Critical heat flux (CHF) and premature tube burnout are the common failure modes observed in the multiphase flows, especially in steam water two-phase flows, frequently observed in heat exchangers and boilers used in nuclear, chemical and power industry. Lot of investigations were carried on CHF previously, giving importance to flow process conditions, fluids used and the geometry through which the two-phase flow fluids are transported. The pipe orientation and the flow direction plays an important role on the CHF. Horizontal flows, inclined flows and vertical flows with flow directed upwards in tubes received due attention earlier. One of the scenarios in which the flow was directed vertically downwards did not receive the same level of focus. Moreover, the investigations carried earlier were limited to understanding the flow pattern maps, void fraction up to high pressures and to some extent on CHF at atmospheric pressures. The complex two-phase flow associated due to the competing nature of gravity and buoyancy acting in opposite directions leads to instabilities and premature tube burnout. This phenomenon makes the vertically downward flows challenging and interesting.  

Numerical simulations using commercially available software were done on vertically upward two-phase flows earlier. Several boiling models were developed and commercially used. But these investigations were simulated at high pressures and validated by experimental data at high pressures. There were hardly any investigations carried at low pressure conditions as well as in vertically downward two-phase flows.

In the current investigations, numerical simulations were carried with commercially available CFD software Fluent and using the available boiling models, more specifically RPI boiling model and its extension to CHF. A set of simulations were carried by varying the flow rates in the range of 0-5LPM, pressures in between 1-5bar and inlet fluid temperature between 30-70^o^C and for a fixed tube length to tube diameter ratio. The simulations were carried for vertically downward two-phase flows. A CHF correlation as a function of these three variables was developed by using non-linear regression analysis. This CHF correlation was compared with the experimental CHF correlation published earlier with the same set of variables and the ranges considered. The results show that at low flow rates and high heat fluxes, both the experimental data and the numerical data shown comparable trends. However, at high mass flow rates, the numerical results tend to disagree by a large magnitude with the experimental data, indicating the importance of refining the available boiling models, especially at low operating pressures.


Development of Critical Heat Flux Correlation in Circular Pipe Sections With Vertically Downward Two-Phase Flows by Numerical Analysis and Comparison With Experimental Correlation

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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