C:\Users\raoh\Desktop\TJER_Vol.12(2)_2015_PDFs_Press\Paper 7.xps The Journal of Engineering Research (TJER) Vol. 12, No. 2 (2015) 60-67 Drag Reduction Properties of Nanofluids in Microchannels H.A. Abdulbari* and F.L.W. Ming Centre of Excellence for Advanced Research in Fluid Flow; Faculty of Chemical Engineering and Natural Resources, Universiti Malaysia Pahang, Gambang 26300, Kuantan, Pahang, Malaysia. Received 21 November 2014; Accepted 1 July 2015 Abstract: An experimental investigation of the drag reduction (DR) individualities in different sized micro channels was carried out with nanopowder additives (NAs) (bismuth(III) oxide, iron(II/III) oxide, silica, and titanium(IV) oxide) water suspensions/fluids. The primary objective was to evaluate the effects of various concentrations of NAs with different microchannel sizes (50, 100, and 200 µm) on the pressure drop of a system in a single phase. A critical concentration was observed with all the NAs, above which increasing the concentration was not effective. Based on the experimental results, the optimum DR percentages were calculated. The optimum percentages were found to be as follows: bismuth III oxides: ~65% DR, 200 ppm and a microchannel size of 100 µm; iron II/III oxides: ~57% DR, 300 ppm, and a microchannel size of 50 µm; titanium IV oxides: ~57% DR, 200 ppm, and a microchannel size of 50 µm, and silica: 55% DR, 200 ppm, and a microchannel size of 50 µm. Keywords: Microchannels, Pressure drop, Drag reduction, Nanopowder additives. خواص // 50100 200 65200100 /5730050 572005055 20050 *Corresponding authors’ e-mail: hayder.bari@gmail.com H.A. Abdulbari and F.L.W. Ming 61 1. Introduction The study of drag reduction (DR) in microchannels has raised considerable interest in researchers all over the world. DR additives (DRAs), such as polymers, surfactants, and microbubbles, have proven their effectiveness and are appreciated by many industries for their economic value (Nghe et al. 2010). The use of diverse polymers such as DRAs has been reported previously (Abubakar et al. 2014; Al- Sarkhi 2012; Matras and Kopiczak 2015; Edomwonyi-Outu, Chinaud and Angeli 2015; Hong et al. 2015; Iaccarino et al. 2010; Resende et al. 2011) and surfactants (Drzazga et al. 2013; Li et al. 2008; Różański 2011; Yu and Kawaguchi 2006; Tuan Mizunuma 2013; Qi et al. 2011) depending upon the polarities and different behaviors in a turbulent flow (Tarn and Pamme 2014), while the impact of other colloidal suspensions, such as nanofluids, in reducing the pressure drop has not been widely studied. The dispersion quality of the nanoparticles in the base fluid and the stability of the suspension play a crucial role in most applications of practical interest (Rivet et al. 2011; Xie et al. 2003; Choi et al. 2007; Ganguly et al. 2009). Zhao et al. (2009) studied the viscosity of silicon dioxide nanofluids with different particle sizes and pH values. The results revealed that the nanoparticle diameter is of crucial importance to the viscosity of a nanofluid. The smaller the nanoparticles, the larger the viscosity and the greater the dependence on the nanofluid volume fraction. Zhao et al. (2009) studied the effect of silica-based nanofluids as DRAs. The results suggested that the DR enhanced increasing nanofluid concentrations up to a critical concentration, above which no significant DR was observed. Pouranfard. (2014) observed an increase in the viscosity of a fluid with increasing concentrations of nanofluid DRAs and decreases in temperature. Kostic (2013) reported a comparative study of silica and carbon nanotubes (CNTs), where the CNTs were found to be better DRAs (75%) than silica with a maximum reduction of 60% (Yang et al. 2005). Very few studies have been done on DR using nanoparticles compared to polymers and surfactants. Therefore, in this study, nanofluids at different operating conditions were injected through microchannels, and the effects of different parameters on DR were investigated. Pressure drop was calculated as a function of flow rate (rpm) in different sized microchannels (50, 100, and 200 µm) by varying the concentration of different types of oxide nanoparticles, including bismuth, iron, silica, and titanium. The pressure drop was later applied in calculating the DR efficacy (%DR) of a particular nanofluid. In addition, the effect of flow rate and channel size on DR was also investigated. 2. Experimental Procedure 2.1 Materials Bismuth(III) oxide, iron(II, III) oxide, titanium (IV) oxide, and silicon(II) oxide were purchased from Sigma-Aldrich (St. Louis. Missouri, USA) and used without any alterations [Table 1]. Deionized water was used to prepare samples and make dilutions. Table 1. Physical properties of nanoparticles used. Working fluids M. weight (g/mol) Size Density (g/mL) Bismuth(III) oxide (Bi2O3) 465.96 90– 210 0.5–1.1 Iron(II,III) oxide (Fe3O4) 251.53 50– 100 4.8–5.1 Silica (SiO2) 60.08 200– 300 0.037 Titanium(IV) oxide (TiO2) 79.87 <100 -- 2.2 Nanofluid Solution Preparation Nanoparticle solutions were prepared in deionized water for each oxide at five different concentrations, ranging from 100–500 ppm. The fluids were homogenized at high speed for 10 minutes with a stirrer and then left overnight on low speed to achieve maximum dispersion. Before running each experiment, the solution was stirred for two hours at 100 rpm to avoid any agglomeration. 2.3 Experimental Setup Straight microchannels of 50, 100, and 200 µm with a length of 58.5 mm (TOPAS Advanced Polymers, Frankfurt-Höchst, Germany) were employed in this study. The solution was transferred through two syringes via a syringe pump (model SN-50F6) connected to a T-junction connection whose outlet was connected to another T-junction connection. A pressure transmitter (model: STK336) and the inlet of the microchannel were connected to the outlet of the second T- Drag Reduction Properties of Nanofluids in Microchannels 62 junction connection. The solutions were pumped into the microchannel via connecting tubes. The flow rate of the solutions was controlled using the syringe pump. The flow of the liquid in the microchannel was observed using an HBO 50 microscope (Zeiss International, Oberkochen, Germany) connected to a camera. The pressure drop across the microchannel was observed using the pressure transmitter and the results were recorded. The %DR was calculated using Eqn. 1. %DR ∆P ∆P∆P 100 1 Where, ∆P = pressure drop before adding DRA ∆P = pressure drop after adding DRA 3. Results and Discussion A series of experiments were performed to study the effects of nanofluid concentration, microchannel size, and flow rate on pressure drop and %DR. It was found that the %DR rises with increasing nanofluid concentration. However, there is a critical concentration above which no more reduction can be attained. In the present study, the critical concentration obtained was almost the same for all oxide nanoparticles (200 ppm) with the exception of Fe3O4 (300 ppm) [Table 2]. Figure 1 presents a comparison of the pressure drop at the critical concentrations of nanofluids and deionized water versus the flow rate in the microchannels. A regular pressure drop trend was observed with an increasing flow rate in all cases. Increasing the flow rate increases the degree of turbulence which provides a better platform for DRAs to perform (Stone and Ajdari 2014). SiO2 showed a maximum pressure drop at its critical concentration, which may be due to the fact that the mechanism of DR by SiO2 nanoparticles is mainly a surface modification. The SiO2 nanoparticles have good rigidity and reduce the friction factor through dispersion into the liquid phase and contact with the pipe surface, so a higher pressure drop was observed (Zhao et al. 2009). Figures 2–5 show the %DR versus the rate of flow in the different sized microchannels and at different oxide nanoparticle concentrations. The DR is greater with smaller particles as the smaller sized particles with larger surface areas result in the migration effect, where a bond is formed between the transporting fluid molecules and the DR agents (Khadom and Abdul-Hadi 2014). It was observed that the use of nanofluids at low concentrations did not cause a serious reduction in the pressure drop which is due to the small size of the nanoparticles (Abdulbari, Shabirin and Abdurrahman 2013). The diameter of the microchannels does not seem to affect the %DR greatly in contrast to the behavior usually observed with a reduction in diameter (Lee and Mudawar 2007). The effect of solution velocity (m) on the %DR was studied in terms of the volumetric flow rate. The results show that the %DR increases with increasing fluid velocity. Increasing the fluid velocity means increasing the degree of turbulence inside the pipe, which will provide a better medium for the drag reducer to be more effective (Byrne, Hart and da Silva 2012). Figure 2 presents the %DR while keeping the concentration constant for all the nanofluids at 100 ppm with varying microchannel sizes (50, 100, and 200 µm). An increase in %DR was observed when increasing the microchannels’ sizes from 50 to 200 µm. This can be explained by the presence of large Figure 1. A comparative study of pressure drop between deionized water and oxides used in the study. H.A. Abdulbari and F.L.W. Ming 63 Table 2. Parameters affecting the drag reduction in different nanofluids. Oxide Channel size Critical Con. Flow rate Pressure drop Drag reduction µm ppm ml/hr kPa % 50 200 1350 9.93 56.9 Titanium 100 200 1250 10.74 53.3 200 200 1350 9.34 51.3 50 200 1400 8.28 63.0 Bismuth 100 200 1400 7.35 64.8 200 200 1200 8.67 57.5 50 200 1350 10.35 55.1 Silicon 100 200 1400 9.99 52.1 200 200 1400 9.68 57.6 50 300 1450 9.59 56.8 Iron 100 300 1350 9.66 56.5 200 300 1350 9.94 48.2 µm = micrometer; Con. = concentration; ppm = parts per million; kPa = kilopascals Figure 2. Variations of %DR with a nanofluid concentration of 100 ppm. 42 44 46 48 50 52 54 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-100ppm-50µm Bismuth (III) oxide-100ppm-50µm Silicon dioxide-100ppm-50µm Iron (II,III) oxide-100ppm-50µm 39 41 43 45 47 49 51 53 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-100ppm-100µm Bismuth (III) oxide-100ppm-100µm Silicon dioxide-100ppm-100µm Iron (II,III) oxide-100ppm-100µm 34 36 38 40 42 44 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-100ppm-200µm Bismuth (III) oxide-100ppm-200µm Silicon dioxide-100ppm-200µm Iron (II,III) oxide-100ppm-200µm (a) (b) (c) Drag Reduction Properties of Nanofluids in Microchannels 64 Figure 3. Variations of %DR with nanofluid concentration of 200 ppm. Figure 4. Variation in %DR at a nanofluid concentration of 300 ppm. 45 50 55 60 65 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-200ppm-50µm Bismuth (III) oxide-200ppm-50µm Silicon dioxide-200ppm-50µm Iron (II,III) oxide-200ppm-50µm 38 43 48 53 58 63 68 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-200ppm-100µm Bismuth (III) oxide-200ppm-100µm Silicon dioxide-200ppm-100µm Iron (II,III) oxide-200ppm-100µm 34 39 44 49 54 59 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-200ppm-200µm Bismuth (III) oxide-200ppm-200µm Silicon dioxide-200ppm-200µm Iron (II,III) oxide-200ppm-200µm 32 37 42 47 52 57 62 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-300ppm-50µm Bismuth (III) oxide-300ppm-50µm Silicon dioxide-300ppm-50µm Iron (II,III) oxide-300ppm-50µm 32 37 42 47 52 57 62 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-300ppm- 100µm Bismuth (III) oxide-300ppm- 100µm 25 30 35 40 45 50 55 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-300ppm-200µm Bismuth (III) oxide-300ppm-200µm Silicon dioxide-300ppm-200µm Iron (II,III) oxide-300ppm-200µm (a) (b) (c) (c) (a) (b) H.A. Abdulbari and F.L.W. Ming 65 Figure 5. Variations in %DR at nanofluid concentrations of 400 ppm. eddies in the larger microchannels which absorb a large amount of energy from the flow. In smaller microchannels, more smaller eddies are formed, but some small eddies are unable to absorb enough energy and are incapable of overcoming the viscosity resistance (Kim et al. 2000; Khadom and Abdul-Hadi 2014; Al-Sarkhi and Hanratty 2001). SiO2 exhibited the highest %DR of all the microchannels, which may be due to the larger sized silica particles settling down quickly and further decreasing the diameter of the microchannels. Figure 3 depicts the maximum DR achieved in all the oxides except for Fe3O4 nanofluids at a concentration of 200 ppm, which is a clear indication of reaching a critical concentration. Bi2O3 exhibited the highest %DR (~65%) at the critical concentration with a microchannel size of 100 µm at 1400 rpm, which reduced to 200 µm-sized microchannels (~58%) with a flow rate of 1200 rpm. In the case of SiO2 and TiO2, the maximum drag achieved at the critical concentration was ~57.5 and ~57% with 200 and 50 µm-sized microchannels, respectively. The iron oxide nanofluid exhibited maximum DR at a concentration of 300 ppm in all the microchannels [Fig. 4]. Maximum DR (~ 56.8 %) was achieved with 50 µm-sized microchannels and a flow rate of 1400 mL/hr. In cases of 100 µm-sized microchannels, a nearly identical increasing trend was observed and maximum drag reduction (~56.5 %) was observed at a slightly lower flow rate (1350 mL/hr). A decreasing trend in drag was observed for all the other oxides, which confirmed that all the oxides had reached their critical concentration limit; increasing the concentration further had no more effect on the pressure drop in a flow. A gradual decrease in the DR properties of the iron oxide nanofluid was observed at 400 ppm [Fig. 5], emphasizing that iron oxide nanofluids are more effective in the concentration range of 300–400 ppm. All the other nanofluids exhibited a very low DR; thus, it is obvious that, after a critical limit has been reached, increasing the concentration has no positive effect on DR efficiency and a negative impact on the flow of a system. 4. Conclusions To conclude, the DR performances of different nanofluids were studied at diverse concentrations in 22 27 32 37 42 47 52 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-400ppm-50µm Bismuth (III) oxide-400ppm-50µm Silicon dioxide-400ppm-50µm Iron (II,III) oxide-400ppm-50µm 32 37 42 47 52 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-400ppm-100µm Bismuth (III) oxide-400ppm-100µm Silicon dioxide-400ppm-100µm Iron (II,III) oxide-400ppm-100µm 22 27 32 37 42 1100 1200 1300 1400 1500 Flowrate (ml/hr) Titanium (IV) oxide-400ppm-200µm Bismuth (III) oxide-400ppm-200µm Silicon dioxide-400ppm-200µm Iron (II,III) oxide-400ppm-200µm Drag Reduction Properties of Nanofluids in Microchannels 66 different sized microchannels. It was observed that all the nanofluids exhibited a certain critical concentration above which increasing the concentration had no greater impact on the pressure drop or the DR of the system. 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