Effect of heat transfer augmentation on two-phase flow instabilities in a vertical boiling channel

The effect of different heater surface configurations on two-phase flow instabilities has been investigated in a single channel, forced convection, open loop, up-flow system. Freon-11 is used as the test fluid, and six different heater tubes with various inside surface configurations have been tested at five different heat inputs. In addition to temperature and pressure recordings, high speed motion pictures of the two-phase flow were taken for some of the experiments to study the two-phase flow behavior at different operating points. Experimental results are shown on system pressure drop versus mass flow rate curves, and stability boundaries are also indicated on these curves. Comparison of different heater tubes is made by the use of the stability boundary maps and the plots of inlet throttling necessary to stabilize the system versus mass flow rate. Tubes with internal springs were found to be more stable than the other tubes.     

Stratification of boiling two-phase flow in a single horizontal channel

The objective of this study is to investigate experimentally the stratification phenomena of boiling two-phase flow in a uniformly heated horizontal channel. Two-phase flow stratification due to gravity effects, and consequently its thermal and hydrodynamic behavior, under steady state conditions, have been determined by measuring 16 top and 16 bottom wall temperatures. Six distinct wall temperature profiles are found, and the corresponding flow patterns are discussed. A dimensionless number has been formulated for the prediction of the occurrence of different flow patterns.     

Dynamic instabilities of boiling two-phase flow in a single horizontal channel

Dynamic instabilities of two-phase flow associated with refrigerant R-11 in a uniformly heated horizontal in-tube boiling system were experimentally investigated. An experimental setup was designed and built to work in a wide range of mass fluxes G [75–1050 kg/(m² s)], heat fluxes q (0–100 kW/m²), and fluid inlet temperatures T_inlet (2–24°C). Dynamic instability data were obtained under various working conditions. The dependence of oscillation amplitude and period on system parameters is discussed, and the boundaries of various oscillations are located on the steady-state characteristic curves.     

PTV experiments of subcooled boiling flow through a vertical rectangular channel

Time resolved Particle Tracking Velocimetry (PTV) experiments were carried out to investigate turbulent, subcooled boiling flow of refrigerant HFE-301 through a vertical rectangular channel with one heated wall. Measurements were performed with liquid Reynolds numbers (based on the hydraulic diameter) of Re = 3309, 9929 and 16,549 over a wall heat flux range of 0.0–64.0 kW/m². Turbulence statistics are inferred from PTV full-field velocity measurements. Quantities such as: instantaneous 2D velocity fields, time-averaged axial and normal velocities, axial and normal turbulence intensities, and Reynolds stresses are obtained. The present results agree well with previous studies and provides new information due to the full-field nature of the technique. This work is an attempt to provide turbulent subcooled boiling flow data for validation and improvement of two-phase flow computational models.     

Parametric analysis of two-phase flow instability in a channel with inlet and outlet hydraulic resistances

The mechanism of low-frequency self-oscillating instability of a one-dimensional two-phase flow in a channel with inlet and outlet hydraulic resistances is considered. The mechanism is based on the sensitivity of the inlet flow rate of the liquid to the pressure variation inside the channel and the sensitivity of the pressure to the variation of the outlet gas flow rate (with a constant mass rate of the liquid-gas phase transition per unit volume). A spectral analysis of the stability of the steady solution of the boundary-value problem for a hyperbolic-type nonlinear system of equations is performed within the framework of a two-velocity model of a gas-liquid flow. Parametric boundaries of the region of instability are obtained. The existence of self-oscillations in this range of parameters is supported by a numerical solution of the unsteady boundary-value problem. Institute of Catalysis, Siberian Division Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 1, pp. 47–53, January–February, 1998.     

Effect of injection on the flow in the inlet region of a channel

Momentum and energy integrals are used to study the effect of injection on the flow in the inlet region of a channel, taking into account the loss of energy due to viscous dissipation in the boundary layer. Analytical expressions for boundary layer development and inlet region pressure distribution are presented. A comparison of the results of the present analysis is made with earlier studies.     

Flooding in vertical two-phase flow

The phenomenon of flooding limits the stability of a liquid film falling along the walls of a tube in which a gas is flowing upward. This paper describes a series of experiments to observe and photograph the flooding process. Flooding in a vertical perspex tube was observed by viewing axially along the tube from the top, and flooding on the walls of a stainless steel rod supported centrally within a perspex tube was observed by viewing normally through the tube walls. The resulting observations show that flooding occurs as a result of the formation and motion of a large disturbance wave on the surface of the liquid film. A simple theoretical treatment of flooding is presented, based on the observations of the flooding disturbance. The resulting equations enable the calculation of the gas velocity and the liquid wave height at flooding, and the predictions are in good agreement with experimental observations reported here and elsewhere.     

Gas-particle two-phase turbulent flow in a vertical duct

Two-phase gas-phase turbulent flows at various loadings between the two vertical parallel plates are analyzed. A thermodynamically consistent turbulent two-phase flow model that accounts for the phase fluctuation energy transport and interaction is used. The governing equation of the gas-phase is upgraded to a two-equation low Reynolds number turbulence closure model that can be integrated directly to the wall. A no-slip boundary condition for the gas-phase and slip-boundary condition for the particulate phase are used. The computational model is first applied to dilute gas-particle turbulent flow between two parallel vertical walls. The predicted mean velocity and turbulence intensity profiles are compared with the experimental data of Tsuji et al. (1984) for vertical pipe flows, and good agreement is observed. Examples of additional flow properties such as the phasic fluctuation energy, phasic fluctuation energy production and dissipation, as well as interaction momentum and energy supply terms are also presented and discussed.

Applications to the relatively dense gas-particle turbulent flows in a vertical channel are also studied. The model predictions are compared with the experimental data of Miller & Gidaspow and reasonable agreement is observed. It is shown that flow behavior is strongly affected by the phasic fluctuation energy, and the momentum and energy transfer between the particulate and the fluid constituents.     

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     

Flow patterns in vertical two-phase flow

This paper is concerned with the flow patterns which occur in upwards gas-liquid two-phase flow in vertical tubes. The basic flow patterns are described and the use of flow pattern maps is discussed. The transition between plug flow and churn flow is modelled under the assumption that flooding of the falling liquid film limits the stability of plug flow. The resulting equation is combined with other flow pattern transition equations to produce theoretical flow pattern maps, which are then tested against experimental flow pattern data. Encouraging agreement is obtained.     

Effect of surfactant concentration on saturated flow boiling in vertical narrow annular channels

Saturated flow boiling of environmentally acceptable nonionic surfactant solutions of Alkyl (8–16) was compared to that of pure water. The concentration of surfactant solutions was in the range of 100–1000 ppm. The liquid flowed in an annular gap of 2.5 and 4.4 mm between two vertical tubes. The heat was transferred from the inner heated tube to two-phase flow in the range of mass flux from 5 to 18 kg/m² s and heat flux from 40 to 200 kW/m². Boiling curves of water were found to be heat flux and channel gap size dependent but essentially mass flux independent. An addition of surfactant to the water produced a large number of bubbles of small diameter, which, at high heat fluxes, tend to cover the entire heater surface with a vapor blanket. It was found that the heat transfer increased at low values of relative surfactant concentration C/C₀, reaches a maximum close to the value of C/C₀ = 1 (where C₀ = 300 ppm is the critical micelle concentration) and decreased with further increase in the amount of additive. The dependence of the maximal values of the relative heat transfer enhancement, obtained at the value of relative concentration of C/C₀ = 1, on the boiling number Bo may be presented as single curve for both gap sizes and the whole range of considered concentrations.     

Slow two-phase flow through a sinusoidal channel

The two-phase flow through a symmetric sinusoidal channel is studied by means of a regular perturbation analysis, where the small parameter is defined as the ratio between the amplitude of variation of the channel wall and the average thickness of the non-wetting phase. Results are valid for Reynolds numbers of the same order of magnitude as that of the expansion parameter. It is thus found that the fluid-fluid interface presents a wavy shape characterized by an amplitude and a phase-shift with respect to the fixed solid-fluid interface. Instabilities of the two-phase flow can arise for large values of the viscosity, flow rate and phase thickness ratios. Results are expected to be a first step towards the understanding of the hydrodynamics of trickle bed reactors, where several flow regimes are possible.     

Convective magnetohydrodynamic flow in a vertical channel

An analysis is presented for the combined forced and free convective magnetohydrodynamic flow in a vertical, finite rectangular channel that is subjected simultaneously to a pressure gradient and a temperature gradient. Exact solutions are found for electrically nonconducting channel walls and perfectly conducting walls. In particular, the case of heating from below is examined and discussed.     

Periodic oscillations in a horizontal single boiling channel with thermal wall capacity

The dynamic behavior of a horizontal boiling channel with a surge tank is investigated through nonlinear analysis. The model involves a surge tank that is subject to inlet mass flow rate and a constitutive model containing a cubic nonlinearity is used to describe the outlet pressure-flow rate relation of the downstream boiling regime. The model also includes boiling heat transfer process and incorporates the effect of the wall thermal capacity which allows the temperature and heat transfer coefficient of the heater wall to vary with time. Within certain operating regimes, the model exhibits self-excited periodic oscillations, which can be identified with pressure-drop oscillations. In this study, these oscillations are described as relaxation oscillation and the qualitative features of the response can be understood in terms of the underlying model. Finally, the present model is compared with the experimental data available in literature to investigate that transient effects of temperature heater walls, pressure, and mass flow rate.     

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.A mathematical model has been developed to predict the transient behavior of boiling two-phase systems. The model is based on homogenous flow assumption and thermodynamic equilibrium between the liquid and vapor phases. The transient characteristics of boiling two-phase flow horizontal system are obtained for various heat inputs, flow rates and exit orifice diameters by perturbing the governing equations around a steady state. Theoretical and experimental results have been compared.     

Statistical characteristics of two-phase gas-liquid flow in a vertical microchannel

This paper presents an experimental study of the structure of an upward gas-liquid flow in a vertical microchannel with a cross-sectional dimension of 0.67 × 2.00 mm and a length of 0.5 m. The tests were performed in the ranges of reduced rates of nitrogen 0.04–11.00 m/sec and water 0.07–0.41 m/sec. Using the method of two-beam laser scanning, we identified the main flow regimes (slug-bubble, slug, transient, emulsion, and annular) and determined the statistical characteristics of the two-phase flow. A map of flow regimes was constructed, the dependence of slug velocity on the reduced mixture velocity was obtained, and the friction factor for an upward gas-liquid flow in a microchannel was measured.     

Visualization of flow boiling of liquid nitrogen in a vertical mini-tube

The flow boiling patterns of liquid nitrogen in a vertical mini-tube with an inner diameter of 1.931 mm are visualized with a high-speed digital camera. The superficial gas and liquid velocities are in the ranges of 0.01–26.5 m/s and 0.01–1.2 m/s, respectively. Four typical flow patterns, namely, bubbly, slug, churn and annular flow are observed. Some interesting scenes about the entrainment and liquid droplet deposition in the churn and annular flow, and the flow reversal with the indication of negative pressure drop, are also presented. Based on the visualization, the two-phase flow regime maps are obtained. Compared with the flow regime maps for gas–water flow in tubes with similar hydraulic diameters, the region of slug flow in the present study reduces significantly. Correspondingly, the transition boundary from the bubbly flow to slug flow shifts to higher superficial gas velocity, and that of churn to annular flow moves to lower superficial gas velocity. Moreover, time-averaged void fraction is calculated by quantitative image-digitizing technique and compared with various prediction models. Finally, three kinds of oscillations with long-period and large-amplitude are found, possible explanation for the oscillations is given by comparing the instantaneous flow images with the data of pressure, mass flux and temperature recorded synchronously.