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。     

Effect of pressure, subcooling, and dissolved gas on pool boiling heat transfer from microporous, square pin-finned surfaces in FC-72

The present research is an experimental study of the effects of pressure, subcooling, and non-condensable gas (air) on the pool nucleate boiling heat transfer performance of microporous enhanced finned surfaces. The test surfaces, solid copper blocks with 1-cm² bases and 5×5 square pin-fin arrays of 2, 4 and 8 mm fin lengths, were immersed in FC-72. The test conditions included an absolute pressure range of 30-150 kPa and a subcooling range of 0 (saturation) to 50 K. Effects of these parameters on nucleate boiling and critical heat flux (CHF) were investigated. In addition, differences between pure subcooled and gas-saturated conditions as well as horizontal and vertical base orientations were also investigated. Results showed that, in general, the effects of pressure and subcooling on both nucleate boiling and CHF were consistent with previously tested flat surface results, however, subcooling was found to significantly affect the high heat flux region of the microporous finned surfaces nucleate boiling curves. The relative enhancement of CHF from increased subcooling was greater for the microporous surface than the plain surface but less than a microporous flat surface. The horizontal orientation (horizontal base/vertical fins) was found to be slightly better than the vertical orientation (vertical base/horizontal fins). Correlations for both nucleate boiling and CHF for the microporous surfaces were also developed.     

Nucleate pool-boiling enhancement outside a horizontal bank of twisted tubes with machined porous surface

Nucleate pool boiling of refrigerants is of important application in the flooded evaporator of refrigeration and air-conditioning system. Many surface geometries involve machined porous surface have been adopted to enhance the nucleate pool boiling heat transfer of refrigerants. Nucleate pool-boiling performance of R134a and R142b outside a horizontal bank of twisted tubes with machined porous surface (T-MPS tubes) was investigated in this paper. The experimental results showed that the T-MPS tube bank could enhance boiling heat transfer evidently. The enhancement ratios of R134a from the T-MPS tube bank were 1.4–1.7 and the maximum enhancement ratio of R142b could reach up to 4.4. Analyzing the tube bank effects of boiling heat transfer for R134a and R142b, the overall trend showed that the boiling heat transfer performance of the T-MPS tube bank was inferior to that of single T-MPS tube slightly.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：part II—Analytical Model for Boiling Heat Transfer of Binary Mixtures on Smooth Tubes and Comparison of Analytical Models for both Pure Liqu

A combined physical model of bubble growth is proposed along with a corresponding bubble growth model for binary mixtures on smooth tubes. Using the general model of Wang et al.^[1] and the bubble growth model for binary mixtures, an analytical model for nucleate pool boiling heat transfer of binary mixtures on smooth tubes is developed. In addition, nucleate pool boiling heat transfer of pure liquids and binary mixtures on a horizontal smooth tube was studied experimentally. The pure liquids and binary mixtures included water, methanol, ethanol, and their binary mixtures. The analytical models for both pure liquids and binary mixtures are in good agreement with the experimental data.     

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.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：Part Ⅱ－Analytical Modelfor Boiling Heat Transfer of Binary Mixtureson Smooth Tubes and Comparison of Analytical Models for both Pure Liquids and Binary Mixture

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A generalized correlation for evaporation heat transfer of refrigerants in micro-fin tubes

A generalized correlation for flow boiling heat transfer in horizontal micro-fin tubes was developed by implementing non-dimensional parameters accounting for heat transfer enhancement over smooth tubes and physical phenomena into the basic form of a smooth tube correlation. The enhancement factor in nucleate boiling consists of surface tension and turbulence effects generated by the liquid flow. A modified Reynolds number having some similarities with the roughness Reynolds number is introduced into the model to estimate heat transfer enhancement in convective boiling. The ratio of the liquid film thickness to the fin height is also employed in the correlation. The database of the present correlation includes 749 data points for five different refrigerants. The present correlation has a relatively simpler form to employ and yields closer fit to the experimental data with a mean deviation of 20.5% as compared to the existing correlations in the literature.     

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.     

Enhancement of boiling heat transfer in restricted spaces in compact horizontal tube bundles

ln desalinization devices and some heat exchangers making use of low‐quality heat energy, both wall temperatures and heat fluxes of heated tubes are quite low and generally cannot cause boiling in flooded‐type tube bundle evaporators with a large tube spacing. But when the tube spacing is very small, boiling in restricted spaces can occur and induce a higher heat transfer than that of a falling film or pool boiling at the same heat flux. This study investigated experimentally the effects of tube spacing, positions of tubes, and heating status of tubes as well as surface status (smooth and roll‐worked) on boiling in restricted spaces in compact horizontal tube bundle evaporators under atmospheric pressure. The experimental results provide a restricted space boiling database for water in smooth and enhanced surface tube bundles. Of particular importance is information concerning the influence of tube spacing of flooded‐type tube bundle evaporators, especially for the case of zero pitch, when the neighboring tubes are contacting each other. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 394–401, 2001     

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.     

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

Pool boiling heat transfer in vertical annular crevices

Effects of vertical annuli on nucleate pool boiling heat transfer of water at atmospheric pressure have been obtained experimentally. Experiments were performed for annuli with a height of 570 mm and gap sizes of 3.9 and 15 mm. Through the tests, tube bottom confinement (open or closed) has been investigated, too, and the whole results are compared with a single unconfined tube. According to the results, the annular condition gives much increase in heat transfer coefficients at moderate heat fluxes. Its effect is observed much greater for the bottom-closed tube condition.     

A nucleate boiling limitation model for the prediction of pool boiling CHF

A nucleate boiling limitation model which is applicable to the heat transfer prediction in the nucleate boiling region and the CHF was proposed for a pool boiling. The present model was developed based on the direct observations of the physical boiling phenomena. The predicted boiling curves for the nucleate boiling region agree well with both the vertical and the horizontal surface data for all the contact angles. The predicted CHF for the vertical surface also agrees well with the experimental data, but the present model underpredicts the CHF by about 30% for the horizontal surface data.     

Experimental Investigations on Boiling Heat Transfer Inside Miniature Circular Tubes Immersed in FC-72

To investigate the size effect on the characteristics of boiling heat transfer, boiling behavior of FC-72 in heated vertical miniature circular tubes immersed in a liquid pool was experimentally studied. Two AISI 304 stainless steel tubes with inner diameters of 1.10 mm and 1.55 mm correspondingly, were heated by swirled Ni-Cr wire heaters and sealed in Lucite blocks by silicon adhesive. Both the top and the bottom ends of the circular test sections were open to the liquid pool. The boiling curves and heat transfer coefficients were obtained experimentally. The boiling behaviors at the outlets of the miniature tubes were also visualized with a digital video camera. Experimental results show that the tube geometry has a significant effect on the boiling characteristics. Vapor blocking at the outlet of the smaller circular tube with a diameter of 1.10 mm caused severe boiling hysteresis phenomena. The CHF decreased with reducing in tube size.     

Boiling heat transfer performance and phenomena of Al2O3-water nano-fluids from a plain surface in a pool

Boiling heat transfer characteristics of nano-fluids with nano-particles suspended in water are studied using different volume concentrations of alumina nano-particles. Pool boiling heat transfer coefficients and phenomena of nano-fluids are compared with those of pure water, which are acquired on a smooth horizontal flat surface (roughness of a few tens nano-meters). The experimental results show that these nano-fluids have poor heat transfer performance compared to pure water in natural convection and nucleate boiling. On the other hand, CHF has been enhanced in not only horizontal but also vertical pool boiling. This is related to a change of surface characteristics by the deposition of nano-particles. In addition, comparisons between the heat transfer data and the Rhosenow correlation show that the correlation can potentially predict the performance with an appropriate modified liquid-surface combination factor and changed physical properties of the base liquid.     

Effects of enhanced surfaces and surface orientation on nucleate and film boiling heat transfer in R-11

A study of pool boiling heat transfer in R-11 is reported as a function of surface characterization and orientation. Two specially prepared metal coated surfaces (UNB#1, UNB#2) and a flat copper surface were subjected to heat fluxes up to 180 kW m⁻² with surface orientations varying from horizontally facing upward (0°), to vertical (90°), to horizontally facing downward (180°). The resulting nuleate boiling curves display considerable boiling hysteresis and enhanced surfaces show 2–3 times better heat transfer than a plain surface. Rohsenow's nucleate boiling equation is used to correlate the data and modified to account for the effects of surface characterization and orientation. In film boiling, enhanced surfaces also reveal better heat transfer characteristics and the role of surface orientation on the motion and stability of the vapor film is clarified.     

紧凑满液型叉排管束蒸发换热器内水的沸腾强化换热特性

采用紧凑满液型蒸发换热器,利用水平传热管叉排管束狭窄空间内早期沸腾强化换热机理将中小热负 荷条件下的自然对流换热转化为旺盛核沸腾换热,换热性能大大优于传统的降膜式蒸发换热器。对水平传热管 管束在受限空间内沸腾强化换热进行实验研究,确认了紧凑满液式水平管蒸发换热器具有良好的换热性能,传 热管在管束中的位置对换热特性已经没有明显影响,随着压力增加,受限空间内沸腾强化换热强化效果显著增 加。     

CaCO3垢的形成对池核沸腾传热的影响

在一水平圆形加热表面上通过实验考察了饱和池核沸腾和过冷池核沸腾时CaCO3垢的生成对传热的影响。结果表明，在饱和池核沸腾和过冷池核沸腾的初始阶段沸腾传热系数均呈先降低后升高、达到一个最大值后稳定降低的趋势，而且在初始阶段出现了负污垢热阻现象。在相同操作条件下，过冷池核沸腾传热系数明显低于饱和池核沸腾传热系数。在分析污垢的生成和生长影响表面活化中心的基础上，对污垢的形成对沸腾传热的影响进行了机理分析。     

Effects of heater size and working fluids on nucleate boiling heat transfer

The present study is an experimental investigation of nucleate boiling heat transfer mechanism in pool boiling from wire heaters immersed in saturated FC-72 coolant and water. The vapor volume flow rate departing from a wire during nucleate boiling was determined by measuring the volume of bubbles from the wire utilizing the consecutive-photo method. The effects of the wire size on heat transfer mechanism during a nucleate boiling were investigated, varying 25 μm, 75 μm, and 390 μm, by measuring vapor volume flow rate and the frequency of bubbles departing from a wire immersed in saturated FC-72. One wire diameter of 390 μm was selected and tested in saturated water to investigate the fluid effect on the nucleate boiling heat transfer mechanism. Results of the study showed that an increase in nucleate boiling heat transfer coefficients with reductions in wire diameter was related to the decreased latent heat contribution. The latent heat contribution of boiling heat transfer for the water test was found to be higher than that of FC-72. The frequency of departing bubbles was correlated as a function of bubble diameters.     

Behavioral study of alumina nanoparticles in pool boiling heat transfer on a vertical surface

Experiments were carried out to investigate the pool boiling of alumina‐water nanofluid at 0.1 g/l to 0.5 g/l of distilled water, and the nucleate pool boiling heat transfer of pure water and nanofluid at different mass concentrations were compared at and above the atmospheric pressure. At atmospheric pressure, different concentrations of nanofluids display different degrees of deterioration in boiling heat transfer. The effect of pressure and concentration of nanoparticles revealed significant enhancement in heat flux and deterioration in pool boiling. The heat transfer coefficient of 0.5 g/l alumina‐water nanofluid was compared with pure water and clearly indicates deterioration. At all pressures the heat transfer coefficients of the nanofluid were lower than those of pure water. Experimental observation revealed particles coating over the heater surface and subsequent SEM inspection of the heater surface showed nanoparticles coating on the surface forming a porous layer. To substantiate the nanoparticle deposition and its effect on heat flux, investigation was done by measuring the surface roughness of the heater surface before and after the experiment. While SEM images of the heater surface revealed nanoparticle deposition, surface roughness of the heater surface confirmed it. Based on the experimental investigations it can be concluded that an optimum thickness of nanoparticles coating favors an increase in heat flux. Higher surface temperature due to the presence of nanoparticles coating results in the deterioration of boiling heat transfer. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20365