Enhancement Of Heat Transfer Coefficient Research Articles

A facile, green fabrication of aqueous nanofluids containing hydrophilic functionalized carbon nanotubes toward improving heat transfer in a closed horizontal flow passage

In the present research, the synthesis of highly stable multiwalled carbon nanotubes in aqueous media is developed using a one-pot, covalent and green functionalization technique to improve the heat transfer and hydrodynamic behavior of a horizontal stainless-steel tube subjected to a uniform heat flow at its outer surface. Instead of using corrosive inorganic acids, the free radical grafting of gallic acids is used in this procedure. In this work, GA-functionalized multiwalled carbon nanotubes water-based colloidal suspensions (nanofluids) were prepared at three different weight concentrations. Various characterization techniques comprising Fourier transform infrared spectroscopy, Raman spectroscopy, High resolution transmission Electron Microscopy, and zeta-potential measurements were performed and confirmed the success of MWCNT functionalization. The thermophysical properties were evaluated experimentally and validated using empirical correlations available in the literature. To prove the colloidal suspension stability, ultraviolet-visible spectroscopy was used, and the results showed that nanofluid was stable for almost 60 days. When the GAMWCNT concentration was increased, there was a significant improvement in the thermal conductivity. Moreover, the nanofluids' dynamic viscosity experienced a slight increment up to 7.30% when the GAMWCNTs were loaded relative to distilled water. Following the confirmation of the experimental setup's analytical correlations, tests for the colloidal GAMWCNT suspension flowing through a heated horizontal tube were carried out in a fully developed turbulent state. A significant enhancement in the convective heat transfer coefficient was obtained, with only minor growth in the relative pumping power by 33.05 and 1.19%, respectively. More importantly, the reported positive performance index indicator for all the Reynolds numbers of ranges shows the possibility of using the synthesized GAMWCNT aqueous suspensions as an alternative working fluid in heat transfer systems.

Experimental and theoretical investigation of forced convection heat transfer with CNTs and CuO water based nano-fluids

Abstract Improving efficiency of the nuclear power plants has always been of interest for researchers. Recently, Nanofluid technology are developed to increase the heat extraction from a hot surface. Throughout this work, the thermal hydraulic behavior of Nanofluid was experimentally investigated and theoretically predicted. For this purpose, an experimental apparatus (setup) was designed and constructed to study the effect of the kind of Nanomaterial and the concentration of nanoparticles. Two kinds of Nanomaterial, Carbon Nano Tubes (CNTs) and Copper Oxide (CuO) were used. In an experimental investigation of CNTs, two nanoparticles concentration was used; 0.05 and 0.1% vol. CNTs/Water. CuO nanofluid was investigated experimentally with 0.1, 0.2, 0.3, 0.4, and 0.5% vol.CuO/Water. These concentrations of nanofluid were investigated at a constant mass flow rate and different heat fluxes, (1000, 3000 and 5000 W/m2). The test rig was modeled theoretically using ANSYS FLUENT 17.2 code and validated to predict the thermal hydraulic performance of the Nanofluid. The five concentrations of CNTs and CuO nanofluids were theoretically investigated. It was found that the measured wall surface temperatures of CNTs and CuO nanoparticles decreased remarkably as heat flux decreased and the nanoparticles concentrations increased. There was an increase in forced thermal efficiency of nanofluids as compared to base fluid. The experimental results showed a maximum heat transfer coefficient enhancement of 26.55% for a nanofluid prepared with 0.1% vol. CNTs and 20.6% with 0.5% vol. CuO. A good agreement was detected after comparing experimental results with the investigated model.

Experimental study of natural convection heat transfer characteristics affected by electrical field with periodically changed direction

The effects of electrical field on natural convection heat transfer have been extensively investigated. While, the heat transfer characteristic enhanced by the electrical field with periodically changed direction is rarely involved. Therefore, the heat transfer performance of transformer oil, the mechanism analysis and prediction correlation for heat transfer characteristic in the electrical field with periodically changed direction are studied in the present study. Individually, the heat transfer coefficient enhancement is compared with different switch periods. Meanwhile, the prediction correlation for heat transfer enhancement is analyzed and proposed. Results show that the heat transfer is significantly enhanced by the direction changed electrical field with the decrease of switch period. Furthermore, mechanism analysis indicates that the positive and negative charges in the fluid, which form a dipole, move in a certain range by the electrical field with periodically changed direction. It results in the violent fluctuations in the fluid, which is beneficial for the internal local heat transfer. Finally, a prediction correlation is proposed for heat transfer in the electrical field, which agrees well with the experimental data.

Heat Transfer Enhancement of Ionic Wind Assisted Slot Jet Reattachment Nozzle: A Numerical Study

Abstract Impinging jet nozzles are omnipresent in industrial applications. Innovative impinging jet nozzles, such as radial jet reattachment (RJR) and slot jet reattachment (SJR) nozzles, have been proven to be highly efficient tools to enhance heat and mass transfer, compared to traditional in-line jet and slot-jet nozzles. However, the heat and mass transfer in the region immediately underneath these reattachment nozzles are relatively inefficient. The ionic wind is a promising technique for heat transfer enhancement. The ionic flow is induced when free ions are accelerated by an electric field, and exchange momentum with neutral air molecules. In this numerical study, the performance of the SJR nozzle is improved by the application of an electric field, specifically, ionic wind, which is generated in the region directly between the nozzle and the exposed impingement surface. The two-dimensional numerical model is based on the flow field generated by an ionic wind-assisted SJR nozzle. The simulation results show a significant secondary flow induced under the nozzle, due to ionic wind. A significant enhancement of local and average heat transfer coefficients is achieved. The heat transfer increases with the applied potential and nozzle exit velocity. However, the SJR flow pattern is altered when the air exit velocity is below a certain threshold. The simulation results provide an in-depth understanding of the heat transfer characteristics under various operating conditions and pave the way for developing this novel impinging nozzle design.

EFFECT OF CORRUGATED PIPE ON LAMINAR CONVECTIVE HEAT TRANSFER BY USING SWCNT NANOFLUID: A NUMERICAL STUDY

Extension of the tube wall’s heat transfer area and mixing nanoparticles with working fluids are the most effective and potential techniques to enhance the heat transfer rate, which is required to remove the excessive load of heat from the heat transfer apparatus. These extreme loads are dangerous threats for heat transfer equipment, which may cause several damages. Therefore, a corrugated pipe was studied numerically along with SWCNT-water nanofluid, to determine the improvement of laminar convective heat transfer rate. Ansys fluent software and steady-state control volume method were applied for simulation purposes. Hence, different volume fractions (1% - 5%) of nanoparticles were considered to mix with water to produce nanofluid. A range of Reynolds numbers from 500 to 1200 with a constant wall heat flux of 1000 W/m2 was considered to calculate the heat transfer rate. Additionally, the corresponding pumping power requirement for such improvements was also calculated. The result demonstrated that by increasing the Reynolds number, the Nusselt number and heat transfer coefficient were raised significantly for corrugated pipe compared to a plain pipe. Consequently, the presence of nanoparticles in the working fluid also showed more enhancement. For the corrugated pipe, at Re =500, SWCNT-water nanofluid showed a maximum 56.52% enhancement of Nusselt number and heat transfer coefficient. Furthermore, SWCNT-water nanofluid showed pumping power advantage (92% for 5% volume fraction). Additionally, correlations to calculate the Nusselt number and heat transfer coefficient of nanofluid were also developed, which showed good agreement with numerical results. However, it can be concluded that corrugated channels, along with nanofluid, provide enhancement of heat transfer rate, Nusselt number, and pumping power advantage for the laminar developed region of a pipe.

Experimental investigation of flow boiling characteristics in counter-flow microchannels with different mass flux distributions

The cooling of high-power electronics urgently requires efficient and controllable heat dissipation techniques. In this work, a novel bidirectional counter-flow microchannels (CFMC) with mass flux distribution being regulated is proposed to achieve this goal. Flow boiling experiments are conducted in the CFMC and compared with traditional unidirectional parallel-flow microchannels (PFMC) by using deionized water as the working fluid. The effects of mass flux distributions on the boiling heat transfer process are experimentally investigated by changing the mass flux on one side of CFMC from 118 kg/m2·s to 370 kg/m2·s while keeping the mass flux on the other side unvaried. It is found that CFMC excels in heat transfer enhancement, as compared to traditional PFMC, the critical heat flux (CHF) can be increased by 42.9%∼53.8%. Furthermore, the average and local heat transfer coefficient (HTC) of CFMC in the convective boiling stage can be enhanced by 33.5%∼62.0% and 39.3%∼89.4%, respectively. In the nucleate boiling stage, the average enhancement of HTC for CFMC can be up to ∼170%. It should be noted that due to the shortening of the drying-out length and ensuring the wetting on one side, compared with the results under even mass flux distribution, the CHF can be further improved for CFMC with uneven mass flux distribution, but HTC has deteriorated. Surprisingly, all heat transfer enhancement results are achieved with a 53.4%∼66.7% reduction in two-phase pressure drop. Equally important, precise regulation of wall temperature distributions, heat transfer characteristics, and two-phase pressure drops can be realized by adjusting the mass fluxes on both sides. The statistical analysis of the visualization results reveals that the mass flux deployment also allows for regulating the wetting and drying times within the CFMC. This work presents essential value for optimizing the flow boiling process in microchannels and has great potential in practical applications.

Enhanced heat transfer coefficient of flow boiling in microchannels through expansion areas

Flow boiling in microchannels is a promising technique to achieve large cooling rates and high heat transfer coefficients for electronic cooling. This work reports an effective approach to enhance flow boiling heat transfer in microchannels through expansion areas. Flow boiling experiments were conducted on plain microchannels and microchannels with single and three expansion areas for comparison. Experimental results show that the flow boiling heat transfer coefficient can be effectively enhanced by up to 43.3% through adding three expansion areas in microchannels, while the pressure drop variation is within 3 kPa. Expansion areas significantly reduce the inlet temperature fluctuation, indicating the suppression of flow boiling instability in microchannels. As disclosed by the visualization of flow patterns, the improved heat transfer coefficient can be attributed to the enhanced bubble nucleation and formation of a nonuniform liquid layer near the corners of expansion areas.

Pool boiling heat transfer enhancement of aqueous solution with quaternary ammonium cationic surfactants on copper surface

Pool boiling is a widely encountered heat transfer phenomenon and the enhancement of boiling heat transfer performance is of vital importance for real applications. Adding surfactant into distilled water is regarded to be a promising and effective approach for heat transfer improvement. The present study selected four kinds of quaternary ammonium cationic surfactants with an increased molecular weight, namely, decyltrimethylammonium bromide (DeTAB), dodecyltrimethylammonium bromide (DoTAB), myristylmethylammonium bromide (MTAB) and cetyltrimethylammonium bromide (CTAB), to investigate the pool boiling performance of their aqueous solutions on a copper heater with and without electric field under different concentrations. Their dynamic surface tensions and viscosities were also measured. The bubble dynamics during pool boiling and apparent contact angle on copper surface before and after boiling test were evaluated. Experimental results reveal that the aqueous surfactant solution can significantly enhance the heat transfer coefficient (HTC) compared with distilled water. The HTC exhibits an increment with concentration until critical micelle concentration (CMC) and then levels off. The maximum relative HTC enhancement can reach up to 209.3%, which is ascribed to the increased nucleate site and bubble departure frequency. In addition to the reduction in surface tension, a repulsion force caused by electrostatic interaction is also responsible for the changes in nucleate site and bubble departure frequency. However, the critical heat flux (CHF) shows a continuous decrement with surfactant concentration. In a low concentration surfactant solution, the CHF is even slightly higher than that of water, which is caused by an improvement in surface wettability after boiling test. The CHF in moderate and high surfactant concentration will be lower. The surfactant with a smaller CMC tends to have a higher HTC and a lower CHF under a low concentration. However, for a relatively high concentration comparative to CMC, the four kinds of quaternary ammonium cationic surfactant solutions tend to have pretty similar boiling curves. The electric field is found to have a negligible influence on the pool boiling performance in aqueous surfactant solutions. Results in present study can provide valuable guidance for surfactant selection during the design of highly effective heat exchangers.

Metal–organic framework coated porous structures for enhanced thermoelectric performance

Self-powered sensors/transmitters can be operated by thermoelectric generators if the temperature difference across the device is maximized. Here, we demonstrate a novel strategy to increase the overall thermoelectric conversion efficiency near room temperature (≈30 °C) through enhancement of the heat transfer between the thermoelectric generator and the atmosphere by utilizing the latent heat of atmospheric water, radiative cooling, and an enhanced surface area. To maximize the sorption and emissivity, we coat porous copper substrates with metal-organic frameworks. Thermoelectric generators interfaced with these heat sinks exhibit a 50%–70% enhancement in the overall heat transfer coefficient owing to the increased surface area due to particle binding and the material properties of metal–organic frameworks. Furthermore, proof-of-concept experiments reveal an ≈100% increase in the total electromotive force generated by the thermoelectric generator within 30 min. Our study not only introduces a novel method to enhance the thermoelectric conversion efficiency, but also provides physical insight into the link between sorption and thermal processes.

Counterflow HE analysis of Cu and SiO2 nanofluids in the developing flow region

AbstractThree concentrations of 0.2, 0.6, and 1.0 vol.% Copper/25 nm and silica/22 nm nanofluids are prepared in a base liquid glycerol–water mixture of 30:70 ratio by volume (GW70). The thermophysical properties of Cu and SiO2 nanofluids are determined with a TPS500S hot disc thermal analyzer and Brookfield viscometer in the temperature range of 20–80°C. The maximum enhancement in Cu and SiO2 nanofluid viscosity (63.4%, 35.7%), thermal conductivity (100.4%, 71.3%), and density (7.5%, 1.5%) while specific heat (7.8%, 2.3%) determined for 1.0% concentration at 80°C compared to base liquid GW70. Heat transfer experiments are conducted in a short‐length double pipe heat exchanger. The flow rates resulted in the lamifnar entry length region. A maximum enhancement in the overall heat transfer coefficient (HTC; 25.0%, 19.7%) and convective HTC (46.2%, 34.8%), respectively for Cu and SiO2 nanofluids is estimated at 1.0% concentration compared to base liquid at a bulk temperature of 35°C.

Effective passive phase-change light-emitting diode cooling system using graphene nanoplatelets coatings

An effective passive two-phase light-emitting diode (LED) cooling system using thermally cured graphene nanoplatelets (GNPs) coatings is introduced. The GNPs-coated surfaces demonstrate significant reduction in the static contact angle as compared to the uncoated aluminium surface. The ultrafast water permeation property of the micro/nano-capillaries-structured GNPs coatings engenders significant cooling performance enhancement of the LED. The water is absorbed by the GNPs coatings and permeating upwards against the gravitation force. The water molecules are propelled through the nanocapillaries formed by the carbon walls and the capillary force dominates over the gravitational force. This induces the filmwise evaporation on the GNPs-coated surfaces and significantly enhances the LED cooling. As compared to the uncoated case, a maximum heat transfer coefficient enhancement of 73% is achieved. The GNPs coatings also significantly lower the operating light emitting surface temperature up to 10 °C reduction, effectively removes potential hotspots which in turn optimise the LED functionality and durability. This study paves the way for the promising employment of thermally cured GNPs coatings in developing a passive phase-change heat transfer device for LED cooling.

Evaluation of Thermal Hydraulic Characteristics of a Two Phase Superhydrophobic Microfluidic Device using Volume of Fluid Method

ABSTRACT The slug flow in a superhydrophobic microchannel with a T-junction was studied computationally. The continuous phase passed through the main channel while the dispersed phase 1 1. was introduced through the side channel. The volume of fluid (VOF) was employed to track the interface to study the dynamics of slug flow. First, a mesh independence study was carried out to select the optimum mesh by comparison of CFD results with experimental data. The developed model of microchannel was used to study slug flow heat transfer enhancement for micro cooling of electronic chips. The constant heat flux was applied on the walls of the microchannels and the axial wall temperature profile was noted. Upon quantification of heat transfer augmentation in terms of wall temperature reduction, Nusselt number and heat transfer coefficient enhancement, it was noted that slug flow performed much better vis-à-vis single-phase flows at similar conditions.

Heat transport limitations and performance enhancement of anodized grooved heat pipes charged with ammonia under gravity and anti-gravity condition

Grooved Heat Pipes (GHPs) are the essential heat transfer devices for space applications, as they can effectively work with and without the aid of gravity. In the present study, the heat transport limitations of anodized grooved heat pipes were modelled by incorporating an anodised surface coating parameters under gravity and anti-gravity conditions. Variations in the heat transport limitations such as capillary limit, boiling limit, and entrainment limits due to the anodization over non-anodized grooved heat pipes are presented. The heat transfer performance of an internally anodized and non-anodized grooved heat pipe charged with ammonia as a coolant is experimentally investigated and compared. Two sets of grooved heat pipes are fabricated with 40 numbers of rectangular grooves. Semi-spherical pores are formed by carefully adjusting the voltage and current supply between the electrodes during the anodization process. Experiments in grooved heat pipes are conducted at different inclinations (0-90°) under gravity and anti-gravity for various heat inputs of both heat pipes. Due to the anodization, thermal resistance reduction of 51.1% and enhancement in heat transfer coefficient of 36.6% was obtained at an optimum inclination of 45° for the grooved heat pipes with 0.4 mm groove width. Also, a maximum of 35 W heat input was transferred due to the anodization under anti-gravity with an inclination of 30°. Results suggest that the grooved heat pipes with semi-spherical pores charged with ammonia as a coolant was the best combination for enhanced heat transfer, preferably for space applications.

Pool boiling of R-134a refrigerant over a thin film aluminum nanostructured coated surface

Experimental investigation of nucleate pool boiling heat transfer analysis of R-134a is carried out on a bare copper surface and over a 50, 100, 150 and 200 nm thickness aluminum nanostructured coated surfaces at 10 °C and 15 °C saturation temperature. Thin film aluminum nanocoating on the polished copper surface is produced by using Thermal Evaporation Physical Vapor Deposition Technique. The coated surfaces are distinguished with the film thickness, roughness by 2D profilometer, imaging by scanning electron microscopy (SEM) and existing of coating material by energy dispersive X-ray spectroscopy (EDX). The effects of thin film aluminum nanocoating over a bare copper surface with increasing heat flux is thoroughly examined. It is observed that maximum enhancement in heat transfer coefficient at 150 nm thickness of thin film aluminum nanocoated surface is about 2.49 times and 2.81 times compared to copper surface over a heat flux range of 6–90 kW m−2 at 10 °C and 15 °C saturation temperatures respectively. Coated surfaces showed a steady increment in heat transfer coefficient values with the increase in the heat flux values. About 26.8% augmentation in heat transfer coefficient of bare copper surface is observed at 15 °C saturation temperature compared to 10 °C saturation temperature. The heat transfer coefficient values of bare copper surface are validated with Rohsenow and Cooper correlations of nucleate boiling and agreed well with measured data with a standard deviation of ±9%, ±13.5% for both correlations at 10 °C and 15 °C saturation temperatures respectively.

THERMAL PERFORMANCE OF SINTERED COPPER WICK HEAT PIPE USING WATER-BASED HYBRID NANOFLUIDS: AN EXPERIMENTAL STUDY

Relatively stable titania and silica-distilled water-based hybrid nano fluid along with surfactant are proved to be more efficient in thermal performance enhancement of heat pipes which are used for efficient cooling in electronic industry. In the current research, sintered copper wick heat pipe is utilized due to its high level of capillary action and low thermal resistance of copper wick. In this article, the vertically mounted copper sintered heat pipe containing hybrid nano fluids is investigated to measure the wall temperature as well as thermal resistance through force convection. The experimentation was carried out at heat loads of 4W, 8W, 12W, 16W and 20W by using water, titania-distilled water nano fluids and titania-silica/distilled water hybrid nano fluid filled in heat pipe in order to compare the results. By increasing the input power, the wall temperature increases from evaporator to condenser section. Maximum reduction in average vapor temperature of titania-silica hybrid nano fluid filled heat pipe and titania-distilled water nano fluid filled heat pipe was observed to be 35.74% and 31.10% as compared to water filled heat pipe at 20 W respectively. The maximum decrement in thermal resistance of titania-silica/distilled water hybrid nano fluid filled heat pipe and titania-distilled water nano fluid was observed to be 30.66% and 17.33% in comparison with water based filled heat pipe at 8 W respectively. The highest enhancement in heat transfer coefficient of titania-silica hybrid nano fluid filled heat pipe and titania-distilled water nano fluid filled heat pipe was achieved to be 29.73% and 16.21% compared to water filled heat pipe at 8 W respectively. A vi

An experimental study on heat transfer and pressure drop analysis of Al2O3/water nanofluids in a circular tube

The aim of this experimental study is to find out the heat transfer and flow characteristics of Al2O3/water nanofluid in a circular tube. For this, synthesis of nanofluids has been done by employing two-step method. Four different nanofluid concentrations such as 0.01, 0.05, 0.1 and 0.3 wt% have been prepared. An experimental test rig is fabricated for measurement of heat transfer coefficient (HTC) and pressure drop (PD) of nanofluids in laminar and constant heat flux (CHF) conditions. Increasing trend of both HTC and Nusselt number has been observed for nanofluids. Maximum enhancement in HTC and Nusselt number is 28.06 % and 29.93 %, respectively for 0.5 L/min flow rate in comparison with 0.15 L/min at 0.3 wt%. Also, penalty in PD is 2.68 times observed for same conditions. It has been observed that heat transfer enhancement dominates compared to increment in pressure losses. Thus, the use of Al2O3/water nanofluids is quite useful in various heat transfer applications.

Nucleate pool boiling heat transfer of R-1234ze(E) and R-134a on GEWA-B5H and smooth tube with the influence of POE oil

• Effect of POE oil (0.28–5%) on pool boiling with R-1234ze(E) is investigated. • Tests were conducted on GEWA-B/smooth tube with heat flux ranging from 10 −90 kW/m 2 . • The performance of R-1234ze(E) is 7–18% lower than R-134a on GEWA-B tube. • The performance of R-1234ze(E) is 27% lower than R-134a on smooth tube. • The HTC enhancement of R-1234ze(E)-oil mixtures is superior to R-134a-oil mixtures. Nucleate pool boiling heat transfer characteristics of pure refrigerants of R-134a, R-1234ze and their corresponding lubricant/refrigerant mixture of R-134a/POEA-68, and R-1234ze/POEA-68 on horizontally arranged enhanced tube (GEWA-B5H) are reported in detail. Tests are conducted at saturation temperatures of 10 °C, 0 °C, and −6 °C having oil concentration from 0.28% to 5%. Tests are conducted with decreasing heat flux range from 90 kW/m 2 to 10 kW/m 2 . It appears that the heat transfer coefficient (HTC) of pure R-1234ze on the GEWA-B5H tube is nearly 18%, 10%, and 7% lower than those of R-134a at saturation temperatures of 10 °C, 0 °C, and −6 °C, respectively. For the smooth tube, it was found that the HTC of pure R-1234ze is nearly 27% lower than that of R-134a at all the saturation temperatures. Generally, the presence of a small amount of lubricant oil impairs the HTC on an enhanced surface. But the present experimental investigation shows that the presence of lubricant oil on an enhanced surface may alter the HTC appreciably, and it is highly dependent on the saturation temperature, reduced pressure, and transport properties of refrigerant-oil mixtures. The results indicated that the highest enhancement ratio prevailed at a heat flux of 30 kW/m 2 and the HTC is about 17% and 35% with a 2% oil concentration for R-134a/POEA-68 oil mixture at saturation temperature 0 °C, and −6 °C respectively. Correspondingly, R-1234ze(E)/POEA-68 oil mixtures showed the highest 26% and 43% enhancement in the HTC at 0 °C and −6 °C saturation temperature subject to 0.28% oil concentration at a heat flux of 70 kW/m 2 . The HTC enhancement ratios of the GEWA-B5H tube were 2.9 – 8.8 for R-1234ze and 2.8 – 6.6 for R-134a compared to the smooth tube.

Fabrication of nano-copper surfaces by thermal evaporation technique to investigate nucleate pool boiling heat transfer performance of R-141b

In the present work, nano-Cu particle coatings, which were fabricated on a bare Cu surface using various thickness coating through a thermal evaporation technique, were used as heating surfaces to study the pool BHT performance of refrigerant R-141b. The heating surfaces, viz, 50 nm, 100 nm, 150 nm, and 200 nm thick nano-Cu particle coated surfaces over bare Cu heating surface are used in this experimentation. To reveal the formation of crystal structure on coated surfaces, scanning electron microscope (SEM) was used. Electron Discharge Spectroscopy (EDS) technology was employed to measure the percentage distribution of elements in the coating surfaces. Maximum heat transfer coefficient (HTC) enhancements of 10.5%, 15.7%, 26.6%, and 45.2% are observed for all the coated surfaces of thicknesses of 50 nm, 100 nm, 150 nm, and 200 nm when compared with bare Cu surfaces. Increases in HTC in nano-Cu coated surfaces were attributed to creation of high-density active nucleation sites and enhanced surface roughness. Other published data were used to predict the experimental data, with some deviations observed in the HTC. Thus, this paper is expected to gain importance due to the development of surface engineering with respect to energy conversion management and heating applications.

Enhanced pool boiling of propane on horizontal U-shaped tubes in a large-scale confined space

• The experimental data of propane pool boiling on a fin-enhanced tube was obtained. • The heat transfer coefficients demonstrated increasing with heat flux and saturation temperature. • The enhancement factors were found in the range of 1.25∼2.28. • Optimum correlations were validated to predict the experimental data within 11.48%. This paper aims to experimentally investigate the heat transfer characteristics of pool boiling on horizontal U-shaped tubes in a large-scale confined space. The saturation temperature ranges from 20 to 40°C and the heat flux varies from 2.5 to 10.5 kWm −2 . Two types of tubes with an outside diameter of 19.05 mm are tested: a smooth tube and a fin-enhanced tube with 0.4 mm fin height, 1220 fpm (fins per meter) and a 0.12 mm fin gap. The tubes are made of titanium with a length of 1165 mm. In case of pool boiling, the heat transfer coefficient for the tested tubes increases with increasing saturation temperature and heat flux. The fin-enhanced tube facilitates the bubble nucleation with the heat transfer coefficient enhancement factor varying in the range of 1.25∼2.28. Furthermore, the experimental heat transfer coefficients are compared with four correlations obtained in the open literature. R-J correlation and Copper correlation are found to be the optimum correlation for the smooth tube and the enhanced tube with the mean relative error of 10.93% and 11.48%, respectively. Findings from this study can supply a fundamental basis for validation and improvement propane heat transfer performances for the intermediate fluid vaporizer (IFV) engineering design.

Effect of nanoparticle shape on cooling performance of boehmite-alumina nanofluid in a helical heat sink for laminar and turbulent flow regimes

This study is motivated by the impacts of nanoparticle shapes on flow and heat transfer characteristics in a microchannel helical heat sink for cooling of electronics in both laminar and turbulent flow regimes. Therefore, effects of five different nanoparticle shapes (platelet, cylindrical, blade, brick and spherical) at five different volume fractions (ϕ = 0, 0.5, 1.0, 1.5, 2.0%) and at eight different Reynolds numbers corresponding to laminar and turbulent flow regimes (Re = 500–20,000) are investigated using finite volume method with two-phase mixture approach for nanofluids. Various performance parameters related with the first law of thermodynamics are considered. It was found that nanofluid utilization remarkably improves heat transfer along with causing an increment in pumping power. Furthermore, it was revealed that these variations in the parameters strongly depend on the utilized nanoparticle shape, and the platelet shaped particles were the most advantageous. They improved convective heat transfer coefficient up to 25% in laminar and up to 22.5% in turbulent flow regimes compared to base fluid. However, they increased pumping power up to 6.3-folds both in laminar and turbulent flow regimes. On the other hand, spherical particles yielded the least enhancement in convective heat transfer coefficient, which can reach up to 3.7% in laminar, and 1.8% in turbulent flow. Nevertheless, they slightly increase pumping power, which maximally reaches up to around 8%. Yet, the performance evaluation criteria showed that the heat transfer improvement effect of platelet shaped particles is more dominant over the increment effect in pumping power, which makes them favourable for applications.