# Thermal engg. Concepts problem

1. Nov 20, 2008

### rahshail

Hello all,
I have come across some questions in thermal engg., of which i have answers also as per the book where i have found the questions but unable the find the concepts and logic behind them to solve the questions. kindly help me to find the solution with the logic also. following are the questions and their answers.
(may be there is some mistake in the way the questions are asked. if yes, then please correct it out also)

1.) Under what conditions it is possible that heat be abstracted from hot source, as well as cold source and positive work be done
(a) impossible
(b) hot reservoir is at negative temperature
(c) cold reservoir is at negative temperature
(d) both hot and cold reservoir are at positive temperature
(e) none of the above
Ans. (b)

2. What should be the limitations of hot and cold reservoir temperatures, if a two temperature machine pumps heat into hot reservoir as well as cold reservoir and work is done on the system
(a) cold reservoir must have +ive temperature
(b) cold reservoir must have –ive temperature
(c) hot reservoir must have +ive temperature
(d) hot reservoir must have –ive temperature
(e) none of the above
Ans. (a)

3. What are the limitations to the signs of heat exchange from hot and cold reservoir if a two temperature machine operates between two positive temperatures
(a) heat must be rejected to hot source and abstracted from cold source
(b) heat must be abstracted from hot source and rejected to cold source
(c) heat must be rejected to both sources
(d) heat must be abstracted from both sources
(e) no limitations
Ans. (b)

4.) An idea gas as compared to a real gas at very high pressure occupies
(a) more volume
(b) less volume
(c) same volume
Ans. (a)

5.) If the value of n is high in the polytropic process, then the compressor between given pressure limits will be
(a)less
(b)more
Ans. (a)

6.) The law PVγ = const. is not applicable for
(a) adiabatic expansion of steam in turbine
(b) adiabatic expansion of steam in reciprocating steam engine
(c) ideal compression of air
(e) free expansion of an ideal gas
Ans. (e)

7.) Reversible adiabatic process can be expressed as (V2-V1)γ-1 .
This is right or wrong. (In my book it is written right.)

8.) Water contained in a beaker can be made to boiled by passing steam through it
(a) at atmospheric pressure
(c) at high pressure
(c) at low pressure
(d) at any pressure
Ans. (c)

9.) Is Joule cycle a reversible cycle? (Yes/No)
In the book it is given ‘Yes’

10. Diesel cycle is called a const. pressure cycle. (Right/Wrong)
In the book – Right

11. What are the limitation to the signs of heat exchange from hot and cold reservoirs if a two temperature machine operates between two negative temperature
(a) Heat must be rejected to hot source and abstracted from the cold source
(b) Heat must be abstracted from the hot source and rejected to cold source
(c) heat must be rejected to both the sources
(d) Heat must be abstracted from both the sources)
(e) No limitations
Ans (e)

Last edited: Nov 20, 2008
2. Nov 21, 2008

### ank_gl

1. a, it is impossible, use 2nd law of thermodynamics. What do you mean by negative temperature?

2. i dont understand +ve & -ve temperature

3.e, it can work either way, ie, an engine & a heat pump or a refrigeration machine.

4. dunno

5.more or less? relative to what?, compressor work would be more than what it is in isothermal case.

6. e, its isenthalpic & is irreversible.

7. again i don't understand what you are asking.

8.d, water can be boiled at any temperature, if its vapor pressure is made to go below the saturation pressure at that temperature. You are asking if steam can do it, well it can at any pressure, here heat is transferred, it can boil water given its temperature is more than saturation temperature at the given pressure.

9.yes it is, all ideal cycles are reversible. reverse of joule cycle is the basis of air refrigeration units

10. i really don't understand it very well myself. so i can't say

11. again i don't know what +ve & -ve means.

3. Nov 21, 2008

### minger

You may get some more responses in the Homework section.