Horizontal Circular Tube Research Articles

The effects of different entrance sections lengths and heating on free and forced convective heat transfer inside a horizontal circular tube

An experimental study was done for hydrodynamically fully developed and thermally developing laminar air flows in a horizontal circular tube has a 30 mm inside diameter and 900 mm heated length ( L/ D = 30) under a constant wall heat flux boundary condition, with different aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths of 600 mm ( L/ D = 20), 1200 mm ( L/ D = 40), 1800 mm ( L/ D = 60), and 2400 mm ( L/ D = 80). The Reynolds number ranged from 400 to 1600 and the heat flux is varied from 60 W m − 2 to 400 W m − 2 . This paper examines the effects of the entrance sections lengths and heating on the free and forced convection heat transfer process. The surface temperature data were measured and heat transfer rates at different heat flux levels as well as different Reynolds numbers were calculated and correlated in the form of relevant parameters. The buoyancy force has a significant effect on the heat transfer and the combined convection factor was approximately varied form 0.13 ≤ Gr/ Re 2 ≤ 7.125. It was found that the surface temperature increases as the entrance section length increases. It was inferred that the heat transfer decreases as the entrance section length increases due to the flow resistance and the mass flow rate. The proposed correlation was compared with available literature and with laminar forced convection and showed satisfactory agreement.

Flow and heat transfer behaviors of phase change material slurries in a horizontal circular tube

The flow and convective heat transfer behaviors of microencapsulated phase change material (MPCM) slurries in a horizontal circular tube have been experimentally investigated. The slurry consisted of microencapsulated 1-bromohexadecane (C 16H 33Br) and water, with the mass fractions of MPCM varying from 5% to 27.6%. The pressure drop and local heat transfer coefficients were measured, and the influences of capsule fractions, heating rates, and flow structures on heat transfer performance were also studied. Heat transfer coefficients measured for MPCM slurry are significantly higher than for those for single-phase fluid flow in laminar flow conditions, but exhibit more complicated phenomena at low turbulent conditions. Moreover, a new simple heat transfer correlation equation was proposed that accurately predicts the local heat transfer coefficients of laminar MPCM slurry flow in a horizontal circular tube.

Forced convection heat transfer of supercritical CO 2 in a horizontal circular tube

The problem of low Reynolds number forced convection of supercritical CO 2 in a horizontal circular tube is studied numerically. The interest has been simulated by an experimental study on solar collector using supercritical CO 2 as working fluid, in which high collector efficiency above 70% has been found for the supercritical CO 2-based collector. In the present study, to reproduce the general features exhibited in the experiments, a circular tube having a diameter of 6.0 mm has been investigated at a pressure of 8.0 MPa and an inlet temperature of 32.0 °C. In particular, this work has been focused upon the convective heat transfer characteristics of CO 2 flowing in a horizontal circular tube with small mass flow rates ranging from 0.003 to 0.03 kg/min and constant heat flux from 100.0 to 800.0 W/m 2. The results show that a threefold increase in Nusselt number has been achieved at the typical Reynolds number of 210 in the experimental tests, compared with that of using water as working fluid in the collector. The heat transfer enhancement is also found to increase with Reynolds number Re and heat flux q for ranges of 210 ≤ Re ≤ 1800 and 100 W/m 2 ≤ q ≤ 800 W/m 2. Furthermore, the mechanisms that are responsible for the heat transfer enhancement are identified. The first is due to thinner thermal boundary layer than that of the water case and the second is that the decrease in viscosity and the increase in heat capacity contribute to the heat transfer enhancement. In addition, the flow does not reach a fully developed velocity and temperature field, which may also contribute to the enhancement phenomena.

Turbulent Film Condensation on a Nonisothermal Horizontal Tube

AbstractA simple model has been developed for the study of turbulent film condensation from downward flowing vapors onto a horizontal circular tube with variable wall temperature. The interfacial shear at the vapor condensate film is evaluated with the help of Colburn analogy. The condensate film flow and local/or mean heat transfer characteristics from a horizontal tube with non-uniform temperature variation under the effect of Froude number, sub-cooling parameter and system pressure parameter has been conducted. Although the non-uniform wall temperature variation has an appreciable influence on the local film flow and heat transfer; however, the dependence of mean heat transfer on the non-uniform wall temperature variation is almost negligible.

An experimental study on swirling flow behind a round cylinder in the horizontal circular tube

An experimental study is performed for turbulent swirling flow behind a circular cylinder using 2-D PIV technique. The Reynolds number investigated are 10,000, 15,000, 20,000 and 25,000. The mean velocity vector, time mean axial velocity, turbulence intensity, kinetic energy and Reynolds shear stress behind the cylinder are measured before and behind the round cylinder along the test tube. A comparison is included with non swirl flow behind a circular and square cylinder. The recirculation zones are showed asymmetric profiles.

Condensation of R-134a vapour over single horizontal integral-fin tubes: effect of fin height

An experimental investigation has been carried out to study the condensation of R-134 vapour over five single horizontal circular integral-fin tubes of 472 fpm fin density, 417 mm length and different fin heights of 0.45, 1.14, 1.47, 1.92 and 2.40 mm. The circular fins are rectangular in shape and the fin thickness of all tubes is 0.70 mm. The tube with the fin height of 0.45 mm has given the highest enhancement in heat transfer coefficient, h, of the order of 3.18 in comparison to that predicted by the Nusselt model for a plain tube.

Local convective heat transfer from a horizontal tube in an acoustic cavitation field

An experiment has been carried out to investigate the local convective heat transfer from a horizontal circular copper tube in an acoustic cavitation field. The effects of acoustical parameters (including sound source intensity, the vibrator location and sound distance), fluid temperature and thermophysical properties of working fluid on heat transfer enhancement were studied, as well as the variation of heat transfer rate with sound source intensity at constant heat flux. Results show that convection heat transfer was remarkably enhanced due to disturbance and impingement by cavitation bubble. Among these cases tested, the maximum augmentation ratio of 3.95 was reached for acetone with a cluster of cavitation bubbles impinging perpendicularly on the tube surface.

Heat Transfer Characteristics of Nanofluids in an Acoustic Cavitation Field

Heat transfer characteristics of nanofluids in an acoustic cavitation field have been investigated experimentally. The effects of acoustical parameters, nanofluids concentration, and fluid subcooling on heat transfer are determined in detail. Results show that acoustic cavitation and nanometer particles have a profound influence on single-phase convection and boiling heat transfer of a horizontal circular copper tube. The former is mainly ascribed to the impingement and disturbance of cavitation bubbles and anticipatory activation of smaller vapor embryos within the cavities, while the latter is caused by roughness modification of the tube surface. The above mechanisms are given based on the Schlieren photographs and optical observation. Acoustic cavitation has been shown to be a good way to reduce or eliminate boiling hysteresis.

Improved Heat Transfer Correlation in the Transition Region for a Circular Tube with Three Inlet Configurations Using Artificial Neural Networks

Local forced and mixed heat transfer coefficients were measured by Ghajar and Tam [5] in a horizontal circular straight tube fitted with reentrant, square-edged, and bell-mouth inlets under uniform wall heat flux boundary condition. For the experiments, the Reynolds, Prandtl, and Grashof numbers varied from about 280 to 49000, 4 to 158, and 1000 to 2.5 × 105, respectively. The heat transfer transition regions were established by observing the change in the heat transfer behavior. The data in the transition region were correlated by using the traditional least squares method. The correlation predicted the transitional data with an average absolute deviation of about 8%. However, about 30% of the data in the transition region were predicted with 10–20% deviation, and about 3% with deviations greater than 20%. This is due to the abrupt change in the heat transfer characteristic and its intermittent behavior in this region. Since the value of the heat transfer coefficient has a direct impact on the size of the heat exchanger, a more accurate correlation has been developed using artificial neural networks. A total of 1290 data points (441 for reentrant, 416 for square-edged, and 433 for bell-mouth) were used. The accuracy of the improved heat transfer correlation is excellent, with the majority of the data points predicted with less than 5% deviation.

Accurate prediction of friction factor and heat transfer coefficients of mixed convection in circular tubes: a simple linear algebraic model

This investigation proposes a novel linear algebraic model (LAM) for deriving simple and accurate empirical formulas solved numerically for a complex convective flow system. A classical mixing convection problem in a uniformly heated horizontal circular tube is selected to demonstrate the validity of the linear algebraic model. Solving the set of linear algebraic equations for characteristic quantities is much easier than solving the original system of partial differential equations. The remaining undetermined weighting factors are then determined using two-data in the traditional numerical solution of the governing partial differential equations. The present LAM technique successfully utilizes parametric analysis to describe the mechanism of transport phenomena. A set algebraic functional relationship is established among these parameters

Turbulent heat transfer and flow characteristics in a horizontal circular tube with strip-type inserts. Part II. Heat transfer

This paper is the second of two papers that presents the results of an extensive study of turbulent heat transfer and pressure drop in a horizontal tubes with strip-type inserts. Experimental data were taken for air for a class of strip inserts (longitudinal strip and crossed-strip). The insert was characterized by the parameters of 1⩽AR⩽5 and R * (=0.5 and 1). Friction factor data (from Part I) and temperature measurements were used to understand the underlying physical phenomena responsible for the heat transfer enhancement for 6500⩽Re⩽19 500 . Nusselt numbers were between four and two times the bare tube values at low Re and high Re, respectively. Performance evaluation index R1 (constant pumping power) and R2 (constant heat duty) were made and an optimum condition would be thus found.

Turbulent heat transfer and flow characteristics in a horizontal circular tube with strip-type inserts. Part I. Fluid mechanics

This paper is the first of two papers that present the result of a study of turbulent flow and pressure drop in a horizontal tube with strip type inserts. Experimental data taken for air for a class of strip type inserts (longitudinal, LS and cross, CS inserts) used as a tube side heat transfer augmentative device for a single-phase cooling mode operation are presented. To broaden the understanding of the underlying physical phenomena responsible for the heat transfer enhancement, flow mechanisms through velocity measurements are combined with pressure drop measurements to develop friction factor correlations for 6500⩽Re⩽19 500 where Re is the Reynolds number. Friction factor increases were typically between 1.1 and 1.5 from low Re(≅6500) to high Re(≅19 500) with respect to bare tubes.

Effect of acoustic cavitation on boiling heat transfer

Boiling heat transfer on a horizontal circular copper tube in an acoustical field is investigated experimentally and the relation between the liquid cavitation, the boiling and the micro bubble radii are analyzed theoretically. The results show that cavitation bubbles have an important influence on the nucleation, growth and collapse of vapor embryo within cavities on the heat transfer surface and that the enhancement of boiling heat transfer by acoustic cavitation mainly depends on whether the vapor embryo is activated by the cavitation bubbles to initiate boiling.

Gas-assisted non-newtonian fluid displacement in circular tubes and noncircular channels

The motion of long bubbles into Newtonian and non-Newtonian fluids confined in horizontal circular tubes, rectangular channels, and square cross-sectional channels has been studied both theoretically and experimentally. Of particular interest is the determination of residual liquid film thickness on the walls. Isothermal experiments have been conducted to measure the displacement of the gas–liquid interface as a function of the applied pressure differential. The velocity of the interface and residual liquid film thickness have been determined for both Newtonian and non-Newtonian (shear thinning and viscoelastic) fluids. These experimental results are in good agreement with similar experimental studies conducted by other investigators. The experimental results indicate that the liquid film thickness of constant viscosity viscoelastic fluids (Boger fluids) deposited on the tube wall is thicker than that of comparable Newtonian fluids. A simple mathematical analysis was developed using a power-law model. The mathematical model successfully captures the gas–liquid dynamics for Newtonian and non-Newtonian fluid displacement in a tube and rectangular channel. The prediction of the liquid fraction deposited on the walls is in qualitative agreement with the experimental observations of previous investigators (Chem. Eng. Sci. 24 (1969) 471; A.I.Ch.E. 16 (1970) 925; Chem. Eng. Sci. 30 (1975) 379). The model gives similar results to a numerical solution (Polm. Eng. Sci. 35 (1995) 877) in which a constitutive equation containing a yield stress is used to model the non-Newtonian behavior. The model is used to determine the location and velocity of the advancing bubble front for the case of a power-law fluid. The results indicate that the gas–liquid interface advances more rapidly with decreasing values of the power-law index above a certain value of dimensionless time ( t/ t b ≈0.75).

Developing 3-D turbulent mixed convection in a circular duct inserted with longitudinal strips

A numerical study was undertaken for developing turbulent mixed convection in a horizontal circular tube with strip-type inserts (longitudinal strip LS and crossed-strip CS inserts) for 6500⩽Re⩽19 500 and 0⩽ Gr⩽10 8. The turbulence model used is a standard version of k– ε model with wall functions. Three-dimensional Navier–Stokes equations were solved using some modifications to the SIMPLE solution procedure. Temperature data and heat transfer coefficients at different heat flux levels were calculated and correlated in terms of relevant parameters. The buoyancy effect was examined in detail for 0⩽ Gr/ Re 2⩽2.37. It is found that the buoyancy effect has less influence for tubes with inserts compared to the bare tubes. Moreover, the heat transfer enhancement was found to be 2–3 times that of bare tubes. The results were also compared with those from the corresponding previous experimental data. A good agreement was found.

K-2211 気泡微細化を伴う円管内サブクール流動沸騰について(J14-3 沸騰伝熱)(J14 & J11 & J12 原子力利用技術と熱問題)

Subcooled flow boiling of water was performed in a horizontal circular tube. The two channels of which inner diameters were 10 mm and 16 mm were employed in the experiment. Tested subcooling of liquid were 20K, 30K and 40K and liquid velocity was 0.5 m/s. Microbubble Emission Boiling, MEB, was observed in transition boiling as observed in a rectangular channel. A periodic type of MEB, which was categorized in the authors' previous works, was observed according to the analysis of pressure fluctuations in both channels. In the periodic MEB, the maximum pressure was higher in the smaller channel and was considerably higher than that of rectangular channel. The heat fluxes in the MEB increased with the increase of frequency of the pressure fluctuations. The frequency is considered to be that of liquid supply into the heating surface.

Condensation of R-134a Vapor over Single Horizontal Circular Integral-Fin Tubes with Trapezoidal Fins

An experimental investigation has been carried out for the condensation of R-134a vapor on four single horizontal circular integral-fin tubes (CIFTs) made of copper. The fins are of trapezoidal shape and the fin density of these tubes has been varied systematically from 934 fpm to 1,875 fpm. All the experimental data have been acquired at the saturation temperature of 312.4 +/- 0.5 K. The CIFT with 1,560 fpm fin density has been found to be the best-performing tube, as it has provided an enhancement factor (EF) of the order of 5.6. The experimental results are in the best agreement with Honda and Nozu model. This model has predicted 80% of the experimental results in a range of 0-30%.

The Nusselt condensation and nonisothermality

We extend Nusselts theory of condensation accounting for subcooling of condensate at a variable wall temperature. Neglecting vapour shear, we derive an equation for heat transfer of a saturated pure vapour condensing on a surface of arbitrary shape. Using this equation, we examine the interaction between heat transfer and nonisothermality specifying the conditions that give the mean heat transfer coefficient independent of the temperature profiles. Applying these conditions to the condensation on a horizontal circular tube and on a sphere, we analytically confirm some numerical results from the literature.

Developing turbulent mixed convection in a horizontal circular tube with strip-type inserts

An experimental study was undertaken for developing turbulent mixed convection in a horizontal circular tube with strip-type inserts (longitudinal strip LS and crossed-strip CS inserts) for 6500 ⩽ Re ⩽ 19,500 and 5.82 × 10 4 ⩽ Gr ∗ ⩽ 9.78 × 10 7 . Temperature data were measured and heat transfer rates at different heat flux levels were calculated and correlated in the form of relevant parameters. The buoyancy effect was detailedly examined for 1.53 × 10 −4 ⩽ Gr ∗/Re 2 ⩽ 2.31 . It is found that the buoyancy effect has less influence for tubes with inserts compared to the bare tubes. Moreover, the heat transfer enhancement was found 2–3 times than those of bare tubes.

Departure-site spacing for liquid droplets and jets falling between horizontal circular tubes

When a liquid film falls from one tube to another below it, the flow can take the form of discrete droplets, individual jets or a continuous sheet. Experiments exploring the effects of thermophysical properties and geometrical parameters on the droplet and jet flow patterns are described. Measurements of droplet and jet departure-site spacing are reported for several fluids over a wide range of liquid flow rates, tube sizes and tube spacing. For the conditions of this study, departure-site spacing increased with decreasing Re for high-Ga fluids and was nearly independent of Re for low-Ga fluids. Departure-site spacing increased slightly with tube diameter for small tubes and was nearly independent of tube size for large tubes. Departure-site spacing was nearly independent of tube spacing for the entire range of experiments; however, a relation between jet shape, jet spacing and tube spacing was observed under some conditions. A qualitative study of liquid-jet shapes shows that this flow feature depends strongly on tube spacing, and comparisons to existing models suggest that further work in this area is needed.