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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 rever...

In the present work the dynamic performance of a natural circulation loop (NCL) has been studied under step, ramp, exponential and sinusoidal excitations. The loop is equipped with two heat exchangers at its lower and upper end for the heating and cooling of the loop fluid. For the analysis, transient one-dimensional conservation equations have been constructed for the loop fluid as well as for the two fluid streams of hot and cold end heat exchangers. The solution of a set of differential equations and one integro-differential equation has been obtained through a finite element method (FEM). For different excitations imposed to the inlet temperature of the hot fluid responses have been studied for the outlet temperature of the two fluid streams and the mass flow rate of the coupling fluid. It has been observed that all these quantities experience some initial transients before reaching the steady state. Time needed for the attainment of steady state varies with the type of excitation. A finite time delay is observed before the cold fluid stream temperature starts responding to the excitation. This delay is related to the time required for the advection of a fluid particle.

Three different arrangements of heat exchange from a hot fluid stream to a cold fluid stream such as, direct heat exchanger and fluid coupled indirect heat exchanger both with forced circulation loop as well as thermally driven natural circulation loop have been considered in the present work. Dynamic performance of these three arrangements has been studied for four different excitations namely, step, ramp, exponential and sinusoidal. These excitations are imposed at the hot fluid inlet temperature. Finite element technique is used to solve the transient one-dimensional conservation equations. A thorough comparison of the dynamic performance of these three arrangements is made. It is found that the direct heat exchanger does not have any time delay between the response and the excitation function. Moreover, the phase difference between the sinusoidal excitation and response is the lowest in this case.

The pressure variation in a natural circulation loop has been investigated for both steady state and transient operating conditions. The loop exchanges heat with hot and cold external fluid streams through counterflow concentric tube heat exchangers. Considering spatial changes only in the direction of the loop length, closed form expressions have been obtained for pressure difference under steady state condition. An iterative scheme based on finite element method (FEM) has been formulated to solve the conservation equations in the transient state. The variation of dynamic pressure and its relationship with the variation of temperature field and circulation rate has been described.

The present investigation describes the stability behavior of NCL with end heat exchangers. The one-dimensional transient conservation equations of the loop fluid and the two fluid streams of cold end and hot end heat exchangers are solved simultaneously using the finite element program. For the stability analysis the loop response is found for an imposed finite perturbation to the loop circulation rate. Though the stability may depend on the number of parameters, variation of two nondimensional parameters, namely Ch* and GrL, is studied. Selecting the specific combinations of the above two parameters three different cases of stability, namely, stable, neutrally stable, and unstable, are demonstrated. The stability behavior is scanned over a wide range of Ch* and GrL values and the stability envelope is also constructed.

The effect of end heat exchanger parameters on the performance of a single phase rectangular natural circulation loop with a constant circular cross section is reported in the present paper. Based on one-dimensional governing equations, the mass flow rate of the coupling fluid has been determined as a function of physical properties and geometrical parameters of the loop. The relevant non-dimensional numbers for the loop performance have also been identified. The model developed has been used for parametric study to determine the circulation rate as a function of (Ntu*, Ntu max , CR h -CR c combinations and θ o ).

In the present investigation, the steady state performance of a rectangular single phase natural circulation loop (NCL) with end heat exchangers is studied. One-dimensional governing equations are considered in developing the mathematical model. Analytical expressions are derived for the circulation rate and temperature profile. However, the individual performance parameters are to be computed iteratively as the equations are strongly coupled. A suitable iterative procedure is given to evaluate the important loop parameters such as steady state flow rate, and riser and downcomer temperatures. Few special cases are discussed where analytical expressions for circulation rate and temperature distribution can be obtained directly without any iterative procedure. It is also shown that both the hot and cold end heat exchangers should have equal conductance (UA) for maximization of circulation rate. This feature of NCL is identical with heat power cycle and can be explained in light of equipartition principle.