Effect of Liquid Reynolds Number on Pressure Drop of Evaporative R-290 in 500µm Circular Tube

Due to certain advantages, natural refrigerants have recently become more popular. Environmental issues motivate this study, focused on the characteristics of propane (R-290) as a replacement for conventional refrigerants. The aim of the present research is to characterize the pressure drop of evaporative R-290 in a microchannel of 500µm diameter and 0.5 m length. The variables of the experimental conditions are mass flux between 155 and 1071 kg/m²s and vapor quality between 0 and unity. The results show a laminar flow for liquid R-290 and a turbulence flow for vapor. Some existing correlations of two-phase flow viscosity were used to predict the pressure drop. For homogeneous model, Dukler et al.’s (1964) prediction viscosity correlation best predicted the present experimental pressure drop.

Microchannel; Propane; Pressure drop; Two-phase flow; Viscosity

Two-phase flows in microchannel applications have become more popular in many industries, recently. Pressure drops on heat exchangers are an important consideration in energy conversation. Therefore, research on pressure drops in microchannels is very important, and more must be conducted. Studies on pressure drops in a tube with homogeneous models have already been developed. The homogeneous models consider the two-phases to flow as a single phase possessing mean fluid properties. Pressure drops, with homogeneous models, consist of a frictional pressure drop, acceleration pressure drop, and static pressure drop:

                                                                       (1)

The frictional pressure drop is a function of the friction factor coefficient, mass flux, hydraulic diameter, and density. The predicted two-phase flow viscosity is significant in influencing the friction factor coefficient. Some existing correlations of two-phase flow viscosity (i.e. Cicchitti et al., 1959; Dukler et al., 1964; McAdams et al., 1942) were used to predict the pressure drop.

There is limited research on two-phase flow boiling using propane in a microchannel. This study aims to characterize the effect of the liquid Reynolds number on pressure drops for two-phase flow boiling of R-290 in a microchannel with a diameter of 500µm.

2.1.    Experimental Set Up

Figure 1 depicts the experimental apparatus. The main observation is of the test section heated by the electrical heater. The test section is a horizontal tube with a diameter of 500 µm and length of 0.5 m.

Figure 2 Comparison of Reynolds number with varying two-phase viscosity from McAdams et al., 1942; Cicchitti et al., 1959; Dukler et al., 1964

Figure 2 shows the comparison of predicted Reynolds numbers using the two-phase viscosity models (McAdams et al., 1942; Cicchitti et al., 1959; Dukler et al., 1964). The data on the figure used mass flux 130 kg/m².s and quality from 0 to unity. Duklerat al.’s (1964) equation generally results in a higher two-phase Reynolds number. The McAdams et al. (1942) equation results in a linear Reynolds number gradient. The Cicchitti et al. (1959) equation results in a lower two-phase Reynolds number.

Predictions for the pressure drop used homogeneous models. The results indicate that the Dukler et al. (1964) correlation was best able to predict pressure drops, with MRD 63%. Figure 3 shows pressure drop comparisons between the experimental results and the predicted pressure drops using calculated viscosity. Predictions of pressure drop using the McAdams et al. (1942) correlation resulted in MRD 79%, and predictions of pressure drop with the Cicchitti et al. (1959) correlation resulted in MRD 89%. The all predicted pressure drop showed the MRDs were higher than 50%. This also means that the experimental pressure drop is lower than the predicted pressure drop. Based on the MRD result, predicted pressure drop using homogeneous models as MRD more than 50%. The Dukler et al. (1964) equation offered a lower MRD because it has, overall, higher predicted two-phase Reynolds numbers. The two-phase Reynolds number from Dukler et al. (1964) is a function of average density, quality, specific volume, and viscosity. The properties’ average densities decrease when quality increases. Lower average density caused higher two-phase viscosity. Clearly, the two-phase Reynolds number predictions resulting from Dukler et al. (1964) are higher than McAdams et al.’s (1942) or Cicchitti et al.’s (1942)  model. The frictional pressure drop is a function of the two-phase Reynolds number. Under constant mass flux test conditions, the increasing of heating will increase vapor quality and the two-phase Reynolds number. The increasing two-phase Reynolds number will increase the frictional pressure drop. 

Figure 4 shows the effect of the liquid Reynolds number on pressure drops with a 0.5 mm diameter tube. In Figure 4, the lower liquid Reynolds number is obtained from higher heat flux conditions. The increasing heat flux can make the liquid became vapor faster. More vapor means higher vapor quality. The higher heat flux means the working fluid has a higher vapor quality at the outlet of the test section. Under constant mass flux test conditions, higher vapor quality results in lower liquid Reynolds numbers. This correlation means that higher heat flux results in lower liquid Reynolds numbers, or, conversely, lower heat flux results in higher liquid Reynolds numbers. Figure 4 shows that the pressure drop decreases with increasing liquid Reynolds numbers. This also means that the pressure drop decreases with decreasing heat flux, and the pressure drop also decreases with decreasing vapor quality. Zhang and Webb (2001) also reported that the two-phase pressure drop increases with vapor quality.

Figure 3 Experimental pressure drop versus predicted pressure drop with calculated viscosity

Figure 4 Effect of Reynolds number liquid on pressure drops with 0.5mm diameter tube

The present experimental result corresponds with the result of the Choi et al. (2009) experimental data using a 3 mm diameter tube. There, the pressure drop decreases when the liquid Reynolds number increases. The author reported that increasing heat flux resulted in more vaporization and an increased pressure drop.

The effect of mass flux on the pressure drops of the present experiment with constant heat flux and constant saturation temperature is explained by the pressure drops increase in connections with increasing mass flux. For G = 295 kg/s.m² the pressure drop occurs on 2540 Pa. When G = 456 kg/s.m², the pressure drop occurs on 4046 Pa. Increasing mass flux at the same diameter will increase the Reynolds number. Similarly, Dario et al. (2016) reported that pressure drops increases in connections with mass flux. 

An experiment on two-phase flow boiling pressure drops in microchannels is presented in this study. Pressure drops are best predicted with homogeneous models when using Dukler at al.’s (1964) viscosity prediction method. Each two-phase viscosity method offers a different prediction for the two-phase Reynolds number. Under the same mass flux conditions, increases in heating will increase vapor quality and increase two-phase Reynolds numbers. The increasing two-phase Reynolds number will increase frictional pressure drop. 

The present study shows that pressure drop decreases with increasing liquid Reynolds number and decreasing heat flux. The lower heat flux results in lower vapor quality. However, the pressure drop decreases with decreasing vapor quality.

This study is funded by a research grant from PUPT 2017 RISTEKDIKTI (2727/UN2.R3.1/HKP05.00/2017).

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