Research PaperTransient model of a refrigerant-to-water helically coiled tube-in-tube heat exchanger with corrugated inner tube

- The homogeneous flow approach is suitable for refrigerant two-phase flow modeling.
- The superposition principle is appropriate where suitable correlations are missing.
- Transient models of refrigeration systems require correct OFF-cycle modeling.
- Helically coiled double tubes with inner fluted tube experience high pressure drop.

A transient model of a refrigerant-to-water helically coiled double tube with corrugations in the inner tube is developed. The refrigerant flows in the annulus, and a secondary fluid (water, glycol/water solution, etc.) flows in the inner tube. The model addresses both the single-phase and the two-phase flows of the refrigerant and the single-phase flow of the secondary fluid. Both the ON/OFF operation of the compressor and of the secondary fluid pump (at compressor shut-down, the pump may be ON or OFF) are considered. To deal with the lack of suitable friction factor and heat transfer correlations, a superposition principle that allows to combine the effects of the curvature of the heat exchanger and of the flutes in the inner tube is adopted. The finite volume method is used to numerically solve the governing equations. The developed model is experimentally validated and achieves reasonable agreement with the experimental data in both steady-state and transient conditions, in both the heating and the cooling modes.