Heat Transfer Investigation Of Aluminum Oxide Nanofluids In Heat Exchangers

Transcription

1 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN Heat Transer Investigation O Aluminum Oxide Nanoluids In Heat Exchangers Roy Jean Issa, Ph.D. West Texas A&M University, USA Abstract An experimental study was conducted to investigate the use o waterbased aluminum oxide nanoluids in enhancing the heat transer perormance o heat exchangers. Two types o heat exchangers were studied: a blocktype heat exchanger or an electronic system cooling, and a radiator-type heat exchanger simulating an automobile cooling system. Tests conducted on the block heat exchanger used 20 nm alumina particles at a concentration o 5% by mass (1.3% by volume), while tests conducted on the radiator-type heat exchanger used 50 nm alumina particles at a concentration o 3% by mass (0.8% by volume). Tests conducted on the electronic heat sink system show an average enhancement o about 20% in heat transer coeicient, while tests conducted on the radiator-type heat exchanger show a substantial enhancement in heat exchanger eectiveness that reaches almost 49%. Results demonstrate that the application o nanoluids in low concentrations is suicient to cause a considerable improvement in the system s thermal perormance. Results also show that the increase in bulk low heat transer coeicient happens at the expense o the increase in luid pumping power caused by the increase in luid viscosity. Keywords: Heat Transer Coeicient, Eectiveness, Nanoluid, AL 2 O 3 Introduction With the ever increasing demand or cooling power in heat exchangers, huge eorts have been devoted to their heat transer enhancement. Research conducted during the last ew years have shown signiicant improvements in the thermal properties o conventional heat transer luids by the addition o nanoparticles to the base luids. Tests conducted on water-based AL 2 O 3 nanoluids have shown enhancement in thermal conductivity that varied rom a modest 1.4% at 0.3% volume concentration with 30 nm particles (Lee, 2008), to 10% at 3% volume concentration with 43 nm particles (Chandrasekar, 2010), to 24% at 4% volume concentration with 33 nm particles (Eastman, 1997), to 30% at 18% 128

2 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN volume concentration with 36 nm particles (Mintsa, 2009), and to a considerable enhancement o 88% at 12% volume concentration with 75 nm particles (Ghanbarpour, 2014). It is clearly evident in those studies that the bulk luid thermal conductivity in general increases with the increase in nanoparticles volume concentration. The beneit o using nanoluids in heat exchanger applications have been investigated by several researchers. In the cooling o a microchannel heat sink, Ijam et al. (2012) has shown that adding Al 2 O 3 nanoparticles to water at 4% volume concentration improved the heat lux by about 3 %, and by about 17.3% when the particle volume concentration was 0.8%. Ijam and Saidur (2012) also showed that the addition o SiC nanoparticles to water at 4% volume raction resulted in an improvement between 7.3 to 12.4% in heat lux. Selvakumar and Suresh (2012) studied the perormance o CuO water-based nanoluid in an electronic heat sink. Their study revealed a 29% improvement in heat transer coeicient or 0.2% volume raction o CuO in deionized water. Hashemi et al. (2012) studied heat transer enhancement in a nanoluid-cooled miniature heat sink application. Their study showed an enhancement in the heat transer coeicient by about 27% when using SiO 2 at a concentration o 5% concentration by volume. Khedkar et al. (2013) studied the heat transer in a concentric tube heat exchanger with dierent volume ractions o water-based Al 2 O 3 nanoluids. It was observed that at 3% volume raction, the optimal overall heat transer coeicient was about 16% higher than water. Sun et al. (2015) analyzed the low and convective heat transer characteristics o Fe 2 O 3 water-based nanoluids inside inner grooved copper and smooth cooper tubes. For the same mass raction o Fe 2 O 3 nanoparticles, the convective heat transer coeicient was better in the inner grooved copper tube than in the smooth copper tube. The enhancement in heat transer coeicient associated with the inner grooved copper tube was about 33.5% or Fe 2 O 3 mass concentration o 0.4%. All o the above researchers have examined the eect o nanoparticles concentration on heat transer enhancement, and have studied dierent types o nanoparticles. However, there are contradictory conclusions on the heat transer enhancement at lower nanoparticle concentrations among dierent researchers. Also, still limited research studies have been conducted on the evaluation o alumina nanoluid properties and their perormance in heat exchanger applications. The current study aims at investigating some o these issues in addition to investigating the thermal and rheological properties o water-based alumina nanoluids. 129

3 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN Experimental Setup: a) Electronic Heat Sink Application A closed-loop cooling system using block heat exchangers was built to evaluate the heat transer perormance associated with the use o a waterbased nanoluid with alumina particles as a cooling luid. A general picture o the experimental setup is shown in igure 1a. The nanoluid was prepared by mixing alumina nanoparticles with 20 nm average size in deionized water or a suspension concentration o 5% by mass (1.3% by volume). A digital geared-pump was used to pressurize the nanoluid or circulation in the closed-loop system. Two block heat exchangers were used in the system: one to heat the nanoluid (HXR1), and the other to cool the luid (HXR2). The interior o the block heat exchanger (igure 1b) consisted o 10 channels through which the cooling luid travelled back and orth. Figure 1a. Experimental setup o the electronic heat sink system. Figure 1b. Block heat exchanger. The heat exchanger that was used to heat the nanoluid sat on top o a 500 W plate heater separated by a 6.2 mm thick aluminium plate. A temperature control system was used to control the input heat to the base plate o the heat exchanger. The heat exchanger that was used or cooling the nanoluid sat approximately ive inches above the base o the closed-loop system. Two cooling ans rated at 120 cm each were used to cool the upper and lower suraces o this heat exchanger. Once the luid exited HXR2 it lowed into a 2-litres reservoir tank. A compact digital mixer system providing a top speed o 2,500 rpm was embedded in the tank. To achieve closed-loop circulation, the outlet rom the reservoir tank ed directly into the pump inlet. Thermocouples were embedded at various locations to record the temperature variation throughout the system. b) Radiator-Type Heat Exchanger Application Another closed-loop cooling system was also constructed to evaluate the perormance o a radiator-type heat exchanger as shown in the sketch o igure 2. The heat exchanger (202 mm x 89 mm x 160 mm) had a 10-pass 130

4 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN cross-low inned-tubes with a single tube inlet and outlet, where the tubes were arranged in a staggered array. Each tube had an inner diameter o 7.73 mm, an outer diameter o 9.5 mm, and a length o 12.7 cm. 80 in plates o 0.15 mm thickness, 120 mm width, and 38 mm depth were packaged normal to the tubes to orm narrow passes having 1.4 mm separation distance where air blowing rom a an passed through. A collection tank with a high-speed agitator thoroughly mixed the nanoluid, alumina-water based using 50 nm AL 2 O 3 particles with a mass concentration o 3% (0.8% by volume), beore it was circulated using a circulation pump. A controlled heating system was installed in the tank to maintain the circulating luid temperature within a desired range. The luid was cooled using a blowing an that was attached on one side o the heat exchanger. Thermocouples and low sensors were installed throughout the system to monitor the luids temperature and pressure, and were connected to a data acquisition device to record the data. Finned-Tubes Heat Exchanger Fan Flow Meter Flow Valve Relux Nanoluid Agitator Pump Heating Tank Heating Element Figure 2. Experimental setup o the radiator-type heat exchanger system. Property Measurements Nanoluid test samples using 20 and 50 nm AL 2 O 3 particles in distilled water o various concentrations were prepared and tested in a laboratory experimental setup or the determination o thermal conductivity. Thermal conductivity was measured using a KD2 Pro thermal properties analyzer by Decagon Devices. Details about the design and working o the KD2 Pro device can be ound in the operator s manual (Decagon Devices, 2010). The analyser consists o a microcontroller with several needle sensors that can be used. KS-1 sensor needle was selected to determine the thermal conductivity o the nanoluids. The needle contains both a heating element and a thermistor. The needle, 1.3 mm in diameter and 6 cm long, was inserted vertically (to minimize natural convection) inside a test tube 131

5 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN containing the nanoluid sample (igure 3). Tests were carried out at a temperature close to 46 o C, and or nanoparticles mass concentration o up to 40%. Tests reveal that the thermal conductivity o the nanoluid increases with the mass raction (igure 4). However, there is a slight dierence in thermal conductivity between the 20 and 50 nm particle suspensions. Mixing Device Test Needle Insulation Water Test Tube Heating Element Nanoluid KD2-Pro Analyzer Figure 3. Experimental setup or thermal conductivity analysis Mass Fraction o Al 2O 3, φ w % Figure 4. Thermal conductivity versus alumina mass concentration. Rheological tests were conducted on the same samples originally used or thermal property evaluation. Rheological properties were conducted using UL adapter attached to LVDVII+Pro viscometer. Suspensions were mixed thoroughly using a high-speed mixing device or about 30 minutes beore the viscosity tests were carried out. All tests were perormed at a temperature ranging rom 45 to 49 o C. Figures 5 and 6 show the variation in the nanoluid viscosity or 20 and 50 nm alumina particles, respectively as unction o the shear rate and nanoparticles concentration. Suspensions with 50 nm particles show an increase in viscosity with the increase in shear rate; thus, a shear thickening luid behaviour (dilatant luid). However, suspensions with 20 nm particles show a decrease in viscosity with the increase in shear rate or the nanoluid having 5% mass concentration (i.e., shear thinning luid), but an increase in viscosity or the nanoluid having 2.5% mass concentration (i.e., shear thickening luid). Thermal Conductivity, λ (Wm -1 K -1 ) AL 2O 3 in Deionized Water T = 46 o C AL 2O 3 in Water AL 2O 3 Particle Size: 20 nm Alumina + Water, 5% wt, 49 deg.c AL 2O 3 + Water AL 2O 3 Particle Size: 50 nm Avg. T = 45 o C Viscosity, µ (10 3 Pa.s) Alumina + Water, 2.5% wt, 48 deg.c Viscosity, µ (10 3 Pa.s) % wt 20% wt 30% wt 40% wt Shear Rate, γ (s -1 ) Figure 5. Viscosity versus shear rate or 20 nm alumina-in-water particles Shear Rate, γ (s -1 ) Figure 6. Viscosity versus shear rate or 50 nm alumina-in-water particles. 132

6 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN Heat Transer Measurements: a) Electronic Heat Sink Application Heat transer tests were carried out on the electronic heat sink system using deionized water-based alumina nanoluid coolant with a 20 nm particles and a mass concentration o 5%. The results were compared to that o a cooling luid consisting o deionized water. The plate heater was set to a constant temperature o 91 o C, and the coolant low rate was varied between 7.8 and 16.1 cm 3 s -1. Interace temperatures, and heat exchangers inlet and outlet temperatures were recorded once steady state temperature in the system was reached. The steady state temperature o the coolant associated with the dierent low rates ranged rom about 47 to 57 o C. The total volume o the coolant in the system was 2 litres. To minimize the precipitation o nanoparticles in time, a stirring device embedded in the reservoir tank was turned on or the duration o the tests. The heat lux supplied by the electric heater at the base plate o HXR1, q ", is determined rom the temperature variation, T, across the plate wall thickness: q T q" = = λ p (1) A x where λ p is the thermal conductivity o the plate, x is the plate thickness, and A is the surace area. The heat transer coeicient associated with the coolant in the heat exchanger, h c, is calculated as: q" hc = (2) Ti T where T i is the heat exchanger base plate interace temperature (interace between the heat exchanger bottom surace and the base plate top surace), and T is the bulk mean temperature o the cooling luid in HXR1. The pumping power o the bulk luid, P power, is calculated as: P power = V P (3) Figures 7 and 8 show the wall heat lux (at the base o HXR1) and the coolant heat transer coeicient as unction o the bulk mass low rate or the case o water-based alumina nanoluid and deionized water, respectively. The wall heat lux and coolant heat transer coeicient are shown to increase with the increase in bulk mass low rate. Comparison between igures 7 and 8, show water-based alumina nanoluid has higher values or both the wall heat lux and heat transer coeicient. An average increase by about 24% is seen in the wall heat lux or the case o water-based nanoluid compared to the case o deionized water. The heat transer coeicient is also shown to increase by about 20% or the case o water-based nanoluid. 133

7 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN Wall Heat Flux, q" (Wm -2 ) d = 20 nm 5% wt AL 2O 3 in Deionized H2O Wall Heat Flux Heat Transer Coe. 1,800 1,600 1,400 1,200 1, Bulk Mass Flow Rate, m (kgs -1 ) Figure 7. Wall heat lux versus mass low rate (5% wt AL 2 O 3 ). Heat Transer Coeicient, h c (Wm -2 K -1 ) Wall Heat Flux, q" (Wm -2 ) Deionized H 2O Wall Heat Flux Heat Transer Coe. 1,600 1,500 1,400 1,300 1,200 1, , Bulk Mass Flow Rate, m (kgs -1 ) Figure 8. Wall heat lux versus mass low rate (Deionized water). Heat Transer Coeicient, h c (Wm -2 K -1 ) Figure 9 shows the decrease in the heated wall cross-section temperature (at the base o HXR1) as unction o bulk mass low rate. For the considered low rates, the decrease in temperature ranged rom 23.7 to 25.8 o C or the nanoluid test case, while it ranged rom 18.8 to 21.7 o C or the deionized water test case. Figure 9 clearly shows an additional temperature drop (i.e., enhancement) between 4.1 and 4.9 o C when alumina nanoluid instead o deionized water is used as a coolant. Wall Temperature Decrease, T w ( o C) % wt AL2O3 in DI H2O Deionized H2O Bulk Mass Flow Rate, m (kgs -1 ) Figure 9. Decrease in wall temperature versus bulk mass low rate. Figure 10 shows the pressure drop across HXR1 heat exchanger as unction o the bulk mass low rate. The pressure drop is shown to increase with mass low rate and with the addition o nanoparticles to the base luid. The relationship between bulk luid heat transer coeicient and pumping power is shown in igure 11. The increase in heat transer coeicient is shown to occur at the expense o pumping power increase. But or the same pumping power, the presence o nanoparticles in the base luid is shown to have a signiicant eect on the increase in heat transer coeicient, and thereore cooling eiciency. 134

8 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN Pressure Drop, P (Nm -2 ) % wt AL2O3 in DI H2O Deionized H2O Bulk Mass Flow Rate, m (kgs -1 ) Figure 10. Pressure drop versus bulk mass low rate. Heat Transer Coeicient, h c (Wm -2 K -1 ) 1,800 1,700 1,600 1,500 1,400 1,300 1,200 1,100 5% wt AL2O3 in DI H2O Deionized H2O 1, Pumping Power, P pump (W) Figure 11. Heat transer coeicient versus pump power. b) Radiator-Type Heat Exchanger Application Heat transer tests were also perormed on the radiator-type heat exchanger. In this case, the water-based alumina nanoluid consisted o 50 nm alumina particles suspensions having a mass concentration o 3%. The perormance o the heat exchanger using the nanoluid as a circulating luid was also compared to that using distilled water. Several test cases were conducted where the tank immersed heater was set to dierent temperature settings ranging rom 200 to 500 o C. Temperature data were recorded or about 45 minutes ater the system reached steady state condition. Figure 12 shows typical results or the temperature drop between the radiator inlet and outlet or the two types o circulating luids. As expected, the temperature drop increases with the decrease in volumetric low rate, and the nanoluid is shown to outperorm distilled water. HXR Temperature Drop (%) (across Inlet & Outlet) AL 2O 3-H 2O Nanoluid d = 50 nm 3% by wt (0.8% by vol) T heater = 200 o C HXR Fluid: Distilled H20 HXR Fluid: AL2O3-H2O Circulating Fluid Flow Rate, gal/min Figure 12. Comparison in radiator temperature drop using aluminum oxide nanoluid versus distilled water. 135

9 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN The radiator heat transer eectiveness, ε, deined as the ratio o the actual heat transer, q, to the maximum possible heat transer that can be achieved, q max, is calculated as ollows: q m cp, ( Ti, Te, ) ε = = (4) q C T T where max min ( ) i, i, a m is the mass low rate o the circulating luid, and c, is its speciic heat. C min is the minimum o m c p, and m ac p, a where index a indicates air. T i, and T e, are the inlet and exit temperatures o the circulating luid, and T, is the air temperature at the an inlet. Figure 13 i a shows a comparison in the radiator heat transer eectiveness between aluminum-oxide water based nanoluid and distilled water. The operating conditions were the same or both luids. All test cases conducted using the nanoluid showed an increase in heat transer eectiveness. A substantial enhancement o up to 49% was achieved. p HXR Eectiveness (AL 2 O 3 -H 2 O Nanoluid) AL 2O 3-H 2O Nanoluid d = 50 nm 3% by wt (0.8% by vol) T_heater = 200 deg.c T_heater = 300 deg.c T_heater = 400 deg.c T_heater = 500 deg.c HXR Eectiveness (Distilled H 2 O) Figure 13. Comparison in radiator heat transer eectiveness using aluminum oxide nanoluid versus distilled water. The radiator overall heat transer coeicient, U, is calculated using the experimental prediction o the heat exchanger log mean temperature dierence, TLM : m c p, ( Ti, Te, ) U = (5) A TLM where A is the peripheral area o the radiator tubes. Figure 14 shows a comparison in the radiator overall heat transer coeicient between the aluminum-oxide nanoluid and distilled water. The conditions are shown or 136

10 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN an average circulating luid temperature o 40 o C. An increase o up to 38% can be seen in the highest volumetric low rate cases. Overall Heat Transer Coeicient W/m 2.K AL 2O 3-H 2O Nanoluid d = 50 nm 3% by wt (0.8% by vol) Avg. T luid = 40 o C HXR Fluid: Distilled H2O HXR Fluid: AL2O3-H2O Circulating Fluid Flow Rate, gal/min Figure 14. Comparison in radiator overall heat transer coeicient using aluminum oxide nanoluid versus distilled water. Conclusion An experimental study was conducted to investigate the heat transer perormance o two types o heat exchangers using an alumina-water based nanoluid as a circulating luid: a block-type heat exchanger or an electronic system cooling, and a radiator-type heat exchanger simulating an automobile cooling system. Tests carried out on the block heat exchanger used 20 nm alumina particles at a concentration o 5% by mass, while tests carried out on the radiator-type heat exchanger used 50 nm alumina particles at a concentration o 3% by mass. In both cases, the suspended particles were thoroughly mixed using a high speed agitator beore the start o each test. Thermal conductivity tests conducted on the alumina nanoluids show an enhancement o less than 4% or the above mentioned particles concentration levels. Results also show that the increase in bulk low heat transer coeicient happens at the expense o the increase in the bulk luid pumping power due to the increase in bulk luid viscosity. Tests on the electronic heat sink system show an average enhancement o about 20% in heat transer coeicient and 24% in the wall heat lux. Results also show an additional decrease in the heated wall crosssection temperature ranging rom 4.1 to 4.9 o C. Tests conducted on the radiator-type heat exchanger also show a substantial enhancement in heat exchanger eectiveness that reaches almost 49%. Based on these results, it seems that the 4% increase in bulk luid thermal conductivity may not have been the only driving orce behind this substantial increase in the systems heat transer perormance. It is likely possible that another eect such as the Brownian motion o the nanoparticles may have been behind this increase. Since the presence o nanoparticles in the bulk luid can reduce the thermal 137

11 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN boundary layer thickness (thus enhancing the bulk luid heat transer capability), it is possible that the size o the nanoparticles may inluence this phenomenon. Even though higher heat transer perormance is achieved in the radiator-type heat exchanger with 50 nm AL 2 O 3 particles, the size o the nanoparticles in the base luid needs to be explored urther to veriy this eect. Reerences: Lee, J. H., Hwang, K. S., Jang, S. P., Lee, B. H., Kim, J. H., Choi, S. U. S., & Choi, C. J. (2008). Eective viscosities and thermal conductivities o aqueous nanoluids containing low volume concentrations o AL 2 O 3 nanoparticles International Journal o Heat and Mass Transer, 51, Chandrasekar, M., Suresh, S., & Bose, A. C. (2010). Experimental investigations and theoretical determination o thermal conductivity and viscosity o AL 2 O 3 /water nanoluid. Experimental Thermal and Fluid Science, 34, Eastman, J. A., Choi, U. S., Li, S., Thompson, L. J., & Lee, S. (1997). Enhanced thermal conductivity through the development o nanoluids. Materials Research Society Symposium Proceedings, 457, Mintsa, H. A., Roy, G., Nguyen, C. T., & Doucet, D. (2009). New temperature dependent thermal conductivity data or water-based nanoluids. International Journal o Thermal Sciences, 48, Ghanbarpour, M., Haghigi, E. B., & Khodabandeh, R. (2014). Thermal properties and rheological behaviour o water based AL 2 O 3 nanoluid as a heat transer luid. Experimental Thermal and Fluid Science, 53, Ijam, A., Saidur, R., & Ganesan, P. (2012). Cooling o minichannel heat sink using nanoluids. International Communications in Heat and Mass, 39, Ijam, A. & Saidur, R. (2012). Nanoluid as a coolant or electronic devices. Applied Thermal Engineering, 32, Selvakumar, P. & Suresh, S. (2012). Convective perormance o CuO/water nanoluid in an electronic heat sink. Experimental Thermal and Fluid Science, 40, Hashemi, S. M. H., Fazeli, S. A., & Zirakzadeh, H. (2012). Study o heat transer enhancement in a nanoluid-cooled miniature heat sink. International Communications in Heat and Mass, 39, Khedkar, R. S., Sonawane, S. S., & Wasewar, K. L. (2013). Water to nanoluids heat transer in concentric tube heat exchanger: Experimental study. Procedia Enineering, 51,

12 European Scientiic Journal July 2016 /SPECIAL/ edition ISSN: (Print) e - ISSN Sun, B., Lei, W., & Yang, D. (2015). Flow and convective heat transer characteristics o Fe 2 O 3 -water nanoluids inside copper tubes. International Communications in Heat and Mass, 64, Decagon Devices. (2010). KD2 Pro Thermal Properties Analyzer Operator s Manual, Washington: Pullman. 139