DA-07-056 Applying the Effectiveness-NTU Method to Elemental Heat Exchanger Models PDF

Elemental heat exchanger models are increasingly used in heat pump design calculations because of their ability to accurately model complex heat exchanger circuits and non-uniform coil face velocities. These models usually assume that refrigerant properties are constant in each element. For transition elements, where both single-phase and two-phase refrigerant are present, this assumption introduces some error in the capacity calculation and, more importantly, results in dualvalued property functions that may trap the numerical solver at an unconverged point in the solution domain. This paper illustrates both the capacity error and the numerical error that can result from improper handling of the transition element in an elemental heat exchanger model. An algorithm that correctly handles the transition element is proposed. Examples are presented to illustrate improvements in the capacity calculation and the simulation performance due to the proposed algorithm. The proposed algorithm is specifically applied to the e-NTU heat exchanger calculation method but can also be applied to the LMTD method without loss of generality.

Units: SI