Experimental study on the flow and heat transfer characteristics of nanofluids in double-tube heat exchangers based on thermal efficiency assessment

Abstract Thermal performance and pressure drop of TiO2-H2O nanofluids in double-tube heat exchangers are investigated. The influence of the thermal fluid (water) volume flow rates (qv = 1–5 L/min), nanoparticle mass frictions (ω = 0.0%, 0.1%, 0.3% and 0.5%), nanofluids locations (shell-side and tube-side), Reynolds numbers of nanofluids (Re = 3000–12000), and the structures of inner tubes (smooth tube and corrugated tube) is analyzed. Results indicate that nanofluids (ω = 0.1%, 0.3% and 0.5%) can improve the heat transfer rate by 10.8%, 13.4% and 14.8% at best compared with deionized water respectively, and the number of transfer units (NTU) and effectiveness are all improved. The pressure drop can be increased by 51.9% (tube-side) and 40.7% (shell-side) at best under the condition of using both nanofluids and corrugated inner tube. When the nanofluids flow in the shell-side of the corrugated double-tube heat exchanger, the comprehensive performance of nanofluids-side is better than that of the smooth double-tube heat exchanger.

that Nusselt number can be increased by 51.4% compared with water. Biglarian et al. 153 [68] reported on a study to explore the forced convection of various nanofluids (Cu, 154 Ag, Al 2 O 3 , TiO 2 ) and obtained that Cu nanofluids show the largest enhancement ratio. 155 Many enhanced tubes and nanofluids are applied to enhance the thermal performance.  Table 1. The experimental procedure is as follows: 220 (1) Place the nanofluids in the low-temperature thermostat bath and deionized 221 water in the hot water tank, and set the temperature to the desired value. 222 (2) Open the all valves, flow meters and pumps of the experimental system, two 223 kinds of working fluids begin to circulate in the two loops, and carefully check the 11 experimental system for leakage. 225 (3) Open the differential pressure transmitters, date acquisition instrument and 226 computer, collect date of the import and export of the two loops, perform more than 227 three experiments on each experimental condition and record the experimental dates. 228 (4) When the experiment is completed, turn off the high-power thermostat, then 229 turn off the pumps and date acquisition instrument, finally turn off the main power. (4) 12 The overall heat transfer coefficient is expressed as: The effectiveness and the NTU are given by: The frictional resistance coefficient can be calculated with:

Error analysis 263
Based on the root-sum-square method presented by Kline [87], the errors of 264 physical parameters can be calculated from following equations (9-11), and the results 265 are shown in Table 2. It is indicated that the maximum uncertainties in the resistance 266 coefficient, NTU and effectiveness are ±1.18%, ±1.77%, and ±2.06% respectively. compared to deionized water [88].
Furthermore, with the increase of the nanofluids mass fraction, the NTU also 358 increases correspondingly, which indicates that nanofluids can improve the heat 19 exchange capacity of heat exchangers. The NTU of the corrugated double-tube heat 360 exchanger with ω=0.1%, 0.3%, 0.5% can be improved by 10.7%, 12.6% and 13.6% at 361 best compared with deionized water respectively. Comparative analysis of the NTU in 362 the two kinds of heat exchangers can be also obtained from Figs. 7-9, the NTU of the 363 corrugated double-tube heat exchanger is higher, and it can be improved by 47.5% at 364 best under the same condition.       Fig. 11. Effects of nanoparticle mass fraction on the effectiveness of the corrugated          Fig. 18 and Fig. 19 533 respectively. The summary graph on the NTU with velocity is shown in Fig. 20.

534
The NTU trends of this experiment don't show a tendency to increase firstly and 535 then decrease. This can be explained that the flow rate of nanofluids in the shell-side 536 is larger than that in the tube-side under the same Reynolds number, the NTU is 30 determined from the overall heat exchanger, which varies with the flow rates in 538 shell-side and tube-side.

561
The variations of effectiveness against Reynolds number in shell-side are 562 depicted in Fig. 21 and Fig. 22, and the summary graph on the effectiveness with 563 velocity is shown in Fig. 23  In the experiment of this section, the shell-side working fluid is nanofluids.   shows that the increase is smaller than that of the previous experiment.

623
Furthermore, the viscosity of the nanofluids is higher than that of the deionized 624 water, so that the nanofluids have a greater flow resistance than the deionized water.

625
In the corrugated double-tube heat exchanger, the pressure drop of nanofluids with 626 =0.1wt%, 0.3wt% and 0.5wt% is improved by 2.77%, 3.89% and 5.97% at best 627 compared with deionized water respectively. Therefore, the influence of the 628 nanoparticle concentration on the pressure drop is smaller than the disturbance of the 629 corrugated tube.  (1) TiO 2 -H 2 O nanofluids with =0.1wt%, 0.3wt% and 0.5wt% have better 668 thermal performance than the deionized water, the heat transfer rate can be improved 669 by 10.8%, 13.4% and 14.8% at best respectively, and the pressure drop of nanofluids 670 can be increased by 2.77%, 4.38% and 6.5% at best respectively.

671
(2) The thermal performance of the corrugated double-tube heat exchanger is 672 significantly stronger than that of the same size smooth double-tube heat exchanger.

673
But the pressure drop of nanofluids in the corrugated double-tube heat exchanger is 674 also significantly stronger, and it can be increased by 51.9% (tube-side) and 40.7% 675 (shell-side) at best.

676
(3) When using both nanofluids and corrugated tube, the overall thermal 677 performance is significantly enhanced, which reflects in the increase of the NTU and