Enhancement of pool boiling heat transfer in water and ethanol/water mixtures (effect of surface‐active agent)

The surface tension of alcohol/water mixtures has been measured over the whole fraction range and then it has been measured when a surface‐active agent was added into the mixtures. The effect of the concentration of alcohol and the surface‐ active agent on surface tension was experimentally clarified, in order to gain base data related to enhancement of the heat transfer coefficient in the mixtures and water. The experiment was also carried out to enhance the boiling heat transfer coefficients of water and alcohol/water mixtures on a horizontal heated fine wire at a pressure of 0.1 MPa by adding a surface‐active agent into the tested liquid. The results show, the coefficients were enhanced in lower alcohol concentration (C ≦ 0.5) and low heat flux range which occur just after the onset of boiling. It was also found that the enhancement effect by the surfactant disappears in concentrations over 1000 ppm. Finally, we demonstrated that the surface tension remarkably affects the heat transfer coefficients in nucleate pool boiling. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(4): 229–244, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20010     

Enhancement of nucleate pool boiling heat transfer in ammonia/water mixtures with a surface-active agent

Nucleate pool boiling heat transfer coefficients of ammonia/water mixtures were measured at a pressure of 0.4 MPa on a horizontal heated fine wire having a diameter of 0.3 mm. The nucleate pool boiling aspects were observed, after the addition of a surface-active agent to the mixtures. The effects of the concentrations of ammonia and the surface-active agent on the coefficients were clarified experimentally for the ammonia fraction range 0.1 ≦ C ≦ 0.9 and surfactant concentration range 0 ≦ C_S ≦ 3500 ppm. The results showed that the coefficients were enhanced at C ≦ 0.5 and in low heat flux ranges just after the onset of boiling. It was also found that the enhancement effect caused by the surfactant disappeared at surfactant concentrations of more than 1000 or 1500 ppm.     

Enhancement of nucleate pool boiling heat transfer in ammonia/water mixtures (Effect of surface‐active agent)

Nucleate boiling heat transfer coefficients of ammonia/water mixtures have been measured when a surface‐active agent was added into the mixtures. The experiment has been carried out on a horizontal heated wire at a pressure of 0.4 MPa. The effect of concentration of the ammonia and the surface‐active agent on the coefficients was investigated experimentally for the ammonia fractions C = 0.3 and 0.7 and the surfactant concentration C_S = 0 to 3500 ppm. The result shows that just after onset of boiling the nucleate boiling heat transfer coefficient enhances at the lower ammonia fraction i.e., C = 0.3. It was found that the enhancement effect by the surface‐active agent disappeared when the surfactant concentration is more than 1000 ppm at an ammonia fraction C = 0.3. It is also shown that the generated heat of mixing near the vaporBliquid interface can be removed easily and the pressure and temperature in the system can be controlled easily by placing a cooling pipe on the vaporBliquid interface, resulting in improvement of accuracy in measuring the heat transfer coefficients. We correlated the effect of the surfactant on the heat transfer coefficient using the improved measurement data in the current paper. 8 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20328     

Effects of Surfactant on Flow Boiling Heat Transfer of Ethylene Glycol/Water Mixtures in a Minitube

This study investigated effects of surfactant sodium dodecylbenzene sulfonate (SDBS) on flow boiling heat transfer of ethylene glycol/water mixtures in a vertical minitube. Experiments were performed using solutions containing 300 ppm by weight of surfactant and the results were compared with those of pure mixture. Local heat transfer coefficient was measured and found to be dependent on the mass quality. Although the surfactant intensifies the vaporization process, it doesn't necessarily enhance the heat transfer coefficient. Heat transfer coefficients were compared at two different mass fluxes, and the results might be explained based on the local flow pattern and the heat transfer mechanism. After a critical quality, higher quality tends to deteriorate heat transfer due to intermittent dryout, and therefore adding a surfactant to generate more vapor may be of negative effect on the flow boiling heat transfer in a minitube, which is contrary to the experience of enhancing nucleate pool boiling heat transfer with trace surfactants.     

Boiling heat transfer and critical heat flux of ethanol–water mixtures flowing through a diverging microchannel with artificial cavities

This paper presents an experimental study on the convective boiling heat transfer and the critical heat flux (CHF) of ethanol–water mixtures in a diverging microchannel with artificial cavities. The results show that the boiling heat transfer and the CHF are significantly influenced by the molar fraction (x_m) as well as the mass flux. For the single-phase convection region except for the region near the onset of nucleate boiling with temperature overshoot, the single-phase heat transfer coefficient is independent of the wall superheat and increases with a decrease in the molar fraction. After boiling incipience, the two-phase heat transfer coefficient is much higher than that of single-phase convection. The two-phase heat transfer coefficient shows a maximum in the region of bubbly-elongated slug flow and deceases with a further increase in the wall superheat until approaching a condition of CHF, indicating that the heat transfer is mainly dominated by convective boiling. A flow-pattern-based empirical correlation for the two-phase heat transfer coefficient of the flow boiling of ethanol–water mixtures is developed. The overall mean absolute error of the proposed correlation is 15.5%, and more than 82.5% of the experimental data were predicted within a ±25% error band. The CHF increases from x_m = 0–0.1, and then decreases rapidly from x_m = 0.1–1 at a given mass flux of 175 kg/m² s. The maximum CHF is reached at x_m = 0.1 due to the Marangoni effect, indicating that small additions of ethanol into water could significantly increase the CHF. On the other hand, the CHF increases with increasing the mass flux at a given molar fraction of 0.1. Moreover, the experimental CHF results are compared with existing CHF correlations of flow boiling of the mixtures in a microchannel.     

Experimental and theoretical studies on subcooled flow boiling of pure liquids and multicomponent mixtures

To improve the design of modern industrial reboilers, accurate knowledge of boiling heat transfer coefficients is essential. In this study flow boiling heat transfer coefficients for binary and ternary mixtures of acetone, isopropanol and water were measured over a wide range of heat flux, subcooling, flow velocity and composition. The measurements cover the regimes of convective heat transfer, transitional boiling and fully developed subcooled flow boiling. Two models are presented for the prediction of flow boiling heat transfer coefficients. The first model is the combination of the Chen model with the Gorenflo correlation and the Schlünder model for single and multicomponent boiling, respectively. This model predicts flow boiling heat transfer coefficients with acceptable accuracy, but fails to predict the nucleate boiling fraction NBF reasonably well. The second model is based on the asymptotic addition of forced convective and nucleate boiling heat transfer coefficients. The benefit of this model is a further improvement in the accuracy of flow boiling heat transfer coefficient over the Chen type model, simplicity and the more realistic prediction of the nucleate boiling fraction NBF.     

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.     

Influence of thermodynamic models on the prediction of pool boiling heat transfer coefficient of dilute binary mixtures

In this paper, large number of experiments has been performed on saturated pool boiling heat transfer to three different dilute binary mixtures at various heat fluxes (up to 100 kW/m²) and five different concentrations (1–5 vol.% of heavier component). The test mixtures include water/glycerol, water/mono‐ethylene glycol (MEG), and water/di‐ethylene glycol (DEG). The effects of the main operating parameters such as heat flux, concentration, and bubble dynamics on the pool boiling heat transfer coefficient are also investigated. Furthermore, it is shown that physical properties of the mixtures have a considerable effect on the prediction of pool boiling heat transfer coefficients using the available correlations. In almost all of the existing correlations, some physical properties are strongly involved which can be estimated using different thermodynamic models. These models for the calculation of specific heat, density, heat of vaporization, and surface tension do not give exactly similar results and consequently, the heat transfer coefficient obtained from a specified predictive correlation can be tolerated according to the method used for the calculation of the physical properties. This point is usually ignored by investigators and they compare their experimental data with the correlations without reporting which thermodynamic models have to be used for the calculation of the physical properties. In this study, the prediction of Schlünder correlation has been compared with the present experimental data. Results show that the prediction ability of the Schlünder correlation is strongly dependent on the method used for the estimation of the required physical properties.     

Investigation of Nucleate Pool Boiling of Saturated Pure Liquids and Ethanol-Water Mixtures on Smooth and Laser-Textured Surfaces

ABSTRACT

Nucleate pool boiling experiments were performed on plain and five laser-textured stainless-steel foils using saturated pure water, 100% ethanol, 0.4% and 4.2% mole fraction ethanol – water mixtures. All laser-textured samples contained untreated, smooth 0.5 mm wide regions and intermediate textured surfaces, that differ in the width of the laser patterned regions (from 0.5 mm to 2.5 mm). For smooth surfaces, we measured significant decreases in average heat transfer coefficients (HTC) and increases in bubble activation temperatures in comparison with the laser-textured surfaces for all the tested working fluids. Significant enhancement in HTC (280%) on a textured heating surface with 2.5-mm-wide laser pattern was recorded using pure water. For pure ethanol, the highest enhancement of 268% was achieved on a heating surface with 1.5-mm-wide laser pattern. The highest enhancement of HTC for the tested binary mixtures was obtained using 2.0-mm wide-laser-textured regions (HTC improved by 235% and 279% for the 0.4% and 4.2% mixtures, respectively). Our results indicate that laser texturing can significantly improve boiling performance when the intervals of the laser-textured patterns are close to the capillary lengths of the tested fluids.     

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.     

Experimental investigation on pool boiling heat transfer of ZnO,and CuO water-based nanofluids and effect of surfactant on heat transfer coefficient

Comparison of boiling performance of nanofluids and mixtures of nanofluids with surfactant is an objective of this research. Experimental investigation has been performed with different heat flux and concentrations of nanoparticles and surfactant. CuO and ZnO water-based nanofluids are used and sodium dodecyl sulfate (SDS) is used as surfactant. The size of nanoparticles is measured from Field Emission Scanning Electron Microscopy pictures. Roughness of rod heater is calculated by using Atomic Force Microscopy picture. The pure water is tested after each run with nanofluid and mixture of nanofluid with SDS and the results are presented. Result from experiments demonstrates that the addition of SDS to nanofluids solution resulted in improving boiling performance. Experimental results also show an inefficient process by excluding surfactant. An optimum value for heat transfer coefficient is found by increasing of surfactant concentration within CuO nanofluid (0.01wt%CuO). Pictures of coated and clean surface in boiling pure water are employed for understanding the dynamics of bubbles. Gorenflo constant (h₀) is utilized to show the effect of addition of nanoparticles and SDS in boiling performance of base fluid.     

Convective boiling heat transfer characteristics of CO2 in microchannels

Convective boiling heat transfer coefficients and dryout phenomena of CO₂ are investigated in rectangular microchannels whose hydraulic diameters range from 1.08 to 1.54 mm. The tests are conducted by varying the mass flux of CO₂ from 200 to 400 kg/m² s, heat flux from 10 to 20 kW/m², while maintaining saturation temperature at 0, 5 and 10 °C. Test results show that the average heat transfer coefficient of CO₂ is 53% higher than that of R134a. The effects of heat flux on the heat transfer coefficient are much significant than those of mass flux. As the mass flux increases, dryout becomes more pronounced. As the hydraulic diameter decreases from 1.54 to 1.27 mm and from 1.27 to 1.08 mm at a heat flux of 15 kW/m² and a mass flux of 300 kg/m² s, the heat transfer coefficients increase by 5% and 31%, respectively. Based on the comparison of the data from the existing models with the present data, the Cooper model and the Gorenflo model yield relatively good predictions of the measured data with mean deviations between predicted and measured data of 21.7% and 21.2%, respectively.     

Pool boiling heat transfer in binary mixtures of ammonia/water: Effect of heat of dilution and dissolution on heat transfer coefficient

Nucleate boiling heat transfer coefficients were measured on a horizontal heated wire during the pool boiling of non‐azeotropic mixtures of ammonia/water. The experiment was carried out at pressures of 0.4 and 0.7 MPa, at heat fluxes below 2.0 × 10⁶ W/m², and over a range of mass fraction. The heat transfer coefficients in the mixtures were smaller than those in single‐component substances. No existing correlation is found to predict boiling heat transfer coefficients over the range of mass fraction of interest. In the mixtures of the ammonia/water, the heats of dilution and dissolution were generated near a liquid surface while vapor with a rich concentration of ammonia was condensed and then was diffused into the bulk liquid; while in most other mixtures, little heat was generated during any dilution and dissolution. In relation to the heat generated, the effect of the heats of dilution and dissolution on pressure and temperature in a system (pressure vessel) is shown herein. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 272–283, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10034     

Nucleate Pool Boiling Heat Transfer of Pure Refrigerants and Binary Mixtures

Heat transfer coefficients in nucleate boiling on a smooth flat surface were measured for pure fluids of R-134a, propane, isobutane and their binary mixtures at different pressure from 0.1 to 0.6 MPa. Series of experiments with different heat flux and mixture concentrations were carried out. The influences of pressure and heat flux on the heat transfer coefficient for different pure fluids were studied. Isobutane and propane were used to make up binary mixtures. Compared to the pure components, binary mixtures show lower heat transfer coefficients. This reduction was more pronounced as the heat flux increasing. Several heat transfer correlations are obtained for different pure refrigerants and their binary mixtures.     

Nucleate boiling of two and three-component mixtures

For three pure fluids and their two- and three-component mixtures, heat transfer coefficients were measured in nucleate pool boiling on the upward facing copper surface of 40 mm diameter. The more-, moderate- and less-volatile components in mixtures are refrigerants R-134a, R-142b and R-123, respectively. Heat transfer coefficients of mixtures were less than the interpolated heat transfer coefficients between pure components, with more reduction at higher heat flux. Two correlations originally developed for two-component mixtures by Thome and Shakir and by Fujita and Tsutsui reproduced well the measured heat transfer coefficients of three- as well as two-component mixtures. This result implies that the boiling range included in the correlations accounts for heat transfer reduction in mixture boiling.     

Subcooled flow boiling heat transfer of R-134a and the associated bubble characteristics in a vertical plate heat exchanger

Subcooled flow boiling heat transfer characteristics of refrigerant R-134a in a vertical plate heat exchanger (PHE) are investigated experimentally in this study. Besides, the associated bubble characteristics are also inspected by visualizing the boiling flow in the vertical PHE. In the experiment two vertical counterflow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Upflow boiling of subcooled refrigerant R-134a in one channel receives heat from the downflow of hot water in the other channel. The effects of the boiling heat flux, refrigerant mass flux, system pressure and inlet subcooling of R-134a on the subcooled boiling heat transfer are explored in detail. The results are presented in terms of the boiling curves and heat transfer coefficients. The measured data showed that the slopes of the boiling curves change significantly during the onset of nucleate boiling (ONB) especially at low mass flux and high saturation temperature. Besides, the boiling hysteresis is significant at a low refrigerant mass flux. The subcooled boiling heat transfer coefficient is affected noticeably by the mass flux of the refrigerant. However, increases in the inlet subcooling and saturation temperature only show slight improvement on the boiling heat transfer coefficient.The photos from the flow visualization reveal that at higher imposed heat flux the plate surface is covered with more bubbles and the bubble generation frequency is substantially higher, and the bubbles tend to coalesce to form big bubbles. But these big bubbles are prone to breaking up into small bubbles as they move over the corrugated plate, producing strong agitating flow motion and hence enhancing the boiling heat transfer. We also note that the bubbles nucleated from the plate are suppressed to a larger degree for higher inlet subcooling and mass flux. Finally, empirical correlations are proposed to correlate the present data for the heat transfer coefficient and the bubble departure diameter in terms of boiling, Froude, Reynolds and Jakob numbers.     

Saturated flow boiling heat transfer of refrigerant R-410A in a horizontal annular finned duct

An experiment is conducted here to investigate the saturated flow boiling heat transfer characteristics of ozone friendly refrigerant R-410A in a horizontal annular finned duct. Meanwhile the associated bubble characteristics in the duct are also inspected from the flow visualization. The experimental data are presented in terms of saturated flow boiling curves, boiling heat transfer coefficients and flow photos. In addition, empirical correlation equations for the saturated flow boiling heat transfer coefficient and mean bubble departure diameter are proposed. The saturated flow boiling curves show that boiling hysteresis is insignificant in the flow and the wall superheat needed for the onset of nucleate boiling is slightly affected by the refrigerant mass flux. Besides, the boiling curves are mainly affected by the imposed heat flux and refrigerant mass flux. Moreover, the measured saturated flow boiling heat transfer coefficient increases with the imposed heat flux and refrigerant mass flux. Furthermore, at a higher refrigerant mass flux the departing bubbles are smaller.     

An experimental study of void fraction and heat transfer under upward and downward flow boiling conditions

Heat transfer coefficient and actual void fraction have been measured during upflow and downflow boiling of water in an annular channel. At the same values of pressure, mass flux, heat flux and flow quality significant difference of void fraction has been established in upflow and downflow. The upflow and downflow heat transfer coefficients did not deviate significantly from each other, if compared at identical values of pressure, mass flux, heat flux and actual void fraction.     

Characteristic curves and the promotion effect of ethanol addition on steam condensation heat transfer

Although most previous studies concerning the condensation of binary vapor mixtures report the condensation rates to be less than that for pure vapor, heat transfer enhancement can be realized by using additives to form a positive system (solutal Marangoni condensation). The objective of the present study was to clarify the effect of mixing ethanol into steam on condensation heat transfer. Precise measurements of the ethanol concentration in the vapor of water-ethanol mixtures were performed over a wide range of ethanol concentrations, and the condensation behavior was observed. The maximum heat transfer coefficients in the condensation characteristic curves were determined to be 0.12 and 0.18 MW/m² K for vapor velocities of 0.4 and 1.5 m/s, respectively, and appeared at an ethanol vapor mass fraction of approximately 1%. The mixing was demonstrated to be extremely effective, particularly in the low-ethanol concentration range. The condensation heat transfer was enhanced approximately 2-8 times compared to pure steam.     

Microjet array flow boiling with R134a and the effect of dissolved nitrogen

An investigation of flow boiling with two arrays of microjets with R134a was conducted. Velocities of 4 and 7 m/s, subcoolings of 10, 20, and 30 °C, and jet-to-heater area ratios of 8.9% and 21% were employed. Lower subcoolings and lower velocities were found to enhance boiling and reduce the onset of nucleate boiling heat flux. The area ratio did not influence the onset of nucleate boiling but did increase the boiling enhancement of the heat transfer coefficient. Also, a previous hypothesis that nitrogen dissolved into the working fluid prevented premature temperature excursion was tested with controlled mixtures and was not substantiated.