Heat Transfer In Circular Tube Research Articles

Post-dryout heat transfer in circular tubes using R-134a: experiment and correlation assessment

Post-dryout heat transfer in circular tubes using R-134a: experiment and correlation assessment

Analysis of the Influence of Convection Heat Transfer in Circular Tubes on Ships in a Polar Environment

Electric heat tracing is the main measure for cold protection of the polar transfer coefficient in marine engineering equipment, but thermal equilibrium is the key problem this technology faces. In this paper, the circular tube was the research object. We studied the influence of convective heat transfer by Fluent software and experiments with a wind speed of 0–40 m/s and temperature of −40–0 °C by constant heat flux heating. The results show that the convective heat transfer increases with increased wind speed and decreased temperature. When the temperature is below −30 °C, the effect of temperature is increased; when the wind speed is greater than 25 m/s and the temperature is lower than −20 °C, the effect of temperature on the convective heat transfer coefficient of the circular tube increases. Based on the simulation data, we established a prediction model, and the rationality of the prediction model was verified by tests. The model provides reference for the design of electric heat tracing of circular tubes on polar ships.

Effects of the configuration of winglet vortex generators on turbulent heat transfer enhancement in circular tubes

Rectangular winglet vortex generators (RWVGs) are put forward to intensify turbulent heat transfer in circular tubes. Two configurations of RWVGs, including parallel type (P-RWVGs) and V-shaped type (V-RWVGs), are considered. Numerical results demonstrate that P-RWVGs generate a single longitudinal vortex, while V-RWVGs induce multiple longitudinal vortices. Both the flow patterns above increase the turbulent kinetic energy of the fluid and thus enhance fluid mixing in the tube. The influences of the attack angle (α) and length ratio (LR) of RWVGs on the thermal-hydraulic performance are examined experimentally. The results indicate that an increase in the length ratio of RWVGs contributes to the augmented pressure drop and heat transfer for both P-RWVGs and V-RWVGs. The heat transfer rate and flow resistance for the tube with P-RWVGs are increased respectively by 54–118% and 152–568 % over those for the smooth tube, and the corresponding values for V-RWVGs are 60–118% and 141–644%, respectively. The overall thermal performance of the tubes with P-RWVGs and V-RWVGs rises with the pressure drop. Overall, P-RWVGs with the combination of LR0.2-α75° and V-RWVGs with the combination of LR0.2-α45° show the higher thermal performance under the same pressure drop than the other cases.

Experimental correlations for oscillatory-flow friction and heat transfer in circular tubes with tri-orifice baffles

Experimental results of isothermal pressure drop and convective heat transfer coefficient are presented for a smooth tube with tri-orifice baffle inserts, under net and oscillatory flow conditions. Using propylene-glycol as working fluid, net Reynolds numbers in the range Ren=10−600 are reproduced, allowing to describe Fanning friction factor in the laminar, transitional and turbulent regimes, with an onset of transition at Ren≈100 in steady-state conditions. Oscillatory Fanning friction factor for 20<Reosc<200 is also reported, based on the maximum oscillatory pressure drop obtained by statistical fitting of the signal. Nusselt number under uniform heat flux conditions is obtained for superimposed net and oscillatory flows in the ranges 10<Ren<600 and 10<Reosc<440, with Prandtl number in the range 190<Pr<470. The existence of buoyancy effects in steady-state conditions and its vanishing with oscillations is analyzed. The oscillatory flow promotes a 4-fold increase of heat transfer for Ren<20. For Ren>100, the effect of the flow oscillation on heat transfer enhancement is negligible.

Comparative Study by Numerical Investigation of Heat Transfer in Circular Tube by using Hybrid Nanofluids

Comparative Study by Numerical Investigation of Heat Transfer in Circular Tube by using Hybrid Nanofluids

Effect of tube material on convective heat transfer of various nanofluids

This work presents the convective heat transfer and friction loss characteristics of novel functionalized graphene-based and metal oxide nanofluids. The convective heat transfer in circular tubes of different materials (copper, aluminium and stainless steel 316) was used at constant wall heat flux of 23,870 W m−2. An innovative approach was used to prepare highly dispersed propylene glycol-treated graphene nanoplatelets–water (GNP1) and trimethylolpropane tris amine–water (GNP2) by functionalization method. The measured thermal conductivity of GNP1 and GNP2 nanofluids showed incredible performance which increased up to 32% and 31% higher than that of basefluid. By comparing material effect, copper tube showed the highest HTC up to 119% in GNP1 at 0.1 mass%, while in aluminium and stainless steel 316 tube the highest heat transfer coefficient (HTC) was 110.2% and 100.68%. Besides, alumina and silicon dioxide nanofluids also presented decent increment in HTC which was up to 29.1% and 31.6%, respectively. The highest rise in friction factor for GNP1 and GNP2 was obtained up to 10.2% and 10%, respectively. For alumina and silicon dioxide nanofluids, the friction factor was measured up to 5.92% and 7.14% at velocity range of 1–3 m s−1. The maximum enhancement in Nusselt number (Nu) for GNP, GNP2, alumina and silicon dioxide nanofluids was achieved up to 84%, 72%, 26% and 28%. The results suggest that the copper tube which is a good conductor of heat could be used in the heat exchangers and functionalized GNP nanofluids can be used as the heat exchanging fluids in heat transfer applications which could give a decent substitute to traditional working fluids in heat exchangers and in thermal fluid systems.

A new method on fluid-to-fluid scaling for heat transfer in tubes at supercritical pressures

Supercritical fluid heat transfer has great significance for the thermal-hydraulic design and safety performance of the SCWR (Super-Critical Water-cooled Reactor). Experimental studies on heat transfer at prototypical conditions of SCWRs would suffer a huge cost on safety and economy, which strongly limits the research of supercritical water heat transfer. One solution is to adopt model fluid technique which has been applied in thermal and fluids engineering widely. Therefore, finding the equivalent relationship between the experimental data in model fluid and those in prototype fluid becomes a key issue. It means a fluid-to-fluid scaling method is necessary for converting experimental data obtained by using model fluid into the prototype fluid. In the present study, from the perspective of thermo-physical properties similarity and conservation equations similarity, considering the comprehensive influence of the experimental conditions and fluid types, a theoretical study on fluid-to-fluid scaling is performed and most importantly, a new scaling method is proposed. The new scaling method adopts a modification on thermo-physical properties. Several dimensionless numbers are obtained according to the dimensionless momentum and energy governing equations. A new dimensionless number En is derived to present the comprehensive influence of experimental conditions and fluid types. According to the theoretical analysis, an entire set of scaling laws applied for supercritical fluid heat transfer in circular tubes was obtained. Based on the heat transfer data of supercritical R134a, the new scaling method was validated, indicating the feasibility and accuracy of the proposed new scaling method.

Investigation of in-tube cooling of carbon dioxide at supercritical pressure by means of direct numerical simulation

To understand the cooling heat transfer behavior of carbon dioxide at supercritical pressure, direct numerical simulations of the flow and heat transfer in circular tubes have been performed at a pressure of 8MPa, an inlet temperature of 342.05K and a moderate inlet Reynolds number of 5400. The tube diameter was 2mm, which is in the range of the hydraulic diameter of a compact heat exchanger. Both forced (gravity neglected) and mixed convection with upward or downward direction of the flow were simulated while the pipe is in vertical orientation. As result of thermal contraction, flow deceleration was observed primarily in the vicinity of the wall, which is opposite to the acceleration observed with wall heating. It is found that combined effects of deceleration and buoyancy in the upward flow enhance the heat transfer while the heat transfer in the downward flow is deteriorated. Further investigations have educed that in downward flow, when the direction of buoyancy force and flow are the same, all turbulent quantities diminish significantly in the axial direction, which is the reason for heat transfer deterioration. Quadrant and octant analyses are presented here to understand the effects of sweep and ejection events on turbulence. Finally, the anisotropy of the Reynolds stress tensor indicates that turbulence is modulated especially in the near-wall region in both upward and downward flow.

Study on two wall temperature peaks of supercritical fluid mixed convective heat transfer in circular tubes

Heat transfer deterioration is numerically studied for supercritical fluids flowing upward in circular tubes at high heat fluxes and low mass fluxes. The simulations are conducted with Shear Stress Transport (SST) k-ω turbulent model in commercial software Fluent 15.0. Both water and CO2 are simulated and the results are consistent well with the experiments. It is found that there are two peaks of wall temperature when the heat transfer deterioration occurs, the first peak is narrow and sharp while the second peak is lower and broader. The mechanism of two wall temperature peaks is analyzed in details based on radial distributions of velocity and turbulent kinetic energy at different axial positions. It is found that the mechanism of the first peak is quite different from the second peak. The first peak is caused by buoyancy effect, which flattens the velocity distribution in the near wall region and leads to the reduction of turbulent kinetic energy and the impairment of heat transfer in the near wall region. For the second peak, it is the shear stress that flattens the velocity distribution in the main flow region and leads to the reduction of turbulent kinetic energy, which again impairs the heat transfer and causes the second peak.

An experimental study of forced convection effectiveness of Al2O3-water nanofluid flowing in circular tubes

In the present study, experimental efforts have been performed to explore the forced convection heat transfer using water-based suspension of Al2O3 nanoparticles (nanofluid) to replace the pure water as the working fluids in circular tubes. The nanofluid was prepared as a functional forced convection fluid and the thermal properties including the density, thermal conductivity, and dynamic viscosity were investigated experimentally. Besides, forced convection heat transfer in circular tubes was investigated with water-based nanofluid containing various mass fractions of the Al2O3 nanoparticles (2, 5, and 10wt%) under the following operating conditions: the volume flow rate Qf=23.6–183.5cm3/min (the Reynolds number Ref,0=188–2095), the heating power applied at the outer wall of the tube qo,eff.″=1908–7362W/m2, and the inlet fluid temperature Tin=24.5–25.5°C or 49.5–50.5°C. Measured data showed that the dispersion of increasing mass fraction of Al2O3 nanoparticles can effectively improve the thermal conductivity relative to the pure water. Besides, higher average heat transfer effectiveness ε−h,btd and figure of merit FOM are noted for the cases with higher inlet fluid temperature Tin.

Computational Fluid Dynamics Investigation of Supercritical Water Flow and Heat Transfer in a Rod Bundle With Grid Spacers

This paper presents a steady-state computational fluid dynamics approach to supercritical water flow and heat transfer in a rod bundle with grid spacers. The current model was developed using the ANSYS Workbench 15.0 software (CFX solver) and was first applied to supercritical water flow and heat transfer in circular tubes. The predicted wall temperature was in good agreement with the measured data. Next, a similar approach was used to investigate three-dimensional (3D) vertical upward flow of water at supercritical pressure of about 25 MPa in a rod bundle with grid spacers. This work aimed at understanding thermo- and hydrodynamic behavior of fluid flow in a complex geometry at specified boundary conditions. The modeled geometry consisted of a 1.5-m heated section in the rod bundle, a 0.2-m nonheated inlet section, and five grid spacers. The computational mesh was prepared using two cell types. The sections of the rods with spacers were meshed using tetrahedral cells due to the complex geometry of the spacer, whereas sections without spacers were meshed with hexahedral cells resulting in a total of 28 million cells. Three different sets of experimental conditions were investigated in this study: a nonheated case and two heated cases. The nonheated case, A1, is calculated to extract the pressure drop across the rod bundle. For cases B1 and B2, a heat flux is applied on the surface of the rods causing a rise in fluid temperature along the bundle. While the temperature of the fluid increases along with the flow, heat deterioration effects can be present near the heated surface. Outputs from both B cases are temperatures at preselected locations on the rods surfaces.

Laminar flow in circular tube with internal solidification of a binary mixture

Laminar flow heat transfer in circular tube with internal solidification of a binary mixture of paraffin is investigated numerically. The problem of the solidification process is formulated using the enthalpy–porosity based method. The influence of various parameters such as: wall temperature, inlet maximal velocity, initial temperature and initial concentration of the binary mixture, on the solidification process is analyzed in detail. It is found that the initial temperature has less important effect on the solidification progress than the wall pipe temperature, the initial concentration and the inlet velocity of the binary mixture.

EFFECT OF TWISTED RATIO ON FLOW STRUCTURE, HEAT TRANSFER AND THERMAL IMPROVEMENT IN A CIRCULAR TUBE WITH SINGLE TWISTED TAPE

The research presents a numerical analysis of laminar fully developed periodic flow and heat transfer in a constant temperature-surfaced circular tube with single twisted tape inserted. The twisted tape is introduced and inserted in the middle of the tested tube. The effects of twisted ratios (y/W = 1, 2, 3, 4, 5 and 6) are presented for Reynolds number (Re) values ranging from Re = 100 to 2000. The SIMPLE algorithm and periodic condition are used in the current study. The computational results are shown in the topology of flow, heat transfer and thermal improvement. It is found that the heat transfer in the circular tube with the twisted tape is more effective than that with no twisted tape inserted. The increase in the y/W ratio leads to decrease in the Nusselt number and friction factor. In addition, the numerical result shows that the reduction in the y/W ratio not only helps to create strong longitudinal vortex flows which enhance heat transfer, but also increase the pressure loss in the tested tube. The computed result reveals that the maximum value of the thermal enhancement factor, TEF is found to be 3.52 for using the twist tapes with y/W = 5 at the highest Reynolds number regime.

Computational Study of Turbulent Heat Transfer for Heating of Water in a Vertical Circular Tube

The steady-state turbulent heat transfer coefficients in a vertical circular Platinum (Pt) test tube for the flow velocities (u=4.22 to 21.45 m/s), the inlet liquid temperatures (Tin=308.20 to 311.77 K), the inlet pressures (Pin=834.04 to 910.23 kPa) and the increasing heat inputs (Q0exp(t/τ), exponential periods, τ, of 6.04 to 32.13 s) were systematically measured by an experimental water loop comprised of a multistage canned-type circulation pump with high pump head. Measurements were made on Pt test tubes of 3, 6 and 9 mm inner diameters, 32.7, 69.6 and 49.6 mm heated lengths and 0.5, 0.4 and 0.3 mm thicknesses, respectively. Theoretical equations for turbulent heat transfer in circular tubes of 3, 6 and 9 mm in diameter and 492, 636 and 616 mm long were numerically solved for heating of water with heated sections of 3, 6 and 9 mm in diameter and 33, 70 and 50 mm long by using PHOENICS code under the same condition as the experimental one considering the temperature dependence of thermo-physical properties concerned. The surface heat flux, q, and the average surface temperature, Ts,av, on the circular tubes solved theoretically under the flow velocities, u, of 4.22 to 21.45 m/s were compared with the corresponding experimental values on heat flux, q, versus the temperature difference between average inner surface temperature and liquid bulk mean temperature, ΔTL [=Ts,av-TL, TL=(Tin+Tout)/2], graph. The numerical solutions of q and ΔTL are almost in good agreement with the corresponding experimental values of q and ΔTL with the deviations less than ±10 % for the range of ΔTL tested here. The numerical solutions of local surface temperature, (Ts)z, and local average liquid temperature, (Tf,av)z, are within ±10 % of the corresponding experimental data on (Ts)z and (Tf,av)z. The thickness of the conductive sub-layer, δCSL [=(Δr)out/2], and the non-dimensional thickness of conductive sub-layer, y+CSL [=(fF/2)0.5ρluδCSL/µl], for the turbulent heat transfer in various vertical tubes are clarified based on the numerical solutions. It was confirmed in this study that authors’ steady-state turbulent heat transfer correlation based on the experimental data [Hata and Noda, 2008] can not only describe the experimental data of steady-state turbulent heat transfer but also the numerical solutions within ±10 % difference for the wide ranges of tube inner diameters (3, 6 and 9 mm), temperature differences between average inner surface temperature and liquid bulk mean temperature (ΔTL=8 to 145 K) and flow velocities (u=4.22 to 21.45 m/s).

Fluid-to-fluid scaling of heat transfer in circular tubes cooled with supercritical fluids

Experimental investigations of heat transfer at prototypical conditions of supercritical water cooled reactors (SCWRs) are strongly limited due to their huge technical and financial efforts required. One of the possible solutions is the application of model fluids, which have much lower critical pressure and critical temperature. Model fluid technique has been widely applied in the thermal-hydraulic studies of nuclear engineering. In spite of growing activities of heat transfer at supercritical conditions using model fluids, there does still not exist any reliable fluid-to-fluid scaling methods, to transfer the test data in model fluids directly to the conditions of prototype fluid. This paper presents a fluid-to-fluid scaling method for heat transfer in circular tubes cooled with supercritical fluids. Based on conservation equations and boundary conditions, one set of dimensionless numbers and the requirements of a complete scaling are determined. Scaling of pressure and temperature ensures the similarity of thermo-physical properties of various fluids. A new dimensionless number, presenting the product of the so-called pseudo Boiling number, Reynolds number and Prandtl number, is applied to scale heat flux. The distortion approach is used to scale mass flux. The scaling of heat transfer coefficient is based on Nusselt number. In addition, a new approach is introduced to validate the scaling law. The validation results show good feasibility and reasonable accuracy of the proposed scaling law. Assessment of scaling factors of various parameters indicates the high feasibility of Freon-134a as model fluid for SC water. Some guidelines can be derived for the future experimental investigations on heat transfer at supercritical pressures using model fluid techniques.

Heat Transfer Coefficients for Turbulent Flow in Concentric Annular Ducts

On the basis of a large number of experimental data from the literature, correlations were developed for the heat transfer coefficient for turbulent flow in concentric annular ducts. A proven correlation for heat transfer in circular tubes was extended by factors that take into consideration the effect of the diameter ratio of the annulus and the different boundary conditions for heating or cooling.

DEVELOPING COMPRESSIBLE TURBULENT FLOW AND HEAT TRANSFER IN CIRCULAR TUBE WITH UNIFORM INJECTION OR SUCTION

In the present work , a numerical study has been made for the developing compressible turbulent flow and heat transfer in circular tube with uniform injection or suction. The study included the numerical solution of the continuity, momentum and energy equations together with the two equations of the (k-ε) turbulence model, by using the Finite Difference Method (FDM). The air was used as the working fluid, and the circular passage was composed of tube with diameter (20.0) cm , and the length was 130 (hydraulic diameter) .The Reynolds number of the flow was (Re=1.78x106 ), and the Mach number (M=0.44) the ratio of the transverse velocity at the wall (vw) to the axial velocity at inlet (Uin), Ω=(vw/Uin), for suction equal(0.001) and for injection (-0.001).. The wall of the tube was heated with constant wall temperature (Tw) and in other case with constant heat flux (Qw) as a thermal boundary condition. The development of both hydrodynamic and thermal boundary layers occurs simultaneously. The computational algorithm is capable of calculating the hydrodynamic parameters such as the velocities , friction factor , turbulence structure which includes the Reynolds stress and the turbulent kinetic energy and eddy viscosity. Besides, the thermal parameters are also predicted, such as the temperature, Nusselt number, and the turbulent heat fluxes.The Results showed that the hydrodynamic and thermal entrance length is increased with the increasing of Reynolds number. The suction caused a flatten for the velocity profile and thus decreasing the hydrodynamic entrance length, and caused an increase in the Nusselt number and decreasing the local coefficient of friction, but injection caused a steeping of the velocity profile , and thus increasing the entrance hydrodynamic length and caused a decrease in the Nusselt number and increase the local coefficient of friction. Turbulent kinetic energy and turbulent heat flux are decreased with suction and increased with injection .Predictions have been obtained which are in good agreement with results obtained by past experimental and theoretical work.

Effect of Surface Roughness on Heat Transfer and Fluid Flow Characteristics at Low Reynolds Numbers in Small Diameter Tubes

The effect of surface roughness on pressure drop and heat transfer in circular tubes has been extensively studied in literature. The pioneering work of Nikuradse [1] established the sand grain roughness as a major parameter in defining the friction factor during laminar and turbulent flows. Recent studies have indicated a transition to turbulent flows at Reynolds number values much below 2300 during single-phase flow in channels with small hydraulic diameters. In the present work, a detailed experimental study is undertaken to investigate the roughness effects in small diameter tubes. The roughness of the inside tube surface is changed by etching it with an acid solution. Two tubes of 1.032 mm and 0.62 mm inner diameter are treated with acid solutions to provide three different roughness values for each tube. The Reynolds number range for the tests is 500-2600 for 1.067 mm tube and 900-3000 for 0.62 mm tube.

A study of heat transfer effectiveness of circular tubes with internal longitudinal fins having tapered lateral profiles

This paper presents a finite-difference based numerical simulation of steady, laminar heat transfer in circular tubes fitted with four identical longitudinal fins having tapered lateral profiles. The lateral view of the tip of each fin is a circular arc. The tube is exposed to constant heat flux. The temperature dependence of thermal conductivity and viscosity has been taken into account. The flow is assumed to be locally fully developed but thermally undeveloped. The momentum equation for the fluid and energy equations for the fluid and the tube wall with/without fins are solved iteratively and simultaneously. At each axial location, bulk velocity, bulk temperature of the fluid and Effectiveness which is an indicator of the enhancement of heat transfer due to addition of fins are calculated. A parametric study of effectiveness vs. axial distance for various combinations of fin materials and coolants reveals interesting results. The velocity profiles, friction factors and comparisons with respect to constant property solutions have also been discussed.

Constant-Wall-Temperature Nusselt Number in Micro and Nano-Channels1

We investigate the constant-wall-temperature convective heat-transfer characteristics of a model gaseous flow in two-dimensional micro and nano-channels under hydrodynamically and thermally fully developed conditions. Our investigation covers both the slip-flow regime 0⩽Kn⩽0.1, and most of the transition regime 0.1&lt;Kn⩽10, where Kn, the Knudsen number, is defined as the ratio between the molecular mean free path and the channel height. We use slip-flow theory in the presence of axial heat conduction to calculate the Nusselt number in the range 0⩽Kn⩽0.2, and a stochastic molecular simulation technique known as the direct simulation Monte Carlo (DSMC) to calculate the Nusselt number in the range 0.02&lt;Kn&lt;2. Inclusion of the effects of axial heat conduction in the continuum model is necessary since small-scale internal flows are typically characterized by finite Peclet numbers. Our results show that the slip-flow prediction is in good agreement with the DSMC results for Kn⩽0.1, but also remains a good approximation beyond its expected range of applicability. We also show that the Nusselt number decreases monotonically with increasing Knudsen number in the fully accommodating case, both in the slip-flow and transition regimes. In the slip-flow regime, axial heat conduction is found to increase the Nusselt number; this effect is largest at Kn=0 and is of the order of 10 percent. Qualitatively similar results are obtained for slip-flow heat transfer in circular tubes.