Effect of surface etching on condensing heat transfer

This study conducted experiments on humid air condensation during heat transfer in an air preheating exchanger attached to a home condensing boiler to improve thermal efficiency. An etchant composed of sulfuric acid and sodium nitrate was used to create roughness on the heat exchanger surface made from STS430J1L. A counter flow heat exchanger was fabricated to test the performance of heat transfer. Results showed that the overall heat transfer coefficients of all specimens treated with etchant improved with respect to the original specimens (not treated with etchant), and the overall heat transfer coefficient of the 60 s etching specimen increased by up to 15%. However, the increasing rate of the heat transfer coefficient was disproportional to the etching time. When the etching time specifically increased above 60 s, the heat transfer coefficient decreased. This effect was assumed to be caused by surface characteristics such as contact angle. Furthermore, a smaller contact angle or higher hydrophilicity leads to higher heat transfer coefficient.

     

R290蒸气珠状凝结传热微尺度特性的研究

通过对自然工质R290蒸气珠状凝结传热过程的微尺度特性分析得出,在一定的过冷度、液珠半径和分形维数下,液珠的导热热阻随接触角的增大而增大,促进层的热阻在接触角为90°时最小,气液界面的热阻随着接触角的增大而减小,单个液珠的总传热热阻随着接触角的增大呈现出先减小后增大的变化规律,即存在最佳接触角,在此最佳接触角下,单个液珠的总传热热阻最小,单个液珠的传热量最大,换热表面的热流密度最高。随着液珠半径的增大,最佳接触角减小。随着过冷度的增加和分形维数的增大,换热表面液珠成核中心密度增大,表面的热流密度增加。在一定的过冷度下,液珠半径增大,液珠分布密度减少,分形维数增大,液珠的分布密度增加。在一定的分形维数和过冷度下,随着液珠半径的减小,换热表面的热流密度增大。     

Theoretical method for estimation of mean pressure on contact area between rolling tools and workpiece in cross wedge rolling processes

The paper gives a new method no determine the mean unit contact pressures on a material—tool contact surface in cross wedge rolling processes (CWR). The dependencies worked out on the basis of the energy and the upper bound methods permits rolling forces to be determined which are comparable to experimentally measured ones. The analysis provides equations which relate the mean contact pressure q_m to the basic process parameters, namely the forming angle α, the spreading angle β, the relative reduction of a portion δ and the shear friction factors m and m_k.     

Two-phase flow heat transfer of R-134a in microtubes

The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of 430 μm and 792 μm were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient for the tube A (D _i = 430 μm) was 47.0% greater than that for the tube B (D _i = 792 μm) at G = 370 kg/m²·s and q″ = 20 kW·m². A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer coefficients within an absolute average deviation of 8.4%.     

Enhancement of pool boiling heat transfer coefficients using carbon nanotubes

In this study, the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer is investigated. Three refrigerants of R22, R123, R134a, and water were used as working fluids and 1.0 vol.% of CNTs was added to the working fluids to examine the effect of CNTs. Experimental apparatus was composed of a stainless steel vessel and a plain horizontal tube heated by a cartridge heater. All data were obtained at the pool temperature of 7°C for all refrigerants and 100°C for water in the heat flux range of 10–80 kW/m². Test results showed that CNTs increase nucleate boiling heat transfer coefficients for all fluids. Especially, large enhancement was observed at low heat fluxes of less than 30 kW/m². With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generation. Fouling on the heat transfer surface was not observed during the course of this study. Optimum quantity and type of CNTs and their dispersion should be examined for their commercial application to enhance nucleate boiling heat transfer in many applications.     

Wetting behaviour during evaporation and condensation of water microdroplets on superhydrophobic patterned surfaces

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water repellent properties. The superhydrophobic surfaces may be generated by the use of hydrophobic coating, roughness and air pockets between solid and liquid. The geometric effects and dynamic effects, such as surface waves, can destroy the composite solid–air–liquid interface. The relationship between the water droplet size and geometric parameters governs the creation of composite interface and affects transition from solid–liquid interface to composite interface. Therefore, it is necessary to study the effect of droplets of various sizes. We have studied the effect of droplet size on contact angle by evaporation using droplets with radii ranging from about 300 to 700 μm. Experimental and theoretical studies of the wetting properties of silicon surfaces patterned with pillars of two different diameters and heights with varying pitch values are presented. We propose a criterion where the transition from Cassie and Baxter regime to Wenzel regime occurs when the droop of the droplet sinking between two asperities is larger than the depth of the cavity. The trends are explained based on the experimental data and the proposed transition criteria. An environmental scanning electron microscopy (ESEM) is used to form smaller droplets of about 20 μm radius and measure the contact angle on the patterned surfaces. The investigation has shown that ESEM provides a new approach to wetting studies on the microscale.     

An analytic study on laminar film condensation along the interior surface of a cave-shaped cavity of a flat plate heat pipe

An analytic approach has been employed to study condensate film thickness distribution inside cave-shaped cavity of a flat plate heat pipe. The results indicate that the condensate film thickness largely depends on mass flow rate and local velocity of condensate. The increasing rate of condensate film for circular region reveals about 50% higher value than that of vertical region. The physical properties of working fluid affect significantly the condensate film thickness, such as the condensate film thickness for the case of FC-40 are 5 times larger than that of water. In comparison with condensation on a vertical wall, the average heat transfer coefficient in the cave-shaped cavity presented 10-15% lower values due to the fact that the average film thickness formed inside the cave-shaped cavity was larger than that of the vertical wall with an equivalent flow length. A correlation formula which is based on the condensate film analysis for the cave-shaped cavity to predict average heat transfer coefficient is presented. Also, the critical minimum fill charge ratio of working fluid based on condensate film analysis has been predicted, and the minimum fill charge ratios for FC-40 and water are about Ψ_crit= 3-7%, Ψ_crit=0.5-1.3%, respectively, in the range of heat fluxq” = 5-90kW/²     

复合材料层合结构在防覆冰/除冰系统中的应用

设计了一种基于复合材料层合结构的新型防覆冰/除冰系统。此多功能复合层合结构由铜基超疏水材料、非对称正交铺设的双稳态可变形结构和聚酰亚胺加热膜构成。铜基超疏水材料起到延迟水滴结冰、减少表面覆冰和降低冰层与接触面之间黏附力的作用;聚酰亚胺加热膜驱动双稳态结构发生稳态突变时,产生的热量会快速融化冰层;双稳态结构形态突变时会产生机械振动,从而形成一套新型电热-机械耦合防覆冰/除冰机制。通过测试,该系统表面水接触角可达155°,具有良好的超疏水特性、可变形性、耐高低温交变性和紫外线耐性等特性。     

Condensation heat transfer coefficients of flammable refrigerants on various enhanced tubes

In this study, external condensation heat transfer coefficients (HTCs) of six flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), dimethylether (RE170), and HFC32 were measured at the vapor temperature of 39°C on a 1023 fpm low fin and Turbo-C tubes. All data were taken under the heat flux of 32- 116 and 42-142 kW/m² for the Iow fin and Turbo-C tubes respectively. Flammable refrigerants’ data obtained on enhanced tubes showed a typical trend that external condensation HTCs decrease with increasing wall subcooling. HFC32 and DME showed up to 30% higher HTCs than those of HCFC22 due to their excellent thermophysical properties. Propylene, propane, isobutane, and butane showed similar or lower HTCs than those of HCFC22. Beatty and Katz’ correlation predicted the HTCs of the flammable refrigerants obtained on a low fin tube within a mean deviation of 7.3%. Turbo-C tube showed the best performance due to its 3 dimensional surface geometry for fast removal of condensate.     

Two-phase flow heat transfer of propane vaporization in horizontal minichannels

Experiments were performed on the convective boiling heat transfer in horizontal minichannels using propane. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and lengths of 1000 mm and 2000 mm, respectively, and it was uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 5–20 kW m⁻², a mass flux range of 50–400 kg m⁻² s⁻¹, saturation temperatures of 10, 5, and 0°C and quality ranges of up to 1.0. The nucleate boiling heat transfer contribution was predominant, particularly at the low quality region. Decreases in the heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux and mass flux, and with a lower saturation temperature and inner tube diameter. Laminar flow appeared in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for propane was developed with 8.27% mean deviation. This paper was recommended for publication in revised form by Associate Editor Jae Young Lee Jong-Taek Oh received his B.S., M.S. and Ph.D. degrees in Refrigeration Engineering from Pukyong National University, Korea. Dr. Oh is currently a Professor at Department of Refrigeration and Air Conditioning Engineering, Chonnam National University at Yeosu, Korea. Dr. Oh’s research interests are in the area of boiling and condensation heat transfer and pressure drop of refrigerants with small tubes, heat pump and transportation refrigeration.     

Nucleate Boiling Heat Transfer Coefficients of Mixtures Containing Propane, Isobutane and HFC134a

Nucleate pool boiling heat transfer coefficient (HTCs) were measured with one nonazeotropic mixture of propane/isobutane and two azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a. All data were taken at the liquid pool temperature of 7°C on a horizontal plain tube of 19.0 mm outside diameter with heat fluxes of 10kW/m² to 80 kW/m² with an interval of 10 kW/m² in the decreasing order of heat flux. The measurements were made through electrical heating by a cartridge heater. The nonazeotropic mixture of propane/isobutane showed a reduction of HTCs as much as 41% from the ideal values. The azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a showed a reduction of HTCs as much as 44% from the ideal values at compositions other than azeotropic compositions. At azeotropic compositions, however, the HTCs were even higher than the ideal values due to the increase in the vapor pressure. For all mixtures, the reduction in heat transfer was greater with larger gliding temperature difference. Stephan and Körner’s and Jung et al’s correlations predicted the HTCs of mixtures with a mean deviation of 11 %. The largest mean deviation occurred at the azeotropic compositions of HFC 134a/isobutane and propane/HFC 134a.     

An experimental study on convective boiling of R-22 and R-410A in Horizontal smooth and micro-fin tubes

Evaporation heat transfer coefficients and pressure drops were measured for smooth and micro-fin tubes with R-22 and R-410A. Heat transfer measurements were performed for 3.0 m long horizontal tubes with nominal outside diameters of 9.52 and 7.0 mm over an evaporating temperature range of ?15 to 5°C, a mass flux range of 68 to 211 kg/m²s, and a heat flux range of 5 to 15 kW/m². It was observed that the heat transfer coefficient increased with mass flux. Evaporation heat transfer coefficients of R-22 and R-410A increased as the evaporating temperature dropped at a lower heat flux. Generally, R-410A showed the higher heat transfer coefficients than R-22 in the range of low mass flux, high heat flux and high evaporating temperature. Pressure drop increased with a decrease of evaporating temperature and a rise of mass flux. Pressure drop of R-22 was higher than that of R-410A at the same mass flux.     

Thermal characteristics of an ammonia-charged closed-loop pulsating heat pipe

At this age, engineering applications are demanding effective ways of heat recovery and energy savings for their optimum performance. Among other cooling techniques, pulsating heat pipes have emerged as a convenient and cost effective thermal design solution due to its excellent heat transfer capability, high thermal efficiency and structural simplicity. The paper presents an experimental study on the operational limit of an aluminum closed-loop pulsating heat pipe (CLPHP) charged with ammonia. It consists of total 14 turns of aluminum pipe with 3 mm inner and 4 mm outer diameter. Ammonia was used as working fluid with 3 different filling ratios such as 0.4, 0.6 and 0.8. Operation orientations were vertical, 30°, 45°, 60°, 90° and 180° inclinations. Constant electric heat input of 36 W was applied to the heating block and temperature rise in various sections was monitored till steady state was reached. Temperature was measured at different locations of the CLPHP by using thermocouples. The effects of operational orientations and filling ratios were investigated on heat transfer by working fluid Q̇_php (Watt), overall heat transfer coefficient U (W/m² °C) and thermal resistance R (°C/W) considering the measured temperature. The result shows that, 0.4 and 0.6 fill ratios and inclination angle of 30º give better result than any other arrangements for CLPHP.

     

Temperature measurement in orthogonal metal cutting

Orthogonal cutting experiments were carried out on steel at different feedrates and cutting speeds. During these experiments the chip temperatures were measured using an infrared camera. The applied technique allows us to determine the chip temperature distribution at the free side of the chip. From this distribution the shear plane temperature at the top of the chip as well as the uniform chip temperature can be found. A finite-difference model was developed to compute the interfacial temperature between chip and tool, using the temperature distribution measured at the top of the chip.Nomenclature contact length with sticking friction behaviour [m] - c specific heat [J kg^–1 K^–1] - contact length with sliding friction behaviour [m] - F _P feed force [N] - F _V main cutting force [N] - h undeformed chip thickness [m] - h _c deformed chip thickness [m] - i,j denote nodal position - k thermal conductivity [W m^–2 K^–1] - L chip-tool contact length [m] - p defines time—space grid, Eq. (11) [s m^–2] - Q _C heat rate entering chip per unit width due to friction at the rake face [W m^–1] - Q _T total heat rate due to friction at the rake face [W m^–1] - Q _% percentage of the friction energy that enters the chip - q ₀ peak value ofq(x) [W m^–2] - q _e heat rate by radiation [W] - q(x) heat flux entering chip [W m^–2] - t time [s] - T temperature [K] - T _C uniform chip temperature [°C] - T _max maximum chip—tool temperature [°C] - T _mean mean chip—tool temperature [°C] - T _S measured shear plane temperature [°C] - x,y Cartesian coordinates [m] - V cutting speed [m s^–1] - V _C chip speed [m/s] - rake angle - ,, control volume lumped thermal diffusivity [m² s^–1] - emmittance for radiation - exponent, Eq. (3) - density [kg m^–3] - Stefan-Boltzmann constant [W m^–2 K⁴] - (x) shear stress distribution [N m^–2] - shear angle     

Nucleate boiling heat transfer in nanofluids with carbon nanotubes up to critical heat fluxes

In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHF) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60°C. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001, 0.001, 0.01, and 0.05%. For the dispersion of CNTs, DISPERBYK 184 is used in distilled water. Pool boiling HTCs are taken from 10 kW/m² to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the nanofluids are lower than those of pure water in entire nucleate boiling regime. On the other hand, critical heat flux is enhanced greatly showing up to 150% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle on the test surface and extends the nucleate boiling regime to very high heat flux range and reduces the formation of large vapor canopy at near CHF. Thus, a significant increase in CHF results.     

Contact angle analysis on polymethylmethacrylate and commercial wax by using an environmental scanning electron microscope

The environmental scanning electron microscope (ESEM) represents one of the most exciting breakthroughs in electron microscopy since the invention of the electron microscope. Its ability to observe uncoated and hydrated samples enhances the possibility for investigating the wettability of surfaces at a microscopic level; by varying the relative vapour pressure or the temperature inside the chamber, it is possible to condense water drops on a micron scale. A large problem in measuring contact angles by ESEM is that the observation angle is not parallel or perpendicular to the surface; thus, the study of the droplets profile using the common algorithms such as spherical approximation or axisymmetric drop shape analysis (ADSA) is not possible, because only a spherical cap shape is commonly observed. In this paper we provide a useful mathematical model to calculate the real contact angle from the initial images. Initially, some simulated spherical caps with different contact and observation angles were created by an appropriate graphic package in order to test the mathematical model. Some real drops obtained by ESEM on wax and polymethylmethacrylate (PMMA) were then studied and the results compared with contact angles measured by common methods on the same materials.     

Numerical investigation on Rayleigh-Bénard-Marangoni flow of water near the maximum density in cylindrical pools with large aspect ratios

We report a set of direct numerical simulation results on Rayleigh-Bénard-Marangoni (R-B-M) flow of cold water in the cylindrical pools. The heat transfer between the free surface and the environment is considered. The aspect ratio Г of the cylindrical pool varies from 2 to 8. Rayleigh (Ra) and Biot (Bi) numbers are respectively confined in Ra ≤ 10⁴ and 0 < Bi ≤ 50. The flow onset critical Ra is determined and the influences of Bi and the density inversion parameter (Θ_m) on the critical Ra are analyzed. The primary bifurcation flow structures of R-B-M flow are shown and the evolution of the flow structures with Ra and Bi at different Θ_m is observed. Furthermore, the heat transfer ability is estimated by Nusselt number. The results indicate that the critical Ra of the flow onset increases with increasing Bi and Θ_m. But it decreases with the increase of Г. The primary bifurcation pattern is multicellular flow. With increasing Г, the number of flow cells in multicellular flow increases fast. With increasing Ra, the up-triangular and up-quadrilateral flow structures appear at Г = 4, and finally transits to the up one-torus. With increasing Ra and Г, and decreasing Θ_m, average Nusselt number increases monotonically. However, with increasing Bi, it first increases, and then decreases.

     

A comparison of the effect of the electrohydrodynamic technique on the condensation heat transfer of HFC-134a inside smooth and micro-fin tubes

The results of the condensation heat transfer enhancement and pressure drop of HFC-134a by using the electrohydrodynamic (EHD) technique are presented. The test section is a horizontal tube-in-tube heat exchanger where the refrigerant flows in the inner tube and water flows in the annulus. The outer tube is a smooth copper tube havign outer diameter of 21.2 mm. Two types of inner tubes, smooth and micro-fin copper tubes, are tested. The outer diameter and length of both inner tubes is 9.52 mm and 2.5 m, respectively. A stainless steel cylindrical electrode of 1.47 mm in diameter is placed in the center of the tube. Experiments are conducted under conditions providing mass flux of 400 kg/m²s, saturated temperature of 40°C, heat flux of 20 kW/m² and applied voltage of 2.5 kV. The experimental results indicate that the EHD enhancements of the smooth tube are higher than those of the micro-fin tube over the range of average quality. The maximum heat transfer enhancements for smooth and micro-fin tubes are 1.1. times and 1.08 times, respectively. For a smooth tube, the pressure drop induced by EHD is considerably small. However, the application of EHD in a micro-fin tube can lead to 10% increase in the pressure drop.     

超疏水表面液滴的冷凝成长特性研究

利用聚二甲基硅氧烷(PDMS)为基底采用光刻技术制备了微方柱状超疏水表面,分析了冷凝条件下超疏水表面液滴的冷凝生长特征,发现液滴的生长过程可分为微液滴成核冷凝独立生长、冷凝微液滴合并生长以及大液滴生长3个阶段。超疏水表面初始合并的液滴呈Wenzel-Cassie状态和Wenzel状态,随着冷凝液滴的成长,液滴的液-固接触面积与粗糙结构表面的表观面积之比f随着冷凝液滴尺寸的增大而增大,Wenzel-Cassie状态向完全Wenzel状态转变。最后分析了超疏水性破坏的原因。     

Measurement and correlation of boiling heat transfer coefficient of R-1234yf in horizontal small tubes

We report experimental data of boiling heat transfer of R-1234yf in horizontal small tubes. The experimental data obtained in the horizontal circular small tubes of 1.5 and 3.0 mm inner diameter, the lengths of 1000 and 2000 mm, the mass flux range from 200–650 kg/m²s, the heat flux range from 5–40 kW/m² and saturation temperature of 10 and 15°C, was used to develop a modified correlation for the heat transfer coefficient. The flow pattern of the experimental data was mapped and analyzed with existing flow pattern maps. The heat transfer coefficient was also compared with some well-known correlations.