Yasunobu Fujita

Nucleate boiling heat transfer and critical heat flux in narrow space between rectangular surfaces

Abstract Pool boiling heat transfer in a confined narrow space is systematically investigated for saturated water at atmospheric pressure between heated and unheated parallel rectangular plates. Experiments are performed at heat flux from boiling inception to the critical heat flux on heating surfaces with a width of 30 mm, lengths of 30 and 120 mm, and gap sizes of 5, 2, 0.6 and 0.15 mm under three surface peripheral conditions. They are all edges open, closed side edges, and closed side and bottom edges. Space inclination is also changed from vertical to facing downwards nearly to the horizontal. On the basis of measurements and observation, boiling behavior and mechanisms in a narrow space are discussed. Measured critical heat flux under two different periphery conditions are also compared with predictions on corresponding burnout models.

Heat transfer in nucleate pool boiling of binary mixtures

Abstract Heat transfer coefficients for nucleate pool boiling of binary mixtures were measured on a circular copper plate facing upwards. Tested mixtures were methanol/water, ethanol/water, methanol/ethanol, ethanol/ n- butanol , and methanol/benzene, each in the saturated state at atmospheric pressure. Special stress was laid on elucidating the dependence of heat transfer reduction on mixture composition, physical properties, phase equilibrium diagram, and heat flux. Experimental data were compared with available correlating methods developed for binary mixtures by Stephan and Korner, and by others. The comparison showed that among the compared correlations, Stephan and Korners correlation and Thomes correlation give fairly good results while their accuracy varies considerably with mixtures and heat flux levels. A modification of Thomes correlation so as to include the effect of heat flux succeeded in correlating the present data within ± 20% accuracy.

Correlation of nucleate boiling heat transfer based on bubble population density

Abstract A general correlating equation of heat transfer in nucleate boiling is derived, based on the assumption that the main driving force for convection in nucleate boiling is the stirring action of the generated bubbles and by means of analogy between pool boiling and free convection. In the theoretical analysis, the degree of superheating is represented not only by the heat flux but also by the bubble population density. Futhermore, the nucleation factor and the pressure factor are introduced into the final correlating equations. Good coincidence has been attained between these equations and the available experimental data in nucleate boiling of various liquids at various pressures.

Critical heat flux of binary mixtures in pool boiling and its correlation in terms of Marangoni number

Abstract Critical heat flux (CHF) in nucleate pool boiling of binary mixtures was newly measured with a horizontal platinum wire, 0.5 mm in diameter, and heated by DC, over the full range of concentrations. Seven mixtures were selected with the intent to cover various types of mixtures: methanol/water, ethanol/water, methanol/ethanol, ethanol/n-butanol, methanol/benzene, benzene/n-heptane and water/ethylene glycol, each in the saturated state at atmospheric pressure. Total 311 raw CHF data were obtained at 75 concentrations including pure components. Aqueous mixtures of methanol and ethanol revealed significant increase of CHF compared to either CHF linearly interpolated between pure components or CHF predicted from a single component correlation with use of the mixture properties. Three organic mixtures showed more or less the same level as an interpolated CHF, while the remaining two mixtures of methanol/benzene and water/ethylene glycol gave the reduced CHF by 20% and 50% at most, respectively. Marangoni number was introduced as a controlling variable to explain the observed increased, invariable, or reduced CHF, and an empirical correlation was developed.

Nucleate Boiling Heat Transfer and Its Augmentation

Publisher Summary Nucleate boiling is a complicated phenomenon accompanied with a phase change from liquid to vapor. The nucleate boiling heat transfer is one of the most important modes of heat exchange occurring in several constituent devices of thermal plant. The augmentation of nucleate boiling heat transfer significantly contributes towards the efficient use of the thermal energy. The chapter describes a correlation method of nucleate boiling heat transfer by considering the factors that affect the heat-transfer processes. It presents the effect of surface configuration on the nucleate boiling along with the nucleate boiling heat transfer in a narrow space and in a liquid film. The chapter also illustrates potential measures for augmentation of nucleate boiling heat transfer and some results obtained. As the nucleate boiling heat transfer is greatly influenced by surface conditions, the unified rule of the nucleation factor for several surface conditions, which is similar to the emissivity encountered in the heat transfer by thermal radiation, needs to be developed in the future.

Flow boiling heat transfer and flow pattern in rectangular channel of mini-gap

Flow boiling in micro- and mini-channels has attracted much attention in recent years. But the phenomena is such confined channels have not been fully understood and explained. Some conclusions reached by different authors are even contradictory. The present research is trying to study some aspects of flow boiling in mini- and micro-channels. In the present paper boiling heat transfer and two-phase flow patterns in rectangular narrow channels were studied. The gap size of the channel was varied as 2, 1, 0.5 and 0.2 mm with the channel width and length being kept at 20 mm and 100 mm, respectively. In the present mini- and micro-channels, four flow patterns were identified; bubbly, intermittent, wavy and annular flow. They can be also divided into several sub-flow patterns. Flow patterns showed strong channel gap size dependence. Smaller gap size deleted bubbly flow, thus induced simpler flow patterns to shift the annular flow at lower vapor quality. The channels can be divided into two groups depending on the gap size; the larger gap group of 2 and 1 mm, and the smaller gap group of 0.5 and 0.2 mm. The larger gap group showed similar heat transfer behavior as conventional size of tubes. The smaller gap group indicated some peculiar phenomena. Heat transfer coefficient in the smaller gap group was relatively high in the low quality region. Then heat transfer coefficient decreased monotonously with increasing vapor quality. This behavior was considered attributable to the micro-bubble generation in the channel corners and an early partial dryout of thin liquid film. Thus the relationship between heat transfer coefficient and flow pattern should be carefully pursued in micro- and mini-channels to develop heat transfer correlations based on flow patterns.© 2004 ASME

Experimental investigation in pool boiling heat transfer of ternary mixture and heat transfer correlation

Abstract Heat transfer coefficients in nucleate boiling on a smooth flat surface were measured for pure fluids, their binary and ternary mixtures under the saturated conditions at 0.6 MPa for a wide range of heat flux and mixture concentration. Refrigerants, R-134a, R-142b and R-123 were used to make up binary and ternary mixtures. Compared to the ideal heat transfer coefficients calculated from a mole fraction average of the wall superheats of pure components, both binary and ternary mixtures showed lower heat transfer coefficients. This reduction was more pronounced as heat flux was increased. The data of binary mixtures were well reproduced by Thome–Shakir and Fujita–Tsutsui correlations. For ternary mixtures, dimensionless heat transfer coefficients plotted on the concentration triangle were very similar to the contour map of boiling range. This similarity suggested the boiling range is an essential parameter to account for heat transfer reduction of ternary mixtures. Fujita–Tsutsui correlation was found applicable to ternary mixtures with a reasonable success.

Boiling and Evaporation of Falling Film on Horizontal Tubes and its Enhancement on Grooved Tubes

Thin liquid films are utilized as an important component in various heat transfer processes because of their high heat transfer rate at low feed rates and with small temperature difference. Evaporators employing such characteristics are widely found in refrigeration systems, natural gas and air liquefying facilities, petrochemical and distillation plants, and more recently are proposed for use in OTEC systems. This paper reviews recent developments on evaporation and boiling heat transfer to falling films on horizontal tubes and their enhancement. Main items focused here are; (1) analysis and experiment for evaporative heat transfer to falling films, (2) empirical formulas for predicting heat transfer coefficient, (3) falling film breakdown and resulting deterioration of heat transfer, (4) analysis of enhanced heat transfer in grooved surface, (5) experimental results for heat transfer enhancement on grooved tubes, and (6) enhanced boiling heat transfer in falling films on grooved tubes.

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance.

A finite element analysis for inverse heat conduction problems

This paper presents an efficient inverse analysis technique based on a sensitivity coefficient algorithm to estimate the unknown boundary conditions of multidimensional steady and transient heat conduction problems. Sensitivity coefficients were used to represent the temperature response of a system under unit loading conditions. The proposed method, coupled with the sensitivity analysis in the finite element formulation, is capable of estimating both the unknown temperature and heat flux on the surface provided that temperature data are given at discrete points in the interior of a solid body. Inverse heat conduction problems are referred to as ill-posed because minor inaccuracy or error in temperature measurements cause a drastic effect on the predicted surface temperature and heat flux. To verify the accuracy and validity of the new method, two-dimensional steady and transient problems are considered. Their surface temperature and heat flux are evaluated. From a comparison with the exact solution, the effects of measurement accuracy, number and location of measuring points, a time step, and regularization terms are discussed.