# MECHANICAL ENGINEERING

## applied in real life applications.

SOLUTION

### (a) Define enthalpy of a system. How is it related to internal energy?

ANS:  Enthalpy is total heat content. It is one of the derived thermodynamic property of a fluid. It is the sum of internal energy (U) and the flow work PV..

For mass m

H =U +PV

For 1 kg mass, specific quantities are used i.e.

h =u +pv

### (b) State compression ratio.

ANS:  Compression ratio is the ratio of cylinder volume to clearance volume.

Let V is the volume of a cylinder

Vc is the clearance volume of a cylinder

V = Clearance volume + stroke volume = Vc + Vs

Then Compression ratio r

=(Clearance volume + stroke volume)/ Clearance volume                   = (Vc + Vs)/ Vc

r = 1 + Vs/ Vc

It is always much greater than 1.

### © Is it possible that W≠ 0 even if dv =0? If so, give an example.

ANS:  Yes, it is possible in case of paddle work. The volume change is zero but work is done due to the motion of the paddle.

### (d) What is entropy principle?

ANS:

Entropy always increases for every real process. Every real process is irreversible. Thus entropy increases for every irreversible process.  Entropy remains constant for a reversible process. In reality no process is completely reversible. Hence entropy always increases.

### (f) What is PMM1?

ANS:

PMM! Is perpetual motion machine of the first kind. In this, output is more than the input. It is a machine which gives work continuously without heat energy input? Obviously PMM1 is not possible.

### (g) Give the relation between COP of a heat pump and a refrigerator.

COPheat pump = COP refrigerator +1

COPheat pump = heating effect/work input =(Cooling effect + work input) / work input

= Cooling effect / work input + work input / work input= Cooling effect / work input +1

= COP refrigerator +1

### (h) Differentiate between machine and mechanism.

ANS:

Machine converts energy input into useful purpose (useful work). For example, human body, drilling machine, washing machine etc.

Mechanism is a combination of links to achieve motion to convert energy input into useful purpose. It is internal part of a machine. It is in between the energy input and the useful purpose. Without a proper mechanism a machine is not possible.

### (i) State the uses of Oldham coupling.

1.  Used in between two shafts having angular and axial misalignment.
2. It is used between a Reciprocating compressor crankshaft and lubricating oil pump shaft in refrigeration and air conditioning units.
3.  Used connect servo motor shaft to the ball screws. It  moves the work piece table to the required positions.

### (j) Explain temperature stresses and strains

Thermal stress exists when contraction due to fall of temperature is restricted.  It will be a tensile stress.

σth =EαΔt

Thermal stress when expansion due to rise of temperature is restricted. It is a compressive stress.

σth = EαΔt

Thermal strain = εth= σth / E=  αΔt

Corresponding Thermal stress in a single material   σth= Eα Δt,

Thermal strain in a single material εth = α Δt

When there is no restriction, there is no thermal stress.

THERMAL STRESSES IN A COMPOSITE MATERIAL

Pth= [(α1 –α2 ) Δt] /(1/A1E1 +1/A2E2) with α1   > α2 )

σth1  = Pth /A1 =  [(α1 –α2 ) Δt] /A1(1/A1E1 +1/A2E2)

σth2  = Pth /A2 =  [(α1 –α2 ) Δt] /A2(1/A1E1 +1/A2E2)

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