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.     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.     

Unified theoretical prediction of fully developed nucleate boiling and critical heat flux based on a dynamic microlayer model

A new dynamic microlayer model has been proposed to predict theoretically the heat flux in fully developed nucleate boiling regions including critical heat flux (CHF). In this model, the heat transfer with boiling is mainly attributed to the evaporation of the microlayers which are periodically formed while the individual bubbles are forming. Since the initial microlayer thickness becomes thinner with the increase of wall superheat, both the local evaporation and the partial dryout speed of the microlayer increase. As a result, the time-averaged heat flux during the period of individual bubble has a maximum point, the CHF, at the predicted continuous boiling curve.     

Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas–liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen–water flow in a microchannel with the size of 1500 × 720 μm were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m²-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m². The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer deterioration occurred for annular flow when the film thickness became small to suppress nucleate boiling. The mechanism of heat transfer deterioration was discussed and a model of heat transfer deterioration was applied to predict the experimental data.     

New theoretical models of evaporation heat transfer in horizontal microfin tubes

A stratified flow model and an annular flow model of evaporation heat transfer in horizontal microfin tubes have been proposed. In the stratified flow model, the contributions of thin film evaporation and nucleate boiling in the groove above the stratified liquid level were predicted by a previously reported numerical analysis and a newly developed correlation, respectively. The contributions of nucleate boiling and forced convection in the stratified liquid region were predicted by the new correlation and the Carnavos correlation, respectively. In the annular flow model, the contributions of nucleate boiling and forced convection were predicted by the new correlation and the Carnavos correlation in which the equivalent Reynolds number was introduced, respectively. The flow pattern transition curve between the stratified-wavy flow and the annular flow proposed by Kattan et al. was introduced to predict the heat transfer coefficient in the intermediate region by use of the two theoretical models. The predictions of the heat transfer coefficient compared well with available experimental data for ten tubes and four refrigerants.     

An onset of nucleate boiling criterion for horizontal flow boiling

A model to predict the onset of nucleate boiling has been successfully developed to differentiate purely convective evaporation from mixed nucleate and convective boiling during evaporation inside a horizontal tube of 14 mm I.D. Based on an extensive database collected for the natural refrigerant ammonia (R-717) over mass velocities from 10 to 140 kg · m ⁻²· s ⁻¹ , the analysis of the stratified, stratified–wavy and mainly annular flow patterns during evaporation with different heat flux ranges showed very accurate predictions in terms of the local heat transfer coefficient using this new onset of nucleate boiling criterion.     

Boiling Heat Transfer Characteristics for Methane,Ethane and Their Binary Mixtures

Abstract

Methane (R50) and ethane (R170) are the dominated components of natural gas and the important components in mixture refrigerants for the mixture Joule–Thomson refrigeration cycle. In this article, experimental investigations on nucleate pool boiling and flow boiling heat transfer characteristics of R50, R170, and their binary mixtures are presented. The effects of saturation pressure, heat flux, mass flux, concentration, and vapor quality on heat transfer coefficients are analyzed and discussed. Firstly, the pool boiling heat transfer data were compared with six well-known correlations. Labuntsov correlation shows the best agreement with a mean absolute relative deviation (MARD) of 11.3%. Secondly, a new flow boiling heat transfer correlation for pure fluids was proposed based on the asymptotic addition of forced convection and pool boiling. The modified enhancement factor and suppression factor were developed to account for their relative contribution. In addition, in order to consider the mass transfer resistance of mixtures, a new mixture factor was deduced. The new flow boiling heat transfer correlations can well predict the experimental data with the MARD of 9.5% for pure fluids and 8.3% for mixtures.     

Effect of pressure, subcooling, and dissolved gas on pool boiling heat transfer from microporous, square pin-finned surfaces in FC-72

The present research is an experimental study of the effects of pressure, subcooling, and non-condensable gas (air) on the pool nucleate boiling heat transfer performance of microporous enhanced finned surfaces. The test surfaces, solid copper blocks with 1-cm² bases and 5×5 square pin-fin arrays of 2, 4 and 8 mm fin lengths, were immersed in FC-72. The test conditions included an absolute pressure range of 30-150 kPa and a subcooling range of 0 (saturation) to 50 K. Effects of these parameters on nucleate boiling and critical heat flux (CHF) were investigated. In addition, differences between pure subcooled and gas-saturated conditions as well as horizontal and vertical base orientations were also investigated. Results showed that, in general, the effects of pressure and subcooling on both nucleate boiling and CHF were consistent with previously tested flat surface results, however, subcooling was found to significantly affect the high heat flux region of the microporous finned surfaces nucleate boiling curves. The relative enhancement of CHF from increased subcooling was greater for the microporous surface than the plain surface but less than a microporous flat surface. The horizontal orientation (horizontal base/vertical fins) was found to be slightly better than the vertical orientation (vertical base/horizontal fins). Correlations for both nucleate boiling and CHF for the microporous surfaces were also developed.     

Determination of the bulk-saturated liquid condition for maximum heat fluxes at boiling crises

In this paper is presented a method of predicting the thermodynamic state of the bulk-saturated liquid at which peak nucleate boiling and minimum film boiling heat fluxes attain a maximum in pool boiling. The data in the form of saturated pressure and temperature in both absolute and reduced quantities are listed for several cryogenic liquids, hydrocarbons and halocarbon refrigerants. It is observed that the bulk liquid condition for a maximum in critical heat flux in nucleate boiling corresponds to intermolecular separation at maximum attractive potential, which in turn can be determined from saturated liquid density data. The predictions are fairly well corroborated by experimental data available in literature.     

Augmentation of nucleate boiling heat transfer and critical heat flux using nanoparticle thin-film coatings

Nanoparticle thin-film coatings applied to boiling surfaces using a layer-by-layer (LbL) assembly method demonstrated significant enhancement in the pool boiling critical heat flux (CHF) and nucleate boiling heat transfer coefficient. Up to 100% enhancement of the critical heat flux and over 100% enhancement of the heat transfer coefficient were observed for pool boiling of nickel wires coated with different thin-films of silica nanoparticles. Surface characterization revealed that the surface wettability changed drastically with the application of these coatings, while causing virtually no change in the surface roughness. It is concluded that the nanoporous structure coupled with the chemical constituency of these coatings leads to the enhanced boiling behavior.     

Evaporation of refrigerants in a horizontal tube: an improved flow pattern dependent heat transfer model compared to ammonia data

Few experimental test data are available for evaporation of ammonia inside tubes and numerous new data have been measured and presented here. An improved approach to the prediction of flow boiling heat transfer in horizontal tubes has been proposed through the study of each flow pattern separately, incorporating a new criterion defining the onset of nucleate boiling as a function of the critical convective heat transfer coefficient representative of the location where nucleate boiling might occur. A new function, based on a pseudo-Biot number delineates two different mean heat fluxes on the perimeter of the tube in stratified types of flow, one in contact with the liquid and one in contact with the vapor. Considering pure convective heat transfer, or mixed convective and nucleate heat transfer, this division allows the use of a common criterion to be applied to each flow pattern. Even if the database showed that the flow conditions in the annular liquid film were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points where purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database of refrigerants HFC-134a and ammonia. Due to the precision of the new flow pattern map and the effectiveness of the onset on nucleate boiling criterion, this new heat transfer model accurately predicts the heat transfer conditions during evaporation.     

Boiling on a Tube Bundle: Part II—Heat Transfer and Pressure Drop

An extensive experimental study was undertaken to measure nucleate pool boiling heat transfer coefficients, local bundle boiling heat transfer coefficients, and two-phase bundle pressure drops for R134a and R236fa on one plain tube bundle configuration. The experimental database allowed the refinement of frictional pressure drop models previously developed at the Laboratory of Heat and Mass Transfer. Together with the new onset of dryout prediction method presented in part I (preceding article in this issue), this constitutes a significant improvement in such prediction methods. The local bundle boiling heat transfer data highlighted the dependency of the heat transfer coefficient on the heat flux as expected for the present conditions. The new method was proposed and worked well versus the present database and was also validated against additional refrigerants from independent studies. It was proven to also work reasonably well for falling film evaporation data, proving the new prediction method is applicable for a wide range of operating conditions.     

Two-tier model for nucleate pool boiling on microconfifured composite surfaces

A new model is developed to describe the heat transfer mechanism in nucleate pool boiling on a microconfigured composite surface. Both the microlayer and macrolayer thickness are determined from the model. This model can be extended to explain the nucleate boiling on plain surfaces. The enhancement mechanisms of heat transfer for the nucleate boiling on the microconfigured surface are analyzed.     

Dry patch formed boiling and burnout in potassium pool boiling

Experimental results are presented on dry patch formed boiling and burnout in saturated potassium pool boiling on a horizontal plane heater for system pressures from 30 to 760 torr and liquid levels from 5 to 50 mm. The dry patch formed boiling is a peculiar boiling state where the dry patch formation and the rewetting are alternately repeated in intermittent boiling at a heat flux smaller than burnout heat flux of continuous nucleate boiling and is considered to be a local phenomenon in transient transition boiling from the observations of the wall and liquid temperature fluctuations. The dry patch formation occurs in the intermittent boiling which is often encountered when liquid alkali metals are used under relatively low pressure conditions. Burnout is caused from both continuous nucleate and dry patch formed boiling. The burnout heat flux together with nucleate boiling heat transfer coefficients are empirically correlated with system pressures. A model is also proposed to predict the minimum heat flux to form the dry patch.     

Pool Boiling on Modified Surfaces Using R-123

Saturated pool boiling of R-123 was investigated for five horizontal copper surfaces modified by different treatments, namely, an emery-polished surface, a fine sandblasted surface, a rough sandblasted surface, an electron beam-enhanced surface, and a sintered surface. Each 40-mm-diameter heating surface formed the upper face of an oxygen-free copper block, electrically heated by embedded cartridge heaters. The experiments were performed from the natural convection regime through nucleate boiling up to the critical heat flux, with both increasing and decreasing heat flux, at 1.01 bar, and additionally at 2 bar and 4 bar for the emery-polished surface. Significant enhancement of heat transfer with increasing surface modification was demonstrated, particularly for the electron beam-enhanced and sintered surfaces. The emery-polished and sandblasted surface results are compared with nucleate boiling correlations and other published data.     

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.     

Two-Phase Flow and Boiling Heat Transfer in Small-Diameter Tubes

For the development of a high-performance heat exchanger using small channels or minichannels for air-conditioning systems, it is necessary to clarify the characteristics of vapor‐liquid two-phase flow and heat transfer of refrigerants in small-diameter tubes. In this keynote paper, the related research works that have already been performed by the author and coworkers are introduced. Based on the observations and experiments of R410A flowing in small-diameter circular and noncircular tubes with hydraulic diameter of about 1 mm, the characteristics of vapor‐liquid two-phase flow pattern and boiling heat transfer were clarified. In low quality or mass flux and low heat flux condition, in which the flow was mainly slug, the “liquid film conduction evaporation” heat transfer peculiar to small-diameter tubes prevailed and exhibited considerably good heat transfer compared to nucleate boiling and forced convection evaporation heat transfer. The effects of the tube cross-sectional shape and flow direction on the heat transfer primarily appeared in the region of the “liquid film conduction evaporation” heat transfer. A new heat transfer correlation considering all of three contributions has been developed for small circular tubes.     

Nucleate pool-boiling enhancement outside a horizontal bank of twisted tubes with machined porous surface

Nucleate pool boiling of refrigerants is of important application in the flooded evaporator of refrigeration and air-conditioning system. Many surface geometries involve machined porous surface have been adopted to enhance the nucleate pool boiling heat transfer of refrigerants. Nucleate pool-boiling performance of R134a and R142b outside a horizontal bank of twisted tubes with machined porous surface (T-MPS tubes) was investigated in this paper. The experimental results showed that the T-MPS tube bank could enhance boiling heat transfer evidently. The enhancement ratios of R134a from the T-MPS tube bank were 1.4–1.7 and the maximum enhancement ratio of R142b could reach up to 4.4. Analyzing the tube bank effects of boiling heat transfer for R134a and R142b, the overall trend showed that the boiling heat transfer performance of the T-MPS tube bank was inferior to that of single T-MPS tube slightly.     

Nucleate boiling heat transfer from a structured surface – Effect of liquid intake

A model of the suction evaporation mode in nucleate boiling from tunnel and pore structures is presented. The model is based on the analysis by Nakayama et al. [W. Nakayama, T. Daikoku, H. Kuwahara, T. Nakajima, Dynamic model of enhanced boiling heat transfer on porous surfaces – Part II. Analytical model, ASME J. Heat Transfer 102 (3) (1980) 451–456] and L.H. Chein and R.L. Webb [A nucleate boiling model for structured enhanced surfaces, Int. J. Heat Mass Transfer 41 (14) (1998) 2183–2195]. Additionally, a detailed phenomenological model of liquid refill has been developed. It has been shown that the process of liquid refill and the time needed for it is strongly dependent on pool height. Effect of liquid pool height on bubble frequency has also been discussed. Finally, a generalized methodology is given for the prediction of boiling data from a structured surface.     

Enhanced boiling heat transfer simulation from structured surfaces: Semi-analytical model

A semi-analytical model of the bubble dynamics is proposed based on the experimental results reported in the literature on boiling from porous enhanced surfaces. The model considers the ‘flooded mode’ regime of enhancement boiling and is validated for data covering a range of tunnel and pore dimensions. The dynamic model accounts for the temporal evaporation rate variation inside tunnels to arrive at the latent heat flux due to internal evaporation and frequency of bubble formation. The population density is predicted using an empirical formulation, and in turn used to estimate the total heat flux from the porous enhanced surface. The model predicts the heat flux for pool boiling from structured surfaces within ±30% of the experimental data. The model is subsequently used in the prediction of the thermal performance of a novel two-phase heat spreader that employs porous structured surfaces for enhancing boiling heat transfer.