Academic literature on the topic 'Filmwise evaporation'

In this paper, the applicability of advanced heat transfer enhancement technology to a paddle dryer was discussed. A computational fluid dynamics (CFD) method was used to simulate condensation heat transfer on the inner surface of a dryer paddle. The effect of surface wettability and rotation on condensation heat transfer and droplet behavior was studied. The results showed that the present CFD model could properly simulate the condensation process on a vertical surface. With a decrease in the contact angle, the filmwise condensation turned into a dropwise condensation, which resulted in a significant increase in heat transfer coefficient and provided an approximately 5% increase in evaporation rate for the paddle dryer by changing the wettability of the inner surface of the paddle. Additionally, with a change in rotational angular velocity, heat transfer performance was almost unchanged under the filmwise condensation condition. However, rotational motion might cause a decrease in wall temperature and the equivalent evaporation rate under the dropwise condensation condition. Only a 2.4% increase in the equivalent evaporation rate was found in dropwise condensation with rotation, which indicated that changing the wettability inside the paddle could not be an effective means to enhance the heat transfer and drying efficiency of a rotating paddle dryer.

Abstract Two-phase flow inside the two-phase closed thermosyphon (TPCT) including evaporator, adiabatic and condenser sections was visually investigated in order to qualitatively analyze the complicated behaviors of both liquid film and vapor flows simultaneously. The semi-cylindrical channel which is 650 mm long was formed in the long copper block and the flat face of the channel was covered with a flat Pyrex glass for visual observation. The inner diameter of the semi-cylindrical channel was 25 mm and distilled water was used as a working fluid. The filling ratio of the thermosyphon was fixed at 0.5 and the inclination angle was set to 60º. As the heat flux increases, nucleate boiling becomes dominant and the bursting motion starts to begin in the liquid pool at the evaporator section. The bursting liquid flow reaches the condenser section and changes the condensation regime from dropwise to filmwise by flooding the condenser wall, which results in the decrease of condensation heat transfer coefficient. In addition, the vigorous vapor generation which occurs in the liquid pool at the evaporator section disturbs the circulation of the condensate film from the condenser to the evaporator section. As a result, the local dry-out occurs on the evaporator section with increasing heat flux, so the boiling heat transfer coefficient is decreased. [This research was supported by the Ministry of Science and ICT through the National Research Foundation of Korea (NRF-2018H1D3A2000929).]

Indirect evaporative cooling is a rapidly developing air-handling technology and has great application potentials for energy recovery in hot and humid regions. The condensation in dry channels of an indirect evaporative cooler (IEC) occurs when the dew point temperature of outdoor air is high. However, the dynamic heat transfer performance of an IEC during the process of condensation evolution was rarely discussed. This paper aims to experimentally investigate the dynamic behaviour of primary air condensation and its effects on the convective and total heat transfer rate of an IEC. A transparent cover plate is placed outside an aluminium heat exchanger plate to visualize the evolution process of droplets retained on the surface. The droplets' falling frequency was recorded and the accumulated condensate mass was correlated based on an analytical method taking into account the contact angle and droplet volume. Results showed that the dynamic dehumidification performance can pose great influences on the convective heat transfer in IEC. The gradually diminished dropwise regions and increase of filmwise regions deteriorate the wet-bulb effectiveness of IEC by 14.8%. The convective heat flux keeps decreasing with the accumulation of condensate retention until a dynamic equilibrium is achieved between the retained and falling droplets.

Interest in the use of heat pipe technology for heat recovery and energy saving in a vast range of engineering applications has been on the rise in recent years. Heat pipes are playing a more important role in many industrial applications, especially in increasing energy savings in commercial applications and improving the thermal performance of heat exchangers. Computational techniques play an important role in solving complex flow problems for a large number of engineering applications due to their universality, flexibility, accuracy and efficiency. However, up to now, computational studies on heat pipes are still at an early stage due to the complexity of multiphase flow characteristics and heat and mass transfer phase changes. Therefore, the main objective of this study is to develop a CFD modelling that includes the complex physical phenomena of both the heat transfer processes of evaporation and condensation and the mass transfer process of phase change during the pool boiling and film condensation. In this thesis, two novel numerical models were developed in ANSYS FLUENT. In the first, a two-dimensional CFD model was developed to visualise the two-phase flow and the evaporation, condensation and heat transfer phenomena during the operation of a wickless heat pipe, that otherwise could not be visualised by empirical or experimental work. An in-house code was developed using user-defined functions (UDFs) to enhance the ability of FLUENT to simulate the phase change occurring inside the heat pipe. Three different fluids, water, R134a and R404a, were selected as the working fluids of the investigated wickless heat pipe. The cooling system of the condenser section was simulated separately as a three-dimensional CFD model of a parallel-flow double pipe heat exchanger to model the heat transfer across the condenser section's heat exchanger and predict the heat transfer coefficients. The overall effective thermal resistance along with the temperature profile along the wickless heat pipe have been investigated. An experimental apparatus was built to carry out a thermal performance investigation on a typical wickless heat pipe for the purpose of validating the CFD simulation. A theoretical model based on empirical correlations was developed to predict the heat transfer thermal resistances in the evaporator and the condenser section. The second model was developed to combine the two-dimensional CFD simulation of the wickless heat pipe and the three-dimensional CFD simulation of the condenser section's heat exchanger to simulate the two-phase flow phenomena of boiling and condensation and the cooling system of the condenser section through a comprehensive three-dimensional CFD model of a wickless heat pipe. Two fluids, water and R134a, were selected as the working fluids of the investigated wickless heat pipe. This model was validated using a transparent glass wickless heat pipe to visualise the phenomena of pool boiling and comparing the results with the three-dimensional CFD flow visualisation. This study demonstrated that the proposed CFD models of a wickless heat pipe can successfully reproduce the complex physical phenomena of both the heat transfer process of evaporation and condensation and the mass transfer process of phase change during the pool boiling that takes place in the evaporator section and the filmwise condensation that takes place in the condenser section. The CFD simulation was successful in modelling and visualising the multiphase flow characteristics for water, R134a and R404a, emphasising the difference in pool boiling behaviour between these working fluids. The CFD simulation results were compared with experimental measurements, with good agreement obtained between predicted temperature profiles and experimental temperature data.

We report an electrowetting-controlled cooling system with site-specific treatments on the heat source (evaporator or hot spot) surfaces. Electrowetting-on-dielectric (EWOD) has great potential in thermal management because EWOD-driven droplet transport has unique characteristics of prompt response, low power consumption and programmable paths without the need for any mechanical moving parts. Prompt and fast droplet transport is necessary for adaptive and active cooling of high heat flux targets. Using a multi-channel DC/AC control system, we carried out sequenced activation of AC voltages on coplanar electrodes and transmitted a droplet to the spot target along a programmable path. With high positioning accuracy at the chip level, we have successfully transmitted a water droplet of 15 μL at speeds as high as ∼10 cm/s. We further improved electrowetting cooling performance by coating a fine copper screen on the cooling targets. The capillarity associated with the copper screen facilitates the delivered droplets automatically spreading and clinging to the target surfaces. As a result, heat transfer is in the more efficient form of filmwise evaporation at the evaporator sites. To maintain a thin film with proper thickness on the hot spots, we implemented EWOD-assisted droplet splitting and merging to precisely control the droplet volume to avoid fluid flooding (accumulation) on the hot spot surfaces. Our investigation indicates that thin-film evaporation is a high-efficiency heat transfer mechanism on a hydrophilized hot spot surface. Based on EWOD technique with surface treatments, the superheat on a hot spot of 4mm × 4mm was maintained well below 30°C even when the heat flux reached as high as 80W/cm2. The closed loop of this novel thermal management system can potentially function as a wickless vapor chamber or heat pipe with enhanced heat dissipation capabilities.