Bench-Scale Experimental Study on the Heat Transfer Intensification

Article pubs.acs.org/IECR

Bench-Scale Experimental Study on the Heat Transfer Intensification of a Closed Wet Cooling Tower Using Aluminum Oxide Nanofluids XiaoCui Xie, Yi Zhang, Chang He,* Tao Xu, BingJian Zhang, and QingLin Chen* School of Chemical Engineering and Technology/Guangdong Engineering Center for Petrochemical Energy Conservation, Sun Yat-sen University, No. 135, Xingang West Road, Guangzhou, 510275, China ABSTRACT: This study was designed to experimentally investigate the intensification effect of Al2O3/water nanofluid used as the spray fluid on the thermal performance of counterflow closed wet cooling towers (CWCTs). The mass fraction of nanoparticles was 0.1−1 wt % with nanoparticle dimensions of 10, 30, and 80 nm. Meanwhile, glycerin as stabilizer was used. After that the thermophysical properties of Al2O3 nanofluids were systematically measured. It was observed that the thermal conductivity, density, and viscosity of Al2O3 nanofluids increased with the increments of mass fraction. The results revealed that the maximum mass transfer coefficient was found to be 0.16 kg·m−2 s−1 for 0.5 wt % nanofluids and spray mass flow rate of 0.278 kg·s−1. Moreover, the heat transfer capacity of the CWCT spraying 0.5 wt % Al2O3 nanofluid could reach 1.22 times that of the water case, while the water consumption reduced about 15% under the same heat transfer capacity.

1. INTRODUCTION There are varieties of heat rejection devices which can remove low-grade waste heat to the atmosphere by means of different cooling technologies in petrochemical enterprises, refineries, metallurgical operations, thermal power plants, etc. Among these devices, a cooling tower is the most common option that either uses the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, relies solely on air to cool the working fluid to near the dry-bulb air temperature.1−3 According to the direct and indirect contact mode between the process water and the inlet air flow, wet cooling towers can be mainly classified into open wet cooling towers and closed wet cooling towers (CWCTs). Compared to open wet cooling towers, there is no direct contact between the process water and air flows in CWCTs, and the mass transfer and heat transfer primarily occur only on the tube surface through the water evaporation and convection effect. This indirect contacting mode ensures that the process water remains clean and is not polluted by the environment. Thus, the heat transfer enhancement of CWCTs has gained growing concern in industrial applications and received substantial research of interest from the experimental aspect.4,5 A variety of research has been conducted to enhance the cooling efficiency and reduce the spray water of CWCTs during the past decade. Some concentrated on the optimizations of operating parameters, including the inlet air flow rate, temperature of process water, spray water, and inlet air,6,7 as well as the structural parameters including the packing types, spray nozzles, and air inlet of CWCTs.8,9 Furthermore, some CWCTs equipped with novel coil tubes, finned tubes and oval tubes, for example, were designed and investigated systematically. Hasan and Sirén10,11 compared the oval tube with a plain tube in CWCTs, and they concluded that the oval tubes were © XXXX American Chemical Society

better than the plain tubes by 1.93−1.96 times in terms of the combined thermal−hydraulic performance index. Zheng et al.12 obtained the mass and heat transfer coefficients in a CWCT with oval tubes, and a suitable mathematical model was used to predict the thermal performance. Jiang et al.6 investigated the influence of operating factors on the cooling capacity of a crossflow CWCT with plate-fin tubes. They also developed the correlation equations for the heat and mass transfer coefficients. Novel CWCTs consisting of copper and plastic tubes were proposed and numerically studied by Xia et al.;13 both the cooling efficiencies and process water outlet temperature under different conditions were evaluated. Sarker et al.14 demonstrated that the cooling capacities of spiral finned tube CWCTs were about 22 and 260% higher than those of the bare tube ones operating in wet and dry modes, respectively. However, note that some optimizations of the operating parameters were not universal and controllable in different seasons, regions, and conditions. Moreover, CWCTs are mostly facility specific, where the tube diameter and spacing, as well as the tube rows, are fixed; consequently, it is costly and difficult to replace or remold the coil tubes. To the best of our knowledge, the conventional heat transfer fluids (water, oil, and ethylene glycol, or their mixture) belong to poor heat transfer fluids, which lead to the low heat transfer capacity and limited cooling range in CWCTs. It is worth mentioning that using some higher thermal conductivity fluid as spray liquid not only will improve the thermal conductivity of the spray water but also will not pollute the process water. Therefore, it is a good choice to intensify the cooling efficiency of CWCTs with nanofluids. Received: Revised: Accepted: Published: A

February 20, 2017 April 13, 2017 May 8, 2017 May 8, 2017 DOI: 10.1021/acs.iecr.7b00724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research A nanofluid is a fluid which contains nanometer-sized particles (1−100 nm) in base liquid. Generally, aluminum oxide (Al2O3), copper oxide (CuO), zinc oxide (ZnO), and titanium dioxide (TiO2) are the most commonly used nanoparticles in experiments. As is well-known, the thermal conductivity of these nanoparticles is an order of magnitude higher than that of the conventional liquid, and the introduction of these solid particles in base fluids to enhance the thermal conductivity has been the subject of research works for many years. Some researchers have applied a variety of preparation methods, characteristics, and different models used for the calculation of thermophysical properties of nanofluids (i.e., stability, thermal conductivity, viscosity, density, specific heat capacity, etc.). The thermophysical properties of nanofluids containing Al2O3, CuO, ZnO,15−20 multiwalled carbon nanotubes (MWCNTs),21−25 silicon dioxide (SiO2),24 Cu,26 and Al27 were systematically investigated. The available literature indicates that the thermal conductivity of low nanoparticle concentration (1−5 wt %) nanofluids is higher than the base fluid and it depends strongly on the mass fraction as well as the type of nanoparticles.28−34 Furthermore, numerous investigations were undertaken to apply the nanofluids to evaluate the heat transfer enhancement potential in different thermal systems. The thermal performance enhancement potential of nanofluids in cooling electronics,35 in refrigeration,36 in plate heat exchangers,37 and in solar energy systems38−40 were summarized and reviewed. Pak and Cho41 investigated the thermal intensification behavior of Al2O3 and TiO2 nanofluids in a circular pipe, and a new correlation for the dispersed fluids Al2O3 and TiO2 particles was suggested as Nu = 0.021Re0.8Pr0.5. Pantzali et al.42 tested the efficacy of nanofluids as coolants in plate heat exchangers. They found that the effectiveness of nanofluids was mainly determined by the physical properties and flow patterns. Askari et al. 21 experimentally investigated the effect of MWCNTs and nanoporous graphene (NPG) nanofluids on the performance of a wet cooling tower. The results indicated the cooling range can increase by 40 and 67%, and the water consumption was reduced by 10 and 19% for the MWCNTs and NPG nanofluids, respectively. The enhanced thermal behavior of nanofluids could provide a basis for enormous heat transfer intensification in a number of industrial sectors. However, the effect of nanofluids on the heat transfer, mass transfer, and hydraulic performance of CWCTs have not been studied before. The main objective of this study is to investigate the thermohydraulic performances of closed wet cooling towers (CWCTs) using Al2O3 nanofluid as the spray fluid, compared with those of CWCTs using water. To achieve this goal, Al2O3 water-based nanofluids (0.1−1 wt %) were prepared and applied in a bench-scale CWCT. In section 3.1, zeta (ζ) potential measurement has been used to assess the stability of nanofluids and the suitable stabilizer is selected. Then the thermophysical properties of the nanofluids, including thermal conductivity, density, and viscosity have been evaluated. Finally, for investigating the effect of nanofluids on heat transfer enhancement and water conservation, the effect of the spray water flow rate and inlet air velocity on the heat and mass transfer, cooling efficiency, and pressure drop of closed wet cooling towers have been systematically investigated.

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation of Nanofluids. The aluminum oxide (Al2O3) nanoparticles and water were used to produce the nanofluids. The Al2O3 nanoparticles (take no account of modifying the properties of nanoparticles) with mean sizes of 10, 30, and 80 nm were purchased from Meryer (Shanghai) Chemical Technology Co. Ltd. Different stabilizers, namely glycerin, polyethylene glycol 400 (PEG400), sodium dodecyl benzenesulfonate (SDBS), Tween 80, and Tween 20, were obtained from Aladdin (Shanghai) Chemical Technology Co. Ltd. For preparation of Al2O3 nanofluids, a two-step procedure was applied. First, the desired amount of stabilizers and the nanoparticles were added to the water, and then intense mechanical stirring was conducted. To ensure uniform dispersion of nanoparticles and prevent initial agglomeration of nanoparticles in the base fluid, an ultrasonic disruptor was employed to homogenize the nanofluid for 40 min. In this way, different Al2O3 nanofluid concentrations of 0.1, 0.3, 0.5, 0.8, and 1 wt % were prepared, and nanofluids with different types of stabilizers were also obtained by a similar method. (The characterization of Al2O3 nanoparticles is depicted in Figure 1.) 2.2. Characterization of Nanofluid. To investigate the stability dispersion of the nanoparticle in different nanofluids,

Figure 1. Characterization of Al2O3 nanoparticles. (a) Scanning electron microscopic (SEM) image of Al2O3 nanoparticles. (b) Transmission electron microscopic (TEM) image of Al2O3 nanoparticles. (c) Density functional theory (DFT) pore size distributions of Al2O3 nanoparticles. B

DOI: 10.1021/acs.iecr.7b00724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 2. (a) Experimental closed wet cooling tower and (b) schema of the utilized experimental system.

the ζ potential was measured by a Nano-ZS90 (Malvern Instrument Inc., England). The effects of different stabilizers, nanoparticle sizes, and nanoparticle mass fractions on the stability of the nanofluids were studied. The thermal conductivity was measured using the transient hot-wire technique with a thermal constant analyzer (TPS2500, Hot Disk Inc., Sweden). The density was calculated by weighing a known volume of the nanofluids by a pycnometer according to ASTM D153, and the average value of density was obtained by three test cases. The nanofluid viscosity was measured by an Ubbelohde viscometer at 30 °C, and the accuracy of the measurement was calculated to be about 5%. 2.3. CWCT Experimental Setup. Figure 2 shows our main experimental apparatus and the schema of the system for the performance study of CWCTs using the Al2O3 nanofluids. The spray fluid is pumped out of a basin to the nozzles above the tube bundle, and then sprayed on the tubes and returned to the basin. Air enters the cooling tower from the bottom and flows successively through the blind window, outside tubes, and mist eliminator, and out of the tower finally with suction from a draft fan. The hot water heated by the heating boiler flows into the cooling tower from the right side, and then passes through the tubes and goes back to the boiler. The heat transfer mainly occurs in the two-tube pass segment, each of which is triangularly arranged and composed of four rows and eight columns of tubes. A tube has a length of 700 mm, external diameter of 12 mm, and wall thickness of 1 mm. Moreover, the flow rates of spray fluids and air are controlled by frequency converters, and the hot water flow rate is changed by the butterfly valve. A PID controller is applied to control the inlet temperature of hot water. In addition, the flow rates of spray fluid and hot water are measured by turbine flow meters, while the outlet air velocity is measured by the vane anemometer, and the temperatures of the air and hot water are acquired by the thermocouple and thermal resistance, respectively. The uncertainties of these measured values have been depicted in Table 1.

Table 1. Uncertainties of Measurement and Performance Parameters parameter

uncertainty

water temperature air temperature water flow rate heat transfer coefficient mass transfer coefficient cooling efficiency pressure drop

±0.24 °C ±0.75% ±5.0% ±6.7% ±5.0% ±0.96% +0.05 Pa

Table 2. ζ Potential of Nanofluids with Different Stabilizers type of stabilizer PEG400 Tween 80 SDBS glycerin Tween 20

ζ potential [mV] 32.3 19.4 14.3 48.0 0.34

± ± ± ± ±

6.04 6.39 3.13 5.6 3.25

Table 3. ζ Potentials of Nanofluids with Different Al2O3 Sizes size [nm]

ζ potential [mV]

10 30 80

48 ± 5.60 30.8 ± 6.45 0.047 ± 5.95

system. Different stabilizers including PEG400, glycerin, Tween 80, Tween 20, and SDBS were introduced to 0.1 wt % Al2O3 nanofluids, and then ζ potential measurement was performed to investigate the stability of Al2O3 nanofluids, as given in Table 2. The ζ potential values of nanofluids are 48.0 ± 5.6, 32.3 ± 6.04, 19.4 ± 6.39, 14.3 ± 3.13, and 0.34 ± 3.25 mV, in sequence, corresponding to glycerin, PEG400, SDBS, Tween 80, and Tween 20, respectively. To the best of our knowledge, it is a stable suspension when the ζ potential is above 30 mV, whereas it is unstable when the ζ potential is below 20 mV.21 According to this principle, it can be seen that the Al2O3 nanofluids prepared using glycerin have the highest ζ potential (>40 mV), which indicates glycerin is more suitable to stabilize Al2O3 nanofluid in comparison to other surfactant candidates. Consequently, glycerin was chosen as the stabilizer in the following experiments. Especially, the ζ potential value of the

3. RESULTS AND DISCUSSION 3.1. Nanofluid Properties. 3.1.1. Nanofluid Stability. The stability of nanoparticles in the base fluid is the main problem in the application because the particles tend to precipitate or agglomerate in suspended state. The utilization of stabilizers can help to stabilize the nanoparticles in base fluids; thus it is important to find a suitable surfactant in the Al2O3 nanofluid C


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Figure 5. Effect of mass fraction on thermal conductivity and viscosity of nanofluids.

Figure 6. Density of different Al2O3 nanofluids. Figure 3. Stability of Al2O3 nanofluids in different mass fractions.

heat transfer enhancement in CWCTs. The nanoparticles which are larger in size are usually heavier than others, leading to a decrease in Brownian motion and random movement of the suspension particles. Thus, it is difficult for them to suspend well in the nanofluids. Hence, as listed in Table 3, the ζ potential decreases significantly with the increase of the mean size of the Al2O3 nanoparticles. For example, the ζ potential of nanofluids declines from 48 ± 5.6 mV for the 10 nm Al2O3 case, to 30.8 ± 6.45 mV for 30 nm Al2O3, and to 0.05 ± 5.95 mV for the 80 nm Al2O3 case. The stability of nanofluids also relates to the concentration of nanoparticles. The nanoparticles in a Brownian movement state collide more easily and precipitate at high concentration, so there is a general tendency that the nanofluids with high mass fractions are less unstable.29,43,44 Figure 3 shows the stability of different mass fraction Al2O3 nanofluids at different times including 30 min, 2 days, 4 days, and 7 days after nanofluid preparation, and it can be seen that nanofluids with higher Al2O3 concentrations tend to precipitate more intensely. Furthermore, the effect of mass fraction on nanofluid stability has been investigated and the results are presented in Figure 4. The ζ potential decreases with the increase of Al2O3 mass fraction; for instance, the ζ potential drops from 48 ± 5.6 to 26.3 ± 19.1 mV as the mass fraction enriches from 0.1 to 1 wt %. 3.1.2. Nanofluid Physical Properties. The utilization of nanofluids in thermal engineering fields is mainly due to their better thermal conductivity (Knf) compared with that (Kbf) of the base fluid. The viscosity related to the Prandtl number (Pr)

Figure 4. ζ potential of Al2O3 nanofluids in different mass fractions.

0.1 wt % Al2O3 nanofluid with glycerin at the highest spray water temperature (lower than 35 °C) is 34.6 ± 5.37 mV, which can ensure good stability during the operating process. Some factors, namely, particle size, morphology, structure, and pore volume, may affect the strong van der Waals interaction between nanoparticles, which will influence the Brownian motion of particles and the stability of nanofluid.47 However, to the best of our knowledge, the particle size is the main factor discussed which affects the nanofluid stability in existing references.48 In our present experiment, only the nanoparticle size has been taken into account to investigate the D


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Figure 8. Water film distribution outside the tubes at the spray water flow rate of (a) 0.139, (b) 0.278, and (c) 0.306 kg·s−1.

Figure 9. Outlet air temperature ta,out and the humidity difference Δω under different spray mass flow rates.

Figure 7. Thermal performances under different spray mass flow rates: (a) heat transfer coefficient, (b) mass transfer coefficient, and (c) cooling efficiency.

and the Reynolds number (Re) is a critical factor which influences the pressure drop and pump shaft power in an engineering system.45,46 The effect of the mass fraction of Al2O3 nanoparticles on the thermal conductivity (at 25 °C) and viscosity (at 30 °C) of nanofluids was investigated, and the results are shown in Figure 5. As expected, the thermal conductivity increases with the addition of mass fraction of nanoparticles; for example, the thermal conductivity ratio (Knf/ Kbf) enhances by 15.9% as the mass fraction increases from 0.1 to 1 wt %, respectively. Meanwhile, the viscosity has a modest increment from 0.800 to 0.833 cP when the mass fraction increases from 0 to 1.0 wt %. This indicates that the pump shaft power for delivering nanofluid increases slightly compared with that of water. However, the thermal conductivity mainly

Figure 10. Humidity and temperature correlation of outlet air under different spray flow rates.

depends on the dispersion uniformity and stability of the nanofluid, and the rapid aggregation and settling of the E


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Figure 11. Temperature distribution on the center section of the lower tube bundle: (a) water case; (b) 0.1 wt % nanofluid case; (c) 0.5 wt % nanofluid case.

Figure 13. Influence of spray mass flow rate on airflow: (a) pressure drop Δp and (b) friction factor f.

CWCT, and the inlet mass flow rate of cooling fluid is a significant parameter affecting the CWCTs’ thermal performance, such as the mass transfer coefficient (β), heat transfer coefficient (α), and cooling efficiency (η) as defined by eqs 1, 2, and 3, respectively. To study the effect of Al2O3 nanoparticles under different spray mass flow rates, the nominal operating conditions are the process water flow rate, 1.5 kg·s−1; inlet process water temperature, 43 °C; spray fluid temperature, 30 °C; inlet air velocity, 2.25 m·s−1; and inlet air temperature, 10 °C. β= Figure 12. Humidity distribution on the center section of the lower tube bundle: (a) water case; (b) 0.1 wt % nanofluid case; (c) 0.5 wt % nanofluid case.

α=

nanoparticles would result in a significant decrease in thermal conductivity. Because of the instability (low ζ potential) of nanofluids at high mass fraction that is observed from Figure 4, the thermal conductivity varies slightly when the Al2O3 mass fraction is above 0.5 wt %. Meanwhile, the density is a significant parameter that affects the Reynolds number and pressure drop. The densities of different Al2O3 nanofluids have been measured at 30 °C, and the results are shown in Figure 6. Dependent on the concentration of Al2O3 nanoparticles, it is observed that the density merely increases 0.64% as the mass fraction of Al2O3 nanoparticles changes from 0.1 to 1.0 wt %. 3.2. Effect of Nanoparticles under Different Spray Mass Flow Rates. 3.2.1. Thermal Performance. In this experiment, the spray water acted as the cooling fluid in the

η=

ma ⎛ iw′ − ia,in ⎞ ⎟⎟ ln⎜⎜ A ⎝ iw′ − ia,out ⎠ ma cpa A

⎛ tw − ta,in ⎞ ⎟⎟ ln⎜⎜ ⎝ tw − ta,out ⎠

(1)

(2)

tw,in − tw,out tw,in − ta,wb

(3)

As given in Figure 7, the heat transfer coefficients of the water case and the nanofluid cases increased about 70% with the spray mass flow rate increasing from 0.083 to 0.306 kg·s−1. This is because the increase in spray mass flow rate enhances the turbulence of falling film, which further improves the convective heat transfer between the spray fluid and air. The heat transfer coefficient, mass transfer coefficient, and cooling efficiency in this case (0.5 wt % Al2O3 nanofluid) are nearly 20, 17, and 19% greater than those of the case for water. The mass transfer coefficient and cooling efficiency, as shown in Figure F


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Figure 15. Air velocity vector fields outside the tubes at inlet air velocities of (a) 1.68, (b) 2.81, and (c) 3.93 m·s−1.

Figure 16. Outlet air temperature ta,out and the humidity difference Δω under different inlet air velocities.

Figure 14. Thermal performance under different inlet air velocities: (a) heat transfer coefficient, (b) mass transfer coefficient, and (c) cooling efficiency.

7b,c, increase when the mass fraction is below 0.5 wt %, while they decrease as the mass fraction exceeds this value. These results are attributed to the facts as follows: (1) The Al2O3 nanoparticles added in the spray fluid could enhance the turbulence of falling film. (2) The thermal conductivity showed a dramatic increase when the nanofluid mass fraction was below 0.5 wt %; however, it showed a slight increase when the nanofluid mass fraction was above 0.5 wt %, as previously mentioned in Figure 5. However, the rise in spray mass flow rate not only enhances the convective heat transfer interaction between falling film and airflow but also increases the falling film thickness. As shown by the results of a computational fluid dynamics (CFD)

Figure 17. Correlation of outlet air humidity and temperature under different inlet air velocities.

simulation, in Figure 8, the tube surface is actually fully wetted and has a thin water film at the spray mass flow rate of 0.278 kg· s−1. Although increasing the flow rate would enlarge the water film area around the tubes, the film thickness is increased as the G


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thickness would decrease the heat transfer in falling film. The heat transfer coefficient and cooling efficiency increase as the spray mass flow rate is below 0.278 kg·s−1, whereas these parameters begin to decrease when the spray mass flow rate exceeds this value, indicating that 0.278 kg·s−1 is a critical value with respect to the spray mass flow rate. It can be concluded that the 0.5 wt % Al2O3 nanofluid case has the best thermal performance with the spray mass flow rate at 0.278 kg·s−1. The heat transfer coefficient, mass transfer coefficient, and cooling efficiency of nanofluid cases are higher than those of the water case. This is mainly because the Brownian diffusion and thermophoresis of Al2O3 nanoparticles added to the spray fluid could increase the chaotic movement and fluctuation of the spray fluid, especially the turbulence of the boundary layer (near the tube wall) of falling film; likewise the enhancement in the turbulence of falling film increases the heat transfer between the tube wall and the falling film, which increases the spray fluid temperature and leads to a higher evaporation rate of water and a greater temperature difference between the spray fluid and air. 3.2.2. Humidity Difference and State of Outlet Air. The outlet air state, as plotted in Figure 9, indirectly reflects the heat and mass transfers in CWCTs. It can be found that the outlet air temperature increases with the increment of spray mass flow rate and mass fraction of nanoparticles. For example, the outlet air temperature rises 4.8 °C at 0.5 wt % Al2O3 nanofluid, while the humidity difference is the greatest at the spray flow rate of 0.278 kg·s−1 and mass fraction of 0.5 wt %. Moreover, the outlet air temperature and humidity difference of nanofluid cases are larger than those of the water case. Especially, the temperature and humidity differences of the 0.5 wt % Al2O3 nanofluid case are 4.5 and 18.0% higher than those of the water case, respectively. It can be demonstrated that the outlet air states, given by Figure 10, of the water case are closer to the humidity line of saturated air. In Figure 10, it is obvious that the increase in air temperature is larger than that of humidity. These indicate that the nanoparticles mainly enhance the heat transfer process in CWCTs, while they have little effect on mass transfer. To gain a temperature profile of the air state inside the CWCTs, a CFD simulation was conducted. The temperature of the water case, 0.1 wt % nanofluid case, and 0.5 wt % nanofluid case, as shown in Figure 11, gradually rises by as the air flows through the tube bundle from the bottom up. It is found that, at the same longitudinal section, the air temperatures of the nanofluid cases are higher than that of the water case; namely, they have a greater increase rate from the bottom up. Thus, the outlet air temperatures of nanofluid cases are higher than water case as mentioned previously. With regard to the humidity given in Figure 12, the increase rates of the water case and nanofluid cases are nearly the same, which agrees well with the experimental results. 3.2.3. Pressure Drop and Friction Factor. In this study, the effect of scaling on the Reynolds number and friction factor is negligible, due to the good stability of the nanofluid and short operation time in the bench-scale CWCT. Figure 13 shows the effect of spray mass flow rate and mass fraction of Al2O3 nanoparticles on the pressure drop and friction factor (defined by eq 5). The friction factor is defined as

Figure 18. Influence of inlet air velocity on pressure drop Δp and friction factor f.

Figure 19. Thermal performance under different inlet air states and spray mass flow rates: (a) heat transfer coefficient and (b) mass transfer coefficient.

spray mass flow rate changes from 0.278 to 0.306 kg·s−1. The effect of film thickness can be illustrated by eq 4: Q = (K nf /δ)A(tw − tsurf )

(4)

where Q is the heat exchange capacity, and δ and Knf are the thickness and thermal conductivity of falling film. In eq 4, the heat exchange capacity takes into account the thermal resistance Knf/δ and temperature differences (tw − tsurf) between the wall and falling film surface, where δ is the thickness of falling film. It is obvious that the increment in

f= H

Δp 0.5ρa vmax 2

(5) DOI: 10.1021/acs.iecr.7b00724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 20. Heat transfer capacity ratio Qnf/Qbf and humidity difference ratio Δω′nf/Δωbf under (a) pump power and (b) fan power.

the friction factor is mainly influenced by the pressure drop, which varies with the change of nanofluid viscosity when the air density and velocity are constant. Nevertheless, the Al2O3 nanofluid used as spray fluid shows a high stability, and no sedimentation was found in our bench-scale CWCT after 4 h of operation. Therefore, it is assumed that the viscosity is a constant property in our study, and the effect of scaling on friction factor is neglected accordingly. The viscosity of nanofluids is larger than that of water, which increases the flow resistance of air. The nanofluid cases have greater pressure drop and friction factor than the water case. It is noticeable that the pressure drop and friction factor increase with the increase of spray mass flow rate and mass fraction of nanoparticles. As shown in Figure 13a, the pressure drop increases more than 60% in the range of spray mass flow rate from 0.083 to 0.306 kg·s−1. Especially, the largest increase of pressure drop occurs as the spray mass flow rate changes from 0.222 to 0.278 kg·s−1,

Table 4. Cost under the Same Heat Transfer Capacity a

spray fluid water 0.1 wt % nanofluid 0.3 wt % nanofluid 0.5 wt % nanofluid 0.8 wt % nanofluid 1.0 wt % nanofluid a

E [kW·h]

W [kg/h]

Cm [RMB]

total cost [RMB/(4 h)]

1.40 0.54

11.63 10.99

0 0.6

3.51 2.03

0.35

9.66

1.8

2.76

0.30

9.18

3.0

3.84

0.34

10.00

4.8

5.74

0.38

10.43

6.0

7.04

Total cost = 4(0.6E + 0.00315W) + Cm.

where Δp is the pressure drop, ρa is the air density, and vmax is the maximum air velocity in CWCTs. It can be clearly seen that I


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would enhance the heat transfer and mass transfer between the spray fluid and air as discussed previously. This can be validated by Figure 17, which shows that the outlet air states the 1.0 wt % Al2O3 nanofluid case is closer to the humidity line of saturated air in contrast with the water case under different inlet air velocities. 3.3.3. Pressure Drop and Friction Factor. Figure 18 illustrates the effect of inlet air velocity on the pressure drop and friction factor of the 1.0 wt % Al2O3 nanofluid and water cases (the scaling is neglected), respectively. It is obvious that the pressure drop and friction factor of the nanofluid case are higher than those of the water case, attributed to the larger viscosity of nanofluid. Since the inertial flow resistance is proportional to the square of air velocity, the pressure drops distinctly from air inlet to outlet. As shown in Figure 18, the pressure drops of the 1.0 wt % Al2O3 nanofluid case and water case quadratically increase around 5 times as the inlet air velocity rises from 1.68 to 3.93 m·s−1. Especially, it is 277 Pa approximately for the nanofluid case. The change in characteristics of the friction factor is analogous to a quadratic curve with the increase in inlet air velocity. With the increase of the inlet air velocity, the friction factor appears to have two district regimes. At low inlet air velocity (

Bench-Scale Experimental Study on the Heat Transfer Intensification

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