竖直矩形狭缝通道内环状流沸腾换热分析模型

对竖直矩形狭缝通道内有液滴卷吸环状流阶段流动沸腾进行分析。以液膜紊流的动量方程和能量方程为基础,加上相应的边界条件和使控制方程组封闭的经验关系式,建立了环状流的教学模型并进行数值计算,得到了矩形狭缝通道内的液膜厚度分布、沸腾传热系数等结果;将模型预测的换热系数同实验关系式作比较,最大相对误差为17．8％。     

Investigation on the characteristics of boiling heat transfer through narrow annular channels

Factors concerning the characteristics of boiling heat transfer are analyzed theoretically. Based on the experimental data of boiling heat transfer through annular channels with the gaps of 1–2 mm, three correlations which will be used to calculate the heat transfer in the similar conditions are given. The results obtained from these correlations are compared with experimental data. The main factors having influence on boiling heat transfer through narrow channels, and the desirable correlation are determined. This correlation can be used to predict the flow boiling heat transfer within the range of this experiment. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(2): 78–84, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20049     

Critical Heat Flux of Boiling Heat Transfer in a Moderate Narrow Space

INTanDUCTI0NBoilingheattransferandcriticalheatflux(CHF)inaconfinednarrowspacehavebeenstudiedexperi-melltallybyanumberofinvestigatorsinthepastfewdecades.However,thereisnoanypopularlyacceptedmodelintheheattransferinnarrowspaceboiling,althoughsomepopularknowledgeabouttheboilingheattransferinthenarrowspacehavebeenacceptedbymanyresearchers.Theknowledgecanbecon-cludedasthatthenucleateboilingheattransferisenhancedatlowheatfluxregionanddeterioratedathighheatfiuxregi0nespeciallyatCHF.Theenhanceme…     

Subcooled flow boiling heat transfer in narrow passages

Subcooled flow boiling heat transfer coefficients for refrigerants R11 and HCFC123 in smooth copper tubes of small diameter have been investigated experimentally. The parameter ranges examined are: tube diameters of 0.92 and 1.95 mm; heat fluxes 11-170 kW m⁻²; mass fluxes 110-1840 kg m⁻² s⁻¹. The range of liquid Reynolds numbers encompassed by the data set is 450 to 12,000.The data in the subcooled and saturated regions are well represented by the simple addition of convective and nucleate boiling heat transfer contributions

     

Estimation local convective boiling heat transfer coefficient in mini channel

The aim of this paper is to present an inverse heat conduction method used for determining the local convective boiling heat transfer coefficient in mini channel for pure water, copper nanofluid with using three different concentrations of nanoparticles: 5 mg/L, 10 mg/L and 50 mg/L. Sequential specification function method is used to solve the IHCP and estimate the space-variable convective heat transfer coefficient. The uncertainties in the estimated in heat transfer coefficient are calculated using Bias and Variance errors. The technique is used in a series of numerical experiments to provide the optimum experimental design for a boiling heat transfer investigation.     

Saturated flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the saturated flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured heat transfer data indicate that the saturated flow boiling heat transfer coefficient increases with a decrease in the gap of the duct. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly at increasing R-134a mass flux. Moreover, the bubble departure frequency increases at reducing duct size mainly due to the rising shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Correlation for the present saturated flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

Flow boiling heat transfer in two-phase micro-channel heat sinks--II. Annular two-phase flow model

This paper is Part II of a two-part study devoted to measurement and prediction of the saturated flow boiling heat transfer coefficient in water-cooled micro-channel heat sinks. Part I discussed the experimental findings from the study, and identified unique aspects of flow boiling in micro-channels such as abrupt transition to the annular flow regime near the point of zero thermodynamic equilibrium quality, and the decrease in heat transfer coefficient with increasing quality. The operating conditions of water-cooled micro-channels fell outside the recommended range for most prior empirical correlations. In this paper, an annular flow model is developed to predict the saturated flow boiling heat transfer coefficient. Features unique to two-phase micro-channel flow, such as laminar liquid and vapor flow, smooth interface, and strong droplet entrainment and deposition effects, are identified and incorporated into the model. The model correctly captures the unique overall trend of decreasing heat transfer coefficient with increasing vapor quality in the low vapor quality region of micro-channels. Good agreement is achieved between the model predictions and heat transfer coefficient data over broad ranges of flow rate and heat flux.     

狭缝中流动沸腾传热过冷沸腾起始点的实验研究

以间隙为1．0mm和1．5mm的环形狭缝通道中流动沸腾传热的实验数据为基础，分析了影响过冷沸腾起始点热负荷的主要因素，给出了计算环形狭缝通道中流动沸腾传热过冷沸腾起始点的经验关联式，并将计算结果与实验值进行了比较。该关联式可以用来预测实验范围内的过冷沸腾起始点的热负荷。     

Subcooled flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct

An experiment is conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and the associated bubble characteristics of refrigerant R-407C in a horizontal narrow annular duct with the gap of the duct fixed at 1.0 and 2.0 mm. The measured boiling curves indicate that the temperature overshoot at ONB is relatively significant for the subcooled flow boiling of R-407C in the duct. Besides, the subcooled flow boiling heat transfer coefficient increases with a reduction in the duct gap, but decreases with an increase in the inlet liquid subcooling. Moreover, raising the heat flux imposed on the duct can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are slighter. Visualization of the subcooled flow boiling processes in the duct reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Raising the imposed heat flux, however, produces positive effects on the bubble population, coalescence and departure frequency. Meanwhile, the present heat transfer data for R-407C are compared with the R-134a data measured in the same duct and with some existing correlations. We also propose empirical correlations for the present data for the R-407C subcooled flow boiling heat transfer and some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density.     

Saturated flow boiling heat transfer of R-410A and associated bubble characteristics in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the R-410A saturated flow boiling heat transfer and associated bubble characteristics in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured data indicate that the saturated flow boiling heat transfer coefficient increases with increasing refrigerant mass flux and saturated temperature and with a decrease in the gap size. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficient. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly with increasing refrigerant mass flux and saturated temperature. Moreover, the bubble departure frequency increases at reducing duct size and increasing mass flux. And at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Meanwhile, comparisons of the present heat transfer data for R-410A with R-407C and R-134a in the same duct and with some existing correlations are conducted. Furthermore, an empirical correlation for the present R-410A saturated flow boiling heat transfer data is proposed.     

Saturated flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the R-407C saturated flow boiling heat transfer and associated bubble characteristics in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured data indicate that the saturated flow boiling heat transfer coefficient increases with a decrease in the duct gap. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases noticeably at increasing R-407C mass flux. Moreover, the bubble departure frequency increases at reducing duct size and at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Meanwhile comparison of the present heat transfer data for R-407C with R-134a in the same duct and with some existing correlations is conducted. Furthermore, correlation for the present R-407C saturated flow boiling heat transfer data is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

垂直矩形窄缝内的过冷流动沸腾换热性能

用高速摄像等方法研究了有压模化介质在单一垂直矩形窄缝流道内的气泡形态和传热情况 ,发现窄缝流动沸腾换热强化的原因在于流道尺寸较小 ,气泡的形状发生变化 ,增加了界面体积浓度 ,并强化了对加热面附近的扰动 ,使换热有所强化。通过与实际测量的壁温数据进行比较 ,发现用于计算大流道和池过冷沸腾换热的 Rohsenow关系式预测窄流道内高热流密度下的过冷流动沸腾换热的误差不大 ,但对于较低热流密度下的过冷流动沸腾时误差较大 ;通过最小二乘法对 Rohsenow关系式进行修正后 ,误差低于± 2 5 %。     

Flow boiling heat transfer in two-phase micro-channel heat sinks--I. Experimental investigation and assessment of correlation methods

This paper is the first of a two-part study concerning measurement and prediction of saturated flow boiling heat transfer in a water-cooled micro-channel heat sink. In this paper, new experimental results are discussed which provide new physical insight into the unique nature of flow boiling in narrow rectangular micro-channels. The micro-channel heat sink contained 21 parallel channels having a m cross-section. Tests were performed with deionized water over a mass velocity range of 135-402 kg/m² s, inlet temperatures of 30 and 60 °C, and an outlet pressure of 1.17 bar. Results indicate an abrupt transition to annular flow near the point of zero thermodynamic equilibrium quality, and reveal the dominant heat transfer mechanism is forced convective boiling corresponding to annular flow. Contrary to macro-channel trends, the heat transfer coefficient is shown to decrease with increasing thermodynamic equilibrium quality. This unique trend is attributed to appreciable droplet entrainment at the onset of annular flow regime development, and the increase in mass flow rate of the annular film by droplet deposition downstream. Eleven previous empirical correlations are assessed and deemed unable to predict the correct trend of heat transfer coefficient with quality because of the unique nature of flow boiling in micro-channels, and the operating conditions of water-cooled micro-channel heat sinks falling outside the recommended application range for most correlations. Part II of this study will introduce a new annular flow model as an alternative approach to heat transfer coefficient prediction for micro-channels.     

Experimental research of pool boiling heat transfer in horizontal narrow spaces

Much progress has been made in high‐performance electronic chips, the miniaturization of electronic circuits and other compact systems recently, which brings about a great demand for developing efficient heat removal techniques to accommodate these high heat fluxes. With this objective in mind, experiments were carried out on five kinds of test elements with distilled water and ethanol as working liquids. The test elements used in these experiments consisted of five parallel discs with diameters varying from 5 mm to 40 mm. The experiments were performed with the discs oriented horizontally and uniform heat fluxes applied at the bottom surfaces. The influence of narrow spacing, space size, working liquid property, and heat flux on boiling heat transfer performance in narrow spaces has been investigated. Experimental results showed that the boiling heat transfer coefficient of a narrow space was 3 to 6 times higher than that of pool boiling when the narrow space size and heat flux combine adequately, but the critical heat flux was lower than that of pool boiling. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 307–315, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20017     

水平窄空间沸腾传热的实验研究

通过对五种尺寸的窄空间试验元件分别以水和乙醇做工质进行实验。研究了窄空间间距、窄空间尺寸、不同工质及不同热流密度对窄空间沸腾性能的影响。结果表明：当窄空间尺寸与热流通等因素组合恰当时。其换热系数可比大空间池沸腾提高3～6倍;临界热流密度有所降低。     

Prediction and measurement of incipient boiling heat flux in micro-channel heat sinks

Experiments were performed to measure the incipient boiling heat flux in a heat sink containing 21 rectangular (231 μm wide and 713 μm deep) micro-channels. Tests were performed using deionized water with inlet liquid velocities of 0.13-1.44 m/s, inlet temperatures of 30, 60 and 90 °C, and an outlet pressure of 1.2 bar. Using a microscope, boiling incipience was identified when the first bubbles were detected growing at, and departing from the micro-channel wall near the outlet. A comprehensive model was developed to predict the incipient boiling heat flux, accounting for the complexities of bubble formation along the flat and corner regions of a rectangular flow channel, as well as the likelihood of bubbles growing sufficiently large to engulf the entire flow area of a micro-channel. The model is based on a bubble departure criterion, which combines both mechanical considerations (force balance on a bubble) and thermal considerations (superheating entire bubble interface). The model shows good agreement with the experimental results.     

常压下狭缝流动沸腾传热的研究

以蒸馏水为工质，在常压下，对间隙为1mm的环形狭缝通道中的流动传热进行了实验研究。分别将狭缝通道中的单相强制对流和过冷沸腾的实验数据与传统的Dittus-Boelter型关系式的计算结果进行了比较。通过分析狭缝通道中流动沸腾的传热特性认为，过冷沸腾传热比单相强制对流传热加强；质量流速对狭缝通道中的流动沸腾传热有较大影响。     

Critical heat flux for subcooled flow boiling in micro-channel heat sinks

Critical heat flux (CHF) was measured and examined with high-speed video for subcooled flow boiling in micro-channel heat sinks using HFE 7100 as working fluid. High subcooling was achieved by pre-cooling the working fluid using a secondary low-temperature refrigeration system. The high subcooling greatly reduced both bubble departure diameter and void fraction, and precluded flow pattern transitions beyond the bubbly regime. CHF was triggered by vapor blanket formation along the micro-channel walls despite the presence of abundant core liquid, which is consistent with the mechanism of Departure from Nucleate Boiling (DNB). CHF increased with increasing mass velocity and/or subcooling and decreasing hydraulic diameter for a given total mass flow rate. A pre-mature type of CHF was caused by vapor backflow into the heat sink’s inlet plenum at low mass velocities and small inlet subcoolings, and was associated with significant fluctuations in inlet and outlet pressure, as well as wall temperature. A systematic technique is developed to modify existing CHF correlations to more accurately account for features unique to micro-channel heat sinks, including rectangular cross-section, three-sided heating, and flow interaction between micro-channels. This technique is shown to be successful at correlating micro-channel heat sink data corresponding to different hydraulic diameters, mass velocities and inlet temperatures.     

水平窄空间沸腾换热的数理模型研究

窄空间只有在间距小于汽泡脱离直径时,对沸腾传热强化才有比较显的效果。窄空间沸腾强化传热的机理在于较大的泡底微层加速了蒸发传热和窄空间中被加热的液体周期性地与池液进行容积交换。水平圆盘窄空间中的汽泡生长分为性质完全不同的自由生长期和抑制长大期;在一个周期内,加热面的总传热量等于壁面传导给窄空间液体的热量与通过合体泡底微层蒸发潜热之和。在对圆形水平窄空间的沸腾传热的现象和机理进行分析的基础上,提出了窄空间的沸腾换热过程的数理模型;进而对窄空间沸腾的本质规律在理论上进行了初步探索,并得到分析解。理论计算结果与实验数据比较表明,该分析解适合于中低壁面过热度的情形。由于问题的复杂性,该模型仍需不断完善。