Self-sufficient Propane-CO2 cascade with two-phase thermosiphon - Simulation of an evaporating two-phase flow with variable heat transfer coefficient"

A thermal energy storage (ice storage) is to be charged by a natural circulation evaporator, also called a two-
phase thermosiphon. When using the natural circulation evaporator, the heat transmittance coefficient from
the freezing water to the refrigerant changes with the loading of the ice storage. For the correct design of
such a system it is important to predict the pressure drop and the flow behavior. In order to use a minimum
of refrigerant, the smallest pipe diameter should be calculated at which the thermosiphon can be operated
even with the required ice thickness on the outside. In this paper, a simulation is described, whereby the flow
patterns, the required pipe length and the pressure drop can be determined for different internal pipe
diameters and increasing ice thickness. A variable length pipe coil is simulated. R-744 (carbon dioxide) is used
as refrigerant. The design procedure is explained by means of an example application in which an ice storage
system is calculated for a cascade refrigeration system inside a refrigerated container.