Heat Transfer Coefficient Of Nanofluids Research Articles

Experimental assessment of convective heat transfer coefficient enhancement of nanofluids prepared from high surface area nanoporous graphene

In this research convective heat transfer coefficient enhancement of nanofluids prepared from high surface area graphene has been investigated in laminar flow in the developing region. The nanofluid has been prepared from nanoporous graphene with high surface area and with concentration of 0.025 to 0.1wt.%. Deionized water has been used as the base fluid and a type of Ter-Polymer has been utilized as the surfactant. The results indicate that thermal conductivity of nanofluid with concentration of 0.1wt.% remains quite constant, with only 3.8% enhancement, while convective heat transfer coefficient improves significantly, with 34% enhancement. The behavior of this enhancement related to the concentration of graphene is linear. Also the results revealed that convective heat transfer coefficient of graphene nanofluids improves by temperature increase and the rate of improvement enhances by temperature increase. Increase in Reynolds number enhances the convective heat transfer.

On the reliable estimation of heat transfer coefficients for nanofluids in a microchannel

Nanofluids (base fluid and nanoparticles) can enhance the heat transfer coefficient h in comparison to the base fluid. This open the door for the design of efficient cooling system for microelectronics component for instance. Since theoretical Nusselt number correlations for microchannels are not available, the direct method using an energy balance has to be applied to determine h. However, for low nanoparticle concentrations the absolute numbers are small and hard to measure. Therefore, the study examines the laminar convective heat transfer of Al2O3-water nanofluids in a square microchannel with a cross section of 0.5 × 0.5 mm2 and a length of 30 mm under constant wall temperature. The Al2O3 nanoparticles have a diameter size distribution of 30-60 nm. A sensitivity analysis with error propagation was done to reduce the error for a reliable heat transfer coefficient estimation. An enhancement of heat transfer coefficient with increasing nanoparticles volume concentration was confirmed. A maximum enhancement of 6.9% and 21% were realized for 0.6% Al2O3-water and 1% Al2O3-water nanofluids.

Nano Fluid in Water as Base Fluid in Flat-Plate Solar Collectors with an Emphasis on Heat Transfer

Objectives: The objective of this paper is to review the heat transfer of Nano fluid in water as a base fluid in Flat-Plate Solar Collectors, and provide an optimal solution to improve the heat transfer. Methods/Statistical Analysis: This study used a sample of the heat exchanger consists of three small solar collectors which a copper tube passes from each of them, and there is a surface area the size of 1 * 0.15m 2 . The current cycle was analyzed using the amount of energy stored to transfer the heat. Findings: In constant Reynolds number, the convective heat transfer coefficient of Nano-fluids in the heat-absorbing pipes (with concentration 1000ppm) is slightly greater than that used the water as working fluid. Also in the concentration 10000ppm, this rate is doubled. Application: The findings concluded that the use of Nano-fluid in flatplate solar collector reduces the total heat loss coefficient of solar collector, and obtains the amount of more heat energy.

Thermal performance evaluation of a helically-micro-grooved heat pipe working with water and aqueous Al2O3 nanofluid at different inclination angle and filling ratio

The present work reports the thermal performance of a helically-micro-grooved heat pipe working with water-based alumina nanofluid. The effects of the inclination angle (0°, 30°, 60° and 90°), filling ratio (20, 40, 60 and 80%) and heat input (45:65W) on heat transfer characteristics of the heat pipe using either distilled water or water-based alumina nanofluid with the volume concentration of Al2O3 nanoparticles of 3.0% are experimentally investigated. Results revealed that the thermal performance of the heat pipe enhances with nanofluid compared to DW and both inclination angle and filling ratio significantly affect the thermal performance of the heat pipe. The evaporation and condensation heat transfer coefficients increase as filling ratio increases, but thermal resistance decreases when filling ratio increases. Moreover, the inclination angle of 60° achieves the best thermal performance of the heat pipe using water or nanofluid. The evaporation and condensation heat transfer coefficients of nanofluid are higher than those of water by 30.4% and 11.1%, respectively, while the thermal resistance of nanofluid is lower than that of water by about 18.2%.

Effects of surface modification on the pool boiling heat transfer of MWNTs/water nanofluids

The influences of covalent and non-covalent functionalization on the pool boiling heat transfer performance of MWNTs nanofluids were experimentally investigated under different weight concentrations. Relevant pool boiling heat transfer coefficients (HTCs) of MWNTs nanofluids were measured on a cooper flat surface in a pool. The obtained results are further compared with that of the deionized water as a base fluid. The results show that the MWNTs nanofluid with CTAB as non-covalent modification presents poor HTCs, which decreases with increasing the MWNTs concentration due to the deposition of nanoparticles and lower contact angle between nanofluids and heating surface. However, the MWNTs-COOH and MWNTs-OH nanofluids using the COOH and OH groups as covalent modifications show higher pool boiling HTCs than the base fluid, and they increase as the MWNTs concentration increases. The maximum HTC enhancements can reach 34.2% and 53.4% for MWNTs-COOH and MWNTs-OH nanofluids, respectively. This implies the covalent modifications for the MWNTs nanofluid could avoid the deposition of nanoparticles and benefit to HTC enhancements.

Toward improved heat transfer performance of annular heat exchangers with water/ethylene glycol-based nanofluids containing graphene nanoplatelets

A novel synthesis procedure is presented for preparing functionalized graphene nanoplatelets (GNPs). Using sonication method, the functionalized GNPs are dispersed in water-ethylene glycol to prepare water–ethylene glycol-based functionalized GNP nanofluids. Meanwhile, the thermophysical properties of the prepared nanofluids, i.e., thermal conductivity, specific heat capacity, and rheological properties are investigated. As the second phase of study, the heat transfer performance of an annular channel is simulated and measured in the presence of the prepared nanofluids. To this end, a computational fluid dynamics study has been carried out to calculate the heat transfer rate as well as pressure drop of the well-dispersed nanofluids. Meanwhile, the effects of concentration and Reynolds number on the convective heat transfer coefficient have been investigated at constant wall temperature boundary condition under turbulent flow regime. Consist with the results, the convective heat transfer coefficient of nanofluids are significantly higher than that of the base-fluid. The novel type of nanofluid reveals promising potential for use as an advanced working fluid in future heat transfer applications.

Effects of working temperature on thermo-physical properties and forced convection heat transfer of TiO2 nanofluids in water – Ethylene glycol mixture

Nanofluids as a new coolant have overcome the limitations in thermal performance faced by conventional fluids such as water, ethylene glycol (EG) and oil based fluids. The objective of the study is to investigate the properties and heat transfer performance of TiO2 nanofluids in water-EG mixture at different operating temperatures. The study is carried out through experimental determination on heat transfer coefficient of TiO2 nanofluids in a circular tube under turbulent flow. The thermal conductivity and viscosity measurements were undertaken for temperature in the range of 30–80°C. The maximum enhancement of thermal conductivity achieved was 15.4% at 1.5vol% concentration and temperature of 60°C. The relative viscosities fluctuate at a range of 4.6–33.3% with variation of temperature. The Nusselt number showed an enhancement up to 22.8% and 28.9% for temperatures of 50°C and 70°C, respectively. The friction factor for the nanofluids is slightly increased with concentration. Thermal conductivity, viscosity and heat transfer coefficient of TiO2 nanofluid are strongly influenced by working temperature and concentration.

An experimental investigation on rheological properties and heat transfer performance of MWCNT-water nanofluid flow inside vertical tubes

The contradictory results in literature regarding water-based carbon nanofluids for the turbulent regime are addressed in this paper. Convective heat transfer and friction factor of MWCNT-water nanofluids inside a vertical circular tube are investigated experimentally. The tested tubes are under constant heat flux boundary condition and the flow regime inside the tubes is turbulent. Distilled water and nanofluids with different particle weight concentrations of 0.05%, 0.1%, and 0.2% are utilized as the working fluids. Such tube and flow conditions are considered to specifically answer the question “whether water-based carbon nanofluids increase the heat transfer rate or friction factor in the turbulent regime". The required thermo-physical properties of the MWCNT–water nanofluids for calculating the convective heat transfer coefficient and pressure drop are measured. Observations clearly show that the heat transfer coefficient of nanofluid is higher than that of the base fluid and increases with the particle concentrations. Moreover, the measurements show that the pressure drop of nanofluid is slightly higher than that of the base fluid and does not increase significantly with increasing the nanoparticles volume fraction. Also, two correlations are proposed using the experimental data to predict the Nusselt number and friction factor of the nanofluid flow inside vertical tubes. Finally, the heat transfer performance analysis of MWCNT-water nanofluids show that the thermal performance factor for all cases are greater than unity which indicate that this nanofluid enhances the heat transfer without huge penalty in pumping power.

Local convective heat transfer coefficient and friction factor of CuO/water nanofluid in a microchannel heat sink

Forced convective heat transfer in a microchannel heat sink (MCHS) using CuO/water nanofluids with 0.1 and 0.2 vol% as coolant was investigated. The experiments were focused on the heat transfer enhancement in the channel entrance region at Re < 1800. Hydraulic performance of the MCHS was also estimated by measuring friction factor and pressure drop. Results showed that higher convective heat transfer coefficient was obtained at the microchannel entrance. Maximum enhancement of the average heat transfer coefficient compared with deionized water was about 40 % for 0.2 vol% nanofluid at Re = 1150. Enhancement of the convective heat transfer coefficient of nanofluid decreased with further increasing of Reynolds number.

Numerical investigation on using of nanofluid in a water-cooled photovoltaic thermal system

Photovoltaic thermal collector or PV/T is a new hybrid technology that produces heat and electricity simultaneously. The objective of the current study is to evaluate the effects of utilizing nanofluid on the performance of PV/T system. The considered coolant fluids in the simulation are pure water, Ag-water nanofluid and Alumina-water nanofluid. The proposed model includes a water riser tube and an absorber plate to consider the conduction and convection heat transfer mechanisms. The numerical results were obtained using the computational fluid dynamics (CFD) by employing conjugate heat transfer. The effects of the nanoparticle volumetric concentration and inlet fluid velocity on the efficiency and heat transfer enhancement were simulated. In this paper, the Second Law analysis of a water-cooled PVT is also represented, based on results of simulations. The results show that the efficiencies (the first and second laws of thermodynamics) and heat transfer coefficient increase by increasing the nanoparticle volume fraction. The maximum increase percentage of heat transfer coefficient versus volume fraction for alumina-water and Ag-water nanofluid are 12% and 43%, respectively. Increasing the inlet fluid velocity improves the heat transfer performance of PV/T system. Increasing the heat transfer coefficient with respect to the inlet fluid velocity for alumina-water nanofluid at φ=5% was in the range of 8–10%, in comparison to pure water. While this value for Ag-water nanofluid at the same volumetric fraction is significantly greater than alumina-water, which varies from 28 to 45%. For the purpose of validating the results of the presented method, a comparison study was carried out with the experimental results in the literature. The comparison shows an appropriate agreement with the results of the literature.

Heat Transfer and Pressure Drop Characteristics of Dilute Alumina–Water Nanofluids in a Pipe at Different Power Inputs

This work addresses the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina–water nanofluid over a wide temperature range (25°C–75°C). Low concentrations (0–0.5% v/v) of alumina nanoparticles (40 nm size) in distilled water were used in this study. The pressure drop and the effective heat transfer coefficient of nanofluids were also estimated for different power inputs and at different flow rates corresponding to Reynolds numbers in the range of 1500–6000. The trends in variation of thermophysical properties of nanofluids with temperature were similar to that of water, owing to their low concentrations. However, the density, viscosity, and thermal conductivity of nanofluids increased, while the specific heat capacity decreased with increasing the nanoparticle concentration. The convective heat transfer coefficient of the nanofluid and the pressure drop along the test section increased with increasing the particle concentration and flow rate of nanofluid. Results showed that the heat transfer coefficient increases, while the pressure drop decreases slightly with increasing the power input. This is because of the fact that increasing power input to heater increases the bulk mean temperature of nanofluids, resulting in a decreased viscosity. The prepared nanofluids were found to be more effective under turbulent flow than in transition flow.

Thermal performance of a counter-current double pipe heat exchanger working with COOH-CNT/water nanofluids

The main aim of this work is to investigate the heat transfer coefficient and pressure drop characteristics of carbon nanotube water-based nanofluids inside the double pipe heat exchanger. Diameters of inner and outer copper tubes (ID and OD) were 6.35 and 12.7mm, respectively (in accordance with ANSI/ASME/API 5L). Nanofluids were prepared using two-step method at mass concentrations of 0.1–0.3% by dispersing the COOH functionalized multi-walled carbon nanotubes, (FCNTs as purchased) into the deionized water. Since this work can technically be important, thermal conductivity of nanofluids were experimentally measured using KD2 Decagon instrument at different mass concentrations and temperatures. To assess the thermal performance of nanofluids, forced convection experiments were conducted at laminar and turbulent flow regimes (900<Re<10,500). Influence of different operating parameters including: flow rate, mass concentration of nanofluid, inlet temperature of nanofluid on the heat transfer coefficient and pressure drop is studied. Results demonstrated that presence of carbon nanotube can enhance the thermal conductivity up to 56% at wt.%=0.3. Likewise, CNT/water nanofluids have higher convective heat transfer coefficient in comparison with water, which is due to internal thermal conduction of CNTs. Longer stability was seen due to the COOH group attached to the CNTs. Small penalty is reported for pressure drop and friction factor as well, which is due to the presence of carbon nanotube inside the bulk of base fluid. Considering the influence of CNTs on heat transfer and pressure drop, it was found that carbon nanotube nanofluids can drastically enhance the thermal performance of heat exchanger in comparison with water up to 44% at maximum mass concentration (wt.%=0.3).

Remarkable enhancement of convective heat transfer with different nanoparticles in N-methyldiethanolamine solution in gas sweetening process

In this work, effective thermal conductivity and convective heat transfer at different Reynolds number of nanofluid containing nanometer-size particles in N-methyldiethanolamine solution is measured. Thermal conductivity and stability of nanostructures in water-based nanofluid, as well as their dependence to temperature and time variation, are of a great concern. These nanofluids consists of 0.1 mass fraction of spherical silica nanoparticles (SiO2), carbon nanofiber (CNF), UiO-66-NH2 MOF, carbon nanotube functionalized with carboxylic acid as ligand (CNT-COOH), MgO nanoparticles, and colloidal silica. These nanofluids mixed with of N-methyldiethanolamine solution and thermal conductivity coefficients were measured. The appropriate result demonstrated the thermal conductivity is highly related to the optimum concentration of nanoparticle in base fluid, velocity, flow rate, and heat flux. It was found that a combination of CNF and SiO2 nanoparticles in a 1:1 ratio with 0.1wt.% could enhanced thermal conductivity which will induces greater enhancement by combined nanoparticle base amine fluid in heat transfer application. It has been shown increase of Reynolds number at the mass concentration 0.1wt.% an increase in convective heat transfer coefficient of nanofluid about 13%. These results improve the thermal conductivity properties of solution in amine plants.

Heat transfer and friction factor of water and ethylene glycol mixture based TiO2 and Al2O3 nanofluids under turbulent flow

It has been a great challenge in heat transfer to provide efficient thermal fluids for cooling purposes especially in engineering practice. The concerns on various operating temperatures become the main concern in the present study to investigate the heat transfer and friction factor of titanium oxide (TiO2) and aluminium oxide (Al2O3) under turbulent flow in a tube. The nanofluids were prepared using the two-step method and dilution process for volume concentrations of 0.5% to 1.0% in a mixture of water (W) and ethylene glycol (EG) at a volume ratio of 60:40 (W:EG). The convective heat transfer investigations were conducted at a constant heat flux boundary condition and operating temperatures of 30, 50 and 70°C. The enhancement of thermal conductivity and viscosity of Al2O3 was found to be influenced by the temperature while the enhancement of the TiO2 nanofluid properties was observed to be independent of temperature. Both Al2O3 and TiO2 nanofluids were observed to have almost the same values of heat transfer coefficients for 1.0% concentration at 50 and 70°C with an average enhancement of 24%. However, the heat transfer coefficients of Al2O3 nanofluids were found to be higher than TiO2 nanofluids at the operating temperature of 30°C. The heat transfer concentrations increased with volume concentration and observed for both types of nanofluids at all operating temperatures. The friction factors for both TiO2 and Al2O3 nanofluids slightly increased with volume concentration.

Convective heat transfer characteristics of water–ethylene glycol mixture with silver nanoparticles

We report the convective heat transfer characteristics of water–ethylene glycol mixture fluids seeded with silver nanofluids under laminar, transitional and turbulent regime in this work. The volume concentrations of silver nanoparticles viz. 0.05%, 0.1%, 0.15%, 0.3% and 0.45% are considered. Thermophysical properties measurements such as thermal conductivity, viscosity, density and specific heat capacity were carried out using conventional techniques. Convection measurements were carried out in a tube in tube counter flow heat exchanger using nanofluids as the hot fluid. The effect of nanofluid mass flow rate ranging from 5g/s to 45g/s and inlet temperature of nanofluid at 35°C and 45°C on the convective heat transfer coefficient were investigated. The convective heat transfer coefficient was considerably increased with increasing Reynolds number, particle concentration and inlet temperature. The maximum enhancement of convective heat transfer coefficient of nanofluid is observed to be ∼42% at 0.45vol% compared with pure basefluid. The pressure drop of nanofluid increases marginally for a nanofluid volume concentration up to 0.15vol%. When compared with the base fluid, the difference of pressure drop is insignificant so that the use of nanofluid has limited penalty on the pressure drop up to 0.15vol% loading of silver nanoparticles. However, beyond 0.15vol% the pressure drop increases significantly which limits the use of these nanofluids at higher concentration for engineering applications.

Adoption of nanofluids in low-enthalpy parabolic trough solar collectors: Numerical simulation of the yearly yield

Energy demand in the world is continuously increasing and fossil fuels resources must be replaced by renewable resources with lower environmental risk factors, in particular CO2 emissions. Concentrating solar collectors appear to be very promising for that purpose. Thus, this work presents a numerical analysis for the evaluation of the yearly yield of a low-enthalpy parabolic trough solar collector (PTC). To increase the thermal efficiency of such systems, six water-based nanofluids at different weight concentrations are investigated: Fe2O3 (5, 10, 20wt%), SiO2 (1, 5, 25wt%), TiO2 (1, 10, 20, 35wt%), ZnO (1, 5, 10wt%), Al2O3 (0.1, 1, 2wt%), and Au (0.01wt%). The simulation environment was validated by experimental tests using water as heat transfer fluid, in two prototypes of PTC located in the city of Ancona (central Italy), while the convective heat transfer coefficient of nanofluids was measured through a dedicated apparatus. A typical meteorological year was built to perform the simulation, which presents a time-resolution of one hour. A specific arrangement for the PTC was defined, while different inlet fluid temperatures were considered at a mass flow rate of 0.50kg/s: 40, 50, 60, 70, and 80°C. For this last temperature, the variation in flow rate was also studied (at 1kg/s and 1.5kg/s). Results show that only Au, TiO2, ZnO, and Al2O3 nanofluids at the lower concentrations, present very small improvements compared to the use of water, while increasing the concentration of nanoparticles no advantage with respect to water appears.

Experimental study of the effect of solid volume fraction and Reynolds number on heat transfer coefficient and pressure drop of CuO–Water nanofluid

This study presents an experimental study of the effect of solid volume fraction and Reynolds number on heat transfer coefficient and pressure drop of CuO–Water nanofluid. Pure Water and nanofluid with particle volume fractions of 0.0625%, 0.125%, 0.25%, 0.5%, 1%, 1.5% and 2% are used as working fluids. Nanofluids were flowed inside a horizontal double-tube counter flow heat exchanger under turbulent flow regime. Flow Reynolds numbers of each volume fraction of nanofluid were between from 2900 to 18,500 during the experiments. The Result shows that generally heat transfer coefficient of nanofluids is higher than that of base fluid. Moreover, it is observed that heat transfer coefficient and Nusselt number of nanofluids increases with an increase in solid volume fraction and Reynolds number. But the rate of this increase in low Reynolds numbers was more than that at high Reynolds numbers. The measurements also show that the pressure drop of nanofluid is slightly higher than that of the base fluid and increases with an increase in the nanoparticles volume fraction. But the rate of this increase in low Reynolds numbers was more than that at high Reynolds numbers. Therefore, it can be concluded that the effect of increasing percentage of nanoparticle in low Reynolds number of this research is stronger than that of high Reynolds number. Moreover, friction factors were calculated and compared with blasious correlation. Finally, in order to find the optimum condition of this nanofluid for practical applications, thermal performance factor was defined to consider increasing Nusselt ratio besides increasing friction ratio simultaneously. The results show that the maximum thermal performance factor of this nanofluid was 1.266, which was calculated for 2% nanoparticle volume fraction at Reynolds number 3677.

Improving the rate of solar heat recovery from nanofluids by using micro heat exchanger

This work investigates the possibility of improving the solar heat recovery from copper–water nanofluids using a micro heat exchanger. This micro heat exchanger consisted of double brass tubes with a micro annular space. The effect of nanoparticles addition to the base fluid and its flow rate on the heat transfer characteristics and heating rate for both nanofluids in solar simulator section and water in shell side of micro heat exchanger were studied. The nanofluids was forced to flow in the tube side, while the fluid in the shell side was pure water. The effect of copper (Cu) weight fraction in nanofluids and the flow rate of nanofluids on the heat transfer coefficient (h), overall heat transfer coefficient (U), thermal resistance (R), and heating rate for both nanofluids in heating section and cold water in micro heat exchanger were studied. Four different concentrations of nanofluids in the range 0.01–0.1 wt.% had been used. The flow rate of nanofluids was changed in laminar region using a control valve. The temperature of heated nanofluids varied from 22 to 73°C. The heat transfer and overall heat transfer coefficients of the nanofluids 0.1 wt.% were found to increase by 49.9 and 42.08%, respectively, when it was compared with pure water. Also to be increased by increasing the nanofluid flow rate. Also, it was observed that the weight fraction has no significant effect on the final temperature of the nanofluid exit from heating section.

Experimental and numerical investigation on natural convection heat transfer in nanofluids

Abstract An experimental study has been undertaken concerning natural convection heat transfer of nanofluids over a cylindrical heater with a constant wall heat flux condition. The cylindrical heater having 7.1 mm O. D. and 0.5 mm thickness with 100 mm heated length was used as heater surface. The heat flux was varied from 0 to 50 000 W/m2 and the corresponding Rayleigh number range is varied from 3 × 104 to 1.65 × 105. Different Nanofluids were used i. e. Aluminium oxide, Titanium dioxide, Silicon dioxide and Copper oxide with concentration varying from 0.0005 % to 0.05 % by weight. Results show that there was a reduction in natural convection heat transfer coefficient of nanofluids as compared to water. Experimental results were compared with existing models for similar geometry. However, the available correlation was found to be unable to predict experimental data as it does not take into account the effect of particle concentration. A new empirical model was developed based on the experimental data including the effect of nanoparticles concentration which predicts the experimental data satisfactorily.

Investigation of heat transfer of nanofluids in turbulent flow in a cylindrical channel

Turbulent flow of nanofluids based on the distilled water with aluminum and silicon oxide particles of different sizes in a cylindrical channel is studied. The results of the measurements of the heat transfer coefficient and the pressure difference are presented. The maximum volume concentration of the particles was not greater than two percents. The dependence of the heat transfer coefficient on the nanoparticle concentration and their sizes and material is studied. It is shown that a considerable increase in the nanofluid heat transfer coefficient, compared with the corresponding value for water, may generally be expected. At the same time, the heat transfer coefficient of a nanofluid depends on the nanoparticle size and material; because of this, under certain conditions the nanofluid heat transfer coefficient can turn out to be lower than that of the baseline fluid. Situations, when this can occur, are established. It is for the first time experimentally shown that the nanofluid viscosity coefficient depends not only on the nanoparticle size but also on its material.