Condensation flow patterns

Condensation occurs whenever a vapor, a vapor mixture, or a vapor containing a noncondensable gas is brought into contact with a surface below the dew point or saturation temperature of the vapor. The condensed liquid is most likely to form a continuous film covering the cooled surface. In some cases, however, dropwise condensation is possible if the fluid does not wet the surface. Filmwise condensation is encountered in most industrial applications and is the only mode of condensation we shall consider further. 

The cooled surface may be of any orientation although vertical and horizontal arrangements are most common. Condensation operations often are carried out inside shell and tube heat exchangers and the condensing vapors may be fed either to the shell side or to the tube side depending on the nature of the fluids involved, their pressure, and their corrosion and fouling characteristics (Webb and McNaught, 1980). 

In vertical devices with condensation on the inside or on the outside of the tubes the condensed liquid-film falls under the influence of gravity. The thickness of the condensed liquid film increases in the direction of flow due to the increased liquid load caused by 435 

On perfectly horizontal tubes a condensed film will increase its thickness towards the bottom of the tube (Fig. 15.2). Some of the condensed liquid will fall onto lower tubes, increasing the liquid load, and decreasing the heat transfer coefficient on those tubes. Even a slight inclination of the tube is sufficient to cause the condensate to drain in the direction of the slope. A horizontal shell and tube condenser is likely to be baffled so as to force the vapor to flow horizontally across the tubes. Other flow arrangements are, however, possible. 

Condensation may also take place inside horizontal tubes, the flow regimes depend strongly on the velocity of the vapor. At low vapor flows, the condensate film tends to collect 

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R. Hashimoto, K. Yanagi, and T. Fujii, Effects of Condensate Flow Patterns upon Gravity-Controlled Condensation of Ethanol and Water Mixtures on a Vertical Surface, Heat Transfer-Japanese Research, 23, pp. 330-348,1994. 

Figure 1 shows the experimental setup for the study of condensation in a microchannel [5], The deionized water in the water tank was pumped into the electric boiler where water was vaporized. Saturated steam from the boiler flowed successively through the valve, filter, and test section and was finally collected by a container at atmospheric pressure. Figure 2 shows the test section of the parallel microchanneis etched in a silicon wafer, which was cooled by circulation of cooling water from the bottom of the wafer. Temperature and pressure of steam at the inlet and the condensate at the outlet were measured by thermocouples and pressure transducers, respectively. WaU temperature distribution along the bottom of the microchaimels was measured by thermocouples embedded in the silicon wafer substrate. The microchanneis were then covered with thin transparent Pyrex glass from the top. To visualize condensation flow patterns... 

To study condensation flow patterns of refrigerant R134a in circular tubes and in minichannels having hydraulic diameters of 1 mm < < 5 mm, Garimella [6] used dig-... 

Wu HY, Cheng P (2005) Condensation flow patterns in silicon microchanneis. Int J Heat Mass Transf 48 2186-2197... 

Design faults in two-pass condensers and heat exchangers that can cause corrosion include poor division plate seals allowing the escape of water at high velocity between the passes, and flow patterns that produce stagnant zones. 

Kawahara A, Chung PM, Kawaji M (2002) Investigation of two-phase flow pattern, void fraction and pressure drop in a micro-channel. Int J Multiphase Plow 28 1411-1435 Kawaji M (1999) Fluid mechanics aspects of two-phase flow Flow in other geometries. In Kand-likar SG, Shoji M, Dhir VK (eds) Handbook of phase change boiling and condensation. Taylor and Francis, Washington, DC, pp 205-259... 

The number of fluorine equivalents (to toluene) was varied the gas and liquid flow velocities were kept constant to maintain the same flow pattern for all experiments. Liquid products were collected in an ice-cooled roimd-bottomed glass flask containing sodium fluoride to trap the hydrogen fluoride. The flask is connected to a cooling condenser to recover the solvent. Samples were typically collected for 1 h. Waste gases were scrubbed in aqueous 15% potassium hydroxide solution. Samples were degassed with nitrogen and filtered before analysis. 

Figure 12.44. Flow patterns, vapour condensing in a horizontal tube...

Figure 12.45. Flow patterns in condensation, (a) Stratified flow (b) Annular flow...

To evaluate the true temperature difference (driving force) in a mixed vapour condenser a condensation curve (temperature vs. enthalpy diagram) must be calculated showing the change in vapour temperature versus heat transferred throughout the condenser, Figure 12.48. The temperature profile will depend on the liquid-flow pattern in the condenser. There are two limiting conditions of condensate-vapour flow ... 

These methods can be used to make a crude estimate of the likely pressure drop. A reliable prediction can be obtained by treating the problem as one of two-phase flow. For tube-side condensation the general methods for two-phase flow in pipes can be used see Collier and Thome (1994) and Volume 1, Chapter 5. As the flow pattern will be changing throughout condensation, some form of step-wise procedure will need to be used. Two-phase flow on the shell-side is discussed by Grant (1973), who gives a method for predicting the pressure drop based on Tinker s shell-side flow model. 

W-3 CHF correlation. The insight into CHF mechanism obtained from visual observations and from macroscopic analyses of the individual effect of p, G, and X revealed that the local p-G-X effects are coupled in affecting the flow pattern and thence the CHF. The system pressure determines the saturation temperature and its associated thermal properties. Coupled with local enthalpy, it provides the local subcooling for bubble condensation or the latent heat (Hfg) for bubble formation. The saturation properties (viscosity and surface tension) affect the bubble size, bubble buoyancy, and the local void fraction distribution in a flow pattern. The local enthalpy couples with mass flux at a certain pressure determines the void slip ratio and coolant mixing. They, in turn, affect the bubble-layer thickness in a low-enthalpy bubbly flow or the liquid droplet entrainment in a high-enthalpy annular flow. 

Soliman, H. M., and N. Z. Azer, 1971, Flow Patterns during Condensation inside a Horizontal Tube, ASHRAE Trans. 77 210. (3)... 

The basic assumptions implied in the homogeneous model, which is most frequently applied to single-component two-phase flow at high velocities (with annular and mist flow-patterns) are that (a) the velocities of the two phases are equal (b) if vaporization or condensation occurs, physical equilibrium is approached at all points and (c) a single-phase friction factor can be applied to the mixture if the Reynolds number is properly defined. The first assumption is true only if the bulk of the liquid is present as a dispersed spray. The second assumption (which is also implied in the Lockhart-Martinelli and Chenoweth-Martin models) seems to be reasonably justified from the very limited evidence available. 

In compact geometries the heat transfer coefficient depends on the two-phase flow pattern (51-67). For low condensation rates, the heat transfer is gravity controlled, and the heat transfer coefficient depends on the liquid film thickness. For higher condensation rates, the heat transfer coefficient depends on the vapor shear effect, and for small passages the liquid-vapor interaction leads to high heat transfer coefficients. 

Observations of the flow patterns inside of fin passage making the set of rectangular channels were done at apparatuses shown at Fig. 9. Subcooled liquid was pumped through electro-heating coil to provide a certain vapor quality of the flow. Then the flow was passed through adiabatic test section and later through the evaporator, for exception pulsation of flow, into the condenser. The test section can operate both in up... 

Figure 9. Experimental apparatus for flow pattern observation and test section with bottom window 1-condenser, 2-pump, 3-flowmeter, 4-heater, 5- test sample, 6-evaporator, 7- camera, 8-PC.

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