Effect of twisted tape insert on heat transfer and pressure drop in horizontal evaporators for the flow of R-134a

An experimental study has been carried out on the heat transfer enhancement and pressure drop characteristics in presence of twisted tape inserts, during flow boiling of R-134a, inside a horizontal evaporator. The test-evaporator was an electrically heated 1260 mm long copper tube with 7.5 mm inside diameter. The experiments were performed for plain flow and four tubes with twisted tapes of 6, 9, 12 and 15 twist ratios and four refrigerant mass velocities of 54, 85, 114 and 136 kg/s m² for each tape. It has been found that the twisted tape inserts enhance the heat transfer coefficient on relatively higher pressure drop penalty, in comparison to that for the plain flow.     

管内扭带插入件强化沸腾换热技术的研究现状及展望

制冷设备对换热器紧凑化和小型化的需求促使人们开发新型的强化传热技术,而管内扭带插入件是一种廉价且易于制造的被动强化传热技术,在制冷系统蒸发器中具备良好的应用潜力。扭带插入两相沸腾换热的管中能够增大表面传热系数,但同时也增大了管内压降。分析发现,通常情况下,质量流量、干度的变化与表面传热系数和压降的变化呈正相关,而管径、扭率、饱和温度的变化与表面传热系数和压降的变化呈负相关。沸腾换热过程复杂、评价指标选取不一、实验工况数量有限等因素是导致各学者总结的扭带插入的最佳条件不一致的主要原因。本文收集了各作者预测的内插扭带管内沸腾换热的表面传热系数和压降的关联式,认为管内扭带插入件还需要进一步明确最佳使用条件,并需要结合蒸发器整机或变频压缩机加以研究。     

Experimental investigation of heat transfer enhancement in helical coil heat exchangers using water based CuO nanofluid

The present work deals with the study of heat transfer enhancement using water based CuO nanofluids in the helical coil heat exchanger. Nanofluids were prepared using two-step method by using wet chemical method. Nanofluids with various volume percentage between 0 and 0.5 of CuO nanoparticles and their flow rate between 30 and 80 LPH (Reynolds number ranging from 812 to 1895, Laminar flow regime) were considered in the present study. The setup consists of a test section (helical coil), cooler, reservoir, pump, flow meter, thermocouples and flow controlling system. The temperature measurements were carried out with the help of thermocouples. The investigation was carried out to study the effect of particle loading and flow rate on heat transfer coefficient and Nusselt number. It has been found that the increase in the loading of CuO nanoparticles in base fluid shows a significant enhancement in the heat transfer coefficient of nanofluid. In the present study, at 0.1 vol% concentration of CuO nanoparticles in nanofluid, enhancement in heat transfer coefficient was 37.3% as compared to base fluid while at 0.5 vol%, it is as high as 77.7%. Also with the increase in the flow rate of the CuO nanofluid, significant increase in heat transfer coefficient was observed.     

Effects of tangential and radial velocity on fluid flow and heat transfer for flow through a pipe with twisted tape insert—laminar flow

The present study reports the numerical analysis of fluid flow and heat transfer in a pipe with full length twisted tape insert. The investigation is carried out for five different twist ratios of 4, 5, 6, 8 and 10 at 100 ≤ Re ≤ 1000. The velocity field in terms of streamwise, tangential and radial velocity and temperature field are studied as a function of Reynolds number and twist ratio. The variation of friction factor and Nusselt number with Reynolds number for different twist ratios is also presented. The heat transfer enhancement due to insertion of twisted tape mainly comes from the tangential and radial components of velocities, which are regarded as secondary fluid motion. It is evident from the results that with increase in Reynolds number the axial convection increases. However, with the decrease in the twist ratio, the tangential and radial convection increases, leading to increased heat transfer. The secondary flow affects the thermal boundary layer inside the tube and increases the cross-flow mixing, which increases the heat transfer. The correlations for prediction of friction factor and Nusselt number based on the numerical data are also proposed.     

一种自动清洗式传热强化新元件 ——旋流口椭圆齿平带

针对斜齿螺旋扭带的制造困难,提出了一种自动清洗式传热强化新元件--旋流口椭圆齿平带,每个椭圆齿都是使平带自转的流体动力结构元素,平带两侧的旋流口使管内的部分液体形成螺旋线流,因而具有近似斜齿螺旋扭带那样能够在较低流速下自转、实现管内污垢在线自动清洗的性能。同时流体经过椭圆齿后产生的大量涡流、管内部分液体转变成的螺旋线流以及螺旋线流与轴向流线不断交错混合等三方面的综合作用,使管内对流传热得到强化。对椭圆齿的齿距、斜角和旋流口三个因素进行了试验研究,结果表明：旋流口椭圆齿平带在管内流速为0.5 m/s左右就能可靠旋转运行;相对于光滑螺旋扭带,旋流口椭圆齿平带的管内侧传热系数可提高171％;其在管内流速为0.625 m/s时的流体阻力为3.25 kPa/m,流体阻力在一般工程许可的范围内。因此,旋流口椭圆齿平带具有良好的综合性能,并且又容易制造,具有较高的工程应用价值。     

Scalable microwave-assisted continuous flow synthesis of CuO nanoparticles and their thermal conductivity applications as nanofluids

We have demonstrated the novel and scalable synthesis of CuO nanoparticles by an integration of microwave and flow synthesis. The shape and size of CuO nanoparticles were tuned by changing the concentration of copper precursor. The production rate of CuO nanoparticles was found to be 5?g/h with 70% conversion of copper acetate into the CuO nanoparticles. The thermal conductivity of CuO nanofluid prepared in ethylene glycol showed linear enhancement with increase in the volume content of CuO nanoparticles produced in batch and flow reactors.     

Improving hydrothermal performance of hybrid nanofluid in double tube heat exchanger using tapered wire coil turbulator

Tapered wire coil insert is proposed as a novel enhancer in the double tube heat exchanger and experimental studies on Al₂O₃ + MgO hybrid nanofluid flowing under the turbulent condition are performed to investigate the hydrothermal characteristics. Effects of using tapered wire coil turbulator and hybrid nanofluid on the hydrothermal behaviors are examined for different coil configurations (Converging (C) type, Diverging (D) type and Conversing-Diverging (C-D) type) and hybrid nanofluid inlet temperatures and volume flow rates. Results show that D-type wire coil insert promotes better hydrothermal performance as compared to C-type and C-D type. Nusselt number and friction factor of hybrid nanofluid using D-type, C-D type and C-type wire coil inserts enhance up to 84%, 71% and 47%, and 68%, 57% and 46%, respectively than that of water in tube without insert. The entropy generation of hybrid nanofluid is lower than that of base fluid in all cases. The thermal performance factor for hybrid nanofluid is found more than one with all inserts. The thermal performance factor is observed a maximum of 1.69 for D-type coil. The study reveals that the hybrid nanofluid and tapered wire coil combination is promising option for improving the hydrothermal characteristics of double pipe heat exchanger.     

Numerical study on heat transfer and entropy generation of developing laminar nanofluid flow in helical tube using two-phase mixture model

Nanofluids and helical tubes are among the best methods for heat transfer enhancement. In the present study, laminar, developing nanofluid flow in helical tube at constant wall temperature is investigated. The numerical simulation of Al₂O₃-water nanofluid with temperature dependent properties is performed using the two-phase mixture model by control volume method in order to study convective heat transfer and entropy generation. The numerical results is compared with three test cases including nanofluid forced convection in straight tube, velocity profile in curved tube and Nusselt number in helical tubes that good agreement for all cases is observed. Heat transfer coefficient in developing region inside a straight tube using mixture model shows a better prediction compared to the homogenous model. The effect of Reynolds number and nanoparticle volume fraction on flow and temperature fields, local and overall heat transfer coefficient, local entropy generation due to viscous dissipation and heat transfer, and the Bejan number is discussed in detail and compared with the base fluid. The results show that the nanofluid and the base fluid have almost the same axial velocity profile, but their temperature profile has significant difference in developing and fully developed region. Entropy generation ratio by nanofluid to the base fluid in each axial location along the coil length showed that the entropy generation is reduced by using nanofluid in at most length of the helical tube. Also, better heat transfer enhancement and entropy generation reduction can be achieved at low Reynolds number.     

Assessment and analysis of binary hybrid nanofluid impact on new configurations for curved-corrugated channel

The current numerical paper introduces the flow and heat transfer characteristics across a new configuration channel, namely: the curved-corrugated channel, using binary hybrid nanofluid. E-shaped baffles with different geometrical parameters have been employed while CuO / MgO-water nanofluid is experimentally prepared with different volume fractions 0.0–5%. Measured thermophysical properties is utilized to simulate the flow and heat transfer characteristics by adopting the κ-ε model. The influences of corrugations, baffles, and geometric parameters; gap ratio (GR = 0.2,0.3,0.4, and 0.5), blockage ratio (BR = 0.2,0.25,0.3, and 0.35), and pitch angle (β = 10°, 12.5°, and 15°) at different Reynolds number (8000–28000) are evaluated using thermal–hydraulic performance method. The outcomes show that vortex flow and increased turbulence will increase heat transfer due to influences of corrugations and baffles. It is confirmed that the flow variations governed by the geometric parameters of the design and the best performance produce at lowest pitch angle 10°, lowest gap ratio (GR = 0.2) and highest blocking ratio (BR = 0.35). Regards the fluid medium, CuO / MgO particles improve the thermophysical properties of the base fluid and thereby boost the thermal performance of the system. It has found new correlations between the Nusselt number, friction Factor and design parameters of tested channel with using binary hybrid nanofluid.     

A new model for flow boiling heat transfer coefficient inside horizontal tubes with twisted-tape inserts

This paper presents a new method to predict the heat transfer coefficient during flow boiling inside horizontal tubes containing twisted tape inserts. The method was developed based on the database presented by Kanizawa et al. This database comprises flow boiling results for horizontal tubes with internal diameters of 12.7 and 15.9 mm, twisted-tape ratios of 3, 4, 9 and 14, mass velocities ranging from 75 to 200 kg m⁻² s⁻¹, heat fluxes of 5 and 10 kW m⁻² and saturation temperatures of 5 and 15 °C. The method is flow-pattern based and considers flow boiling, dryout and mist flow regions. The predictive method also takes into account the physical picture of the swirl flow phenomenon by considering swirl flow effects promoted by the twisted tape insert. The proposed method provides satisfactory predictions and captures the main heat transfer trends of the data of Kanizawa et al. and also of independent data from literature.     

Heat transfer characteristics of refrigerant-based nanofluid flow boiling inside a horizontal smooth tube

The objective of this paper is to investigate the influence of nanoparticles on the heat transfer characteristics of refrigerant-based nanofluid flow boiling inside a horizontal smooth tube, and to present a correlation for predicting heat transfer performance of refrigerant-based nanofluid. For the convenience of preparing refrigerant-based nanofluid, R113 refrigerant and CuO nanoparticles were used. Experimental conditions include an evaporation pressure of 78.25 kPa, mass fluxes from 100 to 200 kg m⁻² s⁻¹, heat fluxes from 3.08 to 6.16 kW m⁻², inlet vapor qualities from 0.2 to 0.7, and mass fractions of nanoparticles from 0 to 0.5 wt%. The experimental results show that the heat transfer coefficient of refrigerant-based nanofluid is larger than that of pure refrigerant, and the maximum enhancement of heat transfer coefficient is 29.7%. A heat transfer correlation for refrigerant-based nanofluid is proposed, and the predictions agree with 93% of the experimental data within the deviation of ±20%.     

Measuring the heat-transfer coefficient of nanofluid based on copper oxide in a cylindrical channel

The heat-transfer coefficient of nanofluid during its flow in a cylindrical channel is studied experimentally. The studied nanofluid was prepared based on distilled water and CuO nanoparticles. Nanoparticle concentration varied in the range from 0.25 to 2% in the volume. The nanofluid was stabilized using a xanthane gum biopolymer the mass concentration of which did not exceed 0.03%. Considerable intensification of heat transfer was found. The nanofluid appeared to be Newtonian when particle concentrations exceeded 0.25%. Estimates for rheological parameters of the nanofluid and thermal conductivity coefficient have been obtained.     

Refrigerant boiling at low heat flux in vertical tubes with heat transfer enhancing fittings

The paper presents the results of thermal and flow analyses of the boiling process in vertical tubes with heat transfer enhancing inserts. Tests were performed for three different geometrical shapes of the inserts: a spiral tape with a core rod, a rib insert and a spring insert. Experimental measurements were performed for R507, R410A and R407C refrigerants at low heat flux. The obtained results indicate the increase ratio of heat transfer and flow resistance coefficients in tubes with inserts, as opposed to a plain one. Finally, the paper presents the dimensionless relationships which enable the calculation of heat transfer coefficients and pressure drops during boiling in vertical tubes with the studied heat transfer enhancing inserts.     

Measurement and correlation of frictional pressure drop of refrigerant-based nanofluid flow boiling inside a horizontal smooth tube

The objective of this paper is to investigate the effect of nanoparticle on the frictional pressure drop characteristics of refrigerant-based nanofluid flow boiling inside a horizontal smooth tube, and to present a correlation for predicting the frictional pressure drop of refrigerant-based nanofluid. R113 refrigerant and CuO nanoparticle were used for preparing refrigerant-based nanofluid. Experimental conditions include mass fluxes from 100 to 200 kg m^?2 s^?1, heat fluxes from 3.08 to 6.16 kW m^?2, inlet vapor qualities from 0.2 to 0.7, and mass fractions of nanoparticles from 0 to 0.5 wt%. The experimental results show that the frictional pressured drop of refrigerant-based nanofluid increases with the increase of the mass fraction of nanoparticles, and the maximum enhancement of frictional pressure drop is 20.8% under above conditions. A frictional pressure drop correlation for refrigerant-based nanofluid is proposed, and the predictions agree with 92% of the experimental data within the deviation of ±15%.     

A comprehensive analysis for second law attributes of spiral heat exchanger operating with nanofluid using two-phase mixture model: Exergy destruction minimization attitude

The present article focuses on the second law attributes of a counter-flow spiral heat exchanger working with an Al₂O₃–H₂O nanofluid with employing the two-phase mixture model. To improve the cogency of the simulations, the turbulence modeling is performed using four-equation transition Shear Stress Transport (SST) model. The simulations are conducted for different nanoparticle volume fractions and nanofluid flow rates. It is shown that by dispersing further nanoparticles in the common fluid, the total entropy generation of the hot nanofluid significantly diminishes, whereas the cold water and the heat exchanger body exhibit higher thermal entropy generation. The overall exergy destruction in the heat exchanger significantly decreases by the increase of the volume fraction, while it tends to increase by the flow rate increment. For instance, an about 9.2% reduction in the overall exergy destruction is observed as the volume fraction increases from 0.01 to 0.04. All the conditions exhibit great second law efficiency so that the minimum second law efficiency is larger than 0.84, and increases with the raise of either the volume fraction or flow rate.     

Oscillatory non-Newtonian flow in tubes of slowly varying radius

Summary The paper studies low Reynolds number flow of a non-Newtonian fluid in an axisymmetric tube of slowly varying radius which is subjected to an axial oscillatory pressure gradient. It is observed that the leading approximation is affected by the visco-elastic coefficient. In the higher approximation particular attention is centered around the steady streaming components for both small and large values of the frequency of oscillation. On the overall the combined effect of visco-elast c and cross viscosity parameter is to induce a radial pressure gradient. For the velocity components the Newtonian and non-Newtonian effects are of the same magnitude when the frequency is small; but when this frequency is large the non-Newtonian effects swamp the flow velociites. When the results are applied to a locally constricted tube, flow reversal is possible downstream of this constriction. The most striking feature however is the condition of zero velocity at a locally constricted tube for the steady streaming velocities—upstream of this constriction the velocity is positive while downstream it is negative. In pathophysiology thrombus formation in constrictions is believed to be caused by aggregation of platelets due to endothelium damage. The condition of zero steady streaming velocity at the constriction is another possible explaination of platelet accumulation and possible blood cloting.With 2 Figures     

Effect of Using CuO–Oil Nanofluid on Surface Roughness and Thermal Performance during Superfinishing Process

This research aims to investigate the effect of adding copper oxide nanoparticles to the oil Gr-6004 base fluid and its concentration changes from 0.1% to 0.4% on the surface roughness of gudgeon pin and the thermal conductivity of the nanofluids during the superfinishing process. The main novelty of this investigation is analyzing the impact of utilizing CuO/oil Gr-6004 nanofluid on thermal conductivity of oil and surface quality of gudgeon pin during superfinishing process. Based on the results, adding nanoparticles to the oil Gr-6004 has significantly reduced the surface roughness. In addition, by increasing the concentration of nanoparticles to 0.4%, the surface roughness has decreased by 57% compared to oil Gr-6004. Also, by adding nanoparticles, the thermal conductivity of the nanofluids has increased to 19.5%. In addition, dispersing CuO nanoparticles into base fluid reduces oil temperature by 17.44%.     

Investigation of turbulent heat transfer and nanofluid flow in a double pipe heat exchanger

In present study, heat transfer and turbulent flow of water/alumina nanofluid in a parallel as well as counter flow double pipe heat exchanger have been investigated. The governing equations have been solved using an in-house FORTRAN code, based on finite volume method. Single-phase and standard k-ε models have been used for nanofluid and turbulent modeling, respectively. The internal fluid has been considered as hot fluid (nanofluid) and the external fluid, cold fluid (base fluid). The effects of nanoparticles volume fraction, flow direction and Reynolds number on base fluid, nanofluid and wall temperatures, thermal efficiency, Nusselt number and convection heat transfer coefficient have been studied. The results indicated that increasing the nanoparticles volume fraction or Reynolds number causes enhancement of Nusselt number and convection heat transfer coefficient. Maximum rate of average Nusselt number and thermal efficiency enhancement are 32.7% and 30%, respectively. Also, by nanoparticles volume fraction increment, the outlet temperature of fluid and wall temperature increase. Study the minimum temperature in the solid wall of heat exchangers, it can be observed that the minimum temperature in counter flow has significantly reduced, compared to parallel flow. However, by increasing Reynolds number, the slope of thermal efficiency enhancement of heat exchanger gradually tends to a constant amount. This behavior is more obvious in parallel flow heat exchangers. Therefore, using of counter flow heat exchangers is recommended in higher Reynolds numbers.     

基于径向基神经网络模型预测CuO-ZnO/(乙二醇-水)混合纳米流体导热系数

采用两步法制备CuO-ZnO质量分数为0~3 wt%的CuO-ZnO/(乙二醇(EG)-水)混合纳米流体。其中,纳米颗粒CuO和ZnO质量比为50∶50不变,基液混合比(EG与去离子水的质量比)变化范围为20∶80~80∶20,分析其导热系数随温度(25~60℃)及基液比的变化规律。然后,以质量分数、温度及基液混合比为自变量,导热系数为因变量,采用径向基神经网络(RBFNN)模型预测导热系数,并与反向传播神经网络(BPNN)模型和多元线性回归(MLR)模型的预测值对比。结果表明,CuO-ZnO/(EG-水)纳米流体导热系数随温度的升高呈非线性增大,当CuO-ZnO质量分数为3 wt%及基液混合比为20∶80时,其导热系数与纯基液相比增大了14.03%~23.47%;但随着基液中的EG含量增大,导热系数非线性下降。总之,CuO-ZnO/(EG-水)纳米流体的导热系数受粒子随机运动和温度变化呈非线性变化。采用RBFNN模型预测CuO-ZnO/(EG-水)纳米流体的导热系数,其结果与BPNN模型和MLR模型对比,RBFNN模型性能评价指标均方根误差(RMSE)、平均相对百分比误差(MRPE)及误差平方和(SSE)更接近于0,多元统计系数R2更接近于1,说明RBFNN模型预测导热系数的精度更高,能够较好地表征不同参数对导热系数的影响,为CuO-ZnO/(EG-水)纳米流体的热物理性能参数的预测提供了一种有效的数据驱动建模方法。      

Non-Newtonian fluid flow model for ceramic tape casting

The current device of miniaturisation and higher device counts in integrated circuit (IC) packages has significantly increased the use of both multilayer ceramic packages (MLCP) and multilayer capacitors (MLC). Currently, one of the main methods used for the manufacture of flat ceramic packages with precise thickness control and consistency is the tape casting technique. Since these tapes can be cast with thickness of about 100 μm, it is crucial that the control of green tape thickness is precise, and that these thickness values are reproducible consistently. The flow of the slurry onto the casting surface can be modelled as a two dimensional fluid flow through a parallel channel. By choosing a suitable constitutive model, the predictions of the proposed model and existing models were compared with experimental results. The proposed model accurately described the fluid flow characteristics of the process, and had good agreement with experimental results.