Impact of classical assumptions in modelling a microchannel gas cooler

Most of the current air-to-refrigerant heat exchanger models use the classic ɛ-NTU approach, or some of its assumptions. These models do not account for longitudinal heat conduction in the tube and the fin, and the heat conduction between different tubes. This paper presents a more fundamental numerical approach to heat exchanger modelling which takes into account the 2D longitudinal heat conduction in any element, does not apply the fin theory, and captures a more detailed representation of air properties. Using the fundamental numerical approach, the paper assesses the impact of the traditional heat exchanger model assumptions when modelling a microchannel gas cooler working with CO2. The study revealed significant differences in capacity predictions depending on the ɛ-NTU relationship adopted. Large errors in capacity prediction of individual tubes occurred due to the adiabatic-fin-tip assumption when the neighbouring tubes were of different temperature.

► We propose a model to evaluate the impact of classical assumptions in gas coolers.
► Two approaches based on ɛ-NTU methodology were compared against the present model.
► Neglecting 2D longitudinal heat conduction introduced an error of as much as 2.5%.
► Assuming adiabatic-fin-tip leads to large errors in heat distribution per tube.
► Temperature of air close to the tube is very different than the bulk air temperature.