Pressure Drop Fluctuations Research Articles

Experimental investigations of flow boiling heat transfer performance in finned micro-channels

The flow boiling heat transfer and pressure drop in rectangular-finned micro-channels (i.e. micro-channel with rectangular fins) were experimentally investigated at five different fin-heights. At three different mass fluxes and a range of heat fluxes, heat transfer coefficients and pressure drops in the micro-channels were measured. The flow patterns during the flow boiling process were visualized to estimate the flow regime transitions in the micro-channels. The experimental results showed that the micro-channel with 500 μm height fins (i.e. tip clearance = 100 μm) performs the highest heat transfer and shows the highest resistance to flow boiling instability among the studied cases. Compared to the smooth micro-channel, the averaged heat transfer coefficient in the micro-channel with 500 μm height fins has been improved by up to 133.9 % at the same wall temperature. Compared to the micro-channel with 600 μm height fins (i.e. tip clearance = 0 μm), the averaged heat transfer coefficient in the micro-channel with 500 μm height fins has been improved by up to 45.8 %. At the initial point of flow instability, the micro-channel with 500 μm height fins has the highest pressure drop fluctuation frequency, the highest surface wetting frequency, and the lowest pressure drop variation coefficient among the studied cases. However, the micro-channel with 600 μm height fins has the lowest pressure drop fluctuation frequency and highest pressure drop variation coefficient, hence experiencing the longest dry-out process among the studied cases.

Appraisal of the Flooding Behaviour of Rotating Packed Beds

Rotating packed beds (RPBs) enhances mass transfer processes because a centrifugal force which is several -times greater than gravity is used as the driving force. The complexity of fluid flow across RPBs has made predicting and accurately determining their hydrodynamic behaviours difficult. The flooding point as a hydrodynamic characteristic is essential for the accurate design and scale-up of RPBs. However, variations in flooding point definitions and methodologies across the literature highlight the need for standardized approaches in studying RPB flooding phenomena. This study compared four approaches based on pressure drop fluctuations and the volume of liquid ejected from the RPB to determine the onset of flooding in RPBs using experimental results from a pilot-scale counter-current RPB. For rotational speeds of 300 -1500 rpm, gas flow rate of 100-300 Nm3/h, and liquid flow rates of 0.39-0.75 m3/h, the pressure drop varied from 314 to 2,100 Pa. Quantitative comparisons of the results based on different flooding point definitions showed wide variations with the values of the pressure drop at the onset of flooding differing by as much as 325 %. A quantitative approach based on virtual observations and the ejection of 8 % of the total liquid flow rate from the rotor’s eye is proposed as the standard method for identifying the onset of flooding in RPBs.

Experimental investigation of different sub-regimes in horizontal stratified and intermittent gas-liquid two-phase flow: Flow map and analysis of pressure drop fluctuations

Horizontal stratified and intermittent gas-liquid two-phase flows exhibit several sub-regimes. Identifying these correctly would enable the development of more robust predictive models. This study reports on experimental investigation, based on observations and the collection of time series of pressure drop in a 40 mm ID pipe. Nine different sub-regimes (SS, 2D wave, 3D wave, RW, ED+RW, PS+RW, plug, LAS, HAS) were revealed. An original flow pattern map for this diameter is proposed including transitory regions between the sub-regime. Comparison of experimental observations with existing flow maps has highlighted the effect of pipe diameter on transition of sub-regimes. As reported in literature the sub-regimes, could be identified by direct visualization of the pressure drop time series obtained from differential pressure sensor a swell as using Probability Density Function. Furthermore, statistical analysis of standard deviation as function of gas superficial velocity enabled detection of the transition of various sub-regimes. Finally, a space feature based on standard deviation and mixture Froude number is proposed as an identification tool between the different sub-regimes investigated.

Physics-data fusion for digital metering of gas-liquid two-phase flow in oil nozzles

Oil well production metering is an important basis for assessing resource reserves and production capacity. Existing metering methods require high investment and involve significant implementation difficulties and high computational costs. A new method of multiphase flow metering method was proposed by combining the fluctuation of pressure drop and temperature difference generated after throttling through the oil nozzle. The throttling equation coupling differential pressure, flow rate and gas fraction was derived. By regulating the gas and liquid superficial velocity, we carried out the experimental test for true nozzle aperture of 10 mm in the loop system, covering various flow patterns. In addition, the deep neural network was constructed for inversion prediction of mass gas fraction. The proposed flow metering equation has an adaptable gas mass fraction range of 0 to 100%, unaffected by the variations in flow patterns, gas-liquid velocities, and system pressure changes. The metering errors of gas mass fraction and mass flow rate is below ±10%. The proposed metering method only relies on the existing temperature and pressure measurement system in the gathering and transportation system, and no additional measuring devices are required. It can be widely used in oil filed production to realize the virtual metering.

Effect of number of channels on the fluctuations and distribution of humid-nitrogen condensing flow in a parallel flow field

Pressure drop and flow rate fluctuations of condensation-induced gas–liquid two-phase flow through a single microchannel have been widely recognized, which are quite different from that through parallel-connected multiple microchannels, as in the flow field of proton exchange membrane fuel cells and heat exchangers. This work investigates the effects of the number of channels on the fluctuations of the humid-nitrogen condensing flow and the uniformity of the flow rate distribution across the channels. A parallel flow field with eight channels is designed; the arrangement of these channels is axisymmetric, ensuring identical flow routes and thermal boundary conditions among individual channels. The number of open channels in the flow field could be adjusted from one to eight. The observed pressure drop fluctuated between 0.20 kPa and 0.68 kPa in the single-channel flow field, compared to 0.22 kPa to 0.43 kPa in the two-channel flow field and 0.28 kPa to 0.48 kPa in the eight-channel flow field. This indicates that the amplitude of pressure drop fluctuations decreases as the number of channels increases. In contrast, both the flow rate fluctuation and maldistribution are most significant in the eight-channel arrangement. It is also found that the effect of channel number on condensing flow dynamics is noticeable when both the velocity and humidity of the inlet gas are low.

Investigation of the Static Pressure Drop and Production that Occurs due to Flow Patterns in a Perforated Horizontal Wellbore

This study examined the intricate interaction between flow patterns and production within a perforated horizontal wellbore. The study precisely assessed the behavior of static pressure drop by utilizing an array of flow regimes encompassing bubble, dispersed bubble, transitional bubble/slug, slug, stratified, transitional slug/stratified wave, and stratified wave. Remarkably, an upward trend in static pressure drop was observed with increasing water phase presence, while the converse was true for the air phase. Besides, the air phase superficial velocity exhibited a direct correlation with the magnitude of pressure drop fluctuations. The liquid production demonstrated a peak during bubble and slug flow regimes, followed by a descent during the transition to stratified and stratified wave flow. This decline can be attributed to mixing pressure drops localized during the perforations. Furthermore, an upward trend in average liquid production was observed with increasing mixture superficial velocity, primarily due to the dominant presence of the water phase. Additionally, the percentage of liquid production was positively associated with the water's superficial velocity when the air's superficial velocity was held constant. While the experimental and numerical results were in agreement for slugs and structured flows, there were discrepancies in the behavior of static pressure for bubbles, small bubbles, and structured waves.

CFD investigation of structural effects of internal gas intake on powder conveying performance in fuel supply systems for aerospace engines

Optimal design of gas intake in powder fuel supply systems is crucial for performance of aerospace engines. There is little research on the impact of intake structure on powder conveying performance. Three novel internal intakes were proposed, which are spherical, cube-shaped, and dome-shaped. After validation, CFD simulations demonstrate that fluctuation of mass flow rate of powders in the dome-shaped intake is reduced by about 73.3% compared with the annular external one. Variation trends of phase velocities are similar for the spherical and cube-shaped intakes, while those are similar for the annular external and dome-shaped internal intakes. Fluctuation of area of gas zone for the annular external and spherical internal intakes is larger than that for the cube-shaped and dome-shaped internal intakes. Pressure and relative pressure drop in the fluidization chamber have a stable stage, and fluctuation of relative pressure drop is small when dome-shaped internal intake is used.

Flow pattern identification for internally threaded tubes based on pressure drop fluctuations

Internally threaded tubes, renowned for their exceptional heat transfer capabilities, are extensively used in heat exchangers. However, directly identifying two-phase flow patterns within internally threaded tubes poses a formidable challenge. To address this, this study conducted two-phase flow experiments with R600a within an internally threaded tube, encompassing a range of mass fluxes spanning from 40 to 130 kg m−2 s−1. The experimental observations unveiled three distinct flow patterns: plug, slug, and annular flow. To validate the accuracy of these observed flow patterns, they were compared with an empirical flow pattern map. Meanwhile, pressure drop signals corresponding to various flow patterns were recorded under different experimental conditions using a high-frequency data acquisition card. The experimental findings indicated that the pressure drop signals fluctuate because of the difference in gas-liquid phase compressibility, while the fluctuation period and flow pattern change simultaneously with the flow rate. Specifically, plug flow exhibited the longest pressure drop fluctuation periods and the sharpest probability density function (PDF) curve. In contrast, annular flow exhibited the shortest fluctuation periods and the least sharp PDF curve. Based on the distinctive characteristics of the PDF curve, a novel parameter, K, was introduced primarily derived from the sharpness of the curve. This led to the formulation of innovative transition criteria, which achieved a prediction accuracy of 86.99% across all datasets in this study. This work furnishes a straightforward methodology for distinguishing flow patterns. The advantages of the proposed criteria include the requirement of measuring only pressure drop fluctuations to identify flow patterns without direct observation.

Methane foam performance evaluation in fractured oil-wet carbonate systems at elevated pressure and temperature conditions

This study investigates the potential of foam flooding for enhanced oil recovery in oil-wet fractured systems under reservoir conditions. Two surfactants with distinct ionic properties were employed in combination with methane to generate foam via simultaneous injection into propped fractures, aiming to optimize the diversion of gas from fractures to the matrix for improved oil recovery. Core flooding experiments demonstrated the critical roles of total injection rate and foam quality in controlling foam performance. Increasing the total injection rate from 1 to 3 cm³/min enhanced foam generation rates and foam lamella stability, facilitating better interactions between fractures and the matrix. However, beyond an injection rate of 4 cm³/min, there was a decline in apparent viscosity and mobility reduction factor. Foam strength was directly proportional to higher foam quality within the 50 to 80% range, but beyond 85% foam quality, fluctuations in pressure drop signaled the formation of weaker, less stable foams. Micro-scale experiments validated these macroscopic findings, revealing that smaller bubbles were associated with higher flow rates and lower foam qualities. This research contributes valuable insights into both micro and macro-scale in-situ foam generation, which can inform the design and application of foam-based enhanced oil recovery methods in fractured reservoirs.

Subcooled flow boiling in ultrahigh-aspect-ratio microchannels for high heat flux cooling

In present work, the numerical simulation and experimental investigations were performed on the subcooled flow boiling behavior in ultrahigh-aspect-ratio microchannel heat sink (AR = 25). As the heat flux is low, the unique discotic bubble expands the condensation area while ensuring an adequate evaporation area in the channel. When the heat flux increases, the violently fluctuating liquid-vapor interface forms a throat shape that effectively reduces the cross-section of vapor flow. Furthermore, the flow pattern of annular flow at the channel bottom and nucleate bubble at the channel top realizes a combined heat transfer mechanism (nucleate boiling and liquid film evaporation occurs simultaneously). Moreover, the phenomenon of “droplet inside the bubble” caused by bubble coalescence also significantly enhances the thermal process. The fluctuation of local pressure drop in UARM acts gently, which effectively inhibits the boiling instability, and its average value can be reduced by 68.9%. The maximum CHF reaches 1258.1 W/cm2 under a mass flux of 410.2 kg/(m2·s) based on the experimental data, indicating a tremendous potential for the efficient heat dissipation.

A novel step-gap design on the top-cover of the microchannel cold plate to improve the flow boiling stability and heat transfer performance

The current study investigates the impact of incorporating the step gap in the cover above the microchannel heat sink subject to two-phase convective boiling. The microchannel heat sink size is 49 mm × 52 mm, with a channel width of 200 μm and a height of 3 mm, with a fin thickness of 200 μm. The effect of the step gap above the microchannels (SGAM) was compared at different heights, one 1 mm, which represents 1/3 fin height, and 3 mm, which means the same fin height with the same sample with no step gap above the microchannels (NSGAM). The working fluid used dielectric fluid HFE-7000 at the mass flux ranges from 30 to 260 kg/m2·s, while the heat flux ranges from 50 to 156 W/cm2. Providing a step gap above the microchannels in the cover reduces pressure drop significantly, around 37 ∼ 48% lower than the NSGAM. The experimental results demonstrate that the SGAM configuration reduces wall superheat temperatures by approximately 4 ∼ 6 °C compared to the NSGAM configuration. Flow visualizations indicate that the SGAM design avoids flow reversal, where the pressure drop fluctuations are significantly reduced by more than three times. The experimental outcomes for excessive temperatures and total thermal resistance exhibit a high level of concordance with the analytical results, with a deviation being less than 11%. The spreading resistance comprises the major portion of the total resistance, accounting for approximately 52∼58%. The SGAM samples effectively reduce local wall temperature downstream for step gap heights 1 mm, and 3 mm are 4 ∼ 6 °C, and 2 ∼ 4 °C, respectively. Overall, these findings demonstrate the potential of the step gap above microchannels to improve flow boiling stability and heat transfer performance appreciably while offering a much lower pressure drop penalty.

A numerical investigation of the thermal–hydraulic performance during subcooled flow boiling in MMCs with different manifolds

Thermal management of high-heat-flux electronics has been a critical bottleneck for integrated circuit miniaturization and increased power density, and manifold microchannel (MMC) heat sinks are expected to be an efficient near-junction thermal solution. A numerical investigation based a self-developed solver has been conducted to explore the effects of manifold geometries on the thermal–hydraulic performance of MMC heat sinks during subcooled flow boiling, and phase, pressure, and mass distribution among microchannels are analyzed combined with temperature distribution. Heat flux makes little difference on the mass maldistribution among microchannels of MMCs regardless of single-phase and flow boiling, and flow patterns in the outlet manifolds of Z-type MMCs changes from bubbly flow, slug flow to vapor columns with segmented bubbles with increasing heat fluxes. As for the effect of manifold numbers, both the maximum temperature and the pressure drop decrease with increasing manifold numbers, and a relatively uniform distribution of mass and void fraction has been found for all cases of HU-type MMCs regardless of heat fluxes with the best thermal–hydraulic performance at N = 10. However, the thermal performance of Z-type MMCs would improve firstly and then flatten out with increasing manifold number, while the pressure drop presents an opposite trend. The differences in thermal–hydraulic performance are attributed to the longer flow path in Z-type manifolds, and the increasing manifold number presents a higher manifold pressure drop and thus would worsen the mass maldistribution. As for tapered manifolds, a tapered ratio of 0.33 could provide a slightly lower base temperature than the original case with a pressure drop decreased by over 25 %, and the pressure drop fluctuation at high heat fluxes could be suppressed by 70 % with significantly improved flow boiling stability. This work can serve as a basis for the further optimization of subcooled flow boiling in MMC heat sinks, and the results indicate that both an even mass distribution among microchannels and a desirable inlet/outlet port ratio for improved flow field are of great importance for the manifold design applied in MMC heat sinks for two-phase heat dissipation.

Heat transfer distribution and pressure drop fluctuations in subcooled flow boiling at subatmospheric system pressure

The transfer of higher heat flux at subatmospheric system pressure is relevant to different applications where the higher surface temperature is a constraint. There is a scarcity of literature on subcooled flow boiling at subatmospheric system pressure. The experiments are performed at 50, 75 and 101 kPa absolute system pressure in an SS-304 tube of 5.85 mm radius, heated length of 1500 mm and 0.5 mm thickness. The mass flux of 90–300 kg/m2s is streamed at the varying heat flux of 48–218 kW/m2. The heat transfer coefficient in single-phase flow is not impacted significantly by the subatmospheric system pressure. The subcooled flow boiling heat transfer coefficient is increased at the lower subatmospheric system pressure. The more nucleation sites due to higher wall superheat are activated at the higher heat flux at subatmospheric system pressure. The surface temperature is radially symmetric at subatmospheric system pressure for all values of the Froude number. The gravitational force is suppressed at subatmospheric system pressure. The onset of nucleation is achieved earlier in the axial direction at the higher heat flux. The mass flux has no significant effect on the subcooled flow boiling heat transfer coefficient. The pressure drop and fluctuations are enhanced at the higher mass flux, higher heat flux and lower subatmospheric system pressure.

Study on the effect of ocean rolling motion on the thermal–hydraulic performance of supercritical carbon dioxide in horizontal tube

The effect of ocean rolling motion on the flow and heat transfer characteristics of the S-CO2 has been numerically investigated in this article. The results showed that increasing the rolling period makes the influence of rolling motion tend to be gentle. Increasing the maximum rolling angle and the rolling radius will increase the centrifugal and tangential forces on the fluid inside the tube, further making the swing motion of the flow field become more intense. Thus, this directly results in the tube wall temperature oscillation of 10 K to 50 K as well as the total pressure drop fluctuation of about 6%. In addition, the effect of three spatial positions relative to the rolling axis is also investigated. This research work can provide a theoretical reference for the development of marine equipment.

Experimental study and prediction on the thermal management performance of SDS aqueous solution based microchannel flow boiling system

This study reports the flow boiling of SDS aqueous solution in straight and tree-shaped microchannel for thermal management. Under different volume flow rate and heat flux, the effect of SDS on the flow pattern, heat transfer and pressure drop in different channel structures is clarified. The results reflect that unique flow patterns like bubble stacking, bubble cluster and activated bubbles are observed in the flow boiling of SDS aqueous solution. 400 mg/kg and 200 mg/kg SDS aqueous solution has the best overall heat transfer enhancement performance in straight (25% improvement) and tree-shaped (22% improvement) microchannels respectively. The heat transfer enhancing mechanism of SDS can be summarized as suppressing the formation of large volume vapor slug and promote the rewetting of the bottom channel surface. Based on this mechanism, SDS can also advance the ONB and extend the stable nucleate boiling state, 100 mg/kg, and 400 mg/kg SDS aqueous solution show longest stable nucleate boiling state in straight and tree-shaped microchannel respectively. Increasing solute concentration of SDS leads to larger pressure drop and stronger pressure drop fluctuation in both microchannels, but increasing volume flow rate will suppress this extra resistance and instability caused by SDS. Targeted heat transfer and pressure drop correlations are also presented.

Channel flow boiling on hybrid wettability surface with lattice Boltzmann method

Channel flow boiling on the hybrid wettability surface is studied numerically using a two-dimensional pseudo potential lattice Boltzmann method (LBM) coupled with a thermal phase transition model. Heat transfer performance between single and hybrid wettability surfaces is compared first, and it is shown that the heat flux on the hybrid surface is higher than that of its single wettability competitor. After analyzing 36 groups of numerical results, it is found that the impacts brought by contact angle and geometrical parameters of the hybrid surface on heat flux is complex. By fixing the hydrophilic contact angle, the heat flux increases first but then decreases with the increase of hydrophobic contact angle. However, the heat flux decreases monotonously with increasing the hydrophilic contact angle under a fixed hydrophobic contact angle. The 30°/120° hybrid wettability surface shows the best heat transfer performance due to its better three-phase contact line spreading and accelerated bubble departure frequency. Aside from the contact angle, the flow boiling heat transfer performance dominated by bubble dynamics is also closely related to the widths of hydrophilic/hydrophobic regions, and there exist optimal widths under various wall superheats. Finally, the pressure drop inside channel is investigated, and it is shown that the hybrid surface is favorable to reduce pressure drop fluctuation on the premise of providing high vapor yield.

Mechanistic model to predict oscillating frequency of flow boiling in large length to diameter ratio micro-channel heat sinks

This study presents a new mechanistic model for density wave oscillation (DWO) of flow boiling in horizontal micro-channel heat sinks that enables prediction of oscillation frequency, utilizing both flow visualization results and transient experimental data. The proposed model is based on the hypothesis that the period of oscillation consists of two sub-periods, flow surge and reversal, and establishes a set of 1D transient conservation equations. The flow surge period is associated with liquid rapidly entering the channels, momentarily establishing high pressure gradient between inlet and outlet plenums before the incoming flow is decelerated by a combination of increased flow resistance and bubble expansion within the channels. The initial location of the bubble nucleation is estimated using a criterion for onset of nucleate boiling (ONB) that governs bubble formation from cavities in superheated near-wall liquid. The flow reversal period is associated with back flow into the inlet and estimated by time elapsed before the reversed flow returns to its original location where bubble expansion and onset of annular flow are initiated. Unlike prior models, the proposed model simulates a full cycle of flow oscillation. In this model, a new mass conservation criterion is introduced to ensure agreement between predicted and experimental mass outflows during one cycle. Also developed is a detailed spatial and temporal description of the high density wave front (HDWF) largely responsible for both pressure drop and mass velocity fluctuations during the oscillation period. Finally, a periodic flow boundary condition is imposed on flow velocities and locations to achieve same flow behavior at the start and end of cycle. The proposed model shows good agreement with measured oscillation frequencies.

Automated detection of loading and flooding points in packed columns

Predictive correlations for packed distillation and absorption columns usually cannot account for changes in operating limits due to phenomena such as fouling. A method for detection of loading and flooding points even under altered operating conditions would therefore be highly desirable. In this work, a new approach for real-time detection of packed column loading and flooding points is introduced. The method utilises Fourier analysis of pressure drop data. Pressure drop fluctuations occurring even under steady-state conditions are sampled and treated with Fast Fourier transform (FFT) to derive the corresponding frequency spectra. Results of examinations with three structured packings (RSP 250Y, RMP N 250X, RMP N 250Y) conducted at three liquid loads each show that operating regions (below loading point, loading point, loading region, flooding point) can be identified from the obtained frequency spectra. Starting at the loading point, certain amplitudes within the spectra grow and increase up to the flooding point. This behavior is consistent for all examined packings. The results from this study suggest that the proposed approach could support column operation as an additional monitoring tool that is low in cost and not reliant on additional measurements besides pressure drop.

Experimental and Numerical Study of Cross-Domain Hydrodynamics in an Integrated Pneumatic-Spouted Bed

As an essential resource, the drying, upgrading, and efficient utilization of lignite are of great significance to the development of the low-carbon economy. In this work, a novel drying system, combining a pneumatic and spouted bed, is designed to achieve lignite deeply dewatering. The system is investigated experimentally and numerical simulated to characterize the hydrodynamics of particles moving across the domain, such as pressure drop, fountain height, and pressure fluctuation. The results show that there is a critical gas velocity that changes the flow pattern of the bed when the static bed is higher than 50 mm. When the gas velocity increases to the critical value, the spouting state becomes unstable and the corresponding pressure drop will undergo a secondary mutation. This work provides a deeper understanding of the flow structure and particle motion in the novel drying system under various industrial operating conditions.

Investigation of the Microscopic Process of the Media Coalescence Treatment of Water-in-Oil Emulsion.

Medium coalescence technology is a research hotspot for the separation of oil-in-water emulsions. However, the coalescence mechanism is still unclear, making it challenging to effectively improve the separation performance. Herein, the microscopic mechanism of medium coalescence was revealed. We found that the effective collision positions under the action of the flow field include the exposed granule surface, adherent droplet surface, and three-phase contact line. Furthermore, a numerical model of the microscopic process of water-in-oil emulsion permeation through a granular bed was established. The effects of different parameters (including the number of medium layers, Reynolds number, and inlet concentration) on the microscopic process of capturing dispersed-phase droplets in the bed and the pressure drop in the coalescence area were studied. The numerical results show that the droplets form the bridging structure between the granules. On the one hand, the bridging structure promotes the capture of the droplets by the bed; on the other hand, it causes pressure-drop fluctuations in the coalescence area and asymmetric distribution of the velocity field.