Things to think about

Aerospace Propulsion Winter 2002

* The mean molecular weight of a gas mixture is 53.75

(a) What is the gas constant for this mixture?

(b) The mixture is heated so that one of the compounds present starts

dissociating. Will the gas constant increase or decrease?

What are the values of the specific heats at constant pressure and volume?

(d) Why is the specific heat at constant pressure greater than that at

constant volume? (Physical reasoning, not "because R is positive"!)

* Does the specific heat of air at constant pressure increase or decrease as

the temperature is decreased? Why?

* Fifty Joules of work is done on an adiabatic system. What is the change in

internal energy of the system?

* What is an exothermic reaction? Give an example.

* Given that a mixture of perfect gases contains 8 possible species, and that

the only elements present are oxygen, hydrogen, and nitrogen, how many independent

equilibrium constants do you need to solve for the final composition? Assume

that the final temperature and pressure are known.

How would you do this, if the final temperature is not known?

* If the polytropic efficiency of a compressor is 0.93, the compressor has 20 stages

with equal pressure ratio per stage, and the overall pressure ratio of the

compressor is 40, find the stage efficiency and the compressor efficiency.

* Why is supersonic combustion unattractive at low supersonic flight Mach numbers?

How would you find the minimum flight Mach number at which it is better than

subsonic combustion, assuming that both kinds of flames (supersonic and subsonic)

can be held equally?

* Why is it possible for as few as 4 turbine stages to take out the work required

to run as many as 20 compressor stages on the same engine?

* The stagnation pressure at the turbine exit is 5 atmospheres, and the stagnation

temperature is 1000K. The mass flow rate through the turbine is 100Kg/sec, and

the last stage of the turbine is choked. Find the max. mass flow rate when the

stagnation pressure is 3 atmospheres, and the stagnation temperature is 800K.

* Why are diffusion flames thin?

* Given a mixture containing 1.5 moles of oxygen, 2 moles of hydrogen, 2.1

moles of nitrogen, 1 mole of carbon dioxide, and 0.3 moles of carbon

monoxide at a pressure of 2 atmospheres and a temperature of 500K,

determine:

(a) the molecular weight of the mixture

(b) the total enthalpy of the mixture

(d) the enthaply per mole of the mixture

(e) the internal energy per mole of the mixture.

Assume that the enthalpy per mole at 300K is h0 and the internal energy per mole

at 300K is e0.

* The rate of a certain chemical reaction can be described by the Arrhenius

rate expression. It activation energy is 25Kcal/mole. If the temperature

increases by 10%, what is the percentage increase in reaction rate?

* Why are compressors required on jet engines?

* What will happen to the nozzle exit velocity of a jet engine when the afterburner

is turned on? Why? (Assume that the nozzle geometry is unchanged)

* The stagnation pressure upstream of a nozzle is 3 atmospheres, and the stagnation

temperature is 2000K. The exit Mach number is 1.5. The nozzle efficiency is 0.98.

Find:

(a) the exit velocity

(b) the stagnation pressure at the exit

(d) the static pressure at the exit.

* Why are compressor blades twisted?

* What is an "impulse" turbine stage? Sketch the velocity diagram

for such a stage.

* The absolute velocity of the gas entering an impluse turbine rotor is

500 m/s at a station where the radius is 0.5m. What is the maximum shaft rpm

at which the turbine is extracting work?

* The air entering a compressor rotor has an absolute velocity of 100 m/s,

a static temperature of 400K, and is directed at 30 degrees to the vertical plane through the engine axis.

The radial component of the velocity is negligibly small. The axial component

remains unchanged as the air leaves the rotor, but the air is now at 60 degrees

to the vertical plane. The shaft rpm is 4000, and the radius at this location is

1 m.

Find the work done per unit time per unit mass flow by this rotor.

* What is the motivation behind conducting cascade tunnel tests?

* The blade tip radius at the first stage of a compressor is 1m, and the hub/tip

radius ratio is 0.8. The compressor pressure ratio is 32. Find the blade height at

the last stage if the mean blade radius is kept constant. Assume that the fraction

of the annulus area available to the flow is the same through all stages, and

that the axial velocity is the same at the mean radius for every stage.

* Given that the stagnation temperature must increase from a specified T01

to a specified T02 through a supersonic combustor, show the effect of

distributing the heat addition over n stages, as opposed to releasing all

the heat at one station. (What IS the effect, if any, and what does it

mean as far as the engine net thrust is concerned?)

* Why does combustor performance deteriorate as the altitude increases?

* What will be the effect on engine efficiency is part of the heat release

occurs inside the turbine, instead of inside the combustor? Show by means

of a T-S diagram.

* Can the exhaust temperature of a jet engine (turbofan/ramjet/turboprop/whatever)

be less than the enlet temperature? Why?

* Why are stator stages used on compressors?

* Sketch a mechanical configuration for counter-rotating turbine stages.

* A bunsen burner has been designed with a rectangular mouth: two of the edges are

much longer than the other two. The flame sheet above the long edges is found to take up a position

which is inclined at 15 degrees to the vertical. The flow velocity inside the flame

sheet is vertical, at 0.5 meters/sec. Find the flame speed.

* Why are multiple ramp inlets used?

* Why are the leading edges of hypersonic inlets swept?

* How would you start a supersonic inlet?

* How do you calculate the optimal ramp angle settings of a 4-ramp 2-D inlet,

given the flight Mach number?

* Derive an expression for the throat area of an inlet as a function of the

mass flow through the engine and the flight Mach number and altitude.

* A commercial airliner engine has an inlet diameter of 3 meters. The total mass

flow rate through the engine is 200kg/sec, as it reaches full takeoff thrust

while stationary at the edge of the runway (the brakes are new, and the wheels

are stuck in a snowdrift, and the maintenance chief is trying to blast the

aircraft out of the snow before the airport manager's snowplows trash the airplane).

The atmospheric pressure is 25" of mercury (worst storm in years) and the temperature

is 0 Fahrenheit. A stricken airliner is circling overhead, waiting for a clear runway.

Inside are 250 passengers who have just opened their fish dinners, only to

find them spoiled, just as they realize that the air-conditioning system failed

along with the refrigerators.

Find the area-averaged velocity in miles/hour at the inlet.

This is important to know, to keep the lead actor from getting his $1000 - hair-do

messed up as he swaggers up to the airplane to be obnoxious to the poor souls shoveling snow

and operating the cameras.

* You are to devise a control system for a fighter engine.

What quantity/quantities would you measure to achieve

each of the following objective, why, and how would you set up procedures

based on the analysis skills that you have learned?

(a) the turbine inlet temperature is to be held constant as altitude and

Mach number vary.

(b) the compressor blades are to be kept from stalling as you increase thrust,

by ensuring that the fuel flow rate increases only as fast as the rpm increases.

(Assuming that the engine was not operating at max thrust to begin with)

(d) the thrust is to be kept constant as the airplane accelerates at constant

altitude in supersonic flight.

while flying