Evaporation of Gravity- and Gas Flow-Driven Thin Liquid Films in Micro- and Minigrooves

2004 ◽  
Author(s):  
T. Gambaryan-Roisman ◽  
P. Stephan
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Gas Flow ◽  
Thin Liquid Film ◽  
Flow Region ◽  
Wave Theory ◽  
Liquid Films ◽  
Wall Material ◽  
Wall Structure ◽  
Ultra Thin Film

Using microstructured wall surfaces may improve the heat transfer performance of falling film or shear-driven film cooling devices enormously. The advantages of the structured surface include the prevention of the formation of dry patches on hot surfaces, the promotion of ultra-thin film evaporation, and a wavy motion of the film that enhances mixing of the liquid. We develop a model describing the hydrodynamics and heat transfer by evaporation of gravity- and gas flow-driven liquid films on grooved surfaces. For low Reynolds numbers or low liquid mass fluxes the heat transfer is governed by the evaporation of the ultra-thin film at a micro region, in the vicinity of the three-phase contact line. We investigate the hydrodynamic stability of the film flow using the long-wave theory. In addition to the films completely covering the wall structure, we study the stability characteristics of a thin liquid film partly covering the grooved wall, so that the flow region is bounded by contact lines. Two cases are analyzed: fully wetting liquids and liquids which form a small but finite contact angle with the wall material.

Experimental Study on Single-Phase Gas Flow in Microtubes

2011 ◽  
Vol 133 (11) ◽  
Author(s):  
T. T. Zhang ◽  
L. Jia ◽  
C. W. Li ◽  
L. X. Yang ◽  
Y. Jaluria
Keyword(s):  
Heat Transfer ◽  
Gas Flow ◽  
Single Phase ◽  
Slip Flow ◽  
Experimental System ◽  
Flow Region ◽  
Flow And Heat Transfer ◽  
Wide Range ◽  
Physical Insight ◽  
Insight Into

An experimental system for single-phase gas flow in microtubes has been developed. The effects of viscous heating and compressibility on the flow and temperature field were studied for a wide range of governing parameters. Also, an analytical/numerical model of the flow was developed. Numerical results for the flow and heat transfer in the slip flow region were found to agree quite well with the experimental data, lending support to the model. The study provides greater physical insight into and understanding the effects of viscous dissipation and compressibility in microtube flow and the associated heat transfer. In addition, the combined experimental and numerical simulation approaches of the process can be used for control and optimization of systems based on microtube heat transfer.

Annular Liquid Film Flow Under Local Heating in Microchannel

2005 ◽  
Author(s):  
Elizaveta Ya. Gatapova ◽  
Vladimir V. Kuznetsov ◽  
Oleg A. Kabov ◽  
Jean-Claude Legros
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Biot Number ◽  
Gas Flow ◽  
Film Flow ◽  
Local Heating ◽  
Local Heat Transfer ◽  
Flow Region ◽  
Local Heat ◽  
Liquid Film Flow

In our previous investigations the formation of liquid bump of locally heated laminar liquid film with co-current gas flow was obtained [1,2]. The evaporation of liquid was left out of account. Heat transfer to the gas phase was approximately specified by a constant Biot number [2,3]. The aim of this work is an investigation of the evaporation effect, the hydrodynamics and the heat transfer of liquid film flow in a channel 0.2–1 mm height. The 2-D model of locally heated liquid film moving under gravity and the action of co-current gas flow with low viscosity in a channel are considered. The channel can be inclined at an angle with respect to horizon. It is supposed that the height of the channel is much less than its width. Surface tension is assumed to depend on temperature. The velocity profiles for gas and liquid regions are found from problem of joint motion of isothermal non-deformable liquid film and gas flow. Using the findings the joint solution of heat transfer and diffusion problem with corresponding boundary condition is calculated. Having the temperature field in the whole of liquid and gas flow region we find a local heat transfer coefficient on the gas-liquid interface and Biot number as a function of flow parameters and spatial variables.

Temporal Effect and Transient Simulation of Thin Liquid Film in Micro Channels

2013 ◽  
Author(s):  
Yu-Yan Jiang ◽  
Da-Wei Tang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Liquid Film ◽  
Flow Boiling ◽  
Thin Liquid Film ◽  
Liquid Films ◽  
Transient Simulation ◽  
Governing Equations ◽  
Flash Evaporation ◽  
Micro Channels

The evaporation and heat transfer of thin liquid film are crucial factors affecting on the heat transfer performance of boiling bubbles or slugs. For boiling in micro-channels, the flash evaporation of the liquid film may give rise to boiling instability, and the dry-out of the film leads to serious deterioration of the heat transport. The thin liquid film has multi-scale transitions, and hence the phase change and fluid dynamics need to be solved by special governing equations and numerical algorithm. The numerical studies to date have solved the steady state distribution of the film, but the difficulty consists in the transient simulation of time-variant liquid films. In the present study, unsteady form governing equations are developed. With inclusion of the temporal terms, we conducted transient simulations for flat liquid films formed during the flow boiling in micro-channels. The model predicts the developing of drying spot during growth of elongated bubbles. The results show that the film thickness and distribution change quickly in a growth period, which are functions of the heat flux, mass flow rate and the other parameters. The quantitative assessment of these effects helps to clarify the mechanism of boiling instability and the conditions for the occurrence of critical heat flux (CHF). The simulation needs special numerical scheme for time marching and stabilization treatment for the nonlinear terms, where the numerical accuracy and the significance of the temporal effects are also discussed.

The Visualization of Thin Film Evaporation on Thin Micro Sintered Copper Mesh Screen

2008 ◽  
Author(s):  
Chen Li ◽  
G. P. Peterson ◽  
Ji Li ◽  
Nikhil Koratkar
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Flux ◽  
Thin Liquid Film ◽  
Heated Wall ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Sintered Copper ◽  
Mesh Screen ◽  
Copper Mesh

The thin film evaporation process through use of thin micro-scale sintered copper mesh screen was proven to be a very effective heat transfer mechanism with high critical heat flux (CHF). This efficient heat transfer mechanism is widely used in designing heat pipe, Capillary Pumped Loops (CPL), and drying process, however, the nucleation process and meniscus dynamics at the liquid-vapor-solid interface are not directly observed and systematically studied. Very few visual investigation in thin film evaporation has been conducted. In the existing two visual studies, the interface thermal resistance between coating and the heated wall was not seriously considered, and the heat flux was limited below 35 W/cm2. In this visualization investigation, the nucleation process and meniscus dynamics from initial condition to drying out were observed and well documented. To minimize the interface thermal resistance, the micro scale wicking was sintered to heated wall directly. High quality images were acquired through a well-designed visualization system. The majority of nucleate bubbles, whose diameters are at a magnitude of 10 μm, were found to form on the top wire surfaces instead of inside the porous media at moderate heat flux. Few large size bubbles were observed to grow inside capillary wicks, however, their presence did not seem to stop the evaporation process as reported before. The menisci receding process was visually captured for the first time. The minimum menisci radius was found to form at the smallest corners and pores. It is also illustrated the thin liquid area increases when the menisci recede and the thin liquid film evaporation is the dominant heat transfer mode at high heat flux. The present work visually confirms the heat transfer regimes of evaporation on micro porous media, which was proposed by Li and Peterson [2], and further improves the understanding to the nucleate boiling and thin liquid film evaporation on the surfaces of micro sintered copper mesh screen.

Heat Transfer Analysis of Flash Evaporation With MEPCM

2019 ◽  
Vol 11 (5) ◽  
Author(s):  
Yang Guo ◽  
Hongbin Ma ◽  
Benwei Fu ◽  
Yulong Ji ◽  
Fengmin Su ◽  
...  
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Phase Change ◽  
Thin Liquid Film ◽  
Heat Transfer Analysis ◽  
Seawater Desalination ◽  
Flash Evaporation ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Evaporation Heat

Several seawater desalination technologies have been developed and widely used during the last four decades. In the current investigation, a new approach to the seawater desalination process is presented, which utilizes microencapsulated phase change materials (MEPCMs) and thin film evaporation. In this process, the MEPCMs were placed into hot seawater. Then, the hot seawater and the MEPCMs containing the liquid phase change material (PCM) were ejected into a vacuum flash chamber. A thin liquid film of seawater was formed on the surface of the MEPCM, which subsequently vaporized. This evaporation significantly increased the evaporation heat transfer and enhanced the desalination efficiency. Film evaporation on MEPCM surfaces decreased their temperature by absorbing sensible heat. If their temperature was lower than the phase change temperature, the MEPCM would change phase from liquid to solid releasing the latent heat, resulting in further evaporation. The MEPCMs were then pumped back into the hot seawater, and the salt residue left on the MEPCMs could be readily dissolved. In this way, the desalination efficiency could be increased and corrosion reduced. A mathematical model was developed to determine the effects of MEPCM and thin film evaporation on desalination efficiency. An analytical solution using Lighthill's approach was obtained. Results showed that when MEPCMs with a radius of 100 µm and a water film of 50 µm were used, the evaporation rate and evaporative capacity were significantly increased.

Falling Films in Micro- and Minigrooves: Heat Transfer and Flow Stability

2003 ◽  
Author(s):  
Tatiana Gambaryan-Roisman ◽  
Peter Stephan
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Liquid Film ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Film Flow ◽  
Wall Structure ◽  
Falling Film ◽  
Thin Film Evaporation ◽  
Film Evaporation

Structured (in particular, micro- and minigrooved) wall surfaces may improve numerous industrial processes, including falling film evaporation, thin film evaporation in lean premixed prevaporized combustion technology (LPP), and spray and jet cooling. The advantages of such surfaces include the promotion of ultra-thin film evaporation at the apparent contact lines and the prevention of dry patches on hot surfaces. However, the behavior of thin film flow on structured surfaces has not yet been comprehensively studied. We derive a model describing the heat transfer in liquid film flowing down inclined micro- or minigrooved walls. The derived model accounts for peculiarities of the evaporation process in the vicinity of the liquid-vapor-solid contact line (“micro region”) and their effect on the overall heat transfer rate. It is shown that the effect of the micro region is to increase the overall heat transfer rate at the constant fluid flow rate. A long-wave stability analysis has been performed to quantify the effect of the capillary structure on the film stability properties. Sinusoidal and triangular longitudinal groove shapes have been considered. Two cases have been studied: (i) the film completely covers the wall structure; (ii) the film partly covers the wall structure. It is shown that the longitudinal grooves completely covered by the liquid have a stabilizing effect on the falling film flow. The performed analysis is a step towards modeling the wavy motion of the liquid film on grooved surfaces.

Experimental Study on the Effect of Viscous Heating on the Temperature Field in Microchannel Gas Flow

2009 ◽  
Author(s):  
T. T. Zhang ◽  
L. Jia ◽  
C. W. Li ◽  
Y. Jaluria
Keyword(s):  
Heat Transfer ◽  
Temperature Field ◽  
Gas Flow ◽  
Superposition Principle ◽  
Slip Flow ◽  
Experimental System ◽  
Flow Region ◽  
Viscous Heating ◽  
Flow And Heat Transfer ◽  
Set Up

An experimental system for single-phase gas flow in microchannels was set up. The effects of viscous heating on the temperature field in the flow were studied experimentally. Also, a theoretical model for the flow and heat transfer in the slip flow region was developed and the resulting equations were solved analytically by using a method based on the superposition principle. The results obtained agree well with the experimental data. The study also provides greater insight into microchannel flow and the associated heat transfer.

Characteristics of Extended Evaporating Meniscus on Nanotextured Rough Solid Substrate for Wenzel States

2011 ◽  
Author(s):  
J. J. Zhao ◽  
Y. Y. Duan ◽  
X. D. Wang ◽  
B. X. Wang
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Liquid Film ◽  
Thin Liquid Film ◽  
Disjoining Pressure ◽  
Roughness Effects ◽  
Maximum Heat ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Nanoscale Roughness

The surface nanostructure determines the system wettability and thus has significant effects on the thin liquid film spreading and phase change heat transfer. A model based on the augmented Young-Laplace equation and kinetic theory was developed to describe the nanoscale roughness effects on the extended evaporating meniscus in a microchannel. The roughness geometries in the model were theoretically related to the disjoining pressure and the thermal resistance across the roughness layer. The results show that the dispersion constant for the disjoining pressure increases with the nanopillar height when the solid-liquid-vapor system is in the Wenzel state. Thus, the spreading and wetting properties of the evaporating thin liquid film are enhanced due to the higher nanopillar height and larger disjoining pressure. Since the evaporating thin film length increases with the nanoscale roughness due to better surface wettability, the total liquid flow and heat transfer rate of the evaporating thin liquid films in a microchannel can be enhanced by increasing the nanopillar height. The effects of the nanopillar on the thin film evaporation are more significant for higher superheats. Hydrophilic nanotextured solid substrates can be fabricated to enhance the thin film evaporation and thus increase the maximum heat transport capability of the two-phase cooling devices.

Numerical and Experimental Analysis of Convection Heat Transfer in a Lean-To Type Greenhouse

Solar Energy ◽  
2004 ◽  
Author(s):  
Wei Chen ◽  
Wei Liu
Keyword(s):  
Heat Transfer ◽  
Mathematical Model ◽  
Moisture Content ◽  
Gas Flow ◽  
Solar Irradiation ◽  
Wall Material ◽  
Insulation Material ◽  
Remarkable Effect ◽  
The North ◽  
North Wall

In this paper, heat transfer and flow in a lean-to passive solar greenhouse has been studied. A mathematical model based on energy equilibrium and a one-dimensional mathematical model for the unsaturated porous medium have been founded and developed to predict the temperature and moisture content in soil and the enclosed air temperature in the greenhouse. On the condition that plant and massive wall is neglected, the air is mainly heated by the soil surface in the greenhouse, which absorbs the incident solar radiation. With increase in depth, the variation of the temperature and moisture content in soil decreases on account of ambient, and the appearance of the peak temperature in soil postpones. Solar irradiation absorber, heat storage and insulation are the main effects of the north massive wall on greenhouse, which is influenced by the structure and the material. The specific heat capacity and thermal conductivity of wall material have a remarkable effect on the north wall temperature. The build-up north wall with thermal insulation material may be chosen for greenhouse. The temperature distribution and gas flow in greenhouse is influenced by the cover material of the inside surface of the north wall. All results should be taken into account for a better design and run of a greenhouse.

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