Engineering guide
This module determines critical boiling conditions in flow boiling inside tubes according to Section H3.6 of the VDI Heat Atlas (VDI-Wärmeatlas, 12th edition 2019), the standard German reference work for heat transfer. It calculates the conditions under which the boiling crisis occurs — the transition from well-cooling nucleate or convective boiling to film boiling, or to dryout of the liquid film at the wall. The key results are the critical heat flux, the critical flow quality xcrit, and the critical tube length lcrit beyond which the boiling crisis is to be expected.
In practice, the critical heat flux is a hard design limit for evaporator tubes, steam generators, waste-heat boilers, and cooled reactor tubes: once it is exceeded, the heat transfer coefficient collapses abruptly and the wall temperature jumps — up to overheating and damage of the tube material. Anyone who wants to calculate the boiling crisis distinguishes two mechanisms: film boiling of the first kind (DNB, Departure from Nucleate Boiling) at high heat fluxes and low vapor quality, and dryout at high vapor qualities in annular flow.
The module follows the calculation recommendations of the VDI Heat Atlas for saturated and subcooled flow boiling and thus provides the basis for designing evaporators with a sufficient margin to the critical heat flux.
Scope
- Flow boiling - Critical boiling states
Typical workflow
Define the operating condition and geometry
The starting point is the system pressure, mass flux, tube diameter, heated length, and the vapor quality at the inlet or its profile along the tube. The local flow quality follows from the heat flux via the energy balance.
Identify the boiling crisis mechanism
Based on vapor quality and heat flux, the module checks whether a boiling crisis of the first kind (DNB, bubble coalescence at the wall at low vapor quality) or of the second kind (dryout, drying out of the wall film in annular flow) is governing.
Calculate the critical heat flux or critical vapor quality
Using the pressure-, mass-flux-, and diameter-dependent correlations of the VDI Heat Atlas, the critical heat flux and the critical vapor quality x<sub>crit</sub> are determined; tabulated values are available for water, and conversion relations for other fluids.
Determine the critical length
The energy balance along the heated tube yields the critical length l<sub>crit</sub> at which the critical vapor quality is reached — that is, the position at which the boiling crisis would occur in the tube.
Assess the design margin
The calculated critical conditions are compared with the operating values; the design is made with a sufficient safety margin to the boiling crisis, since beyond it the wall temperature rises abruptly.
Frequently asked questions
What is the difference between a boiling crisis of the first and of the second kind?
In a boiling crisis of the first kind (DNB), a coherent vapor film forms at the wall at high heat flux and low vapor quality, even though plenty of liquid is still present in the core. In a boiling crisis of the second kind (dryout), the liquid film on the wall dries out in annular flow at high vapor quality. Both lead to a collapse of the heat transfer, but they occur in different ranges of vapor quality and respond differently to pressure and mass flux.
Why is exceeding the critical heat flux so dangerous?
Beyond the boiling crisis, the heat transfer coefficient drops by one to two orders of magnitude. With an imposed heat flux — for example in fired steam generators or electrically heated tubes — the wall still has to reject the heat, and its temperature jumps, often by several hundred kelvin. This can lead to exceeding the allowable material temperature and to tube failure (burnout).
Do the correlations also apply to mixtures?
The basic correlations of the VDI Heat Atlas are formulated for pure fluids; for mixtures, the critical heat flux and the dryout point shift due to mass transfer effects at the phase interface. The VDI Heat Atlas provides correction approaches for this, but the uncertainty is greater than for pure fluids — the design should be correspondingly conservative.
What role does the flow direction play?
The tabulated critical heat fluxes apply primarily to vertical tubes with upward flow. In horizontal tubes, gravity can cause the liquid to accumulate at the bottom, so that the top of the tube dries out significantly earlier; additional criteria (e.g. a minimum mass flux) must be checked for this case.
Technical content source: LV-Solved-Problems-Content 628d313247e7