Effects of thermal conduction in microchannel gas coolers for carbon dioxide

Abstract

A numerical code was developed for an accurate fully three-dimensional simulation of crossflow compact heat exchangers with finned, internally microchannelled flat tubes, which are often used as gas coolers in transcritical refrigerating machines CO 2 operated. The equations describing the system were discretised by means of a finite-volume and finite-element hybrid technique for strictly adhering to the real heat transfer process regarding the finned surfaces. The numerical code uses recent correlations by different authors for predicting the heat transfer coefficients both refrigerant side and air side. The results of simulations are verified against experimental data reported in the open literature. The aim of this work is to investigate the effects of thermal conduction inside metal on the overall performance of the considered gas coolers; high-resolution meshes for the discretisation of separating wall and fins makes it possible to avoid much of the approximations typical of the traditional approaches. In particular the efficiency of finned surfaces, the real distribution of thermal fluxes between the two fin roots and the effects of thermal conduction along the walls of microchannel flat tubes are extensively discussed. The numerical simulations confirm that the traditional approach for describing fins, which assumes them as adiabatic at the middle section in order to decouple the equations accounting for the effect of different temperatures at the fin roots, can be considered acceptable in a wide range of applications. In a similar way, the conduction on fins along the direction of the air velocity and the longitudinal conduction on tubes produce a negligible effect on the performance of the considered class of heat exchangers.

Numerical Model for Microchannel Condensers and Gas Coolers With an Improved Air-Side Approach

“…It does not account for 2D longitudinal heat conduction in the tube and it neglects the longitudinal heat conduction in the fin, along the air flow direction. Asinari et al (2004) concluded that these effects produce a negligible effect on the performance of the class of CO 2 gas cooler they studied.…”

“…This capability means a model able to simulate both scenarios with and without fin cut. Asinari et al (2004) proposed a three-dimensional model for microchannel gas coolers using CO 2 as refrigerant which can evaluate such effects but the model employs a finite-volume and finite-element hybrid technique. This model discretizes the equations by means of a finite-volume and finite-element hybrid technique taking into account longitudinal heat conduction (LHC) along all directions for all elements (fins and tubes), thus it does not employ the adiabatic-fintip assumption.…”

“…For a finned tube gas cooler Singh et al (2010) reported heat load gain of up to 12% and fin material savings of up to 40%. However, improvements no so large have been achieved for MCHXs, namely: Asinari et al (2004) who concluded that the impact of this heat conduction can be assumed as negligible in a wide range of applications; Park and Hrnjak (2007) reported measurements of capacity improvements of up to 3.9% by cutting the fins in a CO 2 serpentine microchannel gas cooler. Also Zilio et al (2007) concluded that heat conduction through fins in a CO 2 gas cooler had a significant impact on the capacity.…”

“…Asinari et al (2004) presented a model for MCHX gas coolers that included these phenomena but an important difference exists between these similar models and the proposed one in this work related to the computational cost. The method usually applied by these models to discretize governing equations is a finite element method (FEM) (Patankar, 1980) or even a hybrid one as Asinari et al (2004) employed, which use both the FEM and the finite volume method (FVM) (Patankar, 1980). …”

“…Fin2D differs from other models referred to previously in the number of classical assumptions made. The model of Asinari et al (2004) is the most similar to the present model regarding the number of assumptions, although the discretization method applied by them is a hybrid one, which uses both the finite element method (FEM) and the finite volume method (FVM), whereas the methodology used in the Fin2D model is the FVM.…”