Two-phase Flow Instabilities during Flow Boiling and Control of Wettability by Micro-structured Surfaces

Abstract

Boiling heat transfer is of relevance in a variety of applications in power and process industries. In spite of the vast research performed in the last decades, fundamental questions about mechanisms related to the boiling regime, heat transfer mechanisms, pressure drop and two-phase flow instabilities still remain unanswered. This thesis deals with two important points within boiling heat transfer: (i) the first part of the thesis is focused on understanding the dynamics of two-phase flow instabilities in small-diameter channels, in particular, pressure drop oscillations and their effect on the heat transfer coefficient; (ii) the second part of the thesis deals with manipulation of surface wettability by modification of the surface structure its effect on evaporative heat transfer, more precisely the Leidenfrost phenomenon. In flow boiling systems, two-phase flow instabilities such as density wave oscillations and pressure drop oscillations have been identified as one of the impediments for achieving high heat flux due to the potential heat transfer deterioration. However, most of the fundamental characteristics of the two-phase flow instabilities and the mechanisms leading to the heat transfer deterioration remain uncharted. In this thesis, contradictions between previous studies regarding the effect of the mass flux on amplitude and period of oscillations during pressure drop oscillations are clarified experimentally. A series of designed experiments are able to show the conditions for the interaction between density wave oscillations and pressure drop oscillations. Especially, it is shown that pure pressure drop oscillations can occur in a horizontal heated tube, something that has not been reported in previous studies. When both types of oscillations occur simultaneously, it is shown that the amplitude of the superimposed density wave oscillations on the pressure drop oscillations is controlled by the surge tank upstream of the test section. This previously unknown effect of the surge tank on the density wave oscillations contributes to improving the understanding of the complex dynamics of twophase flow instabilities. Furthermore, the deterioration of the heat transfer coefficient under controlled flow oscillations is investigated. It is observed that the averaged heat transfer coefficient can be deteriorated by flow oscillations. In particular, the deterioration is negligible until the amplitude of the oscillations become higher than a given threshold. Above this threshold, the deterioration is attributed to the dry-out of the wall during the low mass flux part of the oscillations. Based on these findings, it can be suggested that a severe deterioration of the flow boiling heat transfer coefficient can occur because of two-phase flow instabilities, but only when the amplitude of the oscillations is above a given threshold. Looking more in the microscale level, when it comes to the enhancement of the heat transfer from the surface to the cooling fluid, recent progress in controlling the properties of the surfaces at the micro-nanoscale by micro/nano fabrication techniques has motived a vast amount of research. However, the effect of the micro-nanoscale surface properties into the overall performance remains an open question. In the second part of the thesis, a fabrication process for nature-inspired microstructures is introduced. By mimicking conical microstructures found in nature, which can present superhydrophilic and superhydrophobic properties, a broad range of surface wettability is obtained. The fabricated surfaces show that the wettability can be controlled by adjusting the geometric parameters of the microstructure without external excitation. Remarkably, a drastic wetting transition from the Cassie-Baxter to the Wenzel state is observed by varying the spacing between the microstructures. Furthermore, the effect of the surface topology and chemical coating on the surface wettability is investigated. Silanization and replication of the geometrical microstructures into polydimethylsiloxane (PDMS) are considered to modify the chemical composition of the surfaces and to keep the similar topography of the surfaces. It is shown that the wettability in the Cassie-Baxter state is determined by the geometrical aspect in the microscale while wettability in the Wenzel state is decided by the chemical aspect. Regarding heat transfer, enhancement of the Leidenfrost temperature on the microstructured surfaces is investigated. The tested microstructures are able to increase the Leidenfrost point. Differences between all the definitions of the Leidenfrost point found in the literature are investigated. It is observed that the difference between the Leidenfrost points which is less than 10K in the smooth surface can become larger up to 70K when microstructures exist. Results indicate that with a microstructured surface, not only can the Leidenfrost be shifted to higher temperatures but phenomenological differences compared to what is observed on smooth surfaces occur, namely the observation of the transition film boiling regime.