What dominates heat transfer performance of a double-pipe heat exchanger

Dan Zheng; Zhenwei Hu; Liting Tian; Jin Wang; Bengt Sundén

doi:10.1515/phys-2021-0101

Article Open Access

What dominates heat transfer performance of a double-pipe heat exchanger

Dan Zheng , Zhenwei Hu , Liting Tian , Jin Wang and Bengt Sundén

Published/Copyright: December 31, 2021

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From the journal Open Physics Volume 19 Issue 1

Abstract

ZnO–water nanofluid in an inner pipe was experimentally investigated to improve the thermal efficiency of double-pipe heat exchangers. In this work, a 57.6% increase in the Nusselt number was obtained at Re = 14,340 when the ZnO–water nanofluid flowed through a helically corrugated tube. It was found that helical corrugation played a more important role in heat transfer enhancement than thermophysical properties of the nanofluid at the Reynolds number below 10,221. In addition, with the increase of the Reynolds number, the advantages of nanofluids in thermal performance become obvious, and Nusselt numbers increase by 28.5 and 30.6% with the effects of helical corrugation and ZnO–nanofluid, respectively, at Re = 14,349.

Keywords: nanofluid; structure optimization; Nusselt number; thermal performance; double-pipe heat exchanger

1 Introduction

Energy shortage has become one of the greatest concerns around the world. To improve thermophysical properties of working media, nanofluids have been widely used in energy conversion devices. Zheng et al. [1] investigated the effects of many parameters on heat transfer performance of a double-tube heat exchanger with various nanofluids. They found that the nanofluid type and particle concentration showed important influences on the thermal performance of the heat exchanger, and a maximum of 68.4% increase in the Nusselt number was obtained for the SiC–water nanofluid with a mass fraction of 1.5% compared to water. Zheng et al. [2] performed several experiments to study the effects of magnetic fields on the Nusselt number and pressure drop of a corrugated plate heat exchanger filled with ferrofluids. Results indicated that the distribution and strength of magnetic fields had significant effects on heat transfer performance. When two vertical magnets were arranged side by side around the plate heat exchanger, the average Nusselt number increased by 21.8% compared to the case without a magnetic field. A nanofluid is a type of colloidal suspension containing nanoparticles. Ezzat et al. [3] analyzed the effects of a transverse magnetic field and chemical reaction on an unsteady magnetohydrodynamics micropolar flow. Micropolar fluid theory had guiding significance for the analysis of colloidal suspension behavior. Zhang et al. [4] investigated flow characteristics and thermal performance of the Al₂O₃–water nanofluid in rectangular microchannels with teardrop dimple/protrusion structures. They found that the shear-thinning effect caused a secondary vortex in the dimple region. Ezzat et al. [5] introduced a new mathematical model to provide a reference for the improvement of thermoelectric material efficiency. They found that variable thermal conductivity and fractional orders played important roles in temperature distributions.

From previous studies, it has been confirmed that thermal performance could be significantly improved by the application of nanofluids and optimization of the heat transfer structure. Few findings were obtained for heat transfer enhancement of nanofluids in helically corrugated tubes. In this work, heat transfer performance of the ZnO–water nanofluid in a double-pipe heat exchanger is investigated at different Reynolds numbers. This research aims to analyze potential of ZnO–water nanofluid and helical corrugation on the improvements of thermal performance, which will further provide a scheme for engineering applications.

2 Experimental investigation

2.1 Preparation of experiments

In this work, ZnO nanoparticles were mixed and dispersed in deionized (DI) water by a magnetic stirrer and ultrasonic oscillation. The ZnO nanoparticles with purity of 99% were purchased from Deke Daojin Company (China), and the diameter of the spherical ZnO nanoparticle was 30 nm. In the preparation process, sodium hexametaphosphate with one-fifth particle mass was added to improve the stability of the nanofluid. An experimental system with two loops (hot water loop and nanofluid loop) was established to investigate the thermal performance of a double-pipe heat exchanger. A double-pipe heat exchanger consists of two concentric copper pipes with different diameters, that is, an inner pipe with a diameter of 6 mm and an outer pipe with a diameter of 10 mm. Thermal insulation material is wrapped outside the double-pipe heat exchanger to avoid heat losses. Temperature and pressure drop are measured at the inlet and outlet of the test section to evaluate the thermal performance and rheological properties. A helically corrugated tube is used as the inner pipe of the double-pipe heat exchanger to analyze the effects of the corrugations on thermal performance. Height, width, and pitch of the corrugation are 1, 2, and 2 mm, respectively. The nanofluid preparation process and the experimental system are shown in Figure 1.

Figure 1

Diagrams of nanofluid preparation and experimental system.

2.2 Data analysis

In this work, the Nusselt number (Nu) and friction factor (f) are key parameters to analyze the thermal performance of double-pipe heat exchangers. The processes of analysis of the two parameters are discussed in brief in this section.

Heat transfer rate is calculated by the following equation:

(1) Q = m c p ( T out − T in ) ,

where Q, m, c _p, T _in, and T _out represent heat transfer rate, mass flow rate, specific heat, and temperatures at the inlet and outlet. The final heat transfer rate is calculated by equation (2):

(2) Q ave = ( Q w + Q nf ) / 2 ,

where the subscripts w and nf represent water and nanofluid, and deviation between Q _w and Q _nf is less than 5%. Heat transfer rate is also calculated by

(3) Q ave = h nf A i ( T wall − T nf ) ,

where h _nf is the heat transfer coefficient of the nanofluid and A _i represents the area of the inner tube. T _wall represents the wall temperature of the inner tube. The nanofluid bulk temperature T _nf is the average value of the inlet temperature and outlet temperature of the double-pipe heat exchanger, and it is calculated by

(4) T nf = ( T in + T out ) nf / 2 .

Heat transfer coefficient of the nanofluid is calculated by equation (5):

(5) h nf = ( Q w + Q nf ) / 2 ( T wall − T nf ) A i .

Nusselt number of the nanofluid is calculated by equation (6):

(6) Nu nf = h nf D / k nf ,

where D is the diameter of the pipe. K _nf is the thermal conductivity of the nanofluid, which is calculated based on a typical model proposed by Maxwell [6]. Thermal conductivity of the ZnO–water nanofluid with a mass fraction of 0.5% was obtained as 0.616 W/mK in this work.

Friction factor (f) for the nanofluid is calculated by equation (7).

(7) f nf = 2 Δ P nf D / L ρ nf u nf 2 ,

where ΔP _nf represents the measured pressure drop, ρ _nf, and u _nf represent the density and flow velocity of nanofluid.

2.3 Analysis of experimental uncertainty

In the present study, uncertainties of experimental results are calculated based on the method proposed by Moffat [7]. The uncertainties for the heat transfer coefficient, Reynolds number, Nusselt number, and friction factor are 2.66, 2.6, 3.15, and 2.62%, respectively.

3 Results and discussion

Heat transfer performance of a double-pipe heat exchanger was investigated under different operating conditions. Figure 2 shows differences of Nusselt numbers for DI water with a plain tube and ZnO nanofluid with a helically corrugated tube at various Reynolds numbers.

Figure 2

Nusselt numbers of DI water and nanofluid in various pipes.

It is found that both the Nusselt number of DI water with the plain tube and Nusselt number of ZnO nanofluid with the helically corrugated tube increase with the increase of Reynolds number. The growth proportion of Nusselt number increases with the increase of the Reynolds number. Compared with DI water in a plain tube at Re = 14,340, there is a 57.6% increase in Nusselt number for the ZnO nanofluid with the helically corrugated tube. These phenomena are caused by the greater thermal conductivity of the nanofluid, Brownian motion of the nanoparticles, and pipe structure.

Figure 3 shows the thermal performance and rheological properties of DI water and nanofluid in the double-tube heat exchanger. It is found that in all cases, Nusselt number increases with the increase of Reynolds number, and friction factor decreases with the increase of Reynolds number. There will be a critical value, at a Reynolds number in which ZnO nanofluid in the plain tube and DI water in the helically corrugated tube will have similar values of Nusselt number for Reynolds numbers in the range of 8,925–10,221. This result indicates that the application of the ZnO nanofluid shows a similar heat transfer enhancement to the application of the helical corrugation at Reynolds numbers of 8,925–10,221. Compared with the effect of the helical corrugation at Reynolds numbers above 10,221, the application of the ZnO nanofluid has a more significant role in the improvement of thermal efficiency of heat exchangers. This result indicates that methods of heat transfer enhancement should be applied discriminatively at different conditions.

Figure 3

Comparison of effects of nanofluid and helical corrugation on thermal performance and flow loss.

Figure 4 shows comparisons of Nusselt numbers for the combined effect of nanofluid and helical corrugation. It is found that the increase of Nusselt number caused by using both nanofluid and helical corrugation is a little lower than the sum of increases from independent effects of nanofluid and helical corrugation at high Reynolds numbers.

Figure 4

Comparison of increases in Nusselt number with different conditions.

4 Conclusions

This research experimentally investigated thermal performances of ZnO–water nanofluid and helical corrugation in a double-pipe heat exchanger. The independent roles of nanofluid and helical corrugation were further compared to confirm which factor dominates heat transfer enhancement in the double-pipe heat exchanger. Based on the experimental results, it is concluded that helical corrugation plays a greater role in the increase of the Nusselt number compared with ZnO–water nanofluid at low Reynolds numbers. With the increase of the Reynolds number, the ZnO–water nanofluid in the helically corrugated tube shows a 57.6% increase in the Nusselt number compared with DI water in a plain tube at Re = 14,340. The ZnO–water nanofluid in a helically corrugated tube provides an increase of Nusselt number, which is lower than the sum of increases caused by their respective roles at high Reynolds numbers. The rational selection of the improvement method can effectively increase the thermal performance of devices and decrease pressure losses.

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Funding information: This work is supported by the National Natural Science Foundation of China (Grant No. 52176067), the Natural Science Foundation of Hebei Province of China (Grant Nos. E2021202163 and E2019202184), the Special Project of Science and Technology Winter Olympics in the Hebei Technology Innovation Guidance Plan (Grant No. 21474501D), the Project of Innovation Ability Training for Postgraduate Students of Education Department of Hebei Province (Grant No. CXZZSS2021046), and the Foundation of Key Laboratory of Thermo-Fluid Science and Engineering (Xi’an Jiaotong University), Ministry of Education, Xi’an 710049, P.R. China (Grant No. KLTFSE2018KFJJ01).
Author contributions: All the authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2021-02-28

Revised: 2021-07-03

Accepted: 2021-12-12

Published Online: 2021-12-31

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/phys-2021-0101

Keywords for this article

nanofluid; structure optimization; Nusselt number; thermal performance; double-pipe heat exchanger

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