Investigating the efficacy of nanofluids as coolants in plate heat exchangers (PHE)

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Abstract

The efficacy of nanofluids as coolants is investigated in the present study. For the nanofluids tested, systematic measurements confirmed that the thermophysical properties of the base fluid are considerably affected by the nanoparticle addition. A typical nanofluid, namely a 4% CuO suspension in water, is selected next and its performance in a commercial herringbone-type PHE is experimentally studied. The new experimental data confirmed that besides the physical properties, the type of flow inside the heat exchanging equipment also affects the efficacy of a nanofluid as coolant. The fluid viscosity seems also to be a crucial factor for the heat exchanger performance. It is concluded that in industrial heat exchangers, where large volumes of nanofluids are necessary and turbulent flow is usually developed, the substitution of conventional fluids by nanofluids seems inauspicious.

Introduction

The new technological developments as well as the industrial process intensification have made the need for more efficient heat exchanging systems a contemporary demand. Therefore, the scientific interest is focused both on improving the equipment design and on enhancing the thermal capability of the working fluids. The progress in equipment design has led to the development of compact plate heat exchangers (PHE) with modulated surface. The high surface density of such devices combined with a type of flow that involves successive flow separation and reattachment inside the narrow PHE passages augment heat transfer rate. At the same time the complexity induced by the modulations significantly increases the friction losses, making the design of this type of equipment an optimization problem that must compromise between heat transfer enhancement and pumping power demands (Kanaris et al., 2006).

The need for working fluids with improved performance has increased the scientific interest in nanofluids, i.e., colloidal suspensions of nanometer-sized solid particles. Das et al. (2006), Trisaksri and Wongwises (2007) and Wang and Mujumdar (2007) have recently summarized the work done in this area. The available literature indicates that the thermal conductivity of the nanofluid is higher than that of the base fluid and it depends strongly on the size, shape and volume fraction of the nanoparticles as well as on the type of the nanoparticles and of the base fluid (Trisaksri and Wongwises, 2007). The published rheological studies of nanofluids are limited and the Newtonian (or not) behaviour of the various nanofluids has not been completely clarified (Wang and Mujumdar, 2007). For example, Das et al. (2003a) report the Newtonian behaviour of an Al2O3–water nanofluid with up to 4% particle volume concentration whereas Pak and Cho (1998), who consider similar nanofluids, detected a non-Newtonian behaviour. Nguyen et al. (2007a) reported that the viscosity of Al2O3 and CuO–water nanofluids is significantly affected by the particle concentration and the temperature. They also questioned the applicability of the aforementioned nanofluids in heat exchanging equipment, since the hysteresis phenomenon on viscosity, observed after subsequent heating and cooling of the fluid, strongly affects the entire flow field and consequently the heat transfer behaviour. The heat capacity of nanofluids is usually estimated using theoretical equations, since to the authors’ best knowledge the relevant published experimental studies are limited.

It must be pointed out that the increase of thermal conductivity of nanofluids is necessary but not sufficient condition for achieving high performance in heat exchanging equipment, and hence further investigation is required in this area (Das et al., 2006). The majority of the available experimental studies on the application of nanofluids at single-phase forced convective heat transfer concern laminar flow mostly in externally heated circular tubes or micro-channels. Among the various types of nanoparticles and concentrations used, metal oxides with particle volume concentrations up to 4% are the most common, probably due to their lower price. The reported findings and conclusions vary considerably. For example, Ding et al. (2006) studied the performance of a nanofluid containing carbon nanotubes and reported that the heat transfer coefficient can be 3.5 times higher than the respective water value, while Zeinali Heris et al. (2007), who used Al2O3–water nanofluids, found an enhancement of less than 40%. Wen and Ding (2004) noted that the heat transfer coefficient enhancement is much higher than the increase in the thermal conductivity, which is not in agreement with the observations by Yang et al. (2005). The heat transfer enhancement is reported either to increase with flow rate (e.g. Jung et al., 2009), to remain constant (e.g. Hwang et al., 2009) or even to reduce with flow rate (e.g. Chein and Chuang, 2007). Xuan and Li (2003) studied turbulent flow in a horizontal tube using a copper nanofluid and reported about 40% enhancement, while Williams et al. (2008) observed no abnormal enhancement with oxide nanofluids in turbulent flow and reported that if accurate thermophysical properties are employed the existing correlations are capable to predict the convective heat transfer. Nguyen et al. (2007b), who experimentally studied heat transfer in a miniature PHE, reported an enhancement of the heat transfer rates. Pantzali et al. (2009), who investigated the effect of the use of a CuO nanofluid in a similar apparatus both experimentally and numerically, also confirmed the aforementioned trend. Mansour et al. (2007) based on empirical correlations for heat transfer in a uniformly heated tube have demonstrated that the advantages of utilizing nanofluids may vary significantly, depending strongly not only on their thermophysical properties but also on the geometrical characteristics of the heat exchanging equipment and the operating conditions. Bergman (2009) has shown that the performance of a nanofluid is geometry- and flow rate-dependent and that a particular nanofluid, although efficient for one application, can be proved inadequate for another one. The relevant experimental studies are summarized in Table 1. It is clear that most of the experimental works have been conducted in very simple geometrical configurations (e.g. tubes), whereas limited experiments in commercial heat exchangers are available. Moreover, when laminar flow is encountered, the use of a nanofluid is always accompanied by heat transfer augmentation. The results concerning turbulent flow are very limited and seem rather ambiguous.

In the present study, the performance of a typical nanofluid used as a coolant in a typical commercial herringbone-type PHE (where the flow complexity is also intense) is experimentally investigated and compared to that of the base fluid (i.e., water). Prior to the experiments, the systematic measurement of all the thermophysical properties involved in heat exchanging processes for various nanofluids will lead to the selection of a typical nanofluid, appropriate for the heat transfer study in this commercial PHE. Finally and in an effort to assess the efficacy of nanofluids in heat exchanging equipment the results of the present study will be discussed in conjunction with results of other available relevant studies.

Section snippets

Preparation of nanofluids

Various nanofluids are used in the present study and are presented in Table 2. In all cases water was used as the base fluid. More specifically:

  • Three nanofluids were prepared using commercially available nanoparticles, i.e., Al2O3, CuO nanoparticles and carbon nanotubes (CNT), purchased from Nanostructured and Amorphous Materials Inc. (NanoAmor).

  • Two commercially available suspensions containing Al2O3 and TiO2 nanoparticles were obtained from Sigma Aldrich®.

  • A 50% w/w commercial CuO nanofluid was

Measurement of thermophysical properties

In the present work the thermophysical properties of the nanofluids are systematically measured:

  • The thermal conductivity is measured using the transient hot-wire technique as developed in the Thermophysical Properties Laboratory of the Aristotle University of Thessaloniki. A detailed description of the method, along with an estimation of the uncertainty introduced, which, in the case of nanofluids, is better than 2%, is available elsewhere (Assael et al., 2004).

  • The density is calculated by

Experimental setup

The experiments are conducted in a welded commercial PHE (Alfa Laval). The PHE comprises 16 stainless steel corrugated plates that create two isolated fluid paths for the hot and cold fluid flow respectively, forming eight flow channels per stream. The plates have chevron-type corrugations with a height of 2 mm and a wavelength (in a direction normal to the crests) 8.6 mm. The corrugations form a herringbone pattern (Fig. 4a) with an angle of 50° relative to the direction of the flow. Successive

Heat transfer

The heat removed from the hot water, Qh, and the heat absorbed by the cooling liquid, Qc, are calculated by Eqs. (6), (7) using the temperature and mass flow rate data recorded during the experiments. It is confirmed that they are practically equal, i.e., within the accuracy of the measuring technique, which is found to be less than 15%.Qh=Q=mhcp,h(Thi-Tho)Qc=Q=mccp,c(Tco-Tci)In the above equations m is the mass flow rate, cP the heat capacity and Ti and To the inlet and outlet temperatures for

Concluding remarks

In the present study the efficacy of nanofluids as coolants has been investigated. Prior to this the thermophysical properties of several nanofluids have been systematically measured confirming the general trends reported in the literature, when nanoparticles are added in the base fluid, that is:

  • increase of thermal conductivity,

  • increase of density,

  • decrease of heat capacity

  • increase of viscosity and

  • possibly non-Newtonian behaviour.

The new experimental data concerning the use of nanofluids in a

Notation

Atotal heat transfer area, m2
bcmean spacing between plates, m
cPheat capacity, J/kg K
ffriction factor, dimensionless
kthermal conductivity, W/m K
LMTDlog mean temperature difference, K
mmass flow rate, kg/s
Nnumber of channels per stream, dimensionless
NuNusselt number, dimensionless
ΔPpressure drop, Pa
PrPrandtl number, dimensionless
PPpumping power, W
Qheat flow rate, W
ReReynolds number, dimensionless
Ttemperature, K
Utotal heat transfer coefficient, W/m2 K
ussuperficial velocity, m/s
Vvolumetric flow rate, m3

Acknowledgements

Financial support by the General Secretariat for Research and Technology and the European Union (PENED 2003) is greatly acknowledged. Prof. M.J. Assael, Dr J. Tihon and Mr. K. Antoniadis are acknowledged for their contribution to this work. The authors would also like to thank Mr. A. Lekkas, Mr. M. Bridakis, Mr. T. Tsilipiras and Mr. F. Lambropoulos for the technical support.

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