Critical heat flux
Boiling is a very efficient heat transfer mechanism with a large heat transfer coefficient and it is widely found in industrial systems. However, boiling heat transfer is limited by the critical heat flux (CHF), also termed as boiling crisis. It leads to a rapid decrease of the heat transfer coefficient in temperature controlled heat transfer or to a significant jump in heater surface temperature in power controlled heat transfer cases. While the earlier effect clearly lowers efficiency the latter may even jeopardize safety. A clear understanding of the basic mechanisms leading to CHF is still lacking which is required for the calculation in the Computational fluid dynamics approach.
A new CHF- model and its applications for pool boiling and flow boiling are developed and published recently in our group. This model is one of the very few CHF explanations, which try to explain the boiling crisis from the view point of “on the wall” instead of “near the wall” conditions. The model accounts for the impact of the different parameters, such as pressure, orientation angle, subcooling, and mass flow, hydraulic diameter, length, pressure, orientation angle in the pool or forced convective boiling and further potentially wall thickness, wettability, surface tension and so on. For pool boiling CHF was considered as a local phenomenon, while for flow boiling, there are two different phenomena leading to CHF. CHF at low flow rate is similar to that in pool boiling and considered as a local phenomenon. CHF at high flow rate is found to be a global phenomenon, which strongly depends on the upstream void fraction near wall. The criterion, which one of these phenomena is relevant, is determined by the fact, which one has the main impact on the departure diameter: shear stress (hydrodynamic) or liquid property (thermal property). The CHF-model is derived from the nucleation boiling, which allows the boiling process to continuously change from nucleation boiling to CHF. The initiating mechanism of CHF is able to be explained by this model. CHF will be strongly dependent on the onset wall superheat of the cavity. In the other words, the wettability and roughness impacts the onset point and impact the CHF further. Additionally the wall thickness plays a role in the heat up of the heated liquid trapped in the cavity, which is also considered as an impacting parameter to CHF in the future. Last but not least, our model can easily be implemented in a CFD code, which would allow modelling the whole boiling process covering the nucleation boiling and boiling crisis simultaneously in one model.
Ding W, Krepper E, Hampel U.
A hypothesis of near critical heat flux (CHF-) derived from nucleation for pool and forced convective boiling
NURETH17 (accepted) (2017)