HisTORIC · Heat Transfer Enhancement during Oscillatory Flows: Impact Quantification of Heat Transfer Coefficient

Heat Transfer Enhancement during Oscillatory Flows: Impact Quantification of Heat Transfer Coefficient

1. Over the last century, the research on flow boiling believes that the nucleate flow boiling heat transfer rate is unaffected by flow velocity. We found a strong influence of flow velocity on the nucleate flow boiling heat transfer rates. We found that the existing belief of independence of nucleate flow boiling on flow velocity holds good only above a threshold value. Below the threshold flow velocity, the heat transfer significantly deteriorates due to the limitation in bubble departure and surface-rewetting. Thus we show that to achieve an efficient nucleate boiling and to enhance the heat transfer rates, the flow velocity should be increased proportionally to the heat flux or working pressure. Hence, this research leads into a new understanding of nucleate flow boiling and opens a new strategy of enhancing heat transfer rates in high heat flux removing devices. Thus it demands further research on understanding the relationship between the flow velocity with heat flux during nucleate flow boiling. 2. Since the early development of systems requiring cooling based on flow boiling (e.g.: boilers in steam engines and power plants) understanding of the fundamental mechanisms controlling the heat transfer from the wall to the cooling fluid have motivated vast experimental, numerical and analytical work. In particular, from the early development of systems based on flow boiling, the occurrence of self-sustained oscillations (i.e. oscillations of flow and pressure naturally occurring under specific conditions) has been attributed to the deterioration of the heat transfer, setting limits in the operating conditions of the system to avoid the oscillations even at the price of compromising the efficiency of the plant. During almost 80 years, research efforts have suggested different theories about the physics of heat transfer deterioration with limited agreement among researchers. It is proposed that when a flow oscillates, it is believed that the heat transfer is controlled by the flow rate or flow velocity oscillations. In particular, the amplitude and period of the oscillations are the indicators of the amount of deterioration in the heat transfer rates. Although existing theories have suggested that the heat transfer deterioration observed during self-sustained oscillations is a consequence of the amplitude and period of the flow velocity oscillations. We have shown clear evidence that instead, it is the oscillations of the associated pressure field that are responsible for the heat transfer deterioration. In particular, we have shown that flow variations without pressure fluctuations do not lead to heat transfer deterioration, with the exception of large period oscillations that can cause dry-out of the wall. 3. The existing research over last 60 years consider the liquid Reynolds number to predict convective boiling heat transfer rates by multiplying with an appropriate enhancement factor. We show that, instead of the liquid Reynolds number, the vapor Reynolds number plays an important role in determining the convective boiling heat transfer rates. We show that the vapor Reynolds number is always dominant over the liquid Reynolds number. Interestingly, by using the vapor Reynolds number along with the liquid Reynolds number, the well-known Dittus-Boelter correlation can be used to predict the convective boiling heat transfer rates without any additional adjusting parameter. Thus this research suggests that to predict the convective boiling heat transfer rates, one should consider the influence of vapor Reynolds number. This study also opens new research on the understanding of the influence of the vapor phase in the thermal resistance of conductive sublayer.

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