Pool boiling heat transfer in a vertical annulus with a stepped outside tube

Effects of a stepped outside tube on nucleate pool boiling heat transfer in a vertical annulus with closed bottoms have been investigated experimentally. For the study saturated water at atmospheric pressure and a heated tube of 200 mm length and 19 mm diameter were used. The shape of an outside tube of the annulus has step geometry and the ratio between the upper and lower regions are varied from 0.23 to 4.43. The existence of the stepped outside tube changes heat transfer characteristics and its effect becomes evident as the gap ratio is smaller than 1. For the tested gap ratios heat transfer deterioration is observed as the heat flux is higher than 80 kW/m² comparing to the single unrestricted tube. The major cause for the tendency is attributed to the loss of liquid agitation intensity around the stepped regions of the annulus.     

Pool boiling heat transfer on a vertical tube with a partial annulus of closed bottoms

To improve pool boiling heat transfer in an annulus with closed bottoms, the length of an outer tube has been changed between 0.2 m and 0.6 m. A heated tube of 19.1 mm diameter and the water at atmospheric pressure have been used. Three annular gap sizes of 3.65, 6.35, and 17.95 mm have been investigated. To elucidate effects of the outer tube results of the annulus are compared with the data of a single unrestricted tube. The change in the outer tube length results in much variation in heat transfer. As the outer tube length is much shorter than the heated tube the deterioration point of heat transfer gets moved up to the higher heat fluxes and the possibility of CHF creation is prevented. The major cause of the tendencies is related with the decrease in the intensity of bubble coalescence.     

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 heat transfer in a vertical annulus as the bottom inflow area changes

To identify effects of the bottom inflow area on pool boiling heat transfer in a vertical annulus, three tube diameters (16.5, 25.4, 34.0 mm) and the saturated water at atmospheric pressure has been tested. The inflow area has been changed from 0 to 1060.3 mm². To clarify effects of the inflow area on heat transfer results of the annulus are compared to the data of a single unrestricted tube. The inflow area changes heat transfer coefficients much and moves the deterioration point of heat transfer coefficients to the higher heat fluxes. To quantify effects of the inflow area on heat transfer, a new empirical correlation has been developed in terms of the area ratio, inflow area, and the heat flux. The correlation predicts the heat transfer data of boiling within ±10%.     

Pool boiling heat transfer in a vertical annulus with controlled inflow area at its bottom

To investigate effects of the inflow rate on pool boiling heat transfer in a vertical annulus, the inflow area at its bottom has been changed from 0 to 1060.26 mm². For the test, a heated tube of 16.5 mm diameter and water at atmospheric pressure has been used. To elucidate effects of the inflow area on heat transfer results of the annulus are compared to the data of a single unrestricted tube. The change in the inflow rate at the bottom of the annulus results in variation in heat transfer coefficients. When the inflow area is 176.71 mm² the deterioration point of heat transfer coefficients gets moved up to the higher heat fluxes because of the convective flow at the bottom regions.     

Pool boiling heat transfer to electrolyte solutions

Pool boiling heat transfer coefficients were measured for solutions of salts with positive solubility in water. The effect of the dissolved salts on nucleation site density, bubble departure diameter and bubble frequency was also investigated. The results show that at low heat fluxes heat transfer coefficients can be considerably lower than corresponding values for distilled water. At high heat fluxes the negative effect of the dissolved electrolyte gradually decreased and finally some improvement in heat transfer coefficient was observed. A correlation was developed for nucleate boiling of aqueous solutions from salts with positive solubility. Assuming that the mass transfer resistance is limited to the liquid side, the proposed model allows the prediction of heat transfer coefficients from boiling point data of the respective solutions. Comparison with a significant number of experimental data for different systems indicates that the model should be sufficiently accurate for most practical applications.     

Pool boiling heat transfer of non-Newtonian nanofluids

This paper reports on the investigation of pool boiling heat transfer of γ-Al₂O₃/CMC non-Newtonian nanofluids. To prepare nanofluids, γ-Al₂O₃ nanoparticles were dispersed in CMC solution (carboxy methyl cellulose in water) using ultrasonic mixing and mechanical mixer. Different concentrations of CMC non-Newtonian fluids and γ-Al₂O₃/CMC non-Newtonian nanofluids were tested under nucleate pool boiling heat transfer conditions. Experiments were carried out at atmospheric pressure. Results show that the pool boiling heat transfer coefficient of CMC solutions is lower than water. The decrease in boiling heat transfer is more pronounced at higher CMC concentrations and, as a result, higher solution viscosity. Adding nanoparticles to CMC non-Newtonian solutions results in an improved boiling heat transfer performance. The enhancement in the boiling heat transfer coefficient increases with the nanoparticle concentration; at a concentration of 1.4 wt.%, the boiling heat transfer coefficient increases by about 25% when compared to the base fluid.     

Pool boiling heat transfer on open-celled metallic foam sintered surface under saturation condition

The current study investigated the saturated pool boiling heat transfer of deionized water with added surfactant on a horizontal metallic foam surface with V-shape grooves under atmospheric pressure. The influences of the groove configurations, sodium dodecyl sulfate (SDS) concentration, foam thickness, and thermal conductivity of foam material on heat transfer performance and bubble growth patterns were studied. SDS with concentrations of 100, 400, and 800 ppm was used as the surfactant. The foam porosity was fixed at 0.95. V-shape grooves with various widths and groove number were manufactured in the foam samples, with three pore densities of 20, 100, and 130 PPI. Results showed that the effects of the groove configuration, SDS concentration, and thickness on boiling heat transfer are heavily dependent on foam pore density. The counter-flow between the released bubbles and sucked liquid plays a significant role in heat transfer performance. The existence of sufficient grooves delays the critical heat flux (CHF), whereas SDS can achieve CHF earlier.     

Pool boiling heat transfer in vertical annular crevices

Effects of vertical annuli on nucleate pool boiling heat transfer of water at atmospheric pressure have been obtained experimentally. Experiments were performed for annuli with a height of 570 mm and gap sizes of 3.9 and 15 mm. Through the tests, tube bottom confinement (open or closed) has been investigated, too, and the whole results are compared with a single unconfined tube. According to the results, the annular condition gives much increase in heat transfer coefficients at moderate heat fluxes. Its effect is observed much greater for the bottom-closed tube condition.     

Pool boiling heat transfer experiments in silica-water nano-fluids

Heat transfer measurements taken at atmospheric pressure in silica nano-solutions are compared to similar measurements taken in pure water and silica micro-solutions. The data include heat flux vs. superheat of a 0.4 mm diameter NiCr wire submerged in each solution. The data show a marked increase in critical heat flux (CHF) for both nano- and micro-solutions compared to water, but no appreciable differences in heat transfer for powers less than CHF. The data also show that stable film boiling at temperatures close to the wire melting point are achievable with the nano-solutions but not with the micro-solutions.     

Pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating

We study the pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating. Nanofluids are the mixture of de-ionized water and Al₂O₃ particles with 0.1%, 0.2%, 0.5% and 1.0% weight concentrations. The microheater is a platinum surface by 50 × 20 μm. Three types of bubble dynamics were identified. The first type of bubble dynamics is for the boiling in pure water, referring to a sharp microheater temperature increase once a new pulse cycle begins, followed by a continuous temperature increase during the pulse duration stage. Large bubble is observed on the microheater surface and it does not disappear during the pulse off stage. The second type of bubble dynamics is for the nanofluids with 0.1% and 0.2% weight concentrations. The microheater surface temperature has a sharp increase at the start of a new pulse cycle, followed by a slight decrease during the pulse duration stage. Miniature bubble has oscillation movement along the microheater length direction, and it disappears during the pulse off stage. The third type of bubble dynamics occurs at the nanofluid weight concentration of 0.5% and 1.0%. The bubble behavior is similar to that in pure water, but the microheater temperatures are much lower than that in pure water. A structural disjoining pressure causes the smaller contact area between the dry vapor and the heater surface, decreasing the surface tension effect and resulting in the easy departure of miniature bubbles for the 0.1% and 0.2% nanofluid weight concentrations. For the 0.5% weight concentration of nanofluids, coalescence of nanoparticles to form larger particles is responsible for the large bubble formation on the heater surface. The microlayer evaporation heat transfer and the heat transfer mechanisms during the bubble departure process account for the higher heat transfer coefficients for the 0.1% and 0.2% nanofluid weight concentrations. The shortened dry area between the bubble and the heater surface, and the additional thin nanofluid liquid film evaporation heat transfer, account for the higher heat transfer coefficient for the 0.5% nanofluid weight concentration, compared with the pure water runs.     

Crossflow boiling heat transfer in tube bundles

Experimental studies are conducted for two-phase crossflows in horizontal tube bundles at various mass fluxes and local flow qualities. The results are compared with those for a single tube in a channel [4,6] at two-phase crossflow conditions. The difference between the heat transfer performance for a heated tube in an infinite pool and in a heated or non-heated bundle can be explained in terms of the different flow field geometry and thermal environment.     

Three－Dimensional Numerical Simulation of Natural Convection Heat Transfer in an Inclined Cylindrical Annulus

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多孔表面新型复杂结构优化沸腾传热的实验研究

报道了R11在烧结多孔表面开槽时沸腾传热的实验研究，实验发现，与普通槽道和双空隙层多孔表面相比，沸腾换热增强，沸腾表现为液体灌注、槽道起泡、底部蒸干三个区，对特定的多孔层，开槽可获得更好的换热效果。带槽道的多孔表面实验件与均匀多孔表面相比，在相同壁面过热度（θ）条件下，热流密度（q）提高2－10倍，临界热流密度提高2－4倍。     

Critical heat flux of countercurrent boiling in an inclined small tube with closed bottom

The study was focused on the effect of the inclination angle on the critical heat flux of countercurrent boiling in an inclined uniformly heated tube with open top and closed bottom ends at zero inlet flow. The experimental results show that the CHF data of the small vertical tubes agree reasonably well with the predicting correlation proposed by Tien. The CHF data of the small inclined tubes decrease with reducing the inclination angle. The experimental data of the inclined tubes agrees reasonably well with the modified correlation, which is resulted from the conventional correlation for vertical tubes.     

Effects of outer tube length on saturated pool boiling heat transfer in a vertical annulus with closed bottoms

To improve pool boiling heat transfer in an annulus with closed bottoms, the length of an outer tube has been changed between 0.2 m and 0.6 m. For the test, a heated tube of 19.1 mm diameter and water at atmospheric pressure has been used. To elucidate effects of the outer tube length on heat transfer results of the annulus are compared with the data of a single unrestricted tube. The change in the outer tube length results in much variation in heat transfer coefficients. As the outer tube length is 0.2 m the deterioration point of heat transfer coefficients gets moved up to the higher heat fluxes because of the decrease in the intensity of bubble coalescence.     

Numerical scrutinization of dropwise condensation heat transfer on an inclined surface

Recently, achieving an ameliorated heat transfer rate in dropwise condensation (DWC) has attracted the attention of many researchers. Several parameters, including chemical and physical properties of the substrate, inclination, and interfacial characteristics influence DWC heat transfer rate. The variation of inclination angle is followed by the change in droplet shape and consequently, the heat transfer rate are changed. In this study, the effect of droplet shape variation on diverse inclined substrates is simulated. Moreover, three-dimensional mass, momentum, and energy equations considering the desired boundary conditions on the unstructured grid are utilized for the scrutinization of flow behavior and heat transfer for static and sliding droplets. For the sake of validation, the outcomes obtained from the simulation were compared with existent data in the literature and a proper agreement was attained. Regarding the outcomes, it was of concern that the influence of inclination angle on the droplet shape is more distinct at higher droplet volume; while no considerable change was seen on heat flux of small droplets by increasing the inclination angle. Furthermore, a higher heat transfer rate was noted by exceeding the inclination angle beyond a definite angle. Additionally, the increased heat transfer rate was affirmed by increasing the Marangoni number $\left(\mathit{Ma}\right)$.     

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 heat transfer in binary mixtures of ammonia/water

Nucleate boiling heat transfer coefficients were measured during pool boiling of the mixtures of ammonia/water on a horizontal heated wire. The experiment was carried out at pressures of 0.4 and 0.7 MPa, at heat fluxes below 2000 kW/m² and over all ranges of fraction. The heat transfer coefficients in the mixtures are markedly less than those in single component substances and, in particular, are dramatically deteriorated in the vicinity of both single component substances. An applicability of existing correlations to the present experimental data is discussed. As a result, it is difficult for any existing correlation to predict the coefficients over all ranges of fraction.In the mixtures of ammonia/water, heat of dilution and of dissolution are generated near a vapor-liquid interface, while vapor with a richer concentration of ammonia is condensed and then diffused into a bulk liquid; while in most other mixtures, little heat is generated during any dilution and dissolution. The effect of the heat of dilution and of dissolution on pressure and temperature in a system (pressure vessel) is shown.