Elsevier

Energy Conversion and Management

Volume 75, November 2013, Pages 609-616
Energy Conversion and Management

Experimental study on the heat recovery characteristics of a new-type flat micro-heat pipe array heat exchanger using nanofluid

https://doi.org/10.1016/j.enconman.2013.08.003Get rights and content

Highlights

  • A new-type flat micro-heat pipe array heat exchanger (FMHPAHE) was designed.

  • Al2O3-R141b nanofluids with different volume concentration have different effect on heat recovery effectiveness.

  • The effectiveness increases greatly by using 0.01 vol% nanofluids as working fluid.

  • FMHPAHE with nanofluid as working fluid can effectively save energy.

Abstract

This study aims to investigate the heat recovery characteristics of a new-type flat micro-heat pipe array heat exchanger using δ-Al2O3-R141b nanofluids as working fluids. δ-Al2O3-R141b nanofluids with volume fractions of 0.001%, 0.01%, and 0.1% were prepared. The experiments were performed with air volume flow rates ranging from 60 m3/h to 120 m3/h. The evaporator section inlet air temperature varied from 27 °C to 40 °C while the condenser section inlet air temperature remained at 24 °C. The results indicated that heat recovery efficiency is enhanced remarkably by using 0.01% volume fraction nanofluid as working fluid, and the maximum growth rate of effectiveness can reach 110%. The heat recovery effectiveness of the 0.001 vol% nanofluid was almost similar to that of R141b; however, the efficiency of 0.1% nanofluid declined significantly. The pressure drop ratio of the heat exchanger was tested subsequently, and the energy saved by the exchanger was analyzed to further evaluate the performance of heat exchanger.

Introduction

Nanofluid is a new class of nanotechnology-based heat transfer fluid proposed by Choi [1] in 1995. Given that the thermal conductivity of solid is higher than that of liquid, adding a certain amount of solids into liquids may enhance the thermal properties of the liquids. When small amounts of nano-scale particles, fibers, or tubes with lengths ranging from 1 nm to 50 nm are stably suspended in traditional liquid, they transform into liquid–solid mixture denominated nanofluids. Adding nano-sized particles to base fluid to strengthen conductivity showed the following advantages compared with adding millimeter- or micron-sized particles:

  • (1)

    The thermal resistance of nanofluids during heat transfer was reduced because of the collision between the particles, the liquid, and the tube wall, thus enhancing the heat transfer rate.

  • (2)

    Adding nanoparticles to base fluid could greatly increase the coefficient of thermal conductivity, which would improve internal heat transfer.

  • (3)

    The surface area of nanoparticles far outweighed the surface area of millimeter- and micron-sized particles with the same particle volume fraction; thus, nanofluids had greater effective thermal conductivity coefficients.

  • (4)

    Given the effect of their small size, nanoparticles could reach an important stable suspension through strong Brownian motion.

A number of studies have been carried out on the thermal conductivity and convective heat transfer of nanoparticles in different kinds of base fluids. Xuan and Li [2] measured the thermal conductivity of nanofluids and found that when water–Cu nanoparticles are suspended in base liquid, the ratio of the nanofluid thermal conductivity to the base liquid thermal conductivity varies from 1.24 to 1.78 if the volume fraction of the ultra-fine particles increases from 2.5% to 7.5%. Eastman et al. [3] carried on experiments on suspended nanoparticles at the Argonne National Laboratory show that nanofluids possess extremely high thermal conductivities compared to liquids without dispersed nanocrystalline particles. For example, 5 vol% of CuO nanoparticles suspended in water results in an almost 60% improvement in thermal conductivity. Lee et al. [4] produced oxide nanofluids and measured their thermal conductivities, and the experimental results show that the oxide nanofluids, which contain a small amount of nanoparticles, have substantially higher thermal conductivities than similar liquids without nanoparticles. Xie et al. [5] investigated the thermal conductivity behaviors of nanosized Al2O3 suspensions, and the experimental results show that the addition of nanoparticles to fluids leads to the increase of suspensions thermal conductivities. The enhanced thermal conductivity ratios increase with the volume fraction of nanoparticles. Pak and Cho [6] performed experiments on the turbulent heat transfer performance of the two kinds of nanofluids and turbulent frictions using γ-Al2O3 and TiO2 dispersed in water, and they determined that the Nusselt numbers of the dispersed fluids for fully developed turbulent flow increase when volume concentration increases. Xuan and Li [7] studied single-phase turbulent flow and found that a nanofluid’s conductivity increases with nanoparticle’s volume fraction. He et al. [8] studied new nanofluid phase change materials by suspending TiO2 nanoparticles in BaCl2 aqueous solution, and the experimental results show that the thermal conductivities of nanoparticle PCMs are enhanced by 12.76% and the supercooling degree is reduced by 84.92% when volume fraction is 1.130%. Kakac et al. [9] studied the convective heat transfer enhancement in nanofluids, and they attribute the enhancement to factors such as particle volume concentration, particle material, particle size, particle shape, base fluid material temperature, and additives.

Meanwhile, as the energy issue increases in importance, the demand for small and light heat exchangers with excellent heat transfer performance is urgent. However, the performance of working fluids is a major factor that affects the heat transfer performance of heat exchanger equipment. The idea of infusing nanoparticles into heat pipe working fluid has become an interesting topic in recent years, given the excellent heat transfer performance of nanofluids. Shafahi et al. [10] studied the thermal performance of a cylindrical heat pipe utilizing Al2O3, TiO2, and CuO nanofluids as working fluids, and they found that smaller particles have a more pronounced effect on the temperature gradient along the heat pipe. Hajian et al. [11] carried out an experimental research on the thermal performance of silver–water nanofluid in concentrations of 50 ppm, 200 ppm, and 600 ppm with a cylindrical meshed heat pipe. The thermal resistance and the response time of the heat pipe using 50 ppm nanofluid as a working fluid decreased by 30% and 20%, respectively, compared to deionized water (DI). Liu and Zhu [12] carried out an experimental study to investigate the effects of aqueous CuO nanofluids on the thermal performance of a horizontal mesh heat pipe, and the experimental results show that adding CuO nanoparticles into DI water can significantly enhance the heat transfer coefficients of both the evaporator and the condenser. The nanoparticles also enhance the maximum heat flux of the heat pipe. Mohammed et al. [13] studied the effect of various nanofluid types on a double pipe heat exchanger using a numerical approach, and the results reveal that a slight change in the skin friction coefficient occurs when the nanoparticle diameters of a SiO2 nanofluid are varied.

In this study, the heat transfer performance of δ-Al2O3-R141b nanofluids in three different concentrations was compared with the performance of pure R141b on a new-type flat micro-heat pipe array heat exchanger (FMHPAHE). Compared with the traditional cylindrical copper heat pipe heat exchanger, this new-type aluminum heat exchanger had advantageous characteristics including lightness, compactness, efficiency, and a greater heat exchange area with a given volume. In addition, extensive studies on the thermal performance of nanofluids mainly choose water or methanol as the base fluid; however, R141b was used as the base fluid with which δ-Al2O3-R141b nanofluids were compared in this study. The reverse and negative effects of high concentration nanofluids are also discussed in this study.

Section snippets

Preparation of nanofluids

The nanofluids used in this work were prepared by dispersing commercial δ-Al2O3 nanoparticles with diameters of 20 nm in R141b. The volume concentrations investigated were 0.001%, 0.01%, and 0.1%. The normal procedure is outlined as follows: nanoparticles were first dispersed in the base fluid. SDBS, a dispersant whose mass is four times that of the nanoparticles, was then added to the base fluid. Finally, the solid–liquid mixtures were oscillated in an ultrasonic washer for 1 h. The washer was

Results and discussion

First, the effect of various evaporator inlet temperatures (27–40 °C) and fresh air volume flow rates (60 m3/h, 90 m3/h, and 120 m3/h) on the effectiveness of the similar FMHPAHE working fluids was investigated. The experimental results of R141b are indicated in Fig. 4. Effectiveness was observed to increase with an increase in inlet fresh air temperature, regardless of the volume. This result was attributed to the difference between the inlet fresh air and the inlet return air temperatures, which

Conclusion

The heat recovery characteristic of a new-type FMHPAHE was studied experimentally using nanofluids at different volume concentrations. Al2O3 nanoparticles at volume concentrations of 0.001%, 0.01%, and 0.1% suspended in R141b were prepared for the present investigation, and the following conclusions were reached:

  • (1)

    The experimental results show that using 0.01 vol% nanofluid as a working fluid could greatly enhance heat transfer effectiveness compared with R141b. The maximum growth rate of

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