In this paper, single-phase heat transfer characteristics of water inside internal enhanced tubes were investigated. Refrigerants are boiling or condensing outside the tube. The experimental tubes have internal helical rib heights of 0.25-0.36 mm, helix angles of 40-60 degrees, rib base thicknesses of 0.40-0.79 mm, rib tip thicknesses of 0.078-0.283 mm, and Ns (starts number per circle) of 40-50. It shows that the heat transfer enhanced ratios usually range from 2.3 to 3.64. The friction factors relative to the smooth tube are about 1.8 to 3.3 times higher. Analyzing the effect of rib geometry on flow and heat transfer, it was found that the higher the height of the internal rib, the better the enhancement of convective heat transfer in the tube. Also, the greater the thickness of the rib tip and base, the more detrimental to the friction factor in the tube. There was no noticeable influence on the heat transfer performance as the helix angle increased from 45 degrees to 50 degrees. For the increase of Ns, it appears that 45 ribs per circle is the best value in the present study when considering the increase in pressure loss. The thermal-hydraulic performance of 11 tubes was also evaluated. It shows that Tube-1 had the best performance in the condensing tubes and Tube-7 had the best performance in the boiling tubes.

采用数值模拟方法,对不同工况下内螺纹管的湍流特性进行了分析.通过设置不同螺旋角、不同流速、不同肋高和不同肋条数等内螺纹管参数,对不同工况下的流动阻力和阻力系数进行比较,探讨了各参数对流动阻力和阻力系数的影响.研究结果表明,雷诺数和内螺纹管参数对内螺纹管阻力特性的影响显著;采用数值模拟方法可揭示内螺纹管的湍流特性,为内螺纹管的设计和优化提供参考.

为提高内螺纹管换热效率,采用 Fortran语言编程,以水为介质(其 Pr= 5.42),对在雷诺数 Re= 20000～60000、螺旋角α= 15～45°、齿高 E= 0.20～0.60mm、齿条数 Ns= 20～52条件下,内螺纹管内流体周期性充分发展的湍流传热性能进行了研究.通过分析雷诺数Re、螺旋角α、齿高 E、齿条数 Ns的变化对努赛尔数Nu的影响,以及 Nu关于Re的指数关系变化,得出了在研究范围内增大 Re、α、E、Ns,均可提高内螺纹管换热能力的结论.其中螺旋角α对内螺纹管传热性能的影响最为明显,而齿条数 Ns的增大对内螺纹管传热性能的影响并不显著.

以某公司高精特齿轮减速机扩建搬迁项目中联合厂房的恒温机加车间为研究对象,用数值计算方法模拟车间内单侧侧送风、双侧侧送风、方形散流器送风和旋流送风4种不同送风方式,研究送风方式对车间内速度场、温度场和湿度场的影响.对比分析得出,旋流送风方式的温、湿度场不仅能满足工艺要求,而且其温、湿度场分布情况相较于单侧侧送风、双侧侧送风和方形散流器送风更加均匀,因此提出采用旋流送风作为恒温机加车间的送风方案.

提高人才培养质量的关键是提高课程教学质量.在教学实践中,教师可将人才培养目标分级细化到每一节课的学习目标,以帮助学生、教师、教学管理者检验学习成效.教师通过对学科基本概念的讲解,合理引入教学法,帮助学生突破核心概念,使学生建立起较为完整的知识结构,并找到适合自己的学习方法,提升思维质量,进入深度学习的境界.教学管理者应侧重于目标管理,辅以过程指导,并为课程团队建设、资源建设和课程考核等做好服务工作.

In this work, the condensation of refrigerants on a single, high-density, low-fin tube and full-sized shell and tube condensers were investigated experimentally. The low-fin tube had an external fin density of 56 fins per inch (fpi) and fin height 1.023 mm. Another three-dimensional (3D) finned tube was also tested for comparison. The condensing heat transfer coefficient of the refrigerant R134a was first investigated outside a single horizontal tube at saturation temperature of 40 °C. The overall heat transfer coefficients of the two tubes were similar in magnitude. The condensing heat transfer coefficient of the low-fin tube was 16.3–25.2% higher than that of 3D enhanced tube. The experiments of the two condensers mounted with low-fin and 3D enhanced tubes were then conducted in centrifugal and screw chiller test rigs. It was found that chillers with the two different condensers generally had the same refrigeration capacity under the same experiment conditions. The refrigeration capacity of the screw chiller was smaller. It had fewer tube rows and elicited fewer inundation effects owing to the falling condensate. The heat transfer coefficients of the condensers with R134a in centrifugal chillers equipped with high-density low-finned tubes were higher than those in the screw chillers. The total number of tubes for low-fin tube condensers, in the two chillers, was reduced by approximately 15% compared with the use of domestic advanced condensers equipped with the 3D enhanced tubes.

The effect of vapor flow on the falling film evaporation of refrigerant R134a outside a horizontal tube bundle is investigated with an experimental approach. The test space is a cube with a rectangular cross section of 0.575 m (length) x 38.8 mm (width). The tube bundle was 3 x 6 (columns x rows) of staggered horizontal finned tubes made of copper. The longitudinal tube pitch is 22.5 mm and the transverse is 19.9 mm. The external fin density of test tube is 45 fpi (fins per inch), and outside diameter is 19.05 mm. The vapor flow velocity can be adjusted in the range of 0-3.1 m/s. Liquid falling film flow rate ranges from 0.07 to 0.2 kg/m.s. Experiment is firstly conducted at saturation temperature of 6 degrees C without the effect of additional vapor flow at the heat flux of 20, 60, 100 and 180 kW/m(2) (for the first tube row). Vapor flow effect experiment was carried out at three heat fluxes 20, 40 and 60 kW/m(2). It is found that falling film flow rate is an important factor to influence the evaporating heat transfer coefficient. With the effect of vapor flow, both positive and negative effects are observed as the increment of vapor velocity. Positive effects are predominant for the two tubes in the top positions and higher vapor velocity. (C) 2015 Elsevier Ltd. All rights reserved.

An experimental investigation on the pool boiling heat transfer of refrigerant R134a outside three enhanced tubes is conducted. The heat flux is from 10,000 to 370,000 W/m(2). The heat transfer is substantially enhanced at the heat flux less than 200 kW/m(2). An increase of heat transfer coefficient up to 330% above the plain tube is observed. However, at the heat flux higher than 200 kW/m(2), it is found that the heat transfer coefficient of enhanced tubes is even lower than the plain tube. The same features are also observed for other enhanced tubes in the literature. (C) 2017 Elsevier Ltd. All rights reserved.

根据郑州地区夏季太阳辐射和空调负荷特点,以HFO1234yf为制冷剂,基于实验测试和模拟计算,针对某一太阳能喷射制冷空调系统运行性能进行研究,该系统供冷对象为200 m2别墅,采用40 m2真空管式太阳能集热装置和一个带有辅助加热装置的集热水箱作为热源,计算分析冷负荷、辅助加热量以及太阳能集热量的关系.结果表明:5～9月份,郑州地区典型气象条件下,太阳能集热效率、系统喷射系数、系统COP、太阳能喷射制冷系统综合性能COPo随月份呈波动变化,均在7月份达到最小值;在5～9月份期间,太阳能制冷系统制冷月贡献率在0.46～0.95间波动,9月份制冷贡献率最大达到0.95,7月份制冷贡献率最小达到0.46.

For the flooded evaporator in refrigeration or air conditioning systems,refrigerant is boiling on the shell side and water is flowing in the tube side.The pool boiling heat transfer coefficient of R134a outside one smooth tube and one reentrant cavity enhanced tube No.1 is investigated with an experimental approach.At the saturate temperature of 6,10 and 16℃,the heat flux of 10-250 kW · m-2,the heat transfer coefficient versus heat flux of smooth tube is investigated and compared with Cooper correlation.The external diameter of smooth tube and enhanced tube are 15.93 mm and 25.36 mm,respectively.At the heat flux of 10-250 kW · m-2,it is found that the deviation of experimental result and Cooper correlation is within ± 15％.the average value of m in h ∝qm,is 0.67.For the enhanced tube,the enhanced ratio is the largest at the heat flux less than 40 kW · m-2.The enhanced ratio is decreasing as the increment of heat flux.At the heat flux larger than 250 kW · m-2,the heat transfer coefficient of enhanced tube is approaching the smooth tube,and even smaller than smooth tube.

The thermo-hydraulic performance of two shell and tube condensers was investigated with an experimental approach. The experiment is conducted in a water cooled centrifugal chiller test rig. The condensers are made of three-dimensional (3-D) and high fin density integral-finned (2-D) tubes. 2-D and 3-D tubes all have the diameter of 3/4 inch (19mm). The 2-D tube has external fin density of 56fpi (fins per inch), fin height 1.023mm and 48 internal ribs per circle. The 3-D enhanced tube has the external fin density of 45fpi, fin height of 0.981mm and 45 internal ribs per circle. The 3-D tube is widely used in the water cooled chillers. 2-D tube is a newly designed surface with enhanced external fin density. Condensing heat transfer coefficient of R134a outside single horizontal tube is firstly tested at saturate temperature of 40°C. At the internal water velocity of 2.2m/s, the overall heat transfer coefficients of 2-D tube is in the range of 10364.7 to 12420.9W/m2K, 4.2% ∼ 9.0% higher than 3-D tube. External condensing heat transfer coefficient is 16.3% ∼ 25.2% higher than 3-D tube. The condensers are manufactured with these two types of tubes. Both condensers have the same geometric parameters except the tubes and tube bundle space. The length of tube in the condenser is 4000mm. The tube bundles are arranged in a staggered mode. For the integral-fin tube condenser, the longitudinal tube pitch of tube arrays is 23mm in rows and the transverse is 20mm. At the same power input and cooling water inlet temperature of 32°C, the cooling power of 2-D tube condenser are respectively of 1755.4kW and 1769.4kW; 3-D tube condenser is 1727.5kW and 1770.5kW. The pressure drop increased about 11.2% ∼ 15.9% for the 2-D tube condenser compared with 3-D tube condenser. Generally, the two condensers have the same heat transfer performance, while the integral-fin tube condenser saves 15% of copper material consumption.

Film condensing of refrigerant R134a on single horizontal tube coated with open cell copper foam was investigated at saturation pressures of 10.1 Bar and 8.87 Bar. Two foam thicknesses of 1.6 and 2.5 mm, three pore densities of 40, 80 and 130 PPI(Pores per inch) as well as the porosity of 90%, characterize the six tubes in the experiment. Heat flux in this work ranges from 10 to 60 kW/m(2). It is found that saturation vapor pressure within the measurement range has almost no effect on the condensing heat transfer in the range of 8.87-10.1 Bar. Film condensing depends much on foam thickness with the pore density of 40 PPI, compared with 80 PPI and 130 PPI. The heat transfer coefficient generally ranges from 2 to 6 kW/m(2) K for the six tubes, which decreases linearly up to the heat flux of 60 kW/m(2) on a log log plot. The enhanced ratio of all the coated tubes compared with that of the plain tube ranges from 2 to 3. Minor increment of the enhanced ratio is observed as the increment of heat flux. (C) 2014 Elsevier Ltd. All rights reserved.

在工况温度分别为6℃和10℃,对R134a在光管和三根双侧强化管(F38,F46,F56)外进行池沸腾换热试验研究.结果表明:两种工况下,R134a在光管外沸腾表面传热系数与Cooper公式计算值相对偏差均在±15％以内,R134a在强化管外沸腾表面传热系数变化趋势与光管Cooper公式基本一致.受试验管外表面孔隙直径的影响,当热流密度小于50kW/m2(或壁面温差小于2K)时,孔隙直径越小,沸腾换热效果越好;反之,当热流密度大于50kW/m2(或壁面温差大于2K)时,孔隙直径大的强化表面沸腾换热效果要优于小孔隙直径表面.

为了了解新型制冷剂——HFO1234yf的太阳能喷射制冷系统的运行性能状况,建立了太阳能喷射制冷系统性能分析计算模型,结合西安地区的气象条件,重点研究了HFO1234yf喷射制冷系统的系统性能系数,典型日条件下的系统集热效率、综合性能系数的变化特点.研究表明HFO1234yf喷射制冷系统的系统性能系数随蒸发温度的升高而升高,在典型日气象条件下,HFO1234yf喷射制冷系统的集热效率呈现先上升后下降的趋势,性能系数COP以及系统综合性能系数COP0的逐时变化在不同典型日下的变化趋势接近.在典型日6月28日气象条件下,10:00-16:00时段内,集热面积为40 m2时的太阳能喷射制冷系统可以为西安地区面积为200 m2的住宅建筑提供43％左右的冷量.

为考核内螺纹管阻力系数与对流表面传热系数之间的依变关系,对6根内螺纹管在管内水流速1.0～3.5 m/s范围内进行加热或冷却试验,采用Wilson试验法获得管内对流表面传热系数.基于相同质量流量的评价指标(Nu/Nup)/(f/fp),流体被加热工况接近1,流体被冷却工况在1.2左右,对Gnielinski推广公式作出合理修正.将修正的Gnielinski推广公式的预测结果与6根内螺纹管试验结果进行对比,125组试验数据中90％的数据两者偏差在±10％以内.

Experimental studies of boiling heat transfer of R417A on three single horizontal doubly-enhanced tubes have been conducted at the saturation temperature of 8 ℃. A modified Wilson plot technique was used to obtain the boiling heat transfer coefficients, and the thermal resistance analysis was conducted. The results indicate that the intube heat transfer coefficients of the three enhanced tubes are within 2.524-2.658 times as those of smooth tube, which were rolled 0.32-0.34 mm high spiral channel. For the same type of enhanced heat transfer surface, the trends of boiling heat transfer coefficient with the wall temperature difference are similar. Boiling heat transfer coefficients of tube E30 (42 fin per inch) are 4.5 percent higher than those of tube E32(50 fpi) at the same temperature difference. It is indicated that the boiling heat transfer coefficient and its trend were closely related with fin density (pore diameter) and tube type. Boiling heat transfer of R417A on doubly- enhanced tubes, the outer thermal resistance was about 70% of the total thermal resistance, further enhancement was useful for improving the overall heat transfer performance. Boiling heat transfer coefficient of R417A is only one third of R134a at approximate conditions, R134a is the main component of R417A, indicating that the boiling heat transfer of mixed refrigerants are quite different with pure refrigerants.

Experimental studies of film condensation of R134aand mixed R134a/R125at three different concentrations have been conducted on three tubes,one is smooth tube,the others are two-dimensional and three-dimensional enhanced tubes at the same fin density.The results indicate that the predicted condensation heat transfer coefficients of R134aon smooth tube from Nusselt theory agree with the experimental data within ±10percent.The condensation heat transfer coefficients of pure R134aare consistent with Nusselt theoretical trend for smooth and enhanced tubes.Compared with pure R134a,the condensation heat transfer coefficients of mixed R134a/R125 are all decreased.For smooth tube, condensation heat transfer coefficient of mixed refrigerants decreased with the increase of temperature difference.But for enhanced tubes,condensation heat transfer coefficients increased with the increase of temperature difference,which is close to pure R134aat high temperature difference,the mixed refrigerant including 6 percent and more R125.Indicating that condensation heat transfer of mixed refrigerants is quite different with pure refrigerants.Condensation heat transfer coefficient of three-dimensional enhanced tube is higher than that of the two-dimensional tubes,and two-dimensional tube is significantly higher than that of smooth tube at the same working fluid.The condensation heat transfer enhancement factor of HT-3D, HT-2D are 9.83 and 7.85,respectively, which compared with smooth tube at the temperature difference 8K.