Jakob Number formula |
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\( Ja \;=\; \dfrac{ c_p \cdot ( T_s - T_{sat} ) }{ \Delta h_f }\) (Jakob Number) \( c_p \;=\; \dfrac{ Ja \cdot \Delta h_f }{ T_s - T_{sat} }\) \( T_s \;=\; \dfrac{ Ja \cdot \Delta h_f }{ c_p } + T_{sat} \) \( T_{sat} \;=\; T_c - \dfrac{ Ja \cdot \Delta h_f }{ c_p } \) \( \Delta h_f \;=\; c_p \cdot \dfrac{ T_s - T_{sat} }{ Ja }\) |
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| Symbol | English | Metric |
| \( Ja \) = Jakob Number | \( dimensionless \) | \( dimensionless \) |
| \( c_p \) = Constant Pressure at Specific Heat | \(lbf \;/\; in^2\) | \(Pa\) |
| \( T_s \) = Total Stagnation Temperature | \(F \) | \( K \) |
| \( T_{sat} \) = Temperature Saturation Point | \(F \) | \( K \) |
| \( \Delta h_f \) = Evaporation Enthalpy Change | \(Btu \;/\; lbm\) | \(kJ \;/\; kg\) |
Jakob number, abbreviated as Ja, a dimensionless number, is the ratio of sensible latent heat absorbed or released during liquid vapor phase change. It represents the dominance of convection over conduction in heat transfer processes.
The Jakob number is commonly used in the analysis and design of heat exchangers, cooling systems, and other applications involving convective heat transfer. It helps engineers and researchers understand and optimize heat transfer processes by considering the relative importance of convection and conduction.
Jakob Number Interpretation
- Small Jakob Number (Ja << 1) - The latent heat is much larger than the sensible heat. Most of the energy is being used to drive the phase change (turning liquid into vapor) rather than raising the temperature of the substance. The temperature difference is small compared to the energy needed for vaporization.
- Moderate Jakob Number (Ja ≈ 1) - Sensible heat and latent heat are of comparable magnitude. Energy is split somewhat evenly between heating the liquid (or cooling the vapor) and changing its phase. This might indicate a process where the liquid is significantly superheated before boiling begins or subcooled before condensation starts.
- Large Jakob Number (Ja >> 1) - Sensible heat dominates over latent heat. A lot of energy goes into changing the temperature of the substance rather than driving the phase change. This could mean a large temperature difference (highly superheated liquid) or a relatively small latent heat.
