Flow Patterns and Heat Transfer in Thin Liquid Films on Walls With Straight, Meandering and Zigzag Mini-Grooves

2008 ◽  
Author(s):  
Karsten Lo¨ffler ◽  
Hongyi Yu ◽  
Tatiana Gambaryan-Roisman ◽  
Peter Stephan
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Flow Patterns ◽  
Liquid Films ◽  
Thin Liquid Films ◽  
Transport Processes ◽  
Temperature Fields ◽  
Film Stability ◽  
Falling Films ◽  
Falling Liquid Films

Thin liquid films flowing along solid walls are widely used in technological applications in which high rates of heat and mass transport are required. The transport processes can be further intensified by using structured walls. In the present work hydrodynamics and heat transfer in falling liquid films on heated vertical and inclined walls with mini-grooves are studied experimentally and theoretically/numerically. The experiments are performed with straight, meandering and zigzag mini-grooves. The film dynamics is investigated using a confocal chromatic sensoring (CHR) technique. The flow patterns and the temperature of the liquid-gas interface are visualized using the high-speed infrared thermography. The wall temperature distribution is measured with thermocouples. A numerical model for description of the velocity and temperature fields in the thermal entrance region of the falling films on smooth and structured walls is developed. This model is based on the solution of the Graetz-Nusselt problem for falling films on grooved plates. We show that the mini-grooves significantly affect the flow patterns, film stability and heat transfer in falling liquid films. Using grooved walls leads to the increase of the maximal attainable heat transfer rate.

Study of the Fluid Dynamics of Thin Liquid Films Flowing Down a Vertical String With Counterflow of Gas

2015 ◽  
Author(s):  
Zezhi Zeng ◽  
Gopinath Warrier ◽  
Y. Sungtaek Ju
Keyword(s):  
Heat Transfer ◽  
Fluid Dynamics ◽  
Film Thickness ◽  
Liquid Film ◽  
Direct Contact ◽  
High Speed ◽  
Small Diameter ◽  
Liquid Films ◽  
Thin Liquid Films ◽  
Liquid Coolant

Direct-contact heat transfer between a falling liquid film and a gas stream yield high heat transfer rates and as such it is routinely used in several industrial applications. This concept has been incorporated by us into the proposed design of a novel heat exchanger for indirect cooling of steam in power plants. The DILSHE (Direct-contact Liquid-on-String Heat Exchangers) module consists of an array of small diameter (∼ 1 mm) vertical strings with hot liquid coolant flowing down them due to gravity. A low- or near-zero vapor pressure liquid coolant is essential to minimize/eliminate coolant loss. Consequently, liquids such as Ionic Liquids and Silicone oils are ideal candidates for the coolant. The liquid film thickness is of the order of 1 mm. Gas (ambient air) flowing upwards cools the hot liquid coolant. Onset of fluid instabilities (Rayleigh-Plateau and/or Kapitza instabilities) result in the formation of a liquid beads, which enhance heat transfer due to additional mixing. The key to successfully designing and operating DILSHE is understanding the fundamentals of the liquid film fluid dynamics and heat transfer and developing an operational performance map. As a first step towards achieving these goals, we have undertaken a parametric experimental and numerical study to investigate the fluid dynamics of thin liquid films flowing down small diameter strings. Silicone oil and air are the working fluids in the experiments. The experiments were performed with a single nylon sting (fishing line) of diameter = 0.61 mm and height = 1.6 m. The inlet temperature of both liquid and air were constant (∼ 20 °C). In the present set of experiments the variables that were parametrically varied were: (i) liquid mass flow rate (0.05 to 0.23 g/s) and (ii) average air velocity (0 to 2.7 m/s). Visualization of the liquid flow was performed using a high-speed camera. Parameters such as base liquid film thickness, liquid bead shape and size, velocity (and hence frequency) of beads were measured from the high-speed video recordings. The effect of gas velocity on the dynamics of the liquid beads was compared to data available in the open literature. Within the range of gas velocities used in the experiments, the occurrence of liquid hold up and/or liquid blow over, if any, were also identified. Numerical simulations of the two-phase flow are currently being performed. The experimental results will be invaluable in validation/refinement of the numerical simulations and development of the operational map.

Capillary Flow Separation in 2- and 3-Dimensional Laminar Falling Liquid Films

2010 ◽  
Author(s):  
Georg F. Dietze ◽  
Reinhold Kneer
Keyword(s):  
Heat Transfer ◽  
Flow Separation ◽  
Capillary Flow ◽  
Capillary Wave ◽  
Liquid Films ◽  
Transport Processes ◽  
Gravitational Acceleration ◽  
3 Dimensional ◽  
Wave Region ◽  
Falling Liquid Films

Due to the selective use of liquid films in specialized technical equipment (e.g. new generation nuclear reactors), a fundamental understanding of underlying momentum and heat transport processes inside these thin liquid layers (with a thickness of approximately 0.5 mm) is required. In particular, the influence of surface waves (which develop due to the film’s natural instability) on these transport processes is of interest. For a number of years, experimental and numerical observations in wavy falling liquid films have suggested that momentum and heat transfer in the capillary wave region, preceding large wave humps, undergo drastic modulations. Indeed, some results have indicated that upward flow, i.e. counter to the gravitational acceleration, takes place in this region. Further, evidence of a substantial increase in wall-side and interfacial transfer coefficients has also been noted. Recently, Dietze et al. [1,2] have established that flow separation takes place in the capillary wave region of 2-dimensional laminar falling liquid films, partially explaining the above mentioned observations. Thereby, it was shown that the strong third order deformation (i.e. change in curvature) of the liquid-gas interface in the capillary wave region causes an adverse pressure gradient sufficiently large to induce flow detachment from the wall. In the present paper, a detailed experimental and numerical account of the capillary flow separation’s kinematics and governing dynamics as well as its effect on heat transfer for two different 2-dimensional flow conditions is presented. Experimentally, velocity measurements (using Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV)) and film thickness measurements (using a Confocal Chromatic Imaging technique) were performed in a specifically designed optical test setup. On the numerical side, simulations of the full Navier-Stokes equations as well as the energy equation using the Volume of Fluid (VOF) method were performed. In addition to the 2-dimensional investigations, the characteristics of capillary flow separation under 3-dimensional wave dynamics were studied based on the 3-dimensional numerical simulation of a water film, which was previously investigated experimentally by Park and Nosoko [3]. Results show that flow separation persists over a wide area of the 3-dimensional capillary wave region, with multiple capillary separation eddies occurring in the shape of vortex tubes. In addition, strong spanwise flow induced by the same governing mechanism is shown to occur in this region, which could explain the drastic intensification of transfer to 3-dimensional liquid films.

Boiling Heat Transfer for Freon R21 in Rectangular Minichannel

2006 ◽  
Author(s):  
V. V. Kuznetsov ◽  
A. S. Shamirzaev
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Flow Patterns ◽  
Liquid Films ◽  
Thin Liquid Films ◽  
Modified Model ◽  
Wide Range ◽  
Nucleation Boiling ◽  
The Heat Transfer Coefficient ◽  
Transfer Mechanisms

In this paper we study the boiling heat transfer of upward flow of R21 in a vertical mini-channel with size 1.6×6.3 mm. The heat transfer coefficient was measured as a function of heat flux for a wide range of vapor quality and for two levels of mass flow rate, G = 215 kg/m2s and G = 50 kg/m2s. The temperature dispersions over channel perimeter and in time were calculated. Different heat transfer mechanisms were revealed for different flow patterns. We distinguish the dominant nucleation boiling and the joint mode of nucleation boiling and convective evaporation. We also found the modes when the evaporation of thin liquid films makes the main contribution to heat transfer. The modified model of Liu and Winterton describes the experimental data for the flow patterns when the nucleation boiling and convective transfer make the most contribution to the heat transfer.

HEAT TRANSFER IN THIN LIQUID FILMS FLOWING DOWN HEATED INCLINED GROOVED PLATES

2010 ◽  
Vol 2 (5) ◽  
pp. 455-468 ◽  
Author(s):  
Hongyi Yu ◽  
Karsten Loffler ◽  
Tatiana Gambaryan-Roisman ◽  
Peter Stephan
Keyword(s):  
Heat Transfer ◽  
Liquid Films ◽  
Thin Liquid Films

scholarly journals Influence of Vibration on Diffusion in Liquid Mixtures On-Board the International Space Station

2021 ◽  
Author(s):  
Aram Parsa
Keyword(s):  
Heat Transfer ◽  
Numerical Analysis ◽  
Physical Properties ◽  
International Space Station ◽  
Diffusion Coefficients ◽  
Space Station ◽  
Flow Patterns ◽  
Transport Processes ◽  
International Space ◽  
Different Levels

Experiments on-board the International Space Station experience a convective flow due to the oscillatory g-jitters induced by several sources such as crew activities, mechanical systems, thrusters firing, spacecraft docking, etc. Although g-jitter seems to have a major impact on diffusion-related experiments in Space, very few experimental studies have addressed this topic. This study examined the effect of oscillatory g-jitters on transport processes (fluid flow, heat transfer and mass transfer). Cubic rigid cells filled with water and isopropanol at different concentrations were subjected to thermal gradients and forced vibrations. The cells were exposed to different levels of vibration in terms of frequency and amplitude, which were applied perpendicular to the temperature gradient. The full transient Navier Stokes equations coupled with the mass and heat transfer formulas were solved numerically using the control volume technique. The physical properties of the fluid mixture such as the density were determined using two different models. The effect of different levels of vibration on the flow was analysed and the results were compared in a benchmark study with other scientific groups. The effect of the diffusion coefficients variation and other physical properties on the temperature and concentration distribution was compared to those results obtained with constant diffusion coefficients. Results show that use of variable physical properties in the modelling produces different flow patterns and component concentration. By examining different flow patterns, it was found that the effect of using variable coefficients is much more significant in the cases with high Rayleigh vibration that result in strong flow when compared with numerical analysis using constant variables. The numerical analysis was also performed for the actual experiment on board the International Space Station. The same trend was seen for both the numerical and experimental results. However, the separation of components was higher in the experiment in comparison with the numerical analysis. This was discussed in detail for various scenarios in terms of the applied frequency and amplitude. Recommendations are made according to the findings from this study for the improvement of accuracy in the numerical and experimental analyses of future diffusion experiments in Space.

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