The Influence of Core Capacitance on the Dynamic Performance of a Single-Phase Natural Circulation Loop With End Heat Exchangers

The effect of wall-core capacitance of heat exchangers on the dynamic behavior of a natural circulation loop (NCL) with end heat exchangers is studied under various excitations such as step, ramp, exponential, and sinusoidal. The transient one-dimensional conservation equations are derived for loop fluid, hot and cold fluid streams, and wall core of both heat exchangers. The solution of a set of transient partial differential equations and one integro-differential equation for loop fluid circulation rate is achieved through a finite-element technique. Imposing the excitations to the inlet temperature of hot fluid, the effects of wall-core capacitance on the responses of outlet temperatures of both hot and cold fluid streams and flow rate of loop fluid are studied. Wall-core capacitance diminishes the initial transients and delays the inception of hot and cold fluids outlet temperature profiles as well as loop fluid flow profile. Further, it has the ability to bring even unstable system behavior with reverse flows into a stable system with steady loop flow rate through quickly decaying oscillations. System responses are also greatly influenced by boundary conditions such as hot and cold fluids flow rates and their inlet temperature excitations such as step, ramp, and exponential. As flow stability is an important subject for single-phase NCLs, a stability map is constructed and compared with zero wall-core capacitance. Inclusion of wall-core capacitance in the present study reveals the important fact that the stable state operating zone widens with the wall-core capacitance.