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2170, Page 2 Moreover, fullerene (C60) was studied by Xing et al. (2014) as additive of mineral oil in a domestic refrigerator working with isobutane (R600a). They found about 5-6% COP improvement with a nanoparticles concentration of 3 gL-1. Considering the extraordinary importance of any possible improvement in the HVAC&R applications performance, the employment of nanofluids should be seriously evaluated. In this paper, the use of nanofluids as nanolubricants in compressors for heat pump systems has been tested in order to detect any possible positive effect. Different nano-oils, based on POE and mineral oil added with TiO2 or Single-Wall Carbon Nanohorns (SWCNH) nanoparticles, have been studied in a properly built test rig.POE oil with TiO2 nanoparticles at 0.1 wt%, 0.05 wt% and 0.5 wt%, POE oil with SWCNH nanoparticles at 0.1 wt% and mineral oil with TiO2 nanoparticles at 0.1 wt% were tested. 2. EXPERIMENTAL SECTION 2.1 Nanofluids preparation The optimization of the two-step preparation method of nanofluids has been deeply explained in Colla et al. (2013). A commercial polyolester (POE) oil and a mineral oil were used as base fluids; TiO2 nanoparticles, purchased from Degussa (TiO2, P25), with a spherical shape and a declared 21 nm diameter, and SWCNH nanoparticles, supplied by Carbonium Srl, with an estimated equivalent diameter of 100 nm, were chosen as particles. No dispersant was added. Nanoparticles were dispersed in the lubricant by means of ultrasound irradiation, supplied by a Sonics & Materials VCX130 sonicator, operating at 20 kHz frequency and 130 W maximum power, equipped with a 6 mm diameter Ti-6Al-4V alloy tip. Nanoparticles mass fraction, sonication time, sonication power and sample temperature reached during sonication were optimised in order to obtain the most stable suspension. The best conditions were 180 minutes sonication time at the 50% of the maximum power of sonication (65 W) and the reached preparation temperature of about 69°C. 2.2 Nanofluids characterization As described in Colla et al. (2013), all the considered nanofluids, i.e. POE oil with TiO2 nanoparticles at 0.1 wt%, 0.05 wt% and 0.5 wt%, POE oil with SWCNH nanoparticles at 0.1 wt% and mineral oil with TiO2 nanoparticles at 0.1 wt%, were characterized in order to check their stability, dynamic viscosity and thermal conductivity. All nanofluids showed a good stability, proved by DLS measurements. Thermal conductivity and dynamic viscosity were respectively measured by means of a Hot Disk thermal conductivity-meter and a rotational cone-plate rheometer. Generally, from these experimental results, pure oil and nano-oils seem very similar in terms of thermal conductivity and dynamic viscosity. 2.3 Experimental test rig In order to test the synthesized nanolubricants, the experimental circuit shown in Figure 1 was built; the typical operating conditions of a heat pump system were carried out. The most important parts of the circuit were the rotary compressor, the concentric tube condenser, the thermostatic expansion valve and the concentric tube evaporator. Two see-through tubes made of polycarbonate for visual inspection were installed in order to visually observe the refrigerant and the dispersed nanoparticles flow. The two tubes were installed downstream of the condenser and downstream of the evaporator. All the necessary parameters to control the entire functioning of the system were acquired, as refrigerant flow rate, temperatures and pressures. In Figure 2, all the acquisition points are indicated. Water was involved as secondary fluid in both the heat exchangers; inlet temperature was controlled by means of two different thermostatic baths and flow rate was set through two pumps. Refrigerant and water temperatures were measured by means of thermoresistances Pt100; the total uncertainty includes the measurement uncertainty, the error in the acquisition of the digital multimeter and the data interpolation. An overall uncertainty of 0.07 ̊C was estimated for all the employed Pt100 sensors. Refrigerant pressures were acquired by means of piezoresistive transmitters; three different models with different precision were used: sensors supplied by Wika have a total percentage error equal to 0.75%, sensors supplied by Bell&Howel equal to 0.05% and sensors supplied by CEC Instrument equal to 0.036%. The total percentage error takes into account the non-linearity, non-repeatability, stability over time and the acquisition system accuracy. The water flow rates through the two heat exchangers were measured by means of two magnetic flow meters, with an uncertainty of 0.35%, as declared in the calibration certificate. A Coriolis mass flow meter, supplied by Emerson, was used for refrigerant flow rate measurement; as indicated by the calibration certificate, the overall uncertainty is 0.00000176 kg/s. Moreover, the ambient temperature and the temperature at the base and the head of the compressor were acquired by means of T type thermocouples provided by Tersid with a declared error of ±0.3 ̊C. The experimental data were acquired, elaborated 15th International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2014PDF Image | Nanofluids Application as Nanolubricants in Heat Pumps Systems
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