Investigation of empirical correlations on the determination of condensation heat transfer characteristics during downward annular flow of R134a inside a vertical smooth tube using artificial intelligence algorithms

The heat transfer characteristics of R134a during downward condensation are investigated experimentally and numerically. While the convective heat transfer coefficient, two-phase multiplier and frictional pressure drop are considered to be the significant variables as output for the analysis, inputs of the computational numerical techniques include the important two-phase flow parameters such as equivalent Reynolds number, Prandtl number, Bond number, Froude number, Lockhart and Martinelli number. Genetic algorithm technique (GA), unconstrained nonlinear minimization algorithm-Nelder-Mead method (NM) and non-linear least squares error method (NLS) are applied for the optimization of these significant variables in this study. Regression analysis gave convincing correlations on the prediction of condensation heat transfer characteristics using ±30% deviation band for practical applications. The most suitable coefficients of the proposed correlations are depicted to be compatible with the large number of experimental data by means of the computational numerical methods. Validation process of the proposed correlations is accomplished by means of the comparison between the various correlations reported in the literature.     

R410A在水平双侧强化管外的冷凝换热特性试验研究

对非共沸制冷剂R410A在一根水平光管和2根25.4 mm水平双侧强化管管外冷凝换热特性进行研究.分别研究在变入口水温和变水流量的条件下,制冷剂的管外冷凝换热特性.利用Wilson图解法和Wilson-Gnielinski法计算管外冷凝换热系数,并分析两种处理方法所得到的管外冷凝换热系数的差异,最后根据Nusselt管...     

R134a冷水机组用高效换热管的试验研究及应用

通过换热管测试台对A、B2种双侧强化蒸发管的换热性能进行了测试研究。研究结果显示：对于同一管型而言,在工况不变的情况下,管内流速在1．0-2．0m／s时,换热管的综合换热系数会随着管内水流速的增加而呈上升趋势：两者的综合换热系数与其外齿周向齿数和轴向齿数有很大关系,外内齿数多综合换热系数高：新开发的高效换热蒸发管-B比原仃的蒸发管-A换热性能高6．6％。通过采用新开发的高效蒸发管-B每年可为公司节省成本约256．6余万元。     

Experimental investigation of the reverse heat transfer of R134a flow through non-adiabatic coiled capillary tubes

This research represents an experimental investigation of the metastable flow and re-condensation phenomenon through non-adiabatic lateral helical capillary tubes and suction tube heat exchanger. The results show that mass flux ratio has a vital role: It affects metastable flow and also reverse heat transfer phenomenon through non-adiabatic helical capillary tube. Therefore, by increasing of the mass flux ratio, the rate of heat transfer between them decreases. In contrast to the strong rate condition of heat transfer between them, reverse heat transfer or re-condensation maybe happen. Moreover, experimental results show that for R134 flow with mass flux ratio more than 57.84, metastable flow exists in non-adiabatic capillary tube with 0.9144 mm inner diameter, 30 mm coil diameter, 6.18 m length, 4 mm inner diameter of compressor suction tube.

     

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.     

A comparison of condensation heat transfer performance of MWCNT/Fe composite coatings on steel substrate

In the present study, the effect of MWCNT/Fe coatings on steel substrate was investigated using R134a vapor. Specifically, CNT and Fe₂O₃ powder were used as components in mechanically alloyed powder composites, which were used to fabricate surface coatings. There were two types of coatings used in the tests. The first one was pure Fe coating and the second one was the MWCNT/Fe composite coating. The powders were coated on a plain carbon steel plate by electrostatic spraying. This was followed by sintering at 900 °C. Results showed that the condensation heat transfer can be enhanced by MWCNT/Fe composite coatings, while a pure Fe coating can diminish the enhancement in comparison to the plain carbon steel plate.     

Structure effect of refrigeration polyolester oils for use in HFC-134a compressors

A study of polyolester oils (POE) for compressors using an ozone-friendly refrigerant is presented here. The effect of the chemical structure of POE was studied in relation to three lubricant properties: miscibility with HFC-134a (CF₃-CH₂F), lubricity (steel-on-steel and aluminium-on-steel contacts), and viscosity. The paper shows the strong structure effect of POE on miscibility and lubricity. Based on the data obtained, some model lubricants were blended, and the performance of these lubricants was assessed on refrigeration test rigs.     

A tribological study of refrigeration oils under HFC-134a environment

HFC-134a is a potential alternative material for CFC-12, which depletes the ozone layer. However, problems arise when used together with conventional refrigeration oils due to the extreme polarities of HFC-134a. PAG (polyalkylene glycol) and esters are tested with HFC-134a. This investigation enhances the testing method by taking the compressor's environment into account. Miscibility and material compatibility are proved with sealed glass tests. A testing environment charged with refrigerant gas more closely simulates the conditions of a compressor. The conventional refrigeration oils (mineral oils, alkylbenzene, PAO (polyalpha olefin)) tested here are immiscible with HFC-134a. However the PAG and ester oils are miscible with HFC-134a. The friction coefficient of the PAG/HFC-134a system is similar to that of the mineral oil/CFC-12 system at operating conditions. Ester oils are preferable at the start and stop condition in the lubricity aspect. PAG shows good lubricity in conditions of extreme contact pressure. Consequently, this test provides reliable results on compressor lubricity and material compatibility of refrigeration oils with HFC-134a. It suggests a methodological way for the proper selection of refrigeration oils that may improve the durability and performance of a compressor.     

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%.     

A numerical study on the effect of varying viscosity on the heat transfer inside pipes under low temperature conditions

For a reliable design of a thermal system with oil, which shows drastic change of material properties under low temperature condition, an improved prediction model is necessary because conventional correlations have limited applicability. In this paper, a numerical study on a single and concentric pipes was conducted with i) hydrodynamically fully developed and ii) uniform velocity profiles. Under a low inlet temperature condition, the fully developed velocity profile produced a z-shaped Nusselt number variation with the inverse Graetz number due to the velocity reset and re-developing mechanism. In contrast, the uniform velocity profile showed a large Nusselt number in the entrance region compared to the conventional correlation. Additionally, the simulations were carried out for parallel and counter flow arrangements. Then the Nusselt number correlations were suggested, using a sigmoidal function to reflect the z-shaped pattern. Finally, calculated friction factors were summarized and compared with the conventional correlations.

     

A model for the condensation heat transfer of binary refrigerant mixtures

In the present work turbulent film condensation of nonazeotropic binary mixtures inside a horizontal tube is studied theoretically. The combined heat and mass transfer involved is analyzed through an integral formulation of the continuity, momentum, energy and diffusion equations. As the mass velocity of refrigerant mixtures increases, the condensation heat transfer coefficient increases. The heat transfer coefficient becomes smaller at higher mass quality. As the mole fraction of the more volatile component in binary mixtures increases, the back-diffusion mass flux of the more volatile component reduces in the vapor. As a result the condensation heat transfer coefficient improves with the increase of the inlet mole fraction of the more volatile component especially in the upstream of condenser. The results of the present study show good agreement with the experimental data available.     

Tribological behavior and tribochemical reactions of alumina in HFC-134a environment

The tribological behavior of alumina (Al₂O₃) in CF₃CH₂F (HFC-134a) gas at pressures between 170 Pa and 10⁵ Pa were investigated using a ball-on-disk type tribometer. For comparison, the friction test was also carried out in a vacuum (10⁻⁵ Pa) under the same experimental conditions. X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ions Mass Spectrometry (TOF-SIMS) were used to identify the formation of tribochemical products.It was found that the friction and wear properties of Al₂O₃ were strongly dependent on the pressure of HFC-134a gas. A higher pressure correlated to a lower friction coefficient. When the pressure exceeded 10³ Pa, the wear strikingly decreased. The results of XPS and TOF-SIMS analyses identified that tribochemical films, mainly composed of fluorine-containing organic compounds and Al-oxyfluorides, were produced on the frictional surfaces. In addition, it was found that the amount of the fluorides correlated well to the friction and wear properties, that is, the more the fluorides were, the lower were the friction and wear. The results of research indicate that HFC-134a gas has a good lubricating effect for Al₂O₃ ceramic, which is attributed to the formation of the tribofilms at the frictional interface.     

一种高效热交换用管——内、外螺旋管的加工技术

介绍一种高效热交换用管———内、外螺旋线管的生产方法及其高效传热机理。介绍旋轧成形机和工模具之间的安装结构 ,翅片管成形时各旋轧辊的配置情况。运用本方法 ,用普通光管可一次成形加工出高效热交换用管。     

Comparison of the heat transfer and pressure drop characteristics of fin-and-tube heat exchangers having smooth or herringbone wave fins on 12.7 mm outer diameter tubes

Journal of Mechanical Science and Technology - For cooling or heating coils of a building air conditioning system, large diameter tubes (over 12.7 mm) are frequently used. Furthermore, common...     

Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

Tribochemical Effects on Tribological Properties of Self-Mated Zirconia Ceramic in HFC-134a Gas

Tribochemical effects on the tribological properties of self-mated zirconia ceramic in CF₃CH₂F (HFC-134a) were investigated using a ball-on-disk type environmental tribometer. The friction chamber of the tribometer was attached to a micro-spot X-ray Photoelectron Spectrometer (XPS) for ensuring that surface analysis be conducted without exposuring the frictional surfaces to air. It was found that HFC-134a gas was an effective lubricant for zirconia ceramic, especially at a pressure higher than 10³ Pa. The products of tribochemical reactions between zirconia and HFC-134a molecules were detected. The amount and chemical state of the tribochemical products seemed to control the tribological behaviors. Thus, the role of tribochemical products on the tribological properties of zirconia in HFC-134a gas at 10⁴ Pa was studied in detail under applied loads of 0.6–5.0 N and sliding speed of 0.04–0.35 m/s. It was found that severe tribochemical reactions occurred at low speeds and high loads. The formation of ZrF₄ accelerated the chemical wear of zirconia, and raised the friction. Zirconia ceramic is suitable for use at moderate load and sliding speed under a reactive environment.     

Combustion and heat transfer characteristics inside the combustion chamber of a wood pellet boiler

A grate-firing boiler was developed for wood pellet fuel, and then its combustion characteristics were tested. The flame was stretched to the exit of the combustion chamber, implying insufficient space for complete combustion. As a first step to resolve this problem, a numerical simulation was conducted for the combustion chamber. Turbulent and chemically reacting flow was considered by implementing a homogeneous reaction model. Flow field from the simulation showed strong recirculation flow at the upstream corner of the chamber, along which the flame was stretched to the exit. Based on these results, we suggest possible modification of the combustion chamber to improve combustion characteristics, such as relocating its exit or installing internals like guide vanes.     

波纹管内流动与换热的数值模拟研究

利用计算流体力学软件Fluent,采用数值模拟方法究了幅值不同的两种波纹管传热状况,发现幅值为4mm的波纹管的传热状况优于幅值3mm波纹管的传热状况,这是由前者管内湍流强度高于后者所致.同时,回归了两波纹管的换热准则方程,为波纹管的校核计算及工程应用提供依据.     

Tribological Characteristics and Tribochemical Reactions of Various Ceramics Lubricated with HFC-134a Gas

The role of tribochemical products in the friction and wear reduction of ceramics with different fractional ionic character in CF₃CH₂F (HFC-134a) gas was investigated using a ball-on-disk type tribometer. Without exposure to air, the wear tracks on the disks were characterized with the aid of a micro-spot X-ray Photoelectron Spectroscope (XPS) whose analytical chamber was connected to the friction chamber of the tribometer. Further, the adsorption and desorption behaviors of HFC-134a molecules on the nascent surfaces of the ceramics were studied using an adsorption test apparatus in high vacuum. It was found that the lubricating effect of HFC-134a gas was closely related to the fractional ionic or covalent characters of the ceramics. HFC-134a gas was more effective in lubricating ionic ceramics than the covalent ceramics. XPS analysis revealed that metal fluorides were mainly formed on the frictional surfaces of the ionic ceramics, whereas the composition of the tribochemical products on the frictional surfaces of the other ceramics was complicated. The adsorption tests proved that HFC-134a was decomposed to an olefin CF₂=CHF on the nascent surfaces of the ionic ceramic Al₂O₃ and the covalent ceramics. However, the formation of organic fluorine-containing compounds was not detected on the frictional surfaces of the ionic ceramics by XPS. This result implies that the mechanism of tribochemical reactions is strongly dependent on the bond type of ceramics. It is concluded that the low friction and wear of the ionic ceramics in HFC-134a gas result from the metal fluorides formed with high surface concentration on the sliding surfaces.     

Experimental studies on heat transfer in the annuli with corrugated inner tubes

Experimental heat transfer data for single-phase water flow in the annuli with corrugated inner tubes are presented. In the annuli with parallel flow, ten different annular arrangements are considered. For water (low rate in l,700<Re<13,000 regime, data for Nusselt numbers are presented. The results show significant effects of both the pitch to trough height ratio (P/e) and the radius ratio (r ^*). AsP/e becomes closer to 8 in the range below the radius ratio(r ^*) of 0.5. Nusselt numbers increase. However, Nusselt numbers decrease in the range above the radius ratio (r^*) of 0.5 because flow reattachment position becomes farther in the narrower clearance.