Prediction of pressure drop in a uniformly heated channel under boiling conditions

A theoretical model has been developed to predict the pressure drop in a nuniformly heated channel for a range of conditions from single phase flow through saturated boiling. In the two phase region, the model uses a true mass fraction approach which begins at the point of onset of significant voiding (OSV). In this model the quality is always positive. The region before OSV is divided into two sub-regions at the point of ONB. These sub-regions are defined as the single phase region and the partially developed subcooling boiling region. In the latter region, the friction is based upon the bubble diameter and in the former region the usual surface roughness approach in the Darcy friction factor is used. The model is evaluated by comparing the predicted results with experimental results obtained in a heated vertical channel for both downflow and upflow. The agreement is very good for both the prediction of the minimum point in the demand curve and the overall pressure drop. Comparisons are also presented for low flux upflow conditions where the minimum point in the demand curve does not occur. Again the theoretical predictions are very reasonable. Of particular interest are the results at low flux where the minimum point in the demand curve disappears. The model predicts this response reasonably well.