Flow visualization study of post-critical heat flux in inverted flow (original) (raw)
Related papers
International Journal of Heat and Mass Transfer, 2012
This study explores the mechanism of flow boiling critical heat flux (CHF) in a 2.5 mm  5 mm horizontal channel that is heated along its bottom 2.5 mm wall. Using FC-72 as working fluid, experiments were performed with mass velocities ranging from 185-1600 kg/m 2 s. A key objective of this study is to assess the influence of inlet vapor void on CHF. This influence is examined with the aid of high-speed video motion analysis of interfacial features at heat fluxes up to CHF as well as during the CHF transient. The flow is observed to enter the heated portion of the channel separated into two layers, with vapor residing above liquid. Just prior to CHF, a third vapor layer begins to develop at the leading edge of the heated wall beneath the liquid layer. Because of buoyancy effects and mixing between the three layers, the flow is less discernible in the downstream region of the heated wall, especially at high mass velocities. The observed behavior is used to construct a new separated three-layer model that facilitates the prediction of individual layer velocities and thicknesses. Combining the predictions of the new three-layer model with the interfacial lift-off CHF model provides good CHF predictions for all mass velocities, evidenced by a MAE of 11.63%.
Photographic study of high-flux subcooled flow boiling and critical heat flux
International Communications in Heat and Mass Transfer, 2007
This study examines both high-flux flow boiling and critical heat flux (CHF) under highly subcooled conditions using FC-72 as working fluid. Experiments were performed in a horizontal flow channel that was heated along its bottom wall. High-speed video imaging and photomicrographic techniques were used to capture interfacial features and reveal the sequence of events leading to CHF. At about 80% of CHF, bubbles coalesced into oblong vapor patches while sliding along the heated wall. These patches grew in size with increasing heat flux, eventually evolving into a fairly continuous vapor layer that permitted liquid contact with the wall only in the wave troughs between vapor patches. CHF was triggered when this liquid contact was finally halted. These findings prove that the CHF mechanism for subcooled flow boiling is consistent with the interfacial lift-off mechanism proposed previously for saturated flow boiling.
Experimental and theoretical study of critical heat flux in vertical upflow with inlet vapor void
International Journal of Heat and Mass Transfer, 2012
This study explores the mechanism of flow boiling critical heat flux (CHF) for FC-72 in a 2.5 mm  5 mm vertical upflow channel that is heated along its 2.5 mm sidewall downstream of an adiabatic development section. Unlike most prior CHF studies, where the working fluid enters the channel in liquid state, the present study concerns saturated inlet conditions with finite vapor void. Temperature measurements and high-speed video imaging techniques are used to investigate the influence of the inlet vapor void on interfacial behavior at heat fluxes up to CHF as well during the CHF transient. The flow entering the heated portion of the channel consists of a thin liquid layer covering the entire perimeter surrounding a large central vapor core. Just prior to CHF, a fairly continuous wavy vapor layer begins to develop between the liquid layer covering the heated wall and the heated wall itself, resulting in a complex four-layer flow consisting of the liquid layer covering the insulated walls, the central vapor core, the now separated liquid layer adjacent to the heated wall, and the newly formed wavy vapor layer along the heated wall. This behavior in captured in a new separated control-volume-based model that facilities the determination of axial variations of thicknesses and mean velocities of the four layers. Incorporating the results of this model in a modified form of the Interfacial Lift-off CHF Model is shown to provide fairly good predictions of CHF data for mass velocities between 185 and 1600 kg/m 2 s, evidenced by a mean absolute error of 24.52%.
International Journal of Heat and Mass Transfer, 2013
This study explores the mechanism of flow boiling critical heat flux (CHF) for FC-72 in a rectangular channel fitted along one side with a heated wall. The flow is supplied as a two-phase mixture and the channel is tested at different orientations relative to Earth's gravity. High-speed video imaging is used to identify the CHF trigger mechanism for different orientations, mass velocities and inlet qualities. It is shown that orientation has a significant influence on CHF for low mass velocities and small inlet qualities, with the orientations surrounding horizontal flow with downward-facing heated wall causing stratification of the vapor towards the heated wall and yielding very small CHF values. High mass velocities cause appreciable diminution in the influence of orientation on CHF, which is evidenced by similar flow patterns and CHF trigger mechanism regardless of orientation. The interfacial lift-off model is shown to predict the influence of orientation on CHF with good accuracy. Overall, this study points to the effectiveness of high mass velocities at combating buoyancy effects and helping produce CHF values insensitive to orientation.
Investigation of interfacial behavior during the flow boiling CHF transient
International Journal of Heat and Mass Transfer, 2004
Vertical upflow boiling experiments were performed in pursuit of identifying the trigger mechanism for subcooled flow boiling critical heat flux (CHF). While virtually all prior studies on flow boiling CHF concern the prediction or measurement of conditions that lead to CHF, this study is focused on events that take place during the CHF transient. High-speed video imaging and photomicrographic techniques were used to record the transient behavior of interfacial features from the last steady-state power level before CHF until the moment of power cutoff following CHF. The video records show the development of a wavy vapor layer which propagates along the heated wall, permitting cooling prior to CHF only in wetting fronts corresponding to the wave troughs. Image analysis software was developed to estimate void fraction from the individual video images. The void fraction records for subcooled flow boiling show the CHF transient is accompanied by gradual lift-off of wetting fronts culminating in some maximum vapor layer mean thickness, following which the vapor layer begins to thin down as the transition to film boiling ensues. This study proves the interfacial lift-off model, which has been validated for near-saturated flow boiling CHF, is equally valid for subcooled conditions.
Feasibility Investigation of Experimental Visualization Techniques to Study Subcooled Boiling Flow
International Journal of Multiphase Flow
The purpose of this study is twofold: 1) to explore the feasible implementation of whole-field visualization techniques such as infrared thermometry, particle tracking velocimetry, and high speed shadowgraphy to study subcooled boiling flow through a vertical square channel with a single heated wall and 2) to provide subcooled boiling flow experimental measurements based on the methodology developed in this work. To fulfill the first objective, a series of sensitivity studies and uncertainty analyses was performed, from which recommendations for the proper implementation of these visualization techniques for the study of two-phase flows are given. The purpose of the second objective is to provide reliable information that can be used for the validation of CFD simulations and for the improvement and development of turbulence models under subcooled boiling conditions. Unique in the presented experimental results is the whole-field simultaneous measurement of turbulence characteristics...
International Journal of Heat and Mass Transfer, 2018
The present study investigated the physical processes responsible for the variation in the boiling curve and critical heat flux (CHF) caused by liquid subcooling under atmospheric pressure in a rectangular flow channel; the flow channel was oriented 10°upward from the horizon. Bubble dynamics were examined using a high-speed camera and optical fiber microprobes. A solid copper block was utilized as a test heater and mounted above the flow channel to simulate the passive cooling system of an ex-vessel core catcher designed for nuclear power plants. Low mass flux and subcooling conditions ranging from 40-300 kg/m 2 s and 5-25 K, respectively, were applied at the inlet of the test section. At the mass flux value of 40 kg/m 2 s, large sliding bubbles were attributed to a key criterion for enhanced boiling heat transfer when the liquid subcooling was varied up to 15 K. The results showed that the CHF was weakly dependent on the degree of liquid subcooling, which deviates from the general trend reported by many researchers. A repetitive flow reversal along with a pressure shock appeared, owing to the rapid condensation at the exit, which added complexity to the analysis of the CHF. This study provides physical insights for understanding the subcooled flow boiling heat transfer mechanism (including the CHF) based on sophisticated experimental measurements, such as the visual capture of boiling dynamics using highspeed video and local void fraction.
Flow pattern, heat transfer, and pressure drop data for different flow orientations was presented in this study. The data was obtained based on flow boiling experiments with R-134a flow through a 1 mm diameter channel which was aligned in different orientations, i.e. horizontal flow, vertical upward flow, and vertical downward flow. A constant surface heat flux condition was performed under a saturation pressure of 8 bar, a heat flux range of 1-60 kW/m 2 , and a mass flux range of 250-820 kg/m 2 s. The experimental results showed the importance of the change in the flow direction. The shape of the gas slug during horizontal flow did not look the same as in the vertical orientations. Heat transfer coefficient and pressure drop became increased when the refrigerant flowed in the vertical downward direction. The experimental data was also compared with the existing prediction methods.