THEORETICAL AND EXPERIMENTAL INVESTIGATION OF A DYNAMIC MODEL FOR SMALL REFRIGERATING SYSTEMS.

Abstract

In recent years many models for a simulation of the transients in a refrigeration cycle have been presented; most of them were aimed at an efficiency optimization and concern heat pump applications.These models assume either a three zone division in the heat exchangers and/or thermodynamic equilibrium in the two phase parts.A different dynamic model has been designed for simulating a small refrigerating cycle as e.g.applied in domestic equipment.A multiple node model is used here for the description of the heat exchangers.This multiple node character is necessary for an accurate calculation of superheated and subcooled areas.In the model a mean void fraction approach is used in order to be able to describe the distribution of liquid and vapour in the two phase areas.This distribution is of influence in the calculation of the consequences of a certain fixert refrigerant charge applied.Various assumptions for the basis for calculations using coefficients which take into account heat transfer and flow processes on the "outside" of the cycle; these coefficients are determined from steady state experiments or calculations.The emphasis in this paper is laid on a study of the influence of this approach on the overall accuracy.Experiments and calculations for a start-up/ pull-down phase of an upright freezer with evaporator shelves are given.A sensitivity analysis is carried out in order to investigate the accuracy of the dynamic model itself and the importance of the accuracy of the steady state coefficients used.