Flow Rate Maldistribution in Multi Heated Parallel Pipes

Abstract

Fluids flowing in parallel pipes, undergoing evaporation, take place in heat exchangers, boilers, power plants, cooling systems and in the nuclear industry. Evaporating two-phase flow in parallel micro-channels is considered for heat removal in microelectronic devices. The main motivation of the present work is associated with the use of solar energy collected in long lines. In this technology, an array of parallel pipes is located at the focal center of parabolic mirrors that focus solar radiation on the pipes to generate steam. It is a common knowledge that maldistribution may occur in evaporating liquid flowing in parallel pipes with common inlet and outlet manifolds. This phenomenon occurs due to multiple steady state solutions some of which are unstable. One may obtain uneven flow rate distribution even for the case of equal heating of the pipes. For non equal heating higher flow rates may take place in the less heated pipes. This is quite an unfavorable phenomenon. The theoretical model developed by Minzer et al. [1] for the flow rate distribution is extended to a larger number of pipes and different heating conditions. Stable and unstable solutions are identified and the model predictions are experimentally validated for different configurations involving three pipes. It is shown that the behavior of the system may depend on the history of the process exhibiting a hysteresis phenomenon. Transient simulations are carried out using this model in order to study the time dependent system response to finite disturbances and to changes in operational conditions.