Horizontal Smooth Tube Research Articles

Experiments on Boiling Heat Transfer of Low-pressure Refrigerants Inside a Horizontal Smooth Tube

Experiments on Boiling Heat Transfer of Low-pressure Refrigerants Inside a Horizontal Smooth Tube

Artificial Neural Network Techniques for the Determination of Condensation Nusselt Number in Horizontal Smooth Tubes

In this study, using readily available experimental data in the literature, artificial neural networks (ANN) method is adopted to specify condensation Nusselt number in horizontal smooth tubes. Condensation heat transfer of R22, R134a and 50/50 and 60/40 of the R32/ R125 azeotropic refrigerant mixtures were examined with four different ANN methods. The experimental data is taken from the study of Dobson et al. [1]. The input parameters are mass flux, quality, hydraulic diameter, Soliman's modified Froude number, density of fluid phase and dynamic viscosity of liquid phase where the output parameter is the condensation Nusselt number. In this study the interval for tube diameters is between 3.14-7.04 mm, and the interval for mass flux is between 50-800 kg/m2s. The training algorithms are tested using different neuron numbers and the best algorithm was found as Bayesian regularization having 8 neurons. It is observed that the Nu number evaluated using ANN is ± 15% error margin compared to experimental results. Furthermore, for increasing mass flux rates the error margin is around ± 5%.

Assessment of the low-GWP refrigerants R600a, R1234ze(Z) and R1233zd(E) for heat pump and organic Rankine cycle applications

This paper performs the thermodynamic and the heat-transfer assessment of the low-GWP refrigerants R600a, R1234ze(Z) and R1233zd(E) as alternative to the traditional low-pressure HFC refrigerants, such as R245fa, for Heat Pump (HP) and Organic Rankine Cycle (ORC) applications. The thermodynamic assessment shows that R1234ze(Z) exhibits efficiency very similar to those of R245fa, while R1233zd(E) presents higher efficiency and R600a lower efficiency than those of R245fa both in HP and ORC applications. The heat transfer assessment is carried out in condensation inside a 4 mm ID horizontal smooth tube at three different saturation temperatures (30, 35, and 40 °C), at different vapour qualities and mass velocities. R600a, R1234ze(Z) and R1233zd(E) show very similar condensation heat transfer and pressure drop performances under operating conditions typical for HP and ORC applications. The results of the thermodynamic and the heat transfer assessment show that R600a, R1234ze(Z) and R1233zd(E) are valuable long-term low-GWP substitutes for the traditional low-pressure HFC refrigerants, both in HP and ORC applications.

CFD Simulation of Heat Transfer from Elliptic Tube Bundle Buried in a Bubbling Fluidized Bed

In the present work, numerical simulations are performed to study heat transfer characteristics and fluid flow around elliptic tube heat exchanger. The results for heat transfer coefficient between immersed inline and staggered bundles of horizontal smooth tubes and air-fluidized bed of pulverized coal, dp = 2 mm, are reported within the range of fluidization number, Usup/ Umf, ranges from 1 to 1.4. Heat transfer as well as multiphase flow dynamics in fluidized bed is modelled using the Eulerian-Eulerian and the Two-Fluid Model (TFM) with Kinetic Theory of Granular Flow (KTGF) to describe the granular flow characteristics of the solid phase. The average heat transfer coefficient for the present work is compared with that obtained from the well-known correlation of circular cylinder and a noticed improvement is observed. It is noticed that, the average Nusselt number increases with fluidization number. Also, it is noticed that, the average Nusselt number in case of staggered tubes bundle is higher than the case of inline tubes bundle. The inline tube bundle has lesser pressure drop than the staggered tube bundle.

Numerical Simulation of Steam Condensation in a Steam-Gas Mixture Flow in a Variable-Section Channel with a Bundle of Smooth Horizontal Tubes

In this paper, the results from calculations of heat and mass transfer in a variable-cross-section channel with a bundle of smooth horizontal tubes, on the surface of which steam from a moving steam-gas mixture (SGM) condenses, are presented. The decrease in the channel’s cross section and, accordingly, the number of tubes in the vertical rows along the SGM movement provides the mixture with approximately constant velocity as the steam condenses. The mathematical model used in this study is described in detail in our previous publications. The two-dimensional equations of single-phase hydrodynamics, energy, and diffusion are solved for the external SGM flow. The condensation process is modeled at the level of the boundary conditions on the tube surface, taking into account a moving laminar condensate film. The heat transfer through the tube wall from the film to the cooling water is described using a one-dimensional model of the wall. To account for the irrigation of the bundle’s lower tubes with condensate formed on the upper tubes (inundation effect), a simplified model is used. The data on the velocity fields and impurity concentration in the condenser and the heat transfer characteristics are presented. The calculation results of the heat transfer coefficients on the tubes of the first vertical row of the bundle and the heat transfer coefficients for individual sections of the simulated condenser containing several tube rows at a 0–8.5% volume fraction of air in the SGM at the inlet to the apparatus are compared with experimental data. A quite satisfactory agreement between the calculated and experimental data is obtained, which confirms the efficiency of the used model. The calculated data on the local velocity fields and the composition of the steam-air mixture indicate a significant heterogeneity of these characteristics. This complicates the development of relatively simple engineering methods for calculating the heat load of condensers at high air concentrations. The calculations were performed using in-house CFD-code ANES.

Condensation two-phase flow patterns for zeotropic mixtures of tetrafluoromethane/ethane in a horizontal smooth tube

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) systems have distinct advantages in the temperature range from 80 to 230 K. Tetrafluoromethane (R14) and ethane (R170) are essential components of mixed-refrigerants. In this paper, the adiabatic flow pattern map for R14/R170 mixtures was firstly developed based on previous flow pattern data. Then an experimental investigation on condensation two-phase flow patterns for R14/R170 mixtures (0.193/0.0.807, 0.437/0.563, 0.632/0.368 and 0.799/0.201 by mole) in a horizontal smooth tube with inner diameter of 4 mm was presented. Experiments were carried out at mass fluxes from 100 to 350 kg m−2s−1, saturation pressures from 1.5 to 2.5 MPa and heat fluxes from 8.4 to 42.2 kW m−2 over the entire range of vapor quality. The effects of mass flux, saturation pressure, heat flux and concentration on flow pattern evolutions were discussed and presented. Analysis results indicated that mass transfer resistance and heat flux account for the nonequilibrium effects of zeotropic mixture condensation, which leads to the different evolutionary trends between condensation and adiabatic two-phase flow. Finally, a new condensation flow pattern map was deduced by containing nonequilibrium effects into adiabatic flow pattern map.

Effect of miscible oil on flow boiling heat transfer characteristic of ammonia in a 4 mm small tube

The present work aims to investigate the effect of miscible oil on the flow boiling heat transfer characteristic of ammonia in a 4 mm horizontal smooth tube. In contrast to conventional immiscible oil leading to serious deterioration in heat transfer, the results show that the miscible oil with oil concentration of 1.95% has almost no influence on the heat transfer. For the higher oil concentrations (i.e., 3.99% and 5.78%), the heat transfer coefficient of ammonia-oil mixture also appears to be remarkably similar to that of oil-free condition first, and then shows a slight deterioration trend when the vapor quality is high. Meanwhile, 13 heat transfer correlations are evaluated, and the comparison results indicate the Gungor and Winterton correlation (1987) could give a fair prediction for the experimental data even though it is developed for turbulent flow. Finally, a new correlation is developed with consideration of laminar flow and thermophysical properties change, and it can remarkably improve the prediction accuracy for the experimental data.

Experimental investigation of two phase flow boiling heat transfer of mixtures of refrigerant R410A and nanolubricants in a horizontal smooth copper tube

In air conditioning systems, a small amount of lubricant leaves the compressor and circulates through the system. The lubricant acts as a contaminant, affecting heat transfer and pressure losses in heat exchangers. The mixture of refrigerant and nanolubricants (NLs), that is, nanoparticles dispersed in the lubricant oil, have shown potential to augment the two-phase flow boiling heat transfer in the literature. However, the mechanisms of heat transfer enhancements due to the NLs are still not well understood. Published models of two-phase flow boiling lack experimental validation with refrigerant and NL mixture. This work addresses this gap by presenting new experimental heat transfer coefficient and pressure drop results for the saturated two-phase flow boiling of R410A with two NLs in a 9.5 mm I.D. smooth copper tube: a ZnO nanoparticle laden lubricant and a γ-Al2O3 nanoparticle laden lubricant. In smooth tube, R410A–NL mixtures, which had higher thermal conductivity and kinematic viscosity than R410A–POE lubricant mixture, showed degradation in heat transfer coefficient compared to R410A–POE lubricant case; but they also had lower pressure drops. Relocation of the nanoparticles closer to the inner wall of the tube could lead to enhancement in heat transfer at an expense of increased frictional pressure drop.

Verification of the Mathematical Model of Steam Film Condensation from a Flowing Steam-Air Mixture on a Bundle Made of Horizontal Tubes

A model of steam film condensation from a flowing steam-air mixture on a bundle made of horizontal tubes was described in previous publications. In the model, the bulk flow is simulated using single-phase flow equations, and the condensation is simulated by means of the boundary conditions on the tube surface considering the laminar flow of the condensate film. This model was supplemented with a simplified model to describe irrigation of the lower tubes in the bundle with condensate formed on the upper tubes. The results are presented of the model verification against the published experimental data on condensation on horizontal smooth tubes in a staggered tube bundle at a steam-air mixture pressure of 30 kPa, a forced horizontal flow velocity upstream of the tube bundle between 1.3 and 4 m/s, and an air volume fraction from 0 to 12%. Disagreement between the predicted and the experimental heat-transfer coefficients does not exceed 20% for all considered regimes. The calculations were performed using the ANES CFD-code being developed by the authors.

A new flow pattern map for flow boiling of R410A in horizontal micro-fin tubes considering the effect of the helix angle

This paper presents an improved flow pattern map for flow boiling in horizontal micro-fin tubes considering the effect of helix angle. The proposed map is modified from the one developed for horizontal smooth tubes by Wojtan et al. (2005) and takes the early transition to annular flow due to micro-fin geometry into account. Visualization of R410A flow boiling is carried out through the transparent smooth and micro-fin tubes with 0°, 10° and 18° helix angle and plotted on the proposed map for comparison. The result shows that the flow pattern map for micro-fin tube with 0° helix angle is generally similar to that for the smooth tube. For the tube with 10° and 18° helix angle, the annular flow occurs at lower vapor quality and lower mass flux. The proposed map shows good agreements when compared with the visualization and reflects the downward shift (to lower mass flux and vapor quality conditions) of the SW-WA transition with the increase of the helix angle. In order to further validate the proposed model, the flow pattern transitions calculated from the equations are also compared with the experimental data of the micro-fin tube in literature. The results show that the modified map increase the prediction accuracy of the flow patterns in the micro-fin tube.

Condensation and Evaporation Heat Transfer Characteristics of Low Mass Fluxes in Horizontal Smooth Tube and Three-Dimensional Enhanced Tubes

Abstract An experimental investigation of condensation and evaporation heat transfer characteristics was performed in 15.88-mm-OD and 12.7-mm-OD smooth and three-dimensional enhanced tubes (1EHT, 3EHT) using R134A and R410A as the working fluid. The enhanced surface of the 1EHT tube is made up of dimples and a series of petal arrays; while the 3EHT tube is made up of rectangular cavities. Evaluations are performed at a saturation temperature of 45 °C, over the quality range of 0.8–0.2 for condensation; while for evaporation the saturation temperature was 6 °C and the quality ranged from 0.2 to 0.8. For condensation, the enhancement ratio (enhanced tube/smooth tube) of the heat transfer coefficients was 1.42–1.95 for the mass flux ranging from 80 to 200 kg/m2s; while for evaporation, the heat transfer enhancement ratio is 1.05–1.42 for values of mass flux that range from 50 to 180 kg/m2s. Furthermore, the 1EHT tube provides the best condensation and evaporation heat transfer performance, for both working fluids at the mass flux considered. This performance is due to the dimples in the enhanced surface that produce interface turbulence; additionally, the increased surface roughness causes additional disturbances and secondary flows near the boundary, producing higher heat fluxes. The main objective of this study was to evaluate the heat transfer enhancement of two enhanced tubes when using R134A and R410A as a function of mass flux, saturation temperature, and tube diameter. As a result of this study, it was determined that the heat transfer coefficient decreases with an increase in saturation temperature and tube diameter.

Analysis of the heat transfer characteristics of the liquid-vapor separation condenser based on a condensate growth model in horizontal tube

A theoretical model for horizontal tubes was proposed to analyze heat transfer performance of the condenser with liquid-vapor separation device in this paper. The model was on basis of convection boundary condition and effects of gravity and shear force to express the feature of the condensing film growth inside a horizontal smooth circular tube. In order to validate the model, a set of relevant experiments were carried out to gauge the heat transfer coefficients of condensers with liquid-vapor separation device. The predicted heat transfer coefficients showed a nice agreement with these experimental results. The refrigerant circuitry of a condenser with liquid-vapor separation device was designed according to the principle of equal mass flux with predicted results. Besides, the heat transfer performance of a condenser unit with liquid-vapor separation device in actual operating condition was calculated and analyzed. The influences of refrigerant vapor mass flux and saturation temperature on the performance were investigated and the enhancement of the liquid-vapor separation device on heat transfer was verified.

Single-phase forced convection heat transfer and pressure drop in circular tubes in the laminar and transitional flow regimes

Forced convection experiments are challenging to perform in smooth horizontal circular tubes, because very low heat fluxes are required to ensure that the buoyancy effects do not enhance the heat transfer coefficients. Previous work in the transitional flow regime mainly focused on mixed convection and limited work has been conducted for forced convection heat transfer. It was therefore the purpose of this study to experimentally investigate the heat transfer and pressure drop characteristics during specifically forced convection conditions. Vertical upward and downward flows through a smooth circular test section were considered. Experiments were conducted using water (Prandtl numbers between 3.5 and 8.1) at Reynolds numbers of 400–6000 with constant heat fluxes varying from 1 to 8 kW/m2. Although the flow direction and heat flux had no influence in the laminar flow regime, it was found that the fully developed laminar forced convection Nusselt numbers were not constant at 4.36, but were a function of Reynolds number for Reynolds numbers higher than 1000. Therefore, a revised laminar Nusselt number correlation for smooth circular tubes was developed. The fully developed laminar forced convection friction factors were, as expected, equal to 64/Re. For both the heat transfer and pressure drop characteristics, transition occurred at the same mass flow rates for all the heat fluxes, including isothermal flow, but the critical Reynolds numbers increased with an increase in heat flux. The width of the transitional flow regime in the fully developed region remained constant for all heat fluxes.

R513A flow boiling heat transfer inside horizontal smooth tube and microfin tube

This paper presents experimental results for R513A flow boiling inside a 3.5 mm ID horizontal smooth tube and inside a 3.4 mm ID horizontal microfin tube. R513A is an azeotropic mixture made of R1234yf and R134a (56% and 44% in mass), and it is proposed as a direct substitute of the common R134a. The flow boiling heat transfer coefficients are measured for mass velocity ranging between 150 and 800 kg m−2 s−1, for heat fluxes from 12 to 60 kW m−2, at a saturation temperature at the inlet of the test section of 20°C. The effects of mass velocity, heat flux and vapor quality on the thermal performance are analyzed both for the smooth tube and for the microfin tube. The heat transfer enhancement of the microfin tube compared to the smooth tube is also presented. The comparison reveals that the microfin tube considerably augments the heat transfer coefficient especially at low mass velocity, whereas at high mass velocity microfin tube and smooth tube show almost the same heat transfer coefficient. Finally, the collected experimental heat transfer coefficients are compared against values predicted by empirical correlations available in the open literature.

Two-phase flow condensation heat transfer characteristic of R-600a inside the horizontal smooth and dimpled helical coiled tube in shell type heat exchanger

This paper presents an experimental study on condensation heat transfer of R-600a inside smooth and dimpled helical coiled tubes. The experiments were executed at saturation temperature of 35°, and 45 °C with mass fluxes of 75, 115, 156 and 191 kg m−2s−1. The effect of vapor quality, mass flux and saturation temperature on the heat transfer coefficient is examined. The experiments have been carried out for smooth and dimpled helical tube. The dimpled helical coiled tube showed higher heat transfer coefficient compared to the smooth helically coiled tube. The heat transfer data of R-600a are compared with R-134a from a previous study with the same operating condition for both tubes. We developed correlations to predict the condensation heat transfer coefficients in smooth and dimpled tubes that predict data within ±20%.

An experimental investigation on heat transfer characteristics of graphite-SiO2/water hybrid nanofluid flow in horizontal tube with various quad-channel twisted tape inserts

Turbulent heat transfer characteristics of Graphite-SiO2/Water hybrid nanofluid flow in a horizontal smooth tube with and without quad-channel twisted tape (QCTT) inserts is investigated experimentally. The hybrid nanofluid is obtained using two different nanoparticles: Siliciumdioxid (60%) and Graphite (40%) with pure water as base fluid. Experiments are conducted for two different volume concentrations, 0.5% and 1%, respectively. The length of quad-channel twist tape inserts are between 0 and 42 cm with constant twist ratios of 5. The Reynolds number is varied from 3400 to 11,000. According to the results, Nusselt number of the case with hybrid nanofluid increased with increasing mass flow rate and volume concentration. Also, heat transfer coefficient increased with increasing length of twisted tape insert. Moreover, friction factor increases with increasing volume concentration for increasing Reynolds number whereas and friction factor increases with increasing the length of tape inserts. Pressure drop increases with increasing mass flow rate and increasing volume concentration. Finally, the regression equations are found to be well-matched with the experimental data within the deviation band of ±5% for Nusselt number and ± 10% for friction factor, respectively.

A mathematical model of forced convection condensation of steam on smooth horizontal tubes and tube bundles in the presence of noncondensables

This paper presents a description and the results of verification of an unsteady model for forced convection condensation on a smooth single horizontal tube in the presence of noncondensables. The external flow is described by the two-dimensional (2D) unsteady Navier–Stokes equations for a laminar regime or by the unsteady Reynolds-averaged Navier–Stokes (URANS) equations for a turbulent regime, and the condensate film is considered using a one-dimensional model. Details of the numerical implementation of the developed algorithm are described. The available literature data were employed to verify the proposed model for forced convection condensation for an arbitrary flow direction of vapour with and without noncondensables. The numerical results are in good agreement with the published experimental data. In the proposed model, condensation influences the air-vapour mixture flow only through the boundary condition; therefore, this model can be easily implemented using different computational fluid dynamics (CFD) codes (in this work, an “in-house” CFD code ANES) for modelling more complex configurations, like tube bundles. Results for condensation on five tubes in a tube bundle are also presented in this work. The influence of mutual irrigation of pipes by drops coming off was also approximated.

Single-Phase Heat Transfer of R245fa + Lubricant Oil Mixtures Inside Horizontal Smooth and Microfin Tubes

The effect of lubricant oil on the single-phase heat transfer of R245fa inside horizontal smooth and microfin tubes was experimentally investigated. Tests were conducted using a horizontal smooth tube with an inner diameter of 8.32[Formula: see text]mm and two microfin tubes with equivalent diameters of 8.9 and 8.7[Formula: see text]mm, fin height of 0.12 and 0.18[Formula: see text]mm, and number of fins of 65 and 85, respectively. The heat transfer coefficients were measured for pure R245fa and R245fa + lubricant oil mixture in the Reynolds number range of 2000–10000, heat fluxes of 5 and 10[Formula: see text]kWm[Formula: see text], and refrigerant temperatures of 30∘C and 45∘C at the test section inlet. Oil concentration was varied at 1, 2 and 5[Formula: see text]wt.%. The results showed that the heat transfer of pure R245fa agreed well with previous correlations. However, the heat transfer coefficients of the R245fa + lubricant oil mixtures were lower than those of the pure R245fa, and decreased with an increase in oil concentration for both smooth and microfin tubes. The reduction in heat transfer was more marked for the HF tube than for the smooth and LF tubes. For the HF tube, the decrease in heat transfer was apparently more pronounced at higher oil concentrations from 2 to 5[Formula: see text]wt.% owing to immiscibility between the oil and refrigerant.

Experimental investigation of pressure drop during two-phase flow of R1234yf in smooth horizontal tubes with internal diameters of 3.2 mm to 8.0 mm

In this study, the two-phase pressure drop of R1234yf refrigerant (as an alternative to R134a) in a smooth horizontal tube is investigated. Tests are carried out and under several conditions of vapor quality (0% to 100%), mass velocity (200, 300 and 400 kg/m²s), heat flux (0, 7 and 14 kW/m²) and evaporation temperatures (20 °C and 30 °C). The internal diameters tested are 3.2 mm, 4.8 mm, 6.4 mm and 8.0 mm. A comparison of pressure drop between R1234yf and R134a is also carried out. The results demonstrate that the two-phase pressure gradient of R1234yf is approximately 20% lower than that of R134a. The experimental data is compared with 19 correlations from the literature. From this comparison, it is concluded that the experimental data are in accordance with the correlation proposed by Xu and Fang with a mean absolute error of 20% and with approximately 65% of the data predicted within a ±30% error band.

Two-phase flow condensation pressure drop of R14 in a horizontal tube: Experimental investigation and correlation development

Tetrafluoromethane (R14) is an important fluid in three-stage-cascade refrigeration and mixed-refrigerant Joule-Thomson refrigeration (MJTR). In this work, an experimental investigation on condensation pressure drop of R14 in a horizontal smooth tube with inner diameter of 4 mm was carried out. Experiments were implemented at mass fluxes from 150 to 650 kg/(m2 s), saturation pressures from 1 to 3 MPa and heat fluxes from 8.3 to 28.2 kW/m2 over the entire range of vapor quality. The effects of heat flux, mass flux, saturation pressure and vapor quality were analyzed and discussed. In addition, the frictional pressure drops were compared with twenty-nine well known frictional pressure drop correlations. An improved frictional pressure drop correlation was deduced and predicted R14 experimental data well with a mean absolute relative deviation of 12.18%. Finally, 2317 data points from nineteen published literature were adopted to evaluate the predictive ability of this new model. It achieves a satisfactory predicting result with a mean absolute relative deviation of 17.39% and 87.26% of the experimental data within the deviation bandwidth of ±30%.