Falling Film Transitions on Plain and Enhanced Tubes

2002 ◽  
Vol 124 (3) ◽  
pp. 491-499 ◽  
Author(s):  
J. F. Roques ◽  
V. Dupont ◽  
J. R. Thome
Keyword(s):  
Heat Transfer ◽  
Low Flow ◽  
Film Condensation ◽  
Flow Mode ◽  
Water Mixture ◽  
Falling Film ◽  
Flow Rates ◽  
Enhanced Tubes ◽  
Plain Tube ◽  
Enhanced Boiling

In falling film heat transfer on horizontal tube bundles, liquid flow from tube to tube occurs as a falling jet that can take on different flow modes. At low flow rates, the liquid film falls as discrete droplets. At higher flow rates, these droplets form discretely spaced liquid columns. At still higher flow rates, the film falls as a continuous sheet of liquid. Predicting the flow transitions between these flow modes is an essential step in determining the heat transfer coefficient for the particular flow mode, whether for a single phase process or for falling film condensation or evaporation. Previous studies have centered mostly on falling films on plain tube arrays. The objective of the present study is to extend the investigation to tubes with enhanced surfaces: a low finned tube, an enhanced boiling tube and an enhanced condensation tube. The effect of tube spacing on flow transition has also been investigated. The test fluids were water, glycol and a glycol-water mixture. The adiabatic experimental results show that the flow mode transition thresholds for the enhanced boiling tube are very similar to those of the plain tube while the fin structure of the other two enhanced tubes can significantly shift their transition thresholds.

Wettability Effects on Falling Film Heat Transfer Over Horizontal Tubes in Jet Flow Mode

2019 ◽  
Author(s):  
Avijit Karmakar ◽  
Sumanta Acharya
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Film Thickness ◽  
Mass Flow ◽  
Flow Mode ◽  
Falling Film ◽  
Tube Surface ◽  
Flow Rates ◽  
Horizontal Tubes ◽  
Solid Liquid

Abstract The performance of a falling-film heat exchanger is strongly linked to the surface characteristics and the heat transfer processes that take place over the tubes. The primary aim of this numerical study is to characterize the influence of surface wettability of tubes on the falling film flow mode and its associated surface heat transfer. Surface wettability is generally characterized by the contact angle and, in this study, the wettability characteristics ranged from superhydrophilic to a superhydrophobic tube surface. The dynamic motion of the triple contact line connecting the solid, liquid and gas phases over the tube surface is replicated with the help of the Kistler’s dynamic contact angle model. The volume of Fluid (VOF) simulations was carried out for the flow and heat transfer of liquid flow over horizontal tubes with different surface wettabilities. The mass flow rate is such that the flow is in the jet mode where the liquid flows in the form of jets in between the horizontal tubes. This corresponds to a liquid mass flow rate per unit tube length of 0.06 and 0.18 Kg/m-s, under which the inline and staggered jets modes of flow are observed. Under the two flow rates and different surface wettabilities, the liquid flow hydrodynamics over the tube surfaces was explored in terms of the liquid film thickness, the contact areas (solid-liquid and liquid-air) between the different phases, and the heat transfer coefficient. The axial resistance imposed by the increasing contact angle tends to inhibit the extent of the liquid spreading over the tube surface and this, in turn, influences the liquid film thickness and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle ranged from 2° to 175°. Heat transfer characteristics were quantified over a wide range of contact angles for the two mass flow rates. Fluid recirculations were observed in the liquid bulk which had a major impact on the heat transfer distribution over the tube surface.

Effects of Tube Row on Heat Transfer and Surface Wetting of Microscale Porous-Layer Coated, Horizontal-Tube, Falling-Film Evaporator

2013 ◽  
Vol 135 (4) ◽  
Author(s):  
Batikan Köroğlu ◽  
Nicholas Bogan ◽  
Chanwoo Park
Keyword(s):  
Heat Transfer ◽  
Porous Layer ◽  
Horizontal Tube ◽  
Flow Mode ◽  
Falling Film ◽  
Surface Wetting ◽  
Solution Flow ◽  
Flow Rates ◽  
Film Evaporator ◽  
Evaporation Heat

An experimental study was conducted to investigate the effects of tube row on surface wetting and heat transfer of a horizontal-tube, falling-film evaporator. Two types of the evaporator tubes were used for comparison: plain copper and copper coated with a microscale porous-layer. Distilled water was used as solution and heating fluids. A visual observation experiment performed in ambient with no heat input showed that when solution fluid was dripped onto the evaporator tubes from a solution dispenser, the plain tubes were partially wetted, while the porous-layer coated tubes were completely wetted due to capillary liquid spreading, even at low solution flow rates. It was found from the heat transfer experiment performed in a closed chamber under saturated conditions that the porous-layer coated tubes exhibited superior evaporation heat transfer (up to 100% overall improvement over the plain tubes at low solution flow rates) due to complete solution wetting and thin-film evaporation. It was also observed that the surface wetting and heat transfer are greatly influenced by both intertube flow mode of solution fluid and tube wall superheat. The effects of the tube row on the solution wetting and heat transfer were significant, especially for downstream tubes.

Wettability Effects on Falling Film Flow and Heat Transfer Over Horizontal Tubes in Jet Flow Mode

2020 ◽  
Vol 142 (12) ◽  
Author(s):  
Avijit Karmakar ◽  
Sumanta Acharya
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Nusselt Number ◽  
Film Flow ◽  
Jet Flow ◽  
Equilibrium Contact Angle ◽  
Flow Mode ◽  
Falling Film ◽  
Tube Surface ◽  
Horizontal Tubes

Abstract The performance of a falling-film heat exchanger is strongly linked to the surface characteristics and the heat transfer processes that take place over the tubes. The primary aim of this numerical study is to characterize the influence of surface wettability on the film flow behavior and its associated surface heat transfer in the jet-flow mode. Volume of fluid (VOF) based simulations are carried out for horizontal tubes with different surface wettabilities. The wettability of the tube surfaces is represented using the Kistler's dynamic contact angle model. Surface wettability effects ranging from superhydrophilic to superhydrophobic are studied by varying the equilibrium contact angle from 2 deg to 175 deg. Two different liquid mass flow rates of 0.06 and 0.18 kg/m-s corresponding to the inline and staggered jet flow modes are studied. Results are presented in terms of the liquid film thickness, the contact areas between the different phases (solid–liquid and liquid–air), and the heat transfer coefficient or Nusselt number. The resistance imposed by the increasing contact angles inhibits the extent of the liquid spreading over the tube surface, and this, in turn, influences the liquid film thickness, and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle increased from 2 deg to 175 deg. The local distributions of the Nusselt number over the tube surface are strongly influenced by the flow recirculation in the liquid bulk.

Effects of Taper Configurations on Heat Transfer and Pressure Drop in Single-Phase Flows in Microgaps

2020 ◽  
Author(s):  
Debora C. Moreira ◽  
Gherhardt Ribatski ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Single Phase ◽  
Bubble Nucleation ◽  
Low Flow ◽  
Working Fluid ◽  
Inlet Temperature ◽  
Inlet Section ◽  
Flow Rates

Abstract This paper presents a comparison of heat transfer and pressure drop during single-phase flows inside diverging, converging, and uniform microgaps using distilled water as the working fluid. The microgaps were created on a plain heated copper surface with a polysulfone cover that was either uniform or tapered with an angle of 3.4°. The average gap height was 400 microns and the length and width dimensions were 10 mm × 10 mm, resulting in an average hydraulic diameter of approximately 800 microns for all configurations. Experiments were conducted at atmospheric pressure and the inlet temperature was set to 30 °C. Heat transfer and pressure drop data were acquired for flow rates varying from 57 to 485 ml/min and the surface temperature was monitored not to exceed 90 °C to avoid bubble nucleation, so the heat flux varied from 35 to 153 W/cm2 depending on the flow rate. The uniform configuration resulted in the lowest pressure drop, and the diverging one showed slightly higher pressure drop values than the converging configuration, possibly because the flow is most constrained at the inlet section, where the fluid is colder and presents higher viscosity. In addition, a minor dependence of pressure drop with heat flux was observed due to temperature dependent properties. The best heat transfer performance was obtained with the converging configuration, which was especially significant at low flow rates. This behavior could be explained by an increase in the heat transfer coefficient due to flow acceleration in converging gaps, which compensates the decrease in temperature difference between the fluid and the surface due to fluid heating along the gap. Overall, the comparison between the three configurations shows that converging microgaps have better performance than uniform or diverging ones for single-phase flows, and such effect is more pronounced at lower flow rates, when the fluid experiences higher temperature changes.

Heat Transfer and Pressure Drop Correlations for Twisted-Tape Inserts in Isothermal Tubes: Part I—Laminar Flows

1993 ◽  
Vol 115 (4) ◽  
pp. 881-889 ◽  
Author(s):  
R. M. Manglik ◽  
A. E. Bergles
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Swirl Flow ◽  
Laminar Flows ◽  
Low Flow ◽  
Published Data ◽  
Twisted Tape ◽  
Flow Rates ◽  
Uniform Wall Temperature ◽  
Uniform Wall

Laminar flow correlations for f and Num are developed based on experimental data for water and ethylene glycol, with tape inserts of three different twist ratios. The uniform wall temperature condition is considered, which typifies practical heat exchangers in the chemical and process industry. These and other available data are analyzed to devise flow regime maps that characterize twisted-tape effects in terms of the dominant enhancement mechanisms. Depending upon flow rates and tape geometry, the enhancement in heat transfer is due to the tube partitioning and flow blockage, longer flow path, and secondary fluid circulation; fin effects are found to be negligible in snug- to loose-fitting tapes. The onset of swirl flow and its intensity is determined by a swirl parameter, Sw=Resw/y, that defines the interaction between viscous, convective inertia, and centrifugal forces. Buoyancy-driven free convection that comes into play at low flow rates with large y and ΔTw is shown to scale as Gr/Sw2≫ 1. These parameters, along with numerical baseline solutions for laminar flows with y = ∞, are incorporated into correlations for f and Num by matching the appropriate asymptotic behavior. The correlations describe the experimental data within ±10 to 15 percent, and their generalized applicability is verified by the comparison of predictions with previously published data.

Film Condensation of R-11 Vapor on Single Horizontal Enhanced Condenser Tubes

1990 ◽  
Vol 112 (1) ◽  
pp. 229-234 ◽  
Author(s):  
S. P. Sukhatme ◽  
B. S. Jagadish ◽  
P. Prabhakaran
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Performance ◽  
Film Condensation ◽  
Vertex Angle ◽  
Transfer Performance ◽  
Enhancement Ratio ◽  
Maximum Value ◽  
Enhanced Tubes ◽  
Fin Tubes ◽  
The Heat Transfer Coefficient

The heat transfer performance of R-11 vapor condensing on single horizontal trapezodially shaped integral-fin tubes has been investigated by systematically varying the fin density, the semi-vertex angle, and the fin height. For the nine copper tubes tested, the best performance has been obtained with a tube having a fin density of 1417 fpm, a semi-vertex angle of 10 deg, and a fin height of 1.22 mm. This tube has yielded a maximum value of the heat transfer coefficient of 16,500 W/m2 K at a ΔT of about 3 K, corresponding to an enhancement ratio of 10.3. The performance of the tube has been further improved by fabricating from it “specially enhanced” tubes having axial grooves of varying height. An enhancement ratio of 12.3 has been obtained with this type of tube.

Experimental Investigation of Spray Cooling Heat Transfer on Circular Grooved Surface

2017 ◽  
Author(s):  
Azzam S. Salman ◽  
Jamil A. Khan
Keyword(s):  
Heat Transfer ◽  
Flow Rate ◽  
Cooling System ◽  
Volumetric Flow Rate ◽  
Target Surface ◽  
Spray Cooling ◽  
Operating Conditions ◽  
Low Flow ◽  
Inlet Temperature ◽  
Flow Rates

An experimental study was conducted in a closed loop spray cooling system working with deionized water as a cooling medium, to investigate the effects of surface modification on the spray cooling heat transfer enhancement in the single-phase region. Plain copper surface with diameter 1.5 cm and an enhanced surface with circular grooves were tested under different operating conditions. The volumetric flow rate of the coolant ranged from 115 mL/min to 177 mL/min., and the water inlet temperature was kept between 21–23 °C. Also, the distances between the nozzle and the target surface were varied at 8, 10, and 12 mm respectively. The results show that the distance between the nozzle and the target surface did not have a significant effect on the heat transfer performance for the low flow rates, while it has a slight effect on high flow rates for both surfaces. Also, increasing the liquid volumetric flow rate increases the amount of heat removed, and the heat transfer coefficient for both surfaces. Moreover, the maximum enhancement ratios achieved were 23.4% and 31% with volumetric flow rates of 153 mL/min, and 177 mL/min respectively.

Selective solvent evaporation from binary mixtures of water and tetrahydrofuran using a falling film microreactor

2017 ◽  
Vol 6 (4) ◽  
Author(s):  
Sibylle von Bomhard ◽  
Karl-Peter Schelhaas ◽  
Sabine Alebrand ◽  
Anna Musyanovych ◽  
Michael Maskos ◽  
...  
Keyword(s):  
Mass Transfer ◽  
Gas Flow ◽  
Volume Flow ◽  
Liquid Volume ◽  
Transfer Model ◽  
Pure Liquid ◽  
Water Mixture ◽  
Falling Film ◽  
Operational Parameter ◽  
Flow Rates

AbstractIn this work, a falling film micro reactor was investigated regarding its ability to continuously eliminate tetrahydrofuran (THF) out of a THF-water mixture via nitrogen stripping. Mass transfer measurements were performed at different temperatures and flow rates. The residual content of THF in the eluate was quantified with high precision (<0.1%) via density measurements. Remarkably, complete elimination of THF could be achieved for liquid volume flow rates smaller than 2 ml/min and nitrogen volume flow rates larger than 400 ml/min at all three investigated temperatures (55°C, 60°C, and 65°C). In order to assist future design processes of such binary microstripping systems, we further developed a mass transfer model for this separation process extending an existing model for evaporation of a pure liquid. The good agreement of experimental data and calculations in the overall investigated parameter range (≤20%, for gas flow rates below 500 ml/min ≤11%) shows the potential of the model for the prediction of alternative operational parameter settings, e.g. at different THF entrance concentrations.

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