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Heat Release Rate, abbreviated as HRR, is the characteristic of fire and other combustion processes. It determines the chemical energy released based on other parameters defined during combustion processes. The HRR comprises three types of combustion- premixed, controlled, and late combustions. There are many methods to calculate HRR. One such method to calculate heat release rate (HRR) is defined using the First Law of Thermodynamics. It is given as ΔQc = ΔW + ΔQh + ΔU, when the internal energy (U), heat transferred (Qh), and work done (W) are given.
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Key Terms: Heat Release, Thermodynamics, Work Done, Heat Transfer, Internal Energy, Chemical Energy, Combustion
What is Heat Release Rate (HRR)?
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Heat Release Rate (HRR) is defined as the rate of heat energy released during fire or combustion. With the increase in heat release, increased production of abominable effects of fire and its products occurs.
The orientation, arrangement, geometry, surface area to mass ratio, and other characteristics of fuel can cause influence the rate of heat released.
Heat Release Rate
Read More: Heat Capacity
Heat Release Rate Formula
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In engines, the most common method used to calculate the heat release rate (HRR) is given using the formula,
ΔQc = ΔW + ΔQh + ΔU
Where
- Qc denotes the chemical energy released. (kJ/kg)
- W denotes the work done or work output. (kJ/kg)
- Qh denotes the heat produced or transferred. (kJ/kg)
- U denotes the internal energy. (kJ/kg)
Read More: Heat Transfer
Things to Remember
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- According to the First Law of Thermodynamics, heat energy is neither created nor destroyed.
- Heat Release Rate (HRR) is the rate of heat energy released. It characterizes fire and combustion processes.
- The formula to calculate chemical energy is ΔQc = ΔW + ΔQh + ΔU, where the internal energy (U), heat transferred (Qh), and work done (W) are given.
- The bench-scale calorimeter is used to measure the heat release rate at different heat fluxes.
Previous Year's Questions
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Sample Questions
Ques. Determine the heat release rate when the internal energy, heat transfer, and work done are 144 kJ/kg, 69 kJ/kg, and 177 kJ/kg. (3 Marks)
Ans. Given that the internal energy, U = 144 kJ/kg; heat transfer, Qh = 69 kJ/kg; and work done, W = 177 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have the heat release rate (HRR) as,
Qc = 177+69+144
Qc = 390 kJ/kg,
Therefore, the heat release rate (HRR) equals 390 kJ/kg
Ques. The heat produced in the burner and internal energy is 280 kJ/kg and 430 kJ/kg respectively. Find the heat release rate for a work output of 260 kJ/kg. (3 Marks)
Ans. Given that the internal energy, U = 280 kJ/kg; heat transfer, Qh = 430 kJ/kg; and work done, W = 260 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have the heat release rate (HRR) as,
Qc = 280+430+260
Qc = 970 kJ/kg,
Therefore, the heat release rate (HRR) equals 970 kJ/kg.
Ques. The internal energy, heat transfer, and work output are given as 342 kJ/kg, 123 kJ/kg, and 33 kJ/kg respectively. Calculate the heat release rate. (3 Marks)
Ans. Given that the internal energy, U = 342 kJ/kg; heat transfer, Qh = 123 kJ/kg; and work done, W = 33 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have the heat release rate (HRR) as,
Qc = 33+123+342
Qc = 498 kJ/kg,
Therefore, the heat release rate (HRR) equals 498 kJ/kg
Ques. The heat release rate, internal energy, and heat transfer are 614 kJ/kg, 333 kJ/kg, and 200 kJ/kg. Find the work output. (3 Marks)
Ans. Given that the heat release rate, Qc = 614 kJ/kg; internal energy, U = 333 kJ/kg; and heat transfer, Qh = 200 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have
614 = W+200+333
W = 614-200-333
W = 81 kJ/kg
Therefore, the work done equals 81 kJ/kg.
Ques. The heat produced in the cylinder and internal energy is 420 kJ/kg and 245 kJ/kg respectively. Find the heat release rate for a work output of 280 kJ/kg. (3 Marks)
Ans. Given that the internal energy, U = 245 kJ/kg; heat transfer, Qh = 420 kJ/kg; and work done, W = 280 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have the heat release rate (HRR) as,
Qc = 280+420+245
Qc = 945 kJ/kg,
Therefore, the heat release rate (HRR) equals 945 kJ/kg
Ques. Determine the heat release rate when the internal energy, heat transfer, and work done are 75 kJ/kg, 64 kJ/kg, and 11 kJ/kg. (3 Marks)
Ans. Given that the internal energy, U = 75 kJ/kg; heat transfer, Qh = 64 kJ/kg; and work done, W = 11 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have the heat release rate (HRR) as,
Qc = 11+64+75
Qc = 150 kJ/kg,
Therefore, the heat release rate (HRR) equals 150 kJ/kg.
Ques. The internal energy, heat transfer, and work output are given as 592 kJ/kg, 466 kJ/kg, and 196 kJ/kg respectively. Calculate the heat release rate. (3 Marks)
Ans. Given that the internal energy, U = 592 kJ/kg; heat transfer, Qh = 466 kJ/kg; and work done, W = 196 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have the heat release rate (HRR) as,
Qc = 196+466+592
Qc = 1254 kJ/kg,
Therefore, the heat release rate (HRR) equals 1254 kJ/kg
Ques. Determine the heat release rate when the internal energy, heat transfer, and work done are 333 kJ/kg, 222 kJ/kg, and 111 kJ/kg. (3 Marks)
Ans. Given that the internal energy, U = 333 kJ/kg; heat transfer, Qh = 222 kJ/kg; and work done, W = 111 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have the heat release rate (HRR) as,
Qc = 111+222+333
Qc = 666 kJ/kg,
Therefore, the heat release rate (HRR) equals 666 kJ/kg.
Ques. Determine the heat release rate when the internal energy, heat transfer, and work done are 543 kJ/kg, 991 kJ/kg, and 1000 kJ/kg. (3 Marks)
Ans. Given that the internal energy, U = 543 kJ/kg; heat transfer, Qh = 991 kJ/kg; and work done, W = 1000 kJ/kg
The formula used to calculate the heat release rate is given as,
ΔQc = ΔW + ΔQh + ΔU
Substituting the given values in the above equation, we have the heat release rate (HRR) as,
Qc = 1000+991+543
Qc = 2534 kJ/kg,
Therefore, the heat release rate (HRR) equals 2534 kJ/kg.
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