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Experimental investigation of condensate retention on horizontal pin fin tube with varying pin angle
Journal Pre-proof Experimental investigation of condensate retention on horizontal pin fin tube with varying pin angle Haseeb Ur Rehman, Hafiz Muhammad Ali, Sajjad Ahmad, Mansoor A. Baluch PII:
S2214-157X(19)30382-X
DOI:
https://doi.org/10.1016/j.csite.2019.100549
Reference:
CSITE 100549
To appear in:
Case Studies in Thermal Engineering
Received Date: 18 September 2019 Revised Date:
12 October 2019
Accepted Date: 15 October 2019
Please cite this article as: H.U. Rehman, H.M. Ali, S. Ahmad, M.A. Baluch, Experimental investigation of condensate retention on horizontal pin fin tube with varying pin angle, Case Studies in Thermal Engineering (2019), doi: https://doi.org/10.1016/j.csite.2019.100549. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Experimental investigation of condensate retention on horizontal pin fin tube with varying pin angle Haseeb Ur Rehmana, Hafiz Muhammad Alia*, Sajjad Ahmada, Mansoor A. Baluchb a
Mechanical Engineering Department, University of Engineering and Technology, 47050 Taxila Pakistan b
University of Engineering and Technology, 47050 Taxila Pakistan * [email protected]
Abstract This paper presents the effect of pin fin rotational angle on the condensate retention as function of vapour velocity (0-20m/s) on four different pin fin tubes, by systematically changing the pin fin angle from 0o – 45o with horizontal axis. While other parameters of pin fin tubes like tooth height, tooth thickness, longitudinal spacing inner and outer diameter remain constant for all 4 tubes tested. Three fluids (2-propanol, distilled water and ethylene glycol) with varying surface tension to density ratio are tested. Experimentation is conducted in a vertical wind tunnel by downward supply of air (to simulate vapor) with velocity from 0 - 20 m/s. By accurate small holes on upper side of the tube a constant flow of condensate was assured on a tube to be tested. At almost all vapour velocities; it is observed that retention angle increases with increase in pin fin angle for all fluids tested. As the vapour velocity increased from 0 to 20 m/s, it is observed that the retention angle for tube A1 increased for all test fluids, whereas with increasing velocity, a decrease in retention angle is observed for tubes A2, A3 and A4 for all fluids tested.
Keywords: Condensation Heat Transfer; Condensate Retention, Retention Angle, Pin fin tubes with varying pin angle; Vapour Velocity.
Introduction
Figure 1a: Schematic diagram of pin tube with condensate
Figure 1b: Representation of retention angle
Condensation phenomenon plays an important role in industry such as petroleum & chemical process, refrigeration plants, power plants, as all these plants require a condenser. For this purpose, to reduce the capital cost of a condenser and to make more efficient and high heat transfer enhancement in condensation a continuous effort is required. In modern age, two types of surfaces are generally used for the purpose of condensation of vapors. These are wetted and non-wetted surfaces. On non-wetted surfaces, heat transfer rate is much better as compare to wetted surface. Film-wise condensation is mostly practiced in industrial applications because of low capital cost over non-wetted surfaces. Many researches are reported on filmwise condensation of various fluids on simple horizontal tubes [1-4]. Kharkhu and Borovkhov [1], Mills et al. [2], Beatty and Katz [3], Carnavos [4] experimentally investigated that by increasing the surface area of the tube, heat transfer coefficient was also increased. Wanniarachchi et al. [5] experimentally observed the phenomena of film wise condensation on 6 copper horizontal tubes with external rectangular fins. The thickness and height was kept constant at 1 mm and fin spacing was varied as 9, 4, 2 , 1.5, 1, and 0.5 mm. Test was performed under the vacuum and atmospheric condition. They found maximum enhancement in heat transfer on a tube which was having a fin spacing of 1.5 mm. In industrial condensers, where there is enough vapour velocity, vapour velocity play an important role in enhancement of heat transfer rate in several cases. A systematic study of retention angle measurement on two-dimensional integral-fin tubes by using different types of test fluids was reported by Briggs and coworkers [6-8]. In present days, researcher are more focused on three-dimensional pin fin tubes that are much complex as compared to two dimensional integral fin tubes. It is now discovered that the surface tension can play an important role in drainage of condensate retention. Retention (which is measured by a retention angle, see Figure 1b) significantly affect the heat transfer rate during condensation. Heat transfer rate can be increased by increasing this retention angle (unflooded surface) by varying the pin angle (see Figure 1 for a simple pin fin tube with condensate retention). The shear force of vapour has also significant effect on condensation heat transfer, but limited data is available. The effect of vapour velocity causes an increase in heat transfer due to vapour shear effect on the upper half of the tube. Briggs [7] observed the phenomena of condensate retention when he performed an experimentation on 9 integral fin tubes with various test fluids i.e. ethylene glycol, water and R113. He used a fine spray in experimental setup under the static condition. The experimentation
was performed by two methods which were pin counting and photographic methods. It was concluded that by increasing pin spacing retention was decreased on the lower part of the tube. Honda and Nozu, [9, 10] reported experimental data on the horizontal fin tubes and in-line bundle of horizontal finned tubes for film wise condensation by using R113. Cavallini [11], Bella et al.[12] reported work on the enhancement of condensation heat transfer by using refrigerant R11 & R113. Chen and Yang [13, 14] experimentally investigates the enhancement of heat transfer coefficient on 10mm diameter pipe by using steam ethanol mixture, they observed that by varying vapour velocity heat transfer rate also increased. Ali et al. [15] worked experimentally on the low finned tubes by steam ethanol mixture for enhancement of heat transfer rate. Briggs et al. [16] experimentally reported data for measuring the liquid retention and enhancement of heat transfer rate on pin fin tubes. Baiser and Briggs [17] investigated the effect of varying pin spacing and circumeperial pin thickness on the pin fin tubes, they observed the strong effect on condensate retention by varying the pin spacing and thickness. Ali and Abu baker [18] used R113, ethylene glycol and water on enhanced finned tubes to investigate condensate retention. Fin height, root diameter and thickness was fixed. Ali and Abu baker [19] investigated the effect on condensate retention by varying the geometry of pin-fin tubes and compared with the equivalent integral fin tube, they found strong effect on the retention angle by varying the different geometry parameters and vapour velocity. Experimental work and empirical models on horizontal tubes are reported by various researchers [20-26]. Various important applications where such tubes are used are given in [27-32]. From above review of literature, it is concluded that only few studies are available on threedimensional pin fin tubes that investigated the behavior of condensate retention under the effect of vapor velocity by changing the parameters like spacing between pins, pin thickness and surface area. Another most important geometric parameter that can influence the condensate retention behavior on pin-fin tubes is the pin angle with horizontal axis on horizontal pin fin tubes; yet no research has been reported for this parameter as a function of vapor velocity. This research paper attempts a systematic study to examine the effect of angle of pin on retention angle as a subject of varying vapor velocity. For this purpose, vertical wind tunnel is used (Figure 2a) and four different tubes with varying pin angle are used as shown in Figure 2b.
Figure 2a: Schematic diagram of experimental setup
In this research, the effect of pin fin angle on the condensate retention as function of vapor velocity (0 - 20m/s) on 4 pin fin tubes, by varying pin-fin angle from 0 – 45 degree is examined. While other parameters of pin fin tubes like tooth height, tooth thickness, longitudinal spacing inner and outer diameter were kept same for all four pin-fin tubes (Table 1). Three fluids (2propanol, ethylene glycol and distilled water) with low, intermediate and high surface tension to density ratio were examined. Experimentation was conducted on the vertical wind tunnel by downward supply of air (to simulate vapor) through vertical wind tunnel with velocity from 0 to 20 m/s. By making small holes on upper side of tube a constant flow of condensate was assured on a tube to be tested as shown in Figure 2b.
Figure 2b:A1= 0o, A =15o, A3=30o, A =45o (left to right)
Figure 3 (a): Condensate flooding on test tube using water as a test fluid
Figure 3(b): Condensate flooding on test tube using ethylene glycol as a test fluid
Figure 3(c): Condensate flooding on test tube using 2-propanol as a test fluid
Table 1 : dimensions of pin fin tubes (mm) Tubes A1 A2 A3 A4
Angle (degree) 0 15 30 45
do
dr
di
s
h
t
15.9 15.9 15.9 15.9
12.7 12.7 12.7 12.7
8.5 8.5 8.5 8.5
1 1 1 1
1.6 1.6 1.6 1.6
1 1 1 1
Methodology of experimentation: Schematic diagram of experimental setup is shown in Figure 2a. Experimentation is performed on pin-fin tubes by using distilled water, ethylene glycol and 2-propanol in a vertical wind tunnel. A hand operated damper was installed on the wind tunnel to control the air velocity. Four different pin-fin tubes with varying different pin angle (with horizontal plane) were used in experimentation. Tubes were horizontally mounted in vertical wind tunnel. Tubes were facing cross downward flow of air generated by blower placed at the bottom of experimental setup. Fluid (to simulate condensate) was supplied from holes drilled (0.5mm dia) at the top of the tubes between longitudinal pin-fin spacing. Every tube was closed from one end and other end was kept opened. While opened end was attached to pipe which was connected to fluid reservoir. Vapour velocity was measured with a hot wire anemometer sensor inserted at the center of the wind tunnel. Fluid flow rate was adjusted as per requirement through holes to assure uniform condensate film along the tube. Distilled water, 2-propanol and ethylene glycol were used as test fluids to generate condensate film. After establishing smooth condensate film on the tube, retention angles for different vapor velocities 0 - 20 m/s were measured with the help photographic method. A Green dye was added in all three-testing fluid to get a clear image of
condensate retention starting point via photographic method. An arrow is also placed in Figure 3 (a, b, c) to indicate the condensate retention point under the influence of vapor velocity. Two digital cameras were used to capture the flooding, each on both sides of the tube. These two digital cameras were fixed on the two individual platforms on both sides of the wind tunnel. These cameras were firmly fixed to eliminate vibrations. Platform were tightly attached with the wall of the experimental setup. Retention angle for each testing fluid obtained from these two cameras (photographic method) are demonstrated in Figure 4 (a, b & c) for better understanding.
Figure 4a: Retention angle cam 1 vs cam 2 for water
Figure 4b: Retention angle cam 1 vs cam 2 for ethylene Glycol
Figure 4c: Retention angle cam 1 vs cam 2 for 2-Propanol
Results and Discussions: By varying pin-fin angle systematically on 4 horizontal pin fin tubes namely A1, A2, A3, A4 (corresponding to 0o ,15o, 30o, 45o pin angle with horizontal plane respectively), condensate retention as function of vapor velocity was observed when tested with three different fluids i.e. distilled water, ethylene glycol, and 2-propanol. The effect of vapour velocity on all four pin-fin tubes with all fluids was investigated. It was observed that for tube A1 (where the retention angle is less than 90 degree) in case of distilled water (Figure 5a) by increasing the vapour velocity retention angle is also increased from 62o to 72 o. For other three tubes (A2, A3, A4), retention angle decreases with increase in vapour velocity. However, it should be noted that all over the range of Reynolds number, retention angle increased from tube A1 to A4, which shows that an increasing pin angle from 0o to 45o is considerably helpful to reduce the condensate retention form the tube translating more heat transfer during condensation. This effect of increase in retention angle is more dominant at vapour velocity approaching zero (free convection condensation).
Figure 5a: Retention angle vs Reynold Number for water as a function of vapour velocity
When ethylene glycol is used as a test fluid, same behavior was observed (as in the case of water) however, for ethylene glycol, higher retention angles (Figure 5b) as compared with water are observed, this could be due to the surface tension to density ratio which also influences on retention angle, as ethylene glycol have lower surface-tension to density ratio than water.
Figure 5b: Retention angle vs Reynold Number for ethylene glycol as a function of vapour velocity
Similarly, when 2-propanol is used as a test fluid, similar behavior is observed with even higher retention angles (Figure 5c) as compare to water and ethylene glycol due to lower surface tension to density ratio of 2-propanol than water and ethylene glycol.
Figure 5c: Retention angle vs Reynold Number for 2-propanol as a function of vapour velocity
Conclusion: Effect of pin-fin angle on condensate retention on horizontal pin fin tubes is examined as function of vapor velocity (0 - 20 m/s). Four pin-fin tubes are tested systematically by using 2propanol, ethylene glycol and water as low, intermediate and high surface-tension to density ratio fluids. Salient results are summarized below; Retention angle is a function of test fluid properties, tube geometry and vapor Reynolds number. Retention angle strongly depends upon the surface tension to density ratio of testing fluid. Over the range of vapor velocity; it is observed that retention angle increases with the increase in pin angle for all tested fluids. As the vapor velocity increases from 0 to 20 m/s, retention angle of tube A1 also increased for all test fluids, however, a decrease in retention angle is noted for tubes A2, A3 and A4 for all test fluids with increasing vapour velocity. All over the range of Reynolds number, retention angle increased from tube A1 to A4, which shows that an increasing pin angle from 0o to 45o is considerably helpful to reduce the condensate retention form the tube translating more heat transfer during condensation. This effect of increase in retention angle was more dominant at vapour velocity approaching zero (free convection condensation) for all fluids tested.
Nomenclature Test tube root diameter Test tube outer diameter h
radial pin or fin height
s
fin spacing at fin root
t
fin tip thickness or longitudinal pin tip thickness
Greek Letters Ø
Condensate flooding or retention angle measured from the top of the tube to the point at which the tube flank or integral fin is fully flooded.
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Karkhu, V. and V. Borovkov, Film condensation of vapor at finely-finned horizontal tubes. Heat Transfer-Soviet Research, 1971. 3(2): p. 183-191.
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Mills, A., et al., Experimental study of film condensation on horizontal grooved tubes. Desalination, 1975. 16(2): p. 121-133.
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Beatty, K.O., Condensation of vapors on outside of finned tubes. Chem. Eng. Prog., 1948. 44: p. 55-70.
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Ali, H.M. and M. Abubaker, Effect of vapour velocity on condensate retention on horizontal pin-fin tubes. Energy conversion and management, 2014. 86: p. 1001-1009.
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Conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this paper.
S2214-157X(19)30382-X
DOI:
https://doi.org/10.1016/j.csite.2019.100549
Reference:
CSITE 100549
To appear in:
Case Studies in Thermal Engineering
Received Date: 18 September 2019 Revised Date:
12 October 2019
Accepted Date: 15 October 2019
Please cite this article as: H.U. Rehman, H.M. Ali, S. Ahmad, M.A. Baluch, Experimental investigation of condensate retention on horizontal pin fin tube with varying pin angle, Case Studies in Thermal Engineering (2019), doi: https://doi.org/10.1016/j.csite.2019.100549. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Experimental investigation of condensate retention on horizontal pin fin tube with varying pin angle Haseeb Ur Rehmana, Hafiz Muhammad Alia*, Sajjad Ahmada, Mansoor A. Baluchb a
Mechanical Engineering Department, University of Engineering and Technology, 47050 Taxila Pakistan b
University of Engineering and Technology, 47050 Taxila Pakistan * [email protected]
Abstract This paper presents the effect of pin fin rotational angle on the condensate retention as function of vapour velocity (0-20m/s) on four different pin fin tubes, by systematically changing the pin fin angle from 0o – 45o with horizontal axis. While other parameters of pin fin tubes like tooth height, tooth thickness, longitudinal spacing inner and outer diameter remain constant for all 4 tubes tested. Three fluids (2-propanol, distilled water and ethylene glycol) with varying surface tension to density ratio are tested. Experimentation is conducted in a vertical wind tunnel by downward supply of air (to simulate vapor) with velocity from 0 - 20 m/s. By accurate small holes on upper side of the tube a constant flow of condensate was assured on a tube to be tested. At almost all vapour velocities; it is observed that retention angle increases with increase in pin fin angle for all fluids tested. As the vapour velocity increased from 0 to 20 m/s, it is observed that the retention angle for tube A1 increased for all test fluids, whereas with increasing velocity, a decrease in retention angle is observed for tubes A2, A3 and A4 for all fluids tested.
Keywords: Condensation Heat Transfer; Condensate Retention, Retention Angle, Pin fin tubes with varying pin angle; Vapour Velocity.
Introduction
Figure 1a: Schematic diagram of pin tube with condensate
Figure 1b: Representation of retention angle
Condensation phenomenon plays an important role in industry such as petroleum & chemical process, refrigeration plants, power plants, as all these plants require a condenser. For this purpose, to reduce the capital cost of a condenser and to make more efficient and high heat transfer enhancement in condensation a continuous effort is required. In modern age, two types of surfaces are generally used for the purpose of condensation of vapors. These are wetted and non-wetted surfaces. On non-wetted surfaces, heat transfer rate is much better as compare to wetted surface. Film-wise condensation is mostly practiced in industrial applications because of low capital cost over non-wetted surfaces. Many researches are reported on filmwise condensation of various fluids on simple horizontal tubes [1-4]. Kharkhu and Borovkhov [1], Mills et al. [2], Beatty and Katz [3], Carnavos [4] experimentally investigated that by increasing the surface area of the tube, heat transfer coefficient was also increased. Wanniarachchi et al. [5] experimentally observed the phenomena of film wise condensation on 6 copper horizontal tubes with external rectangular fins. The thickness and height was kept constant at 1 mm and fin spacing was varied as 9, 4, 2 , 1.5, 1, and 0.5 mm. Test was performed under the vacuum and atmospheric condition. They found maximum enhancement in heat transfer on a tube which was having a fin spacing of 1.5 mm. In industrial condensers, where there is enough vapour velocity, vapour velocity play an important role in enhancement of heat transfer rate in several cases. A systematic study of retention angle measurement on two-dimensional integral-fin tubes by using different types of test fluids was reported by Briggs and coworkers [6-8]. In present days, researcher are more focused on three-dimensional pin fin tubes that are much complex as compared to two dimensional integral fin tubes. It is now discovered that the surface tension can play an important role in drainage of condensate retention. Retention (which is measured by a retention angle, see Figure 1b) significantly affect the heat transfer rate during condensation. Heat transfer rate can be increased by increasing this retention angle (unflooded surface) by varying the pin angle (see Figure 1 for a simple pin fin tube with condensate retention). The shear force of vapour has also significant effect on condensation heat transfer, but limited data is available. The effect of vapour velocity causes an increase in heat transfer due to vapour shear effect on the upper half of the tube. Briggs [7] observed the phenomena of condensate retention when he performed an experimentation on 9 integral fin tubes with various test fluids i.e. ethylene glycol, water and R113. He used a fine spray in experimental setup under the static condition. The experimentation
was performed by two methods which were pin counting and photographic methods. It was concluded that by increasing pin spacing retention was decreased on the lower part of the tube. Honda and Nozu, [9, 10] reported experimental data on the horizontal fin tubes and in-line bundle of horizontal finned tubes for film wise condensation by using R113. Cavallini [11], Bella et al.[12] reported work on the enhancement of condensation heat transfer by using refrigerant R11 & R113. Chen and Yang [13, 14] experimentally investigates the enhancement of heat transfer coefficient on 10mm diameter pipe by using steam ethanol mixture, they observed that by varying vapour velocity heat transfer rate also increased. Ali et al. [15] worked experimentally on the low finned tubes by steam ethanol mixture for enhancement of heat transfer rate. Briggs et al. [16] experimentally reported data for measuring the liquid retention and enhancement of heat transfer rate on pin fin tubes. Baiser and Briggs [17] investigated the effect of varying pin spacing and circumeperial pin thickness on the pin fin tubes, they observed the strong effect on condensate retention by varying the pin spacing and thickness. Ali and Abu baker [18] used R113, ethylene glycol and water on enhanced finned tubes to investigate condensate retention. Fin height, root diameter and thickness was fixed. Ali and Abu baker [19] investigated the effect on condensate retention by varying the geometry of pin-fin tubes and compared with the equivalent integral fin tube, they found strong effect on the retention angle by varying the different geometry parameters and vapour velocity. Experimental work and empirical models on horizontal tubes are reported by various researchers [20-26]. Various important applications where such tubes are used are given in [27-32]. From above review of literature, it is concluded that only few studies are available on threedimensional pin fin tubes that investigated the behavior of condensate retention under the effect of vapor velocity by changing the parameters like spacing between pins, pin thickness and surface area. Another most important geometric parameter that can influence the condensate retention behavior on pin-fin tubes is the pin angle with horizontal axis on horizontal pin fin tubes; yet no research has been reported for this parameter as a function of vapor velocity. This research paper attempts a systematic study to examine the effect of angle of pin on retention angle as a subject of varying vapor velocity. For this purpose, vertical wind tunnel is used (Figure 2a) and four different tubes with varying pin angle are used as shown in Figure 2b.
Figure 2a: Schematic diagram of experimental setup
In this research, the effect of pin fin angle on the condensate retention as function of vapor velocity (0 - 20m/s) on 4 pin fin tubes, by varying pin-fin angle from 0 – 45 degree is examined. While other parameters of pin fin tubes like tooth height, tooth thickness, longitudinal spacing inner and outer diameter were kept same for all four pin-fin tubes (Table 1). Three fluids (2propanol, ethylene glycol and distilled water) with low, intermediate and high surface tension to density ratio were examined. Experimentation was conducted on the vertical wind tunnel by downward supply of air (to simulate vapor) through vertical wind tunnel with velocity from 0 to 20 m/s. By making small holes on upper side of tube a constant flow of condensate was assured on a tube to be tested as shown in Figure 2b.
Figure 2b:A1= 0o, A =15o, A3=30o, A =45o (left to right)
Figure 3 (a): Condensate flooding on test tube using water as a test fluid
Figure 3(b): Condensate flooding on test tube using ethylene glycol as a test fluid
Figure 3(c): Condensate flooding on test tube using 2-propanol as a test fluid
Table 1 : dimensions of pin fin tubes (mm) Tubes A1 A2 A3 A4
Angle (degree) 0 15 30 45
do
dr
di
s
h
t
15.9 15.9 15.9 15.9
12.7 12.7 12.7 12.7
8.5 8.5 8.5 8.5
1 1 1 1
1.6 1.6 1.6 1.6
1 1 1 1
Methodology of experimentation: Schematic diagram of experimental setup is shown in Figure 2a. Experimentation is performed on pin-fin tubes by using distilled water, ethylene glycol and 2-propanol in a vertical wind tunnel. A hand operated damper was installed on the wind tunnel to control the air velocity. Four different pin-fin tubes with varying different pin angle (with horizontal plane) were used in experimentation. Tubes were horizontally mounted in vertical wind tunnel. Tubes were facing cross downward flow of air generated by blower placed at the bottom of experimental setup. Fluid (to simulate condensate) was supplied from holes drilled (0.5mm dia) at the top of the tubes between longitudinal pin-fin spacing. Every tube was closed from one end and other end was kept opened. While opened end was attached to pipe which was connected to fluid reservoir. Vapour velocity was measured with a hot wire anemometer sensor inserted at the center of the wind tunnel. Fluid flow rate was adjusted as per requirement through holes to assure uniform condensate film along the tube. Distilled water, 2-propanol and ethylene glycol were used as test fluids to generate condensate film. After establishing smooth condensate film on the tube, retention angles for different vapor velocities 0 - 20 m/s were measured with the help photographic method. A Green dye was added in all three-testing fluid to get a clear image of
condensate retention starting point via photographic method. An arrow is also placed in Figure 3 (a, b, c) to indicate the condensate retention point under the influence of vapor velocity. Two digital cameras were used to capture the flooding, each on both sides of the tube. These two digital cameras were fixed on the two individual platforms on both sides of the wind tunnel. These cameras were firmly fixed to eliminate vibrations. Platform were tightly attached with the wall of the experimental setup. Retention angle for each testing fluid obtained from these two cameras (photographic method) are demonstrated in Figure 4 (a, b & c) for better understanding.
Figure 4a: Retention angle cam 1 vs cam 2 for water
Figure 4b: Retention angle cam 1 vs cam 2 for ethylene Glycol
Figure 4c: Retention angle cam 1 vs cam 2 for 2-Propanol
Results and Discussions: By varying pin-fin angle systematically on 4 horizontal pin fin tubes namely A1, A2, A3, A4 (corresponding to 0o ,15o, 30o, 45o pin angle with horizontal plane respectively), condensate retention as function of vapor velocity was observed when tested with three different fluids i.e. distilled water, ethylene glycol, and 2-propanol. The effect of vapour velocity on all four pin-fin tubes with all fluids was investigated. It was observed that for tube A1 (where the retention angle is less than 90 degree) in case of distilled water (Figure 5a) by increasing the vapour velocity retention angle is also increased from 62o to 72 o. For other three tubes (A2, A3, A4), retention angle decreases with increase in vapour velocity. However, it should be noted that all over the range of Reynolds number, retention angle increased from tube A1 to A4, which shows that an increasing pin angle from 0o to 45o is considerably helpful to reduce the condensate retention form the tube translating more heat transfer during condensation. This effect of increase in retention angle is more dominant at vapour velocity approaching zero (free convection condensation).
Figure 5a: Retention angle vs Reynold Number for water as a function of vapour velocity
When ethylene glycol is used as a test fluid, same behavior was observed (as in the case of water) however, for ethylene glycol, higher retention angles (Figure 5b) as compared with water are observed, this could be due to the surface tension to density ratio which also influences on retention angle, as ethylene glycol have lower surface-tension to density ratio than water.
Figure 5b: Retention angle vs Reynold Number for ethylene glycol as a function of vapour velocity
Similarly, when 2-propanol is used as a test fluid, similar behavior is observed with even higher retention angles (Figure 5c) as compare to water and ethylene glycol due to lower surface tension to density ratio of 2-propanol than water and ethylene glycol.
Figure 5c: Retention angle vs Reynold Number for 2-propanol as a function of vapour velocity
Conclusion: Effect of pin-fin angle on condensate retention on horizontal pin fin tubes is examined as function of vapor velocity (0 - 20 m/s). Four pin-fin tubes are tested systematically by using 2propanol, ethylene glycol and water as low, intermediate and high surface-tension to density ratio fluids. Salient results are summarized below; Retention angle is a function of test fluid properties, tube geometry and vapor Reynolds number. Retention angle strongly depends upon the surface tension to density ratio of testing fluid. Over the range of vapor velocity; it is observed that retention angle increases with the increase in pin angle for all tested fluids. As the vapor velocity increases from 0 to 20 m/s, retention angle of tube A1 also increased for all test fluids, however, a decrease in retention angle is noted for tubes A2, A3 and A4 for all test fluids with increasing vapour velocity. All over the range of Reynolds number, retention angle increased from tube A1 to A4, which shows that an increasing pin angle from 0o to 45o is considerably helpful to reduce the condensate retention form the tube translating more heat transfer during condensation. This effect of increase in retention angle was more dominant at vapour velocity approaching zero (free convection condensation) for all fluids tested.
Nomenclature Test tube root diameter Test tube outer diameter h
radial pin or fin height
s
fin spacing at fin root
t
fin tip thickness or longitudinal pin tip thickness
Greek Letters Ø
Condensate flooding or retention angle measured from the top of the tube to the point at which the tube flank or integral fin is fully flooded.
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Conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this paper.