GATE AE Combustors: Previous Year Questions

GATE AE 2026, Question 52 (2 marks, NAT)

Propulsion - Combustors

Isobutane (C$_4$H$_{10}$) is burnt completely in pure oxygen as per the reaction given below. Given that the standard heats of formation (in kcal/mole) of isobutane, carbon dioxide, and water vapour are $-31.489$, $-94.052$, and $-60.150$, respectively, the heat of reaction is ________ kcal (rounded off to 2 decimal places). $$\text{C}_4\text{H}_{10} + 6.5\,\text{O}_2 \rightarrow 4\,\text{CO}_2 + 5\,\text{H}_2\text{O}$$

Official answer: -645.50 to -645.44

GATE AE 2025, Question 31 (1 mark, MCQ)

Propulsion - Combustors

$\delta Q$ and $\delta W$ are the heat and work interactions of a system with its surroundings, and $dU$ is the change in the internal energy of the system. For an adiabatic process in a closed, constant pressure combustor, which one of the following options is correct?

1. $|\delta Q| = |dU| \neq 0$ and $|\delta W| = 0$
2. $|\delta Q| = |\delta W| = 0$ and $|dU| \neq 0$
3. $|\delta Q| = |\delta W| = |dU| = 0$
4. $|\delta W| = |dU| \neq 0$ and $|\delta Q| = 0$

Official answer: D

GATE AE 2025, Question 33 (1 mark, MCQ)

Propulsion - Combustors

A gaseous fuel mixture comprising 3 moles of methane and 2 moles of ammonia is combusted in X moles of pure oxygen in stoichiometric amount. Assuming complete combustion, with only CO$_2$, H$_2$O and N$_2$ in the product gases, the value of X is ___. $$3\,\text{CH}_4 + 2\,\text{NH}_3 + X\,\text{O}_2 \rightarrow \text{Products (CO}_2\text{, H}_2\text{O, N}_2\text{)}$$

1. 7.5
2. 5.5
3. 8.5
4. 9.5

Official answer: A

GATE AE 2024, Question 17 (1 mark, MCQ)

Propulsion - Combustors

A surrogate liquid hydrocarbon fuel, approximated as C$_{10}$H$_{12}$, is being burned in a land-based gas turbine combustor with dry air (79% N$_2$ and 21% O$_2$ by volume). How many moles of dry air are required for the stoichiometric combustion of the surrogate fuel with dry air at atmospheric temperature and pressure?

1. 61.9
2. 30.95
3. 13
4. 10

Official answer: A

GATE AE 2024, Question 19 (1 mark, MCQ)

Propulsion - Combustors

In an airbreathing gas turbine engine, the combustor inlet temperature is 600 K. The heating value of the fuel is $43.4 \times 10^6$ J/kg. Assume $C_p$ to be 1100 J/kg K for air and burned gases, and fuel-air ratio $f \ll 1.0$. Neglect kinetic energy at the inlet and exit of the combustor and assume 100% burner efficiency. What is the fuel-air ratio required to achieve 1300 K temperature at the combustor exit?

1. 0.0177
2. 0.0215
3. 0.0127
4. 0.0277

Official answer: A

GATE AE 2023, Question 61 (2 marks, NAT)

Propulsion - Combustors

A gas turbine combustor is burning methane and air at an equivalence ratio $\phi = 0.5$, where $\phi = \dfrac{F/A}{[F/A]_{stoich}}$ and $[F/A]_{stoich}$ is the ratio of mass flow rate of fuel to the mass flow rate of air at stoichiometry. If the air flow rate is $\dot{m}_{air} = 20$ kg/s then the mass flow rate of methane is ______ kg/s. (round off to two decimal places)

Official answer: 0.57 to 0.60

GATE AE 2023, Question 65 (2 marks, NAT)

Propulsion - Combustors

Consider a cryogenic liquid rocket engine using an expander cycle with liquid hydrogen and liquid oxygen as the two propellants. The mass flow rate of hydrogen $\dot{m}_{H_2}$ into the combustion chamber is 32 kg/s, and the mass flow rate of oxygen $\dot{m}_{O_2}$ into the chamber is such that $\dot{m}_{O_2}/\dot{m}_{H_2} = 8$. The combustion of hydrogen and oxygen is at stoichiometry. Assuming that the rate of the forward reaction is much larger than that of the reverse reaction, the rate of formation of H$_2$O is ________ kmol/s. (round off to the nearest integer)

Official answer: 16

GATE AE 2022, Question 42 (2 marks, MCQ)

Propulsion - Combustors

Which of the following is NOT always true for a combustion process taking place in a closed system?

1. Total number of atoms is conserved
2. Total number of molecules is conserved
3. Total number of atoms of each element is conserved
4. Total mass is conserved

Official answer: B

GATE AE 2018, Question 35 (1 mark, NAT)

Propulsion - Combustors

A combustor is operating with a fuel-air ratio of 0.03. If the stoichiometric fuel-air ratio of the fuel used is 0.06, the equivalence ratio of the combustor will be _______ (accurate to two decimal places).

Official answer: 0.5

GATE AE 2016, Question 29 (1 mark, MCQ)

Propulsion - Combustors

Combustion in gas turbine engines is ideally represented as the following process:

1. Adiabatic
2. Isentropic
3. Isobaric
4. Isochoric

Official answer: C

GATE AE 2016, Question 43 (2 marks, NAT)

Propulsion - Combustors

A gaseous mixture of air and fuel enters a constant area combustion chamber at a velocity of 100 m/s and at a static temperature of 300 K. The heat release due to combustion is 1000 kJ/kg. The specific heat at constant pressure of the calorically perfect gas is 1000 J/kgK. The total temperature of air-fuel mixture after combustion is ______ K.

Official answer: 1305