Flow In Minichannels Research Articles

Experimental study on condensing flow and heat transfer characteristics in minichannels under mechanical vibration conditions

Minichannel heat exchangers are often used in the condensers of vehicle air conditioners, and vibration is inevitable in vehicle driving. However, the influence of mechanical vibration on the characteristics of minichannel condensing flow and heat transfer has been rarely studied. Therefore, the experimental study of condensing flow and heat transfer in minichannels under mechanical vibration was carried out in this paper. With R134a as the working medium, the condensation heat transfer coefficient, frictional pressure drop and flow pattern images were measured in the saturation temperature of 40 °C, mass flux density of 200 kg/(m2·s), vapor quality range of 0–1, vibration frequency of 15–50 Hz and amplitude of 0–2.4 mm. For annular flow, intermittent flow and bubble flow, the vibration can enhance the wave of gas-liquid interface, and promote the breakup of long bubbles and the polymerization of small bubbles. Vibration enhances heat transfer in most cases, but weakens heat transfer in some specific cases. The frictional pressure drops under vibration conditions are greater than those under static conditions, which indicates that the influence mechanism of vibration on heat transfer and pressure drop is different. This paper attempts to analyze the complex mechanism of vibration affecting minichannel condensing heat transfer and pressure drop, which provides theoretical support for further study of condenser optimization design under vibration conditions.

Thermal-hydraulic performance of R1234yf in brazed plate heat exchanger at low saturation temperature and mass flux conditions: Experimental investigation

This study investigated the heat transfer coefficient (hr) and two-phase frictional pressure drop during the evaporation of R1234yf within an offset strip fin (OSF)-structured brazed plate heat exchanger (BPHE). The work is conducted at a saturation temperature (Ts) of −3 to 3 °C, with mass flux (G) ranging from 25 to 45 kg m−2 s−1 and heat flux (q) between 15 and 23 kW m−2 across a vapor quality (x) range of 0.1–0.8. Understanding these behaviors is critical for optimizing BPHEs in low-temperature applications, such as heat pumps, where energy efficiency is essential. The novelty of this work lies in its exploration of operating conditions that are less studied in the literature, particularly the impact of low Ts, low G, and high q on hr and two-phase frictional pressure drop in an OSF-BPHE with R1234yf. Unlike prior studies focusing on convection-dominated boiling, our findings reveal a coexistence of nucleate and convective boiling mechanisms, especially at different vapor qualities. The hr is significantly influenced by q at x < 0.6, G at x > 0.6, and Ts across all x. In contrast, evidence of dryout at high x highlights the importance of these parameters in managing heat transfer efficiency. The results indicate a strong correlation between the two-phase frictional pressure drop and G, with a marked increase in it at higher G. Besides, this study reveals flow characteristics commonly linked with both macrochannel and minichannel flows, enhancing novelty by bridging the gap between these two regimes. These findings challenge existing literature correlations that predominantly emphasize convection-dominated behavior and contribute to a better understanding of the dual boiling mechanisms in BPHEs.

Experimental investigation of flow boiling instability and associated pressure fluctuations in a mini-channel heat sink

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.

Application of roughness models to stationary and rotating minichannel flows

PurposeThe performance of a bladeless Tesla turbine is closely tied to momentum diffusion, kinetic energy transfer and wall shear stress generation on its rotating disks. The surface roughness adds complexity of flow analysis in such a domain. This paper aims to assess the effect of roughness on flow structures and the application of roughness models in flow cross sections with submillimeter height, including both stationary and rotating walls.Design/methodology/approachThis research starts with the examination of flow over a rough flat plate, and then proceeds to study flow within minichannels, evaluating the effect of roughness on flow characteristics. An in-house test stand validates the numerical solutions of minichannel. Finally, flow through the minichannel with corotating walls was analyzed. The k-ω SST turbulent model and Aupoix's roughness method are used for numerical simulations.FindingsThe findings emphasize the necessity of considering the constricted dimensions of the flow cross section, thereby improving the alignment of derived results with theoretical estimations. Moreover, this study explores the effects of roughness on flow characteristics within the minichannel with stationary and rotating walls, offering valuable insights into this intricate phenomenon, and depicts the appropriate performance of chosen roughness model in studied cases.Originality/valueThe originality of this investigation is the assessment and validation of flow characteristics inside minichannel with stationary and corotating walls when the roughness is implemented. This phenomenon, along with the effect of roughness on the transportation of kinetic energy to the rough surface of a minichannel in an in-house test setup, is assessed.

Extended analytical model of Tesla turbine with advanced modelling of velocity profile in minichannel between corotating disks with consideration of surface roughness

Consideration of roughness effects in the flow in a minichannel has been a major scientific problem for many decades. Roughness can alter the momentum diffusion, heat transfer conditions or laminar-turbulent transition. These effects are even more pronounced in minichannel flows where the roughness can occupy a large part of the cross-section. Modelling methods applied so far usually fall short in internal flows with high relative roughness. The presented paper pertains to the analytical flow model in Tesla turbine components with consideration of roughness on the rotor walls. The model uses the equivalent sand grain approach to modify dynamic viscosity in the governing equations to achieve a desired downward shift of the dimensionless velocity profile. The model solves two-dimensional equations of continuity, momentum and energy. The semi-empirical function was derived to consider the change in the shape of the radial velocity. The applied model incorporates Euler's turbomachinery equation to determine the influence of roughness on the turbine performance under varied operating conditions. Roughness shortens the streamlines inside the rotor, but the overall turbine's performance is improved. The roughness equal to 10−5 m increased the power generated and isentropic efficiency by factors of 3 and 2.5, respectively, compared to the smooth rotor.

Measurements of diffusion coefficient and kinetic diameter of acetone vapor via molecular tagging

The Molecular Tagging (MT) technique is a promising methodology for locally measuring velocity and temperature fields in rarefied gas flows. Recently, Molecular Tagging Velocimetry (MTV) has been successfully applied to gas flows in mini-channels in the continuum regime at high pressure and early slip-flow regime at lower pressure. As the operating pressure decreases, diffusion effects become more pronounced, and in MTV, they hinder the extraction of the correct velocity profile by simply dividing the displacement profile of the tagged molecular line by time of flight. To address this issue, a reconstruction method that considers Taylor dispersion was previously developed to extract the velocity profile, considering the diffusion effects of the tracer molecules within the carrier gas. This reconstruction method successfully extracted the correct velocity profile in the continuum flow regime. However, the method still faces challenges in the slip-flow regime. Since there is currently no consensus in the literature regarding the kinetic diameter value of acetone vapor, the diffusion coefficient estimation is uncertain especially at low pressures. This is why, in this study, we propose an original optical method to measure the diffusion coefficient of acetone vapor. This is achieved by linking the temporal evolution of the spatial photoluminescence distribution of acetone vapor to the diffusion coefficient via the Chapman-Enskog theory. Our research provides measurements of these parameters for a wide range of pressures (0.5–10 kPa) at ambient temperature.

Compound effect of EHD and nanofluid on flow boiling characteristics in minichannels

In this study, electrohydrodynamic (EHD) and nanofluid were synergistically employed to improve minichannel flow boiling. R141b, TiO2/R141b and TiO2/Span80/R141b nanofluids are employed as working fluids. Flow boiling characteristics in minichannels under electric field were evaluated experimentally. Laser confocal microscope was used to examine the effect of EHD and surfactant on nanofluid stability, particularly nanoparticle deposition on heated surfaces. Electric field can effectively enhance heat transfer, and the enhancement is more significant for nanofluids compared to pure fluid. Heat transfer performance is higher for TiO2/R141b than TiO2/Span80/R141b, while TiO2/Span80/R141b shows higher pressure drop than TiO2/R141b. The maximum enhancement ratio of 1.58 and 1.85 is found for TiO2/R141b and TiO2/Span80/R141b under electric field. Besides, heat transfer mechanism enhanced by EHD and nanofluid was discussed. Nanofluid stabilization induced by electric field and surfactant play crucial role in EHD enhancement for nanofluid. The findings suggest that TiO2/Span80/R141b nanofluid is well-suited for integration with EHD.

Experimental study of convective heat transfer in additive manufactured minichannels: the impact of the roughness and Prandtl number

Additive Manufacturing, especially Laser-based Powder Bed Fusion (L-PBF), can fabricate internal channels with enhanced cooling properties. Roughness is a natural consequence of the L-PBF process that can increase flow friction and also influence heat transfer in these cooling channels. While existing literature predominantly explores the impact of roughness on flow friction, less attention has been given to the effects on heat transfer. In this study, a novel experimental setup employing Joule heating was developed to investigate water flow in minichannels fabricated by L-PBF. The impact of roughness and different Prandtl numbers on flow friction and heat transfer was studied. The results indicated that the Nusselt number in rough channels scales with the Prandtl number to the power of 0.8 (Pr 0.8), suggesting greater heat transfer with higher Prandtl numbers for rough channels compared to those of smooth channels. At a specific combination of relative roughness and Reynolds number, the enhancement of heat transfer due to roughness is maximized.

Low reynolds number carbon-containing composite liquid fuel pipeline transportation under sub-ambient and subzero temperatures

An experimental study of the effect of lowered temperatures (from 25 °C to −5 °C) on the velocity and viscosity of composite liquid fuel (CLF) flow in mini-channels was performed at low Reynolds numbers. Examined CLFs are composed of water, coking coal, and sludge from coking coal processing, mixed with lignite polymer, acidic sodium pyrophosphate, isononylphenol and a mixture of mono-and dialkylphenols with ethylene oxide. The method of adjusting the critical ratio between the Reynolds numbers at the actual liquid flow rate of the local consumer and at the pump flow rate has been developed for CLF pipeline transport in off-season and winter temperature conditions. It is based on the constant regulation of the pump flow rate, depending on the current external temperature. Under a controlled decrease in the pump flow rate and a CLF temperature of −5 °C, its transportation by pipeline was successfully performed at laboratory conditions.

Numerical study and multi-optimization of heat transfer performance in counter flow minichannel heat sink with slots on ribs using NSGA-Ⅱ

High temperature and nonuniformity temperature cause thermal failure and thermal deformation for high-heat flux electronic devices. To address this issue, a counter flow minichannel heat sink with slots on ribs is designed. A 3D conjugated heat transfer model based on computational fluid dynamics (CFD) was developed to investigate effect of slots number (i) and growth rate of slot width (j) on average Nusselt number (Nu), local temperature distribution and temperature standard deviation (σ). The results show that design parameters i and j significantly influence heat transfer capability and temperature uniformity. However, there is no clear pattern between design parameters (i and j) and heat transfer performance (Nu and σ) to guide design. Furthermore, a non-dominated sequencing genetic algorithm (NSGA-Ⅱ) multi-objective optimization coupled Artificial Neural Network (ANN) is conducted with the consideration of maximum Nusselt number Nu and minimum temperature standard deviation (σ) as objectives. Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method is employed to determine the best solution from the Pareto front, i.e., multi-objective optimization solutions set. The optimized solution obtains 2.1 times increase in average Nusselt number and 0.36 times decrease in temperature standard deviation compared with conventional counter flow minichannel heat sink. This work serves as a guideline for designing minichannel heat sinks for thermal management of electronic components.

Effects of the position and perforation parameters of the delta winglet vortex generators on flow and heat transfer in minichannels

Minichannel with delta winglet vortices generator (VG) is widely used in heat exchangers. However, there is limited research in the literature on using VG with holes in opposite wall surfaces within a channel, and there are few reports on whether the use of VG with holes in minichannel will necessarily enhance heat transfer. In this work, we consider two different VG arrangements placed on opposite walls of a channel and three parameters for opening hole diameter (0.05, 0.15, and 0.25mm). Additionally, three different pitch values (15mm∼30mm∼45mm) are examined. Under a Reynolds (Re) number ranging from 3000 to 5500, the influence of VGs placement and hole parameters on the flow and heat transfer performance through calculations of parameters such as Nusselt number (Nu), friction coefficient (f), and thermal performance factor (TPF) are evaluated. Furthermore, the vortex structures are checked using the Q method and secondary vortex strength (Se). The results are shown to demonstrate that different VG placements mainly affect the main longitudinal vortices (MLVs), secondary longitudinal vortices (SLVs), and induced corner vortices (ICVs) structures downstream of the VGs. As the opening area increases, the shape of both vortices downstream of the VGs is significantly suppressed due to the generation of a jet flow, and the ICVs may even disappear (r=0.25mm). Case 1 (the right angle of the triangular wing is close to the center of the channel) has a larger TPF value. The opening of the hole reduces the heat transfer performance compared to the non-punched case, and their Nu number and f value are reduced by 0.2%−6.9% and 0.1%−8.3%, respectively. Case 1 obtained the maximum TPF value at Re=3000, which is 1.15, and P=15mm.

Contact temperature measurement using selected thermoelements for studies of heat transfer during fluid flow in minichannels – metrology investigations

Contact temperature measurement using selected thermoelements for studies of heat transfer during fluid flow in minichannels – metrology investigations

Analysis of hybrid active-passive prismatic Li-ion battery thermal management system using phase change materials with porous-filled mini-channels

The rapid expansion of electric vehicles (EVs) has created a significant demand for lithium-ion batteries (LIBs). However, ensuring safety and preventing thermal degradation of LIBs pose challenges to their overall performance and lifespan. Efficient battery thermal management systems (BTMS) are crucial in achieving long-lasting batteries with high energy density. Thermal management procedures are categorized into active and passive approaches. While active cooling methods are crucial, they consume power. Instead, passive cooling methods are not as influential in fulfilling all roles. This study examines a combination of active (mini-channel) and passive (PCM heat sink) techniques. The effects of employing a porous medium inside the mini-channel are also analyzed to enhance the heat exchange. Influences of different factors, including Darcy number, material, the thickness of porous media, and inlet velocity of mini-channel flow in multiple discharge rates (1C, 2C, 3C), are examined. Outcomes show the PCM heat sink itself can lessen the maximum temperature by about 4.2 to 5 K in different inlet velocities. The hybrid porous-filled channel and PCM heat sink can mitigate the maximum temperature by about 13 K in different inlet velocities and at the maximum discharge rate (3C) at the expense of about 14 % higher pumping power in different conditions.

Design and optimization of molten salt printed circuit steam generators

Among the array of heat exchangers available, printed circuit steam generators emerge as a particularly promising choice for high-temperature applications. This is primarily attributed to their augmented heat transfer surface area, which enables the efficient transfer of thermal energy within the Rankine power conversion cycle, especially in the context of molten salt-based thermal power systems, such as those employed in molten salt reactors and concentrated solar power plants. It is imperative to analytically model the heat transfer characteristics of mini-channel flow boilings for a printed circuit steam generator. To resolve new technical design issues that the single hot fluid flow channel is cooled by multiple cold fluid flow channels, an improved heat exchanger model is proposed to assist the numerical modeling of printed circuit steam generators for molten salt based on the equilibrium thermodynamics quality of water/steam flow. The existing flow boiling heat transfer coefficient correlations of mini-channels are leveraged to optimally design the straight-passage based printed circuit layouts and narrow down their optimal operational parameters. With various selections of subcooled water, saturated water, and superheated steam heat transfer correlations, The best combination of heat transfer correlations is found to assess the axial distributions of cold fluid temperatures and pressure drops. Two multi-objective optimizations are formulated to respectively solve thermal sizing and scaling problems for the optimal designs and operations of printed circuit steam generators. The computational results indicate that the overall heat transfer coefficient for an optimized 50 MW(th) printed circuit steam generator assessed at the transition point is about 3,828 W/(m2 K), which remarkably surpasses the 1,544 W/(m2 K) value of the conventional shell-tube steam generators by a factor of three. This implies that printed circuit steam generators would be a promising replacement of the traditional shell-tube steam generator.

Numerical Analysis of the Boiling Heat Transfer Coefficient in the Flow in Mini-Channels

Abstract This paper deals with boiling heat transfer in the flow of water through an asymmetrically heated horizontal rectangular mini-channel. The mini-channel was made by gluing three transparent glass plates and a copper block. Through the glass window, the variable along the length of the mini-channel two-phase flow structures were recorded to determine local values of the void fraction. Four resistance heaters were attached to the copper block, powered by direct current, generating the heat initiating the flow boiling inside the channel. During the experiment, the following were measured: water volumetric flow rate, inlet pressure with pressure drop, inlet and outlet water temperature, copper block temperatures at three points inside its body, voltage and current supplied to the heaters. Stationary and laminar fluid flow with low Reynolds numbers were assumed in the mathematical model of heat transfer in selected elements of the measuring module. The temperature distributions in the copper block and flowing water were described by the appropriate energy equations: the Laplace equation for the copper block and the Fourier–Kirchhoff equation with parabolic fluid velocity for the flowing water. These equations were supplemented with a set of boundary conditions based on measurement data; moreover, data from experimental studies were the basis for numerical calculations and their verification. Two-dimensional temperature distributions of the copper block and water were calculated with the Trefftz method (TM). The main objective of this study was to determine the heat transfer coefficient on the contact surface of the copper block and water, which was calculated from the Robin boundary condition. The results of the calculations were compared with the results of numerical simulations performed using the Simcenter STAR-CCM+ software, obtaining consistent values. Computational fluid dynamics (CFD) simulations were verified based on experimental data including void fraction and temperature measurements of the copper block and flowing water.

Numerical study and performance analyses of counter flow minichannel heat sink with slots on ribs

Various minichannel heat sinks are developed to cope with the increasing demand for thermal dissipation of electronic equipment. However, with the rapid development of microelectronics technology, temperature uniformity has gradually become a thorny problem needing more attention. Counter flow minichannel heat sinks with slots on ribs (CSF) were designed to enhance heat transfer and improve temperature uniformity. A 3D conjugated heat transfer model was developed in laminar flow regime, water has been selected as the working fluid, the constant heat flux was applied on the bottom surface of minichannel heat sinks. The counter flow minichannel heat sinks with slots on ribs not only possess higher heat transfer capacity, but also have better temperature uniformity. The average Nusselt number maximum increases to 2.01 times and the maximum temperature of bottom surface reduces 14.6 K for counter flow minichannel heat sink with five slots on ribs (CSF5) compared with parallel flow minichannel heat sink at Reynolds numberRe = 904. The temperature standard deviation of bottom surface maximum reduces 4.3 K for CSF3 minichannel heat sink atRe = 904. The effect mechanism of setting slots on ribs on the performance of counter flow minichannel heat sink was analyzed, it can be attributed to (i) interrupting and re-developing thermal boundary layer, and (ii) heat transfer between adjacent channels in which working fluid flows in opposite directions. Due to more uniform temperature of bottom surface and better heat transfer performance, the novel minichannel heat sink has potential application for thermal dissipation of electronic equipment.

Evaluation of dynamic correction of turbulence wall boundary conditions to simulate roughness effect in minichannel with rotating walls

PurposeThe purpose of this study is an assessment of the existing roughness models to simulate the flow in the narrow gap between corotating and rough disks. A specific configuration of the flow through the gap, which forms a minichannel with variable cross sections and rotating walls, makes it a complex problem and, therefore, worth discussing in more detail.Design/methodology/approachTwo roughness models were examined, the first one was based on the wall function modification by application of the shift in the dimensionless velocity profile, and the second one was based on the correction of turbulence parameters at the wall, proposed by Aupoix. Due to the lack of data to validate that specific case, the approach to deal with was selected after a systematic study of reported test cases. It started with a zero-pressure-gradient boundary layer in the flow over a flat plate, continued with flow through minichannels with stationary walls, and finally, focused on the flow between corotating discs, pertaining each time to smooth and rough surfaces.FindingsThe limitations of the roughness models were highlighted, which make the models not reliable in the application to minichannel flows. It concerns turbulence models, near-wall discretization and roughness approaches. Aupoix’s method to account for roughness was selected, and the influence of minichannel height, mass flow rate, fluid properties and roughness height on the velocity profile between corotating discs in both smooth and rough cases was discussed.Originality/valueThe originality of this study is the evaluation and validation of different methods to account for the roughness in rotating mini channels, where the protrusions can cover a substantial part of the channel. Flow behavior and performance of different turbulence models were analyzed as well.

Flow transitions during laminar stratified two-phase flow in mini channels

Flow transitions during laminar stratified two-phase flow in mini channels

Enhancing the performance of supercritical CO2-cooled heat sink by converging–diverging minichannels

Heat sinks are broadly utilized in micro-electromechanical systems (MEMSs) to optimize their performance and durability. Due to the ongoing rapid development in this realm, miniaturization has been an enviable trend in these systems. As a result of this revolution, the heat flux over the unit area has increased exponentially, necessitating the need for modified geometries and efficient coolants. In this study, heat sinks with eight different structures including convergent and divergent minichannels are proposed, and the heat transfer is analyzed in the turbulent flow of supercritical CO2, being a promising candidate coolant in the considered cases. 3D numerical simulations are carried out based on the finite volume method, and the turbulent flow is simulated by the SST k-ω model. The results illustrate that the CO2 flow in non-uniform minichannels leads to diverse nature of heat transfer and pressure drop due to drastic variation of thermophysical properties of CO2 near its critical point. Converging the minichannels enhances the ratio of the heat transfer coefficient up to 2.37, and diverging the minichannels reduces the ratio of the pressure drop down to 0.31. The trends in the performance index suggest operating conditions are of utmost importance to obtain the optimal performance of the CO2-cooled heat sink. In general, increasing the CO2 mass flux improves the overall performance of the heat sink with convergent minichannels. At the same time, it worsens the overall performance of the heat sink with divergent and hybrid minichannels. The simultaneous convergence of the minichannels in two directions reduces the base temperature of the heat sink by 27.2%, leading to a maximum performance index of 1.56 at the operating pressure of 10 MPa and the mass flux of 750 kg m−2 s−1.

Comparative analysis of fluid flow in mini channel with nano fluids and base fluid

Comparative study is carrying out between nano fluid Al2O3 + CuO + ZnO + H2O and water as solitary fluid through experimental and simulation result. The outcome of nano fluid and solo base fluid H2O flowing in middle rectangular mini channel with or without insert were admit, for study the conduct of fluid flow and heat exchange features. During composing of nano fluid 0.05%-0.1% volume fraction, 10–20 nm size nano particles were mixed with 0.5 ml CTAB surfactant. Experimentation were manage for different operating parameters under counter flow conditions, where nano fluid is flowing inside mini channel of 2 mm diameter with flow rate 0.0001562 to 0.006255 ml/min and hot water is flowing inside the concentric tube of 3.6 cm diameter with flow rate of 0.000782 ml/min. The operating temperature of nano fluid and hot water were 308 K and 323 K. As per obtained results, the proposed composition of nanofluid showed better performance than normal water due to better thermal conductivity and extra molecular area gain in nanofluid due to addition of nano particle with base fluid, where as better optimum results were observed in case of rectangular mini channel with macro insert compared with other geometry because of rich turbulency gain and extra exposed area due to macro insert.