Heat Transfer In Horizontal Tube Research Articles

Experimental investigation of non-uniform heating effect on flow and heat transfer of supercritical carbon dioxide :An application to solar parabolic trough collector

In order to reduce the carbon dioxide emission, the supercritical carbon dioxide (sCO2) Brayton cycle is a good choice to convert solar energy into power using solar parabolic trough collectors (PTCs). Because the earth rotation induced non-uniform flux distribution threatens the safe operation of absorber tubes, we investigate the flow and heat transfer of sCO2 in a horizontal tube under non-uniform heating, with the mass fluxes, average heat fluxes and pressures in the ranges of (397∼1244) kg/m2s, (99∼413) kW/m2, and (7.5∼15.7) MPa, respectively. On the contrary to the classical treatment of supercritical fluid (SCF), SCF is treated in a multiphase framework. It is found that, compared with the sCO2 heat transfer in horizontal tubes under uniform heating, the non-uniform heating not only enhances heat transfer, but also increases friction factors. Besides, the non-uniform heating weakens the stratification effect, but the difference of heat transfer between the top and bottom generatrixes cannot be completed eliminated. It is observed that the larger vapor-like-layer thicknesses on tube walls may switch the sCO2 heat transfer from normal heat transfer (NHT) to heat transfer deterioration (HTD). Three heat transfer mechanisms are commented by using the dimensionless numbers regarding the pseudo-boiling in supercritical pressures.

Supercritical carbon dioxide heat transfer in horizontal tube based on the Froude number analysis

The heat transfer and pressure drop (ΔP) of supercritical fluid are crucial for the safety of advanced power cycles. Here, experiments were performed for sCO2 in a horizontal tube with inner diameter (din) of 10 mm, covering in the ranges of (G): 496.7–1346.2 kg/m2, heat flux (qw): 97.4～400.3 kW/m2 and pressure (P): 7.53–23.51 MPa. Inspired by the similarity of the outer wall temperature (Tow) distributions between the sCO2 flow and those of stratified-wavy-flow in subcritical pressures, the multiphase flow theory is introduced in subcritical pressures to deal with the sCO2 heat transfer. In the two-phase-like (TPL) regime, heat transfer coefficients (HTC) and friction factors (f) of sCO2 are found to significantly deviate from the correlations based on the single-phase flow theory. Further, the vapor-like Froude number FrVL is proposed to develop new correlations for both the heat transfer and the flow resistance covering the whole data range. The present paper establishes a connection between the flow resistance and heat transfer in supercritical pressures, which is important for the design and operation of sCO2 power cycles.

A universal correlation for flow condensation heat transfer in horizontal tubes based on machine learning

Accurate and universal prediction of in-tube condensation heat transfer coefficients (HTCs) is vital for designing compact condensers. This study presents machine learning (ML) methods for predicting flow condensing HTCs inside horizontal tubes based on an assembled database. The database contains 6064 experimental data of 28 pure fluids and covers broad operating conditions. Furthermore, the database is serviced to train and evaluate five ML models based on K-nearest neighbors (KNN), artificial neural network (ANN), convolutional neural network (CNN), random forest (RF), and Extreme gradient boosting (XGBoost) algorithms. The designed ML models show excellent predictive performances for 1213 test data points with the best mean absolute relative deviation (MARD) of 5.82% achieved by CNN and the coefficient of determination (R2) of 0.98 or higher for both XGBoost models. Based on the parametric importance analysis conducted by the trained XGBoost models, a new universal correlation is developed by incorporating several key parameters to characterize condensation heat transfer. This correlation shows reliable predictions for all data points in the consolidated database with the mean relative deviation (MRD) of −1.77% and MARD of 19.21%. An additional excluded database is collected to validate the universality of the new correlation and these ML models. It illustrates that ML models are effective predictive tools for HTCs, and can also help develop the heat transfer correlation with high accuracy and universality.

Heat transfer characteristics of supercritical water in a horizontal tube considering local temperature fields: An experimental study

A comprehensive understanding of heat transfer to supercritical fluids is essential to improve the thermal efficiency of some advanced thermal systems. However, the mechanism remains unclear that how the heat transfer is affected by local temperature, especially the near-wall temperature. In this study, heat transfer experiments of supercritical water are conducted in a horizontal tube. Five local water temperatures from the near-wall region to the core in a cross-section are measured to clarify the heat transfer process with the operating pressure of 23.5–26.5 MPa, mass flux of 200–400 kg·m–2 s–1, and heat flux of 50–180 kW·m–2. The tube length and inner diameter are 6000 mm and 26 mm, respectively. The results show that the heat transfer is suppressed and the enthalpy increase is no longer linear before the pseudo-critical point. Heat transfer is dominated by the relative magnitude of buoyancy and inertia force, which are affected by heat flux and mass flux, respectively. Moreover, the heat transfer process is divided into four parts, and the heat transfer deterioration occurs at the end of part II and restores in part III. Finally, a new correlation which is suitable for supercritical heat transfer in horizontal tubes with large diameters is developed based on the present experimental data.

Influence of porous inserts on flow boiling heat transfer in horizontal tubes

In this paper we want to assess the mechanisms by which metal porous inserts influence flow boiling heat transfer in horizontal tubes. For this purpose, measurement data of the local and circumferentially averaged heat transfer coefficient of CO2 flow boiling in horizontal tubes with two different porous inserts are presented. As inserts, different sponges (open-cell network structures) and a wire matrix element are chosen and compared with each other. Different operating conditions were investigated. The mass flux was varied in a range from 25 kg m−2 s−1 to 190 kg m−2 s−1 and the vapor quality from 10% to 100%. Saturation pressures of 12 bar, 19 bar and 26.5 bar and average heat fluxes up to 65 kW m−2 were realized. By comparing the local heat transfer coefficient with empty tube data, wetting and convective heat transfer can be identified as the main influencing factors. Thus, we can obtain guidelines for the development of a model to describe the heat transfer coefficient of two-phase flow in porous inserts. A model derived in this way describes the measurement data of flow boiling of CO2 in horizontal tubes with inserts with a high degree of accuracy. The model is based on established sub-models without any additional fitting parameter. More than 90% of all measured data are predicted with less than 30% deviation

Flow-pattern based prediction of flow boiling heat transfer in horizontal tubes with circumferentially varying heat flux

In order to describe the heat transfer coefficient during flow boiling in horizontal tubes with circumferentially varying heat flux, flow pattern-based models from the literature are validated and adapted using measurement data of the local and the circumferentially averaged heat transfer coefficient. Such boundary conditions occur when the wall temperature at the circumference is constant and the tube is partially wetted. The most important adaption is the calculation of the nucleate boiling contribution as a function of the local heat flux and not of the circumferentially averaged heat flux. Further adjustments concern the heat transfer coefficient in the dryout regime and during wavy flow. The database used to validate the method encompasses heat transfer coefficients of flow boiling of CO2 at saturation pressures from 10 bar to 60 bar, mass fluxes from 25 kg m-2 s-1 to 300 kg m-2 s-1, local heat fluxes up to 230 kW m-2, and vapor qualities from 10 % to 99 %. By applying the proposed adjustments, the quality of the prediction of the data is improved from 56 % to more than 80 % of the data within an error interval of ± 30 %. In addition, the comparison of local heat transfer coefficients provides a validation of the basic structure of the model and identifies the potential and the need for further refinement.

Experimental study of condensation heat transfer for R141b in intermittent flow regime within a smooth horizontal tube

This study investigates condensation of R141b in an 8 mm ID horizontal smooth tube. Condensation of R141b is conducted at an average saturation temperature of 40°C with mass fluxes ranging from 40 to 400 kg m−2 s−1. The plant runs with low vapor quality of refrigerant where intermittent flow is expected to be the dominant flow regime. Visualization images presented for intermittent flow regime. Observation results compared with four types of flow regime map and a modified version of El Hajal-Thome map is presented. Condensation heat transfer coefficient in intermittent zone is evaluated. Comparison between the available experimental data and the values predicted by the most popular and current correlations for condensation heat transfer in horizontal tubes are reported. Finally, a new correlation is proposed to predict condensation heat transfer coefficient within intermittent region.

Effect of different magnetic field distributions on laminar ferroconvection heat transfer in horizontal tube

The forced convection heat transfer of ferrofluid steady state laminar flow through a circular axisymmetric horizontal pipe under different magnetic field is the focus of this study. The pipe is under constant heat flux while different linear axial magnetic fields were applied on the ferrofluid with equal magnetic energy. In this scenario, viscosity of ferrofluid is temperature dependent, to capture ferrofluid real behavior a nonlinear Langevin equation was considered for equilibrium magnetization. For this purpose, the set of nonlinear governing PDEs was solved using proper CFD techniques: the finite volume method and SIMPLE algorithm were used to discretize and numerically solve the governing equation in order to obtain thermohydrodynamic flow characteristics. The numerical results show a promising enhancement of up to 135.7% in heat transfer as a consequence of the application of magnetic field. The magnetic field also increases pressure loss of up to 77% along the pipe; but effectiveness (favorable to unfavorable effect ratio) of the magnetic field as a performance index economically justifies its application such that higher magnetic field intensity causes higher effectiveness of up to 1.364.

The phase separation concept condensation heat transfer in horizontal tubes for low-grade energy utilization

This paper reports condensation heat transfer in a horizontal tube with a 14.81 mm inside diameter and a 1200 mm heat transfer length, by using the phase separation concept. The hollow mesh cylinder was formed by packaging two layers of mesh screen surface. The outer layer had the pore size of 76 μm. This invention constructs a multiscale condenser tube. The modulation of stratified flow and annular flow was focused on. Liquid was held within the mesh cylinder to increase the vapor covered tube wall surface to enhance the heat transfer for stratified flows. For annular flows, liquid droplets in the vapor core were captured by the mesh screen surface to increase the vapor void fractions near the tube wall, enhancing the hat transfer. The measurements showed that the heat transfer coefficients could be more than two times of those in the bare tube. The total thermal resistance was decreased by 45.6%, maximally. The heat transfer enhancement ratios were increased with the vapor mass fluxes, Gxin, for the condensation flow along the whole tube length. A pressure analysis explained the increased heat transfer trend. The increment of heat transfer enhancement ratios was suppressed with a liquid flow part in the tube.

Analysis of Annular and Stratified Flows for Vapor Condensation Heat Transfer in Horizontal Tubes

The greater part of the horizontal condenser tube is occupied by the stratified and annular flows, which play important roles in condensation heat transfer coefficients. Both a volume of fluid (VOF) interface tracking method and k-ε two equations model was applied to analyze the characteristics of the stratified and annular flows for horizontal tubes, obtaining distribution of velocity, contours of both temperature and local condensation heat transfer coefficients. The computation shows that an annular flow having thinner liquid film attached on the inner surface of tubes appears at the inlet of the horizontal condenser tube because vapor exerts a higher interfacial shear stress on the gas-liquid interface, therefore, there are a higher local condensation heat transfer coefficients there. Next, as vapor quality decreases and condensate gravity increases along the condenser tube length, the condensation flow pattern transforms from the annular flow into the stratified flow in which local condensation heat transfer coefficients decreases and distributes unevenly along the circumference as liquid film at the bottom of the condenser tube become thicker. In addition, in the stratified flow, a wave structure is formed in the middle of the condensate pool at the bottom of the condenser tube because condensate on the top of the condenser tube slides into the bottom of the condenser tube and collapse each other. The heat transfer rate calculated by the present method is compared to those predicted from a Shah correlation, the agreement is found to be good, and both errors is within 7%.

Experimental investigation of ice slurry heat transfer in horizontal tube

Heat transfer of ice slurry flow based on ethanol–water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 °C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus–Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. It was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus–Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results.

Evaporation of refrigerants in a horizontal tube: an improved flow pattern dependent heat transfer model compared to ammonia data

Few experimental test data are available for evaporation of ammonia inside tubes and numerous new data have been measured and presented here. An improved approach to the prediction of flow boiling heat transfer in horizontal tubes has been proposed through the study of each flow pattern separately, incorporating a new criterion defining the onset of nucleate boiling as a function of the critical convective heat transfer coefficient representative of the location where nucleate boiling might occur. A new function, based on a pseudo-Biot number delineates two different mean heat fluxes on the perimeter of the tube in stratified types of flow, one in contact with the liquid and one in contact with the vapor. Considering pure convective heat transfer, or mixed convective and nucleate heat transfer, this division allows the use of a common criterion to be applied to each flow pattern. Even if the database showed that the flow conditions in the annular liquid film were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points where purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database of refrigerants HFC-134a and ammonia. Due to the precision of the new flow pattern map and the effectiveness of the onset on nucleate boiling criterion, this new heat transfer model accurately predicts the heat transfer conditions during evaporation.

Heat transfer in horizontal tubes during two phase natural circulation with presence of noncondensing gas

The condensation process of steam inside horizontal tubes during natural circulation gains in importance regarding the reactor safety research for existing and future nuclear power plants. Experimental investigations due to the condensation process were realized with the rig HORUS to study the behaviour of water-steam-gas mixtures in horizontal tubes. The paper includes statements regarding the flow and heat transfer conditions inside the tube and the temperature distribution inside the small tube wall. The experiments showed a blockade of the heat transfer area with Nitrogen which is connected with an increasing primary pressure followed by a compression of the Nitrogen and a reentry of steam into the tube. The experiments serve for the creation of an experimental data base. A model development for calculation of the heat transfer is described. The model was implemented in the German thermal-hydraulic code ATHLET. The comparison of calculated data and the measured parameters of HORUS rig show the code improvement for the simulation of noncondensing gases.

Prediction Methods for Entry Length Heat Transfer by Combined Laminar Convection in Horizontal Tubes

Numerical prediction methods for calculating velocity and temperature distributions in heated ducts can be made accurately but are time consuming. The paper shows possible simplifications, including the neglect of the buoyancy term, and the resultant accuracy attained. The case studied is for laminar flow, entry length heat transfer in horizontal tubes with constant rate heat flux. Comparison is made between experimental results, a full solution and an approximation based on a series of truncated versions of the fully developed temperature profile. Calculations are made both with and without variation of thermophysical properties with temperature.

Evaporating Heat Transfer in Horizontal Internal Spiral-grooved Tubes in the Region of Low Flow Rates

This paper presents some experimental data about evaporating heat transfer in horizontal tubes about 4.75mm I.D. with small internal spiral grooves using R efrigerant R12. The groove inclinations of the tested internal spiral-grooved tubes are 4, 7, 15 and 30 degrees. It is found that a spiral-grooved tube can increase the evaporating heat transfer coefficient in tubes from one and a half times to twice as much as that of a smooth tube for annular flow region, and about ten times due to capillary phenomenon in grooves for stratified flow region.

Two phase heat transfer in horizontal tube with jet mixing

The paper deals with experimental studies on heat transfer coefficients in two phase cocurrent flow in a horizontal tube where liquid jet momentum has been used for gas-liquid dispersion and heat transfer. Empirical equations have been developed to predict augmentation ratio of heat transfer as a function of various physical and dynamic variables of the system.

Forced Convective Heat Transfer in Horizontal Tubes : 2nd Report, Theoretical Study on the Effect of Buoyancy

Forced Convective Heat Transfer in Horizontal Tubes : 2nd Report, Theoretical Study on the Effect of Buoyancy

Forced Convective Heat Transfer in Horizontal Tubes : 1st Report, Experimental Study on the Effect of Buoyancy

In forced convective heat transfer in a horizontal straight pipe, buoyancy is considered to cause a serious difference between theoretical and experimental results. This report is devoted to the clarify the effect of buoyancy on this problem by accurate experiment on fully developed flow of air with constant heat flux. Velocity and temperature distributions are measured for large ReRa and these are shown to be essentially different from those known so far, for example, Poiseuille profile for velocity field. Nusselt numbers are calculated from measured velocity and temperature distributions and shown to be about twice as large as those calculated by neglecting buoyancy at ReRa=4×105. In the turbulent region, buoyancy gives little effect on velocity and temperature fields, but the critical Reynolds number is experimentally proved to be affected by buoyancy

The effect of natural convection on viscous-flow heat transfer in horizontal tubes

Experimental work dealing with the laminar flow of relatively non-viscous Newtonian liquids through a horizontal jacketed tube is described. The extent of the heat transfer by natural convection differs from that previously predicted [7, 8] and data from the literature are used in order to further examine the effect. It is shown that when the group ( Gr m Pr m D/L) is taken as the factor controlling natural convection, the information obtained in tubes of different dimensions is inconsistent. Better agreement is obtained when the ratio D/L is omitted from the group and further improvement results from the incorporation of the ratio L/D, all the data being adequately represented by the equation ▪ This equation becomes inaccurate when Gz m < π Nu am and it should be mentioned that the power of L/D is only provisional. Finally, the equations for heat transfer to fluids in both vertical and horizontal tubes are discussed with reference to the probable flow patterns obtained, and possible means of improving the form of the equations briefly described.