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

Periodic Reporting for period 1 - HisTORIC (Heat Transfer Enhancement during Oscillatory Flows: Impact Quantification of Heat Transfer Coefficient)

Reporting period: 2018-06-18 to 2020-06-17

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.
Main scientific and/or technological achievements

1. To model the convective boiling heat transfer coefficients, the vapor Reynolds number play a dominant role over the liquid Reynolds number

2. To achieve an optimum nucleate boiling heat transfer process, the flow velocity or flow rate must be above a certain threshold. Below the threshold flow velocity, the surface re-wetting process is limited and thus the heat transfer rates can not be enhanced.

3. The existing approach of finding the stability thresholds experimentally during the density wave oscillations need to be revised. Contrary to the existing approach where the applied powers are increased in steps and the oscillation amplitudes are recorded, it is proposed that at every applied power, one should apply a high perturbation to the flow rate and observe the oscillations behavior. If the oscillations die, then the corresponding working conditions should be noted as stable. Elseif the oscillations do not die, the working conditions should be treated as unstable.

4. During a flow oscillation, in the particular case of density wave oscillations, the associated pressure oscillations trigger the heat transfer deterioration. The existing knowledge that the flow velocity oscillations control the heat transfer rates is dubious. Thus the amplitude and period of flow velocity oscillations are not the indicators of the heat transfer deterioration. It is found that, in the absence of pressure oscillations, even a high amplitude flow velocity oscillations does not deteriorate the heat transfer rates.
The work carried out in this project addresses two very important topics related to the heat transfer mechanism. The first one is the steady-state heat transfer mechanism and the other one is the transient
heat transfer mechanism. Although the results obtained has wide use in the process and power generation industries, the project outcomes are not immediately creating any new market opportunities. However, the knowledge of the heat transfer mechanisms will improve the heat transfer process in the said industries.