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 pool boiling heat transfer in water and ethanol/water mixtures with surface-active agent

Surface tension of ethanol/water mixtures is measured over the whole ethanol fraction range and the effect of the surface-active agent on surface tension is also measured in the mixtures, in order to gain basic data related to enhancement of the heat transfer coefficient in water and the mixtures. The boiling heat transfer coefficient, the onset of boiling and the critical heat flux in water and ethanol/water mixtures, with and without the surface-active agent, have been measured on a horizontal fine heated wire at a pressure of 0.1 MPa. The experiment was carried out in the whole range of the ethanol fraction and in a surfactant concentration of 0-5000 ppm. The experiment shows that the coefficients were enhanced in a lower ethanol fraction (C ? 0.5) and in low heat flux which is slightly higher than heat flux at the onset of boiling. It is also found that the enhancement due to the surfactant disappears over 1000 ppm. Finally, we clarify that depression of the surface tension by the surfactant remarkably enhances the heat transfer coefficients in the nucleate pool boiling.     

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.     

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     

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.     

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.     

Heat transfer coefficients in two-phase flow for mixtures used in solar absorption refrigeration systems

This paper describes the experimental results obtained on the heat transfer in saturated nucleate boiling of refrigerant mixtures used in solar absorption refrigeration systems flowing upward in a uniformly heated vertical tube. The mixtures analysed were water/ammonia, ammonia/lithium nitrate and water/lithium bromide. The concentration range for the water/ammonia was from 38 to 48 wt%, for the ammonia/lithium nitrate was from 38 to 48 wt% and for the water/lithium bromide was from 48 to 58 wt%. It was observed that the average heat transfer coefficients increased for the mixtures with an increase in the quality, the heat flux and the solution concentration. Comparing the values of the average heat transfer coefficients for the three mixtures, it was observed that the highest values were obtained for the ammonia/water mixture.     

Experimental Research on the Effect of 2‐Ethylhexanol on Water Boiling Heat Transfer at Subatmospheric Pressure

The addition of small amounts of surfactant to water can enhance boiling heat transfer substantially. In this experiment, 2‐ethylhexanol is used as the surfactant, and the critical micelle concentration (CMC) of the 2‐ethylhexanol aqueous solution is determined to be 1000 mg/L. The boiling heat transfer coefficients of the solutions in different concentrations on a horizontal copper rod surface at subatmospheric pressures (1.8 kPa to 3.3 kPa) are calculated according to the experiment results. The results indicate the following: when the concentration is 500 mg/L (lower than the CMC), the boiling heat transfer can be enhanced by up to 22%; when the concentration is 1000 mg/L (equal to the CMC), enhancement is up to about 60%; when the concentration is 2000 mg/L (two times the CMC), boiling heat transfer is diminished by about 60%. To a certain extent, this experiment will guide the enhancement of evaporator heat transfer in water vapor compression refrigeration and heat pump systems.     

Characteristic of local boiling heat transfer of ammonia and ammonia / water binary mixture on the plate type evaporator

Power generation using small temperature difference such as ocean thermal energy conversion (OTEC) and discharged thermal energy conversion (DTEC) is expected to be the countermeasures against global warming problem. As ammonia and ammonia/water are used in evaporators for OTEC and DTEC as working fluids, the research of their local boiling heat transfer is important for improvement of the power generation efficiency. Measurements of local boiling heat transfer coefficients were performed for ammonia /water mixture (z = 0.9−1) on a vertical flat plate heat exchanger in a range of mass flux (7.5–15 kg/m² s), heat flux (15–23 kW/m²), and pressure (0.7–0.9 MPa). The result shows that in the case of ammonia /water mixture, the local heat transfer coefficients increase with an increase of mass flux and composition of ammonia, and decrease with an increase of heat flux.     

Flow boiling and condensation of tetrary refrigerant mixtures inside water/refrigerant enhanced surface tubing

This work presents the results of an experimental study concerning the heat transfer characteristics of two-phase flow condensation and boiling of tetrary (R-32/R-125/R143a/R134a) refrigerant mixtures inside water/refrigerant horizontal enhanced surface tubing. Heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of the tetrary refrigerant mixtures, have been predicted and compared with other mixtures during flow condensation and boiling inside enhanced surface tubing. It was found that the tetrary refrigerant blend has higher transfer coefficients than R-502, and the lowest pressure drop among the refrigerants studied. © 1997 by John Wiley & Sons, Ltd.     

Forced convective boiling of nonazeotropic refrigerant mixtures inside enhanced surface tubing

An experimental study on the characteristics of two phase flow boiling of pure refrigerants such as R12 and R22 as well as nonazeotropic refrigerant mixtures R22/R114 and R22/R152a inside horizontal enhanced surface tubing is presented. The enhanced surface tubing results showed a significant improvement of the heat transfer over that of an equivalent smooth tube, depending on the mixture components and their concentrations. Correlations are proposed to predict the heat transfer characteristics such as average heat transfer coefficients as well as pressure drops of nonazeotropic refrigerant mixture flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixtures pressure drop is a weak function of the mixture compositions.     

Influence of magnetic field on two‐phase flow convective boiling of some refrigerant mixtures

In this paper, an experimental study on the influence of magnetohydrodynamic (MHD) on heat transfer characteristics of two‐phase flow boiling of some refrigerant mixtures in air/refrigerant horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of MHD on the heat transfer characteristics such as average heat transfer coefficients, and pressure drops of R‐507, R‐404A, R‐410A, and R‐407C in two‐phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition. It was also evident that the proposed correlations for predicting the heat transfer characteristics were applicable to the entire heat and mass flux, investigated in the present study. The deviation between the experimental and predicted value using new and improved correlations for the heat transfer coefficient and pressure drop were less than ±20%, for the majority of data. Copyright © 2005 John Wiley & Sons, Ltd.     

Boiling heat transfer coefficients inside a vertical smooth tube for the water/lithium bromide mixture

This paper describes the experimental results obtained on the heat transfer in forced convective boiling for the water/lithium bromide mixture flowing upward in a vertical tube uniformly heated. The concentration range for the mixture was from 48.1 to 57.7 wt%. Correlations were proposed to correlate the experimental local heat transfer coefficients. The results showed that the local heat transfer coefficients are strongly dependent on Bo, 1/X_tt and 1/x at the analysed conditions. It was observed that the average heat transfer coefficients increased for the mixture with a decrease of the solution concentration or an increase of the mass flux. Copyright © 2002 John Wiley & Sons, Ltd.     

Characteristics of heat transfer coefficients during nucleate pool boiling of binary mixtures

Experimental studies were made on heat transfer on a horizontal platinum wire during nucleate pool boiling in nonazeotropic refrigerant binary mixtures at pressures of 0.25 to 0.7 MPa and at heat fluxes up to CHF. The boiling features of the mixtures and the single-component substances were observed by photography. The relationship between the boiling behavior and the reduction of heat transfer coefficients in binary mixtures is discussed in order to propose a correlation useful for predicting the present experimental data over a wide range of low to high heat fluxes. It is shown that the correlation is applicable to alcoholic mixtures. The physical meaning of k, which was introduced to evaluate the effect of heat flux on the reduction of a heat transfer coefficient, is clarified based on measured nucleate pool boiling heat transfer data and visual observations of the boiling features. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 535–549, 1998     

Performance characteristics of an ammonia–water absorption heat pump system

The influences of the performance parameters and the heat transfer characteristics of the absorption heat pump using ammonia–water mixture are theoretically carried out. There is a pronounced effect of the ammonia concentration ξ after rectifier on the temperature glides that has been investigated. At ξ = 0.9000 and saturation pressures of 75 and 0.5 bar, the temperature glides are 64.4°C and 81.21°C, respectively, whereas these glides are 0°C and 16.1°C at ξ = 0.9999 and at the same pressures. This mixture property considerably affects the absorption system performance and the design of the rectifier as well as other absorption components. A correlation of the Nusselt number, Nu, is developed and compared with some published work in the literature for plate type heat exchanger. The effects of ammonia concentration ξ, mass fraction spread Δξ, specific solution circulation ratio f, and pressure ratio R_p on the refrigerant mass flow rate, the pressure drop, and the heat transfer coefficients during the condensation, the evaporation, and the absorption processes are investigated. It was found that increasing ammonia mass fraction spread Δξ results in both specific circulation ratio f and R_p that have insignificant effects on the refrigerant mass flow rate. Mounting Δξ at constant f reduces the pressure drop gradually and subsequently starts to increase as Δξ escalates. The ammonia concentration ξ has insignificant effect on the evaporation heat transfer coefficient but has a little effect on the condensation and the absorber heat transfer coefficients. The ammonia mass fraction spread Δξ and f have considerable effects on the heat transfer coefficient for different absorption heat pump components. R_p has a pronounced effect on the evaporation heat transfer coefficient, although it has a slight effect on the condensation and the absorber heat transfer coefficients. The effect of R_p on the heat transfer coefficient may be eliminated in the absorber for Δξ > 0.18. Copyright © 2013 John Wiley & Sons, Ltd.     

Heat transfer in forced convection boiling of oil–non-azeotropic binary refrigerant mixtures

In this paper, the results of the heat transfer, forced convection, boiling characteristics of non-azeotropic refrigerant mixtures and oil are presented. This includes heat transfer coefficients for pure and binary mixtures under boiling conditions outside enhanced surface tubing. Local convective heat transfer coefficients have been determined using a modified Wilson-plot technique. Heat transfer correlations were established as a function of the binary mixture mass flow rate, and oil concentration, as well as key flow parameters.     

Visualization of pool boiling heat transfer of magnetic nanofluid

To increase heat transfer, ferrofluids have been utilized to study the effective parameters of pool boiling. Changes and possible enhancement of pool boiling heat transfer of magnetic fluids is a function of magnetic field and concentration of nanoparticles. To the best knowledge of the authors, no systematic experiments have been conducted to visualize the phenomena during the boiling of ferrofluids with different concentrations. In this study an experimental investigation has been conducted, by designing and fabricating a novel hele‐shaw vessel with glass sides, to explore via visualizations some details in the pool boiling of ferrofluids. Boiling patterns of ferrofluids at various concentrations have been visualized –both in the presence of a constant magnetic field and without any magnetic field. Pure water tests were performed as a baseline, and the experimental program has been conducted at four different concentrations, namely 30, 40, 50, and 500 ppm. The primary focus of the visualization is to study how different concentration of ferrofluid affects the boiling ebullition cycle through a high‐speed camera. The results showed that in the boiling process of ferrofluids with a low concentration (10 to 50 ppm), the rising bubbles lead to enlarge the active nucleation sites and create cavities. The formation of cavities changes the solid layer of the surface to a porous medium and enhances the wettability of the surface and boiling heat transfer coefficient. In the ferrofluid boiling with high concentration (500 ppm), bubbles rising is hindered by nanoparticles.     

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.     

Pool Boiling Heat Transfer to Phosphoric Acid Solutions

Phosphoric acid is a weak electrolyte with complex physical properties. This complexity combined with its industrial importance has necessitated intensive studies into its heat transfer behavior. In this investigation, pool boiling heat transfer coefficients of phosphoric acid solutions have been measured over a wide range of acid concentrations. The effects of various operating parameters such as heat flux, temperature, and acid concentration have been investigated. Also, the bubble departure diameter and the number of active nucleation sites for phosphoric acid solutions are compared with those for pure water under identical conditions. A model was developed for pool boiling heat transfer of phosphoric acid solutions, which can easily be adapted for other weak electrolyte solutions. In this model, the correct boiling temperature at the vapor/liquid interface is determined rather than applying an arbitrary correction to the boiling heat transfer coefficient. The proposed model is confirmed by comparison between calculated and experimental data.     

Behavioral study of alumina nanoparticles in pool boiling heat transfer on a vertical surface

Experiments were carried out to investigate the pool boiling of alumina‐water nanofluid at 0.1 g/l to 0.5 g/l of distilled water, and the nucleate pool boiling heat transfer of pure water and nanofluid at different mass concentrations were compared at and above the atmospheric pressure. At atmospheric pressure, different concentrations of nanofluids display different degrees of deterioration in boiling heat transfer. The effect of pressure and concentration of nanoparticles revealed significant enhancement in heat flux and deterioration in pool boiling. The heat transfer coefficient of 0.5 g/l alumina‐water nanofluid was compared with pure water and clearly indicates deterioration. At all pressures the heat transfer coefficients of the nanofluid were lower than those of pure water. Experimental observation revealed particles coating over the heater surface and subsequent SEM inspection of the heater surface showed nanoparticles coating on the surface forming a porous layer. To substantiate the nanoparticle deposition and its effect on heat flux, investigation was done by measuring the surface roughness of the heater surface before and after the experiment. While SEM images of the heater surface revealed nanoparticle deposition, surface roughness of the heater surface confirmed it. Based on the experimental investigations it can be concluded that an optimum thickness of nanoparticles coating favors an increase in heat flux. Higher surface temperature due to the presence of nanoparticles coating results in the deterioration of boiling heat transfer. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20365