Identifying the dominant heat transfer mechanism during two-phase flows

The unprecedented demand for air conditioning and refrigeration worldwide demands to improve the knowledge about how heat is transferred. Although heat transfer during phase change has been studied intensively since the 40s, no agreement has been achieved about which is the dominant physical mechanism transferring heat from the surface to the fluid. The design and optimization of such systems depend on accurate models. However, the incomplete understanding of the governing physical mechanisms during phase change heat transfer processes, namely evaporation and condensation, has restricted the development of satisfactory theoretical based models giving rise to many empirically based models with limited application. The objective in this project was to investigate how the heat is transferred close to the wall in a two-phase flow system. In this work, it is postulated that the heat transfer process is equivalent in flow condensation, convective flow boiling, and non-boiling two component systems, and thus it can be described by the same model. For investigating this problem, the project implemented a micro-particle image velocimetry and two-colours laser induced fluorescence thermography capable of resolving the flow and the temperature fluctuations close to the wall. This experimental technique can help us to understand how the flow and the two-phase flow structures can affect the temperature gradients close to the wall. The goal on the project was to identify if the temperature gradients close to the wall for flow condensation, convective flow boiling, and non-boiling two component systems shows a similarity. Challenges in the experiments has limited the fully evaluation of this hypothesis. However, analysis of available experimental studies in the literature gathered during the project has suggested the validity of the hypothesis. This result suggests that flow condensation, convective flow boiling, and non-boiling two component systems can be described by a common model that can help to reduce the uncertainties in the design and optimization of related systems. In addition, the project studied the heat transfer deterioration during flow boiling. It was observed that as the heat flux to the system is increased, the heat transfer reaches a limit or deterioration. By studying the process experimentally, it was found that the deterioration can be attributed to the accumulation of vapour at the wall, and that for preventing this it is required to increase the velocity of the flow. Thus, the project provides new evidence of the role of the flow in the heat transfer deterioration during flow boiling.

The project has provided three main results. First, it was shown the heat transfer deterioration observed during self-sustained flow oscillations can be attributed to the oscillations in the pressure. In the absence of pressure oscillations, a high amplitude flow velocity oscillation does not deteriorate the heat transfer. Hence, to avoid heat transfer deterioration during flow oscillations, the pressure oscillations in the system need to be minimised. Second, it was shown that the mass flux has a major role in the deterioration of the heat transfer coefficient during nucleate flow boiling in a horizontal heated pipe. In this way, in the quest of thermal management system for high heat fluxes, the relationship between mass flux, heat flux, and pressure during nucleate boiling needs to be reconsidered. Third, it was shown that the heat transfer coefficient of binary and single component fluids during flow condensation inside plain pipes is similar. This similarity is attributed to an equivalent heat transfer mechanisms between them and the single-phase flow case. The results of this project contribute to reduce the uncertainties in the design of heat transfer equipment which will result in lower costs and energy consumption.

As the world is facing an unprecedented demand for air conditioning and refrigeration knowledge about how heat is transferred gains paramount relevance. World power consumption for air conditioning alone will surge 33 times by 2100. Global data centres power consumption will double again over the next 15 years. Global food demand will grow 50% by 2050 intensifying the need of refrigerated food transportation. At this pace, by mid-century more energy for cooling than for heating will be used. Cold is still produced by vapour-compression refrigeration where a refrigerant fluid undergoes a condensation and vaporization cycle occurring in heat exchangers consisting of tubes or channels. More efficient technologies need to be developed for satisfying the bursting need of cooling. Heat transfer during phase change has been studied intensively since the 40s without achieving a common agreement about the dominant physical mechanism transferring heat from the surface to the fluid. This knowledge gap has resulted in empirical-based models. A novel study has provided indication that heat transfer might be controlled at a thin layer close to the surface, called conductive sublayer whose significance has been overlooked by all other research to date. The objective in this project is to establish conclusively whether the conductive sublayer is controlling the heat transfer, providing new knowledge in the front of the state of the art while elucidating this long-lasting research question. This will be done by temperature measurements close to the wall and numerical studies. The project involves a high risk but its success will pave the way for developing theoretically-based models representing a tipping point in relation to the actual dominant empirical-based modelling approach. The new knowledge and theory will contribute to guide the development of new heating/cooling concepts that is an important societal issue due to its impact in energy consumption and environmental impact.