Engineering task and calculation objective
The HABA module provides the reference value of the heat transfer coefficient for nucleate boiling on smooth tubes and plane walls. If you want to calculate nucleate boiling heat transfer — whether for pool boiling in kettles, evaporators and reboilers, or as the nucleate boiling contribution in saturated flow boiling — you need this substance-specific reference value α0 as the starting point of the normalized calculation methods, as also used by the VDI Heat Atlas.
The basis is the work of W. Leiner, "Wärmeübergang beim Blasensieden" (heat transfer in nucleate boiling; Chemie-Ingenieur-Technik 64, 1992, No. 1). There, the heat transfer is described via a dimensionless, thermodynamically similar representation that obtains the reference value at defined reference conditions (normalized boiling pressure, reference heat flux, standard roughness of the heating surface) from the property data of the boiling medium. This also makes it possible to treat media for which no measured reference values are available.
In practice, the reference value is the foundation of every evaporator design: starting from α0, the actual heat transfer coefficient is converted to the operating point via pressure, heat flux and surface corrections.
Standard and calculation basis: W. Leiner: "Wärmeübergang beim Blasensieden", Chemie-Ingenieur-Technik 64 (1992) Nr. 1
Calculation workflow
- Define the boiling medium: The user selects the medium from the built-in substance list. This fixes the property data required for the similarity representation, in particular the critical pressure and critical temperature, molar mass, enthalpy of vaporization and the transport properties of the boiling liquid.
- Normalize the reference conditions: The reference value is tied to fixed reference conditions: a normalized reduced pressure p* = p/p_crit, a reference heat flux and a standard roughness of the heating surface. This normalization makes measured values from different substances and heating surfaces comparable.
- Calculate the reference value from the similarity relation: Following Leiner's method, the reference value of the heat transfer coefficient is determined from a generalized, dimensionless correlation (Nusselt representation of nucleate boiling) based on the theorem of corresponding states — the substance dependence lies essentially in the critical data and the molar mass.
- Convert to the operating point: From the reference value, the heat transfer coefficient at the real operating point follows via correction functions for the reduced boiling pressure, the actual heat flux (α ~ q̇<sup>n</sup>) and the surface roughness of the heating tube — the usual procedure of the normalized nucleate boiling methods.
- Use in evaporator design: The heat transfer coefficient thus determined enters the design of pool boilers (kettle reboilers, immersed heating surfaces) or forms the nucleate boiling contribution in superposition models of saturated flow boiling.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Material file | Stoffname | - |
| Critical pressure | pc | Pa |
| Critical temperature | Tc | K |
| Critical density | ρc | kg/m³ |
| Molar mass | M | g/mol |
| Boiling point at 0.1∙pc | T | K |
| Reduced pressure | pr= p / pc | - |
| Boiling pressure at 0.1∙pc | p | Pa |
| Heat flux | q0 | W/m² |
| Roughness (DIN 4762 of 08.69) | Rp0 | m |
| Roughness (DIN 4762 of 01.89) | Ra0 | m |
| Critical pressure | pc | Pa |
| Critical temperature | Tc | K |
| Critical density | ρc | kg/m³ |
| Molar mass | M | g/mol |
| Boiling point at 0.1∙pc | T | K |
| Specific heat capacity of boiling liquid at 0.1∙pc | cp | J/(kg·K) |
| Specific heat capacity of boiling liquid at 0.1∙pc | cp | J/(kg·K) |
| Medium | Bauform | – |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Standard heat transfer coefficient at q0 = 20000 W/m² and pr = p/pc = 0.1 | α0 | W/(m²·K) |
Calculation options
Medium
Property calculation by PROPER · Free input of properties
Frequently asked questions
What exactly is the "reference value" of the heat transfer coefficient?
The heat transfer coefficient in nucleate boiling depends strongly on pressure, heat flux and surface condition. To make substances comparable, it is normalized to a uniform reference state: a fixed reduced pressure, a fixed heat flux and a standard roughness of the heating surface. This normalized value α<sub>0</sub> is a pure substance property. The conversion to the operating point is then carried out with dimensionless correction functions — cleanly separating the substance influence from the operating influence.
Why does nucleate boiling heat transfer increase with heat flux?
With increasing heat flux, more nucleation sites are activated, the bubble frequency rises and the stirring action of the departing bubbles at the heating surface increases. Empirically, α ~ q̇ⁿ with n typically between 0.6 and 0.8. However, this holds only up to the critical heat flux (burnout): beyond it, a vapor film closes over the surface, the heat transfer collapses and the wall temperature jumps — for heat-flux-controlled heating surfaces an acute damage risk that must be avoided with sufficient margin in the design.
What are the limits of the nucleate boiling reference value?
The reference value applies to fully developed nucleate boiling of saturated, clean liquids on smooth technical heating surfaces. It does not apply to quiescent boiling/free convection at small superheats, not to film boiling beyond the critical heat flux, and not directly to mixtures, in which mass transfer resistances reduce the heat transfer. In flow boiling, it provides only the nucleate boiling contribution; the convective contribution must be calculated separately and superimposed.
What influence does the surface roughness of the heating surface have?
Rougher surfaces offer more active nucleation sites for bubble formation and noticeably increase the heat transfer coefficient; very smooth (e.g. electropolished) surfaces degrade it. In the normalized methods, this is captured via the ratio of the actual roughness to the standard roughness. Note that the surface changes in operation — deposits and fouling can both improve and block the nucleation site structure, which is why designs should not be built on optimistic roughness gains.