Pressure-drop Type Research Articles

Flow Boiling Heat Transfer and Two-Phase Flow Instability of Nanofluids in a Minichannel

Single-phase convective heat transfer of nanofluids has been studied extensively, and different degrees of enhancement were observed over the base fluids, whereas there is still debate on the improvement in overall thermal performance when both heat transfer and hydrodynamic characteristics are considered. Meanwhile, very few studies have been devoted to investigating two-phase heat transfer of nanofluids, and it remains inconclusive whether the same pessimistic outlook should be expected. In this work, an experimental study of forced convective flow boiling and two-phase flow was conducted for Al2O3–water nanofluids through a minichannel. General flow boiling heat transfer characteristics were measured, and the effects of nanofluids on the onset of nucleate boiling (ONB) were studied. Two-phase flow instabilities were also explored with an emphasis on the transition boundaries of onset of flow instabilities (OFI). It was found that the presence of nanoparticles delays ONB and suppresses OFI, and the extent is correlated to the nanoparticle volume concentration. These effects were attributed to the changes in available nucleation sites and surface wettability as well as thinning of thermal boundary layers in nanofluid flow. Additionally, it was observed that the pressure-drop type flow instability prevails in two-phase flow of nanofluids, but with reduced amplitude in pressure, temperature, and mass flux oscillations.

The effect of internal surface modification on flow instabilities in forced convection boiling in a horizontal tube

This paper presents the experimental result of a study on the effects of heat transfer enhancement on two-phase flow instabilities in a horizontal in-tube flow boiling system. Five different heat transfer surface configurations and five different inlet temperatures are used to observe the effect of heat transfer enhancement and inlet subcooling. All experiments are carried out at constant heat input, system pressure and exit restriction. Dynamic instabilities, namely pressure-drop type, density-wave type and thermal oscillations are found to occur for all the investigated temperatures and enhancement configurations, and the boundaries for the appearance of these oscillations are found. The effect of the enhancement configurations on the characteristics of the boiling flow dynamic instabilities is studied in detail. The comparison between the bare tube and the enhanced tube configurations are made on the basis of boiling flow instabilities. Differences among the enhanced configurations are also determined to observe which of them is the most stable and unstable one. The amplitudes and periods of pressure-drop type oscillations and density-wave type oscillations for tubes with enhanced surfaces are found to be higher than those of the bare tube. The bare tube is found to be the most stable configuration, while tube with internal springs having bigger pitch is found to be the most unstable one among the tested tubes. It is found that system stability increases with decreasing equivalent diameter for the same type heater tube configurations; however, on the basis of effective diameter there is no single result such as stability increase/decrease with increasing/decreasing effective diameter.

Theoretical analysis of pressure-drop type instabilities in an upflow boiling system with an exit restriction

In this work, two-phase flow pressure-drop type instabilities in an upflow boiling system are studied theoretically. Dynamic simulations of the pressure-drop type instabilities require the knowledge of the steady state characteristics of the system in terms of the pressure drop versus the mass flow rate. In a boiling system with an exit restriction at the outlet of the boiling channel, the pressure drop through the system concentrates at the exit restriction. Therefore, the correlation of the pressure drop of the two-phase mixture flowing through the exit restriction (i.e. a sharp-edged orifice) is essential in the calculation of the total pressure drop of the system. A model for the exit restriction is developed and compared with the experimental results covering a wide range of vapor quality with different heat inputs and inlet subcoolings. The drift-flux model is adopted to predict the steady state characteristics of the boiling system. The dynamic oscillations of the quasi-static pressure-drop type instabilities in the boiling system are simulated and good predictions of the system stability boundary and oscillatory characteristics are obtained when compared with the experimental results.

Dynamical analysis of pressure-drop type oscillations with a planar model

A planar system is formulated for pressure-drop type oscillations in a single-channel boiling system, and the pressure-drop type oscillations are analyzed from the perspective of dynamical system theory. The planar system is developed by using a lumped parameter model, in which the system inlet pressure and the mass flow rate are coupled. The results of Hopf bifucation analysis, the stability criteria and the limit cycles of the pressure-drop type oscillations obtained from the model agree well with experimental results.

Bifurcation analysis of pressure-drop oscillations and the Ledinegg instability

Pressure-drop oscillations and the Ledinegg instability are analyzed from the perspective of dynamical systems theory. An integral formulation is developed to model the two-phase flow system. Instability criteria independent of the actual two-phase flow model are derived for the two phenomena. It is shown that the pressure-drop oscillation limit-cycles occur after a super-critical Hopf bifurcation. In an extension of the analysis, an effort is made to clarify the mechanisms of the pressure-drop type oscillations and Ledinegg instability. The two phenomena are classified from the angle of bifurcation theory, and the differences are outlined.

Two-phase flow pressure-drop type and thermal oscillations

This work focuses on the theoretical investigation of thermal pressure-drop type instabilities in forced convection boiling in a vertical single channel system, with Freon-11 as the working fluid. Experiments with two nichrome tubes of 7.5 mm inner diameter and 9.5 mm outer diameter, one bare and one coated with Linde High-Flux coating, have been carried out. One series of experiments was conducted with constant fluid inlet temperature and various heat inputs, and another with constant heat input and varying inlet liquid temperature. Under the experimental conditions of the study, pressure-drop type and thermal oscillations, as well as pressure-drop type oscillations with superimposed density-wave oscillations, have been observed. A numerical model has been developed to predict the steady-state characteristics of the forced convective two-phase flow and the pressure-drop type and thermal oscillations in a boiling single channel. The drift-flux model is used for numerical predictions. Good agreement between the theory and experiments is obtained.

Two-phase flow instabilities in a horizontal single boiling channel

A study of the stability of an electrically heated single channel, forced convection horizontal system was conducted by using Freon-11 as the test fluid. Two major modes of oscillations, namely, density-wave type (high frequency) and pressure-drop type (low frequency) oscillations have been observed. The steady-state operating characteristics and stable and unstable regions are determined as a function of heat flux, exit orifice diameter and mass flow rate. Different modes of oscillations and their characteristics have been investigated. The effect of the exit restriction on the system stability has also been studied.

Investigation of thermal instabilities in a forced convection upward boiling system

This work covers investigations of thermal instabilities in forced convection boiling in a vertical single channel system, with Freon-11 as the working fluid. Experiments with various heat inputs and inlet fluid temperatures, as well as for different heater tube sizes, have been conducted to investigate the effects of these parameters on the instabilities. Under the experimental conditions of this study, pressure-drop type and thermal oscillations have been observed. In the theoretical study, the one-dimensional governing equations have been solved using the explicit finite difference technique to determine the wall conditions, fluid properties, and flow conditions at any point along the test section. The model is based on the assumption of homogeneous two-phase flow, with thermodynamic equilibrium between the phases. The conditions under which thermal oscillations occur are predicted. The periods and amplitudes predicted by the model are in good agreement with experimental results.

Transient boiling flow instabilities in a multi-channel upflow system

In this study, the transient boiling flow instabilities in a four parallel channel upflow system with and without cross-connections have been experimentally investigated using Freon-11 as the test fluid. Several series of transient experiments have been run using a step increase in power input with a constant flow control valve setting and a step decrease in flow rate with a constant heat input. Two major modes of oscillations, namely density-wave type and pressure-drop type oscillations, have been observed. Total system pressure drop has been plotted as a function of mass flow rate for various heat inputs and flow control valve settings, and the stable and unstable regions for transient density-wave type and pressure-drop type oscillations have been determined and the boundaries of sustained instabilities have also been indicated. Experiments have been conducted with equal and unequal heat inputs and the results have been compared with each other. A comparison of the transient instabilities observed in four parallel channel system with and without cross connections has also been made.