- 2071 Magh · 4 marks
Define critical point, quality, enthalpy, cv of ideal gas
Short answer
Critical point: saturated liquid and vapor become identical, top of dome (22.09 MPa, 374.1°C). Quality x = m_g/m. Enthalpy H = U+PV (h = u+Pv). cv = (∂u/∂T)_v; for ideal gas du = cv dT (air 0.718 kJ/kg·K).
Define the following terms: critical point, quality, enthalpy, specific heat of an ideal gas at constant volume.
- Critical point: the state at which saturated liquid and saturated vapour become identical; it lies at the top of the saturation dome. For water: P_c = 22.09 MPa, T_c = 374.1 °C. At pressures above P_c there is no distinct boiling.
- Quality (dryness fraction), x: the mass of vapour divided by the total mass of a liquid-vapour mixture, x = m_g/m (0 ≤ x ≤ 1). It is used in v = v_f + x v_fg.
- Enthalpy: a property defined by H = U + PV (kJ); per unit mass, h = u + Pv. It is the energy carried by a flowing fluid per unit mass (internal energy plus flow work), and its change equals the heat transferred in a reversible constant-pressure process.
- Specific heat of an ideal gas at constant volume (c_v): the heat required to raise the temperature of unit mass of the gas by 1 K at constant volume: c_v = (∂u/∂T)_v. For an ideal gas du = c_v dT; for air c_v = 0.718 kJ/kg·K.
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