Part 4 Thermo

Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby river is 50 MW, determine the net power output and thermal efficiency for this heat engine:

• 30 MW & 37.5 %

• 40 MW & 0.375 %

• 50 MW & 35.7 %

• 60 MW & 0.357 %

A car engine with a power output of 50 kW has a thermal efficiency of 24 percent. Determine the fuel consumption rate of this car if the fuel has a heating value of 44,000 kJ/kg

• 0.00273 kg/s

• 0.00373 kg/s

• 0.00473 kg/s

• 0.00573 kg/s

A machine which can do work by making use of heat that flows out spontaneously from a high temperature source to a low temperature sink is called:

• Carnot engine

• Cyclic machine

• Heat engine

• Heat machine

What is true about entropy?

• if it increases in one place, it must decrease elsewhere

• If it decreases in one place, it must increase elsewhere

• It must decrease in the universe

Efficiency of a heat engine is the ratio of:

• Heat taken from the high temperature reservoir (Q) to the work obtained in a cyclic process (W)

• Work obtained in a cyclic process (W) to the heat taken from the high temperature reservoir (Q_H)

• Work obtained in a cyclic process (W) to the heat taken from the low temperature sink (Q_L)

• Heat taken from the low temperature sink (Q_L) to the work obtained in a cyclic process (W)

In a heat engine, what is the relationship between Q_H, Q_L and W?

• W = Q_H + Q_L

• W = Q_L - Q_H

• W = Q_H - Q_L

• W + Q_H + Q_L = 0

An air conditioner provides 1 kg/s of air at 15 oC cooled from outside atmospheric air at 35 oC. Estimate the amount of power needed to operate the air-conditioner:

(Cp of air = 1.005 kJ/kgK)

• 1.09 kW

• 1.19 kW

• 1.29 kW

• 1.39 kW

A car engine operates with a thermal efficiency of 35 %. Assuming the air-conditioner has a coefficient of performance (COP) of 3 working as a refrigerator cooling the inside using engine shaft work to drive it. How much extra fuel needed to remove 1 kJ from the inside of the car?

• 0.752 kJ

• 0.952 kJ

• 0.852 kJ

• None of the above

A cyclic machine receives 325 kJ from a 1000 K energy reservoir. It rejects 125 kJ to a 400 K reservoir and the cycle produces 200 kJ of work as output. This cycle is

• reversible

• irreversible

• impossible

• None of the above

The increase in entropy when one mole of ice melts to form water is: (Latent heat of fusion of ice is 80 cal/g)

• 0.293 Cal/molK

• 0.65 Cal/molK

• 5.275 Cal/molK

• 3.41 Cal/molK