Heat Transfer Performance of Heat Pipe Radiator with SiO2/Water Nanofluids

The effect of nanofluids on thermal performance of the miniature heat pipe radiator which was assembled by two heat pipes containing 0.6 vol.% SiO₂/water nanofluids and 30 pieces of rectangular aluminum fins was investigated experimentally and theoretically. The wall temperatures of the miniature heat pipe and fin surface temperatures were measured. Results showed that the utilization of SiO₂/water nanofluids as a working fluid in the heat pipe enhanced the heat performance by reducing wall temperature differences. Compared with Deionized water (DI water), the thermal resistance of the miniature heat pipe with SiO₂/water nanofluids decreased by about 23% to 40%. Furthermore, the theoretical calculation on a basis of one dimension found that the fin heat dissipation in the miniature heat pipe radiator charged SiO₂/water nanofluids was about 1.17 times of that of the DI water radiator.     

Oil fouling in double‐pipe heat exchanger under liquid‐liquid dispersion and the influence of copper oxide nanofluid

Dispersions of oil in water are encountered in a variety of industrial processes leading to a reduction in the performance of the heat exchangers when thermally treating such two phase fluids. This reduction is mainly due to changes in the thermal and hydrodynamical behavior of the two phase fluid. In the present work, an experimental investigation was performed to study the effects of light oil fouling on the heat transfer coefficient in a double‐pipe heat exchanger under turbulent flow conditions. The effects of different operating conditions on the fouling rate were investigated including: hot fluid Reynolds number (the dispersion), cold fluid Reynolds number, and time. The oil fouling rate was analyzed by determining the growth of fouling resistance with time and through pressure drop measurements. The influence of copper oxide (CuO) nanofluid on the fouling rate in the dispersion was also determined. It was found that the presence of dispersed oil causes a reduction in heat transfer coefficient by percentages depending on the Reynolds number of both cold and hot fluids and the concentration of oil. In addition, the time history of fouling resistance exhibited different trends with the flow rates of both fluids and its trend was influenced appreciably by the presence of CuO nanofluid.     

Experimental investigation of heat transfer enhancement in a double pipe heat exchanger with a twisted inner pipe

In the present work, an experimental investigation is conducted to address the influence of inner pipe twisting on the overall performance of a double pipe heat exchanger. With the fluid to fluid heat exchange, both parallel and counter flow directions are examined as well. In addition to the original elliptical pipe, three pipes with different numbers of twisting (3, 5, and 7 twists per unit length) constructed from the elliptical pipe are considered where the heat transfer rate and pressure drop are addressed. All tests are carried out in the turbulent flow regime where the Reynolds number (Re) ranging from 5000 to 26,000 and water is used as the working medium. The obtained outcomes show that for both flow directions, there is an enhancement in the heat exchanger overall performance with all considered twisting pipes. The maximum enhancement in the Nusselt number is found to be 1.8 for the parallel flow and around 2.2 for the counter flow compared with the original pipe. The inner pipe with 7 twists, however, improves the overall performance the most, where a maximum performance enhancement factor of 1.63 and 1.9 are observed at Reynolds number of 26,000 in the parallel and counter flow configurations, respectively.     

Numerical evaluation and the effects of geometrical and operational parameters on thermal performance of the shell and double coil tube heat exchanger

Heat exchangers are extensively used in various industries. In this study, the impact of geometric and flow parameters on the performance of a shell and double helical coil heat exchanger is studied numerically. The investigated geometric parameters include external coil pitch, internal coil pitch, internal coil diameter, and coil diameter. The influences of considered geometrical parameters are analyzed on the output temperature of the hot and cold fluid, convective heat transfer coefficient, pressure drop, and average Nusselt number. Water is considered as working fluid in both shell and tube. As an innovation, double helical coils are used instead of one in the heat exchanger. To compare the obtained results accurately, in each section, the heat transfer area (coil outer surface) is kept constant in all models. The results show that the geometrical parameters of double helical coils significantly affect the heat transfer rate.     

Estimation of Nusselt number and effectiveness of double‐pipe heat exchanger with Al2O3–, CuO–, TiO2–, and ZnO–water based nanofluids

This paper deals with experimental studies carried out to analyze heat transfer characteristics of Al₂O₃–, CuO–, TiO₂–, and ZnO–water based nanofluids in a double‐pipe, counter flow heat exchanger for different volume concentrations (0.025%, 0.05%, 0.075%, and 0.1%) of the nanofluids. The fabricated double‐pipe heat exchanger is made up of two different materials, viz., copper as the inner tube and unplasticized polyvinyl chloride as the outer tube. The density, viscosity, and thermal conductivity were calculated, and were used to estimate dimensionless numbers, such as Reynolds number, Prandtl number, and Nusselt number, and also to estimate heat exchanger effectiveness. High‐energy ball milling technique was used to prepare nanoparticles and were characterized using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. Polyvinyl alcohol (3%) was used as a surfactant for making the nanofluids stable. It was observed from the experiment that with the increase in the volume concentration, thermal conductivity, viscosity, and friction factor increase, whereas the Reynolds number decreases. The experimentally observed data for Nusselt number were formulated into a correlation that matches the data for all these nanofluids within an error of 11.4%. It was found that the highest effectiveness was obtained while using TiO₂–water nanofluids than other nanofluids.     

Influence of air‐flow imbalance and refrigerant flow path pattern on an evaporator's performance

This paper presents the influence of air flow imbalance on performance of an evaporator and the effect of the refrigerant flow path pattern. To estimate the influence, we applied a thermal network method that takes refrigerant quality distribution in the heat exchanger into account. The prediction shows that performance of the evaporator falls by 6% with a 40% imbalance of air flow rate for a simple 2 rows and 2 paths heat exchanger. Also, the prediction shows that the performance level will be recovered to when the air flow distribution is uniform by optimizing refrigerant flow path pattern. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20260     

Heat transfer and flow characteristics of offset fins in the low-Reynolds-number region

The characteristics of heat exchangers with offset-type plate fins for space stations are studied for Reynolds numbers less than 300 based on the hydraulic diameter. A three-dimensional analysis is carried out to study the effects of the following parameters on the heat transfer and the flow characteristics: (a) the thermal boundary layer developing on the bottom plate and on the fins on the plate, (b) the aspect ratio (height/pitch) of the cross section of the flow passage, the fin thickness, the fin length in the direction of the flow, the thermal conductivity of the fluid and the fins, and the Prandtl number of the fluid. The results obtained are as follows. (1) The heat-transfer coefficient on the fin surface is characterized by the thermal-conductivity ratio of fluid to fin material. When the thermal conductivity of the fin material approaches that of the fluid, the heat-transfer coefficient on the fin surface becomes low. (2) The optimum condition of the aspect ratio depends on the value of the thermal-conductivity ratio between the fluid and the fins. (3) When the aspect ratio becomes large or small, the friction factor of offset fins approaches that of fully developed duct flow with the same aspect ratio as the Reynolds number decreases. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(4): 249–261, 1997     

多管型套管式换热器传热与流阻性能试验研究

多管型套管式换热器是在大尺寸外管的内部布置多根内管所构成的换热设备 ,与单根内管的套管式换热器相比 ,流量大幅增加 ,选用螺纹内管和管间折流板可以强化传热。对于多管型套管式换热器的传热性能试验 ,采用修正威尔逊法进行试验计算 ,得出了两种多管型套管式换热器的传热与流动阻力性能试验结果。     

The hydraulic‐thermal performance of miniature compact heat sinks using SiO2‐water nanofluids

Ever since the rapid increase in both the demand for the miniature electronic devices and their applications, heat dissipation in the electronic components has been a serious issue. A miniature plate‐pin heat sink model with square, circular, and elliptic pins is considered to enhance the hydrothermal performance of this kind of compact heat sink (CHS). Water and 3% of SiO₂‐water nanofluids of volume fraction were used with different Reynolds number ranges (100‐1000). The findings show that the base temperature of heat sink reduces while the Nusselt number enhances by using nanofluids and increasing Reynolds number. The lowest value of the base temperature is nearly 25% for the square pins and circular pins CHSs compared with a plate–fin heat sink at 3% of nanofluids. Furthermore, the highest value of the Nusselt number is about 98% at 3% SiO₂ for circular pin CHSs compared with the plate–fin heat sink. However, the pressure drop of CHSs is higher than that of plate–fin heat sink. Moreover, the most significant hydrothermal performance value is about 1.44 for water and around 1.51 for SiO₂ as using the CHS with circular and elliptic pins depends on the Reynolds number. Thus, applying CHSs with nanofluids instead of the traditional heat sinks might produce a substantial enhancement in the hydrothermal performance of heat sinks.     

A numerical investigation of the effect of sinusoidal temperature on mixed convection flow in a cavity filled with a nanofluid with moving vertical walls

The aim of this study is to investigate numerically the effect of sinusoidal temperature on mixed convection flow in a cavity filled with nanofluid and moving vertical walls by using a new temperature function, where thermal heating takes the form of the sinusoidal temperature; and could be found in various natural processes and industries such as solar energy, and cooling of electronic components. The heating is concentrated in the center and then distributed to both ends at different values of Rayleigh numbers, Reynolds numbers, and volumetric fractions of nanoparticles ranging from 1.47 × 10³ to 1.47 × 10⁶, 1 to 100, and 0 to 0.1, respectively. The impact of nanoparticle size on thermal characteristics and hydrodynamics was considered and evaluated. From the results, the volume fraction concentration of nanoparticles affects the flow shape and thermal performance in the case of a constant Reynolds number. Moreover, the effect of nanoparticles decreases with the increase of the Reynolds number. Besides this, with increasing the volume percentage of nanoparticles, the rate of heat transmission increases. It is worth noting that the presence of nanoparticles results in height convective heat transfer coefficient. On the other hand, the thickness of thermal boundary layers decreases with increasing Rayleigh number. The current investigation found that the (sinusoidal) temperature change significantly affects heat transfer.     

Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions

This paper reports an experimental work on the convective heat transfer of nanofluids, made of γ-Al₂O₃ nanoparticles and de-ionized water, flowing through a copper tube in the laminar flow regime. The results showed considerable enhancement of convective heat transfer using the nanofluids. The enhancement was particularly significant in the entrance region, and was much higher than that solely due to the enhancement on thermal conduction. It was also shown that the classical Shah equation failed to predict the heat transfer behaviour of nanofluids. Possible reasons for the enhancement were discussed. Migration of nanoparticles, and the resulting disturbance of the boundary layer were proposed to be the main reasons.     

Impact of volume fraction and fluid layer thickness on heat transfer effectiveness of water-based nanofluids

The heat transfer effectiveness of nanofluids is adversely affected by the delay in convection onset. The lesser effectiveness, when compared to that of base fluid, is observed in a range of nanofluid layer thickness. The heat transfer coefficient of water–Al₂O₃ nanofluid can be enhanced by sustaining the equilibrium between Rayleigh number, temperature, particle volume fraction, and enclosure aspect ratio. In this paper, the specific correlation of fluid layer thickness and the onset of convection, which can significantly dominate the heat transfer characteristics of nanofluids are investigated using the concept of critical Rayleigh number. The water layer thickness for convection onset is first experimentally assessed for different real-life heat flux densities. It is then performed for Al₂O₃–water nanofluid for varying volume fractions. With the increase in volume fraction even though thermal conductivity increases, the overall heat transfer enhancement of the nanofluid is reduced. Temperature involved (heat flux density), the volume fraction of the nanofluid used, nanofluid layer thickness (space availability for the cooling system), and mass of the nanoparticle influence heat transfer enhancement. A higher volume fraction may not always result in enhancement of heat transfer as far as nanofluids are concerned.     

流动参数对双级螺旋套管换热器传热性能的数值分析

高效热泵系统性能研究一直是热泵空调领域普遍关注的热点问题,针对设计开发的双级套管串联式热泵系统。采用3D有限容积法和可实现的k-ε模型,数值分析入口流体温度、流动速度对换热系数以及内外管努塞尔数的影响规律。结果表明,降低入口水温或者增加入口制冷剂温度能够提高整体传热性能,Nui随着水和制冷剂流率的增加有所增加,而Nuo随着水流率的增加而增加,但随着制冷剂流率则增加而减小,Nui 和Nuo都随着水温的减小或制冷剂温度的增大而增大。     

Enhancement of shell side forced convective heat transfer in the shell tube‐type heat exchanger using thin plate‐type supports

The shell side heat transfer and pressure drop in counterflowing water were experimentally investigated on the basis of the overall heat transfer coefficient. The investigation was intended to identify ways to get higher performance for the cooler in a BWR nuclear power plant. The following three conclusions were reached in the study. (1) Predicted performance of the heat exchanger, using the overall heat transfer coefficient based on the outside area of the tube K_o, indicated an enhancement by 92% compared with the measured performance of the conventional segmental baffle‐type heat exchanger. (2) The tube side pressure drop ΔP_t=20 kPa and the shell side pressure drop ΔP_s=70 kPa were obtained, and were within the allowable value ΔP_a=80 kPa. The shell side pressure drop of the low‐pressure drop spacer could be decreased by 20% as compared with that of the standard spacer. (3) The enhancement constant of the shell side heat transfer using the low‐pressure drop spacer was about 1.2 times as large as that of the standard spacer, regardless of the pumping power. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 455–471, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10097     

Heat transfer and fluid flow of nanofluids in laminar radial flow cooling systems

Nanofluids are considered as interesting alternatives to conventional coolants. It is well known that traditional fluids have limited heat transfer capabilities when compared to common metals. It is therefore quite conceivable that a small amount of extremely fine metallic particles placed in suspension in traditional fluids will considerably increase their heat transfer performances. A numerical investigation into the heat transfer enhancement capabilities of coolants with suspended metallic nanoparticles inside a radial, laminar flow cooling configuration is presented. Temperature dependant nanofluid properties are evaluated from experimental data available in recent literature. Results indicate that considerable heat transfer increases are possible with the use of relatively small volume fractions of nanoparticles. Generally, however, these are accompanied by considerable increases in wall shear-stress. Results also show that predictions obtained with temperature variable nanofluid properties yield greater heat transfer capabilities and lower wall shear stresses when compared to predictions using constant properties.     

椭圆管散热器传热及阻力性能试验研究

对空气横掠片距不相等的叉排椭圆翅片管散热器的传热及阻力性能进行了试验研究,得到试件在一系列工况下的传热与管外流动阻力数据,并对试验数据进行分析计算,从总传热系数K中分离出管外空气侧的对流换热系数h,给出有工程应用价值的管外换热准则关系式及管外阻力准则关系式。认为椭圆管管外的平均换热效果优于圆管。在相同的流通截面积下椭圆管传热周边较长,换热面积相应增加,因此结构上允许布置得更紧凑。     

Efficiency and effectiveness enhancement of an intercooler of two‐stage air compressor by low‐cost Al2O3/water nanofluids

The present study was aimed to utilize low‐cost alumina (Al₂O₃) nanoparticles for improving the heat transfer behavior in an intercooler of two‐stage air compressor. Experimental investigation was carried out with three different volume concentrations of 0.5%, 0.75%, and 1.0% Al₂O₃/water nanofluids to assess the performance of the intercooler, that is, counterflow heat exchanger at different loads. Thermal properties such as thermal conductivity and overall heat transfer coefficient of nanofluid increased substantially with increasing concentration of Al₂O₃ nanoparticles. Specific heat capacity of nanofluids were lower than base water. The intercooler performance parameters such as effectiveness and efficiency improved appreciably with the employment of nanofluid. The efficiency increased by about 6.1% with maximum concentration of nanofluid, that is, 1% at 3‐bar compressor load. It is concluded from the study that high concentration of Al₂O₃ nanoparticles dispersion in water would offer better heat transfer performance of the intercooler.     

A numerical study of flow behavior in the shell and helical finned-tube heat exchanger

Extended surfaces mostly aim to improve the heat transfer upon increasing the area of heat transfer. In this paper, the influence of using fins on flow behaviors and the heat transfer of the shell and tube heat exchanger has been investigated. In this regard, the present results are verified with available experimental data in the literature for a helical tube without fins. The effects of fin density (fin per inch), its height, and material have been studied on the heat transfer rate. In addition, the effects of radial pitch and the number of coil loops are studied. The results indicate that implementing extended surfaces significantly increases the heat transfer rate. The increase of fin density from 8 to 12 and the height from 11.5 to 13.5 mm enhances heat transfer up to 48% and 43% depending on Dean number, respectively. The rise of coil pitch augments the overall heat transfer, and it is more efficient at lower Dean numbers. The predicted results also show that the fin material does not have any significant effect on heat transfer.