Flow and heat transfer characteristics in pulsating pipe flows (effects of pulsation on internal heat transfer in a circular pipe flow)

Effects of pulsation on flow and heat transfer characteristics are experimentally examined in the pulsating pipe flows having sinusoidal velocity fluctuations around a nonzero mean. By systematically varying three pulsation parameters (the amplitude, frequency, and mean velocity), time-averaged and fluctuating temperature profiles are measured under the heating condition of constant wall temperature using saturated vapor. The mean Nusselt number, Nu_p, is calculated, and compared with that in ordinary turbulent pipe flows without pulsation. The results show that Nu_p, decreases initially as the pulsation amplitude increases, then recovers gradually, and finally becomes much greater than the original value. In pulsating pipe flows with a nonzero mean velocity, therefore, pulsation cannot always promote heat transfer, but sometimes suppresses it, depending mainly on the pulsation amplitude and mean velocity. It is also found that these heat transfer characteristics of a pulsating pipe flow are controlled by the transition of flow patterns with pulsation amplitude from a fully turbulent flow to a conditionally turbulent flow via a transitional flow. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(5): 323–341, 1996     

Laminar pulsating flow of nanofluids in a circular tube with isothermal wall

In this study, two-dimensional pulsating flow of nanofluids through a pipe with isothermal walls is numerically investigated. In order to solve Navier–Stokes and energy equations, the finite volume approach with collocated grid is employed. The momentum interpolation technique of Rhie and Chow is applied in SIMPLE algorithm. Pulsating flows have potential for research as many aspects of such flows are still unclear and require further investigation because of many upcoming applications. In the past decade, nanofluids have attracted much interest because of their reported superior thermal performance and many potential applications. Pulsation in nanofluid is a new idea in case of fluid mechanics and heat transfer. The authors have not found any records similar to the present study. One of the advantages of using pulsation in nanofluid is a delay in nanoparticles sedimentation process. The simulation performed at with different pulse parameters (Amplitude, Strouhal and Reynolds numbers) and volume fractions of nanoparticles for unsteady flow. Increasing both the frequency and amplitude leads to a slight increase in Nusselt number but by increasing Reynolds and volume fraction, more rate of heat transfer is observed.     

Numerical investigation of pulsating flow around a discrete heater in a channel

In this study, the forced convection heat transfer around a discrete heater located in a channel subjected to laminar pulsating air flow is numerically investigated. Simulations are conducted for six different frequencies and three different amplitudes, while the Reynolds number (Re = 125) and Prandtl number (Pr = 0.71) remain constant for all cases. The impact of the important governing parameters such as the Womersley number (Wo) and the amplitude of flow pulsation (A_o) on heat transfer rate from discrete heaters is examined in detail. The instant velocity and temperature profiles are obtained to determine of the role of dimensionless parameters for pulsating flow. The numerical results show that thermal transport from the heater is greatly affected by the frequency and amplitude of the flow pulsation. The results given are dimensionless parameters.     

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.     

Numerical investigation of effective parameters in convective heat transfer of nanofluids flowing under a laminar flow regime

This article presents a numerical investigation on heat transfer performance and pressure drop of nanofluids flows through a straight circular pipe in a laminar flow regime and constant heat flux boundary condition. Al₂O₃, CuO, carbon nanotube (CNT) and titanate nanotube (TNT) nanoparticles dispersed in water and ethylene glycol/water with particle concentrations ranging between 0 and 6 vol.% were used as working fluids for simulating the heat transfer and flow behaviours of nanofluids. The proposed model has been validated with the available experimental data and correlations. The effects of particle concentrations, particle diameter, particles Brownian motions, Reynolds number, type of the nanoparticles and base fluid on the heat transfer coefficient and pressure drop of nanofluids were determined and discussed in details. The results indicated that the particle volume concentration, Brownian motion and aspect ratio of nanoparticles similar to flow Reynolds number increase the heat transfer coefficient, while the nanoparticle diameter has an opposite effect on the heat transfer coefficient. Finally, the present study provides some considerations for the appropriate choice of the nanofluids for practical applications.     

Heat transfer characteristics in double tube helical heat exchangers using nanofluids

In the present work a three-dimensional analysis is used to study the heat transfer characteristics of a double-tube helical heat exchangers using nanofluids under laminar flow conditions. CuO and TiO₂ nanoparticles with diameters of 24 nm dispersed in water with volume concentrations of 0.5–3 vol.% are used as the working fluid. The mass flow rate of the nanofluid from the inner tube was kept and the mass flow rate of the water from the annulus was set at either half, full, or double the value. The variations of the nanofluids and water temperatures, heat transfer rates and heat transfer coefficients along inner and outer tubes are shown in the paper. Effects of nanoparticles concentration level and of the Dean number on the heat transfer rates and heat transfer coefficients are presented. The results show that for 2% CuO nanoparticles in water and same mass flow rate in inner tube and annulus, the heat transfer rate of the nanofluid was approximately 14% greater than of pure water and the heat transfer rate of water from annulus than through the inner tube flowing nanofluids was approximately 19% greater than for the case which through the inner and outer tubes flow water. The results also show that the convective heat transfer coefficients of the nanofluids and water increased with increasing of the mass flow rate and with the Dean number. The results have been validated by comparison of simulations with the data computed by empirical equations.     

Numerical study of turbulent forced convection flow of nanofluids in a long horizontal duct considering variable properties

The turbulent flow of nanofluids with different volume concentrations of nanoparticles flowing through a two-dimensional duct under constant heat flux condition is analyzed numerically. The nanofluids considered are mixtures of copper oxide (CuO), alumina (Al₂O₃) and oxide titanium (TiO₂) nanoparticles and water as the base fluid. All the thermophysical properties of nanofluids are temperature-dependent. The viscosity of nanofluids is obtained on basis of experimental data. The predicted Nusselt numbers exhibit good agreement with Gnielinski's correlation. The results show that by increasing the volume concentration, the wall shear stress and heat transfer rates increase. For a constant volume concentration and Reynolds number, the effect of CuO nanoparticles to enhance the Nusselt number is better than Al₂O₃ and TiO₂ nanoparticles.     

Heat transfer in rectangular microchannels heat sink using nanofluids

The effect of using nanofluids on heat transfer and fluid flow characteristics in rectangular shaped microchannel heat sink (MCHS) is numerically investigated for Reynolds number range of 100–1000. In this study, the MCHS performance using alumina–water (Al₂O₃-H₂O) nanofluid with volume fraction ranged from 1% to 5% was used as a coolant is examined. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using finite volume method. The MCHS performance is evaluated in terms of temperature profile, heat transfer coefficient, pressure drop, friction factor, wall shear stress and thermal resistance. The results reveal that when the volume fraction of nanoparticles is increased under the extreme heat flux, both the heat transfer coefficient and wall shear stress are increased while the thermal resistance of the MCHS is decreased. However, nanofluid with volume fraction of 5% could not be able to enhance the heat transfer or performing almost the same result as pure water. Therefore, the presence of nanoparticles could enhance the cooling of MCHS under the extreme heat flux conditions with the optimum value of nanoparticles. Only a slight increase in the pressure drop across the MCHS is found compared with the pure water-cooled MCHS.     

Effect of Vibration on Forced Convection Heat Transfer for SiO2–Water Nanofluids

In the process of heat transfer, the fluid type and external parameters have a significant impact on heat transfer performance. For this reason, the physical properties, pressure differences, and heat transfer rates of SiO₂–water nanofluids have been experimentally investigated in a straight circular pipe. Experimental results revealed a great difference in physical properties between SiO₂–water nanofluids and purified water. The friction factor of low-volume-concentration nanofluids was slightly increased for laminar flow and tended to be almost independent of the Reynolds number for turbulent flow. The heat transfer coefficient can be enhanced either by adding nanoparticles to purified water or by imposing a transverse vibration on the heat transfer surface. Using these two methods at the same time (compound heat transfer enhancement), heat transfer performance is much better than that with either method alone. The largest increase of about 182% was observed under conditions of compound heat transfer enhancement.     

A dispersion model for predicting the heat transfer performance of TiO2–water nanofluids under a laminar flow regime

Nanofluids are a suspension of particles with ultrafine size in a conventional base fluid that increases the heat transfer performance of the original base fluid. They show higher thermal performance than base fluids especially in terms of the thermal conductivity and heat transfer coefficient. During the last decade, many studies have been carried out on the heat transfer and flow characteristics of nanofluids, both experimentally and theoretically. The purpose of this article is to propose a dispersion model for predicting the heat transfer coefficient of nanofluids under laminar flow conditions. TiO₂ nanoparticles dispersed in water with various volume fractions and flowing in a horizontal straight tube under constant wall heat flux were used. In addition, the predicted values were compared with the experimental data from He et al. [14]. In the present study, the results show that the proposed model can be used to predict the heat transfer behaviour of nanofluids with reasonable accuracy. Moreover, the results also indicate that the predicted values of the heat transfer coefficient obtained from the present model differ from those obtained by using the Li and Xuan equation by about 3.5% at a particle volume fraction of 2.0%.     

Numerical study of turbulent flow and heat transfer characteristics of nanofluids considering variable properties

Turbulent flow and heat transfer of three different nanofluids (CuO, Al₂O₃ and SiO₂) in an ethylene glycol and water mixture flowing through a circular tube under constant heat flux condition have been numerically analyzed. New correlations for viscosity up to 10% volume concentration for these nanofluids as a function of volume concentration and temperature are developed from the experiments and are summarized in the present paper. In our numerical study, all the thermophysical properties of nanofluids are temperature dependent. Computed results are validated with existing well established correlations. Nusselt number prediction for nanofluids agrees well with Gnielinski correlation. It is found that nanofluids containing smaller diameter nanoparticles have higher viscosity and Nusselt number. Comparison of convective heat transfer coefficient of CuO, Al₂O₃ and SiO₂ nanofluids have been presented. At a constant Reynolds number, Nusselt number increases by 35% for 6% CuO nanofluids over the base fluid.     

Analysis of heat transfer in simultaneously developing pulsating laminar flow in a pipe with constant wall temperature

Numerical studies of flow and heat transfer in a circular tube under pulsating flow condition were carried out in the laminar regime. The flow at the inlet consists of a fixed part and a pulsating component, which varies sinusoidally in time. The flow was both thermally and hydrodynamically developing while the tube wall was kept at a uniform temperature. The solution of two-dimensional Navier–Stokes equation was performed using the SIMPLE algorithm with the momentum interpolation technique of Rhie and Chow. By analysing the data generated from the simulation, it is observed that in the range of present study (frequency: 0–20 Hz; amplitude: < 1.0), pulsation has no effect on time-averaged heat transfer, although the Nu distribution varies in time in the near-entry region of the pipe.     

Experimental Study of the Heat Transfer Enhancement from a Circular Cylinder in Laminar Pulsating Cross-flows

Experiments were designed to investigate the heat transfer enhancement from a single heated circular cylinder in laminar pulsating cross-flows. Several parameters were explored, including Strouhal number between 0.18 and 2.80, Reynolds number between 205 and 822, and pressure amplitude p_rms between 40 Pa and 276 Pa. It was observed that the heat transfer enhancement factor increased up to 2.10, and an empirical correlation between the convection heat transfer coefficient and pulsating flows was developed. Results found that the heat transfer enhancement factor decreases with Strouhal number. Increasing Reynolds number was found to have a negative impact on the heat transfer enhancement factor when the pulsating frequency is relatively low, but with a higher pulsating frequency, an increasing Reynolds number increases the heat transfer enhancement factor. Larger pressure amplitude was found to continuously produce larger heat transfer enhancement factor when both Strouhal number and Reynolds number are kept constant.     

Numerical and Experimental Investigation of Heat Transfer of α-Al2O3/Water Nanofluid in Double Pipe and Shell and Tube Heat Exchangers

This research presents an experimental and numerical study on the heat transfer of α-Al₂O₃/water nanofluid flowing through the double pipe and shell and tube heat exchangers, under laminar flow conditions. Effects of important parameters such as hot and cold volume flow rates, nanofluid temperature, and nanoparticles concentration on the heat transfer characteristics are investigated. The results indicated that the heat transfer performance of both double pipe and shell and tube heat exchangers increases with increasing the hot and cold volume flow rates, as well as the particle concentrations and nanofluid inlet temperature. Compared with pure water, the results indicated that the heat transfer coefficients of nanofluid in the double pipe and shell and tube heat exchangers are higher than those of water by 13.2% and 21.3%, respectively. Also, the heat transfer performance of nanofluid in a shell and tube heat exchanger is 26.2% higher than the double pipe heat exchanger. A computational fluid dynamics (CFD) technique was used for heat transfer simulation in the previously mentioned heat exchangers. Computed overall heat transfer coefficients of the nanofluids are in good agreement with the experimental data.     

Thermal Enhancement of Triangular Ducts Using Compound of Vortex Generators and Nanofluids

Laminar flow and heat transfer of three different types of nanofluids; Al₂O₃, CuO, and SiO₂ suspended in ethylene glycol, in a triangular duct using delta-winglet pair of vortex generator are numerically simulated in three dimensions. The governing equations of mass, momentum and energy are solved using the finite volume method. The effects of types, concentrations, and diameter of solid nanoparticles and Reynolds number on thermal and hydraulic performance of triangular duct are examined. The range of Reynolds number, volume fraction and nanoparticles diameters is 100–1200, 1–4%, and 25–85 nm, respectively. The results indicate that the average Nusselt number increases with the particles volume fraction and Reynolds number associated with an increase in the pressure drop. The heat transfer enhancement and pressure drop penalty reduce with increasing the particles diameters. However, a reduction in the pumping power required is observed to force the nanofluids when the volume fraction increases, assuming the heat transfer coefficient remains constant.     

Two‐phase natural convection analysis and hybrid nanoparticle migration around micromixer blades

In this paper, numerical investigations are presented for hybrid nanoparticle migration and free convection heat transfer of two kinds of nanofluids in a micromixer at the fixed propeller condition. The inner blades and outer crust of the micromixer are kept at constant hot and cold temperatures, respectively. Two kinds of hybrid nanofluids, TiO₂‐CuO water and ethylene glycol‐(MoS₂‐SiO₂), are considered. The governing equations including velocity, pressure, temperature formulation, and nanoparticle concentrations are solved by a partial differential equation solver based on the Galerkin finite element method. The results are discussed based on the governing parameters, such as nanoparticle volume fraction, thermal and solutal Rayleigh numbers. The average Nusselt number was found to increase with the increasing nanoparticle volume fractions. Also, increasing the thermal Rayleigh number enhanced heat transfer while the solutal Rayleigh number has an insignificant effect on it. More importantly, increasing the thermal Rayleigh number assisted avoiding the agglomeration of nanoparticles around the blades and ensured more uniform nanoparticle distribution.     

Investigation of thermohydraulic performance of triple concentric-tube heat exchanger with CuO/water nanofluid: Numerical approach

In this study, numerical investigation of CuO/water nanofluids in a triple concentric-tube heat exchanger has been carried out using a commercial CFD software. The primary objective of this study is to conduct a heat transfer and pressure drop characteristics of water-based CuO nanofluids under turbulent flow regime. Reynolds number for the nanofluid has also been considered in the range of 2500 to 10,000 with a nanoparticle volume concentration of 0% to 3%. The effects of flow rate, volume concentration of nanoparticles, and flow arrangement on heat transfer performance of nanofluid have been studied for four flow arrangements. The comparison of the performance with and without nanofluid has been done. It was found that thermal performance and overall effectiveness increased with the increase in Reynolds number and volume concentration of nanoparticles in all the four flow arrangements for the considered range of operating parameters.     

Heat transfer enhancement with the use of nanofluids in radial flow cooling systems considering temperature-dependent properties

Heat transfer enhancement capabilities of coolants with suspended metallic nanoparticles inside typical radial flow cooling systems are numerically investigated in this paper. The laminar forced convection flow of these nanofluids between two coaxial and parallel disks with central axial injection has been considered using temperature dependent nanofluid properties. Results clearly indicate that considerable heat transfer benefits are possible with the use of these fluid/solid particle mixtures. For example, a Water/Al₂O₃ nanofluid with a volume fraction of nanoparticles as low as 4% can produce a 25% increase in the average wall heat transfer coefficient when compared to the base fluid alone (i.e., water). Furthermore, results show that considerable differences are found when using constant property nanofluids (temperature independent) versus nanofluids with temperature dependent properties. The use of temperature-dependent properties make for greater heat transfer predictions with corresponding decreases in wall shear stresses when compared to predictions using constant properties. With an increase in wall heat flux, it was found that the average heat transfer coefficient increases whilst the wall shear stress decreases for cases using temperature-dependent nanofluid properties.     

Experimental investigation of free single jet impingement using Al2O3-water nanofluid

Four volume fractions Al₂O₃-water nanofluids (0.5%, 1%, 1.5% and 2%) are introduced into free single jet impingement experiment as working fluids. The Reynolds numbers, impact angles and nozzle-to-plate distances (H/D) are variable for investigating the heat transfer performance. As to get observation of flow characteristics in nanofluid, heat transferring performance would be studied in this case. Experimental results show that there is a relationship between convective heat transferring coefficient and nanoparticles suspendability within base fluid. Convective heat transfer coefficient is proportional to the extent of nanoparticles concentration, Reynolds number while it decreases with the increasing angle of impacting. In addition, considering the influence of the suspended nanoparticles and the condition of impinging jet, a heat transfer correlation has been proposed combining the influence of the suspended nanoparticles and the condition of impinging jet.     

An experimental study on the heat transfer performance and pressure drop of TiO2-water nanofluids flowing under a turbulent flow regime

Nanofluid is a new class of heat transfer fluids engineered by dispersing metallic or non-metallic nanoparticles with a typical size of less than 100 nm in the conventional heat transfer fluids. Their use remarkably augments the heat transfer potential of the base liquids. This article presents the heat transfer coefficient and friction factor of the TiO₂-water nanofluids flowing in a horizontal double tube counter-flow heat exchanger under turbulent flow conditions, experimentally. TiO₂ nanoparticles with diameters of 21 nm dispersed in water with volume concentrations of 0.2–2 vol.% are used as the test fluid. The results show that the heat transfer coefficient of nanofluid is higher than that of the base liquid and increased with increasing the Reynolds number and particle concentrations. The heat transfer coefficient of nanofluids was approximately 26% greater than that of pure vol.%, and the results also show that the heat transfer coefficient of the nanofluids at a volume concentration of 2.0 vol.% was approximately 14% lower than that of base fluids for given conditions. For the pressure drop, the results show that the pressure drop of nanofluids was slightly higher than the base fluid and increases with increasing the volume concentrations. Finally, the new correlations were proposed for predicting the Nusselt number and friction factor of the nanofluids, especially.