The influence of oil on nucleate pool boiling heat transfer

The influence of various oil contents in R134a is investigated for nucleate pool boiling on copper tubes either sandblasted or with enhanced heating surfaces (GEWA-B tube). Polyolester oils (POE) (Reniso Triton) with medium viscosity 55 cSt (SE55) and high viscosity 170 cSt (SE170) were used. Heat transfer coefficients were obtained for boiling temperatures between −28.6 and +20.1°C. The oil content varied from 0 to 5% mass fraction. For the sandblasted tube and the SE55 oil the heat transfer coefficients for the refrigerant/oil-mixture can be higher or lower than those for the pure refrigerant, depending on oil mass fraction, boiling temperature and heat flux. In some cases the highest heat transfer coefficients were obtained at a mass fraction of 3%. For the 170 cSt oil there is a clear decrease in heat transfer for all variations except for a heat flux 4,000 W/m² and −10.1°C at 0.5% oil content. The heat transfer coefficients are compared to those in the literature for a smooth stainless steel tube and a platinum wire. For the enhanced tube and 55 cSt oil the heat transfer coefficients are clearly below those for pure refrigerant in all cases. The experimental results for the sandblasted tube are compared with the correlation by Jensen and Jackman. The calculated values are within +20 and −40% for the medium viscosity oil and between +50% and −40% for the high viscosity oil. A correlation for predicting oil-degradation effects on enhanced surfaces does not exist.     

Review of nucleate pool boiling bubble heat transfer mechanisms

Enhanced convection, transient conduction, microlayer evaporation, and contact line heat transfer have all been proposed as mechanisms by which bubbles transfer energy during boiling. Models based on these mechanisms contain fitting parameters that are used to fit them to the data, resulting a proliferation of “validated” models. A review of the recent experimental, analytical, and numerical work into single bubble heat transfer is presented to determine the contribution of each of the above mechanisms to the overall heat transfer. Transient conduction and microconvection are found to be the dominant heat transfer mechanisms. Heat transfer through the microlayer and at the three-phase contact line do not contribute more than about 25% of the overall heat transfer.     

A study of nucleate boiling and critical heat flux with EHD enhancement

The paper describes results from an experimental and theoretical study of the effect of an electric field on nucleate boiling and the critical heat flux (CHF) in pool boiling of R123 at atmospheric pressure on a horizontal wall with a smooth surface. Two designs of electrode (parallel rods and wire mesh) were used. The experimental data exhibit some differences from the data obtained by other researchers in similar experiments on a wall with a different surface finish and with a slightly different design of wire mesh electrode. The hydrodynamic model for EHD enhancement of CHF cannot reconcile the differences. A theoretical model has been developed for the growth of a single vapour bubble on a superheated wall in an electric field, leading to a numerical simulation based on the level-set method. The model includes matching of sub-models for the micro- and macro-regions, conduction in the wall, distortion of the electric field by the bubble, the temperature dependence of electrical properties and free-charge generation. In the present form of the model, some of these effects are realised in an approximate form. The capability to investigate dry-spot formation and wall temperature changes that might lead to CHF has been demonstrated.     

Effects of ultrasonic vibration on subcooled pool boiling critical heat flux

The effects of ultrasonic vibration on critical heat flux (CHF) have been experimentally investigated under natural convection condition. Flat bakelite plates coated with thin copper layer and distilled water are used as heated specimens and working fluid, respectively. Measurements of CHF on flat heated surface were made with and without ultrasonic vibration applied to working fluid. An inclination angle of the heated surface and water subcooling are varied as well. Examined water subcoolings are 5°C, 20°C, 40°C and the angles are 0°, 10°, 20°, 45°, 90°, 180°. The measurements show that ultrasonic wave applied to water enhances CHF and its extent is dependent upon inclination angle as well as water subcooling. The rate of increase in CHF increases with an increase in water subcooling while it decreases with an increase in inclination angle. Visual observation shows that the cause of CHF augmentation is closely related with the dynamic behaviour of bubble generation and departure in acoustic field.     

Lattice Boltzmann simulation of the effects of cavity structures and heater thermal conductivity on nucleate boiling heat transfer

The boiling heat transfer technology with cavity surfaces can provide higher heat flux under lower wall superheat, which is of great significance for the cooling of electronic chips and microelectromechanical devices. In this paper, the boiling characteristics of the cavity surfaces are investigated based on the lattice Boltzmann(LB) method, focusing on the effects of cavity shapes, sizes, and heater thermal conductivity on the heat transfer performance. The results show that the triangular cavi...     

Enhancement of the critical heat flux in saturated pool boiling using honeycomb porous media

Enhancement of the critical heat flux in pool boiling by the attachment of a honeycomb-structured porous plate on a heated surface is investigated experimentally using water under saturated boiling conditions. As the height of the honeycomb porous plate on the heated surface decreases, the CHF increases to 2.5 MW/m², which is approximately 2.5 times that of a plain surface (1.0 MW/m²). Automatic liquid supply due to capillary action and reduction of the flow resistance for vapor escape due to the separation of liquid and vapor flow paths by the honeycomb-structure are verified to play an important role in the enhancement of the CHF. A simplified one-dimensional model for the capillary suction limit, in which the pressure drops due to liquid and vapor flow in the honeycomb porous plate balances the capillary force, is applied to predict the CHF. The calculated results are compared with the measured results.     

Effect of subcooling on critical heat flux during pool boiling on a horizontal heated wire

Critical heat flux(CHF) is measured during pool boiling of water and R113 on a heated horizontal wire submerged in a subcooled liquid. Experiments are conducted over a pressure range from 0.1 to 3.0 MPa and subcooling up to 220 K. CHF data reveal that the CHF increases in a linear fashion with an increase in subcooling, and that the increment of the CHF with increasing subcooling becomes larger with increasing pressure. The characteristics of the CHF obtained differ from those of existing correlations at high pressures, although it is a similar tendency to them in that the CHF is proportional to the subcooling. A new correlation is derived by taking into account the effect of both the density ratio, ρ _L /ρ _V , and the Peclet number, Pe, and it succeeds in predicting the CHF data up to higher pressure and higher subcooling ranges, more effectively than previous studies using existing applicable ranges. Received on 23 July 1997     

Effect of ionic additive on pool boiling critical heat flux of titania/water nanofluids

TiO₂/water nanofluids were prepared and tested to investigate the effects of an ionic additive (i.e., nitric acid in this study) on the critical heat flux (CHF) behavior in pool boiling. Experimental results showed that the ionic additive improved the dispersion stability but reduced the CHF increase in the nanofluid. The additive affected the self-assembled nanoparticle structures formed on the heater surfaces by creating a more uniform and smoother structure, thus diminishing the CHF enhancement in nanofluids.     

Experimental study of pool temperature effects on nucleate pool boiling

Nucleate pool boiling experiments with constant wall temperature were performed using pure R113 for subcooled, saturated, and superheated pool conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant wall temperature and to measure the instantaneous heat flow rate accurately with high temporal and spatial resolutions. Images of bubble growth were taken at 5000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble geometry was obtained from the captured bubble images. The effect of the pool conditions on the bubble growth behavior was analyzed using dimensionless parameters for the initial and thermal growth regions. The effect of the pool conditions on the heat flow rate behavior was also examined. The bubble growth behaviors during subcooled, saturated, and superheated pool boiling were analyzed using a modified Jakob number that we newly defined. Dimensionless time and bubble radius parameters with the modified Jakob number characterized the bubble growth behavior well. These phenomena require further analysis for various pool temperature conditions, and this study will provide good experimental data with precise constant wall temperature boundary condition for such works.     

Compositive effects of orientation and contact angle on critical heat flux in pool boiling of water

An experimental study was carried out to investigate the effects of heat transfer surface orientation and the solid–liquid contact angle on the boiling heat transfer and critical heat flux (CHF) in water pool boiling using a smooth heat-transfer surface under atmospheric pressure. The orientation angle was ranged from 0° (up-facing horizontal position) to 180° (down-facing horizontal position) with a pace of 45°. The three kinds of heat transfer surfaces having different solid–liquid contact angles were the normal surface with a contact angle of 55°, the hydrophilic surface with a contact angle of 30° and the superhydrophilic surface with a contact angle of 0°. The experimental results indicate that orientation and contact angle have complex, coupling effects on heat transfer and CHF. A predicting correlation for the CHF which takes the effects of both orientation and contact angle into account is established. The predicting correlation agrees reasonably well with the experimental data.     

Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid

Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid was investigated experimentally. Three types of surfactants including Sodium Dodecyl Sulfate (SDS), Cetyltrimethyl Ammonium Bromide (CTAB) and Sorbitan Monooleate (Span-80) were used in the experiments. The refrigerant-based nanofluid was formed from Cu nanoparticles and refrigerant R113. The test surface is horizontal with the average roughness of 1.6 μm. Test conditions include a saturation pressure of 101.3 kPa, heat fluxes from 10 to 80 kW m⁻², surfactant concentrations from 0 to 5000 ppm (parts per million by weight), and nanoparticle concentrations from 0 to 1.0 wt.%. The experimental results indicate that the presence of surfactant enhances the nucleate pool boiling heat transfer of refrigerant-based nanofluid on most conditions, but deteriorates the nucleate pool boiling heat transfer at high surfactant concentrations. The ratio of nucleate pool boiling heat transfer coefficient of refrigerant-based nanofluid with surfactant to that without surfactant (defined as surfactant enhancement ratio, SER) are in the ranges of 1.12-1.67, 0.94-1.39, and 0.85-1.29 for SDS, CTAB and Span-80, respectively, and the values of SER are in the order of SDS > CTAB > Span-80, which is opposite to the order of surfactant density values. The SER increases with the increase of surfactant concentration and then decreases, presenting the maximum values at 2000, 500 and 1000 ppm for SDS, CTAB and Span-80, respectively. At a fixed surfactant concentration, the SER increases with the decrease of nanoparticle concentration. A nucleate pool boiling heat transfer correlation for refrigerant-based nanofluid with surfactant is proposed, and it agrees with 92% of the experimental data within a deviation of ±25%.     

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

Experimental studies were conducted on heat transfer on a horizontal platinum wire during nucleate pool boiling in nonazeotropic binary mixtures of R12+R113, R134a+R113, R22+R113 and R22+R11, at pressures of 0.25 to 0.7 MPa and at heat fluxes up to critical heat flux. The substances employed were chosen such that the components of a given mixture had a large difference in saturation temperatures. The boiling features of the mixtures and the pure substances were observed by photography. The relationship between the boiling features and the reduction in heat transfer coefficient in binary mixtures is discussed in order to propose a correlation useful for predicting the experimental data measured over a wide range of low and high heat fluxes. It is shown that the correlation is applicable also to alcoholic mixtures. The physical role of k, which was introduced to evaluate the effect of heat flux on the reduction in heat transfer coefficient, is clarified based on the measured nucleate pool boiling heat transfer data and the visual observations of the boiling features. Received on 13 May 1997     

Mechanistic models for pool nucleate boiling heat transfer: input and validation

Correlations for nucleate boiling heat transfer should be improved, or in the long term possibly be replaced, by the development of mechanistic simulations that include the non-uniform spacing and variable characteristics of the nucleation sites and non-linear interactions between the sites. This paper discusses the interactions that should be included in simulations and some lessons from a first attempt to validate a particular simulation against experimental spatio-temporal data for wall temperature. Input data for nucleation site positions and characteristics are a particular problem and the prospects for obtaining this data from measurements that are independent of boiling are discussed.     

Enhancement of nucleate pool boiling heat transfer coefficient by reentrant cavity surfaces

The pool boiling of acetone, isopropanol, ethanol and water at atmospheric pressure has been carried out on a plain tube, and five different reentrant cavity (REC) heating tubes. The heat flux has remained in a range of 11–42 kW/m² for all the heating tubes. The enhancement factor, E, has been found to increase with the rise in heat flux, irrespective of the boiling liquid and the test-section tube combinations. For the pool boiling of acetone and isopropanol, the maximum enhancement factor has been attained for REC-2 tube with mouth size of 0.3 mm and for ethanol and water the mouth size could not be optimized, however, the maximum enhancement factor has been attained for REC-4 tube with mouth size of 0.2 mm. A correlation has also been developed to predict the enhancement factor, E, for the pool boiling of the test-liquids on REC heating tubes. This correlation has predicted the enhancement factor, E, in an error band of +12.5 to –7.5%.     

Critical heat flux in pool boiling on a vertical heater

Critical heat flux during pool boiling on a vertical heater of wire or plate has been measured employing water and R113. The experiment was made for a wire of 0.5 to 2 mm in diameter and for a plate of 5, 7 and 30 mm in width and from 20 to 300 mm in height. The pressure was 1 and 2 bar for water and 1, 2, 3 and 4 bar for R113. The experiment shows that for the case of both wire and plate of 5, 7 mm, a large coalesced bubble entirely surrounds the vertical heater and rises surrounding it, while for the case of w = 30 mm, a large bubble cannot surround and rises along its surface. The characteristic of CHF can be divided into two regimes depending on the flow condition when CHF takes place. Correlations are proposed for the CHF of the wire and the plate of w = 5, and 7 mm, yielding good accuracy. The CHF for the plate of w = 30 mm has a similar tendency to that in one side headed surface and can be predicted reasonably by existing correlation for one side heated surface.     

Theoretical analyses on boiling critical heat flux with porous media

Porous media has been widely applied to enhance boiling heat transfer in industry, especially for increasing the value of critical heat flux (CHF). Two cases were considered in the paper: boiling within porous bed and boiling above on porous coatings. For boiling within porous bed, simplified Rayleigh–Taylor stability was analyzed and parametric effects of porous media on boiling critical heat flux were revealed. For boiling above on porous coatings, a simple new critical heat flux model was proposed basing on the analysis of liquid film stability and parametric effect of porous coatings on CHF was elaborated.