Pressure Drop Oscillations Research Articles

Flow boiling of ammonia and flow instabilities in mini-channels

A simultaneous visualization and measurement study has been carried out to investigate the flow boiling and two-phase flow instabilities of ammonia in a mini-channel evaporator at various heat and mass fluxes. The evaporator consisted of 4 parallel 1×1.1mm channels with a uniformly heated length of 250mm. The visualization study showed that periodic reverse flow occurred in the mini-channel evaporator with liquid rewetting stage and annular film evaporating stage repeated alternately. Both damped and stable oscillations of pressure drop, wall temperature and system pressure were observed. The liquid rewetting stage approximately coincided with the decreasing stage of system pressure. Three typical two-phase flow patterns: bubbly flow, elongated bubble/slug flow and annular flow were identified. Low-frequency high-amplitude oscillation and high-frequency low-amplitude oscillation were identified through wavelet analysis. The high-frequency oscillation mainly occurred at the wave crest of the low-frequency oscillation. In addition, high-order harmonics were found in almost all the low-frequency oscillation waves. The effects of mass flux and saturation temperature on flow boiling instabilities were also investigated.

Experimental investigation on two-phase flow instabilities of ultra supercritical pressure boiler with spirally water wall tubes

For a better design of 1000MW ultra supercritical pressure boiler with spirally water wall tubes, an experimental study has been performed in the State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University to investigate the instability of the steam-water two-phase flow inside a smooth tube with an inclined angle of 20degree from the horizon. The static hydrodynamic characteristic curves are obtained and the dynamic instabilities are also analyzed. The boundary conditions of the pressure drop oscillation and the density wave oscillation are proposed. The effects of pressure, mass flux, inlet and outlet throttle, inlet subcooling and the compressible volumes are analyzed primarily. The results demonstrate that the above parameters significantly influence the critical heat flux for the mentioned two types of oscillations and the density wave oscillations are the major type of oscillations in boilers and steam generators. The density wave oscillations occur at a higher heat flux than the pressure drop oscillations with the other operating parameters remaining constant. The inducing mechanisms of the above two types of oscillations are clarified. The variation ranges of critical heat flux and critical vapor quality are illustrated. Two empirical correlations predicting the onset of pressure drop oscillation and density wave oscillation are derived from the experimental data.

Effect of heating profile on the characteristics of pressure drop oscillations

The effect of the heating profile on the characteristics of pressure drop oscillations (PDO) are studied. The experiments are performed in a 2m horizontal test section of 5mm I.D. and R134a as working fluid. The PDOs are characterised by a low frequency oscillation with a superimposed high frequency oscillation at the minimum of the flow oscillation for some conditions. This work has focused on identifying how the heating profile can modify the presence of the high frequency oscillations. In particular it was observed that the high frequency oscillations appear in a given range of heat flux, while at low and high heat fluxes with a uniform heating profile the high frequency oscillations vanish. In addition, a decreasing power distribution can increase the presence of high frequency oscillations, and at high heat fluxes only high frequency oscillations are observed.

Theoretical analysis and modeling of flow instability in a mini-channel evaporator

Pressure drop oscillations in micro/mini-channel evaporators and corresponding flow instabilities, temperature fluctuations have received copious of investigations during the last decade. This paper presents a transient lumped model and theoretical analysis for the pressure drop oscillation in a mini-channel evaporator. Based on the model, the effects of saturation temperature, heat and mass flux on the oscillation are investigated. Experimental studies of ammonia and water flow boiling instabilities are conducted. The mini-channel evaporator consists of 4 parallel 1×1.1mm channels with a uniformly heated length of 250mm. A nonlinear system stability analysis is presented. Apart from upstream compressibility, the inlet sub-cooling degree has a significant effect on the pressure drop oscillation. A maximum allowable inlet sub-cooling degree causing no pressure drop oscillation is proposed. The oscillation period is comprehensively studied, and it is found that the upstream compressible volume required sustaining the oscillation decreases with channel length/diameter ratio dramatically. Despite this, the internal compressibility of the long channel is insufficient to sustain the pressure drop oscillation. In addition, the mass flow rate of the upstream pump can greatly affect the oscillation and the flow boiling system may show different behaviors due to the variation of upstream mass flow rate.

Time-averaged and transient pressure drop for flow boiling with saturated inlet conditions

This study explores flow boiling pressure drop of FC-72 in a rectangular channel subjected to single-side and double-sided heating for vertical upflow, vertical downflow, and horizontal flow with positive inlet quality. Analysis of temporal records of pressure transducer signals is used to assess the influences of orientation, mass velocity, inlet quality, heat flux, and single-sided versus double-sided heating on magnitude of pressure drop oscillations, while fast Fourier transforms of the same records are used to capture dominant frequencies of oscillations. Time-averaged pressure drop results are also presented, with trends focusing on the competing influences of body force and flow inertia, and particular attention paid to the impact of vapor content at the test section inlet and the rate of vapor generation within the test section on pressure drop. Several popular pressure drop correlations are evaluated against the present pressure drop database. Predictions are presented for subsets of the database corresponding to low and high ranges of inlet quality and mass velocity. The correlations are ranked based on mean absolute error, overall data trends, and data spread. While most show general success in capturing the data trends, they do so with varying degrees of accuracy.

Dynamic characteristics of the recessed chamber within a gas–liquid coaxial injector

In this article, a linear dynamic characteristic model of the recessed chamber in a gas–liquid shear coaxial injector is developed. The gaseous injector flow before the recessed chamber is considered to be steady, and we just investigate the response of pressure drop across the recessed chamber to the mass flow rate fluctuation of the liquid injector. The transfer function of the recessed chamber is obtained, considering the distribute characteristics of liquid droplets within the recessed chamber. The results show that the amplitude of pressure drop pulsation would decrease as the liquid flow pulsation frequency increases, when the variation of liquid velocity is not taken into account. While considering the variation of liquid velocity, the result is opposite. The amplitude of pressure drop oscillation on the boundary of the recessed chamber increases with the increase in liquid velocity oscillation frequency.

Bubble formation in co-fed gas–liquid flows in a rotor-stator spinning disc reactor

The gas–liquid flow in a rotor-stator spinning disc reactor, with co-feeding of gas and liquid, is studied for high gas volumetric throughflow rates and high gas/liquid volumetric flow ratios. High speed imaging and spectral analysis of pressure drop signals are employed to analyse the flow. Two mechanisms of bubble formation are observed, one due to gas overpressure leading to large irregular bubbles, and one due to liquid turbulent vortices leading to small, well-defined bubbles. The two mechanisms lead to three distinct gas dispersion regimes, distinguished by their characteristic oscillations in pressure drop. At low rotational Reynolds numbers (Reω < 0.4 · 106), in the gas spillover regime, the gas is dispersed as large bubbles only. Above this critical Reω, small bubbles are sheared off as well, thus forming a heterogeneous dispersion. At sufficiently high Reω, depending on the gas flow rate, the gas is homogeneously dispersed as small bubbles. The maximum gas flow that can be dispersed as small bubbles is linearly proportional to the local energy dissipation rate. The understanding of the bubble formation mechanisms and pressure signature allows prediction and detection of the prevailing hydrodynamic regime in scaled up spinning disc reactors and for different reaction fluids.

Two-phase flow instabilities of forced circulation at low pressure in a rectangular mini-channel

An experimental study has been carried out in a rectangular mini-channel to investigate the two-phase flow instabilities of a forced flow at low pressure. The inlet mass flux is gradually reduced by decreasing the rotational speed of the primary pump until the flow becomes unstable, while maintaining all other thermal parameters including system pressure, inlet temperature and heat flux unchanged. Three types of flow instabilities, density wave oscillation (DWO), pressure drop oscillation (PDO) and Ledinegg instability (LED) are identified during the experiment. The onset of flow instability (OFI) is determined by the demand curves and shows good agreement with Roach and Stoddard’s correlations. The OFI is consistent with the boiling incipience and saturated condition. The stability map is obtained on the plane of subcooling number (Nsub) and phase change number (Npch). DWO and PDO occur at the region between the outlet steam quality xe=−0.001 and xe=0.012, while the LED region becomes narrow with increasing subcooling. Finally, the mechanisms of flow instabilities in our experiment are analyzed. The results show that the PDO observed in this work is not accompanied by CHF (dryout) which is supposed to be the most common phenomenon of flow instabilities in mini-/micro-channels. DWO occurrence is determined by the relation between external forces (gravitational force and pump driving force) and surface tension. As the external forces are dominant, Type-I DWO at low outlet steam quality (DWOI) occurs. However, with reduction of the inlet mass flux, the surface tension begins to play a dominant role. Thus Type-II DWO at high outlet quality (DWOII), which is expected in most ordinarily sized channels, has not been observed during this work.

Interactions Between Parallel Unevenly Heated Minichannels During Flow Boiling of R134A

Transient pressure drop of individual channels during flow boiling of R134a in four 0.54 mm square parallel minichannels was experimentally studied in this work. The design of the test section enabled the experimenter to control and to vary heat flux independently in each channel in the range from 3.82 to 18.66 kW/m2 at five different overall flow rates from 86 to 430 kg/m2-s. Flow rate fluctuation in parallel channels due to the formation of bubbles under the nonuniform heat flux conditions caused significant oscillations in local pressure drop. Statistical analysis indicated that the pressure drop signal was normally distributed when boiling was stable with no incoming flow disturbance. Pressure drop distribution was highly skewed and multimodal when significant evaporation rate at low mass fluxes led to rapid annular flow formation, reducing the free flow of incoming fluid. Cross-correlation analysis revealed a strong interaction between minichannels having the highest heat flux difference among the set of channels. The least heated channel was more sensitive to the fluctuations in other channels. Cross-correlation between the most heated channel and the adiabatic one was estimated to be 39% when the total flow rate was the lowest, 86 kg/m2-s. The power of the relationship between channels dropped significantly as the flow rate increased. Less than 5% of data points could be considered cross-correlated at the highest flow rate of 430 kg/m2-s. Increasing the two-phase pressure drop across each channel caused higher resistance to the incoming disturbances and led to less interchannel interaction. This study of the channels interaction in a system of parallel, nonuniformly heated minichannels can be used as a tool to identify and quantify instabilities and reversed flow conditions.

Experimental investigation on flow instability of forced circulation in a mini-rectangular channel under rolling motion

An experimental study was performed in a mini-rectangular channel under rolling condition to investigate the flow instability of forced circulation. In order to analyze the influence of rolling motion on two-phase flow instability, experiments under both static condition and rolling condition were conducted using the same thermodynamic parameters, such as flow rate, system pressure and inlet fluid temperature. During the experiments, typical pressure drop oscillation (PDO) was observed in static condition, while abundant experimental phenomena including single-phase flow fluctuation, trough-type flow oscillation, coupled flow oscillation and two-phase flow fluctuation were found in rolling condition. Single-phase flow fluctuation, which fluctuates in sinusoidal wave, is directly induced by rolling motion, while two-phase flow fluctuation is supposed to be in annular flow regime which is accompanied with fluctuation caused by rolling motion. Trough-type flow oscillation is confirmed as the boiling incipience according to the pressure drop and heat transfer characteristics. As intermittent boiling occurs at the section close to the outlet of the test channel, during trough-type flow oscillation, two wave peaks in one rolling period are found in the pressure drop fluctuation of the outlet section of the channel. The method of Fast Fourier Transform (FFT) is used to analyze the coupled flow oscillation, and two frequencies, including the rolling frequency and PDO frequency, are found in the amplitude spectrum. Thus, the coupled flow oscillation is supposed to be superposition of trough-type flow oscillation and PDO. Besides, the coupling mechanisms of the coupled flow oscillation are discussed and the evolution characteristics of flow instabilities are presented.

Wavelet decomposition method decoupled boiling/evaporation oscillation mechanisms over two to three timescales: A study for a microchannel with pin fin structure

Boiling/evaporation heat transfer in a microchannel with pin fin structure was performed with water as the working fluid. Simultaneous measurements of various parameters were performed. The chip wall temperatures were measured by a high spatial-time resolution IR image system, having a sensitivity of 0.02°C. The flow pattern variations synchronously changed wall temperatures due to ultra-small Bi number. The wavelet decomposition method successfully identified the noise signal and decoupled various temperature oscillations with different amplitudes and frequencies. Three types of temperature oscillations were identified according to heat flux q and mass flux G. The first type of oscillation occurred at q/G<0.62kJ/kg. The approximation coefficient of wavelet decomposition decided the dominant cycle period which was ∼3 times of the fluid residence time in the microchannel, behaving the density wave oscillation characteristic. The detail coefficients of wavelet decomposition decided the dominant cycle period, which matched the flow pattern transition determined value well, representing the flow pattern transition induced oscillation. For the second type of oscillation, the wavelet decomposition decoupled the three oscillation mechanisms. The pressure drop oscillation caused the temperature oscillation amplitudes of 5–10°C and cycle periods of 10–15s. The density wave oscillation and flow pattern transition induced oscillation are embedded with both the pressure rise and decrease stages of the pressure drop oscillation. The third type of oscillation happened at q/G>1.13kJ/kg, having the density wave oscillation coupled with the varied liquid film evaporation induced oscillation. The liquid island, retention bubble induced nucleation sites and cone-shape two-phase developing region are unique features of microchannel boiling with pin fin structure. This study illustrated that pressure drop oscillation and density wave oscillation, usually happened in large size channels, also take place in microchannels. The flow pattern transition and varied liquid film evaporation induced oscillations are specific to microchannel boiling/evaporation flow.

Visual experimental investigation on oscillations of pressure drop and flow rate of two-phase flow in mini-channel

Visual experimental investigation on oscillations of pressure drop and flow rate of two-phase flow in mini-channel

Experimental study of pressure drop oscillations in parallel horizontal channels

Two-phase flow instabilities are an undesirable phenomenon present in many fields and scales, ranging from large heat exchangers and boilers in industrial applications to micro-scale heat exchangers for high density power electronics. In the present study pressure drop oscillations in a two parallel horizontal channels system have been experimentally investigated, focusing in the individual behavior of each channel. The balanced (same characteristic pressure drop curve) and unbalanced cases have been analyzed finding different limit cycles than the typical single channel case. No pressure drop oscillations with both channels following the typical limit cycle were found. The oscillation mode detected consisted in one channel performing the usual limit cycle, while the other was always oscillating in the superheated vapor region.

Dynamic flow response of crude oil-in-water emulsion during flow through porous media

Injection of crude oil-in-water emulsion in tertiary mode has been recognized as a potential method to increase oil recovery. Better understanding of emulsion flow through porous media is needed to develop practical improved oil recovery processes based on emulsion flooding mechanisms. In this study, two sets of experiments were carried out to further investigate the flow of emulsion in consolidated sandstone core plugs. First, the efficiency of emulsion injection to improve oil recovery was evaluated in two-phase flow experiments. Second, single-phase flow experiments were designed to understand the flow of emulsion at pore scale. In these experiments, a crude oil-in-water emulsion with known drop-size distribution was injected in water-saturated Berea sandstone cores followed by water injection at increasing flow rate. The overall pressure drop and its Fast Fourier Transform were analyzed to unveil potential connections to pore level flow. The pore throat blockage-release mechanism was inferred to be linked to the acquired pressure drop oscillation. The pore-scale dynamics has been shown to depend on local capillary number and emulsion drop size. The blockage-release mechanism of the trapped drops becomes less effective as the water flow rate or equivalently, the capillary number, increases above a critical value. Also, a larger drop-to-pore size ratio results in higher required capillary number, i.e. larger viscous force to mobilize the drops in the porous media.

Flow fluctuation behaviors of single-phase forced circulation under rolling conditions

Marine reactors are received increasing attention recently in the field of ocean engineering. The effect of platform motion on the coolant flow of an offshore nuclear power plants is very complicated, which is required further analysis. In this paper, effects of rolling motion on single-phase flow fluctuation with different loops were investigated experimentally. And a mathematical model was also developed to study the mechanism of the influence of rolling motion on flow fluctuation with different pressure head, rolling parameters and layouts of flow loops. For the closed loop, the flow rate was adjusted by two ways, i.e. regulating the pump rotation speed and changing the state of regulating valve. While for the opened loop, the pressure head was always supplied by elevated water tank. The Reynolds number ranged from 300 to 8000, and ranges of the rolling period and amplitude were 8–20s and 10–30°, respectively. The results show that increase of the loop resistance and pressure head results in smaller flow fluctuation amplitudes. Effects of rolling motion on single-phase flow fluctuation weaken as the oscillatory pressure drop decreases or the pressure head increases. If the pressure head is high enough, the effect of rolling motion on flow fluctuation could be neglected. Rolling motions influence the flow fluctuation behaviors in different ways for the opened loop and the closed loop, due to the inequable oscillatory pressure drop. The flow fluctuation model shows a good agreement with the experiments and the analyzing method is expected to other oscillating flow system.

Numerical analysis of pressure drop oscillations in parallel channels

In this study pressure drop oscillations in two parallel channels are analyzed taking into account the thermal capacity of the pipes. A different limit cycle than the one that takes place in a single channel system is found. During the instability one channel always follows the typical pressure drop oscillations limit cycle while the other channel oscillates always in the superheated vapor region. This behavior leads to very high wall temperatures at the outlet of the heated pipe. This undesirable situation with one channel operating in the superheated vapor region takes place also for the maldistributed stable solutions.

On the influence of heat flux updating during pressure drop oscillations – A numerical analysis

In the present study pressure drop oscillations are analyzed with the aim of providing a better understanding on the dynamics of heat interaction between the fluid and the heated pipe wall. The influence of different approaches for updating the instant heat flux applied are analyzed, obtaining an imperceptible effect of the axial heat conduction along the pipe and a negligible influence of the instant heat flux axial distribution along the pipe. Lastly, a criterion was established for determining when the thermal capacity of the wall plays an important role in the dynamics of the oscillations. This criterion allows us to determine with a simple calculation whether the thermal capacity in the modeling can be neglected without losing considerable accuracy in the results.

Experimental Research of Dynamic Instabilities in the Presence of Coiled Wire Inserts on Two-Phase Flow

The aim of this study is to experimentally investigate the effect of the coiled wire insertions on dynamic instabilities and to compare the results with the smooth tube for forced convection boiling. The experiments were conducted in a circular tube, and water was used as the working fluid. Two different pitch ratios (H/D = 2.77 and 5.55) of coiled wire with circular cross-sections were utilised. The constant heat flux boundary condition was applied to the outer side of the test tube, and the constant exit restriction was used at the tube outlet. The mass flow rate changed from 110 to 20 g/s in order to obtain a detailed idea about the density wave and pressure drop oscillations, and the range of the inlet temperature was 15–35°C. The changes in pressure drop, inlet temperature, amplitude, and the period with mass flow rate are presented. For each configuration, it is seen that density wave and pressure drop oscillations occur at all inlet temperatures. Analyses show that the decrease in the mass flow rate and inlet temperature causes the amplitude and the period of the density wave and the pressure drop oscillations to decrease separately.

Nonlinear Analysis of Chaotic Time Series in a Natural Circulation Boiling Loop

The present study reports chaotic flow oscillations observed in a natural circulation boiling loop. The periodic oscillations of wall temperature (thermal oscillations) initiate at a certain condition of heat flux and inlet subcooling in addition to geysering instability and pressure-drop oscillations. We observed that boiling regime changes from nucleate boiling to transition boiling as a result of decrease in inlet subcooling and increase in heat flux. The thermal oscillations are strongly coupled with pressure-drop oscillations. Nonlinear analysis of the time series of loop flow rate at various heater power and inlet subcooling have been carried out using statistical analysis, fast Fourier transform (FFT), time delay embedding for attractor reconstruction, autocorrelation, and correlation dimension. The analysis confirms that the oscillations are more chaotic at relatively low heater power and high inlet subcooling. The complexity of the oscillations strongly depends on boiling heat transfer regime. Our observations and analysis have been supported by other relevant experiments.

Measurements and flow pattern visualizations of two-phase flow boiling in single channel microevaporators

Two-phase flow boiling is being used in different applications because of its high heat flux capacity compared to single phase flow. However, the fundamentals of boiling fluid flow and heat transfer in microscale geometries are not yet fully understood. The aim of this work is to contribute to a better understanding of the underlying physical phenomena in flow boiling of water in small channels. For this purpose experiments were conducted to investigate flow patterns, boiling curves, and heat transfer coefficients in single channel, microevaporators with a channel depth, width and length of 198×241×21900μm and 378×471×21900μm. High speed visualization (up to 30,000fps) were performed simultaneously with heat transfer and pressure drop measurements to support the quantitative experimental data for better understanding of two-phase flow characteristics in microchannel evaporators. The influence of the heat flux and mass flux in the flow patterns, boiling curve and heat transfer coefficient were studied. Six different flow patterns were observed and classified using the most commonly accepted terms as bubbly, slug, churn, annular, wavy-annular, and inverted annular flow. The flow patterns were closely coupled with mass flux, heat flux, and channel size. Bubbly flow was mainly developed at lower heat fluxes and progressed to slug, churn, and annular flow as the heat fluxes increased. High speed visualization provided a means to characterize the formation of intermittent flows and the subsequent re-wetting of the channels that lead to pressure drop oscillations, the evolution of flow patterns and provided a physical explanation for the occurrence of reversed flow.