Experimental study of horizontal flattened tubes performance on condensation of R600a vapor

An empirical setup has been established to study heat transfer and pressure drop characteristics during condensation of R600a, a hydrocarbon refrigerant, in a horizontal plain tube and different flattened channels. Round copper tubes of 8.7 mm I.D. were deformed into flattened channels with different interior heights of 6.7 mm, 5.2 mm and 3.1 mm as test sections. The test conditions include heat flux of 17 kw/m², mass velocity in the range of 154.8–265.4 kg/m²s and vapor quality variation from approximately 10% to 80%. Results indicate that flattening the tubes causes significant enhancement of heat transfer coefficient which is also accompanied by simultaneous augmentation in flow pressure drop. Therefore, the overall performance of the flattened tubes with respect to heat transfer enhancement considering the pressure drop penalty is analyzed. It is concluded that the flattened tube with 5.2 mm inner height tube has the best overall performance. Due to the failure of pre-existing correlations for round tube condensation heat transfer, a new correlation is proposed which predicts 90% of the entire data within ± 17% error.     

大管径下R152a冷凝流动与传热特性研究

工质在大管径通道内的流动与传热特性对于有机朗肯循环系统冷凝器设计至关重要。本文实验研究了内径为9 mm的水平光滑管内R152a在质量流速131～306 kg/(m²·s)、饱和温度303～323 K下的冷凝流动与传热特性,在实验数据与传统传热流动关联式对比基础上,对关联式进行了修正。结果表明：R152a冷凝传热系数和摩擦压降梯度在干度、质量流速和饱和温度的影响下呈现相同的变化趋势和变化幅度;质量流速增大对传热的强化作用在高干度区域更加明显;修正后的传热和摩擦压降梯度关联式对实验数据的平均预测偏差分别为5.3%和6.3%,预测精度显著提高。     

铝扁管内R410A冷凝传热特性研究

为了探究制冷剂在多孔铝扁管内的冷凝传热特性,采用实验方法对 R410A在多孔铝扁管内的冷凝传热和压降特性进行了研究。冷凝温度分别为47、40和30℃,单位截面质量流率在200~600kg/(m2·s)。给出了实验 测 试 结 果,并 采 用 公 开 发 表 的 学 术 文 献 中 的 模 型 与 这 些 实 验 测 试 结 果 进 行 了 对 比。 Müller-Steinhagen和 Heck模型预测压降的精度最高,它预测93.3%测试点的偏差在±20%之内,预测100%测试点的偏差在±30%之内。Koyama等模型预 测 传 热 系 数 的 精 度 最 高,它 预 测 93.3%测 试 点 的 偏 差 在 ±10%之内,预测100%测试点的偏差在±20%之内。     

Experimental investigation of the forced convective boiling heat transfer of R-600a/oil/nanoparticle

In this paper an experimental study of convective boiling heat transfer of R-600a/oil/nanoparticle mixtures is investigated. The experimental setup was prepared with a smooth horizontal tube as a test section with the length and diameter of 9.5 and 103 mm, respectively, and pure R-600a was applied for evaluating the heat transfer enhancement. Six mixtures containing 1% weight fraction of R-600a/oil with different concentrations of CuO nanoparticles including 0.0, 0.5, 1.0, 1.5, 2.0 and 5.0% weight fraction of R-600a/oil/nanoparticle were used in our study.The mass velocity per cross area was considered at the range of 50–700 kg/m² s for low vapor quality (ϕ < 0.25). The results showed that the convective boiling heat transfer coefficient will be increased by increasing the mass fraction of nanoparticles up to 2%, while by increasing the mass fraction of nanoparticles up to 5% the heat transfer coefficient will be reduced.     

Experimental investigation of condensation of hydrocarbon refrigerants (R600a) in a horizontal smooth tube

In this study, the experimental results of the condensing heat transfer coefficients of R600a, a hydrocarbon refrigerant, in a horizontal smooth copper tube with an inner diameter of 4 mm and outer diameter of 6 mm are presented at different vapor quality and different mass fluxes during condensation under annular flow conditions, by adjusting the desired vapor qualities at the test area. A specially-designed sight glass has been fitted to the inlet and outlet of the test tube to identify the flow type by naked eye after the inlet vapor quality of the refrigerant to be fed to the test area during the test is adjusted in the system. Thanks to a new method developed in the measuring system, the condensing heat transfer coefficients could be calculated by measuring the difference value (T_s − T_w) directly from the data collection unit. The experimental findings have shown that the condensing heat transfer coefficients drops down with reduction in vapor quality and the coefficient rises with the increase in the mass flux at constant vapor quality. A correlation has been developed from the data obtained. The condensing heat transfer coefficients obtained from the experimental study were seen to be consistent by ± 20% with the correlations developed by Shah, Travis and Cavallini–Zecchin.     

Experimental investigation of condensation heat transfer and pressure drop of R22, R410A and R407C in mini-tubes

Condensation heat transfer and pressure drop of R22, R410A and R407C were investigated experimentally in two single round stainless steel tubes with inner diameter of 1.088 mm and 1.289 mm. Condensation heat transfer coefficients and two phase pressure drop were measured at the saturation temperatures of 30 °C and 40 °C. The mass flux varies from 300 to 600 kg/m² s and the vapor quality 0.1–0.9. The effects of mass flux and vapor quality were investigated and the results indicate that condensation heat transfer coefficients increase with mass flux and vapor quality, increasing faster in the high vapor quality region. The experimental data was compared with the correlations based on experimental data from large diameter tubes (d_h > 3 mm), such as the Shah and Akers correlations et al. Almost all the correlations overestimated the present experimental data, but Wang correlation and Yan and Lin correlation which were developed based on the experimental data from mini-tubes predicted present data reasonably well. Condensation heat transfer coefficients and two phase pressure drop of R22 and R407C are equivalent but both higher than those of R410A. As a substitute for R22, R410A has more advantages than R407C in view of the characteristics of condensation heat transfer and pressure drop.     

Experimental investigation of convective heat transfer coefficient during downward laminar flow condensation of R134a in a vertical smooth tube

This paper presents an experimental investigation of laminar film condensation of R134a in a vertical smooth tube having an inner diameter of 7–8.1 mm and a length of 500 mm. Condensation experiments were performed at mass fluxes of 29 and 263 kg m^?2 s^?1. The pressures were between 0.77 and 0.1 MPa. The heat transfer coefficient, film thickness and condensation rate during downward condensing film were determined. The results show that an interfacial shear effect is significant for the laminar condensation heat transfer of R134a under the given conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

Saturated flow boiling heat transfer and critical heat flux in small horizontal flattened tubes

This paper presents experimental results for flow boiling heat transfer coefficient and critical heat flux (CHF) in small flattened tubes. The tested flattened tubes have the same equivalent internal diameter of 2.2 mm, but different aspect height/width ratios (H/W) of ¼, ½, 2 and 4. The experimental data were compared against results for circular tubes using R134a and R245fa as working fluids at a nominal saturation temperature of 31 °C. For mass velocities higher than 200 kg/m²s, the flattened and circular tubes presented similar heat transfer coefficients. Such a behavior is related to the fact that stratification effects are negligible under conditions of higher mass velocities. Heat transfer correlations from the literature, usually developed using only circular-channel experimental data, predicted the flattened tube results for mass velocities higher than 200 kg/m²s with mean absolute error lower than 20% using the equivalent diameter to account for the geometry effect. Similarly, the critical heat flux results were found to be independent of the tube aspect ratio when the same equivalent length was kept. Equivalent length is a new parameter which takes into account the channel heat transfer area. The CHF correlations for round tubes predicted the flattened tube data relatively well when using the equivalent diameter and length. Furthermore, a new proposed CHF correlation predicted the present flattened tube data with a mean absolute error of 5%.     

油浓度对小管径水平内螺纹管内R404A冷凝换热影响的实验研究

研究了强制对流条件下水平内螺纹管内R404A气液两相流冷凝换热特性,主要讨论油浓度对外径为5 mm的内螺纹管内R404A冷凝换热的影响。实验中油浓度变化范围为0~5%,设置入口平均饱和冷凝温度为40℃,质量流密度变化范围为200~400 kg/(m~2·s),热流密度变化范围为5~45 kW/m~2。实验研究表明:油的出现恶化了换热,在油浓度为1%以下时恶化作用可以忽略,但随着油浓度的增加换热恶化作用越来越明显;对于纯R404A和油浓度为1%的R404A-油混合物,冷凝换热系数随着制冷剂蒸汽干度的降低而逐渐减小;对于油浓度为3%和5%的R404A-油混合物,随着制冷剂蒸汽干度的下降,冷凝换热系数先增加然后逐渐减小,在干度为0.7~0.75之间呈现出一个冷凝换热系数的峰值。同一质量流密度下,换热系数惩罚因子会随着干度的增加而减小,即干度越大,换热恶化作用越大;当质量流密度从200 kg/(m~2·s)增加到400 kg/(m~2·s)时,同一油浓度下油对换热系数的恶化作用相对变小。     

Experimental study of condensation heat transfer of R-134a flow in corrugated tubes with different inclinations

This experimental study is performed to investigate condensation heat transfer coefficient of R-134a flow inside corrugated tube with different inclinations. Different inclinations of test condenser ranging from − 90^° to + 90^° and various flow mass velocities in the range of 87 to 253 [kg/m²s] are considered in this study. Data analysis showed that change in the tube inclination had a significant effect on condensation heat transfer behavior. At low mass velocities, and low vapor qualities, the highest condensation heat transfer coefficient was obtained for α = + 30^° which was 1.41 times greater than the least one obtained for α = − 90^°. The results also showed that at all mass velocities, the highest average heat transfer coefficients were achieved for α = + 30^°. Based on the experimental results, a new empirical correlation is proposed to predict the condensation heat transfer coefficient of R134a flow in corrugated tubes with different inclinations.     

Modeling of condensation heat transfer of refrigerant mixture in micro-fin tubes

This paper presents a new refrigerant-mixture model for condensation in micro-fin tubes. The new refrigerant-mixture model is developed based on a theoretical analysis of turbulent film condensation inside smooth tubes. Several modifications have been implemented in the original smooth-tube model to account for mass transfer thermal resistance between the liquid and vapor phases. The new condensation model is compared with a set of around 200 experimental data points for refrigerant mixtures. The comparison shows that the new model is capable of producing consistent prediction results with a mean absolute deviation (MAD) less than 22% for most of the available data sets. The MAD values obtained with the new model are lower compared to the results obtained using another models.     

Modeling of condensation heat transfer of pure refrigerants in micro-fin tubes

A new semi-empirical condensation model for heat transfer coefficient of pure refrigerants flowing inside micro-fin tubes is presented. The new model is developed based on a theoretical analysis of turbulent film condensation inside smooth tubes. Several modifications have been implemented in the original smooth-tube model to account for the heat transfer enhancement effects due to the presence of micro-fins on the internal wall surface. The new condensation model is compared with a set of around 400 experimental data points. The comparison shows that the new model is capable of producing consistent prediction results with a mean absolute deviation less than 20% for most of the available data sets.     

An Experimental Study of Condensation Heat Transfer in Sub-Millimeter Rectangular Tubes

The characteristics of local heat transfer and pressure drops were experimentally investigated using condensing R134a two-phase flow, in single rectangular tubes, with hydraulic diameter of 0.494, 0.658, and 0.972 mm. New experimental techniques were used to measure the in-tube condensation heat transfer coefficient especially for the low heat and mass flows. Tests were performed for a mass flux of 100, 200, 400, and 600 kg/m2s, a heat flux of 5 to 20 kW/m2, and a saturation temperature of 40℃. In this study, effect of heat flux, mass flux, vapor qualities, and hydraulic diameter on flow condensation were investigated and the experimental local condensation heat transfer coefficients and frictional pressure drop are shown. The experimental data of condensation Nusselt number are compared with previous correlations, most of which are proposed for the condensation of pure refrigerant in a relatively large inner diameter round tubes.     

Experimental investigation of heat transfer coefficient of R134a during condensation in vertical downward flow at high mass flux in a smooth tube

The two-phase heat transfer coefficients of pure HFC-134a condensing inside a smooth tube-in-tube heat exchanger are experimentally investigated. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The test runs are performed at average saturation condensing temperatures between 40–50 °C. The mass fluxes are between 260 and 515 kg m^− 2s^− 1 and the heat fluxes are between 11.3 and 55.3 kW m^− 2. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy transferred from the test section. The effects of heat flux, mass flux and condensation temperature on the heat transfer coefficients are also discussed. Eleven well-known correlations for annular flow are compared to each other using a large amount of data obtained from various experimental conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

An experimental investigation on heat transfer characteristics of multi-walled CNT-heat transfer oil nanofluid flow inside flattened tubes under uniform wall temperature condition

An experimental investigation on heat transfer characteristics of MWCNT-heat transfer oil nanofluid flow inside horizontal flattened tubes has been carried out under uniform wall temperature condition. Nanoparticle weight fractions were 0%, 0.1%, 0.2%, and 0.4%. The copper tubes of 14.5 mm I.D. were flattened and used as the test section of oblong shape with inside heights of 13.4 mm, 11.7 mm, 10.6 mm, and 8.6 mm. The nanofluid flowing inside the tube was heated inside a steam chamber to keep the temperature of the tube wall constant. The required data were acquired for laminar hydrodynamically fully developed regime. The effects of different parameters such as volumetric flow rate, nanoparticle weight fraction, and hydraulic diameter on the heat transfer behavior of the tested systems have been investigated experimentally. For a given flattened tube at a constant nanoparticle weight fraction, increasing volumetric flow rate results in heat transfer enhancement. In addition, as the tube profile becomes more flattened and the hydraulic diameter decreases, the heat transfer coefficient goes up at constant volumetric flow rate. Utilizing nanofluids instead of the base fluid, the heat transfer rate enhances remarkably. The higher the nanoparticles weight fraction, the more the rate of heat transfer enhancement. Finally, the results show that the amount of increase in heat transfer coefficient caused by employing nanofluid instead of the base fluid is comparable to what caused by flattening the tube.     

Experimental investigation of R290/R600a mixture as an alternative to R134a in a domestic refrigerator

R134a is the most widely used refrigerant in domestic refrigerators. It must be phased out soon according to Kyoto protocol due to its high global warming potential (GWP) of 1300. In the present work, an experimental investigation has been made with hydrocarbon refrigerant mixture (composed of R290 and R600a in the ratio of 45.2:54.8 by weight) as an alternative to R134a in a 200 l single evaporator domestic refrigerator. Continuous running tests were performed under different ambient temperatures (24, 28, 32, 38 and 43 °C), while cycling running (ON/OFF) tests were carried out only at 32 °C ambient temperature. The results showed that the hydrocarbon mixture has lower values of energy consumption; pull down time and ON time ratio by about 11.1%, 11.6% and 13.2%, respectively, with 3.25–3.6% higher coefficient of performance (COP). The discharge temperature of hydrocarbon mixture was found to be 8.5 to 13.4 K lower than that of R134a. The overall performance has proved that the above hydrocarbon refrigerant mixture could be the best long term alternative to phase out R134a.     

管外冷凝强化换热管的结构及发展趋势

介绍了各种类型的管外冷凝强化换热管,分析了其强化机理及结构特点,并总结得出:管外冷凝强化管的换热系数与管型有关,且各管型的结构参数对强化传热具有重要的作用.对国内外管外冷凝强化技术研究工作进行分析,结果表明,目前管外冷凝强化换热管的研究主要集中于翅片形状、翅片密度、翅片高度等结构参数对换热性能的影响.强化换热管的冷凝传热性能不仅与翅片结构参数有关,而且也与管材的表面特性和导热系数有关.管外冷凝强化换热管的研究重点是开发新型三维结构翅片的双侧强化管并研究其传热关联式,以及研究不锈钢等低成本材料制造的强化管换热管的传热性能和强化结构的优化.     

Theoretical investigation of stable dropwise condensation heat transfer on a horizontal tube

The heat transfer model of stable dropwise condensation for saturated vapor on a horizontal tube is developed based on previous theoretical models. Through a comprehensive analysis of all the contributing thermal resistances, the convection effect inside the droplet itself is taken into consideration in the model. For the stable dropwise condensation process in dynamic conservation, a method of double integration of heat flux through numerous inclined plates with different inclination angles is introduced to obtain the overall heat flux through the horizontal tube surface. The model can predict the variation of heat transfer of stable dropwise condensation with different contact angles outside a horizontal tube. The influences of contact angle, temperature difference, and other typical parameters on both a single droplet and the whole condensation process are discussed. The results indicate that a high contact angle can cause a size reduction of falling droplets from condensing surface and thus taking more heat away. The adsorbed condensate film adds an extra thermal resistance and its thickness plays a significant role on the dropwise condensation heat transfer.     

Experimental investigation of twisted tape inserts performance on condensation heat transfer enhancement and pressure drop

A comprehensive experimental investigation is conducted on the augmentation of heat transfer coefficients and pressure drop during condensation of HFC-134a in a horizontal tube at the presence of different twisted tape inserts. The test section is a 1.04 m long double-tube counter-flow heat exchanger. The refrigerant flows in the inner copper and the cooling water flows in annulus. The experiments are performed for a plain tube and four tubes with twisted tapes inserts of 6, 9, 12 and 15 twist ratios. The pressure drop is directly measured by a differential pressure transducer. It is found that the twisted tape with twist ratio of 6 gives the highest enhancement in the heat transfer coefficient and the maximum pressure drop compared to the plain tube on a nominal area basis. For this case the enhancement in heat transfer and the pressure drop are increased by 40 and 240% in comparison with to the plain tube. It is observed that the twisted tape with the twist ratio of 9 has the best performance enhancing the heat transfer with the minimum pressure drop. Also empirical correlations are developed to predict smooth tube and swirl flow pressure drop. Predicted results are compared to experimental data and it is found that these correlations are reliable for pressure drop estimation.