单头“W”形螺旋槽管传热与流体力学特性研究

对不同结构参数的6根铜制“W”形螺旋槽管中的湍流摩擦阻力与换热犊 性进行了实验研究，将大量的实验数据进行了回归，分析了影响“W”形螺旋槽管传热与流阻性能的主要因素，得到了具有较高精度的用于“W”形螺旋槽管的摩擦系数及传热系数的统计关系联式，研究结果为换热器设计及改造提供了理论依据。     

Turbulent heat transfer in a horizontal helically coiled tube

An experiment has been conducted in detail to study the turbulent heat transfer in horizontal helically coiled tubes over a wide range of experimental parameters. We found that the enhancement of heat transfer in the coils results from the effects of turbulent and secondary flows. With Reynolds number increasing to a high level, the contribution of the secondary flow becomes less to enhance heat transfer, and the average heat transfer coefficient of the coil is closer to that in straight tubes under the same conditions. The local heat transfer coefficients are not evenly distributed along both the tube axis and the periphery on the cross section. The local heat transfer coefficients on the outside are three or four times those on the inside, which is half of the average heat transfer. A correlation is proposed to describe the distribution of the heat transfer coefficients at a cross section. The average cross-section heat transfer coefficients are distributed along the tube axis. The average value at the outlet section should not be taken as the average heat transfer coefficient. © 1999 Scripta Technica, Heat Trans Asian Res, 28(5): 395–403, 1999     

Experimental investigation of in-line tube bundle heat transfer process to vertical downward foam flow

Usage of two phase flow as a coolant allows to reach more intensive heat transfer process. This paper presents the results of the investigation of the heat transfer between the in-line tube bundle and statically stable aqueous foam flow. The flow of the foam was generated at the top of the experimental channel and crossed down the tubes of the bundle (the diameter of the tubes was 0.02 m; horizontal and vertical distances between the centers of the tubes – 0.03 m). This investigation allowed determining the dependence of the heat transfer intensity of the tubes to foam flow on the volumetric flow rate and volumetric void fraction of the foam. The influence of tube position in the bundle on the intensity of heat transfer was found in this experimental study: the intensity of the heat transfer between the middle tube and the downward foam flow was determined to be higher comparing to the intensity between the tubes in both sideways of the bundle and the downward foam flow. The results of investigation showed that an average heat transfer intensity of the tubes of in-line bundle to downward without any turning foam flow was higher than that in the case of the downward after 180° turning foam flow. The results of this experimental study were generalized using the empirical equation between heat transfer coefficient from one side and the volumetric flow rate and volumetric void fraction of the foam from the other side.     

Augmentation of heat transfer during filmwise condensation of steam and R-134a over single horizontal finned tubes

An experimental investigation has been carried out to augment the heat transfer rate by enhancing the heat transfer coefficient during the condensation of pure vapours of steam and R-134a over horizontal finned tubes. The study was conducted for plain tubes, circular integral-fin tubes (CIFTs), spine integral-fin tubes (SIFTs) and parially spined circular integral-fin tubes (PCIFTs). The SIFT out performed the CIFT for the condensation of R-134a by approximately 16%. However, the spines were found most effective in the bottom side of the CIFT. The PCIFTs with the spines only in the bottom side of the tube augmented the heat transfer coefficient by 20% and 11% for the condensation of steam and R-134a, respectively, in comparison to the CIFT.     

Heat transfer characteristics of SA-178 boiler tubes covered with erosion resistance coatings

An experimental investigation has been conducted to determine the heat transfer characteristics of SA-178 boiler tubes coated with various erosion resistant coatings. Four tubes were tested to gather data for this investigation. The three outer surface coatings employed consisted of a 0.20 mm ceramic coating, a 0.70 mm black plasma coating, and a 0.70 mm metallic coating. The improved performance of two of the coated tubes over that of the uncoated tube indicated that the addition of material to the outer surface of these tubes did not decrease the tubes overall heat transfer coefficient. This would be expected if conduction were in fact the sole heat transfer mode experienced by the tubes. Radiation heat transfer in conjunction with the outer surface finish of the tubes were the dominant factors leading to the improved heat transfer capability of the ceramic and black plasma coated tubes.     

Performance evaluation of flattened tube in boiling heat transfer enhancement and its effect on pressure drop

An experimental investigation has been carried out to study heat transfer and pressure drop characteristics of R-134a flow boiling inside a horizontal plain tube and different flattened tubes. Round copper tubes with an inner diameter of 8.7 mm were flattened into an oblong shape with an internal height of 6.6 mm, 5.5 mm, 3.8 mm, and 2.8 mm. The test apparatus was basically a vapor compression refrigeration system equipped with all necessary measuring instruments. Analysis of the collected data showed that, by flattening the round tube, the heat transfer coefficient and pressure drop increased simultaneously. The performance of these tubes from the point of view of heat transfer enhancement and pressure drop increasing were evaluated. It was concluded that, the tube with an internal height of 5.5 mm has the best performance compared with the other flattened tubes. Finally, based on the present experimental pressure drop data, a correlation was developed to estimate the pressure drop in flattened tubes. This correlation predicts the experimental data of the present study within an error band of ± 20%.     

椭圆管内对流换热与流动阻力特性研究

通过CFD技术,分别对5种长短轴之比的椭圆管管内湍流和层流状态时的换热与流动进行数值研究,分析了流体流动状态和椭圆管长短轴之比对换热系数与流动阻力的影响,并根据数值计算结果拟合出湍流区椭圆管管内换热系数的准则关系式,最后绘制每种类型椭圆管的局部换热系数曲线。研究结果表明:数值计算结果与实验值吻合良好;采用当量直径的方法计算椭圆管内换热系数误差较大;随着雷诺数的增加,每种类型的椭圆管管内阻力系数逐渐减小;而在相同的雷诺数下,随着长短轴之比K的增大,管内阻力系数逐渐增加;每种类型的椭圆管具有类似的局部换热特性,即长半轴两端点处局部换热系数最低,而短半轴两端点处具有最大局部换热系数。     

An Experimental Study on the Air-Side Heat Transfer Coefficient and the Thermal Contact Conductance in Finned Tubes

The objective of this experimental study is to evaluate the heat transfer coefficient outside a tube with annular transverse fins, derived from strips of copper mechanically bound and coupled outside. Water is used as the heating medium, in turbulent conditions and flowing at different temperatures inside the tube. Petukhov's correlation has been selected to calculate the water heat transfer coefficient in the tube. The experimental data obtained are compared with a correlation from literature, and a similar trend is observed. A fitting of the data provides a correlation for the three tubes of different external diameter (30 mm, 22 mm, and 15.6 mm) that agrees very well with the experimental values. The thermal contact conductance is identified as the main reason for the difference between data and the original Briggs and Young correlation. An estimation of the contact conductance between fins and tubes provides values between 3500 and 11000 W/m²-K, slightly increasing with the air Reynolds number (based on the external diameter of the tube), whose range is 2000 to 8000. The thermal contact resistance is estimated and its importance is confirmed, contributing 30 to 50% to the total air-side thermal resistance in the tubes used in the experiments.     

Numerical simulation of convective heat transfer between air flow and ceramic foams to optimise volumetric solar air receiver performances

Porous ceramic foams are used to achieve high performance in solar heat recovery systems. Understanding the convective heat transfer between the air flow and the ceramic foam is of great importance when optimising the volumetric air receiver. In this work, the convective heat transfer was numerically studied. The present approach was designed to compute the local convective heat transfer coefficient between the air flow and a porous ceramic foam. For that purpose, the energy balance and the flow inside the porous ceramic foam were solved. In addition, a detailed geometry of the porous ceramic foam was considered. The ceramic foams were represented by idealised packed tetrakaidecahedron structures. The numerical simulations were based on the three dimensional Reynolds-averaged Navier–Stokes (RANS) equations. A sensitivity study on the heat transfer coefficient was conducted with the porosity, velocity and mean cell size as parameters. Based on the numerical simulation results, a correlation for the volumetric local convective heat transfer coefficient between air and ceramic foams was developed. The resulting correlation covers a wide range of porosities, velocities, cell sizes and temperatures. The correlation results were compared with experimental data from the literature, and the comparison shows good agreement. The correlation is intended to be used in the design of volumetric solar air receivers.     

表面强化管外池沸腾传热特性研究

为实现节能降耗,开发了多种强化沸腾传热的高效换热管。以水为工质,在0.1MPa下对垂直光管、烧结多孔管和T槽管进行了池沸腾传热实验研究,并分析了沿管子轴向的温度分布。实验结果表明,烧结多孔管与T槽管能显著降低起始沸腾过热度、强化沸腾传热:烧结多孔管和T槽管的起始沸腾过热度比光管的低1.5K左右;烧结多孔管和T槽管的核态沸腾传热系数分别为光管的2.4～3.2倍和1.6～2.0倍。此外,烧结多孔管和T槽管能降低相同热流密度下的壁面温度,且有利于降低管子轴向的温差。     

A General Correlation for Heat Transfer During Dispersed-Flow Film Boiling in Tubes

A general correlation for heat transfer during film boiling in tubes is presented. It is based on the two-step model. It has been verified with data for nine fluids flowing up in tubes. The fluids include water, cryogens, refrigerants, and chemicals. The range of data includes pressures from 1 to 215 bar, reduced pressures from 0.0046 to 0.97, mass velocities from 4 to 5,176 kg/m² s, tube diameters from 1.1 to 24.3 mm, and qualities from 0.1 to 2.4. The 546 data points are predicted with a mean deviation of 15.2%. Deviation is defined as the difference between the measured and predicted heat transfer coefficients divided by the measured heat transfer coefficient, the heat transfer coefficients being based on the saturation temperature. Three other well-known correlations are also compared to the same data and found to have much larger deviations. The correlation is also compared with a limited amount of data from horizontal tubes; the results are encouraging.     

Heat transfer during air flow in aluminum foams

This paper presents experimental heat transfer coefficients measured during air flow heating in seven different aluminum open-cell foam samples with different number of pores per inch (PPI), porosity and foam core height under a wide range of air mass velocity. Three imposed heat fluxes are considered for each foam sample: 25.0, 32.5 and 40.0 kW m^?2. The collected heat transfer data are analyzed to obtain the global heat transfer coefficient and the normalized mean wall temperature. A model from the open literature has been selected and compared with the experimental database. A new simple heat transfer model, based on the present data, for the global heat transfer coefficient and the foam-finned surface area efficiency estimations has then been developed and compared against the experimental measurements.     

A new set of correlations for EHD enhanced condensation heat transfer of tubular systems

An improved set of correlations for evaluating the heat transfer coefficient of EHD assisted condensation heat transfer for tubular systems is presented in this paper. The correlations were developed by curve fitting a wide range of experimental data and then optimising the empirical constants so that the percentage deviation is less than ±30%. The experimental data was classified in four categories: (I) condensation outside horizontal smooth tubes, (II) condensation inside horizontal smooth tubes, (III) condensation outside vertical tubes and (IV) condensation inside vertical tubes. The experimental data in each category included results obtained using various refrigerants and various electrode systems. The improved correlations were used to predict a total of 166 experimental data points.

A comprehensive literature review of all available correlations for EHD assisted condensation heat transfer is also described in this paper. Two correlations that were identified to be simple were used to predict the above-mentioned data bank. These are the correlations of Choi and Reynolds and Didkovesky and Bologa. The Choi and Reynolds correlation was found to successfully predict heat transfer rates of condensation outside horizontal and vertical tubes and of condensation inside vertical tubes when high intensity electric field is applied. However, the Didkovesky and Bologa correlation was found to successfully predict only the results of condensation heat transfer outside vertical surfaces.     

流体纵掠多孔泡沫金属换热器中顺列传热管外表面的试验研究

对壳程流体纵掠多孔泡沫金属中的传热管外表面的流动和传热进行了试验研究,用Brinkman-Forchheimer-extended Darcy流动模型和局部非热平衡传热理论对纵掠传热管外的对流传热进行了理论分析,最后对理论分析得到的结果进行验证和修正,得出了流体层流纵掠传热管外表面的对流传热平均努塞尔特数(Nussel...     

Pressure drop and thermal characteristics of CuO-base oil nanofluid laminar flow in flattened tubes under constant heat flux

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of nanofluid flow inside horizontal flattened tubes under constant heat flux. The nanofluid is prepared by dispersion of CuO nanoparticle in base oil and stabilized by means of an ultrasonic device. Nanofluids with different particle weight concentrations of 0.2%, 0.5%, 1% and 2% are used. Copper tubes of 11.5 mm I.D. are flattened into oblong shapes and used as test sections. The nanofluid flowing inside the tube is heated by an electrical heating coil wrapped around it. Required data are acquired for laminar and hydrodynamically fully developed flow inside round and flattened tubes.The effect of different parameters such as flow Reynolds number, flattened tube internal height and nanofluid particle concentration on heat transfer coefficient and pressure drop of the flow is studied. Observations show that the heat transfer performance is improved as the tube profile is flattened. Flattening the tube profile resulted in pressure drop increasing. In addition, the heat transfer coefficient as well as pressure drop is increased by using nanofluid instead of base fluid. Furthermore, the performance evaluation of the two enhanced heat transfer techniques studied in this investigation shows that applying flattened tubes instead of the round tube is a more effective way to enhance the convective heat transfer coefficient compared to the second method which is using nanofluids instead of the base liquid.     

Enhancement of boiling heat transfer in a 5 × 3 tube bundle

The results of an experimental investigation on nucleate boiling heat transfer in an electrically heated 5 × 3 in-line horizontal tube bundle under pool and low cross-flow conditions of saturated water near atmospheric pressure are presented here. It is observed that the heat transfer coefficient is minimum on bottom row tubes and increases in the upward direction with maximum values on top row tubes. Also, heat transfer coefficient on central column tubes was found to be slightly higher than those on the corresponding side tubes. Further, a Chen-type relation has been used to determine the local boiling heat transfer coefficient on a tube in a heated tube bundle.     

New theoretical models of evaporation heat transfer in horizontal microfin tubes

A stratified flow model and an annular flow model of evaporation heat transfer in horizontal microfin tubes have been proposed. In the stratified flow model, the contributions of thin film evaporation and nucleate boiling in the groove above the stratified liquid level were predicted by a previously reported numerical analysis and a newly developed correlation, respectively. The contributions of nucleate boiling and forced convection in the stratified liquid region were predicted by the new correlation and the Carnavos correlation, respectively. In the annular flow model, the contributions of nucleate boiling and forced convection were predicted by the new correlation and the Carnavos correlation in which the equivalent Reynolds number was introduced, respectively. The flow pattern transition curve between the stratified-wavy flow and the annular flow proposed by Kattan et al. was introduced to predict the heat transfer coefficient in the intermediate region by use of the two theoretical models. The predictions of the heat transfer coefficient compared well with available experimental data for ten tubes and four refrigerants.     

Thermal energy recovery from high-temperature blast furnace slag particles

Dry granulation is an attractive alternative to water quenching because of saving energy and reducing water consumption. In many dry granulation technologies, the high-temperature molten slag is atomized to slag particles, and the thermal energy from high-temperature slag particles is recovered by physical and chemical methods. In the present study, a technique for thermal energy recovery from high-temperature slag particles using a gravity bed waste heat boiler was exploited. The heat transfer characteristics of a laboratory-scale gravity bed waste heat boiler were investigated. An increase in Reynolds number (Re) showed no effect on the heat transfer coefficient or recovery efficiency. However, the heat transfer coefficient and recovery efficiency increased with a decrease in particle diameter and an increase in the velocity of descending particles. Compared with those observed with an aligned arrangement of the boiler tubes, the heat transfer coefficient and the recovery efficiency were higher in a staggered arrangement. The semi-empirical relations regarding the heat transfer coefficient between slag particles and boiler tubes were obtained based on experimental data and can be used to calculate the heat transfer coefficient within a certain range.     

Evaluation of the Heat Transfer Correlations for Supercritical Pressure Water in Vertical Tubes

A large number of studies have been carried out on the flow and heat transfer of supercritical pressure fluids in the past decades. However, there are still some uncertainties and deficiencies in the accurate prediction for supercritical fluid heat transfer coefficient due to the large and fast variations of fluids properties in the so-called pseudo-critical region. In this paper, 15 correlations were selected from the literature and were compared with each other to verify their capability in predicting heat transfer coefficient of supercritical pressure water in vertical tubes. Based on the comparison between the calculation results of the existing heat transfer correlations and the experimental data obtained from the open literature, it was found that the Swenson et al. correlation and the Hu correlation can reasonably predict the heat transfer coefficient of supercritical water in the pseudo-critical region. After evaluating these correlations, the authors conducted polynomial fitting for the collected experimental data and got a new correlation for heat transfer coefficient of water at supercritical pressures. The new correlation can fit well with the experimental data even in the neighborhood of pseudo-critical temperature.     

The impact of bed temperature on heat transfer characteristic between fluidized bed and vertical rifled tubes

In the present work, the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes (38mm-O.D.) in a commercial circulating fluidized bed (CFB) boiler. Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246mm. The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler. A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach. The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.