Particulate fouling of CuO–water nanofluid at isothermal diffusive condition inside the conventional heat exchanger-experimental and modeling

Highlights

• CuO–water nanofluid is utilized as possible energy saving coolant inside the HEX.

• Fouling behavior of nanoparticles on the surface is experimentally investigated.

• Asymptotic behavior of particulate fouling is registered in nucleate boiling domain.

• A correlation based on the isothermal-diffusive particulate flow is proposed.

Abstract

Because of potential of energy saving and wide utilizations of nanofluids in industrial and commercial aspects, sedimentation and scale formation of nanoparticles inside the heat transfer media has been an indispensible challenge. In the present work, experimental investigation is conducted on the convective boiling of dilute CuO–water nanofluids in an upward flow inside the conventional heat exchanger. Nanofluids are stabilized using the stirring, pH control and sonication processes. For different operating conditions, influences of heat and mass flux, concentration of nanofluids on scale formation in term of fouling resistance are investigated in convective and nucleate boiling heat transfer heat transfer domains. Since, particulate fouling has not been correlated for nanofluids, a new correlation based on the experimental data at isothermal/diffusion flow conditions is introduced which can predict the fouling resistance of CuO/water nanofluids with Absolute Average Deviation of (less than) 30% which implies on a fair agreement to experimental data. Since, experiments are carried out in a heat exchanger with conventional dimensions; interpreted results can be generalized and applicable for any present/future industrial heating tools engaged with nanofluids as coolant.

Introduction

Heat exchangers are widely used in many engineering applications such as chemical industry, power plants, food industry, environment engineering, waste heat recovery and air conditioning. In order to reduce the prices of energy, industries tend to apply energy saving methods as much as possible in their facilities. Using nanofluids are one of the best possible solutions for energy saving and improving the efficiency of heating tools which is still under study by researchers [1]. Convective heat transfer is one of the most widely investigated thermal phenomena in nanofluids [2], [3], [4], [5], [6], [7], [8], [9], relevant to a number of engineering applications. Due to the observed improvement in the thermal conductivity, nanofluids are expected to provide enhanced convective heat transfer coefficients as well as better thermal performance when they are used as working fluid. However, as the suspensions of nanoparticles in the base fluids affect the thermo-physical properties other than thermal conductivity also, such as the viscosity and the thermal capacity, quantification of the influence of nanoparticles on the heat transfer performance is essentially required [1], [10]. Fouling of heat transfer surfaces during sub-cooled flow boiling is an important challenge in thermal engineering and process industries. Nevertheless, only few experimental and theoretical investigations on this subject can be found in the literature. For almost all the previous studies, fouling resistance has been investigated when occurring on the heat transfer surfaces of boilers and evaporators as crystalline deposit caused by precipitation from solutions of mineral salts which have inverse solubility behavior, however, lack of literature for particulate fouling of nanofluid is sensible. When Najibi et al. [11] conducted many experiments to investigate the calcium sulfate scale formation during sub-cooled flow boiling; he suggested the mechanistic model based on the nucleate boiling fraction for prediction of fouling resistances, which was in good agreement with the obtained experimental data. They postulated that the deposition in the boiling zones is reaction rate controlled due to the agitation created by bubble interactions around the heating section. Helalizadeh et al. [12] carried out experiments to study the mechanisms of salt solutions and crystallization fouling on heating surfaces in convective boiling and sub-cooled flow boiling conditions. Al Mutairi [13] demonstrated that fouling deposition rate of mixed salts increases with the increase of concentration of the fouling solution. Mwaba et al. [14] have noticed that the scale layer growth increased with the surface temperature and decreased with the flow velocity. After researches that were conducted by Demopoulos [15] it was found that supersaturation is the key parameter in controlling the rate of aqueous precipitation. Fahiminia [16] conducted experimental fouling investigation on tube and reported that the highest rate of fouling occurred at the location of the highest temperature. Vatani et al. [17] conducted experimental investigation inside the annulus to investigate the different operating condition on heat transfer reduction and fouling resistance and applied the asymptotic model for predicting the fouling resistance for mixed salts. Recently, an investigation on micro-particles (alumina 1–2 μm) has been conducted by Vatani et al. [18] and influences of bubble formation and particulate fouling of alumina inside the small vertical (annular) heat exchanger fouling were studied. Arsenyeva et al. [19] investigated thermal resistance of cooling water fouling in plate heat exchangers and the asymptotic behavior of water fouling was examined specifically and the net rate of fouling accumulation was described as the difference between the fouling deposition rate and the fouling removal rate. An equation was proposed to account for how the fouling resistance varies with time.

According to the literatures, no great attention has been paid to fouling of nanoparticles in heat transfer media. On the other hand, most of previous studies have been conducted inside the small annulus, i.e. with hydraulic diameter of 20 mm or less or inside the confined spaces. In our previous works [20], [21], [22], it has been shown that scale formation can deteriorate the heat transfer performance of nanofluids in an isothermal diffusive layer due to formation of a polarized layer around the heating surface in a compact heat exchanger. In this work, a conventional vertical annulus is fabricated and influence of different operating parameters such as heat and mass flux, concentration of nanofluids, wall temperature and sub-cooled level on the fouling resistance parameter and heat transfer coefficient of CuO/water are experimentally investigated and a new correlation for estimating the fouling resistance of nanoparticles based on the isothermal diffusive condition is proposed.