Enhancement of nucleate boiling on composite surfaces

A composite heating surface composed of materials with different thermal conductivities can be expected to enhance heat transfer in nucleate boiling. This is because the end surface, with higher conductivity, will attain a higher temperature and as a result will serve to provide preferential nucleation sites. To confirm this idea, several composite surfaces were fabricated by uniaxially imbedding thin copper cylinders in the heat flow direction on a stainless steel circular plate 30 mm in diameter and 5 mm thick. The imbedded copper cylinders ranged from 1 mm to 4 mm in diameter and one to 77 in number. The heat transfer performance of these composite surfaces was investigated for pool boiling of saturated water at atmospheric pressure. It was confirmed that the copper cylinder surfaces exposed to water functioned as local hot spots to initiate preferential nucleate boiling, leading to higher boiling heat transfer coefficients than those on a homogeneous stainless steel surface. The measured void fraction above the heating surface verified intensive bubble generation on the surface of the copper cylinders. This situation continued up to a certain heat flux level and was then followed by nucleation on the mother surface of stainless steel around the copper cylinders. A numerical analysis of heat conduction within a composite wall simulated the temperature distribution within the wall and the variation in surface heat flux at the time of boiling incipience. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 216–228, 1998     

Two-tier model for nucleate pool boiling on microconfifured composite surfaces

A new model is developed to describe the heat transfer mechanism in nucleate pool boiling on a microconfigured composite surface. Both the microlayer and macrolayer thickness are determined from the model. This model can be extended to explain the nucleate boiling on plain surfaces. The enhancement mechanisms of heat transfer for the nucleate boiling on the microconfigured surface are analyzed.     

Study on onset of nucleate boiling in bilaterally heated narrow annuli

Onset of nucleate boiling (ONB) experiments using deionized water as working fluid have been conducted in a range of pressure from 1 to 4 MPa, mass flow velocity from 56 to 145 kg/m² s and wall heat flux from 9 to 58 kW/m² for vertical narrow annuli with annular gap sizes of 0.95, 1.5 and 2 mm. We found that the ONB sometimes occurs only on outer annulus surface, sometimes occurs only on inner annulus surface and sometimes occurs on both annulus surfaces. The heat flux of the other side has great influence on the heat flux of the ONB and the latter will decrease with the increase of the heat flux of the other side. It is also found that the heat flux of the ONB increases with the increase of the pressure, the mass flux and wall superheat. However, the heat flux of the ONB will decrease as the gap size increases in narrow annuli. The heat flux of the ONB in narrow annuli is much lower than that calculated by correlations for conventional channels and a new correlation, which has good agreement with the experimental data, has been developed for predicting the heat flux of the ONB in narrow annuli.     

Experimental study of nucleate boiling of halocarbon refrigerants on cylindrical surfaces

The present paper reports the results of an experimental investigation of saturated pool boiling of halocarbon refrigerants on cylindrical surfaces of different materials. Experiments covered a wide range of reduced pressures and heat fluxes, being carried out on copper, brass and stainless steel surfaces with different finishing conditions. The obtained results are discussed with regard to the controlled physical and operational parameters of the investigation. An empirical correlation is proposed in terms of reduced pressures. The performance of the correlation can be deemed adequate, considering that it compares well with experimental results of different authors.     

加热板上液滴沸腾实验研究

采用高速摄像技术研究了不同加热表面上液滴蒸发和沸腾的相变特性和壁面温度变化特性,讨论了局部相变行为对壁面温度变化的影响.同时定量的研究了三种不同表面特性的加热板对沸腾和传热的影响,以及液滴初始体积对相变的影响.结果表明,表面特性和液滴尺寸对沸腾传热有较大影响.     

The effect of heated wall thickness and materials on nucleate boiling at high heat flux

The present work is to numerically investigate the effect of heater side factors on the nucleate boiling at high heat flux, which is characterized by the existence of macrolayer. Two-region equations are proposed to study both thermo-capillary driven flow in the liquid layer and heat conduction in the solid wall. The numerical results indicate that the thermo-capillary driven flow in the macrolayer and evaporation at the vapor-liquid interface constitute a very efficient heat transfer mechanism to explain the high heat transfer coefficient of nucleate boiling heat transfer near CHF. For a very thin wall and/or wall with a poor thermal conductivity (heat side factors) are found to have significant effect on flow pattern in the liquid layer and the temperature distribution in the heated wall.     

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 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.     

A new mechanistic model for pool boiling CHF on horizontal surfaces

This work proposes a new mechanistic model for predicting the critical heat flux (CHF) in horizontal pool boiling systems. It is postulated that when the vapor momentum flux is sufficient to lift the liquid macrolayer from the heating surface, wetting is no longer feasible, and a transition from nucleate to film boiling occurs. This is the same mechanism that has found success in predicting CHF in flow boiling systems. An experimental investigation of CHF with pentane, hexane, and FC-72 in saturated horizontal pool boiling with chamber pressures of 150, 300, and 450 kPa provides evidence that the new model captures the variation of CHF with pressure reasonably well compared with other well known models. The new model is also compared with existing data from the literature over a reduced pressure range of 2 × 10^?5–2 × 10^?1. The mean deviation between the predicted and measured CHF is typically within 20% over the parameter space covered.     

Reexamination of Correlations for Nucleate SiteDistribution on Boiling Surface by Fractal Theory

INTRODUCTIONThenumberandspatia1distributionofnucleatesiteplaysaparticu1arimportantroleinmodelingorpred-icatingofnucleateboilingprocess.Thenucleatesitedistributiondensityappearsasanimportantterminalmostallthemechanisticnucleateboilingmodels[1].Inpastdecades,manyresearchersproposedvariousexperimentalcorrelations.Forexample,MikicandRohsenowl2],Biereta1.[3],WangandDhirl4]devel-opedsomeempiricalcorrelationsbetweenthecumula-tiveactivenucleatesitenumberandthecharacteristicsizesuchasthemouthdia…     

Effects of surface wettability on nucleate pool boiling heat transfer for surfactant solutions

Experiments for pool boiling of deionised water and acetone with different surfactant, 95% sodium dodecyl sulfate (SDS), Triton X-100 and octadecylamine, have been conducted under atmospheric pressure to investigate the effect of surface wettability. The boiling curves for different concentrations of surfactant solution on both smooth and roughened surfaces were obtained. The results show that the addition of surfactant can enhance the water boiling heat transfer, and the enhancement is more obvious for SDS solution; but has little influence on the acetone boiling curve. While the roughened surface enhanced the heat transfer for Triton X-100 solution, it also decreased the heat transfer coefficient for SDS solution. All these can be explained by including the changing of surface wettability, which has been neglected for a long time and should be an important parameter influencing boiling heat transfer. By incorporating such effects, the modified Mikic-Rohsenow pool boiling model, we proposed, can predict these experimental data well.     

The rate of heat transfer for boiling on porous surfaces

It is shown that the constant, C_sf, introduced by Rohsenow⁶ for correlating pool boiling heat transfer data, may be used in order to characterize plates which have been made porous.     

A new comprehensive model for nucleate pool boiling heat transfer of pure liquid at low to high heat fluxes including CHF

Based on the fractal distribution of nucleation sites present on heating surfaces, a new comprehensive model is developed for the nucleate pool boiling of pure liquid at low to high heat fluxes including the critical heat flux (CHF). The proposed model is expressed as a function of total number, minimum and maximum sizes of active nucleation sites, fractal dimension, superheat temperature, and properties of fluids. No additional empirical constant is introduced in the proposed model. This fractal model contains less empirical constants than the conventional models. The model predictions are in good agreement with the available experimental data.     

3D isotropic approximation for solar diffuse irradiance on tilted surfaces

A well known 2D approach of Liu and Jordan allows computing isotropic solar diffuse irradiance on a tilted surface. It is a 2D theory as the position of a sky element is characterized by a single (zenith) angle. A more realistic 3D model (that uses both zenith and azimuth angles to describe sky element's position) is developed in this paper for both isotropic diffuse irradiance and ground reflected irradiance incident on an arbitrary oriented surface. The 3D formula predicts a lower diffuse irradiance than the 2D relationship while the ground reflected irradiance is higher in case of the 3D model than in case of the 2D approach. In case of a small tilt angle, the 2D and 3D approximations predict comparable values, higher than the mean of the results obtained with a (reference) non-isotropic model. However, the 3D model is slightly more precise. When a larger tilt angle is considered, the 3D model predicts a few percent larger value than the mean of the values estimated by the reference model while the 2D model gives a significantly higher value.     

紧凑式两相换热器中管式换热元件沸腾传热实验研究

以烃类物质（丙烷和正戊烷）作为工质,进行了紧凑式换热器中带有加工配置表面的管式换热元件池沸腾实验研究。其中,单管实验温度工况为２５３Ｋ￣２９３Ｋ（饱和工质）。实验中所采用的换热元件为重入式结构加工配置表面的强化传热管和光管以及低助管。针对由４５根光管或带有加工配置表面的管子所构成的叉排管束进行了实验研究,实验工质为丙烷和正戊烷,实验温度分别为两种工质在２６３Ｋ和３０８Ｋ之间的饱和和温度。并将所得实     

3D multiphase lattice Boltzmann simulations for morphological effects on self-propelled jumping of droplets on textured superhydrophobic surfaces

Coalescence induced droplets self-propelled jumping on textured superhydrophobic surfaces (SHS) is numerically simulated using multiple–relaxation–time (MRT), and three dimensional (3D) multiphase isothermal lattice Boltzmann method. Symmetric boundary conditions and parallel computation with OpenMP algorithm are used to accelerate computational speeds. Simulation results for velocity field show that the downward velocity of the droplet is reverted to upward direction due to the counter action of the wall to the contact base of the droplet during the period of droplet deformation on the texture. For a fixed droplet diameter, the spacing of the microstructure is found to play a key role on jumping velocity of the coalescence droplet, and an optimal spacing of the microstructure exists for a maximum jumping velocity. For a texture with small spacings, the adhesion force due to surface tension is large because of the large contact area which results in a decrease of its jumping velocity. On the other hand, for a texture with large roughness spacings, the lower contour of the droplet will fall into the texture, which will also decrease droplet jumping velocity. Simulation results for jumping velocities are used to explain large differences in measured jumping velocities of small droplets (with radius less than 20 μm) on hierarchical textured and nanostructured surfaces in existing experiments.