**Publish at : ** 31 Oct 2017 - 00:00

**IJtech :** IJtech
Vol 8, No 5 (2017)

**DOI :** https://doi.org/10.14716/ijtech.v8i5.867

Novianto, S.., Pamitran, A.S.., Koestoer, R.., Oh, J.., Saito, K.., 2017. Effect of Liquid Reynolds Number on Pressure Drop of Evaporative R-290 in 500µm Circular Tube.

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Sentot Novianto | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia |

Agus S. Pamitran | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia |

Raldi Koestoer | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia |

Jong-Taek Oh | Department of Refrigeration and Air Conditioning Engineering, Chonnam National University, San 96-1,Dunduk-Dong, Yeosu, Chonnam 550-749, Republic of Korea |

Kiyoshi Saito | Department of Applied Mechanics and Aerospace Engineering, Waseda University, 1-104, Totsuka-machi, Shinjuku-ku, Tokyo 169-8050, Japan |

Abstract

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/m^{2}s 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

Introduction

Experimental studies of pressure drops with propane (R-290) in
microchannels are still limited. Maqbool et al. (2013) research two-phase heat
transfers and pressure drops of propane (R-290) in a vertical mini channel, with an inner diameter of 1.7 mm and a heated length 245 mm. (Del Col et al., 2014) research
two-phase heat transfers and pressure drops of R-290 in a minichannel with an
internal diameter of 0.96 mm and
a rough inner surface. The predictions for these pressure drops used separate
models, namely Friedel (1979), Del Col et al. (2013), Zhang and Webb (2001).

Natural
refrigerants have become more popular, and more intensively discussed, because
of the increasing awareness of environmental issues. Propane is an
environmentally-friendly refrigerant with zero ODP (Ozone Depletion Potential) and
low GWP (Global Warming Potential) (Choi et al., 2009). Moreover, the natural
refrigerant R-290 is situated to replace R-22, in part because of its
hydrodynamic performance (Ghazali et al., 2016). The properties of a given
refrigerant contribute to the channel classification, whether microchannel or
conventional channel (Kew &
Cornwell, 1997). The authors introduced a confinement number (Co) as a ratio of
capillary length and hydraulic diameter. The selected channel, with a diameter
500µm and with R-290 as the working fluid, can be classified as a microchannel.

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.

Experimental Methods

**2.1. ****Experimental ****S****et ****U****p**** **

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/m^{2}.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.m^{2} the pressure drop occurs on 2540 Pa. When G = 456 kg/s.m^{2}, 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.

Conclusion

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.

Acknowledgement

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

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