Effects of flow orientation on condensation in tubes

This study explores the interfacial and heat transfer characteristics of annular condensation of FC-72 in vertical downflow, vertical upflow and horizontal flow as well as the effects of channel orientation on flow condensation. Two separate condensation test modules are employed, one for high-speed video imaging of the film interface and the second for heat transfer measurements. Condensation in both test modules is achieved by rejecting the heat to a counterflow of cooling water. The heat transfer measurements are obtained along the inner wall of an 11.89-mm i.d. and 1,259.84-mm long stainless steel tube. For vertical downflow, the film at very low FC-72 flow rates is observed to be both smooth and laminar. The film turns turbulent with a very wavy interface as the flow rate of FC-72 is increased, especially for exit film Reynolds numbers above 1,800. The heat transfer coefficient decreases axially because of a gradual thickening of the liquid film. However, the data show a downstream minimum before the heat transfer coefficient increases again towards the outlet as the film transitions to turbulent flow, enhanced by the more intense downstream waves. A control-volume-based model is proposed, which incorporates an eddy diffusivity profile for the liquid film that accounts for interfacial dampening of turbulence due to surface tension. The model shows good accuracy in predicting the average condensation heat transfer coefficient data, evidenced by a mean absolute error of 12.59%. Upflow condensation is complicated by the relative magnitude of the opposing vapor shear and gravity. This study also examined the different flow regimes for condensation of FC-72 in vertical upflow. Four regimes are identified, falling film, where the condensing film drains downwards by gravity opposite to low velocity vapor flow, oscillating film, corresponding to film flow oscillating between upwards and downwards, flooding, where film begins to be sheared upwards by the vapor core, and climbing film, where high vapor velocity causes the film to be sheared upwards. The four flow regimes are well segregated in a flow regime map based on dimensionless superficial velocities of the vapor and liquid. The condensation heat transfer coefficient is shown to decrease axially because of gradual thickening of the film, except for high mass velocities, where turbulence and intensified interfacial waviness cause downstream heat transfer enhancement. The annular flow model is modified to account for the reversed orientation of gravity, and shows fair predictions for the climbing film regime. The predictive accuracy of the model is influenced by flow oscillations occurring downstream of the climbing film region and inability of the model to account for interfacial waves. For condensation of FC-72 in horizontal tubes, dominant condensation flow regimes are identified for different combination of mass velocities of FC-72 and cooling water using high-speed video motion analysis.. Additionally, detailed heat transfer measurements are used to explore both axial and circumferential variations of the condensation heat transfer coefficient. Four different regimes are identified: stratified, stratified-wavy, wavy-annular with gravity influence, and wavy-annular without gravity influence. In the latter regime, which is achieved at high FC-72 mass velocities, annular film transport is dominated by vapor shear with negligible gravity effects. Using different types of regime maps, prior relations for transitions between regimes are assessed, and new, more accurate transition relations developed. The heat transfer coefficient is shown to be highest near the inlet, where quality is near unity and the film thinnest, and decreases gradually along the condensation length because of axial thickening of the liquid film. This study also explores the predictive capabilities of prior heat transfer correlations and a control-volume-based annular flow model. The experimental data of both the local and average condensation heat transfer coefficients show fair to good agreement with predictions of prior and popular correlations. But superior predictions in both trend and magnitude are achieved with the annular flow model. The study of orientation effects on flow condensation explores condensation of FC-72 in a circular tube at three different flow orientations including horizontal flow, vertical downflow, and vertical upflow with the aid of detailed heat transfer measurements and high-speed video motion analysis. Using the video analysis, the behavior of liquid film and influence of gravity are investigated for different combinations of mass velocity of FC-72 and cooling water for three flow orientations. Utilizing the condensation module for heat transfer measurements, axial and circumferential variations of the condensation heat transfer coefficient for different flow orientations are explored. Local and average condensation heat transfer coefficients from the three flow orientations are compared to each other to assess the influence of body force on condensation heat transfer. Flow conditions that negate the influence of body force are identified. Using the annular flow model, the magnitudes of different forces acting on the liquid film are compared to each other for different combinations of mass velocity of FC-72 and cooling water for each orientation