Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids)

This paper is mainly concerned about the heat transfer behaviour of aqueous suspensions of multi-walled carbon nanotubes (CNT nanofluids) flowing through a horizontal tube. Significant enhancement of the convective heat transfer is observed and the enhancement depends on the flow conditions (Reynolds number, Re), CNT concentration and the pH, with the effect of pH smallest. Given other conditions, the enhancement is a function of axial distance from the inlet, increasing first, reaching a maximum, and then decreasing with increasing axial distance. The axial position of the maximum enhancement increases with CNT concentration and Re. Given CNT concentration and the pH level, there appears to be a Re above which a big increase in the convective heat transfer coefficient occurs. Such a big increase seems to correspond to the shear thinning behaviour. For nanofluids containing 0.5 wt.% CNTs, the maximum enhancement reaches over 350% at Re = 800, which could not be attributed purely to the enhanced thermal conduction. Particle re-arrangement, shear induced thermal conduction enhancement, reduction of thermal boundary due to the presence of nanoparticles, as well as the very high aspect ratio of CNTs are proposed to be possible mechanisms.     

Experimental investigation of small diameter two-phase closed thermosyphons charged with water,FC-84, FC-77 and FC-3283

An experimental investigation of the performance of thermosyphons charged with water as well as the dielectric heat transfer liquids FC-84, FC-77 and FC-3283 has been carried out. The copper thermosyphon was 200 mm long with an inner diameter of 6 mm, which can be considered quite small compared with the vast majority of thermosyphons reported in the open literature. The evaporator length was 40 mm and the condenser length was 60 mm which corresponds with what might be expected in compact heat exchangers. With water as the working fluid two fluid loadings were investigated, that being 0.6 ml and 1.8 ml, corresponding to approximately half filled and overfilled evaporator section in order to ensure combined pool boiling and thin film evaporation/boiling and pool boiling only conditions, respectively. For the Fluorinert? liquids, only the higher fill volume was tested as the aim was to investigate pool boiling opposed to thin film evaporation. Generally, the water-charged thermosyphon evaporator and condenser heat transfer characteristics compared well with available predictive correlations and theories. The thermal performance of the water-charged thermosyphon also outperformed the other three working fluids in both the effective thermal resistance as well as maximum heat transport capabilities. Even so, FC-84, the lowest saturation temperature fluid tested, shows marginal improvement in the heat transfer at low operating temperatures. All of the tested Fluorinert? liquids offer the advantage of being dielectric fluids, which may be better suited for sensitive electronics cooling applications and were all found to provide adequate thermal performance up to approximately 30–50 W after which liquid entrainment compromised their performance.     

Pool boiling characteristics of multiwalled carbon nanotube (CNT) based nanofluids over a flat plate heater

This paper is mainly concerned about the pool boiling heat transfer behavior of multi-walled carbon nanotubes (CNTs) suspension in pure water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS). Three different concentrations of 0.25%, 0.5% and 1.0% by volume of CNT dispersed with water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS) were prepared and boiling experiments were conducted over a stainless steel flat plate heater of size 30 mm² and 0.44 mm thickness. The test results exhibit that the addition of carbon nanotubes increases boiling heat transfer coefficients of the base fluids. At a given heat flux of 500 kW/m², the enhancement of heat transfer coefficient was found to be 1.5, 2.6 and 3.0 times of water corresponding to 0.25%, 0.5% and 1.0% concentration of CNT by volume in water, respectively. In water–CNT–surfactant nanofluid, it was found that 0.5% of CNT concentration gives the highest enhancement of 1.7 compared with water. In both water and water–surfactant base fluids, it was observed that the enhancement factor for 0.25% of CNT first increases up to the heat flux of 66 kW/m² and then decreases for higher heat fluxes. Further, the overall heat transfer coefficient enhancement in the water–CNT nanofluids is approximately two times higher than that in the water–CNT–surfactant nanofluids. With increasing heat flux, however, the enhancement was concealed due to vigorous bubble generation for both water–CNT and water–CNT–surfactant nanofluids. Foaming was also observed over the liquid-free surface in water–CNT–surfactant nanofluids during the investigation. No fouling over the test-section surface was observed after experimentation.     

High heat flux phase change on porous carbon nanotube structures

Carbon nanotube (CNT) forests are investigated as porous wick structures for chip-scale heat pipe cooling systems. An analytical model is developed to demonstrate the merits of phase change heat transfer on nanoscale porous structures, compared to that on microscale porous wick. Results indicate that nanoscale porous structures increase the thin-film evaporation surface area by one order of magnitude, which can significantly increase phase change heat transfer efficiency. The pertinent wick structure properties of the CNT forest are experimentally measured. Results show that the CNT forest is highly porous (～95% porosity), and possesses large variations in effective thermal conductivity ranging from 0.8 to 180 W/m K. Effective pore size of the CNT wick structure varies between 50 and 180 nm, which can generate capillary pressure up to two orders of magnitude higher than the microscale wick structure. However, its low permeability, about three to four orders of magnitude lower than the traditional wicks, underscores the necessity of bi-porous CNT wick structures. The bi-porous CNT wick structures are composed of nanoscale porous CNT clusters, separated by microscale (～50 μm wide) passages. Experimental results show a maximum heat flux of 770 W/cm² over a 2 mm × 2 mm heating area. With enhanced thin-film evaporation, heat transfer coefficients are improved by up to 100%, compared to the microscale wick. In contrast, the low CHF ～140 W/cm² over a 10 × 10 mm² heating area is caused by vapor occupation of the microscale pores and the reduction of wick permeability.     

Experimental analysis of flow and heat transfer in a miniature porous heat sink for high heat flux application

A novel miniature porous heat sink system was presented for dissipating high heat fluxes of electronic device, and its operational principle and characteristics were analyzed. The flow and heat transfer of miniature porous heat sink was experimentally investigated at high heat fluxes. It was observed that the heat load of up to 280 W (heat flux of 140 W/cm²) was removed by the heat sink with the coolant pressure drop of about 34 kPa across the heat sink system and the heater junction temperature of 62.9 °C at the coolant flow rate of 6.2 cm³/s. Nu number of heat sink increased with the increase of Re number, and maximum value of 323 for Nu was achieved at highest Re of 518. The overall heat transfer coefficient of heat sink increased with the increase of coolant flow rate and heat load, and the maximal heat transfer coefficient was 36.8 kW(m² °C)^?1 in the experiment. The minimum value of 0.16 °C/W for the whole thermal resistance of heat sink was achieved at flow rate of 6.2 cm³/s, and increasing coolant flow rate and heat fluxes could lead to the decrease in thermal resistance. The micro heat sink has good performance for electronics cooling at high heat fluxes, and it can improve the reliability and lifetime of electronic device.     

Heat transfer performance in 3D internally finned heat pipe

An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 °C–95 °C, heat flux 5.0 kw/m²–40 kw/m², inclination angle 2–90°), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50–100% and 100–200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance.     

Experimental study on condensation and evaporation flow inside horizontal three dimensional enhanced tubes

Experimental investigations of tube side condensation and evaporation in two 3-D enhanced heat transfer (2EHT) tubes were compared to the performance of a smooth surface copper tube. The equivalent outer diameter of all the tubes was 12.7 mm with an inner diameter of 11.5 mm. Both the inner and outer surfaces of the 2EHT tubes are enhanced by longitudinal grooves with a background pattern made up by an array of dimples/embossments. Experimental runs were performed using R410A as the working fluid, over the quality range of 0.2–0.9. For evaporation, the heat transfer coefficient ratio (compares the heat transfer coefficient of the enhanced tube to that of a smooth tube) of the 2EHT tubes is 1.11–1.43 (with an enhanced surface area ratio of 1.03) for mass flux rate that ranges from 80 to 200 kg/m² s. For condensation, the heat transfer coefficient ratio range is 1.1–1.16 (with an enhanced surface area ratio of 1.03) for mass flux that ranges from 80 to 260 kg/m² s. Frictional pressure drop values for the 2EHT tubes are very similar to each other. Heat transfer enhancement in the 2EHT tubes is mainly due to the dimples and grooves in the inner surface that create an increased surface area and interfacial turbulence; producing higher heat flux from wall to working fluid, flow separation, and secondary flows. A comparison was performed to evaluate the enhancement effect of the 2EHT tubes using a defined performance factor and this indicates that the 2EHT tubes provides a better heat transfer coefficient under evaporation conditions.     

Effects of carbon nanotube arrays on nucleate pool boiling

Experiments were performed to assess the impact coating silicon and copper substrates with nanotubes (CNTs) have on pool boiling performance. Different CNT array densities and area coverages were tested on 1.27 × 1.27 mm² samples in FC-72. The CNT preparation techniques used provided strong adherence of CNTs to both substrate materials. Very small contact angle enabled deep penetration of FC-72 liquid inside surface cavities of smooth uncoated silicon surfaces, requiring unusually high surface superheat to initiate boiling. Fully coating the substrate surface with CNTs was highly effective at reducing the incipience superheat and greatly enhancing both the nucleate boiling heat transfer coefficient and critical heat flux (CHF). Efforts to further improve boiling performance by manipulating CNT area coverage of the substrate surface proved ineffective; best results were consistently realized with full surface coverage. Greater enhancement was achieved on silicon than on copper since, compared to uncoated copper surfaces, the uncoated silicon surfaces were very smooth and void of any sizeable nucleation sites to start with. This study is concluded with detailed metrics to assess the enhancement potential of the different CNT array densities and area coverages tested.     

Experimental study of R-134a evaporation heat transfer in a narrow annular duct

An experiment is carried out here to investigate the evaporation heat transfer and associated evaporating flow pattern for refrigerant R-134a flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm. In the experiment, the effects of the duct gap, refrigerant vapor quality, mass flux and saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. For the duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m² s, imposed heat flux q from 5 to 15 kW/m², vapor quality x_m from 0.05 to 0.95, and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m² s with the other parameters varied in the same ranges as that for δ = 2.0 mm. The experimental data clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficient also rises substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in T_sat, but the effects are less pronounced in the narrower duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficient increases substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the duct gap causes a significant increase in h_r. In addition to the heat transfer data, photos of R-134a evaporating flow taken from the duct side show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. Finally, an empirical correlation for the present measured heat transfer coefficient for the R-134a evaporation in the narrow annular ducts is proposed.     

Effect of CNT concentration and agitation on surface heat flux during quenching in CNT nanofluids

In this article, the effect of Carbon Nanotube (CNT) concentration and agitation on the heat transfer rate has been studied during immersion quenching in CNT nanofluids. For this purpose, CNT nanofluids were prepared by suspending chemically treated CNTs (TCNT) at four different concentrations in deionized (D.I) water without using any surfactant. Quench probes with a diameter of 20 mm and a length of 50 mm were machined from 304L stainless steel (SS) and quenched in water and CNT nanofluids with the CNT concentration ranging from 0.25 to 1.0 wt.%. The heat flux and temperature at the quenched surface were estimated based on the Inverse Heat Conduction (IHC) method using the temperature data recorded at 2 mm below the probe surface during quenching. The computation results showed that the peak heat flux increased with an increase in the CNT concentration up to 0.50 wt.% and started decreasing with further increase in the CNT concentration. The enhanced heat transfer performance of CNT nanofluids during quenching at lower concentration of CNTs is attributed to their higher effective thermal conductivity. The reduced heat transfer performance of CNT nanofluids having higher concentration of CNTs is due to the increased viscosity of CNT nanofluids. The effect of agitation on heat transfer rate during quenching has also been studied in this work by stirring the CNT nanofluid prepared with 0.50 wt.% of CNTs which recorded the maximum peak heat flux among the four concentrations. The effect of CNT nanofluid agitation was counter-intuitive and resulted in decreased heat transfer rate with the increase in agitation rate.     

Falling water film evaporation on newly-designed enhanced tube bundles

Experimental studies are presented on falling film evaporation of water on 6-row horizontal enhanced tube bundles in a vacuum condition. Turbo-CAB (19 fpi and 26 fpi), Korodense, and smooth tubes were tested in a range of film Reynolds number from about 10 to 110 and in the condition of only convective evaporation, without nucleate boiling. The flow modes and heat flux will affect the transition Reynolds numbers. Tubes with enhanced inner surface provide better heat transfer performance. Hotter heating water may lead to better heat transfer performance mainly due to higher heat fluxes. Correlations were also derived to predict the heat transfer coefficients and the enhancement ratio.     

Heat transfer and single-phase flow in internally grooved tubes

This study investigates heat transfer and flow characteristics of water flowing through horizontal internally grooved tubes. The test tubes consisted of one smooth tube, one straight grooved tube, and four grooved tubes with different pitches. All test tubes were made from type 304 stainless steel. The length and inner diameter of the test tube were 2 m and 7.1 mm, respectively. Water was used as working fluid, heated by DC power supply under constant heat flux condition. The test runs were performed at average fluid temperature of 25 °C, heat flux of 3.5 kW/m², and Reynolds number range from 4000 to 10,000. The effect of grooved pitch on heat transfer and pressure drop was also investigated. The performance of the grooved tubes was discussed in terms of thermal enhancement factor. The results showed that the thermal enhancement factor obtained from groove tubes is about 1.4 to 2.2 for a pitch of 0.5 in.; 1.1 to 1.3 for pitches of 8, 10, and 12 in., respectively; and 0.8 to 0.9 for a straight groove.     

Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes

An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m² s, imposed heat flux q from 5 to 15 kW/m², inlet vapor quality x_in from 0.2 to 0.8 and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m² s with the other parameters varied in the same ranges as those for D_i = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (D_i = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same x_in, T_sat, G, q and D_i, refrigerant R-407C has a higher h_r when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed.     

Experimental investigation of two-phase closed thermosyphon used in heavy duty extrusion pelleting line

Efficient cooling system is an essential part of heavy duty extrusion pelleting line which plays an important role in production engineering of megaton polyolefin. In this paper, a new cooling system based on two-phase closed thermosyphon (TPCT) used in heavy duty extrusion pelleting line was presented. Comparative experimental results show that thermal performance of TPCT is more efficient, and the temperature uniformity is much better than traditional extruder (similar to coiled heat exchanger) in preheating process and extrusion reaction process. The effects of different operation conditions: filling ratio (0.2, 0.35, 0.5, 0.65 and 0.8), flow rate of cooling water (120 L/h, 180 L/h, 240 L/h, 300 L/h and 360 L/h) and heating power (7 kW, 9 kW and 11 kW) on thermal characteristics were experimental investigated, respectively. The results show that temperature of barrel inner wall increased significantly as filling ratio increased and evaporator appeared dry out when filling ratio is 0.2. The flow rate of cooling water affected the condenser section obviously, but had little influence on evaporation section. With increase of heating power, the start-up time decreases and the heat transfer coefficient increased. An ideal cooling scheme was concluded: the liquid filling ratio was 0.35, the cooling water flow rate was 180 L/h and the heating power was 11 kW when working medium was water.     

Effect of solid volume fraction and tilt angle in a quarter circular solar thermal collectors filled with CNT–water nanofluid

Solar thermal collectors have significant importance due to its wide use in solar thermal technology. Augmentation of heat transfer is a key challenge for solar thermal technology. A quarter circular solar thermal collectors is investigated throughout the paper introducing carbon nanotube (CNT)–water nanofluid in the cavity. Tilt angle of this type of collector plays a vital role and heat transfer can be maximized for a particular tilt angle and solid volume fraction of the nanofluid. Galerkin weighted residual of FEM has been applied for the numerical solution of the problem. Grid independency test and code validation have been assessed for the accuracy of numerical solution. In this paper a wide range of solid volume fraction (δ = 0 to δ = 0.12) and tilt angle (ϕ = 0 to ϕ = 60°) has been investigated for Rayleigh number (Ra = 10⁵–10⁸) with varying dimensionless times. It has been found that both solid volume fraction and tilt angle play vital roles for the augmentation of heat transfer and a good heat transfer characteristic can be obtained by compromising between these two parameters. The results are shown using streamline, isotherm contour and related graph and chart.     

The effects of geometrical parameters of a corrugated channel with in out-of-phase arrangement

Numerical investigations of forced turbulent convective flow and heat transfer in a corrugated channel of plate heat exchanger are carried out. The continuity, momentum and energy equations were solved by means of a finite volume method (FVM). The top and bottom walls of the corrugated channel are heated at constant heat flux boundary conditions. The effects of geometrical parameters of corrugated tilt angles, channel heights and wavy heights using water as a working fluid on the thermal and flow fields as well as on the performance of evaluation criterion are studied. The corrugated channel with three different corrugated tilt angles of 20°, 40° and 60° with different channel heights of 12.5, 15 and 17.5 mm and different wavy heights of 2.5, 3.5 and 4.5 mm are tested. This investigation covers Reynolds number and heat flux in the range of 8000–20,000 and 0.4–6 kW/m², respectively. The numerical results indicate that the wavy angle of 60° and wavy height of 2.5 mm with channel height of 17.5 mm are the optimum parameters and they have a significant effect on the heat transfer enhancement. It is found that using wavy channel is a suitable method to increase the thermal performance and getting higher compactness of the heat exchanger.     

An experimental study on CHF enhancement in flow boiling using Al2O3 nano-fluid

The critical heat flux (CHF) is one of the most important thermal hydraulic parameters in heat transfer system design and safety analyses. CHF enhancement allows higher limits of operation conditions such that heat transfer equipment can be operated safely with greater margins and better economy. The application of nano-fluids is thought to have strong potential for enhancing the CHF. In this study, zeta potentials of Al₂O₃ nano-fluids were measured and flow boiling CHF enhancement experiments using Al₂O₃ nano-fluids were conducted under atmospheric pressure. The CHFs of Al₂O₃ nano-fluids were enhanced up to ～70% in flow boiling for all experimental conditions. Maximum CHF enhancement (70.24%) was shown at 0.01 vol% concentration, 50 °C inlet subcooling, and a mass flux of 100 kg/m² s. Inner surfaces of the test section tube were observed by FE–SEM and the zeta potentials of Al₂O₃ nano-fluids were measured before and after the CHF experiments.     

Experimental investigation of flow boiling heat transfer of jet impingement on smooth and micro structured surfaces

Flow boiling heat transfer experiments using R134a were carried out for jet impingement on smooth and enhanced surfaces. The enhanced surfaces were circular micro pin fins, hydrofoil micro pin fins, and square micro pin fins. The effects of saturation pressure, heat flux, Reynolds number, pin fin geometry, pin fin array configuration, and surface aging on flow boiling heat transfer characteristics were investigated. Flow boiling experiments were carried out for two different saturation pressures, 820 kPa and 1090 kPa. Four jet exit velocities ranging from 1.1–4.05 m/s were investigated. Flow boiling jet impingement on smooth surfaces was characterized by large temperature overshoots, exhibiting boiling hysteresis. Flow boiling jet impingement on micro pin fins displayed large heat transfer coefficients. Heat transfer coefficients as high as 150,000 W/m² K were observed at a relatively low velocity of 2.2 m/s with the large (D = 125 μm) circular micro pin fins. Jet velocity, surface aging, and saturation pressure were found to have significant effects on the two-phase heat transfer characteristics. Subcooled nucleate boiling was found to be the dominant heat transfer mechanism.     

Heat transfer and entropy generation for laminar forced convection flow of graphene nanoplatelets nanofluids in a horizontal tube

The results are reported of an investigation of the heat transfer characteristics and entropy generation for a graphene nanoplatelets (GNP) nanofluid with specific surface area of 750 m²/g under laminar forced convection conditions inside a circular stainless steel tube subjected to constant wall heat flux. The analysis considers constant velocity flow and a concentration range from 0.025 wt.% to 0.1 wt.%. The impact of the dispersed nanoparticles concentration on thermal properties, convective heat transfer coefficient, thermal performance factor and entropy generation is investigated. An enhancement in thermal conductivity for GNP of between 12% and 28% is observed relative to the case without nanoparticles. The convective heat transfer coefficient for the GNP nanofluid is found to be up to 15% higher than for the base fluid. The heat transfer rate and thermal performance for 0.1 wt.% of GNP nanofluid is found to increase by a factor of up to 1.15. For constant velocity flow, frictional entropy generation increases and thermal entropy generation decreases with increasing nanoparticle concentration. But, the total entropy generation tends to decrease when nanoparticles are added at constant velocity and to decrease when velocity rises. Finally, it is demonstrated that a GNP nanofluid with a concentration between 0.075 wt.% and 0.1 wt.% is more energy efficient than for other concentrations. It appears that GNP nanofluids can function as working fluids in heat transfer applications and provide good alternatives to conventional working fluids in the thermal fluid systems.     

Experimental investigation on heat transfer characteristics of low mass flux rifled tube with upward flow

An experiment for heat transfer of water flowing in a vertical rifled tube was conducted at subcritical and supercritical pressure. The main purpose is to explore the heat transfer characteristics of the new-type rifled tube at low mass flux. Operating conditions included pressures of 12–30 MPa, mass flux of 232–1200 kg/(m² s), and wall heat fluxes of 133–719 kW/m². The heat transfer performance and wall temperature distribution at various operating conditions were captured in the experiment. In the present paper, the heat transfer mechanism of the rifled tube was analyzed, the effects of pressure, wall heat flux and mass flux on heat transfer were discussed, and corresponding empirical correlations were also presented. The experimental results exhibit that the rifled tube has an obvious enhancement in heat transfer, even at low mass flux. In comparison with a smooth tube, the rifled tube efficiently prevents Departure from Nucleate Boiling (DNB) and delays dryout at subcritical pressure, and also improves the heat transfer of supercritical water remarkably, especially near pseudo-critical point. An increase in pressure or wall heat flux impairs the heat transfer at both subcritical and supercritical pressure, whereas the increasing mass flux has a contrary effect.