Comparative investigations of the condensation of R134a and R404A refrigerants in pipe minichannels

Abstract

The present paper describes the results of experimental investigations of heat transfer and pressure drop during the condensation of the R134a and R404A refrigerants in pipe minichannels with internal diameters d = 0.31–3.30 mm. The results concern investigations of the local heat transfer coefficient and a pressure drop in single mini-channels. The results were compared with calculations according to the correlations proposed by other authors. Within the range of the examined parameters of the condensation process in mini-channels produced from stainless steel, it was established that the values of the heat transfer coefficient may be described with Akers et al. and Shah correlations within a limited range of the mass flux density of the refrigerant and the mini-channel diameter. A pressure drop during the condensation of these refrigerants is described in a satisfactory manner with Friedel and Garimella correlations. On the basis of the experimental investigations, the authors proposed their own correlation for the calculation of local heat transfer coefficient α_x.

Introduction

The turn of the 21st century saw an abrupt technological advance that was evident in the qualitative and quantitative growth in the production of space technology and electronics devices. These two technologies are inseparable, and they mark a trend toward the miniaturisation of these devices. It is worth noting that the absolute value of thermal power in computer systems is not significantly large, while the density of heat fluxes, i.e., the amount of heat transferred by a field unit of the heat transfer area, reaches substantial values, even as much as 1000 W/cm², as indicated by Baummer et al. [1]. The traditional methods for transferring such heat flux densities are either minimally useful or not useful at all. A paper by Obhan and Garimella [2] discusses methods presently used and those recommended for the future. In view of the above, the use of two-phase media, which mediate in the heat exchange process, becomes a priority. The practical realisation of these media involves an implementation of the intensification methods of convective heat transfer with phase changes. One of the passive methods used to intensify the convective heat transfer process includes, among others, a reduction of the diameter of those channels through which agents flow that realise the heat transfer process. An increase in the value of heat transfer coefficients serves as a measure of the effectiveness of this intensification. The following criteria need to be fulfilled for the construction of modern refrigeration and air conditioning heat exchangers: small dimensions, a high efficiency of heat transfer and environmentally friendly technology.

For this purpose, the concept of so-called compact heat exchangers (including refrigeration evaporators and condensers) was proposed. In these compact heat exchangers, the ratio of the heat transfer area to the overall volume is greater than 700 m²/m³ [3]. The fulfilment of this criterion requires the use of channels with hydraulic diameters d_h = 1–6 mm. Such a criterion was proposed by Mehendale et al. [4]. At present, the classification proposed by Kandlikar [5] is commonly used. This classification specifies in detail the measurements of channels in the construction of heat exchangers. According to this classification, the following can be used in compact exchangers: for micro-channels, d_h = 10–200 μm; for mini-channels, d_h = 200 μm–3 mm. In comparison, conventional channels have hydraulic diameters d_h ⩾ 3 mm.

Recent years have seen an increased interest in and a growing number of papers about heat and flow processes in compact exchangers where channels are used with dimensions smaller than conventional ones [6], [7], [8]. Also, the number of studies concerning heat transfer and flow resistance in channels with small diameters is growing as is the number of studies that deal with the condensation process. One of the reasons for this increased interest might be that, initially, the boiling and condensation phenomena in these types of channels were treated as being symmetrical to one another, which, even concerning the flow in conventional channels, proved to be too great of a simplification [9]. This problem was confirmed in papers by Thome et al. [10], [11], which presented a new heat transfer model in horizontal pipes with hydraulic diameters d_h = 3.1–21.4 mm, and in Cavallini et al. [12], who dealt with heat transfer in mini-channels. Many authors warn against transferring to micro- and mini-channels those correlations that have been specified and verified for the flow in a phase change in conventional channels. However, it is to be noted that the correlations developed for low- or medium-pressure refrigerants (e.g., R134a) do not have to be proper for high-pressure refrigerants such as R404A and R410A.

Designing compact heat exchangers is also made difficult because, according to the present state of knowledge, there is no unequivocal indication of a unified model of the condensation of refrigerants in micro- and mini-channels. Investigations concerning the realisation of this postulate are being conducted in many countries, while an introductory attempt to summarise these was provided, among others, in papers [13], [14].

This study presents research problems that occur in compact condensers with R134a and R404A refrigerants. Homogenous R134a refrigerant belongs to the HFC group and is one of the substitutes of R12, while the near-azeotropic mixture (composed of three refrigerants from the HFC group: R134a, R125 and R134a) is a substitute for R22. On 1 January 2010, R22 freon was withdrawn from active application in refrigeration technology (it is still possible to service devices that were designed prior to that date).