Sure! Below are detailed explanations and calculations based on the prompts you provided.
(a) Define the heat value of a fuel
The heat value of a fuel, also known as the calorific value, is the amount of energy released per unit mass or volume when the fuel is completely combusted. It is typically expressed in kilojoules per kilogram (KJ/kg) or megajoules per cubic meter (MJ/m³). It can be classified into two types: the higher heating value (HHV), which includes the total energy content including latent heat of vaporization of water, and the lower heating value (LHV), which excludes this energy.
(b) Explain how the heat value of a fuel can be used as a factor to consider when choosing a good fuel
The heat value of a fuel is a critical consideration when selecting an appropriate fuel for a particular application. High heat values indicate that a fuel can produce more energy per unit mass, which can lead to greater efficiency in energy production. Choosing a fuel with a higher heat value can result in lower fuel consumption rates, which translates to reduced costs and emissions. Additionally, fuel selection based on heat value may also consider safety, storage, and handling characteristics that may differ among fuels with varying calorific values.
(c) Differentiate between burning velocity and pyrometric burning effect of a fuel
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Burning Velocity: Burning velocity refers to the speed at which a flame front propagates through a combustible mixture of fuel and oxidizer. It is typically measured in centimeters per second (cm/s) and is influenced by various factors, including fuel composition, temperature, pressure, and initial concentration of the reactants. Higher burning velocities indicate a more reactive fuel mixture, which can affect combustion stability and efficiency.
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Pyrometric Burning Effect: The pyrometric burning effect refers to the thermal and pressure effects resulting from the combustion of fuels, particularly in the context of how well a fuel maintains high temperatures during burning. This term encompasses factors like flame temperature, thermal radiation, and heat release rates. The pyrometric effect impacts the efficiency of thermal systems and influences the design of combustion equipment. It can be more reflective of the combustion characteristics of a fuel under practical operating conditions rather than just the speed of flame propagation.
(d) To boil 20 liters of water, 250 cm³ of kerosene as a fuel were completely burnt. Determine the temperature in Celsius from which water started to boil to 121.5 °C.
Given:
- Volume of water = 20 liters = 0.020 m³ = 20 kg (since the density of water = 1000 kg/m³)
- Volume of kerosene = 250 cm³ = 0.00025 m³
- Density of kerosene = 810 kg/m³
- Heat value of kerosene = 42430 KJ/kg
- Specific heat capacity of water \( C \) = 4.18 KJ/kg/K
Steps:
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Calculate the mass of kerosene: \[ \text{Mass of kerosene} = \text{Density} \times \text{Volume} = 810 , \text{kg/m}^3 \times 0.00025 , \text{m}^3 = 0.2025 , \text{kg} \]
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Calculate energy produced by burning kerosene: \[ \text{Energy (Q)} = \text{Mass of kerosene} \times \text{Heat value of kerosene} \] \[ Q = 0.2025 , \text{kg} \times 42430 , \text{KJ/kg} = 8588.175 , \text{KJ} \]
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Calculate the energy needed to raise the temperature of water: First, note that the water will turn from its starting temperature \( T_1 \) to boiling at \( 121.5 \degree C \):
\[ Q = \text{mass of water} \times C \times (T_2 - T_1) \] Where:
- \( T_2 = 121.5 \degree C \)
- \( T_1 \) is the starting temperature.
Rearranging gives: \[ T_1 = T_2 - \frac{Q}{\text{mass of water} \times C} \] Substituting: \[ T_1 = 121.5 \degree C - \frac{8588.175 , \text{KJ}}{20 , \text{kg} \times 4.18 , \text{KJ/kg/K}} \] First, calculate the energy contribution factor: \[ Q_{\text{water}} = 20 , \text{kg} \times 4.18 , \text{KJ/kg/K} = 83.6 , \text{KJ/K} \] Now, complete the calculation: \[ \frac{Q}{Q_{\text{water}}} = \frac{8588.175 , \text{KJ}}{83.6 , \text{KJ/K}} \approx 102.3 , \text{K} \]
Finally: \[ T_1 = 121.5 \degree C - 102.3 \degree C \approx 19.2 \degree C \]
Conclusion:
The temperature from which the water started to boil was approximately 19.2 °C.