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If a single stream calorically perfect gas flow is steady, adiabatic, and free of shaft power, which quantities DO NOT change along the flow direction?
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If a single stream calorically perfect gas flow is steady, adiabatic, reversible, and free of shaft power, which quantities DO NOT change along the flow direction
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COMP
EX 1 quiz 1-8
| Question | Answer |
|---|---|
| If a single stream calorically perfect gas flow is steady, adiabatic, and free of shaft power, which quantities DO NOT change along the flow direction? | Total enthalpy, total temperature, critical temperature, critical pressure, critical density, and entropy. |
| If a single stream calorically perfect gas flow is steady, adiabatic, reversible, and free of shaft power, which quantities DO NOT change along the flow direction | Total enthalpy, total temperature, total pressure, total density, critical temperature, critical pressure, critical density, and entropy. |
| What is the definition of a compressible flow? | If the relative density change is greater than 5% |
| Are all liquid flows incompressible? | Yes |
| Are all gas flows compressible? | Not all gas flows are compressible |
| What is a wave? | A wave is a disturbance that carries energy through a material without moving the material itself. Molecules near a disturbance gain excess KE then transfer it out to neighboring molecules. |
| Does M->inf imply V->inf | No |
| Above what Mach number will changes in air density exceed 5%? | Greater than 0.3 |
| What are the most important and distinctive affects of compressibility on flow? | Choking - where in the duet is shapely limited by the sonic flow Shockwaves - which are nearly discontinuous properties in supersonic flow |
| What are the factors that affect wave speed? | Wave speed depends on the material, thermodynamic state, and the strength of the wave. The stronger the wave, the faster it moves. |
| How is kinetic energy distinguished from internal energy? | Kinetic Energy - Direct or Bulk motion Internal Energy - Random motions of molecules in a system |
| How do the possible disturbance propagation directions differ in subsonic and supersonic flows? | At SUBsonic flow, the source remains within the zone of action, but in SUPERsonic flow, the source lies ahead in the zone of silence. |
| Why does thermodynamics play an important role in the study of compressible flows? | Consequences of motion; Have common variables - Pressure, Temperature, Density, Specific Volume, Specific Internal Energy, Specific Entropy, Specific Enthalpy |
| What is the principal of state? | It principal states that there exists relations among state variables (Equation of State) |
| How does the speed of a shock wave compare with the speed of a sound wave? | The speed of a shock wave is greater than the speed of a sound wave in order for them to propagate. |
| Why do shockwaves sometimes form in a supersonic flow? | The disturbances cannot travel against supersonic flow. |
| How is a shock wave formed in supersonic flow? | Compression waves coalesce and reinforce each other. |
| What is the relationship between the compressibility and the speed of sound? | inversely related. |
| Is the flow upstream of a stationary normal shock wave always supersonic or subsonic? Downstream? | Flow upstream is supersonic, and flow downstream is subsonic for stationary normal shock. |
| The thermodynamic state of a unit mass pure substance is determined by how many state variables? | You need 2 of the 3 independent variables |
| State the thermal equation of state | Look at Quiz #1 Question #9 |
| Does the Mach number after a stationary normal shock wave decrease or increase as the Mach number before the shock wave increases? | The Mach number behind the shock wave decreases as the mach number ahead increases. |
| For a stationary normal shock, as M1 --> infinity, does M2 --> 0? | No, M2--> sqrt((gamma-1)/2gamma) |
| What is a calorically perfect gas? State the caloric equations of state. When does this assumption fail? | A perfect gas with constant specific, Cv and Cp. If there is NO chemical change, it obeys e=CvT; h=CpT. It fails when there are verry large temperature change in the flow or if the gas temperature is high. |
| What is an isothermal process? | The temperature remains constant |
| What is a adiabatic process? | There is no heat transfer between the system and surroundings |
| What is a reversible process? | There is no dissipative phenomena that occurs |
| What common agent is required for irreversibility? | Its required to have a significant spatial gradient field |
| An isentropic process is ALWAYS | reversible and adiabatic |
| State the 1st law of thermodynamics applied to a system | This deals with energy conservation q + w =Δε or Q + W = ΔE |
| In steady adiabatic flow, why is an "expansion shock" impossible? | There is no shaft power, so energy isn't added or removed. The useful energy decreases and the internal energy increases. The total amount of energy in the system remains the same. |
| The loss in stagnation pressure across a shock wave is directly related to what other important thermodynamic variable? | energy loss (useful energy decreases) |
| Can entropy of a system ever decrease? If so give an example. | Yes, turning the the heater within a room |
| Can entropy of an adiabatic system ever decrease? If so give an example. | No |
| Shock wave is a dissipative phenomena. Across a shock wave, where did the lost useful energy go? | The total amount of energy in a system must remain the same, so when useful energy decreases that means the the internal energy is increasing or there is heat loss. |
| Name the three governing equations of fluid dynamics | Conservation of mass, momentum, and energy |
| What is the physical interpretation of the time rate of change of a fluid property contained in a fixed volume in the integral form of governing equations? | It represents the accumulation/decumulation of a flow quantity in a CV due to flow unsteadiness |
| What is the physical interpretation of the surface integral in the integral form of governing equations? | It represents the net efflux of the property across the control surface. |
| The static property ratios across a normal shock wave are one to one functions of the Mach number before or after the normal shock wave. True or False? | True |
| Do these quantities increase, decrease, or stay the same across a stationary steady adiabatic shock wave: Mach number, velocity, pressure, temperature, stagnation temperature, stagnation pressure, entropy, and enthalpy. | Mach number decreases, velocity decreases, density increases, pressure increases, temperature increases, stagnatioin temperature stays the same, stagnation pressure decreases, entropy increases, and enthalpy stays the same. |
| What complicating factor must be taken into account in reading a pitot tube in a supersonic flow. | The total pressure behind a normal shock wave. |
| What is the advantage of the control volume approach over the differential equation approach for flow analysis? | Its not necessary to know the details of the flow inside the CV in order to compute useful information such as thrust and efficiency of a jet engine. |
| What is the limitation of the control volume approach? | it doesn't predict the details of the flow within the CV |
| What is the one-dimensional flow assumption? | at any given section all fluid properties are constant/uniform across the cross section. It is not necessarily unidirectional flow |
| State the energy equation for steady one-dimensional single stream, flow | h2-h1 + (1/2)(V2^2 - V1^2) = q + wshaft |
| Why does the familiar Bernoulli Equation not hold in compressible flow? | Because in order to use Bernoulli it is assumed that its steady and incompressible flow. So it cant have changes in velocity or density. |
| How are stagnation properties defined? How are they determined? | When fluid is not flowing, its at a stagnation state, and its thermodynamic properties are stagnation properties(total properties for a gas) zero speed relative to observer. Its determined by a pitot tube. |
| Is there any difference between stagnation property and total property for gas flow? | No |
| What is the difference between static and stagnation properties of a moving fluid? | Stagnation properties are defined at a point compared to static properties which are defined by the entire system |
| According to the steady energy equation for a single stream adiabatic flow free from shaft power, what thermodynamic properties are constant along a streamline | Stagnation Enthalpy and Stagnation Temprature |
| According to the steady energy equation for a single stream adiabatic and reversible flow free from shaft power, what are the thermodynamic properties constant along a streamline? | Pressure, Density, Temperature |
| If the flow through a gas is steady adiabatic and reversible, will stagnation temperature and stagnation pressure change from the inlet to the exit of the machine? | No, because it is locally adiabatic and reversible (no energy is added or removed and there is no change in entropy) |
| Irreversibilities cause entropy and stagnation pressure to increase or decrease? | Entropy will increase, and Stagnation Pressure will decrease |
| The total to static property ratio is a function of what dimensionless quantities? | Mach number and ratio of specific heats |
| What is the sonic reference state? What type of process is assumed to hold in bringing the fluid to M=1? | A sonic reference state is a condition where the fluid would reach the speed of sound if it were accelerated or slowed down in a way that doesn’t involve heat transfer or energy loss, and without using any shaft work. |
| What quantities does a pitot tube measure? What additional information is needed to measure the speed of a compressible subsonic flow. | Total and static pressure is enough to measure mach number, but you need speed of sound to determine flow velocity. |
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emmaliza