Pre- and post-boiling nucleation thermal and fluid flow transients during the startup of capillary pumped loops (CPLs)
Abstract
During the fully flooded startup of a Capillary Pumped Loop (CPL), accelerated vapor growth caused by the boiling incipient superheat results in rapid pressure and fluid flow transients within the loop. A number of factors influence the vapor growth rate and the induced variations in the differential pressure across the evaporator wick, which is important for determining whether or not the startup will be successful. The primary objectives of the current study are to evaluate the effects of procedural and design variations on the startup performance of CPLs and to develop predictive tools that will result in an improved fundamental understanding of the startup. An analytical solution for the temperature profile in a cylindrical evaporator subject to a uniform heat flux prior to the initiation of boiling is derived using the Green's Function method. Also, an approximate solution is derived for the case in which the evaporator is heated by means of a constant conductance to a heat dissipating device. Results of this model for the preheating stage of startup suggest potential design and procedural modifications that can improve startup success for the CPL. A three-dimensional transient conduction model of the evaporator following vapor formation is developed to determine the effect that design parameters have upon the vapor phase growth. A system level fluid flow model of the CPL is coupled to the vapor growth model, which allows the differential pressure spike across the evaporator wick to be calculated. Key design parameters are identified from the model results that have the potential to significantly reduce the bubble growth rate and the corresponding differential pressure spike. All of the model results indicate that design modifications are available that can significantly improve the chances for successful startup of CPLs under flooded startup conditions.
Degree
Ph.D.
Advisors
Mudawar, Purdue University.
Subject Area
Mechanical engineering
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