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A promising nanomaterial, magnesium oxide (MgO) and a commonly studied nanomaterial, aluminum oxide (Al2O3) were used to enhance the thermal conductivity of two ionic liquids, i.e., 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCa]) and 1-ethyl-3-methylimidazolium tricyanomethanide ([emim][TCM]) of potential as heat transfer medium. Effects of nanoparticle material, size, shape as well as mass concentration on thermal conductivity enhancement were investigated experimentally, along with stability analysis of the nanoparticle suspensions. The thermal conductivity of [emim][TCM] can be enhanced by up to 40% by adding 15 wt.% of polyhedral MgO nanoparticles. The thermal conductivity of Al2O3 ionanofluids does not vary much with nanoparticle size while that of MgO ionanofluids tends to increase as the nanoparticle size decreases. The Maxwell-Garnett model and the Hamilton–Crosser model could estimate the thermal conductivity of [emim][DCa]-based nanofluids containing Al2O3 and MgO nanoparticles, respectively, while under-prediction prevailed for [emim][TCM]-based nanofluids. Besides, challenges were encountered during zeta potential measurements created by the ionic liquids themselves. New methods need to be developed to correctly measure the zeta potential of ionic liquid-based nanofluids. [ABSTRACT FROM AUTHOR]

In this study, microparticle coatings and nanoparticle coatings were fabricated on copper surfaces by an electrochemical deposition method and an electrophoretic deposition method, respectively. Pool boiling of NOVEC-649 was experimentally studied on the coated surfaces, concerning heat transfer, bubble dynamics, and critical heat fluxes. Compared with a smooth surface, heat transfer coefficients and critical heat flux (CHF) were improved, achieving a maximum heat transfer enhancement of 460% on the nanoparticle-coated surface and a maximum CHF enhancement of 60% on the microparticle-coated surface. Based on high speed visualizations, bubble departure diameters were measured and compared with several correlations, and then the heat transfer was analyzed by a mechanistic model, considering natural convection, transient heat conduction and microlayer evaporation. The mechanistic model demonstrated a good ability to predict the present results. In addition, wickability, representing a liquid supplement ability, was measured, indicating that the wickability enhancement was probably responsible for the CHF improvement. [ABSTRACT FROM AUTHOR]

Due to the merits of higher heat transfer efficiency, spirally corrugated tubes are widely used in a range of applications for heat transfer enhancement, especially in coaxial heat exchangers. This article studied a novel hybrid smooth and six-start spirally corrugated tube and focused on its heat transfer performance as well as its pressure drop. Firstly, we focused on the secondary flow derived from the centrifugal force due to the continuous spiral structure, and discovered the flow patterns of fluids in the hybrid tube. In this part, we applied models with different lengths of the corrugation part. Then, the corrugation parts connected with a smooth part can be arranged in identical rotation or opposite rotation of the corrugations, where the fluid affected by the corrugation part is still whirling. For the above different models, both heat transfer and pressure drop performances of two corrugation arrangements are investigated. This work reveals the enhanced heat transfer mechanism of the hybrid smooth and spirally corrugated tube and should be of interest to researchers in heat transfer issues of corrugated tubes. [ABSTRACT FROM AUTHOR]

Fouling on heat exchanger surfaces can weaken the heat-transfer capability, increase the energy consumption, and even cause the failure of the whole system. In coaxial heat exchangers, spirally corrugated tubes perform better than smooth ones concerning heat transfer and antifouling. In this article, a parametric study on the antifouling performance of a six-start spirally corrugated tube is carried out with a solid–liquid two-phase model. First, comparisons between a smooth tube and a specific sample six-start spirally corrugated tube on the solid particle volume fraction distributions are carried out. Then, the effects of solid particle diameter, the main geometric parameters, including pitch and the corrugation depth, are investigated. Analyzing the solid particle volume fraction with different geometries, solid particle diameters and Reynolds number, the roles played by the centrifugal force, drag force, and gravity of solid particles on fouling performance in six-start spirally corrugated tubes are obtained. In addition, the corrugation depth affects the volume fraction range more while the pitch affects more on the steady range of particle volume fraction. This work is of significance for further design of spirally corrugated tubes and analysis of fouling problems in heat exchangers. [ABSTRACT FROM AUTHOR]

Experimental single-phase, condensation and flow boiling heat transfer data from the literature and our previous studies were collected to evaluate existing heat transfer correlations for microfin tubes of different geometries. The Ravigururajan and Bergles correlation modified by using the hydraulic diameter proposed by Li et al. (2012) can predict single-phase heat transfer data relatively well. Among the four reviewed condensation heat transfer correlations, the Yu and Koyama (1998) correlation presents the best prediction. However, all the four condensation correlations are prone to overpredict the carbon dioxide data. For flow boiling in microfin tubes, the general semiempirical correlation developed by Wu et al. (2013), applicable for intermittent and annular flow patterns, is the most reliable predictive method among the five evaluated correlations. It can predict 90% of the overall 754 data points within a ±30% error band, with a mean absolute deviation and a standard deviation equal to 18.2% and 21.9%, respectively, covering pure halogenated refrigerants, near azeotropic refrigerant mixtures, and carbon dioxide with the following applicable range: fin root diameter 2.1 to 14.8 mm, mass flux 100 to 800 kg/m2s, heat flux 4.5 to 59 kW/m2, and reduced pressure 0.07 to 0.7. [ABSTRACT FROM PUBLISHER]