Flow In Minichannels Research Articles

Analysis of mixed convection heat transfer of nanofluids in a minichannel for aiding and opposing flow conditions

In this study, mixed convection heat transfer characteristics of nanofluid flow in a circular minichannel were investigated experimentally. The effects of the particle volume ratio (0, 0.25, and 0.75%) and Reynolds number (200-60) on heat transfer by mixed convection were investigated for aiding and opposing flow conditions. Water and water based SiO2 nanofluids were used as working fluid in a minichannel with a diameter of 1.9 mm, and constant heat flux was applied to the outer surface of the minichannel. Temperature-dependent thermophysical properties such as thermal conductivity and viscosity, obtained by experimental measurements, were used in heat transfer calculations. Pressure based numerical computations for all experimental cases were also made by using single phase approach. The results were analyzed separately for aiding flow condition in which secondary flows originating from natural convection are in the same direction with the forced flow, and for opposing flow condition in which secondary flows are in the opposite direction with forced flow. For detailed analysis of mixed convection heat transfer, temperature contours and velocity profiles were obtained by the numerical computations which were compared and validated with the experimental results. According to the data obtained, it was determined that the addition of nanoparticles into the pure water increased the Nusselt number by around 21–64% for the aiding flow, and 18–58% for the opposing flow. The heat transfer in the aiding flow was observed to be minimum 4% and maximum 16% higher in comparison with the opposing flow condition. It was concluded that the direction of secondary flows significantly affects heat transfer.

Visualization Study of Air-Water Two-Phase Flow Pattern in Upward Vertical Flow of Square Channel

An important two-phase flow experiment study is carried out as a consideration in the application of piping system design that depends on two-phase flow. The purpose of this study was to determine the air-water flow pattern formed in a small square channel with vertical flow direction and to know the flow configuration. Channel 5 mm hydraulic diameter was used in this study. Bubble, slug, churn, and annular flow patterns are formed in the air superficial velocity range 0.33 - 7.31 m/s and superficial velocity of water is 0.13 - 1.2 m/s. A flow pattern map produced has not similarities with the mini-channel flow pattern map in previous studies. The velocity of bubbles and void fractions increases with the increase of superficial air velocity. The void fraction of present research has a good correlation to the results of predictions proposed by previous studies.

Parametric study on thermal and hydraulic characteristics of inter-connected parallel and counter flow mini-channel heat sink

The paper presents three-dimensional numerical investigation of fluid flow and heat transfer performance of parallel and counter flow mini-channel heat sink where cross flow between parallel channels was enabled with inter-connectors. The aspect ratio (height to width) and hydraulic diameter of the investigated mini-channel were 0.33 and 750 µm respectively. In this study, water was employed as the coolant, and the flow was in the single-phase regime under laminar flow condition at Reynolds numbers ranging from 150 to 1044. A constant heat flux of 20 W/cm2 was applied at the bottom surface of the heat sink. To analyze the effect of cross flow on the overall thermo-hydraulic performance of the mini-channel heat sink, five different widths of the inter-connector were studied. The non-dimensional pressure, velocity, temperature, friction factor, overall Nusselt number (Nu), and thermal resistance were calculated to evaluate the overall performance of the inter-connected mini-channel heat sink. Finally, the performance of the inter-connected mini channel was compared with the conventional mini-channel by calculating the performance evaluation criteria (PEC). The results show that the inter-connector has a significantly larger effect on the counter flow mini-channel heat sink compared with the parallel flow mini-channel. For the counter flow mini-channel heat sink, Nu was enhanced by a maximum of 36% at Re = 1044 as compared to the conventional parallel flow mini-channel while a maximum of 31.13% reduction in friction factor was recorded at Re = 150. The PEC of the counter flow mini-channel went up to 1.33, and its value shows an increasing trend with as Reynolds number increases.

Numerical simulation on fluid motion and heat transfer within porous medium considering its structural irregularity in minichannel flow

Numerical simulation on fluid motion and heat transfer within porous medium considering its structural irregularity in minichannel flow

Numerical simulation of flow in triangular minichannel

Minichannel cooling systems show great potential for small scale electronics. In this paper we analytically and numerically investigate flow with evaporation in open triangular minichannels without upper gas flow. It models processes either in a groove or in an individual channel. From the laws of mass, momentum and energy conservation we derive equations for mean velocity and for the depth of the liquid. These equations have been solved numerically. The viscosity limits maximum fluid speed and causes the formation of dry spots. The heat flux reaches 40kW/m2 for initial liquid velocity 2 m/s. It is demonstrated that triangular or grooved channels even without upper gas flow are perspective for cooling systems.

Accurate and inexpensive thermal time-of-flight sensor for measuring refrigerant flow in minichannels

Thermal flow meters are good candidates for measuring flow rates in small diameter channels because they can be integrated into compact geometries and consist of only a few simple components. A new thermal time-of-flight (TOF) flow sensor is developed here for measuring liquid refrigerant velocities in minichannels. The sensor consists of a small heater and two downstream thermocouples. The heater is pulsed to induce a temperature rise in the fluid, and the time for the temperature pulse to reach the downstream thermocouples is measured. Insights from the data and a simplified one-dimensional transient model are used to calibrate the device. The calibration relates the time of flight to both the average refrigerant velocity and local temperature gradients in the fluid. The calibrated TOF device can detect refrigerant velocities between 1 and 20 mm s−1, measuring 98.7% of the data to within ±3% of the full scale (±0.6 mm s−1).

Optimal shape design of a minichannel heat sink applying multi-objective optimization algorithm and three-dimensional numerical method

In the present study, an optimal laminar flow minichannel heat sink (MCHS) has been determined through three-dimensional simulations and the multi-objective optimization algorithm. The cross-sectional shape described by six variables is optimized by the multi-objective genetic algorithm (MOGA) and multi-objective particle swarm optimization (MOPSO). During the optimization, the thermal resistance θ and pumping power P are two conflicting objectives for evaluating the performances of the MCHS. After obtaining the non-inferior solutions, the technique for order preference by similarity to an ideal solution (TOPSIS) is applied as a decision-making method to determine the best compromise one. Results indicate that the TOPSIS can effectively reduce the P of the Pareto solutions without significantly increasing θ. Compared with the straight channel, the TOPSIS optimal solution could reduce θ by 7.47% or P by 31.54%. Meanwhile, the mechanism of performance improvement is analyzed by comparing the TOPSIS optimal solution and the straight channel with the same P. It is observed that the optimized channel shape changes the fluid distribution by increasing the heat transfer coefficient slightly and the heat transfer area by 12.22%, respectively.

Heat transfer and pressure drop of air/water mist flow in horizontal Minichannels

The heat transfer, friction factor, and flow pattern of air/water mist flow in a rectangular minichannel heat sink were experimentally investigated. The channel size effect was studied using three horizontal minichannels exhibiting cross-sections measuring 0.5 mm × 3 mm, 1 mm × 3 mm, and 3 mm × 3 mm. The gas Reynolds number ranged from 1000 to 6000, and the wall temperature ranged from 40 to 60 °C. For the single-phase air flow, the flow transition was comparable with the conventional flow channel. For the mist flow, the flow patterns observed in the current minichannel were dry-wall mist flow and wavy annular flow. The mist cooling performance decreased with increase in wall temperature, mainly because of the extended dryout regions. The heat transfer from the mist flow was 1.5–4 times higher than the air flow, and higher enhancement ratios were observed in larger minichannels at lower gas Reynolds numbers. Because of droplet accumulation in the minichannel, the friction factor due to the mist flow was 2–3 times higher than the air flow. The friction factor decreased with increase in wall temperature because of the low volume of liquid in the minichannel.

Experimental study on the flow pattern and pressure gradient of air-water two-phase flow in a horizontal circular mini-channel

Experimental studies on the flow pattern and the pressure gradient of gas-liquid co-current two-phase flow in a mini-channel were conducted. The test section was a transparent circular channel of 1.6 mm inner diameter. The working fluids were air and water. The superficial velocities of gas and liquid were in the range of 0.025-66.300 m/s and 0.033-4.935 m/s, respectively. In the present works, the flow pattern and the pressure gradient data were obtained by analyzing the flow images captured bya high-speed camera, and by using the pressure transducer, respectively. As a result, it was found that (1) the obtained flow patterns were bubbly, plug, slug-annular, annular, and churn flows, (2) new experimental correlations on the bubble and plug lengths were proposed, whereas the lengths are the function of the homogeneous void fraction, (3) both gas and liquid superficial velocities affect proportionally to the pressure gradient, whereas it increases with the increase both of JG and JL. In addition, the obtained flow patterns are in a good agreement with that of the available flow pattern maps in the open literatures, such as, Triplett et al. (1999) and Chung and Kawaji (2004).

Analytical solution to heat transfer in compressible laminar flow in a flat minichannel

Heat transfer in compressible laminar flow in mini-/micro-channels, a classical and general topic in fields of fuel cells, electronics, micro heat exchanger, etc., is revisited. Based on a two-dimensional continuum flow model, analytical solutions of the dimensionless model are achieved in closed-form symbolic algebras of Whittaker eigenfunctions, corresponding to two kinds of boundary conditions with arbitrarily prescribed wall temperature or wall heat flux. As the eigenvalues and eigenfunctions are independent on the dimensionless quantities, which influence the along-the-channel behaviors, the algorithm reveals the common features of compressible laminar thermal flows. The algorithms do not require the assumption of a linear pressure distribution, which is proved to be untenable in some cases (e.g. constant wall heat flux). The algorithms are validated well by the exact (numerical) computations in exemplary cases of both small and moderate Reynolds number, Mach number and Eckert number of air. Although expressed in a series of eigenfunctions, only several terms (sometimes one or two terms) of solutions are required for a practical computation.

Heat Transfer Coefficient Identification in Mini-Channel Flow Boiling with the Hybrid Picard–Trefftz Method

This paper summarizes the results of the flow boiling heat transfer study with ethanol in a 1.8 mm deep and 2.0 mm wide horizontal, asymmetrically heated, rectangular mini-channel. The test section with the mini-channel was the main part of the experimental stand. One side of the mini-channel was closed with a transparent sight window allowing for the observation of two-phase flow structures with the use of a fast film camera. The other side of the channel was the foil insulated heater. The infrared camera recorded the 2D temperature distribution of the foil. The 2D temperature distributions in the elements of the test section with two-phase flow boiling were determined using (1) the Trefftz method and (2) the hybrid Picard–Trefftz method. These methods solved the triple inverse heat conduction problem in three consecutive elements of the test section, each with different physical properties. The values of the local heat transfer coefficients calculated on the basis of the Robin boundary condition were compared with the coefficients determined with the simplified approach, where the arrangement of elements in the test section was treated as a system of planar layers.

Heat transfer enhancement strategies in a swirl flow minichannel heat sink based on hydrodynamic receptivity

The swirl flow minichannel heat sink has shown to be a promising alternative for thermal management of high heat flux applications, such as electronics and concentrated photovoltaics. Effective heat transfer enhancement strategies for this device are identified by studying the receptivity of temperature disturbances to a momentum forcing input. Steady state laminar flow is calculated numerically and experimental measurements are carried out to validate the results for subcritical Reynolds numbers. Using the framework of nonmodal stability theory, a harmonically driven linear perturbation problem is formulated, and the methodology to apply the local and parallel flow approximations based on order of magnitude arguments is detailed. The input-output response of temperature perturbations to forcing of the radial, azimuthal, and wall-normal momentum components is calculated for a range of wavenumbers, waveangles and temporal frequencies. The largest amplification is presented by streamwise vortices and streaks, followed by axisymmetric inward travelling waves, and then by streamwise propagating waves. Micromachining the channel walls with streamwise spiral grooves is proposed as a heat transfer enhancement technique. Excitation of streamwise independent structures in the wall-normal direction is expected, therefore maximum amplification should be obtained. Due to its simple implementation, we also propose using a pulsating flow rate as a heat transfer enhancement technique. Receptivity results for streamwise propagating waves of radial forcing show a response curve with moderate amplification for a wide range of actuation frequencies. Experimental work is conducted to measure the performance of the swirl flow channel heat sink using flow pulsations at the range of forcing frequencies suggested by the receptivity study. Compared to the unforced case, a lower wall temperature (up to 5°C cooler) was observed with pulsations, at the same imposed heat flux and flow rate. To get the same wall temperature as in the unforced case, a pumping power reduction of up to 26.6% was observed, and using the same pumping power resulted in up to a 10.3% Nusselt enhancement. Hydrodynamic receptivity was successfully used to identify effective heat transfer enhancement strategies, resulting in a significant performance improvement for the swirl flow channel heat sink. This physics based approach can be extended to other techniques, for instance, to select the wavelength of a wavy surface, the periodicity of surface roughness elements, or the frequency of acoustic vibrations.

A Review on Thermal Characteristics of Single and Two-phase Flow inMini channel for Refrigeration

This paper presents an exercise in applying the bacterial foraging algorithm (BFA) optimisation method on a proportional—integral-derivative controller (PID) of a DC motor circuit. The paper presents the system description of the DC motor transfer function and the simulation of the close loop system using MATLAB. The BFA algorithm is described and discussed with the simulation results presented to illustrate the enhancement of the system response that in result enhances the operation of the DC motor system.

Development of flow in a square mini-channel: Effect of flow oscillation

In this research paper, we present a numerical prediction of steady and fully oscillatory flows in a square mini-channel connected between two plenums. Flow separation occurs at the contraction of the plenum into the channel which causes an asymmetry in the development of flow in the entrance region. The entrance length and recirculation length are found, for both steady and fully oscillatory flows. It is shown that the maximum entrance length decreases with an increase in the oscillating frequency while the maximum recirculation length and recirculation area increase with an increase in oscillating frequency. The phase of a velocity signal is shown to be a strong function of its location. The phase difference between the velocities with respect to the different points along the centerline and those at the middle of the channel show a significant dependence on the driving frequency. There is a significant variation in the phase angles of the velocity signals computed between a point near the wall and that at the centerline. This phase difference decreases along the channel length and does not change beyond the entrance length. This feature can then be used to determine the maximum entrance length, which is otherwise problematic to ascertain in the case of fully oscillatory flows. The entrance length, thus obtained, is compared with that obtained from the velocity profile consideration and shows good similarity. The phase difference between pressure and velocity is also brought out in this work.

The difference in flow pattern, heat transfer and pressure drop characteristics of mini-channel flow boiling in horizontal and vertical orientations

Flow pattern, heat transfer, and pressure drop data for different flow orientations was presented in this study. The data was obtained based on flow boiling experiments with R-134a flow through a 1 mm diameter channel which was aligned in different orientations, i.e. horizontal flow, vertical upward flow, and vertical downward flow. A constant surface heat flux condition was performed under a saturation pressure of 8 bar, a heat flux range of 1–60 kW/m2, and a mass flux range of 250–820 kg/m2s. The experimental results showed the importance of the change in the flow direction. The shape of the gas slug during horizontal flow did not look the same as in the vertical orientations. Heat transfer coefficient and pressure drop became increased when the refrigerant flowed in the vertical downward direction. The experimental data was also compared with the existing prediction methods.

Volume-of-fluid simulations of gas-liquid-liquid flows in minichannels

Three-phase segmented gas-liquid-liquid (G/L/L) flows in minichannels are important to several chemical process applications involving gas-liquid-liquid reactions. In the present work, we have investigated segmented G/L/L flows in a double T-junction minichannel, with cross-section of 0.95 mm × 1 mm, through high-speed imaging experiments and Volume-of-Fluid (VOF) simulations. The dynamics of bubble/slug formation at the 1st T-junction and importantly that of water drop/slug formation at the 2nd T-junction was simulated under different flow conditions (Caoil=2.63×10-3-1.101; Weair=4.24×10-4-2.62×10-3; Wewater=0.0431-7.14) and different surfactant concentrations (0.3 and 2 wt/wt.%) in aqueous phase. The predicted formation mechanisms, three-phase flow regimes, and drop/bubble/slug lengths were compared quantitatively with the measurements. Different mechanisms of bubble/slug formation observed for the aforementioned range of the Caoil and Weair, and bubble/slug lengths were predicted in a satisfactory agreement with the measurements. The complex formation mechanism of water drops/slugs that was governed by viscous force exerted by continuous oil phase, inertial force exerted by water phase, interfacial tension force acting on the oil-water interface and also by incoming air bubbles/slugs; could be predicted in a satisfactory agreement with those observed in the experiments. Different three-phase flow regimes, e.g., drop-bubble, drop-slug and slug-slug, observed for different oil, air and water flow rates; and for different values of oil-water interfacial tensions are also predicted in satisfactory agreement with the measurements. The results reported in the present work help to understand the hydrodynamics of complex three-phase gas-liquid-liquid flows in minichannels, which in turn is crucial to device microreactor systems for process applications involving G/L/L flows.

Experimental investigations on pressure loss and heat transfer of two-phase carbon dioxide flow in a horizontal circular pipe of 0.4 mm diameter

The miniaturisation process in electronics, mechanics and other disciplines requires efficient, light and small cooling devices. Especially in the case of high thermal loads, mini-channels have been found to be appropriate candidates to remove heat efficiently from heated structures. The associated convective heat transfer is usually limited by the pressure loss which complicates the increase of flow rates and the achievement of an improved turbulent heat transfer. In this regard, increasing heat transfer of boiling flow in mini-channels by using CO2 as refrigerant opens an interesting new approach for such applications. The inner diameter (I.D.) of minichannels ranges from 0.2 mm to 3 mm. Two-phase carbon dioxide flow in mini-channels with I.D. >1 mm has been extensively experimentally and numerically investigated. Because advanced technologies now allow for the precise manufacture of smooth mini-channels with I.D. <0.5 mm, new trends intend to implement thinner tubes in order to further reduce hardware size. The necessary instrumentation of such thin tubes for the experimental investigation of two-phase flow carbon dioxide becomes delicate. In the present contribution, a modified 2-Phase Accumulator Loop (2-PACL) was used in order to measure heat transfer coefficients and pressure loss of boiling CO2 flowing through a horizontal circular pipe with an I.D. of 0.4 mm and a length of 140 mm. A specific part of the pipe was electrically heated and the resulting heat transfer coefficients were measured. Inlet conditions have been selected to be single phase with a saturation pressure of about 60 bar and a saturation temperature of about 22 °C. Experiments have been performed under high flow and heat load conditions. Mass flux and heat flux density have been systematically increased from 1100 to 4400 kgm2s and from 119.4 to 397.9 kWm2, respectively. Furthermore the measurements were compared to some of the latest published correlations to get an impression on how accurate two-phase flow with heat transfer using CO2 can be predicted for the design of mini-channel based cooling devices.

The solution of a two-dimensional inverse heat transfer problem using two methods

PurposeThis paper aims to focus on flow boiling heat transfer in an asymmetrically heated minichannel. Two-dimensional inverse heat transfer problem was solved using the Trefftz and Beck methods. The primary purpose was to find an enhanced surface that could help intensify heat transfer.Design/methodology/approachThe experimental set-up and methodology for FC-72 boiling heat transfer in two parallel vertical rectangular minichannels with smooth or enhanced heated surfaces was presented. The heat transfer coefficient was calculated using the Trefftz and Beck methods.FindingsThe results confirm that considerable heat transfer enhancement takes place when selected enhanced heated surface is used in the minichannel flow boiling and that it depends on the type of surface enhancement. The analysis of the experimental data revealed that the values and distributions of the heat transfer coefficient obtained using the Beck and Trefftz methods were similar.Practical/implicationsMany studies have been recently devoted to flow boiling heat transfer in minichannels because of the rapid development of high-performance integrated systems generating large amounts of heat. Highly efficient small-size cooling systems for new-generation compact devices are thus in great demand.Originality/valueThe present results are original and new in the study of cooling liquid boiling in minichannels with enhanced heated surfaces that contribute to heat transfer enhancement. The paper allows the verification of state-of-the-art methods of solving the inverse problem by using empirical data from the experiment. The application of the Trefftz and Beck methods for finding a solution of the inverse heat transfer problem is promising.

Joint 12th International Conference: “Two-Phase Systems for Space and Ground Applications” and 2nd International School of Young Scientists “Interfacial Phenomena and Heat Transfer”

PrefaceJoint 12-th International conference «Two-Phase Systems for Space and Ground Applications» and 2nd International School of Young Scientists «Interfacial Phenomena and Heat Transfer» were held at Kutateladze Institute of Thermophysics SB RAS, Academgorodok, Novosibirsk, Russia, during 11-16 of September 2017. The third largest city in Russia, often referred to as “the capital of Siberia”, Novosibirsk is home to a number of research centres and institutes, including the Kutateladze Institute of Thermophysics. The long tradition of research in both fluid mechanics and heat transfer makes this institute a natural venue for the Conference and School.The Conference and School provide a platform for researchers to exchange information and identify research needs in this important area encompassing several engineering, mechanical and physical disciplines. A wide range of topics in the rapidly developing and highly interdisciplinary field of interfacial phenomena and heat transfer were covered, including boiling, shear-driven films, droplet evaporation, contact line phenomena, thermocapillary flows, two-phase flows in microchannels and minichannels, and effect of gravity on various processes.The Conference and School were conducted under the financial support of the Russian Science Foundation (№ 15-19-30038).The editors would like to express their sincere appreciations and thanks to all the committee members of the Conference and School for their enormous efforts, to all the authors for their research contribution and to all expert referees for peer review of the articles. Thanks also to IOP Conference Series for producing the volume with articles.Editors,Prof. Oleg KabovDr. Yuriy LyulinDr. Dmitry ZaitsevFor more information about the Conference and School please visit the websites:http://www.itp.nsc.ru/htl/spaceconf-2017/http://www.itp.nsc.ru/htl//school-conference-2017/

Generalized Heat Transfer and Entropy Generation of Stratified Air-Water Flow in Entrance of a Mini-channel

In the present study a rule-based fuzzy inference system is used to predict heat transfer and entropy generation of stratified air-water flow in horizontal mini-channel as a function of a wide range of important parameters. Numerical data of our recent study are used to develop and test the system. The GK clustering algorithm is used to cluster the data. Fuzzy rules are generated based on the Sugeno-Yasukawa algorithm by using trapezoidal membership functions. The FATI and FITA approaches are implemented in the inference engine and finally the combination of the two approaches is defuzzified. The Mamdani and logical methods with the Yager operators are used and unified in both approaches. The parametric form of the system is a feature of the present study which can be used as an effective tool to improve the accuracy of the results. The novelty of the present study is the presentation of the generalized diagrams for the developing region of the channel which seems to be useful for engineering applications. In addition, generalized diagrams of average Nusselt numbers as well as total entropy generation can identify the appropriate range of volumetric flow rate ratio and the Reynolds number.