Flow Regimes in Horizontal and Vertical Tubes

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Forced convective boiling in tubes finds application in evaporators for air conditioners and refrigerators, as well as in boilers. The two most common configurations in practical applications are convective boiling in horizontal tubes and upward flow boiling in vertical tubes. The flow regimes and heat transfer correlations for these two configurations are different and will be discussed separately. The typical sequence of flow regimes for upward flow forced convective boiling in a uniformly-heated vertical tube (q'' = const) is given in Collier and Thome, 1994. Since gravity acts parallel to the flow direction, the upward flow boiling in the tube is axisymmetric. The gravitational force plays a more dominant role while the liquid-vapor interfacial shear force is less important. If the fluid enters the tube as subcooled liquid and leaves the tube as superheated vapor, all of the flow regimes for horizontal tubes will be encountered in the interim. The void fraction increases from zero at the inlet of the tube to one at the outlet of the tube. Since the density of the vapor is significantly lower than that of the liquid, the density of the two-phase mixture significantly decreases along the flow direction. To maintain constant mass flux along the flow direction, the mean velocity must increase substantially to match the significant increase in vapor phase velocity. The growing disparity between vapor and liquid velocities along the flow direction will result in changing flow patterns along the flow direction.

Subcooled liquid enters the vertical tube and prior to initiation of any bubbles (Zone A), the heat transfer mechanism is single-phase forced convection. When the rising wall temperature exceeds the saturation temperature, vapor bubbles are generated and dispersed in the continuous liquid phase. While the fluid temperature in Zone B (subcooled boiling) is still below saturation temperature, the fluid temperature in Zone C (saturated nucleate boiling) is equal to the saturation temperature. The vapor bubbles in the bubbly flow region rise due to buoyancy force and the external pressure difference. As the vaporization process continues, more liquid is converted to vapor, the void fraction increases, then the flow regime progresses from bubbly to slug, churn, and then to annular (Zones D through F). When the liquid film on the inner surface of the tube is completely evaporated, the flow pattern becomes drop or mist flow (Zone G) and the wall temperature abruptly increases. When the last droplet of the liquid is vaporized, the flow in the tube becomes single-phase (Zone H). Under constant heat flux conditions, both wall and fluid temperatures increase linearly in Zone H.

 Flow regimes for convective boiling in a horizontal tube
Figure 1: Flow regimes for convective boiling in a horizontal tube ( Grey=liquid, White=vapor).

Figure 1 shows the typical sequence of flow regimes for forced convective boiling in a horizontal tube. Two-phase flow with boiling in a horizontal tube is no longer axisymmetric, because the gravitational force acts perpendicularly to the flow direction. When the subcooled liquid enters the horizontal tube, because no bubbles are present, the initial heat transfer mechanism is single-phase forced convection. After vapor is generated, the bubbles are dispersed in the continuous liquid phase and tend to rise to the top portion of the tube due to buoyancy effects. As boiling continues in the horizontal tube, the flow regime changes to plug, annular, and mist flow. As with the vertical flow case, the flow becomes single-phase in the horizontal tube after the last droplet of the liquid is vaporized.


Chang., J.S., Watson, A., 1994, “Electromagnetic Hydrodynamics,” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 1, pp. 871-895.

Collier, J.G., and Thome, J.R., 1994, Convective Boiling and Condensation, 3rd Ed., Oxford University Press, Oxford, UK.

Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA

Thome, J.R., 2004, Engineering Data Book III, Wolverine Tube, Inc., Huntsville, AL.

Thome, J.R., and El Hajal, J., 2003, “Two-Phase Flow Pattern Map for Evaporation in Horizontal Tubes: Latest Version,” Heat Transfer Engineering, Vol. 24, pp. 3-10.

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