mass transfer coefficient in the case of brake-up of a continuous falling film. - stainless steel name plate

by:ShunDing     2019-10-15
mass transfer coefficient in the case of brake-up of a continuous falling film.  -  stainless steel name plate
Introduce thin, falling liquid films encountered in many industrial applications (e. g.
Distillation facilities, cooling tower, vertical tube evaporator, falling film chemical reactor, food processing facilities, etc. )
Due to their high efficiency in heating and heating.
Critical conditions occur during the start-up of the facility-
Because a large amount of liquid is needed to completely cover the dry surface with a continuous liquid film.
When continuous liquid film is formed, it is very important to keep the minimum mass flow rate of the fluid above its critical value to prevent the surface from drying. Liquid-
The flow rate corresponding to these states is called the minimum wet rate, and the film thickness of the response is called the minimum film thickness [1].
The theoretical model used to predict the minimum moistening and the corresponding film thickness is mainly based on two criteria: the force balance of the stationary point and the minimum total energy of the rivulets [1-5].
In three states, the constant temperature distilled water on the vertical plate breaks down the film (
Wet, wet and broken dry plates-
Up of continuous falling films)
Check in [6]and [7].
Influence of contact angle (
Static and dynamic)
The effect of the minimum humidity and the angle of the plate tilt [is determined]7].
In the process of air flowing at the countercurrent orco-
The current of the membrane flow, the problem is described as two-
Phase flow, and studies have been carried out in many experimental and theoretical studies [8].
Effect of airflow on thin film brakes-
The up process and related parameters were studied in [theory]9].
In the proposed experiment, the influence of the counter
Current airflow associated with two parameters-
In the case of a thin film brake, the phase flow accurately on the masstransfer coefficient-Check it out.
Minimum flow rate in case of thin film brake-
In the absence of air flow, the rising speed is very low, because the measured back contact point is zero, which indicates a high adhesion force between the plate and the liquid.
Therefore, the falling film remains stable even at a very small flow rate (
Liquid Reynolds number less than 1).
Experimental results of this liquid
The board material combination shows that the minimum moisture rate, dry surface area and mass transfer coefficient change suddenly in the case of braking
When the counter
Current flow is introduced. 2.
Experimental settings the experimental settings used in this exam are shown in the figure1.
It consists of two lines: the empty line and the clearing line, as shown in Figure 1.
These two lines are combined in the test section, and in the test section, the two fluids directly contact the exchange quality in the counter. current flow.
The air is supplied by the compressor.
The following equipment is installed on the air pipe line: air purifier, pressure regulator, flow regulator and-
Line flow meter.
Air purifiers are installed to prevent equipment from scale.
Presureregulator is used to reduce the pressure from the 7 rods supplied by the compressor to the expected value.
The flow regulator is consistent with the flowmeter to control the air flow at the entrance of the test section.
The water supply line leads from the liquid reservoir to the target surface through the dispenser.
The laboratory pumps the working fluid into the dispenser.
In order for the liquid to spread better on the plate, a narrow strip of filter paper is placed on the higher edge of the plate.
The narrow target plate is placed vertically or reclined on the stand.
The stainless steel sheet size is 500x100 and the wet length is 410mm.
The plates are sandblasted and Table 1 gives an average.
The working solution is distilled water;
The water temperature measured in the inlet section is 12 [degrees]C.
Experiments were carried out in the vertical upward air
The water flow inside the enclosed vertical rectangular channel.
Figure 1 shows a schematic diagram of the loop. 1.
The experimental procedure is unified for all measurements.
The plate was completely soaked before, and then the air was introduced into the channel.
The water flow is set on a constant value and the air flow rate is gradually reduced in order to form a dry zone on the plate.
The degree of immersion surface was observed within two and a half hours, as this required a complete measurement process.
Mass transfer occurs in direct contact when the fluid flows upstream. 3.
Fluid Dynamics of flow (observation)
The developed experimental procedure ensures an analysis of the effects of flow and airflow on the wet surface reduced to standard conditions, defined as the ratio of the wet surface to the overall flat surface.
According to the flow calculation measured on the flowmeter, the flow rate corrected to the standard condition is as follows :[[? ? ]. sub. A,s]= [[? ? ]. sub. A][
Square root of PA, out Pat/1,013288, 15/173, 15 [t. sub. A,out](1)where [? ? ]
Is the volume air flow rate measured on the flowmeter ,[p. sub. A,out]
What is the pressure at the entrance of Testsection [p. sub. at]
It's atmospheric pressure and [t. sub. A,out]
As shown in the figure, the temperature at the inlet of the flowmeter. 1.
The pressure at the outlet of the flowmeter is measured using a water meter (U-tube). In Fig. 2.
A set of measurements is displayed.
The liquid flow rate is constant at 1 mL/min, which indicates that the flow rate is higher than the minimum wet rate without air flow rate.
The measurement duration is two and a half hours.
When the air is supplied to the test panel, de-
Wet rapidly (Fig. 2a. ). De-
The wet area increases with the increase of time, which can be seen in the fig. 2 (b-c).
Finally, as shown in the figure, two narrow cracks are formed2. d.
The degree of wet surface is a function of the time required to perform a complete measurement program, as shown in the figure2.
In order to ensure a stable state and parameters, it is concluded that the time cannot be less than two and a half hours. Fig. 3.
It shows a significant effect of liquid flow rate on the degree of wet surface.
It is worth noting that at the same air flow rate, the degree of wet surface increases as the liquid flow rate increases.
For smaller flow rates, only about 30% of the plates are soaked.
For a small flow rate, this small portion of the wet surface confirms previous observations of a liquid Microfilm covering the surface of the sandblasted hydrophilic surface of the plate, which dries at the beginning of the airflow.
The average wind speed of the Test segment ranges from 0,548 m/s to 0,736 m/s. 4.
Two analysis considerations
Considering the phase flow, the infinite number is used: the Sherwood number is used as the representative of mass transfer, and the Reynolds number is used as the representative of the fluid dynamics of the flow.
Similar experiments conducted on this experimental line show that sensitive heat transfer is much smaller than potential heat transfer, which results in the following relationships [10]: Splate-
Liquid material with zero back contact angle and high adhesion force.
For different plate surface treatments, other contact points of the film brake and liquid flow rate-Measure up.
Similar experiments were conducted on other materials and surface treated plates, and the study was conducted on the-
Current flow. DOI: 10. 2507/27th. daaam. proceedings. 037 7. References [1]M. S. El-Genk and H. H. Saber (2001).
The minimum thickness of the vertical surface extending down, Int. J.
Heat Transfer, 44: 2809-2825 [2]D. E. Hartely, W. Murgatroyd (1964).
Standard of rest-
Up, Int of a thin liquid layer that flows in heat such as the solid surface. J.
Heat Transfer, 7: 1003-1015 [3]T. Hobler (1964).
The smallest surface is wet and chemical. Stosow,2B:145-159 [4]S. G. Bankof (1971).
Minimum thickness of drain liquid film, Int. J.
Heat Transfer, 14: 2143-2146 [5]J.
Mickey leviz and J. R. Moszynski (1976).
The minimum thickness of the liquid film flowing vertically down the solid surface, Int. J.
19: 771-hot MassTransfer776 [6]D. Moalem-Maron, N. Brauner (1983).
Transmission properties of wave film on inclined surface, two kinds of progress
Phase flow and heat transfer, coil. 1:155-181 [7]Dz. Kadric, M. Alispahic, S. Sikalo, S. Jakirlic and E. N. Ganic (2015)
E181, record of eight international workshops on turbulence, heat transfer and mass transfer, K. Hanjalic, T. Miyauchi, D. Borello. M. Hadziabdic, P. Venturini (Editors); ISBN: 978-1-56700-428-
8. Berger House[8]Sikalo, Sefko;
Berberovic, Aden (2014).
Analysis of deposition in vertical air
Water dispersed flow, record of 25 DAAAM International Symposium, pagexxxx-xxxx, B. Katalinic (Ed. )
Publishing DAAAM international book number 9783-902734-08-2, ISSN1726-
9679 Vienna, Austria [9]M. S. El-Genk, H. H. Saber (2004). On the break-
Up of thin liquid film affected by interface shear, J. Fluid Mech. 501:113-133 [10]Kadric, Dz. , Sikalo, S. , Delalic, N. and Ganic, E. N. (2007).
Experimental study on the flow film of the sixth international multi-phase flow conference, ISBN 978-3-86010-913-7 description: Figure1.
Experimental setting instructions: Figure2.
Plate drying over time: water flow rate 1 mL/min, average wind speed [u. sub. A]= 0,566 m/s (
Duration of the experiment [tau]= 2,5 h)Caption: Fig. 3.
Wetting properties of flat plates at different volume liquid flow rates and mean wind speeds [u. sub. A]
Title: Figure = 0,552/second4.
According to the ratio of liquid to air flow rate, the degree of wet surface Title: Fig. 5.
Mass transfer coefficient calculated according to the liquid film Reynolds number header: Fig. 6.
According to the ratio between liquid and air volume flow rate, the percentage of evaporation liquid in the net liquid flow rate.
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