INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET)

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1 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN (Print) ISSN (Online) Volume 3, Issue 2, May-August (2012), pp IAEME: Journal Impact Factor (2012): (Calculated by GISI) IJMET I A E M E PARAMETRIC STUDIES OF DEPARTURE SITE SPACING FOR DROPLETS/ JETS IN INTERTUBE FALLING FILM N.U. Korde 1*, A. T. Pise 2 1 Department of Mechanical Engg. G.H.Raisoni Inst. of Engg. & Tech., Pune M.S (India) 2 Department of Mechanical Engg. Govt. College of Engineering. Karad M.S (India) ABSTRACT The wetting characteristics of the flow over the tube are closely related to the departure-site spacing between the adjacent droplets/jets mode. Experiments were conducted exploring the effects of different parameters; thermo physical properties like viscosity, surface tension, inertial, gravitational effects and geometrical parameters on departure site spacing between the adjacent droplets/jets mode. Distilled water, Distilled water with n-heptanol as surfactants and SAE40 oil served as test liquids. The effect of geometrical parameters like tube diameter and spacings between the consecutive tubes on the departure-site spacing was investigated for the smooth tubes. The flow modes and its behaviour over the tubes were observed visually and photographed with clippings by digital video camera. Measurements of departure-site spacing between the adjacent droplets/jets were reported for the test liquids. For the conditions of this study, departure-site spacing increased with decreasing Re f for high - Ga fluids and was nearly independent of Re f for low-ga fluids. Departure site spacing increased slightly with tube diameters. This paper aims to find the effect of various parameters on departure site spacing between the adjacent droplets/jets. Keywords:- Falling film, Tube bank, Departure site spacing Nomenclature d - Diameter, mm d* - Normalized tube diameter, d/a, dimensionless Ga - Modified Galileo number = ρ σ 3 /µ 4 g, dimensionless g - Gravitational acceleration, m /s 2 Re f - Film Reynolds number = 2 Г / µ, dimensionless 470

2 s - Tube spacing, mm s* - Normalized tube spacing, s/a, dimensionless a - Capillary constant m σ ρ g Greek Symbol λ - Departure-site Spacing λ* - Normalized departure-site spacing, dimensionless Г - Mass flow rate per unit length of tube, kg /ms µ - Dynamic viscosity, N s/m 2 ρ - Mass density, kg/m 3 σ - Surface tension at gas/liquid interface, N/m * Corresponding author: N.U. Korde, Assistant Professor; nukorde08@rediffmail.com 1. INTRODUCTION Falling film heat exchangers with horizontal tubes, arranged one upon the other, thus forming a bundle have been used in chemical, refrigeration, petroleum, refining, desalination, and food industries. They provide higher heat transfer coefficients and operate with smaller liquid inventories than flooded heat exchangers. Also they offer advantages in dealing with liquid distribution, noncondensable gases, fouling and other problems. In a typical falling film heat exchanger, the liquid film is introduced at the top of the tube bank through nozzles or drip-tray distribution system. The liquid flows as a film around the tubes. It begins to extend from the underside of the tube and eventually detaches and falls freely until it impacts the top of the next tube. This process is repeated through the entire tube bank. The behavior of real film involves many complex interactions such as the waves and ripples generated by an impacting drop. When a liquid film falls from one horizontal tube to another below it, the flow may take the form of discrete droplets, jets, or a continuous sheet [1][2]. The mode plays an important role in the heat transfer. The flow pattern and wetting characteristics in the falling film heat exchangers have significant impact on the sensible and latent heat transfer. The mode transitions for flow patterns are correlated with Reynolds and Modified Galileo number by the various researchers[1][3][4]. The Reynolds number is defined as the ratio of the inertia force to viscous force and is given by Re f = 2Γ/µ Where µ is the fluid density and Γ is the mass flow rate. The modified Galileo number is given by 3 4 Ga = σρ/µ g Where σ is surface tension and g is acceleration due to gravity[5]. The wetting characteristics and film complex over the tube depends upon distance between adjacent droplets/jets is called departure site spacing λ (Fig. 1). The departure site spacing is considered to be an important parameter to understand the falling film flow behavior. This paper is 471

3 6340(Print), ISSN (Online) Volume 3, Issue 2, May-August (2012), IAEME aimed to analyze the effects of thermo physical properties like viscosity, surface tension, inertial, gravitational effects and geometrical parameters on departure site spacing between the adjacent droplets/jets mode. Fig.1. Stationary columns and flow of liquid on the tube. [2] 1.1 Departure Site Spacing: The processes and conditions of formation of droplets and jets are usually treated on the basis of Taylor instability. According to which the droplet or jet spacing is approximately equal to the so called most dangerous wavelength. This is the wavelength of the most rapidly growing disturbance imposed on an originally smooth but unstable interface. The average distance between the two adjacent droplets/jets can be explained with the Taylor instability theory [8] nσσ λ = 2π ρ g (1) Where n is a factor 2. n was taken equal to 3, assumed to be thick films, n was taken equal to 2, assumed to be thin film. The influence of inter tube spacing and flow rate on falling film heat transfer on horizontal plain tube bundles for subcooled liquid without evaporation was investigated by Mitrovic [5], for isopropanol and water. He measured the distance between the two adjacent columns from still photographs, obtaining a value of 22 mm for water at 25 C and 13.5 mm for iso propanol at 21.5 C. These values fall between the critical wavelength λ crit and most dangerous wavelength λ d. The departure site spacing can be calculated according to equations given by the Lienhard and Wang [6]. λ c = 2 π ( ρl ρg ) g σ D -1 (2) λ d = λ cric 3 (3) 472

4 Hu and Jacobi [7] experimentally investigated the effects of parameters like tube diameters, tube spacing and liquid flow rate on the film wavelength using four working fluids. They reported that the wavelength depends on flow rate and decreases for an increase in Re f and this dependence is higher for fluids with large Ga 1/4. A weak dependence on tube diameter was observed, with wavelength increasing with tube diameter. The dependence of wavelength on the tube spacing was also observed to be the weakest. They correlate the wavelength with Re f, Ga 1/4 and tube diameter and provide a new widely applicable correlation. 2π 3 Re λ = Ga 1 2 a + d o f 1/ 4 a (4) Yung et al. [8], developed an equation to estimate the wavelength of low viscosity liquids like water, ethyl alcohol and ammonia. Ganic and Roppo [9], observed wavelength to be lower than predicted by the Taylor instability and unaffected by the flow rate and tube spacing for the jet mode. Armbruster and Mitrovic [10] provided a correlation for jet spacing; which was based on data with two fluids namely water and isopropyl alcohol. Using current nomenclature, the correlation can be written as λ = g( ρ ρ σ v 2π 2 ) Ref / Ga d (5) This expression is somewhat unique, in that it explicitly accounts for a flow rate effects on the jet spacing. Siyoung et al [11], reported that, the departure site spacing is closer as the flow rate increases and the tube diameter decreases for smooth tube. They proposed a non dimensionalized departure site spacing relation with Reynolds number as 1.1 2Π 3 λ = 1.62 Re 3 1+ d * 0.22 *2 (6) The normalized departure site spacing is defined by [12] with respect to capillary length as λ * = λ / a where a is capillary constant which is given by a = σ / ρ g 2. EXPERIMENTAL METHOD 2.1 Experimental Setup The test facility as shown in Fig. 2, is provided with a closed circuit forced circulation loop of test liquid. The purpose of liquid loop was to supply liquid medium to test section. The liquid 473

5 circulation loop consisted of distributor, reservoir, flow pump and flow meter. The test liquid in reservoir was delivered by pump to distributor via flow meter then passed over the tubes assembly. In the experimental set up to damp the vibrations in the distributor, flow through the flow meter first passed to an inner circular tube having continuous slot of 400 mm along its periphery. The flow is then allowed to fall in the outer distributor tube. Fig.2. Schematic of Experimental Setup At the bottom of distributor small holes of 2 mm diameter were drilled at 10 mm apart. The distance between bottom of distributor & top of first tube is kept 5 mm. Commercially available tubes of different diameters viz. 26, 32 and 42 mm with intertube spacing of 10, 15 and 20 mm were kept in the experiment. The arrangement of the tube side stands is made such that the spacing between the tubes can be changed easily. For this experimentation tube bank of 5 tubes are aligned inline to form a liquid column. The first tube below distributor is taken as dummy tube and the measurement of flow and departure is done on the tube below dummy tube. 2.2 Experimental Procedure Before test, tubes with desired diameter were aligned horizontally in the test section at prescribed tube spacing. As the falling film flow is gravity driven, a tube inclination would cause a non-uniform distribution of the flow. Leveling was checked after the liquid started circulating in the system. The liquid is allowed to fall from the distributor on the top side of first dummy tube. Liquid falls at the centre of the dummy tube, so that it is distributed uniformly on both sides, flows over the surface and from the bottom side falls on the top side of next tube aligned below it. The liquid flow rate was adjusted to obtain a jet mode, and the tubes were considered leveled if the jets 474

6 fell at fixed sites without drifting toward either end of the test tube. The test liquid was circulated through the system for 15 to 30 minutes to ensure that the tubes were fully wetted and the piping system was free of air. Then the fluid flow rate was adjusted, mode existed was droplets/jets. Similar tests were carried for water, distilled water with n-heptanol (200 ppm) and SAE40 oil fluids. Every fluid is tested for various diameter tubes with spacing ranging from 10 to 20 mm. Some clippings and images were taken for different flow rates. All the experiments reported were conducted over adiabatic tubes at atmospheric pressure having temperature of the liquids at 25 C. 3. RESULTS & DISCUSSION 3.1 Departure Site Spacing The measurement of departure site spacing was carried out from the frames, which were excerpted from the Digital video camera for three different fluids viz. distilled water, distilled water with n-heptanol and SAE40 oil. Software Adobe Photoshop 7.0 was used to measure the distance between the two adjacent droplets / jets. Fig.3.Departure site spacing for various fluids on 27 mm diameter tube Fig. 3 shows the images of departure site spacing of test liquids. It shows that λ* is higher for distilled water as compared to distilled water with n hepatnol and SAE 40 oil due to surface tension. Also jet diameter for distilled water is higher as compared with distilled water with n heptanol which is due to higher surface tension. Surface tension reduced due to presence of additive (n-heptanol) in distilled water reduces jet diameter, film thickness and λ*. Viscosity effect is more pronounced for oil. For oil film is thick and jet is thin which becomes broader towards bottom. Crest can be easily visualized for oil due to high viscosity. The decreased in Re f from distilled water towards oil results in increased in λ*. Effect of tube diameter are more pronounced and λ* increases with diameter. Fig. 4 shows shapes of jet at different spacing between the tubes of diameter 26 mm. For small values of spacing the jet appears to take a shape of pendent drop. With an increase in spacing, the jet becomes elongated, taking the shape of an inverted bell. For further increase in tube spacing 475

7 results in jet diameter decreasing from bottom of first tube to a thin jet impacting on the top of next tube. This shape is like the upper frustum of cone. Fig. 4. Liquid jet shapes for distilled water at 10mm, 15mm and 20 mm spacing To account for properties of fluids, normalized diameter and normalized tube spacing are used. Measured λ* are compared with equation stated by Hu & Jacobi [7], Armbruster & Mitrovic [10] and Siyoung [11] and presented in Fig. 5,6 and 7 for distilled water, distilled water + n heptanol and SAE 40 oil respectively. For distilled water the results of λ* are in good agreement with Siyoung [11] as compared with Hu & Jacobi [7] and Armbruster & Mitrovic [10]. While for distilled water + n-heptanol measured λ* are at higher side as compared with Hu & Jacobi, Armbruster & Mitrovic and Siyoung. For SAE 40 oil the results are at lower side as compared with the all researcher, this might be due to higher viscosity of fluid. The equation stated by the [11] is independent of the properties of fluid and they suggested it depends purely on diameter of tube and Reynold number. While it is observed that the λ* is insensitive to tube diameter. Armbuster & Mitrovic suggested equation based on experimentation of two fluids without considering the highly viscous fluids. So the deviations with the literature is obvious The data clearly show that λ* depends on Re f especially at low Re f and high Ga 1/4. A decrease in Re f is accompanied by increase in λ*. At low Ga 1/4, λ* appears to be insensitive to Re f over a wide range. 476

8 Fig. 5. Departure site Spacing (Distilled Water) Fig. 6. Departure site Spacing (Distilled Water + n-heptanol) 477

9 4. CONCLUSIONS Fig. 7. Departure site spacing (SAE 40 Oil) From the results of this study the following major conclusions are drawn. Departure-site depends on mass flow rate and decreases for increase in Re f ; this dependence is higher for fluids with a large Ga 1/4. Using Taylor s instability theory, departure-site spacing was under predicted for thick films and over predicted for thin films. In the experimental data, a weak dependence on tube diameter was observed, λ* is insensitive to the tube diameter. The dependence of λ on tube spacing was observed to be the weakest. These results provide deeper understanding of the falling film mode transitions and a useful tool for designing and modeling heat exchangers. ACKNOWLEDGEMENT Authors want to acknowledge BCUD, of Pune University for providing research grant No. BCUD/OSD/ 184 to carryout this work. REFERENCES [1] X. Hu, A.M. Jacobi, The Intertube Falling Film Part 2- Mode Effects on Sensible Heat Transfer to a Falling Liquid Film, ASME Journal of Heat Transfer, 118 pp (1996) [2] X. Hu, A.M. Jacobi, The Intertube Falling Film Part 1- Flow Characteristics, Mode Transition and Hysteresis, ASME Journal of Heat Transfer, 118 pp (1996). [3] J. Mitrovic, Influence of tube spacing and flow rate on heat transfer from a horizontal tube to a falling liquid film, Proceedings of the eighth international heat transfer conference, SanFrancisco, vol , pp

10 [4] J.R. Thome, J.F. Roques, V. Dupont, Falling Film Transitions on Plain and Enhanced Tubes. ASME Journal of Heat Transfer, Vol. 124, 2002, [5] J. Mitrovic, Flow Structures of a Liquid Film Falling on Horizontal Tubes. Chemical Engineering Technology, Vol. 28, 2005, [6] J. H. Lienhard and P. T. Wong, The Dominant Unstable Wavelength and Minimum Heat Flux during Film Boiling on a Horizontal Cylinder, Transactions of ASME, J. of Heat Transfer, 86, 1964,pp [7] X. Hu, A.M. Jacobi, Departure Site Spacing for Liquid Droplets and Jets Falling Between Horizontal Circular Tubes, Experimental Thermal and fluid Science pp , (1998). [8] D. Yung, J.J. Lorenz E.N. Ganic, Vapor/ liquid interaction and entrainment in falling film evaporators, J. Heat Transfer, 102(1980) pp [9] E.N. Ganic, M.N. Roppo, An experimental study of falling liquid breakdown on a horizontal cylinder during heat transfer, Journal of Heat Transfer, 102, 1980, pp [10] R. Armbruster, J. Mitrovic, Evaporative Cooling of a Falling Water Film on Horizontal Tubes. Experimental Thermal and Fluid Science 18, Elsevier, pp (1998). [11] J. Siyoung, H.K. Kyung, K. Kyongmin, Eun S.J., Effect of Surface Roughness on The Wetting of a Horizontal Tube Falling Film Heat Exchanger, ISHPC 02, Proceeding of the International Sorption Heat Pump Conference, Shangai, China, September 24-27, pp [12] A.M. Jacobi, G. Ribatski, Falling Film Evaporation on Horizontal Tubes A Critical Review. International Journal of Refrigeration, 2005,