Nadia Caney

Surface Coating with Nanofluids: The Effects on Pool Boiling Heat Transfer

Recent research has confirmed a buildup of a thin layer of nanoparticles on the heated surface during nucleate boiling in nanofluids. Most of these studies report no change of heat transfer and, even worse, the presence of heat transfer deterioration. However, a few others report a heat transfer enhancement. In order to understand these controversial results, experiments were performed to explore the mechanism of surface coating during nucleate boiling in nanofluids. The thickness of the nanoparticle layer was observed to depend on the nanoparticles concentration and the experiment duration. Compared to a clean surface, the wettability of the surfaces with a TiO2 nanoparticle layer has been significantly improved. However, up to 50% of heat transfer coefficient deterioration was observed with TiO2-coated surfaces in water pool boiling. An explanation is proposed that involves the role of adhesion energy on heat transfer.

Flow Boiling of Water on Nanocoated Surfaces in a Microchannel

Experiments were performed to study the effects of surface wettability on flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 kg/m2 s and the base heat flux varied from 30 to 80 kW/m2. Water enters the test channel under subcooled conditions. The samples are silicone oxide (SiOx), titanium (Ti), diamond-like carbon (DLC) and carbon-doped silicon oxide (SiOC) surfaces with static contact angles of 26{\deg}, 49{\deg}, 63{\deg} and 103{\deg}, respectively. The results show significant impacts of surface wettability on heat transfer coefficient.

Study of a vertical boiling flow in rectangular mini-channels

Mini-channel heat exchangers with boiling flows present optimal performances: they are highly efficient and compact and require low fluid mass. However, classical correlations for two-phase flow in macro-channels fail in predicting the heat transfer coefficient and the eventual premature dry-out in mini-channels. Therefore, new studies are needed to provide better knowledge on flow boiling phenomena in small, confined spaces. The proposed paper presents an experimental study of vertical flow boiling in mini-channels. The pressure drop and the heat transfer coefficient in the test section have been measured for a variety of conditions. Different heat flux, inlet vapor quality, and mass flow rate values have been tested. A critical dry-out vapor quality depending on the mass flow rate has been found. Nevertheless, the superficial velocity appears to be much more appropriate than the vapor quality or the mass flow rate for the dry-out occurrence prediction. A clean dependence with a single critical velocity value has been found.

Flow Boiling of Water on Titanium and Diamond-like carbon coated Surfaces in a Microchannel

Experiments were performed to study the effects of surface wettability on flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 and 120 kg/m² s and the base heat flux was varied from 30 to 80 kW/m². Water enters the test channel under subcooled conditions. The sample surfaces are titanium (Ti) and diamond-like carbon (DLC) surfaces having a contact angle of 49° and 63°, respectively. The experimental results show different flow patterns that impact the heat transfer significantly. Compared to the Ti surface, the DLC surface shows a deterioration of 10% in heat transfer.

Influence of Surface Wettability on Pool Boiling Heat Transfer

Although boiling process has been a major subject of research for several decades, its physics still remain unclear and require further investigation. This study aims at highlighting the effects of the surface wettability on pool boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20 to 110° by modifying nanoscale surface topography and chemistry. The experimental results obtained disagree with the predictions of the classical models. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer. In this approach, we introduce the concept of macro- and micro-contact angles to explain the observed phenomenon.Copyright

Effects of Surface Wettability on Heterogeneous Boiling

Although boiling process has been a major subject of research for several decades, its physics still remain unclear and require further investigation. This study aims at highlighting the effects of the surface wettability on pool boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20 to 110° by modifying nanoscale surface topography and chemistry. The experimental results obtained disagree with the predictions of the classical models. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer. In this approach, we introduce the concept of macro- and micro-contact angles to explain the observed phenomenon.© 2009 ASME

Flow Boiling of Water on Titanium and Diamond-Like Carbon Coated Surfaces in a Microchannel

Experiments were performed to study the effects of surface wettability on flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 and 120 kg/m² s and the base heat flux was varied from 30 to 80 kW/m². Water enters the test channel under subcooled conditions. The sample surfaces are titanium (Ti) and diamond-like carbon (DLC) surfaces having a contact angle of 49° and 63°, respectively. The experimental results show different flow patterns that impact the heat transfer significantly. Compared to the Ti surface, the DLC surface shows a deterioration of 10% in heat transfer.

Flow Boiling Study in Mini-Channels

Mini-channel heat exchangers improve thermal performance in comparison to conventional macro-channel heat exchangers, being highly efficient, compact and requiring low fluid mass. However, classical correlations for two-phase flow in macro-channels fail in predicting the heat transfer coefficient in mini-channels. Therefore, new studies are needed in order to provide better knowledge on flow boiling phenomena in confined spaces. The proposed paper presents an experimental study on two-phase vertical flow boiling in mini-channels. The aim of this work is to determine the heat transfer coefficient and to study the pressure drop in a mini-channel heat exchanger (hydraulic diameter of 840μm) in order to obtain better understanding of the flow boiling mechanisms. A vertical upward flow test section is connected to a primary HFE-7100 circuit. A preheater imposes a given sub-cooled fluid temperature or a given two-phase vapour quality at the inlet. Downstream in the test loop, the fluid is condensed and pumped again into the test section. The pressure drop and the heat transfer coefficient in the test section have been measured for a variety of conditions. Different heat flux, inlet vapour quality and mass flow rate values have been tested. For the heat transfer coefficient, a correlating model is proposed as a function of the superficial velocity. This parameter appears to be much more appropriate than the vapour quality or the mass flow rate for dry-out occurrence prediction. A single critical velocity value has been found.Copyright