Question

Thermodynamics: It is not always possible or practical to measure a thermodynamic property directly. In the three following situations, select the measurements that are necessary to obtain the desired property. Use as few measurements as possible to get full credit. Justify your answers briefly.

1. You want to find the pressure inside a small tank of R134a refrigerant that you found in your lab. Assume the tank is entirely filled with refrigerant in liquid and vapor states. The system is at equilibrium. To this end you need to measure:

(a) Temperature inside the tank

(b) Total volume of refrigerant in the tank,

(c) Volume of liquid inside the bottle,

(d) Mass of liquid inside the tank.

2. You want to find the specific volume of an air-fuel mixture (modeled as air in a gaseous state) entering a gas turbine. To this end you need to measure:

(a) Temperature of the air-fuel mixture,

(b) Pressure of the air-fuel mixture,

(c) Mass flow rate of the air-fuel mixture,

(d) Speed of the air-fuel mixture.

3. Consider ice and liquid water in a glass sitting on your kitchen counter. You want to find the water level in the glass at the instant when the last piece of ice melts. The geometry of the glass is known. To this end you need to measure:

(a) Pressure in the kitchen,

(b) Temperature in the kitchen,

(c) Mass of ice,

(d) Mass of water (liquid and ice

Answer 1

Answer :-

1. (a) Temperature inside the tank

• Since the tank is entirely filled with refrigerant in liquid and vapor states i.e., it is in saturated mixture state.
• If the temperature of mixture is known, it will be saturation temperature of mixture and from the property table of R-134a , the corresponding saturation pressure value can be found out from the measured saturation temperatue value.
• This presure value from property table will be required pressure.

2. (a) Temperature of the air-fuel mixture, (b) Pressure of the air-fuel mixture,

• An air-fuel mixture (modeled as air in a gaseous state) entering a gas turbine can be consider as a ideal gas.
• Therefore by ideal gas equation, v = specific volume, R = gas constant at T
• , if we measure P and T, we can find out specific volume,v.

3. (b) Temperature in the kitchen, (d) Mass of water (liquid and ice)

• If we calculate the temperature of room, we can find density of water and ice at this temperature from property tables.
• if we calculate the mass of water(liquid and ice), then we can calculate the volume of liquid present in the glass(V) by
• 
• we know geometry of glass, therefore by knowing volume V, we can found out water level in the glass.