Flow of evaporating fluid in multiple parallel pipes-vertical downwards flow

Evaporation of liquid in parallel heated pipes with common inlet and outlet manifolds was investigated mainly for co-current horizontal and upward flow. Almost no work was published on parallel heated pipes with an upper inlet header where sub cooled liquid is injected. In the present work such a system has been investigated. It seems that the Ledinegg excursion instability and the transient simulations used for co- current horizontal flow (Hayat et al. [1]) are not fully sufficient to predict the behavior of parallel heated pipes with vertical downward flow. In the present case the flow rate distribution between the parallel pipes include also solutions for counter current flow, where the flow in one of the pipe is directed upwards. Experimental results of the characteristics pressure drop curve, the steady state flow rate distribution as well as the transient response of the system to changes in the operational conditions are presented. At certain regions of inlet flow rates a cyclic process, similar to a geysering phenomenon, was observed where pressure drop and flow rate oscillations take place in both pipes. It occurs when upward flow is observed in one of the pipes. A theoretical model based on the local instantaneous flow pattern was used for calculating the characteristic pressure drop curve for both downward and upward flows. The theoretical pressure drop curve compares quite well with the experimental results. The characteristic pressure drop, a linear stability analysis and transient simulations were used to predict the flow rate distribution between the pipes at steady state and under transient conditions. The model predicts correctly the experimental results as long as there is no upward flow in one of the pipes.