An experimental investigation of heat transfer in forced convective boiling of R134a,R123 and R134a/R123 in a horizontal tube

This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5–50 kW/m², vapor quality 0–100 percent and mass velocity of 150–600 kg/m²s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen’s superposition model, a new correlation is presented for heat transfer coefficients of mixture.     

润滑油对R32在水平光管内流动沸腾换热特性及压降的影响

对R32在φ5mm和φ7mm的水平光管内的流动沸腾时,润滑油对换热与压降特性的影响进行了试验研究,试验的质量流量范围为100~500 kg/(m~2·s),润滑油的含量在0~5%之间。结果表明,沸腾换热系数随着质量流量的增大而增大。在低干度区,换热系数随干度的增大而增大,当干度达到0.7~0.8时,换热系数达到最大。随着润滑油含量的增大,局部换热系数在减小。压降随着管径的减小和质量流量的增大而增大。润滑油含量的增大,导致压降的增大。在5mm管内,润滑油含量对换热系数和压降影响比较明显。     

管内流动沸腾传热关系式对R410A的适应性研究

R410A作为一种替代制冷剂,已经大量用在工业生产中。R410A制冷系统的设计和研发需要进行R410A管内流动沸腾换热计算。目前有很多公式预测两相流流动沸腾换热系数,它们对R410A的适应性需要判断。本文从10篇论文中收集了1268组R410A流动沸腾传热实验数据,用这些数据对27个两相流流动沸腾换热关系式进行了评价,选出了较为精确的R410A管内流动沸腾换热关系式,为R410A管内流动沸腾换热计算的公式选择提供了依据,为提出精确度更高的R410A管内流动沸腾公式提供了参考。     

非圆截面小通道内R113的流动沸腾换热特性

针对非圆截面小通道流动沸腾换热研究报道较少的现状,以R113为工质,对4种不同水力直径的正方形、三角形截面小通道内的流动沸腾换热特性进行试验研究,试验参数范围：入口干度,过冷～1.0;质量流速400～ 3 300 kg/(m2?s);热流密度20～150 kW/m2,并将试验结果与相近水力直径的圆通道内流动沸腾试验结果进行了对比分析。试验结果表明：非圆小通道内饱和流动沸腾局部壁面温度与质量流速密切相关,并受热负荷与流动沸腾换热状况的影响;质量流速和壁面热负荷是非圆小通道内流动沸腾换热特性的主要影响因素;与相近水力直径的圆通道内流动沸腾试验数据对比显示,非圆截面小通道具有明显的强化传热作用。     

R410A和R22在一种内螺纹强化管管内蒸发性能研究

为了研究单管管内蒸发性能,搭建了管内蒸发性能实验台,用隔膜泵代替了传统压缩机作为系统动力。研究了在冷却水量0.6m3/h,0.8m3/h和1.0m3/h下,9.52mm内螺纹管内10℃蒸发的制冷剂侧换热性能。结果表明,R22和R41OA的总换热系数,换热系数hr和压降均随着制冷剂流量的增加而增加,在小质量流量下,R410A比R22有更好的换热性能,看起来可以替代R22。但当制冷剂流速增大到300～400kg/(s.m2)时,R22的换热系数增加显著,而R410A趋于平缓,所以在大质量流量下,R410A没有R22换热性能好,替代工作仍待研究。     

微尺度通道内混合物流动沸腾特性研究

对非共沸混合工质R32／R134a(25％／75％)在微尺度管内的流动沸腾换热特性进行了试验研究。试验结果表明,在较高热流密度下,微尺度管内流动沸腾换热与质量干度和质量流量基本无关,热流密度对换热有着很大的影响,在较宽的热流密度范围内,核态沸腾在换热过程中占据主导地位。和细小管道相比,在相同条件下,微尺度管道内的流动沸腾表面传热系数高于细小管道。     

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%.     

微通道中液氮的流动沸腾——换热特性分析

对微通道中液氮流动沸腾换热特性进行试验研究和分析。给出典型的沸腾曲线,分析壁温、干度和换热系数沿微通道管程的变化规律,考察热流密度、质量流量和压力对流动沸腾换热的影响。将126个试验数据点与四个换热关联式比较,并对微通道中流动沸腾换热机理进行分析。结果表明,在多数情况下干度和热流密度对沸腾换热系数的影响较小,换热系数主要决定于质量流量和压力,随两者增加而增加,换热以对流蒸发为主导机理。KLIMENKO关联式预测效果最好,TRAN微通道关联式次之,对常规管道得到广泛使用的CHEN关联式和SHAH关联式都远远高估了试验值。基于两相流压降和换热特性分析,推知微通道中的两相流流型不同于常规管道：在低干度情况下,流型以弥散泡状流为主;而在高干度情况下,流型以由雾状汽芯和不规则液膜组成的环状流为主。     

R410A在内螺纹强化管管内冷凝的传热性能试验研究

为了研究水平强化单管的管内冷凝性能,搭建了实验台。研究了在冷却水量不变的情况下,R410A在不同冷凝温度(35℃和40℃)和不同管径(5mm和9.52mm)下的换热情况。结果表明:总换热系数和压降随工质质量流量的增大而增大,质量流量对管内换热系数影响不是很大。冷凝温度40℃,5mm铜管的换热系数最高;冷凝温度40℃,9.52mm铜管的压降最小。     

Prediction of forced convective boiling heat transfer coefficient of pure refrigerants and binary refrigerant mixtures inside a horizontal tube

Forced convective boiling heat transfer coefficients were predicted for an annular flow inside a horizontal tube for pure refrigerants and nonazeotropic binary refrigerant mixtures. The heat transfer coefficients were calculated based on the turbulent temperature profile in liquid film and vapor core considering the composition difference in vapor and liquid phases, and the nonlinearity in mixing rules for the calculation of mixture properties. The heat transfer coefficients of pure refrigerants were estimated within a standard deviation of 14% compared with available experimental data. For nonazeotropic binary refrigerant mixtures, prediction of the heat transfer coefficients was made with a standard deviation of 18%. The heat transfer coefficients of refrigerant mixtures were lower than linearly interpolated values calculated from the heat transfer coefficients of pure refrigerants. This degradation was represented by several factors such as the difference between the liquid and the overall compositions, the conductivity ratio and the viscosity ratio of both components in refrigerant mixtures. The temperature change due to the concentration gradient was a major factor for the heat transfer degradation and the mass flux itself at the interface had a minor effect.     

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.     

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.     

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...     

Electrohydrodynamic (EHD) enhancement of boiling heat transfer of R 113+WT4% ethanol

Nucleate boiling heat transfer for refrigerants, R113, and R113+ wt4% ethanol mixture, an azeotropic mixture under electric field was investigated experimentally in a single-tube shell/ tube heat exchanger. A special electrode configuration which provides a more uniform electric field that produces more higher voltage limit against the dielectric breakdown was used in this study. Experimental study has revealed that the electrical charge relaxation time is an important parameter for the boiling heat transfer enhancement under electric field. Up to 1210% enhancement of boiling heat transfer was obtained for R113+wt4% ethanol mixture which has the electrical charge relaxation time of 0.0053 sec whereas only 280% enhancement obtained for R113 which has relaxation time of 0.97 sec. With artificially machined boiling surface, more enhancement in the heat transfer coefficient in the azeotropic mixture was obtained.     

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.

     

氨/水混合物管内强制对流沸腾换热计算

对几组混合工质强制对流沸腾换热系数计算的实验关联式进行分析比较。利用收集到的实验数据，在Bennett和Chen关联式与Rivera关联式的基础上进行管内强制对流沸腾传热的计算。并与Rivera的实验结果进行比较，得到适合氨／水混合物沸腾换热的计算式，并分析影响换热系数的因素。     

Heat transfer characteristics of liquid-solid suspension flow in a horizontal pipe

Particles in liquid-solid suspension flow might enhance or suppress the rate of heat transfer and turbulence depending on their size and concentration, The heat transfer characteristics of liquid-solid suspension in turbulent flow are not well understood due to the complexibility of interaction between solid particles and turbulence of the carrier fluid. In this study, the heat transfer coefficients of liquid-solid mixtures are investigated using a double pipe heat exchanger with suspension flows in the inner pipe. Experiments are carried out using spherical fly ash particles with mass median diameter ranging from 4 to 78 μm. The volume concentration of solids in the slurry ranged from 0 to 50% and Reynolds number ranged from 4,000 to 11,000. The heat transfer coefficient of liquid-solid suspension to water flow is found to increase with decreasing particle diameter. The heat transfer coefficient increases with particle volume concentration exhibiting the highest heat transfer enhancement at the 3% solid volume concentration and then gradually decreases. A correlation for heat transfer to liquid-solid flows in a horizontal pipe is presented.     

CO_2/润滑油混合物在水平管内流动沸腾换热的试验研究

为了测试润滑油对CO2流动沸腾换热特性的影响,以指导CO2换热器的设计,对CO2/润滑油混合物在水平管内的流动沸腾换热系数进行了试验研究,试验工况质量流量为2.74~5.61kg/h,热流密度为3.2~5k W/m2,测试段入口干度为x=0.2~0.5,蒸发温度在-4~8℃之间,选择PAG作为润滑油,浓度为0~6%。试验结果表明,润滑油浓度越大,CO2的局部换热系数越小;润滑油浓度较低时(<3%),换热系数下降较大,再增大含油量,换热系数下降的趋势减缓。增大蒸发温度可以延迟干涸的发生,相反地,大的热流密度和质量流量可以使管内提前出现干涸。CO2/润滑油混合物的换热系数随蒸发温度的升高而增大,随热流密度和质量流量的增大而减小。     

Convective heat transfer analysis of refined kerosene with alumina particles for rocketry application

A detailed experimental study of the fuel flow characteristic and the convective heat transfer of a nano fluid consisting of refined kerosene have been studied in the present work. Low volume fraction of 0.01 and 0.05 of Al₂O₃-refined kerosene has been studied. A horizontal tube of 4 mm in diameter was used for the fluid flow and the heat flow under various Reynolds number ranging from 500 to 5500 were investigated. SEM analysis showed particle size of alumina nanoparticle ranged from 32 nm–50 nm. It was found that, higher the volume fraction of the refined kerosene- Al₂O₃ nano fluid, larger is the heat transfer coefficient. Moreover it was found that with increase in volume fraction of nano particles in the fluid, the nano fluid’s viscosity also increased which results in an increase in friction factor. The friction factor and pumping power were also investigated under various Reynolds number and volume fraction. An increase of 16.33 % in convective heat transfer was observed at Reynolds number of 5500 for a volume fraction of 0.05 %. All results provide strong proof that the addition of nanoparticles intensified the above properties to a considerable rate.

     

Convective heat transfer behaviour of water-ethylene glycol-mixture with silver nanoparticles under laminar flow conditions

In this work, we report the forced convective heat transfer performance and pressure drop of aqueous ethylene glycol seeded with silver nanoparticles for low temperature applications. Experiments were performed in a tube in tube counter-current heat exchanger using silver nanofluid as the hot fluid under laminar flow conditions. In this study, water-ethylene glycol mixture with 70:30 volume percent was used as the base medium. Silver nanofluid was allowed to flow through inner tube of the heat exchanger for varying nanofluid mass flow rates from 5 g/s to 30 g/s and three inlet temperatures of nanofluid viz. 2 °C, 5 °C and 10 °C. The increments in thermal diffusivity and viscosity are found to be ~37 % and ~69 % at 0.45 vol%, respectively. The enhancement in heat transfer coefficient at highest mass flow rate is found to be ~94 % for 0.45 vol%. The pressure drop in the silver nanofluid increases with respect to increase in volume percentage of nanoparticles due to increase in viscosity.