Pool boiling characteristics of multiwalled carbon nanotube (CNT) based nanofluids over a flat plate heater

This paper is mainly concerned about the pool boiling heat transfer behavior of multi-walled carbon nanotubes (CNTs) suspension in pure water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS). Three different concentrations of 0.25%, 0.5% and 1.0% by volume of CNT dispersed with water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS) were prepared and boiling experiments were conducted over a stainless steel flat plate heater of size 30 mm² and 0.44 mm thickness. The test results exhibit that the addition of carbon nanotubes increases boiling heat transfer coefficients of the base fluids. At a given heat flux of 500 kW/m², the enhancement of heat transfer coefficient was found to be 1.5, 2.6 and 3.0 times of water corresponding to 0.25%, 0.5% and 1.0% concentration of CNT by volume in water, respectively. In water–CNT–surfactant nanofluid, it was found that 0.5% of CNT concentration gives the highest enhancement of 1.7 compared with water. In both water and water–surfactant base fluids, it was observed that the enhancement factor for 0.25% of CNT first increases up to the heat flux of 66 kW/m² and then decreases for higher heat fluxes. Further, the overall heat transfer coefficient enhancement in the water–CNT nanofluids is approximately two times higher than that in the water–CNT–surfactant nanofluids. With increasing heat flux, however, the enhancement was concealed due to vigorous bubble generation for both water–CNT and water–CNT–surfactant nanofluids. Foaming was also observed over the liquid-free surface in water–CNT–surfactant nanofluids during the investigation. No fouling over the test-section surface was observed after experimentation.     

Preparation and pool boiling characteristics of copper nanofluids over a flat plate heater

The copper nanoparticles of average size of 10 nm have been prepared by the sputtering method and characterized through atomic force microscopy (AFM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The pool boiling heat transfer characteristics of 0.25%, 0.5% and 1.0% by weight concentrations of copper nanoparticles has been studied. Different copper based nanofluids were prepared in both, distilled water and distilled water with 9.0 wt% of sodium lauryl sulphate anionic surfactant (SDS). The pool boiling heat transfer data were acquired for the boiling of nanofluids over a 30 mm square and 0.44 mm thick stainless steel plate heater. The experimental results show that for the critical heat flux of pure water is 80% higher than that of water–surfactant fluid. Also, it was found that the critical heat flux for 0.25%, 0.5% and 1.0% concentrations of copper nanoparticles in copper–water nanofluids are 25%, 40% and 48% higher than that of pure water. But in the case of copper–water with surfactant nanofluids comparing with pure water, the CHF decreases to 75%, 68%, and 62% for respective concentrations of copper nanoparticles. The heat transfer coefficient decreases with increase of nanoparticles concentration in both water–copper and water–copper with surfactant nanofluids.     

多壁碳纳米管纳米流体的大容积沸腾

通过在直径为12 mm的沸腾表面进行的多壁碳纳米管阿拉伯树胶水溶液的大容积沸腾实验,研究其沸腾传热特性。纳米流体的沸腾传热效果弱于阿拉伯树胶水溶液,烧毁点的过热度增加而临界热流密度减小,同时,阿拉伯树胶水溶液的传热效果劣于水。纳米颗粒在沸腾加热表面富集、结垢引起液体密度、沸腾表面上活化核心数目的变化,随传热时间的延长,垢层结构包括毛细孔直径、空隙率、垢层厚度不断发生变化,进而引起蒸汽在毛细孔中的流动阻力不断增加、加热表面和垢层间热阻增加,沸腾表面的活化核心数目减小,阿拉伯树胶在蒸发表面的局部富集、黏度大大增加,最终导致沸腾传热恶化。     

Free convection film boiling over a discrete flat heater with different surface orientations

In the present numerical study, a two-dimensional, laminar, free convection saturated film boiling of water at near critical conditions over a discrete heater surface flush mounted over a flat plate has been investigated. For the numerical simulations, a multi-directional advection algorithm based Coupled Level Set and Volume of Fluid (CLSVOF) interface capturing method is used. In this study, both the flow and heat transfer characteristics are evaluated for various heater sizes, wall superheats, and the angle of orientations of the flat plate. The numerical modeling of the three phase moving contact lines is evaluated with the experimental data of a single droplet impact and spreading over a horizontal flat solid surface. From the numerical study, it is observed that the heat transfer rate from a small size discrete heater surface is higher compared to that of a large size heater surface. It is also observed that the variation of the plate orientation angle, α of the heater surface from upward facing horizontal position (α?=?0°) to vertical position (α?=?90°) shows increase in the magnitude of upward rising vapor velocity due to increase in the buoyancy force. It leads to higher rate of heat transfer at vertical position compared to that of horizontal position.     

Pool boiling on a superhydrophilic surface

Titanium Dioxide, TiO₂, is a photocatalyst with a unique characteristic. A surface coated with TiO₂ exhibits an extremely high affinity for water when exposed to UV light and the contact angle decreases nearly to zero. Inversely, the contact angle increases when the surface is shielded from UV. This superhydrophilic nature gives a self-cleaning effect to the coated surface and has already been applied to some construction materials, car coatings and so on. We applied this property to the enhancement of boiling heat transfer. An experiment involving the pool boiling of pure water has been performed to make clear the effect of high wettability on heat transfer characteristics. The heat transfer surface is a vertical copper cylinder of 17 mm in diameter and the measurement has been done at saturated temperature and in a steady state. Both TiO₂-coated and non-coated surfaces were used for comparison. In the case of the TiO₂-coated surface, it is exposed to UV light for a few hours before experiment and it is found that the maximum heat flux (CHF) is about two times larger than that of the uncoated surface. The temperature at minimum heat flux (MHF) for the superhydrophilic surface is higher by 100 K than that for the normal one. The superhydrophilic surface can be an ideal heat transfer surface. Copyright © 2002 John Wiley & Sons, Ltd.     

Pool boiling heat transfer of functionalized nanofluid under sub-atmospheric pressures

A water-based functionalized nanofluid was made by surface functionalizing the ordinary silica nanoparticles. The functionalized nanoparticles were water-soluble and could still keep dispersing well even at the mass concentration of 10% and no sedimentation was observed. An experimental study was carried out to investigate the pool boiling heat transfer characteristics of functionalized nanofluid at atmospheric and sub-atmospheric pressures. The same work was also performed for DI water and traditional nanofluid consisted of water and ordinary silica nanoparticles for the comparison. Experimental results show that there exist great differences between pool boiling heat transfer characteristics of functionalized and traditional nanofluid. The differences mainly result from the changes of surface characteristics of the heated surface during the boiling. A porous deposition layer exists on the heated surface during the boiling of traditional nanofluid; however, no layer exists for functionalized nanofluid. Functionalized nanofluid can slightly increase the heat transfer coefficient comparing with the water case, but has nearly no effects on the critical heat flux. It is mainly due to the changes of the thermoproperties of nanofluids. Traditional nanofluid can significantly enhance the critical heat flux, but conversely deteriorates the heat transfer coefficient. It is mainly due to effect of surface characteristics of the heated surface during the boiling. Therefore, the pool boiling heat transfer of nanofluids is governed by both the thermoproperties of nanofluids and the surface characteristics of the heated surface.     

Pool boiling characteristics of nano-fluids

Common fluids with particles of the order of nanometers in size are termed as ‘nano-fluids’ which have created considerable interest in recent times for their improved heat transfer capabilities. With very small volume fraction of such particles the thermal conductivity and convective heat transfer capability of these suspensions are significantly enhanced without the problems encountered in common slurries such as clogging, erosion, sedimentation and increase in pressure drop. This naturally brings out the question whether such fluids can be used for two phase applications or in other words phase change in such suspensions will be assistant or detrimental to the process of heat transfer. The present paper investigates into this question through experimental study of pool boiling in water-Al₂O₃ nano-fluids. The results indicate that the nano-particles have pronounced and significant influence on the boiling process deteriorating the boiling characteristics of the fluid. It has been observed that with increasing particle concentration, the degradation in boiling performance takes place which increases the heating surface temperature. This indicates that the role of transient conduction in pool boiling is overshadowed by some other effect. Since the particles under consideration are one to two orders of magnitude smaller than the surface roughness it was concluded that the change of surface characteristics during boiling due to trapped particles on the surface is the cause for the shift of the boiling characteristics in the negative direction. The results serve as a guidance for the design of cooling systems with nano-fluids where an overheating may occur if saturation temperature is attained. It also indicates the possibility of such engineered fluids to be used in material processing or heat treatment applications where a higher pre-assigned surface temperature is required to be maintained without changing the fluid temperature.     

Performance of suspended flat plate air heater

In this communication, a heat transfer model to predict the transient behaviour of a suspended flat plate solar collector with constant flow of fluid (air) above the absorber has been presented. A reflecting sheet with an air gap between the absorber plate and bottom insulation to reduce heat loss has been used. The effect, on performance of the air heater, of the parameters viz, spacing between cover and plate, heat capacity of air and absorber plate, flow rate of fluid and collector length have been studied. The effect of changing the averaging inlet temperature with varying collector length has also been studied.     

蜂窝热管平板式太阳能热水器的实验研究

把蜂窝技术和热管技术结合起来应用于平板式太阳能热水器，可以大大减少热损失，提高日平均效率。经多次实验证明，蜂窝热管平板式太阳能热水器的日平均效率可以比真空管式热水器的高，热损系数则要小很多，具有推广应用价值。     

Pool boiling heat transfer of water and nanofluid outside the surface with higher roughness and different wettability

In order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m² to 300kW/m². Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m², and then the heat transfer degraded at higher heat flux.     

Pool boiling of saturated FC-72 on nano-porous surface

Pool boiling heat transfer from nano-porous surface immersed in a saturated FC-72 dielectric fluid has been experimentally studied at atmospheric pressure (101 kPa). The data obtained from nano-porous surface (Anodisc 25) of thickness about 70 μm made from aluminum oxide (Al₂O₃) obtained from Whatman, were compared to that of a plain surface (aluminum) of thickness about 105 μm. From the experimental data obtained it was evident that there is a reduction of about 30% in the incipient superheat for the applied power for nano-porous surface over plain surface. SEM photographs of the nano-porous coating were taken for determining the size of the pores.     

Pool boiling heat transfer with nano-porous surface

Anodizing technique has been recognized as an efficient way to grow the well-ordered oxide nano-structures on metal substrate. In the present experimental study, the nucleate pool boiling heat transfer coefficient and long-term performance of nano-porous surface fabricated by the cost-effective and simple anodizing technique were investigated with water. The incipient wall superheat of pool boiling in nano-porous surface was lower than that in non-coating surface. The nucleate boiling heat transfer coefficient of nano-porous coating surface appeared higher than that of non-coating surface particularly at the low heat flux condition. The higher coefficient remained throughout 500 h of operation.     

Pool boiling characteristics of low concentration nanofluids

The pool boiling behavior of low concentration nanofluids (?1 g/l) was experimentally studied over a flat heater at 1 atm. Boiling of nanofluids produces a thin nanoparticle film, on the heater surface, which in turn is believed to increase the critical heat flux. The present study also indicates that the nanoparticle deposition results in transient characteristics in the nucleate boiling heat transfer. Finally, this study investigates possible causes responsible for the deposition of nanoparticle on the heater surface. Experimental evidence shows that microlayer evaporation, during nanofluid boiling, is responsible for the nanoparticle coating formed on the heater surfaces.     

Fluid oscillations in flat plate boiling water collectors

Boiling water collectors with an internal two-phase thermosiphon and buoyancy driven circulation are apt for process steam generation in the temperature range 70–150°C. Exploiting the advantages of boiling heat transfer requires mastering the instabilities of two-phase flow natural convection. With electrically heated mock-up test devices the boundaries for stable flow conditions were investigated. Between these boundaries, periodic mass flow fluctuations occur and may stimulate resonances of collector components. The fluctuation frequencies measured with the collector mock-up were below 1 Hz. The origin of the oscillations is still under investigation.     

Numerical and experimental analysis of the flat plate solar water heater systems' thermal performance

In this paper, improving the thermal performance of flat plate solar water heater systems by inserting different tube configurations inside the riser pipes has been numerically and experimentally studied. This study is focused on increasing the moving of energy from riser pipes to the operating fluid within the riser pipes. To achieve that, the diameter of the riser pipes was increased along with the insertion of different tube configurations within them, namely, smooth, helical, and wavy tubes, keeping the same amount of the operating liquid. A comparison was performed to determine the best in terms of coefficient of heat transfer of the operating liquid, mass flow rate of the operating liquid, pressure drop, and water temperature in the storage tank, as a thermal performance indication of the system under study. The findings show the model consisting of a straight tube inside the riser pipe provides the best thermal performance. In terms of thermal performance, the straight model outperforms the conventional model by 12.3%. An experimental and numerical comparison between the optimum model (straight model) was conducted. The study proves that the average difference between numerical results and experimental findings is 7.2%.     

Performance of a two channel suspended flat plate solar air heater

In the present communication, an analytical model to study the performance of a two channel suspended flat plate air heater is presented. The effect of different parameters, e.g. spacing between the plates, plate length, same and different flow rates of air in the two channels of the air heater, on the air temperature has been studied. A comparison of single channel and two channel air heaters has also been made. Numerical calculations have been made for a typical cold day in Delhi, viz. Jan. 26, 1980.     

Performance studies on flat plate solar air heater subjected to various flow patterns

The performance of flat plate solar air heater subjected to various flow patterns (over flow, under flow and double pass) were experimentally investigated. An aluminium flat plate of 1.2. × 0.7 m² size was employed as absorber plate. Two glass plates of similar size were used as protection for heat loss from absorber plate to atmosphere. Performance of the solar air heater at various mass flow rates (0.014, 0.0279 and 0.042 kg/s) was also reported. Thermal efficiency is recorded higher during double pass experimental conditions. Heat gained by air is found higher at experimental conditions having double pass when compared with over flow and under flow.     

Pool boiling heat transfer on the tube surface in an inclined annulus

An experimental study was carried out to investigate the pool boiling heat transfer in an inclined annular tube submerged in a pool of saturated water at atmospheric pressure. The outer diameter and the length of the heated inner tube were 25.4 mm and 500 mm, respectively. The gap size of the annulus was 15 mm. For the tests, annuli with both open and closed bottoms were considered. The inclination angle was varied from the horizontal position to the vertical position. At a given heat flux, the heat transfer coefficient was increased with the inclination angle increase. Effects of the inclination angle on heat transfer were more clearly observed in the annulus with open bottoms. The main cause for the tendencies was considered as the difference in the intensity of liquid agitation and bubble coalescence due to the enclosure by the outer tube. One of the important factors in the annulus with open bottom was the convective fluid flow.     

Effect of porous media on the performance of the double-pass flat plate solar air heater

The heat transfer characteristics and performance of the double-pass flat plate solar air heater with and without porous media are studied numerically. The mathematical models described the heat transfer characteristics of the double-pass flat plate solar air heater are derived from the energy conservation equations. The implicit method of finite-difference scheme is employed to solve these models. The effect of the thermal conductivity of the porous media on the heat transfer characteristics and performance is considered. The results obtained from the model are validated by comparison with experimental data of previous researchers. There is reasonable agreement between the present model and experiment.