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.     

Heat transfer characteristics of falling film evaporation on horizontal tube arrays

A falling film heat transfer test facility has been built for the measurement of falling film evaporation in a vacuum of about 1000 Pa. At this condition, only convective evaporation occurred in the liquid film. The Reynolds numbers of falling film over a range from 21.6 to 108.1 were tested on six-tube arrays made of enhanced or smooth tubes. Results show that the tubes with both enhanced outer and inner surfaces give high heat flux. Besides, as the Reynolds number increases, the heat transfer enhancement ratio of falling film evaporation decreases. A semi-analytical correlation is established to predict the heat transfer coefficients of falling film evaporation on smooth tube arrays, considering the contributions of partially dryout and fully wet regimes, respectively. For enhanced tubes, the heat transfer enhancement ratios to the smooth tubes were also correlated.     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.     

Heat Transfer and Crisis Phenomena at the Film Flows of Freon Mixture over Vertical Structured Surfaces

Results on experimental investigation of heat transfer in the liquid films dichlorofluoromethane R21 and dichlorotetrafluoroethane R114 Freon mixture over the vertical tubes are presented. We have studied the film flow over the outer surface of tubes with 50-mm diameter and different configurations: smooth surface, horizontal ribs, and diamond-shape knurling. Heat transfer coefficients were measured under the conditions of evaporation and nucleate boiling together with wave characteristics of the falling film, binary mixture composition, and critical heat fluxes corresponding to dry spots formation. The film Reynolds number at the inlet to the test section was varied from 15 to 250. At evaporation regime the heat transfer coefficient for a smooth surface decreases classically with an increase of Reynolds number. Dependence of heat transfer coefficient on irrigation density for the surface with diamond-shape knurling is similar to dependence for the smooth surface with insignificant heat transfer intensification. The heat transfer coefficients at nucleate boiling for the studied structured surfaces are close to those obtained for the smooth tube. Development of critical phenomena is determined by regularities of dry spots formation typical for evaporation of the wavy liquid film.     

CO2不同螺纹管管外强化换热的实验研究

通过对CO2的物理特性及水平光管与不同螺纹管管外沸腾换热进行实验研究,得出了换热系数随蒸发压力和热流密度的变化关系。拟合得出CO2在蒸发压力的范围为2.6~3.6MPa、热流密度为10~50 kW.m-2的换热关联式h=A.qn。与Cooper预测值的偏差在±15%之内,与Ribatski关联式预测值的偏差在±7%之内,与Ye实验关联式预测值的偏差在±9%之内。在CO2在光管管外沸腾换热的基础上进一步研究其在螺纹管管外沸腾对换热的强化效果,为CO2强化换热进一步发展提供依据,具有一定工程实践意义。     

Modeling and numerical investigation of hydrogen nucleate flow boiling heat transfer

Liquid hydrogen flow boiling heat transfer in tubes is of great importance in the hydrogen applications such as superconductor cooling, hydrogen fueling. In the present study, a numerical model for hydrogen nucleate flow boiling based on the wall partition heat flux model is established. The key parameters in the model such as active nucleation site density, bubble departure diameter and frequency are carefully discussed and determined to facilitate the modeling and simulation of hydrogen flow boiling. Simulation results of the numerical model show reasonably well agreement with experimental data from different research groups in a wide operation condition range with the means absolute error (MAE) of 10.6% for saturated and 5.3% for subcooled flow boiling. Based on the model, wall heat flux components and void fraction distribution of hydrogen flow boiling are studied. Effects of mass flow rate and wall heat flux on the flow boiling heat transfer performance are investigated. It is found that in the hydrogen nucleate flow boiling, the predominated factor is the Boiling number, rather than the vapor quality. A new simple correlation is proposed for predicting hydrogen saturated nucleate flow boiling Nusselt number. The MAE between the correlation predicted and experimentally measured Nusselt number is 13.6% for circular tubes and 12.5% for rectangular tubes. The new correlation is applicable in the range of channel diameter 4–6.35 mm, Reynolds number 64000–660,000, saturation temperature 22–29 K, Boiling number 8.37 × 10^?5–2.33 × 10^?3.     

Heat transfer augmentation in 3D internally finned and microfinned helical tube

Experiments are performed to investigate the single-phase flow and flow-boiling heat transfer augmentation in 3D internally finned and micro-finned helical tubes. The tests for single-phase flow heat transfer augmentation are carried out in helical tubes with a curvature of 0.0663 and a length of 1.15 m, and the examined range of the Reynolds number varies from 1000 to 8500. Within the applied range of Reynolds number, compared with the smooth helical tube, the average heat transfer augmentation ratio for the two finned tubes is 71% and 103%, but associated with a flow resistance increase of 90% and 140%, respectively. A higher fin height gives a higher heat transfer rate and a larger friction flow resistance. The tests for flow-boiling heat transfer are carried out in 3D internally micro-finned helical tube with a curvature of 0.0605 and a length of 0.668 m. Compared with that in the smooth helical tube, the boiling heat transfer coefficient in the 3D internally micro-finned helical tube is increased by 40-120% under varied mass flow rate and wall heat flux conditions, meanwhile, the flow resistance is increased by 18-119%, respectively.     

Falling water film evaporation on newly-designed enhanced tube bundles

Experimental studies are presented on falling film evaporation of water on 6-row horizontal enhanced tube bundles in a vacuum condition. Turbo-CAB (19 fpi and 26 fpi), Korodense, and smooth tubes were tested in a range of film Reynolds number from about 10 to 110 and in the condition of only convective evaporation, without nucleate boiling. The flow modes and heat flux will affect the transition Reynolds numbers. Tubes with enhanced inner surface provide better heat transfer performance. Hotter heating water may lead to better heat transfer performance mainly due to higher heat fluxes. Correlations were also derived to predict the heat transfer coefficients and the enhancement ratio.     

The Effect of Tube Flattening on Flow Boiling Heat Transfer Enhancement

The present study investigated the effect of smooth tube flattening on heat transfer enhancement in an evaporator. The tubes with internal diameter of 8.7 mm were flattened into an oblong shape with different inside heights. The test setup was basically a vapor compression refrigeration system equipped with all necessary measuring instruments. Refrigerant R-134a flowing inside the tube was heated by an electrical coil heater wrapped around it. The ranges of mass velocities were from 74 to 106 kg/m²-s and vapor quality varied from 25% to 95%. Analysis of the collected data indicated that the heat transfer coefficient elevates by increasing the mass velocity and vapor quality in flattened tubes just like the round tube. The flow boiling heat transfer coefficient increases when the flattened tube is used instead of the round tube. The highest heat transfer coefficient enhancement of 172% was achieved for the tube with the lowest inside height at mass velocity of 106 kg/m²-s and vapor quality of 85%. Finally, based on the present experimental results, a correlation was developed to predict the heat transfer coefficient in flattened tubes.     

Heat Transfer Measurements inside Narrow Channels with Ribs and Trenches

Detailed heat transfer coefficient distributions are obtained for high aspect ratio (width/height = 12.5) duct with rib and trench enhancement features oriented normal to the coolant flow direction. A transient thermochromic liquid crystal technique has been used to experimentally measure heat transfer coefficients from which Nusselt numbers are calculated on the duct surface featuring heat transfer enhancement features. Reynolds number (calculated based on duct hydraulic diameter) ranging from 7100 to 22400 were experimentally investigated. Detailed measurements of heat transfer provided insight into the role of protruding ribs and trenches on the fluid dynamics in the duct. Experimentally obtained Nusselt numbers are normalized by Dittus-Boelter correlation for developed turbulent flow in circular duct. The triangular trenches provide heat transfer enhancement ratios up to 1.9 for low Reynolds numbers. The in-line rib configuration shows similar levels to the trench whereas staggered rib configuration provides heat transfer enhancement ratios up to 2.2 for a low Reynolds number of 7100.     

Interfacial heat transfer of condensing bubble in subcooled boiling flow at low pressure

The interfacial heat transfer coefficient is an important parameter for the analysis of multi-phase flow. In subcooled boiling flow, bubbles condense through the interface of phases and the interfacial heat transfer determines the condensation rate which affects the two-phase parameters such as void fraction and local liquid temperature. Thus, the present experiments are conducted to correlate the interfacial heat transfer coefficient at low pressure in the subcooled boiling flow. The local liquid temperature is measured by microthermocouple and the bubble condensation rate is estimated by orthogonal, two-image processing. The condensate Nusselt number, which is a function of bubble Reynolds number, local liquid Prandtl number, and local Jacob number, is obtained from the experimental results. The bubble history is derived from the newly proposed correlation and the condensate Nusselt number is compared with the previous models.     

Two-Phase Flow and Boiling Heat Transfer in Small-Diameter Tubes

For the development of a high-performance heat exchanger using small channels or minichannels for air-conditioning systems, it is necessary to clarify the characteristics of vapor‐liquid two-phase flow and heat transfer of refrigerants in small-diameter tubes. In this keynote paper, the related research works that have already been performed by the author and coworkers are introduced. Based on the observations and experiments of R410A flowing in small-diameter circular and noncircular tubes with hydraulic diameter of about 1 mm, the characteristics of vapor‐liquid two-phase flow pattern and boiling heat transfer were clarified. In low quality or mass flux and low heat flux condition, in which the flow was mainly slug, the “liquid film conduction evaporation” heat transfer peculiar to small-diameter tubes prevailed and exhibited considerably good heat transfer compared to nucleate boiling and forced convection evaporation heat transfer. The effects of the tube cross-sectional shape and flow direction on the heat transfer primarily appeared in the region of the “liquid film conduction evaporation” heat transfer. A new heat transfer correlation considering all of three contributions has been developed for small circular tubes.     

Boiling heat transfer correlations for refrigerant mixtures flowing inside micro-fin tubes

Based on experimental results of ternary non-azeotropic refrigerant mixture R417A flowing and boiling in one smooth and two internally grooved horizontal tubes with different geometrical parameters, a boiling heat transfer correlations was developed for refrigerant mixtures flowing inside micro-fin tubes by applying the enhancement factor in the present modified-Kattan model which was modified by the experimental data of R417A in a smooth tube. The comparison between the calculation and the experimental results indicates that the prediction by the present correlations is in good agreement with the experiment of refrigerant mixtures inside different micro-fin tubes with a standard deviation of ± 30% for vapor qualities below 80%.     

Influence of gas/liquid injection on two‐phase flow convective boiling of refrigerant mixtures

An analytical study on the influence of gas/liquid injection on heat transfer characteristics of two‐phase flow boiling of some refrigerant mixtures in horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of the gas/liquid injection on the heat transfer characteristics such as average heat transfer coefficient of R‐507, R‐404A, R‐410A and R‐407C in two‐phase flow boiling inside enhanced surface tubing. The data also revealed that gas/liquid injection is beneficial at certain gas/liquid injection ratios to the heat transfer coefficient depending upon the Reynolds number and the boiling point. It was also evident that the proposed correlations and the experimental data that the gas/liquid injection has significant impact on the heat transfer coefficient. In addition, the proposed correlations were applicable to the entire heat and mass flux, investigated in the present study under gas/liquid injection conditions. The deviation between the experimental and predicted under gas/liquid injection were less than ±20, for the majority of data. Copyright © 2004 John Wiley & Sons, Ltd.     

Heat transfer enhancement in dimpled tubes

Heat transfer enhancement was investigated in a coaxial-pipe heat exchanger using dimples as the heat transfer modification on the inner tube. Tube-side Reynolds numbers were in the range of 7.5×10³–5.2×10⁴ for water flow. A constant annular mass flow rate was chosen to obtain the highest possible Reynolds number of 1.1×10⁴. Typically, the heating water inlet temperature was 68.1±0.1^∘C.All six variants with inward-facing, raised dimples on the inner tube increased the values of heat transfer coefficient significantly above those for the smooth tube. Heat transfer enhancement ranged from 25% to 137% at constant Reynolds number, and from 15% to 84% at constant pumping power. At a constant Reynolds number, the relative J factor (ratio of heat transfer coefficient to friction factor, relative to smooth tube values), had values from 0.93 to 1.16, with four dimpled tube configurations having values larger than unity. Despite the extremely simple design, this outperforms almost all heat transfer enhancements recommended in the literature. A correlation based on the results of the present work appears to be sufficiently accurate for predicting heat transfer coefficients and friction factors for the design of dimpled-tube heat exchangers.     

Enhanced boiling heat transfer in restricted spaces of a compact tube bundle with enhanced tubes

In flooded-type tube bundle evaporators with smooth tubes and general tube gaps, both wall superheat and heat flux are generally quite low and boiling cannot occur on the heated tubes. But when the tube gap is quite small or the enhanced heat transfer tubes are employed, the incipient boiling can occur at low heat flux levels and results in a significant heat transfer enhancement effect. This study investigates experimentally enhancement effects by the restricted space comprising the compact tube bundle and the enhanced tubes for boiling heat transfer of pure water and salt-water mixtures under atmospheric pressure. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer for the compact enhanced tube bundle.     

A numerical correlation development study for the determination of Nusselt numbers during boiling and condensation of R134a inside smooth and corrugated tubes

In this study, the closed form of artificial neural network method is used to have a reliable empirical correlation to estimate the measured Nusselt numbers of R134a flowing downward and horizontally inside smooth and corrugated copper tubes by means of some dimensionless numbers. R134a and water are used as working fluids flowing in the tube side and annular side of a double tube heat exchanger, respectively. The training sets have the experimental data of in-tube condensation and in-tube boiling tests including various mass fluxes and saturation temperatures of R134a. Inputs of the formula are the dimensionless numbers obtained from measured values of test section such as Froude number, Weber number, Bond number, Lockhart and Martinelli number, void fraction, the ratio of density to dynamic viscosity, liquid, vapor and equivalent Reynolds numbers, surface tension parameter and liquid Prandtl number, while the output of the formula is the experimental Nusselt numbers in the analysis. Nusselt numbers of R134a are modeled using closed form of multi-layer perceptron (MLP) method of artificial neural network (ANN). Analyses of the ANN method are accomplished by means of 1177 data points. The performance of the closed form of multi-layer perceptron (MLP) with three inputs and one hidden neuron architecture was found to be in good agreement, predicting the experimental Nusselt numbers with their deviations being within the range of ± 30% for all tested conditions. Empirical correlations are proposed for both condensation and boiling flows separately. A single empirical correlation is found to be capable of predicting the experimental Nusselt numbers of both condensation and boiling flows together. Dependency of output of the ANNs from input values is also investigated in the paper. Vapor Reynolds number, equivalent Reynolds number, Weber number and Froude number are found to be the most affective parameters as a result of the dependency analyses.     

A generalized numerical correlation study for the determination of pressure drop during condensation and boiling of R134a inside smooth and corrugated tubes

The measured pressure drop of R134a, flowing downward and horizontally inside smooth and corrugated copper tubes, is estimated by the closed form of artificial neural network method to have a reliable empirical correlation using some dimensionless numbers. The working fluids are R134a and water flowing in the test tube and annular tube, respectively. This paper is a continuation of the authors' previous work and includes all their previous works about condensation and boiling in tubes. All data used in the present paper are obtained from the authors' previous studies. The training sets have the experimental data of convective condensation and boiling experiments including various mass fluxes and saturation temperatures of R134a. Froude number, Weber number, Bond number, Lockhart and Martinelli number, void fraction, the ratio of density to dynamic viscosity, liquid, vapor and equivalent Reynolds numbers, surface tension parameter and liquid Prandtl number are the inputs of the formula as the dimensionless numbers obtained from measured values of test section, while the output of the formula is the measured pressure drops in the analysis. A closed form of multi-layer perceptron (MLP) method of artificial neural network (ANN) is used to estimate the experimental pressure drop of R134a numerically. 1177 data points are used in the analyses of the ANN method to be able to have a single generalized empirical correlation for both condensation and boiling flows. The evaluation of the closed form of multi-layer perceptron (MLP) with two or three inputs and one hidden neuron architecture was successful predicting the measured pressure drops with their error bands being within the range of ± 30% for all used data. The proposition of empirical correlations are performed for both condensation and boiling flows separately. A single empirical correlation is able to calculate the measured pressure drop of both condensation and boiling flows together. Moreover, the dependency of output of the proposed formula from input values is examined in the study. By means of the dependency analyses, liquid Prandtl number, Butterworth's void fraction and Lockhart and Martinelli parameter are found to be the most dominant parameters among other dimensionless numbers.     

表面微结构对流动沸腾中核沸腾抑制因子的影响

用两根内表面微结构不同的水平光滑管环状流区流动沸腾换热实验数据，采用叠加模型分别建立了流动沸腾换热关系式，并比较它们的抑制因子。结果表明，表面微结构对抑制因子有显的影响；当表面的平均凹腔半径较大时，抑制因子明显增大。表明表面微结构改变对流动沸腾换热能起到较好的强化作用。