Experimental and numerical study of forced convection heat transfer in different internally ribbed tubes configuration using TiO2 nanofluid

TiO₂/water nanofluid is used together with a ribbed tube for heat transfer augmentation. This paper presents an experimental and numerical investigation to study the influence of the ribs' pitch distance and ribbed tube configuration on heat transfer using TiO ₂ nanofluid in a turbulent regime with Reynolds numbers of 5000‐40 000. Meanwhile, the fluid properties are assumed to be constant with temperature under uniform heat flux. The average nanoparticle size is 50 nm and volume fractions of 0% to 1% are adopted. The study is accomplished by using the finite volume method, and its objective involves finding a low friction factor and high heat transfer enhancement in the presence of TiO ₂/water nanofluids. In comparison with the plain tube, a helical ribbed tube provides higher performance evaluation criteria (about 2.0%), while circumferentially ribbed tube provides 1.9% and longitudinal ribbed tube provides 1.88%. The helical ribbed tubes with a 5.89 mm pitch distance gave higher turbulent kinetic energy due to a stronger swirl intensity, resulting in a thinner thermal boundary layer and a higher Nusselt number with uniform distribution. The nonlinear models of friction factor and Nusselt number have been predicted with a maximum deviation of ±3% and ±2%, respectively.     

Experimental Study of TiO2-Water Nanofluid Flow in Corrugated Tubes Mounted with Semi-Circular Wing Tapes

Heat transfer of corrugated tubes mounted with semi-circular wing tapes has been experimentally investigated. TiO₂-water nanofluids were employed as working fluids. Experiments were conducted in the following ranges: (i) TiO₂-water nanofluids with three TiO₂ concentrations (0.05 to 0.15% by volume), (ii) semi-circular wing tapes with three wing angles (45 to 75°), and (iii) two wing arrangements, and (4) Reynolds numbers between 8000 and 15,000. The experiment using water as the working fluid in a corrugated tube without tape was also performed, for comparison. Evidently, the heat transfer enhancement increases with increasing wing angle. Moreover, the corrugated tubes combined with semi-circular wing tapes in parallel arrangement show better thermal performance than the ones in counter arrangement. The highest thermal enhancement factor of 1.98 is obtained by the use of the combined devices in parallel pattern at wing angle of 75°, concentration of TiO₂-water nanofluid of 0.15% by volume and Reynolds number of 8000.     

WINGLET-TYPE VORTEX GENERATORS FOR PLATE-FIN HEAT EXCHANGERS USING TRIANGULAR FINS

Vortex generators in the form of delta winglet pairs have already been proposed by many researchers for enhancement of the heat transfer rate in plate-fin heat exchangers. In this work, the enhancement potential of triangular fins (which are widely used inserts between the plates of the plate-fin heat exchanger) having delta winglets mounted on their slant surfaces has been computed. The performance of this combination is evaluated for varying angles of attack of the winglet and different thermal boundary conditions. The performance of the combination of triangular fins and winglets with stamping on the slant surfaces also has been evaluated.     

Effect of thermophysical properties models on the predicting of the convective heat transfer coefficient for low concentration nanofluid

The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail.     

Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape insert

Experiments to evaluate heat transfer coefficient and friction factor for flow in a tube and with twisted tape inserts in the transition range of flow with Al₂O₃ nanofluid are conducted. The results showed considerable enhancement of convective heat transfer with Al₂O₃ nanofluids compared to flow with water. It is observed that the equation of Gleninski applicable in transitional flow range for single-phase fluids showed considerable deviation when compared with values obtained with nanofluid. The heat transfer coefficient of nanofluid flowing in a tube with 0.1% volume concentration is 23.7% higher when compared with water at number of 9000. Heat transfer coefficient and pressure drop with nanofluid has been experimentally determined with tapes of different twist ratios and found to deviate with values obtained from equations developed for single-phase flow. A regression equation is developed to estimate the Nusselt number valid for both water and nanofluid flowing in the transition flow Reynolds number range in circular plain tube and with tape inserts. The maximum friction factor with twisted tape at 0.1% nanofluid volume concentration is 1.21 times that of water flowing in a plain tube.     

Impact of volume fraction and fluid layer thickness on heat transfer effectiveness of water-based nanofluids

The heat transfer effectiveness of nanofluids is adversely affected by the delay in convection onset. The lesser effectiveness, when compared to that of base fluid, is observed in a range of nanofluid layer thickness. The heat transfer coefficient of water–Al₂O₃ nanofluid can be enhanced by sustaining the equilibrium between Rayleigh number, temperature, particle volume fraction, and enclosure aspect ratio. In this paper, the specific correlation of fluid layer thickness and the onset of convection, which can significantly dominate the heat transfer characteristics of nanofluids are investigated using the concept of critical Rayleigh number. The water layer thickness for convection onset is first experimentally assessed for different real-life heat flux densities. It is then performed for Al₂O₃–water nanofluid for varying volume fractions. With the increase in volume fraction even though thermal conductivity increases, the overall heat transfer enhancement of the nanofluid is reduced. Temperature involved (heat flux density), the volume fraction of the nanofluid used, nanofluid layer thickness (space availability for the cooling system), and mass of the nanoparticle influence heat transfer enhancement. A higher volume fraction may not always result in enhancement of heat transfer as far as nanofluids are concerned.     

Experimental investigation on frequency pulsation effects on a single pass plate heat exchanger performance

This paper investigates combined heat transfer improvement methods. These methods include introducing pulsating flow, adding nanofluids, and manipulating the flow's characteristics in a corrugated plate heat exchanger. Tests are carried out with multi-walled carbon nanotube (MWCNT), graphene nanoplate (GNP), and a mixture of GNP and MWCNT meeting the requirement of 0.01% nanofluids volume fraction and exposed to pulsation. Results demonstrated that the use of pulsating frequencies from 0 to 30 Hz of GNP-water, MIX nanofluids–water, and MWCNT–water nanofluids with a constant concentration of 0.01 wt% leads to a significant improvement in heat transfer. Using pure water at frequency f = 0 Hz as a benchmark, the Nusselt number of the mixture nanofluid increases by 15.2%, 27.5%, 40.4%, and 52.8% with the increase of frequency pulsation from 0 to 30 Hz with a slight effect on the pressure-drop at this low used constant nanofluid concentration = 0.01%. The highest Nusselt number value for GNP-water nanofluid improved by an amount of 58.3% at the highest frequency compared with pure water at f = 0 Hz.     

Thermal performance enhancement studies using graphite nanofluid for heat transfer applications

Enhanced boiling heat transfer using nanofluids is highly relevant due to its potential applications in thermal management of systems producing large heat fluxes. However, the sedimentation of nanoparticles limits their application in heat transfer systems. So, the preparation of a stable nanofluid remains a big research challenge. The stability issues arise due to the large difference in the density of nanoparticle and the base fluid. Graphite nanoparticle is selected in this study, as it has 4.5 times lower density than copper and comparable thermal conductivity. An experimental study is conducted to evaluate the suitability of graphite nanofluid in mesh wick heat pipes, which are devices that utilize boiling and condensation principles to transfer high heat fluxes. Thermal transport properties and boiling heat transfer characteristics showed enhancement and the effect of nanofluid on the device level thermal performance is thoroughly assessed. Experimental results are compared with the published literature. A reduction in thermal resistance by 32.5% and an improvement in the heat transfer coefficient by 48.02% in comparison with base fluid clearly indicate the superiority of the graphite nanofluid for heat transfer applications.     

CFD analysis of a flat tube with semi-circular fins using graphene nanofluid and to compare performance with square type of fins

CFD analysis on a flat tube with semi-circular fins under laminar flow conditions was performed with graphene-based nanofluids considering the nanofluids as incompressible. Different simulations were performed at four different concentrations of nanofluids (0.01%, 0.1%, 0.2%, and 0.4%) and at different volume flow rates (4, 6, 8, and 10 LPM) and at four different forced convective heat transfer coefficients at different wind velocities at 300 K (50, 100, 150, and 200 W/m² K). It was observed that with an increase in the concentration of nanoparticles in nanofluids, the thermal conductivity of base fluid water was increased (at 353 K the nanofluid of 0.4% volume concentration, the thermal conductivity of nanofluid increased by 200% with respect to base fluid). Graphene-based nanofluids have higher effectiveness than most nanofluids hence it is considered for the analysis, at 0.4% concentration of nanofluid the effectiveness observed was 36.84% at 4 LPM, and for water, the effectiveness was 28.22% under similar conditions. The effect of flow rate on temperature drop was significant. At 4 LPM and at 0.4% of nanofluid, an outlet coolant temperature of 333 K was observed whereas the water outlet temperature at 10 LPM is 346.13 K. The effect of forced convective air heat transfer coefficient was significantly high. At h = 50 W/m² K the outlet temperature of 0.4% nanofluid at 4 LPM was 345.25 K and at h = 200 W/m² K, the outlet coolant temperature was 333.47 K. A single tube of the radiator was considered for the analysis whereas the original radiator consists of 50 tubes due to problems of Ansys in meshing.     

Experimental study of exergetic performance of Al2O3 nanofluid in a double‐pipe heat exchanger fitted by a new modified twisted tape

The use of nanofluids and surface enhancers today are among the new technologies used to increase heat transfer. In this study, heat transfer phenomena in heat exchanger were investigated using Al₂O₃ nanoparticles and modified spiral band as flow turbulator. Results are verified with well‐known correlations. The results show that the tube with cross‐hollow twisted tape inserts has the best exergetic performance for different hollow widths of the tape. Clearance, which is defined as the width between the tube and twisted tape, also affects the heat transfer performance. The smaller the clearance, the better is the exergetic performance. The tube can achieve the best exergetic performance when the number of unilateral twisted tapes is four. The results showed that increasing nanofluid concentration improves exergetic performance.     

纳米流体强化传热研究分析

文中综述了目前国内外对于纳米流体强化传热技术的研究情况,分析了纳米流体的强化传热机理及添加纳米粒子后对液体的物性参数--粘度、比热、密度、流体流动的影响;说明了石墨/水纳米流体及Fe3O4/水纳米流体导热系数和对流换热系数测量实验的原理及结果,并对结果进行了分析,实验结果表明纳米流体强化了传热.     

Advances of nanofluids in heat exchangers—A review

Recently, many researchers have focused on their studies on the analysis of nanofluid flows due to their participation in the enhancement of heat transfer rates in industrial processes. The ordinary fluids, such as water, mineral oils, and so on, are known for their low thermal conductivity in heat transfer processes. A significant enhancement in the thermal properties of ordinary fluid may be obtained by adding nanoparticles having a diameter of less than 100 nm or suspension of fibers. Better spreading, wetting, dispersion, and stability and with acceptable viscosity are the main advantageous properties of nanofluids on a solid surface. The nanofluids are encountered in various thermal engineering systems such as in heat exchangers, refrigeration, thermal management of fuel cells, cooling of nuclear reactors, microelectromechanical systems, and others. In particular, the thermal conversion is known as a great application of nanotechnology, and many studies have been achieved with such fluids in heat exchangers. Therefore, this paper aims to present a global insight into the different applications of nanofluids in various heat exchangers, that is, heat pipe and plate-fin heat exchangers. All research works have been summarized into three main parts: laminar, transition, and turbulent nanofluid flow regimes.     

Indirect thermosiphon flat-plate solar collector performance based on twisted tube design heat exchanger filled with nanofluid

Stationary solar collector such as flat-plate collector is a thermal device, which traps solar energy and converts it into heat that can be used in industrial and domestic applications such as water heating. Flat-plate collector thermal performance enhancement is investigated in this research paper. Two cross-sectional geometries of the tube in the heat exchanger were investigated; a normal circular tube and a twisted tube were used in the experiment. The aim of the twisted tube exchanger is to enhance the performance of heat transfer of the tubes and to reduce the shell pressure drop; flat-plate solar collector is the used application to study the heat exchanger performance. Both twisted tubes heat exchanger and normal circular tubes heat exchanger were examined in the same location and conditions with the same solar collector, both were used in the heat exchanger to study their effect, with two different working fluids, which are distilled water and multiwalled carbon nanotube (MWCNT)/water nanofluid. The system shows an increase in the performance when twisted tubes were used in the system compared with the circular tubes in both distilled water and MWCNT/water nanofluid by 12.8% and 12.5%, respectively, with an improvement by 34% for twisted tubes with MWCNT compared with normal circular tubes with distilled water.     

Experimental and analytical investigation on an automobile radiator with CuO/EG‐water based nanofluid as coolant

In the present study, experimental and analytical thermal performance of automobile radiator using nanofluids is investigated and compared with performance obtained with conventional coolants. Effect of operating parameters and nanoparticle concentration on heat transfer rate are studied for water as well as CuO/EG‐water based nanofluid analytically. The results are presented in the form of graphs showing variations of net heat transfer rate for various coolant flow rate, air velocity, and source temperature for various CuO/EG‐water based nanofluids. Experimental results indicate that with the increase in coolant flow rate and air velocity, heat transfer rate increases, reaches maximum and then decreases. Experimental investigation of a radiator is carried out using CuO/EG‐water based nanofluids. Results obtained by experimental work and analytical MATLAB code are almost the same. Maximum absolute error in water and air side is within 12% for all flow condition and coolant fluids. Nusselt number of nanofluid is calculated using equation number 33[9]. The results obtained from experimental work using 0.2% volume CuO/EG‐water based nanofluids are compared with the results obtained from MATLAB code. The results show that the maximum error in the outlet temperature of the coolant and air is 12% in each case. Thus MATLAB code can be used for different concentration of nanofluids to study the effect of operating parameters on heat transfer rate. Thus MATLAB code developed is valid for given heat exchanger applications. From the results obtained by already validated MATLAB code, it is concluded that increase in coolant flow rate, air velocity, and source temperature increases the heat transfer rate. Addition of nanoparticles in the base fluid increases the heat transfer rate for all kind of base fluids. Among all the nanofluid analyzed in this study, water‐based nanofluid gives highest value of heat transfer rate and is recommended for the heat exchanger applications under normal operating conditions. Maximum enhancement is observed for ethylene glycol‐water (4:6) mixture for 1% volume concentration of CuO is almost equal to 20%. As heat transfer rate increases with the use of nanofluids, the heat transfer area of the radiator can be minimized.     

A comparison of experimental heat transfer characteristics for Al2O3/water and CuO/water nanofluids in square cross-section duct

The present paper is a comparison between heat transfer characteristics of Al₂O₃/water and CuO/water nanofluids through a square cross-section cupric duct in laminar flow under uniform heat flux. Sometimes because of pressure drop limitations the need for noncircular ducts arises in many heat transfer applications, and a testing facility has been constructed for this purpose and experimental studies were performed on both nanofluids under different nanoparticles concentrations in distilled water as a base fluid. The results indicate that a considerable heat transfer enhancement has been achieved by both nanofluids compared with base fluid. However, CuO/water nanofluid shows better heat transfer augmentation compared with Al₂O₃/water nanofluid through square cross-section duct.     

Unsteady squeezing flow of magnetic hybrid nanofluids within parallel plates and entropy generation

Nowadays, due to the novel thermal effectiveness, a new class of fluid, named “hybrid nanofluid,” is used. It has significant applications in domestic and industrial fields. In this study, we investigated the entropy generation and heat transfer of unsteady squeezing magnetic hybrid nanofluid flow between parallel plates by considering heat source/sink and thermal radiation. In this analysis, carbon nanotubes (CNTs) (single‐walled carbon nanotube and multiwalled carbon nanotube) are considered as nanoparticles that are dispersed in water‐ethylene glycol (EG) mixtures (ie, 70%W + 30%EG and 50%W + 50%EG). For the analysis of the physical behavior of hybrid nanofluids, new models related to hybrid nanofluids are incorporated. From this study, it has been observed that as the hybrid nanofluids moved away from the surface, the entropy generation outlines accelerated with an increase in magnetic field values. Moreover, an increase in the volume fraction of CNTs, the thermal conductivity of 50%W + 50%EG + CNTs hybrid nanofluid is greater than 70%W + 30%EG + CNTs hybrid nanofluid.     

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

Three 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.     

Experimental investigation of pool boiling characteristics of low-concentrated CuO/ethylene glycol–water nanofluids

Boiling heat transfer performance of nanofluid has been studied during the past few years. Some controversial results are reported in literature about the potential impact of nanofluids on heat transfer intensification. Whereas the mixtures of ethylene glycol and water are considered the most common water-based antifreeze solutions used in automotive cooling systems, the present study is an experimental investigation of boiling heat transfer of CuO/ethylene glycol–water (60/40) nanofluids. The results indicate that a considerable boiling heat transfer enhancement has been achieved by nanofluid and the enhancement increases with nanoparticles concentration and reaches 55% at a nanoparticles loading of 0.5%.     

Numerical Analysis of Heat Transfer and Nanofluid Flow in a Triangular Duct with Vortex Generator: Two‐Phase Model

Laminar forced convection heat transfer and nanofluids flow in an equilateral triangular channel using a delta‐winglet pair of vortex generators is numerically studied. Three nanofluids, namely; Al₂O₃, CuO, and SiO₂ nanoparticles suspended in an ethylene glycol base fluid are examined. A two‐phase mixture model is considered to simulate the governing equations of mass, momentum and energy for both phases and solved using the finite volume method (FVM). Constant and temperature dependent properties methods are assumed. The single‐phase model is considered here for comparison. The nanoparticle concentration is assumed to be 1% and 4% and Reynolds number is ranged from 100 to 800. The results show that the heat transfer enhancement by a using vortex generator and nanofluids is greater than the case of vortex generator and base fluid only, and the latest case provided higher enhancement of heat transfer compared to the case of a base fluid flowing in a plain duct. Considering the nanofluid as two separated phases is more reasonable than assuming the nanofluid as a homogeneous single phase. Temperature dependent properties model provided higher heat transfer and lower shear stress than the constant properties model.     

Experimental Study on Laminar Forced Convection of Al2O3/Water and Multiwall Carbon Nanotubes/Water Nanofluid of Varied Particle Concentration with Helical Twisted Tape Inserts in Pipe Flow

In this study an experimental investigation has been carried out to analyze the laminar forced convection of Al₂O₃/water and multiwall carbon nanotubes (MWCNT)/water nanofluids through uniformly heated horizontal circular pipe with helical twisted tape inserts. Tests were conducted for varied range of nanoparticle volume concentration (0.15%, 0.45%, 0.60%, and 1%) and helical tape inserts of twist ratios of 1.5, 2.5, and 3. The heat transfer enhancement and the increase of friction factor of nanofluids with helical inserts are compared with that of pure water results with plain tube without inserts. The Nusselt number is found to increase with the increase in Peclet number and nanofluid concentration. The MWCNT/water nanofluids with helical screw tape inserts exhibits higher thermal performance compared to Al₂O₃/water nanofluid. The maximum thermal performance factor was found to be 1.79 and 1.99 for Al₂O₃/water and MWCNT/water nanofluids with helical twisted tape inserts, respectively. The pressure drop for Al₂O₃ nanofluid is found to be higher compared to the MWCNT nanofluid for all the twist ratio of helical screw tape inserts.