Ex. kolvpump, skruvpump.
105
Förklara begreppen: Värmeledning (konduktion):
Värme som färdas genom ett material (gas, solid vätska), ingen fluidrörelse.
Ex. Om du håller i en sked och någon eldar på andra delen så sprids tillslut värmen till din del.
Påverkas av olika saker:
Geometri: Större area ger högre hastighet Tjockleken: Tjockare material ger lägre hastighet Temperaturskillnad: Stort ger hög hastighet Material
106
Förklara begreppen: Konvektion
Värme som färdas mellan en vätska/gas och en solid, fluidrörelse. Högre fluidrörelse, högre konvektionshastighet.
Ex. Du står ute en kall vinterdag och det blåser. Värme förs bort mha molekylrörelse och blir kall.
107
Förklara begreppen: Strålning
Sker mellan två eller flera solider, mellan deras ytor. Behöver inget medium att verka i. Sker bäst i vakuum.
För strålning gäller att för varje våglängd är absorptionsförhållandet = emissionsförhållandet, vilket innebär att alla den strålning som en kropp tar upp kan den också sända ut.
Ex. Solen strålar mot oss, även om vi strålar mot solen så tittar vi bara på skillnaden.
108
Förklara begreppen: Svartkropp
Absorberar/emitterar all strålning vid alla våglängder. ε = 1
109
Förklara begreppen: Termisk diffusivitet
Hur snabbt värme sprids gnom ett material Högt α : Bra ledningsförmåga => Stor spridning
Lågt α : Bra lagringsförmåga => Bra på att bevara energi => Lite spridning
110
Förklara begreppen: Termisk konduktivitet
K är ett mått på ett materials förmåga att lagra termisk energi.
111
Förklara begreppen: Emissivitet
Hur effektivt en yta strålar ut energi i form av elektromagnetisk strålning. Mellan 0-1. 0 = ingen utstrålning, 1 = svartkropp
112
Förklara begreppen: Solarkonstanten
Ett mått på den totala strålningseffekten per areaenhet från solen mätt utanför jordatmosfären och på jordens medelavstånd från solen.
113
Vad är ett prandtl-tal och vad betyder det om Pr=1
Den relativa tjockleken mellan hastighetens- och temperaturens gränssikt beskrivs bäst med Pr.
114
Vad beskriver Nusselts-tal?
Beskriver förhållandet mellan konvektiv- och konduktiv värmetransport
Högt Nu: Konvektiv transport effektivt
Nu=1: Endast konduktion
115
Vad är filmtemperaturen?
Används vi extern strömning över ytor då temperaturen varierar mellan T s och T oändlig. Då temperaturen varierar, så varierar även fluiden egenskaper, och därmed används ett medelvärde.
116
När används bulktemperaturen?
När man ska fram egenskaper för fluiden som strömmar genom ett rör. En fluids egenskaper
varierar med temperaturen och därför används fluidens genomsnittliga temperatur.
117
Vad är drivkraften vid naturlig konvektion?
Fluidens flytkrafter som styrs av densitetsskillnader. Fluid med högre T stiger pga. det är lättare, dvs. lägre densitet.
118
Vad är en regenerativ värmeväxlare?
Involverar alternerande väg för den varma och kalla fluiden genom samma flödesyta, ex. med hjäp av en roterande trumma. Finns statisk och dynamisk. Vanligtvis gas-gas.
119
Vad beskriver U-värdet för en värmeväxlare?
Overall heat capacity, U, beskriver hur bra värme transporteras genom värmeväxlaren.
120
Vad är fouling factor R f och correction factor F vid beräkning av överförd effekt i en värmeväxlare?
Fouling factor tar hänsyn till det motstånd som uppstår av ex. smuts, slitningsskador etc inne i röret.
P,R=>F, korrektionsfaktor: ΔT lm = F * ΔT lm, CF . Ansätter counterflow-flow-värmeväxlare och korrigerar med F, eftersom det är trassligt att lösa pga. geometrin.
121
Vad är NTU-metoden?
Number of transer units, NTU, är ett mått på värmeöverföringen och metoden används då T ut är okänt. Hur effektiv värmeövergången mellan fluiderna är, samt hur effektiv värmeväxlaren är.
122
Bernoullis ekvation kan ses som energiekvation mellan två punkter där det strömmar en vätska, förklara hur?
Om ett fluidpaket antas vara inkompressibelt och färdas mellan a till b är dess energi bevarad och därmed konstant, då den inte utsätts för irreverisibiliter. Bernoullis ekvation beskriver förändring i potentiell-, kinetisk- och flödesenergi.
Summan av flödes-, kinetska- och potentiella energin för ett fluidpaket är konstant utmed en strömlinje vid stationära förhållanden, samt då kompression och friktion är försumbart.
123
I praktiken är det mycket lättare att omvandla tryckenergi till hastighetsenergi än tvärt om. Vad är den grundläggande orsaken till detta?
Tryckenergi har högre kvalité än hastighetsenergi.
124
Vilket Reynolds-tal blir större, det för en person som springer i vatten eller för en person som springer i luft?
Högre reynolds-tal för luft pga. högre viskösa friktionskrafter i vatten.
125
Kan alla tre mekanismer som överför värme ske parallellt i samma medium?
Nej, antingen överförs värme via en fluid i rörelse, konvektion, eller via en stilla fluid, konduktion, samtidigt kan det även ske överföring av värme vid strålning.
126
Ett Pitotrör mäter skillnaden mellan två tryck, vilka?
Mäter stagnationstryck, dvs. statiskt och dynamiskt tryck.
127
Om man seriekopplar pumpar adderas dess tryckökningar vid det aktuella flödet?
Uppfordringshöjden ökar vilket medför tryckökning.
128
Vid varvtalsreglering ändrar man enbart systemkurvan?
Pumpens hjul går saktare, pumpkurvan ändras. Mer miljövänligt.
129
Vad händer vid strypning?
Systemkurvan ändras, brantare, förlusterna ökar, mindre flöde
130
Vad är viskositet och vilka mekanismer påverkar viskositeten för en gas och för en vätska?
Viskositet beskriver interna motstånd mot flöde och kan ses som ett mått på friktion inom vätskor.
Vätska: Viskositeten minskar då T ökar Gas: Viskositeten ökar då T ökar
131
Vilken approximation görs i kursen för vätskors kompressibilitet?
Att de är inkompressibla.
132
Vid strömning runt kroppar så inträffar att flödet släpper (flow separation), vad menar man då, vad är orsaken och hur påverkar det strömningsmotståndet (drag coefficient)?
En fluid inkommer med så hög hastighet mot en yta att den inte klarar av att följa formen på ytan, utan fluiden avlägsnar sig från ytan och det bildas en separationspunkt. Flow separation kan påverkas Reynolds tal, ytråhet, föremålets geometri etc.
Vid flow separation bildas ett lågtrycksområde bakom föremålet där det bildas strömvirvlar. Desto mer separering, desto större område, desto större pressure drag.
