Two-phase flow instability, inherently accompanied by hydraulic and thermal oscillations, is a complex phenomenon that must be overcome to design an efficient flow boiling heat transfer system. The potential factors influencing two-phase flow instability are so diverse and complex that many efforts have been devoted to identifying and classifying flow instabilities. Density wave oscillation and pressure drop oscillation in a single channel represent classical flow instabilities, and their mechanisms are relatively well established. However, parallel channel instability observed in multi-channel heat sinks is challenging to analyze due to flow interaction between neighboring channels through the inlet/outlet plenum. Even the number of channels, the size and shape of the plenum, and the inlet/outlet arrangement influence the fluid flow in multi-channel heat sinks, complicating the analysis of previous experimental data from the literature. Therefore, to address the complexity of multi-channel flow boiling and enhance the understanding of its behavior under specific conditions, additional experimental studies are essential. This study conducts flow boiling experiments using FC-72 with a mini-channel heat sink consisting of 22 parallel channels, each with dimensions of 1.6 mm in width and 4.8 mm in height, operating under two distinct pressure conditions. Flow instability occurring within the present channel flow boiling is investigated by visualizing the flow in the channel upstream and inlet plenum, as well as through spectral analysis of the pressure fluctuation. The fast Fourier transform was used to characterize the oscillation characteristics of transient pressure measured at various locations in the test loop. In addition, visualization data and fast Fourier transform results identified flow oscillation caused by vapor backflow in the channel. As a result, the present flow boiling data are classified as stable or unstable based on the frequency of pressure oscillation, and a new stability map for mini-channel flow boiling is proposed.

Improving the performance of a refrigerated container is crucial in effectively preserving thermal -sensitive cargo. The combined effect of vertical airflow resistance and airflow short-circuiting in air gaps play a vital role in cargo cooling. This study numerically investigates the effects of cargo vertical airflow resistance and air gaps between cargo pallets using two commonly used apple packaging boxes, each stacked in six different cargo pallet arrangements. A heat transfer enhanced refrigerated container design and a numerical model developed from the present authors ' previous studies are used. The numerical model uses the porous medium model to incorporate the refrigeration unit and cargo within the refrigerated container. Additionally, the numerical domain includes the T -bar floor in the refrigerated container. Initially, the airflow characteristics based on vertical airflow resistance and air gaps are discussed. Then, the corresponding effects on cargo cooling are addressed. Results show that the air gaps between the cargo pallets have positive and negative effects on cargo cooling. In cargos with lower vertical airflow resistance, an increase in air gaps leads to reduced cargo cooling and elevated temperature non -uniformity. Conversely, cargos with higher vertical airflow resistance experience increased cooling rates and reduced temperature non -uniformity with the increase in air gaps. However, avoiding airflow short-circuiting through the air gaps between the cargo pallets and improving the vertical airflow inside the cargo pallets can provide better cargo cooling, which outweighs the positive cooling effect of the air gap.

Improving the performance of a refrigerated container is crucial in effectively preserving thermal-sensitive cargo. The combined effect of vertical airflow resistance and airflow short-circuiting in air gaps play a vital role in cargo cooling. This study numerically investigates the effects of cargo vertical airflow resistance and air gaps between cargo pallets using two commonly used apple packaging boxes, each stacked in six different cargo pallet arrangements. A heat transfer enhanced refrigerated container design and a numerical model developed from the present authors’ previous studies are used. The numerical model uses the porous medium model to incorporate the refrigeration unit and cargo within the refrigerated container. Additionally, the numerical domain includes the T-bar floor in the refrigerated container. Initially, the airflow characteristics based on vertical airflow resistance and air gaps are discussed. Then, the corresponding effects on cargo cooling are addressed. Results show that the air gaps between the cargo pallets have positive and negative effects on cargo cooling. In cargos with lower vertical airflow resistance, an increase in air gaps leads to reduced cargo cooling and elevated temperature non-uniformity. Conversely, cargos with higher vertical airflow resistance experience increased cooling rates and reduced temperature non-uniformity with the increase in air gaps. However, avoiding airflow short-circuiting through the air gaps between the cargo pallets and improving the vertical airflow inside the cargo pallets can provide better cargo cooling, which outweighs the positive cooling effect of the air gap.

Improving the performance of a refrigerated container is crucial in effectively preserving thermal-sensitive cargo. The combined effect of vertical airflow resistance and airflow short-circuiting in air gaps play a vital role in cargo cooling. This study numerically investigates the effects of cargo vertical airflow resistance and air gaps between cargo pallets using two commonly used apple packaging boxes, each stacked in six different cargo pallet arrangements. A heat transfer enhanced refrigerated container design and a numerical model developed from the present authors' previous studies are used. The numerical model uses the porous medium model to incorporate the refrigeration unit and cargo within the refrigerated container. Additionally, the numerical domain includes the T-bar floor in the refrigerated container. Initially, the airflow characteristics based on vertical airflow resistance and air gaps are discussed. Then, the corresponding effects on cargo cooling are addressed. Results show that the air gaps between the cargo pallets have positive and negative effects on cargo cooling. In cargos with lower vertical airflow resistance, an increase in air gaps leads to reduced cargo cooling and elevated temperature non-uniformity. Conversely, cargos with higher vertical airflow resistance experience increased cooling rates and reduced temperature non-uniformity with the increase in air gaps. However, avoiding airflow short-circuiting through the air gaps between the cargo pallets and improving the vertical airflow inside the cargo pallets can provide better cargo cooling, which outweighs the positive cooling effect of the air gap.

This study experimentally explores the subcooled flow boiling pressure drop of FC-72 in a multiple parallel rectangular micro-channel heat sink with a channel hydraulic diameter of 666 µm (width 400 µm and height 2000 µm). Detailed experimental procedures and data reduction methodology are presented. The study analyzes and reports the impact of inlet subcooling on the total pressure drop during subcooled flow boiling of FC-72 in rectangular micro-channels heat sink. Additionally, a consolidated subcooled flow boiling pressure drop database is created by incorporating the additional data points assimilated from the study of Lee and Mudawar (2008), which used HFE-7100 as the coolant fluid. The consolidated database is methodically reviewed and evaluated using the seminal pressure drop correlations described in the literature for pressure drop during subcooled flow boiling. For the micro-channels heat sink adopted in this study, existing subcooled flow boiling pressure drop correlations fail to predict the consolidated database. Based on the consolidated pressure drop data points, a new subcooled flow boiling pressure drop correlation for the rectangular micro-channels heat sink is suggested. The new pressure drop correlation well predicts the consolidated subcooled flow boiling pressure drop database with a mean absolute error of 16.6% and 86.3% of data points predicted inside ±30%.

A large number of studies have been conducted to understand the slug flow characteristics of air-liquid two-phase flow in horizontal pipes and to capture parametric trends of critical flow parameters associ-ated with slug flow, such as the liquid slug length, elongated bubble length, and elongated bubble veloc-ity. Based on these findings, many attempts have been made to develop empirical correlations to predict the slug flow parameters. However, the validity of these correlations is limited to a specific working fluid and narrow ranges of geometrical or operating conditions, thereby reducing their effectiveness as useful general predictive tools. This limitation has created a need to develop empirical correlations that offer high accuracy over a broad application range, i.e., a wide range of liquid viscosities, superficial liquid and gas velocities, and pipe diameters. In this study, two-phase slug flow experiments were performed to ex-amine the hydrodynamic features of slug flow under two different pipe diameters (2 and 4 cm) and two different liquid tem peratures (25 and 45 degrees C). In addition, a new consolidated database consisting of 1563 slug flow data points was amassed from 16 sources, including the present experimental data, covering a wide range of operating conditions and fluid properties. In particular, this study includes various liquids with a wide range of viscosities, such as heavy oil, light oil, and water. The parametric study of air-liquid slug flow at various viscosity levels revealed that as the mixture velocity increases, the liquid slug length tends to decrease when the liquid viscosity is high and increase when the liquid viscosity is low. On the other hand, the elongated bubble length and elongated bubble velocity increase with increasing superfi-cial gas velocity, regardless of the liquid viscosity. Based on these findings, three empirical correlations were developed to predict the liquid slug length, elongated bubble length, and elongated bubble velocity. These correlations provide excellent predictive capability against the consolidated database, with overall MAE values of 18.1%, 20.6%, and 9%, respectively.(c) 2023 Elsevier Ltd. All rights reserved.

An experimental pool boiling study on multistage cross-flow porous structure was conducted under at-mospheric pressure using saturated FC-72 as the working fluid. The novel design comprises two stages: each stage contains cross-flow rectangular channels of 1.5 mm in height and 1 mm wide connected by thorough circular pores 0.325 mm in diameter. The dimensions of the pores and channels in both stages were kept identical. Tests were conducted in three positions, enhanced surface horizontal position ( EH ), enhanced surface vertical position (EV -a ), where upper stage channels were parallel to the buoyancy force, and enhanced surface vertical position (EV -b ), where lower stage channels were parallel to the buoyancy force. The critical heat flux was significantly enhanced with an improved boiling heat trans-fer rate in all three positions using the multistage cross-flow porous structure. The highest heat flux of 107 W/cm2 was recorded in EV-b, followed by EV-a and EH with a wall superheat of 42 degrees C. The heat trans-fer coefficient was improved by 300% to 700%, and the critical heat flux was enhanced by 6.5 times com-pared to the plain surface. Improved performance is attributed to the cross-flow of multistage porous structure and orientation of the test section, which induced a breathing phenomenon and prevented the drying out of the test section by resisting the merging of large vapor bubbles for a longer period and decreasing its lingering time over the surface. Bubble behaviors were investigated utilizing a high-speed camera. Two separate modes of the boiling mechanism were observed. At low heat fluxes below 50 W/cm2, bubble discharge and liquid intake co-occurred at random pores and channels, whereas at high heat fluxes, volatile bubbles exhibited a breathing phenomenon.(c) 2023 Elsevier Ltd. All rights reserved.

High heat flux and good axial temperature uniformity of channel flow boiling demonstrate that it is ideally suited for next-generation cooling systems. Such systems, however, are hampered by undesirable flow fluctuations, making channel flow boiling ineffective due to the rapid bubble growth within the channels. Furthermore, especially in the mini-channel heat sink, the interactions between multiple par-allel channels and inlet/outlet plenum induce complex hydraulic and thermal oscillations. Therefore, lots of consistent and detailed experimental data are still needed to design a reliable flow boiling system. However, only limited studies have examined the effects of channel interaction on flow boiling insta-bility. Thus, to provide a better understanding of the flow boiling instabilities, a mini-channel heat sink was tested with near-saturated inlet conditions using FC-72 as the working fluid. The mini-channel heat sink consisted of a 150 mm long and 70.4 mm wide base area, having 22 identical rectangular channels measuring 1.6 x 4.8 mm2. The flow instabilities were analyzed by observing the vapor backflow and flow fluctuation in mini-channels with variations in the mass velocity, pressure, and heat fluxes. The analy-sis is based on the detailed flow visualization in the parallel channels and inlet plenum. It was found that the rapid bubble generation in individual channels promotes vapor backflow and hydraulic oscilla-tions. Further observation of the high-speed camera images shows that the vapor crossing over between parallel channels through the inlet plenum led to the synchronization of vapor oscillations, which is re-sponsible for the parallel channel flow instability. This visualization study revealed six dominant flow patterns showing multi-channel flow instability. Based on the classification of dominant flow patterns, a better understanding of the operating conditions for stable flow boiling was obtained.(c) 2023 Elsevier Ltd. All rights reserved.

It is crucial to design multi-functional structure to improve heat transfer characteristics. Here we report significantly enhanced saturated flow boiling heat transfer of water using the two-tier vertically aligned multiwalled carbon nanotube (VAMWNT) channel (tube diameter: 11.5 nm). The channel size is 10 x 10 x 2 mm3 (height: 2 mm). The tube aggregation-generated large voids act as micropores (10200 & mu;m) whereas the interstitial spaces between the tubes act as nanopores (< 72 nm). The microcavities at the periphery of aggregated nanotubes increase bubble nucleation sites while constraining bubble size. The high nanotube thermal conductivity induces more uniform bubble nucleation, and departed bubbles are swept away through the micropores. On the other hand, water is effectively replenished through the nanopores by high capillary pumping. Overall, the path separation of bubble departure and water replenish effectively prevents dryout and increases critical heat flux (CHF). The experiment is carried out at 3 different mass velocities (62, 83, and 104 kg m -2 s -1). The VAMWNT channel provides a high heat transfer coefficient ( h = 100,666 W m -2 K -1 at 183 W cm -2), even before reaching CHF without any sign of local dryout, compared with microchannels and porous media in literature (under the similar heat flux and mass velocity). In contrast, the CHF condition is observed for a plain channel of the same size without the nanotubes, resulting in smaller h values than those of the VAMWNT channel. The novel two-tier structure separated liquid-vapor pathways, significantly enhancing heat transfer characteristics.& COPY; 2023 Published by Elsevier Ltd.

This study examines two-phase flow and pressure drop characteristics associated with slug flow in a tube with an inner diameter of 4 cm. Experiments are conducted using air and low viscosity mineral oil as working fluids at atmospheric pressure and two different oil temperatures, 25 and 45 degrees C. The superficial gas and liquid velocities are in the range of 0.33-2.43 m/s and 0.20-1.20 m/s, respectively. Flow visualization images and temporal records of pressure drop oscillation are presented for different gas and liquid superficial velocities. The relevant flow parameters associated with slug flow, such as the elongated bubble velocity, elongated bubble length, liquid slug length, and liquid film thickness, are measured, and the relationship between the slug flow parameters and the superficial gas and liquid velocities is investigated. The measured pressure gradient data are also compared to predictions of previous homogeneous equilibrium models and semi-empirical correlations for two-phase frictional pressure gradient. A theoretical slug flow model for predicting the total pressure gradient of slug unit is presented using empirical correlations for the slug flow parameters. The new model accurately predicts the experimental pressure gradient data, evidenced by an overall mean absolute error of 19.3%. (c) 2021 Elsevier Ltd. All rights reserved.

This study examines the air-oil two-phase flow regime and frictional pressure drop in a horizontal circular pipe with a diameter of 40 mm. The pressure drop fluctuation characteristics for different air and oil superficial velocities are investigated. Observed flow regimes and frictional pressure drop data are compared with previous flow regime maps and frictional pressure drop correlations, respectively.

This study provides a new mechanistic model for air–oil plug flow for predicting two-phase pressure drop in horizontal circular pipes. Experiments were conducted with a two-phase air–oil mixture in a horizontal circular pipe with an inner diameter of 40 mm by varying the air and oil superficial velocities within ranges of 0.04–0.27 m/s and 0.20–0.94 m/s, respectively. Closer empirical correlations were developed for the bubble body velocity, bubble body length, bubble tail length, and liquid film thickness based on the flow visualization data. The new model shows good agreement with experimental pressure drop data, evidenced by an overall mean absolute error value of 5.66%.

Compared to a single-phase cooling system, which uses the sensible heat of the coolant, a pool boiling cooling system can achieve a greater heat transfer coefficient by capitalizing on the latent heat of vaporization of the coolant. Nanofluids are known to provide the added benefit of an increase in the heat transfer coefficient due to their high thermal conductivity. In order to simulate pool boiling with water-based nanofluids, a computational model is constructed using the volume of fluid (VOF) model in the FLUENT computational fluid dynamics code. The simulations are performed with saturated pure water and water-Al2O3 nanofluids using a two-dimensional computational domain with a width of 5 cm and a height of 20 cm. An isothermal condition is applied to a vertical surface with a heated length of 2 cm which is exposed to the working fluids, the surface temperature of which is 5 K, 10 K, 15 K and 20 K higher than the initial temperature of the working fluids. The numerical results are verified by comparing the experimental volume fraction data in the pool boiling. The fluid flow and heat transfer characteristics of pool boiling are examined for different wall superheats and nanoparticle concentrations.