Influence of the Operating Time of a Surface Heated by Electric Current on the Heat-Transfer Coefficient of Kerosene and Water Boiling on It

Results of experimental investigation of the influence of the operating time of a heating surface on the intensity of heat transfer in the kerosene and water boiling on it are presented. The experimental data on the heat transfer in the indicated liquids boiling on unrun heating surfaces have been generalized using different relations. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 3, pp. 134–140, May–June, 2005.     

Evaporative heat transfer and pressure drop of R410A in microchannels

Convective boiling heat transfer coefficients and two-phase pressure drops of R410A are investigated in rectangular microchannels whose hydraulic diameters are 1.36 and 1.44 mm. The mass flux was varied from 200 to 400 kg/m²s, heat flux from 10 to 20 kW/m², as the saturation temperatures were maintained at 0, 5 and 10 °C. A direct heating method was used to provide heat flux into the fluid. The boiling heat transfer coefficients of R410A in the microchannels were much different with those in single tubes, and the test conditions only slightly affected the heat transfer coefficients before dryout vapor quality. The present heat transfer correlation for microchannels, which was developed by introducing non-dimensional parameters of Bo, We_l, and Re_l used in the existing heat transfer correlations for large diameter tubes, yielded satisfactory predictions of the present data with a mean deviation of 18%. The pressure drops of R410A in the microchannels showed very similar trends with those in large diameter tubes. The existing two-phase pressure drop correlations for R410A in microchannels satisfactorily predicted the present data.     

Review of correlations of flow boiling heat transfer coefficients for carbon dioxide

Carbon dioxide (CO₂) has quite different flow boiling heat transfer characteristics from conventional refrigerants due to its much higher reduced pressures that make its thermodynamic and transport properties very different. There were some studies evaluating the correlations of flow boiling heat transfer coefficient for CO₂. However, either the number of correlations covered or the number of data used was limited, resulting in inconsistent conclusions. This work presents a comparative review of existing correlations for flow boiling heat transfer coefficient of CO₂. There are 34 correlations analyzed and evaluated using 2956 experimental data points of CO₂ flow boiling heat transfer from 10 independent laboratories. The Fang (2013) correlation performs best with a mean absolute deviation of 15.5%. The evaluation analysis sets a channel transition criterion for flow boiling heat transfer of CO₂. Several topics worthy of attention for future studies are identified.     

High heat flux transport by microbubble emission boiling

In highly subcooled flow boiling, coalescing bubbles on the heating surface collapse to many microbubbles in the beginning of transition boiling and the heat flux increases higher than the ordinary critical heat flux. This phenomenon is called Microbubble Emission Boiling, MEB. It is generated in subcooled flow boiling and the maximum heat flux reaches about 1 kW/cm²(10 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s for a small heating surface of 10 mm×10 mm which is placed at the bottom surface of horizontal rectangular channel. The high pressure in the channel is observed at collapse of the coalescing bubbles and it is closely related the size of coalescing bubbles. Periodic pressure waves are observed in MEB and the heat flux increases linearly in proportion to the pressure frequency. The frequency is considered the frequency of liquid-solid exchange on the heating surface. For the large sized heating surface of 50 mm length×20 mm width, the maximum heat flux obtained is 500 W/cm² (5 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s. This is considerably higher heat flux than the conventional cooling limit in power electronics. It is difficult to remove the high heat flux by MEB for a longer heating surface than 50 mm by single channel type. A model of advanced cooling device is introduced for power electronics by subcooled flow boiling with impinging jets. Themaxumum cooling heat flux is 500 W/cm² (5 MW/m²). Microbubble emission boiling is useful for a high heat flux transport technology in future power electronics used in a fuel-cell power plant and a space facility.     

Influences of refrigerant-based nanofluid composition and heating condition on the migration of nanoparticles during pool boiling. Part I: Experimental measurement

Influences of refrigerant-based nanofluid composition and heating condition on the migration of nanoparticles during pool boiling were investigated experimentally. The nanoparticles include Cu (average diameters of 20, 50 and 80 nm), Al and Al₂O₃ (average diameters of 20 nm), and CuO (average diameter of 40 nm). The refrigerants include R113, R141b and n-pentane. The mass fraction of lubricating oil RB68EP is from 0 to 10 wt%, the heat flux is from 10 to 100 kW m⁻², and the initial liquid-level height is from 1.3 to 3.4 cm. The experimental results show that the migration ratio of nanoparticles during the pool boiling of refrigerant-based nanofluid increases with the decrease of nanoparticle density, nanoparticle size, dynamic viscosity of refrigerant, mass fraction of lubricating oil or heat flux; while increases with the increase of liquid-phase density of refrigerant or initial liquid-level height.     

Subcooled liquid boiling heat transfer under conditions of stepwise heat-load release

The results of experimental investigations of the effect of liquid subcooling below the saturation temperature on boiling heat transfer under conditions of an increase in a heating load are presented. Water and TS-1 kerosene were used as heat carriers. The obtained experimental data on subcooled liquid boiling heat transfer has been generalized by an empirical relation. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 1, pp. 136–139, January–February, 2007.     

Observation and analysis of the detachment frequency of coalesced bubbles in pool boiling liquid nitrogen

The detachment frequency of coalesced bubbles is one of the important parameters in the boiling heat transfer model at high heat flux. Although some researches on detachment frequency of coalesced bubbles have been done for common liquids, the work on cryogenic liquids is relatively insufficient. A visual experimental apparatus was built for observing the boiling in liquid nitrogen. The growth process of coalesced bubbles was recorded by a high-speed camera, emphasizing on the detachment frequency of coalesced bubbles from the circular, horizontal heating surfaces. The effects of the heating surface material, the surface diameter and the heat flux on the detachment frequency were analyzed. The available empirical correlations from common liquids are compared with our experimental data to check their applicability in liquid nitrogen, where the detachment frequency is 13.47 s⁻¹.     

CO_2水平管外池沸腾换热的实验研究

对自然工质CO2在不同沸腾压力下的光管、机械加工表面强化管(Turbo-EHP)水平单管管外电加热池沸腾进行了实验研究。从核态沸腾的角度分析了光管、强化管管外沸腾换热系数随热流密度、沸腾压力的变化规律,通过对热流密度在10~50k W/m2、蒸发压力在2~4 MPa范围内的换热数据分析拟合得出光管时CO2在该范围下的换热关联式,拟合关联式的计算值和实验值的误差在±8.73%以内。新的拟合关联式的计算值与已有关联式的预测值的偏差在±15%之内。在热流密度范围内强化管的强化倍率在1.50~1.72之间。研究结果对进一步深入研究CO2池沸腾换热及蒸发器的设计具有指导意义。     

Heat transfer in nucleate boiling of different low boiling substances

Heat transfer coefficients for nucleate boiling of methane, ethane, ethylene, argon and carbon dioxide were determined using an apparatus for the precise investigation of pool boiling heat transfer in the low temperature range. The apparatus used a horizontal cylinder as the heating element. The influence of the thermophysical properties of the boiling liquid was established by comparing the absolute values of the heat transfer coefficients in a normalized boiling state, i.e. a saturation pressure equal to 10% of the critical pressure and a heat flux density equal to 2 × 10⁴ W m⁻². By including the results for a number of higher boiling liquids, which were investigated previously under similar experimental conditions, and using literature data for three very low boiling liquids, an empirical correlation is established which allows an approximate prediction of the absolute value of the heat transfer coefficient at nucleate boiling for substances of different molecular structure.     

Nanofluids for power engineering: Emergency cooling of overheated heat transfer surfaces

The possibility of emergency cooling of an overheated heat transfer surface using nanofluids in the case of a boiling crisis is explored by means of synchronous recording of changes of main heat transfer parameters of boiling water over time. Two nanofluids are tested, which are derived from a mixture of natural aluminosilicates (AlSi-7) and titanium dioxide (NF-8). It is found that the introduction of a small portions of nanofluid into a boiling coolant (distilled water) in a state of film boiling (t _heater > 500°C) can dramatically decrease the heat transfer surface temperature to 130–150°C, which corresponds to a transition to a safe nucleate boiling regime without affecting the specific heat flux. The fact that this regime is kept for a long time at a specific heat load exceeding the critical heat flux for water and t _heater = 125–130°C is particularly important. This makes it possible to prevent a potential accident emergency (heater burnout and failure of the heat exchanger) and to ensure the smooth operation of the equipment.     

Flow boiling heat transfer characteristics of CO2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO₂ at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO₂ at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO₂ is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO₂ in good agreement with the experimental data, suggesting the need for further research in this area.     

加热方位对流动沸腾临界热流密度影响

为了研究重力场对流动沸腾临界热流密度的影响,搭建了两相沸腾换热实验系统。以蒸馏水为工质,采用单侧加热的窄缝通道,通过改变质量流速、入口过冷度和重力场与加热方位的夹角,考察不同加热方位临界热流密度特性和实验段流阻特性。分析了质量流速、入口过冷度、加热方位对流动沸腾临界热流密度的影响,并将实验数据与Ivey-Morris模型、Sudo模型和Wojtan模型的计算值进行了验证对比。结果表明:加热面呈0°放置时的临界热流密度最大,呈180°放置时最小,质量流速和入口过冷度的增大会加大临界热流密度。Sudo模型对本实验条件不适用;Ivey-Morris模型和Wojtan模型在加热面呈0°放置时与实验值符合情况良好,相对误差约在30%以内,其他加热方位时,计算值均大于实验值。     

Critical thermal load in saturated boiling of liquids

The critical velocity of vapor flow from the heating surface in boiling is found based on the analogy of bubbling and boiling phenomena. The relation for the critical thermal load q_cr is found from the assumption that this quantity corresponds to the maximum vapor velocity. The expression obtained in this work for q_cr describes experimental data on the saturated boiling of water, ethanol, and benzene at different pressures. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 2, pp. 259–263, March–April, 2008.     

Partial Nucleate Pool Boiling at Low Heat Flux: Preliminary Ground Test for SOBER-SJ10

Focusing on partial nucleate pool boiling at low heat flux, SOBER-SJ10, one of 27 experiments of the program SJ-10, has been proposed to study local convection and heat transfer around an isolated growing vapor bubble during nucleate pool boiling on a well characterized flat surface in microgravity. An integrated micro heater has been developed. By using a local pulse overheating method in the experimental mode of single bubble boiling, a bubble nucleus can be excited with accurate spatial and temporal positioning on the top-side of a quartz glass substrate with a thickness of 2 mm and an effective heating area of 4.5 mm in diameter, and then grows under an approximate constant heat input provided by the main heater on the back-side of the substrate. Ten thin film micro-RTDs are used for local temperature measurements on the heating surface underneath the growing bubble. Normal pool boiling experiments can also be carried out with step-by-step increase of heating voltage. A series of ground test of the flight module of SOBER-SJ10 have been conducted. Good agreement of the measured data of single phase natural convection with the common-used empirical correlation warrants reasonable confidence in the data. It is found that the values of the incipience superheat of pool boiling at different subcooling are consistent with each others, verifying that the influence of subcooling on boiling incipience can be neglected. Pool boiling curves are also obtained, which shows great influence of subcooling on heat transfer of partial nucleate pool boiling, particularly in lower heat flux.     

Characteristic features of the boiling of emulsions having a low-boiling dispersed phase

The results of investigation of heat transfer from thin wires to a boiling emulsion, the dispersed phase of which is formed from a liquid with a boiling temperature much lower than the boiling temperature of the dispersion medium, are presented. Two variants of boiling of such an emulsion are possible: boiling of the dispersed phase alone and simultaneous boiling of the dispersed phase and dispersion medium. In the present work, only the first variant has been studied; it is distinguished by the following most important features: high superheat of the dispersed-phase droplets ΔT_sup of the emulsion and a wide temperature range of bubble boiling (50–200°C). For conventional heat carriers (pure liquids and solutions), the value of ΔT_sup does not exceed 1–10°C, with the bubble-boiling interval lying within the range from 5 to 20°C. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 6, pp. 81–84, November–December, 2006.     

Flow boiling heat transfer to carbon dioxide: general prediction method

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO₂. Because CO₂ has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO₂ at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO₂ database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m⁻² s⁻¹, heat fluxes from 5 to 36 kW m⁻², saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87).     

沸腾表面对液氮临界热流密度的影响实验

采用液氮作为沸腾工质,通过可视化液氮沸腾实验台,对不同材料和直径的沸腾表面在液氮中进行相关稳态沸腾实验研究,总结分析沸腾表面材料、表面直径对临界热流密度及其对应过热度的影响规律.     

Results of experimental investigation of heat transfer with emulsions with low-boiling disperse phase

The results are given of a theoretical investigation of heat transfer from thin wires to emulsions in which the disperse phase is low-boiling compared to the dispersion medium. A special feature of heat transfer to such emulsions is observed in the mode of nucleate boiling, when only the droplets of the disperse phase of emulsion are boiling. The transition from the convective mode of heat transfer to nucleate boiling requires high values of temperature gradient ΔT = T _W ? T ₀ at which the temperature of the heat-transfer surface T _W is higher than the temperature T _S of saturated vapors of the disperse phase by 100 °C and more. Here, T ₀ is the temperature of the heat-transfer agent away from the heat-transfer surface. Note that, for pure liquids and solutions, the value of delay of the beginning of boiling ΔT _i = T _W ? T _S as a rule does not exceed 1–5 °C. Another feature of heat transfer to the emulsions being treated is a wide temperature range of the mode of nucleate boiling (over 100 °C); the upper limit of this temperature range is defined by the temperature of transition to film boiling of the dispersion medium of the emulsion being investigated that has already boiled by this instant of time. The value of delay of the beginning of boiling is affected significantly by additions of surfactants and adsorbents.     

添加表面活性剂的沸腾换热强化研究进展

从沸腾换热特性及其影响因素、沸腾汽泡行为和沸腾换热关联式等方面综述了添加表面活性剂的沸腾换热强化研究现状。现有研究指出界面吸附、分子结构、粘度、溶解特性等因素对表面活性剂溶液沸腾换热的作用机制与表面活性剂种类和溶液浓度密切相关,但是蒸汽携带活性剂、非离子活性剂浊点、加热方法、系统压力、参数耦合等因素对表面活性剂溶液沸腾换热的影响规律的研究还需深入开展。在沸腾汽泡行为方面,表面活性剂溶液沸腾汽泡行为与水存在较大差异且与活性剂种类有关,表面活性剂溶液沸腾汽泡行为的理论研究还需加强。此外,现有文献建立的表面活性剂溶液沸腾换热模型及关联式存在验证所用的实验数据较少、模型参数难以确定等不足。最后,在总结现有研究进展的基础上对表面活性剂溶液沸腾换热的后续研究工作提出了建议。     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.