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Performance augmentation of a water chiller system using nanofluids.


Recently, there has been an interest in using nanoparticles as additives to modify heat-transfer fluids to improve their performance. The new class of heat-transfer fluid is called nanofluids. Nanofluids are formed by combining nanomaterials with heat-transfer fluids (Choi 1995). Through dispersion or suspension, nanoparticles of high thermal conductivities in heat-transfer fluids give rise to higher thermal conductivity of the mixtures, thus increasing their heat transfer coefficient. The thermal conductivity of heat-transfer fluid plays an important role in the development of energy-efficient heat transfer equipments including electronics, HVAC&R, and transportation. Development of advanced heat-transfer fluids is essential to improve the effective heat transfer behavior of conventional heat-transfer fluids. The nanofluids are capable of improving thermal conductivity and heat transfer considerably.

Thermal conductivity enhancements in ethylene glycol and synthetic engine oil with the addition of multiwalled carbon nanotubes (MWCNTs) had been investigated previously (Liu et al. 2005). The results show that MWCNTs nano-fluids have noticeably higher thermal conductivities than the base fluid without MWCNTs. For MWCNT-ethylene glycol suspensions at a volume fraction of 0.01 (1 volume percent), thermal conductivity is enhanced by 12.4%. On the other hand, for MWCNT-synthetic engine oil suspension, thermal conductivity is augmented by 30% at a volume fraction of 0.02 (2 volume percent). The rates of increase are, however, quite different, subject to change of the base fluids. The MWCNT-synthetic engine oil suspension has a much higher enhanced thermal conductivity ratio than that of the MWCNT-ethylene glycol suspension. Thermal conductivity enhancements with water as the base fluid in the presence of MWCNTs have also been investigated (Liu et al. 2006). The results for MWCNT-water suspensions also exhibit the same trend. For a MWCNT-water suspension at a volume fraction of 0.015 (1.5 volume percent), thermal conductivity is increased up to 17.8%.

The thermal conductivity of nanoparticles strongly affects the enhancement of the thermal conductivity of nano-fluids. The largest increase in thermal conductivity has been observed in suspensions of carbon nanotube in oil investigated by Choi et al. (2001). Their results showed that the measured thermal conductivity is anomalously greater than the theoretical predictions, and revealed a nonlinear characteristic with nanotube loadings (Wang and Mujumdar 2005). They reported a 160% enhancement in thermal conductivity at only 1 volume percent. Experimental data collected by Xie et al. (2003) for suspensions of MWCNTs in water, ethylene glycol, and decene also indicated that the thermal conductivity enhancement increased with the rise of nanotube concentration, but decreased when the thermal conductivity of the base fluid is increased (Wang and Mujumdar 2007). Gao et al. (2007) reported that the strong agreement with the experimen tal results of Choi et al. (2001) is obtained using the Brugge-man Effective Medium Theory. The nonlinear dependence of thermal conductivity on the nanotube volume fraction in nano-fluid is well predicted.

For the application of nanofluid in a heat transfer device, Xuan and Li (2003) investigated the convective heat transfer coefficient and frictional characteristics of the nanofluids for both laminar and turbulent flow in a tube. Their experimental results illustrated that the convective heat transfer coefficient of the nanofluids varies with the flow velocity and with volume fraction and is higher than that of the base fluid at the same conditions. Compared with water, the Nusselt number of the nanofluid with 2 volume percent of copper particles is 60% higher (Daungthongsuk and Wongwises 2007). Moreover, Yang et al. (2005) reported experimental results that illustrated the convective heat transfer coefficient of graphite nanoparticles dispersed in liquid for laminar flow in a horizontal tube heat exchanger. The experimental results illustrated that the heat transfer coefficient increased with the Reynolds number and the particle volume fraction. At a concentration of 2.5 weight percent, the heat transfer coefficient is 22% higher than that of the base fluid at 50[degrees]C and is 15% higher at 70[degrees]C (Daungthongsuk and Wongwises 2007).

Considerable efforts have occurred over the past few years. However, these studies were primarily focused either on the measurement/calculation of basic physical properties such as thermal conductivity/viscosity, or on the overall heat transfer/frictional characteristics of nanofluids. Until now, there were no studies associated with the overall system performance or with a field test in which some dynamic characteristics of the system may be missing. In that regard, the purpose of this study is to examine the overall system performance of a water chiller (air conditioner) subject to the influence of nanofluids.


MWCNTs/water nanofluids were prepared using a two-step method described in detail elsewhere (Liu et al. 2006). MWCNTs were prepared first. MWCNTs were produced using the catalytic chemical vapor deposition method. MWCNTs powders were then added to the water base fluid. The volume fraction of MWCNTs/water was 0.001 (0.1 volume percent). The thermal conductivity was increased to 1.3%, without surface treatment, and was measured with a transient hot wire method at room temperature (see Figure 1).


The system performance of a 10 RT water chiller located in a well-controlled chamber was observed. Figure 2 shows a schematic diagram of the experimental test system for the water chiller with a nominal 10 RT capacity. Tests were conducted with and without the addition of MWCNTs/water nanofluid. The test system primarily consisted of an air-cooled chiller with 10 RT capacity, a plate-type heat exchanger with 10 RT capacity, a water thermostat with 6000 L capacity, MWCNTs/water nanofluid, and measuring devices.


The test system included a base fluid loop and a water loop. The base fluid could be supplied with either water or with nanofluid. It consisted of an air-cooled chiller, a circulation pump for delivering chilled water being generated, an injection port of nanofluid, and a plate-type heat exchanger. The air-cooled chiller included a compressor, a power meter, a fin-and-tube air-cooled condenser, a shell-and-tube evaporator, and an expansion valve. R-22 was the working refrigerant for the air-cooled chiller. The water loop provided the load for the fluid loop chilled water. The flow rate of the base fluid was controlled by the inverter. The tubing of the water loop was made of stainless steel tube that had an outer diameter of 38 mm. (A water loop is designed to balance the chilled water energy from the air-cooled chiller and contains a circulation pump and a water thermostat.)

Calibrated resistance temperature detectors (RTDs) with 0.02[degrees]C accuracy were used to measure the inlet and outlet temperature of each water loop. A differential pressure transducer was used to measure the pressure difference of the refrigerant loop. The maximum pressure difference (YAKOGAWA MODEL EJA110A) was as high as 10,000 mm [H.sub.2]O, and the corresponding maximum uncertainty was less than 2.4%. The maximum flow rate of the magnetic flowmeter was 300 L/h. All the measuring devices were precalibrated. Furthermore, all the data signals were collected via the data acquisition system connected to a personal computer. The data acquisition system includes a hybrid multipoint recorder (YAKOGAWA DR230), a power distributor, a NI GPIB interface, and a personal computer. The measured cooling capacity and consumed electric power could be used to calculate the overall system performance subject to the addition of nano-fluid.

The system performance of the air-cooled chiller was conducted in a well-controlled chamber capable of maintaining an environment that meets the requirements of ARI 550/590 (the standard for water-chilling packages using the vapor compression cycle). The standard outdoor conditions were 35[degrees]C (dry bulb) and 24[degrees]C (wet bulb), whereas the indoor ambient was fixed at 27[degrees]C (dry bulb) and 19[degrees]C (wet bulb). The maximum temperature deviation was within 0.05[degrees]C, and the airflow uniformity within the chamber was less than 0.05 m/s. Following the standard test of the chiller, the test was first performed with the standard water chiller rating condition: water inlet temperature at 7[degrees]C ([T.sub.1]), water outlet temperature at 12[degrees]C ([T.sub.2])and at a flow rate of 85 L/min.

Tests were performed to get comparisons between water-based fluid and MWCNTs/water nanofluid. In the first run, the water-based fluid was used as the heat transfer medium in the evaporator. The outlet temperature of the heat exchanger was maintained at 12[degrees]C ([T.sub.2]). The inlet temperature located on the left-hand side of the plate heat exchanger ([T.sub.1]), shown in Figure 2, varied in association with the flow rate from 80 to 140 L/min. In the second run, the nanofluids (MWCNTs/water nanofluid) were used for testing. The circulation concentration was checked again at the end of the experiments, yet the concentration remained unchanged. Ranges of the flow rate were from 60 to 140 L/min. The inlet temperature of the cooling water was maintained at 14[degrees]C ([T.sub.3]) by a water thermostat. The outlet temperature ([T.sub.4]) of the plate heat exchanger also changed under the variations of the flow rate from 80 to 140 L/min. The uncertainty of the measured cooling capacity of the test span ranged from [+ or -]0.9% to [+ or -]1.1%. The highest uncertainty occured at the maximum flow rate of 140 L/min.

In order to gain good control of the stability of the flow rate, the inverter-fed pump was used. The electric power of the circulation pump and inverter were supplied externally by an independent power source, and were not counted in the consumed electric power of the experimental water chiller test system. The consumed electric power included a compressor, a condenser fan, and the controller.


The volume fraction of MWCNTs/water used in this study was only 0.001 (0.1 volume percent), and the relevant increase in thermal conductivity was only up to 1.3% at room temperature condition. Note that there is no surfactant or dispersant used for the nanofluids. The standard test of the chiller was performed with the standard water chiller rating condition: water inlet temperature at 7[degrees]C([T.sub.1]), water outlet temperature at 12[degrees]C ([T.sub.2]), and at a flow rate of 85 L/min. For the temperature dependence of thermal conductivity with temperature, Ding et al. (2005) showed that the effective ther mal conductivity increases with increasing temperature in MWCNTs/water suspensions (Wang and Mujumdar 2007).Zhang et al. (2007) showed that the thermal conductivity of the [Al.sub.2][O.sub.3]/water nanofluid increases with an increase of the particle concentration and with the temperature. Furthermore, as for the temperature dependence, the slope was the same as that for pure water. This indicated that the increase of thermal conductivity for the MWCNTs/water at the standard chiller rating condition was even lower than 1.3%. Following an estimation of the linear relationship, the thermal conductivity was increased to be about 0.9% at 10[degrees]C.

The cooling capacity vs. the flow rate that was subject to the influence of nanofluids is shown in Figure 3. For the water-based fluid, the cooling capacity increased with the rise of the flow rate from 60 to 120 L/min. The cooling capacity, however, did not change as the flow rate was further increased to 140 L/min. On the other hand, for MWCNTs/water nano-fluid, the cooling capacity showed a similar trend but leveled off early when the flow rate was increased to more than 100 L/ min. The cooling capacity reached the maximum value at a flow rate of 100 L/min. From the comparison of the cooling capacity rate of water-based fluid and MWCNTs/water nano-fluid, one can see that the cooling capacity of MWCNTs/water nanofluid was higher than that of the water-based fluid over the entire testing range. The increased cooling capacity spanned between 2% and 6%. The maximum difference occurred at the smallest flow rate of 60 L/min. The results were quite surprising. The foregoing measurement of the thermal conductivity for MNWT/water nanofluid showed only a marginal increase in the thermal conductivity (1.3% at the room temperature and 0.9% at the rating condition) of the nanofluid relative to that of pure water, whereas the maximum capacity difference (shown in Figure 3) is increased by more than 6%. Hence, certain dynamic characteristics of nanofluids must be present. Similar results are also reported by Ding et al. (2005) who studied the heat transfer performance of MWCNTs nanofluid in a tube with a 4.5 mm inner diameter. They found that the observed enhancement of the heat transfer coefficient was much higher than that of the increase in effective thermal conductivity. They came up with several possible reasons for the abnormal increase in the heat transfer coefficient including shear-induced enhancement in flow, reduced boundary layer, particle rearrangement, and high aspect ratio of MWCNTs. These observations suggest that the aspect ratio should be associated with the high enhancement of the heat transfer performance of MWCNT-based nanofluids. In addition to the postulated explanations, Majumder et al. (2005) also proposed that the flow of molecules inside carbon nano-tubes could be much faster. Water should be able to flow faster through hydrophobic single-walled carbon nanotubes because the process creates ordered hydrogen bonds between the water molecules. Ordered hydrogen bonds between water molecules and the weak attraction between the water and smooth carbon nanotube graphite sheets should then result in slip flow between the carbon nanotubes and the neighboring water molecule.


Apart from the explanations of the possible causes, one should be aware that the measurement of thermal conductivity is performed under static conditions, whereas the measurement of cooling capacity is carried out at dynamic fluid flow conditions. Hence, interactions of the flow field with nanopowders may be another reason for substantial rises in cooling capacity. A recent numerical investigation concerning the fluid flow behaviors of nanofluid via a two-phase approach was conducted by Behzadmehr et al. (2007). They clearly showed that the presence of nanopowder can absorb the velocity fluctuation energy and reduce the turbulent kinetic energy. However, this phenomenon becomes less pronounced when the Reynolds number is further increased. This is due to the fact that the corresponding velocity profiles become more uniform as the Reynolds number is increased. In that sense, one can see the difference in cooling capacity is reduced between nanofluid and the base fluid when the flow rate is increased.

In addition to the cooling capacity, the associated pressure drop vs. flow rate for the nanofluid and base fluid is shown in Figure 4. For both the water base fluid and the MWCNTs/water nanofluid, the pressure drop increased with the increase of flow rate from 60 to 150 L/min. There was a negligible difference in pressure drop between the MWCNTs/water nanofluid and the base fluid water. The results were in line with the calculation made by Behzadmehr et al. (2007). Their two-phase modeling showed that adding 1% nanopowder results in an increases of the Nusselt number by more than 15% without an appreciable increase of pressure drop. This can be attributed to the absorption of turbulence caused by the nanopowders. Furthermore, Lu et al. (2007) also reported that a novel and stable MWCNTs/ polystyrene hybrid miniemulsion can be used as a water-based lubricant additive. The antiwear performance and load-carrying capacity of the base stock were significantly raised and the friction factor was decreased. As a consequence, the present nano-fluid with MWCNTs revealed a negligible pressure drop penalty pertaining to the system performance of the water chiller. Moreover, at the standard rating condition (water inlet temperature at 7[degrees]C [[T.sub.1]], water outlet temperature at 12[degrees]C [[T.sub.2]], and at a flow rate of 85 L/min), consumed power of the nano-fluid system is reduced up to 0.8% and the coefficient of performance (COP) is increased by 5.15%. The increase of COP is mainly related to the increase of cooling capacity. Notice that the power consumption used in the calculation of COP includes all the consumed electricity (liquid pump, compressor, control system, and the like). This is because the rise of cooling capacity inevitably slightly increases the low-side refrigeration pressure, leading to a very minor reduction in system power consumption (0.8%). In essence, the system COP is increased by 5.15% at a standard rating condition.



This study examined the system performance of a water chiller subject to the addition of MWCNTs/water nanofluids. The test system was an air-cooled water chiller with a nominal capacity of 10 RT. The augmentation of the thermal conductivity of the nanofluids in this study relative to the base fluid was only marginal (0.9%). Surprisingly, the cooling capacity of the nanofluids could be increased by 4.2% at the standard rating condition. The enhancement was increased at a lower flow rate. A 6.7% increase in the capacity was encountered at a flow rate of 60 L/min. The unexpected rise in the cooling capacity of the nanofluids was related to the dynamic interaction of the flow field and the nanopowder. The nanopowders were capable of absorbing the fluctuation of the turbulent kinetic energy, giving rise to a better heat transfer characteristic under dynamic conditions, thereby leading to better system performance. At the standard rating condition, the introduction of nanofluids gave rise to an increase in the COP by 5.15%, relative to a condition without nanofluids. Furthermore, the pressure drop penalty of the addition of nanofluids was almost negligible.


This research was made possible with help from a grant from the Bureau of Energy, Ministry of Economic Affairs, Taiwan. The authors also wish to express their great appreciation to the nanofluid research team.


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M.S. Liu

R. Hu, PhD


M.C.C. Lin, PhD

C.C. Wang, PhD


J.S. Liaw

M.S. Liu, M.C.C. Lin, and J.S. Liaw are researchers, R. Hu is a senior researcher, and C.C. Wang is senior lead researcher at the Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan.
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Author:Liu, M.S.; Hu, R.; Lin, M.C.C.; Wang, C.C.; Liaw, J.S.
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2009
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