Fluid flow and heat transfer in Microchannels

In this paper, flow of slightly rarefied compressible nitrogen in microchannels has been investigated numerically for low values of Reynolds and Mach numbers. The 2D governing equations were solved using Finite Element Method with first-order slip boundary conditions (Comsol Multiphysics software). A validation was performed by comparing with similar configuration from the literature. It was found that our model can accurately predict the pressure driven flow in microchannels. Several interesting findings are reported about the Relative pressure, longitudinal velocity, Mach number, effect of gas rarefaction and flow rate.

Cette thèse présente les résultats de deux études : la première concerne la convection naturelle turbulente dans une cavité rectangulaire chauffée uniformément par le bas et remplie d’un nanofluide et la seconde concerne l’investigation du transfert de chaleur conjugué dans un échangeur de chaleur à tubes ailetés.L’enceinte de la première étude a un faible rapport d’aspect. Ses parois gauche, droite et supérieure sont maintenues à une température relativement basse. Le fluide de travail est un nanofluide constitué d’eau et de nanoparticules, soient d’alumine (Al2O3), ou de cuivre (Cu) ou d’oxyde de cuivre (CuO). L’influence des paramètres tels que le nombre de Rayleigh (basé sur la hauteur H de la cavité et la densité de flux de chaleur), le type de nanofluide et la fraction volumique des nanoparticules sur la performance de refroidissement est présentée. Les équations de Navier-Stokes et les équations de conservation de la masse et de l'énergie sont résolues pour une géométrie ...

This work describes the thermal characterization on ground and under a varying gravity field (parabolic flights) of a large diameter Pulsating Heat Pipe (PHP) especially designed for its future implementation on the heat transfer host module of the International Space Station (ISS) for long term microgravity experiments. A multi-turn compact closed loop PHP is made of aluminum and partially filled with FC-72 (50% vol.). The 3mm tube internal diameter is larger than the static capillary limit evaluated on ground conditions for the above working fluid, with the objective of dissipating larger heat power inputs compared to smaller diameter channels, under microgravity conditions, allowing the typical slug flow pattern of PHPs to occur. To provide a detailed insight on the thermo-hydraulics phenomena during the device start-up under the occurrence of microgravity, the PHP is equipped with a transparent sapphire tube insert, two miniature pressure transducers and two microthermocouples. ...

The numerical solution of unsteady two-dimensional, laminar, boundary layer flow of a viscous, incompressible, electrically conducting fluid along a semi-infinite vertical plate in the presence of thermal and concentration buoyancy effects under the influence of uniform magnetic field applied normal to the flow, has been obtained. A time-dependent suction is assumed and the radiative flux is described using the differential approximation for radiation. Asymptotic series expansion about a small parameter ε is performed to obtain the flow fields. The Galerkin finite element method is used to solve the equations governing the flow. The results obtained are discussed for cooling (G r > 0) and heating (G r < 0) of the plate with the help of graphs and tables to observe the effects of various parameters such as P r , Sc, M, Q, K, Nr, k r , A, G m and G r. It has been found that when the thermal and solutal Grashof numbers increase, the thermal and concentration buoyancy effects are enhanced and the fluid velocity increases. Furthermore, when the Prandtl and Schmidt numbers increase, the thermal and concentration levels decrease resulting in reduced fluid velocity. These results are in good agreement with earlier results.

Efficiency of a PV cell is strongly dependent on its surface temperature. The current study is focused to achieve maximum efficiency of PV cells even in scorching temperatures in hot climates like Pakistan where the cell surface temperatures can even rise up to around 80 °C. The study includes both the CFD and real time experimental investigations of a solar panel using micro channel cooling. Initially, CFD analysis is performed by developing a 3D model of a Mono-Crystalline cell with micro-channels to analyze cell surface temperature distribution at different irradiance and water flow rates. Afterwards, an experimental setup is developed for performance investigations under the real conditions of an open climate of a Pakistan's city, Taxila. Two 35W panels are manufactured for the experiments; one is based on the standard manufacturing procedure while other cell is developed with 4mm thick aluminum sheet having micro-channels of cross-section of 1mm by 1mm. The whole setup also includes different sensors for the measurement of solar irradiance, cell power, surface temperature and water flow rates. The experimental results show that PV cell surface temperature drop of around 15 °C is achieved with power increment of around 14% at maximum applied water flow rate of 3 LPM. Additionally, a good agreement is also found between CFD and experimental results. Therefore, that study clearly shows that a significant performance improvement of PV cells can be achieved through the proposed cell cooling technique.

Advancements in the field of electronics due to miniaturization and compaction have not only improved the performance of the electronic devices but have resulted in increased heat evolution from the devices. This heat evolution if not properly managed can lead to failure of the devices due to overheating. Hence, thermal management plays a key role in deciding the performance of such devices and is also a challenge for engineers and scientists. A gradual evolution in the field of thermal management has led to the development of highly efficient heat transfer devices. Pulsating heat pipes is one such promising device that can be used for better thermal management of the modern day electronic devices. The important task of heat transfer in a pulsating heat pipe is due to the working fluid present inside the heat pipe. Different types of working fluids have been used for testing the thermal performance of pulsating heat pipes. Nano fluids which are the recent additions in the family of working fluids are gaining importance due to their increased heat transfer abilities. This paper considers the review of the working fluids under various aspects.

Trade names and trademarks are used in this report for identifi cation only. Their usage does not constitute an offi cial endorsement, either expressed or implied, by the National Aeronautics and Space Administration. Level of Review: This material has been technically reviewed by technical management. Contents were reproduced from author-provided presentation materials.

Ces travaux présentent une alternative numérique au problème de simulation du procédé de perçage par laser. L'utilisation des éléments finis pour modéliser la propagation du trou au cours du temps montre des limites face à un problème de frontières mobiles induit par un changement de phase rapide et des forts gradients thermiques. L'utilisation d'un code C-NEM a donc été testé avec comme objectif de résoudre ces difficultés numériques et d'utiliser le fort potentiel de cette méthode originale. Le principe physique du perçage laser sera rappelé et le modèle mathématique choisi pour le modéliser sera précisé. Un cas test de simulation a été réalisé avec les grandeurs physiques du fer pur afin de valider le choix du code C-NEM.

This study presents a 3-D numerical analysis of complex heat exchanger systems featuring innovative fin ge-
ometries. These geometries are created by modeling and placing micro fins with perforated square, circular, and
rectangular designs on the heat sink. The objective of the study is to minimise thermal resistance in a hybrid heat
exchanger featuring various fin architectures. The hydraulic diameter ranges from 0.036 to 0.047 mm, with fin
dimensions of 0.04 mm in height and length, and 0.02 mm in width. The fins are designed with square perfo-
rations of 0.01 mm, circular perforations with a diameter of 0.015 mm, and rectangular perforations measuring
0.04, 0.012, and 0.006 mm. A high-density heat flux qʹʹ is applied to the bottom wall, and fluid with a Reynolds
number ranging from 400 to 500 is used to dissipate the heat in the cooling channel. The finite volume method
and computational fluid dynamics code are employed to discretise and solve for thermal and fluid flow solutions.
The impact of Reynolds number on minimised temperature and thermal resistance is analysed for void fraction
ratio (VFR) of 0.25, 0.44, and 0.18.The numerical analysis reveals that the hybrid microchannel, featuring
double square, circular, and rectangular perforations, demonstrates a reduction in resistance by 15 %, 21 %, and
19 %, respectively, when the coolant Reynolds number is increased by 20 %. Notably, the heat sink with square
perforations exhibits the most substantial decrease in resistance, followed by circular and rectangular perfora-
tions. The numerical code is validated with what is available in the existing literature.

This research aims to study the effect of inclination angle on the heat transfer performance of a Closed-Loop Oscillating Heat-Pipe with Check Valve (CLOHP/CV) with fi ns on the tube wall. The heat pipe was made from a copper pipe, and the capillary tube had a 5.0 mm inside diameter. There were 24 meandering turns with two check valves. The lengths of the evaporator section and condenser section were 200 mm and the adiabatic section was 100 mm. The working fl uid used was water with a fi lling ratio of 50% of the total volume of the tube. The temperatures for the evaporator section were 60, 70 and 80 o C. Inclination angles were 0, 20, 40, 45, 60, 80 and 90 degrees from the horizontal axis were established. It was found that when the variable temperature increased from 60, 70 to 80 o C heat fl ux and thermal effi ciency increased. In addition, when the inclination angle increased from 0, 20, 40, 45, 60, 80 and 90 degrees heat fl ux and thermal effi ciency increased. Therefore, this research concluded, from the experiment that the heat pipe was a CLOHP/CV. The maximum specifi c heat fl ux equaled 1,926.97 W/m 2 and the maximum thermal effectiveness equaled 0.44, the operating temperature was 80 °C and an angle of inclination to the horizontal axis was 90 o

In this study, high-fidelity conjugate heat transfer simulations are used to model a microchannel heat exchanger (MCHE) in a crossflow to study its thermal-hydraulic performance. Three different microchannel geometries, namely circular, triangular, and square with louver-shaped fins, are considered. The local flow field showed a strong coupling between the microchannel flow, solid domain, and crossflow. The flow separation and wake regions formed near MCHE resulted in a large variation in the velocity field and temperature in the crossflow. The interaction of the wake region with the fins introduces significant spanwise variations, leading to variations in the thermal boundary layer. The heat conduction in the solid structure provided a non-uniform temperature field with a higher temperature near the microchannel and a slightly lower temperature near the surface exposed to the crossflow. The microchannel flow analysis showed that the internal geometry affects the pressure drop, which is highest for the triangular MCHE and lowest for the circular MCHE. Based on the effectiveness and performance index calculations, the circular MCHE exhibited the best performance, while the rectangular MCHE showed higher heat transfer effectiveness.

In this study, high-fidelity conjugate heat transfer simulations are used to model a micro-channel heat exchanger (MCHE) in a crossflow to study its thermal-hydraulic performance. This study considers three different microchannels (internal flow) geometries (circular, triangular, and square) with louver-shaped fins. The local flow field showed a strong coupling between the microchannel flow, solid domain, and crossflow. The flow separation and wake regions formed near MCHE resulted in a large variation in the velocity field and temperature in the crossflow. The wake region had a significant spanwise variation due to its interaction with fins, which also causes variations in the thermal boundary layer. The heat conduction in the solid structure provided a non-uniform temperature field with a higher temperature near the microchannel and a slightly lower temperature near the surface exposed to the crossflow. The microchannel flow analysis showed that the internal geometry affects the pressure drop, which is highest for the triangular MCHE and lowest for the circular MCHE. However, the microchannel flow temperature change was relatively similar for all microchannels. Results showed that for the same volume of the microchannel, the circular shape microchannel has a higher performance index value than the triangular and square shapes.
This study also considers three different fin (external flow/crossflow) geometries (louver, step, and saw) with the same tube and circular shape microchannel and identifies the corrosion hot spot. Crossflow shows higher temperatures near the boundary layer of the tube, which results in higher corrosion rates. A predicted flow field also identifies crevices between fins and tube surfaces as critical corrosion hot spots often associated with low-velocity regions.
Electrochemical Impedance Spectroscopy (EIS) analysis was done on AA3102 (Alloy used in the circular channel and louver fin) alloy in corrosive environments containing low and high concentrations of the combination of sodium chloride and ammonium sulfate. Electrolytes used in this research have pH values ranging from 4.0 to 5.8, closer to nearly neutral environments encountered in many atmospheres. EIS results are presented, including Rsol, Rpore, and Rct of AA3102 with very thin arc evaporated porous Zinc film on AA 3102 along with their equivalent circuit.

Experiments were carried out to investigate the subcooled¯ow boiling heat transfer and visualize the associated bubble characteristics for refrigerant R-134a¯owing in a horizontal annular duct having inside diameter of 6.35 mm and outside diameter of 16.66 mm. The eects of the imposed wall heat¯ux, mass¯ux, liquid subcooling and saturation temperature of R-134a on the resulting nucleate boiling heat transfer and bubble characteristics were examined in detail. In the experiment signi®cant hysteresis was noted in the boiling curves during the onset of nucleate boiling (ONB) especially at low saturation temperature and high subcooling. The temperature undershoot at ONB is rather large for most cases. The boiling heat transfer was slightly higher for a lower saturation temperature and was little aected by the mass¯ux. However, for a higher subcooling of the refrigerant better heat transfer results. Furthermore, the¯ow visualization revealed that at higher imposed wall heat¯ux the heated surface was covered with more bubbles and the bubble generation frequency is higher. But the size of the bubbles departing from the heated surface was only slightly aected by the imposed heat¯ux. At high mass¯ux and subcooling the bubble generation was suppressed to a noticeable degree. Besides, the bubbles are much smaller at a higher subcooling. Finally, empirical correlations for the heat transfer coecient and bubble departure diameter in the subcooled¯ow boiling of R-134a were proposed.

This paper presents a numerical simulation and optimisation of a complex microchannel featuring innovative fin designs. The primary objective of the study is to minimise resistance in the heat sink by utilizing intricate fin structures. Three different approaches are explored: firstly, cylindrical solid fins are designed and placed on the heat sink; secondly, the solid fins are drilled halfway (50 %); and in the third scenario, the solid fins are drilled 87.5 % and mounted on the heat sink. In the simulation setup , the heat sink has a heat load of 250 W imposed on the bottom wall and single-phase water of Reynolds number between 400 and 500 flows in a forced convection laminar condition to remove the heat at the bottom and internally within the fins walls surface area, while an air stream of Reynolds number between 3 and 6 flows convectively across the cylindrical fins to dissipate excess heat externally. The finite volume method and computational fluid dynamic code, are employed to discretise the geometry with heat and fluid fields solved. The optimisation is performed for parallel and counter flows and the outcomes compete favorably. Similarly, the influence of the Reynolds number on minimised temperature and resistance results is discussed. The results show that in parallel flow the integrated heat sink with half hollow fins is best with a minimised resistance of 27.2 %, while in the counter flow the hollow fins are superior with a declined resistance of 19 %. The study is validated with experimental results in open literature.

Due to the many benefits offered by Microchannel Heat Exchangers (MCHX), such as compactness, high heat transfer coefficients, reduced refrigerant charge, and energy and material cost savings, microchannel condensers and evaporators continue to be increasingly applied and investigated in the HVAC&R fields. One of the practical challenges associated with MCHX is the uniform distribution of two-phase refrigerant in the headers and tubes of the heat exchanger. In MCHX, which typically have port sizes about 1 mm or less, to maintain the pressure drop at reasonable levels while providing fairly uniform two phase flow distribution, an appropriate header size and number of tubes need to be chosen. In this paper, a critical review of experimental and analytical investigations of two-phase flow maldistribution in MCHX is presented. The influence of header and microchannel tube geometry, heat exchanger orientation, flow and operating conditions, fluid properties and flow patterns on the MCHX flow distribution is discussed. Researchers have investigated upward and/or downward two-phase flow in MCHX with horizontal and vertical headers, for which the microchannel tubes/ports are, respectively, vertical and horizontal. Traditionally, compared to investigations in horizontal headers, the studies on vertical headers have been relatively few. However, recently, due to applications involving automotive evaporators, more studies on vertical headers are reported. In all these studies, gravity is seen to profoundly affect the two-phase flow distribution. Various fluids such as R410A, R134a, R245fa, CO2, air-water, etc. have been studied in published works. Fluid thermophysical properties and flow patterns greatly influence the flow distribution in MCHX. Very few investigators have studied the effects of fluid properties on two-phase flow distribution. Zou and Hrnjak (2014) [16] speculated that fluids with high liquid to vapor density ratio would provide better flow distribution. However, this hypothesis needs to be experimentally confirmed. Most studies agree that in headers, compared to annular flow, churn flow is desirable for better flow distribution. Most of the experimental investigations on flow distribution have been conducted for adiabatic flow. However, the applicability of such investigations to practical situations is dubious as the flow will be accompanied by condensation or boiling heat transfer. Refrigerant mass flux (G) and inlet quality (x) are seen to have a significant impact on flow distribution which is discussed in detail. Tube protrusion into the headers, and tube spacing also greatly affect the flow distribution. Since these interacting factors make the prediction of two-phase flow distribution very complex, a limited number of semiempirical models/correlations have been proposed to quantify the two-phase flow maldistribution in MCHX. Five correlations for predicting the liquid takeoff ratio in MCHX headers were assessed and among these five, Zou and Hrnjak (2013b)[15] correlation for R410A was found to perform reasonably. Based on the current study, recommendations regarding the applicability of these correlations to practical problems have been provided. Having identified and examined the key factors influencing two-phase flow maldistribution in MCHX, recommendations for further study are made.

In this work, the effect of temperature-dependent thermal conductivity (k(T)) and viscosity (\mu (T)) variation on entropy generation in circular channels with an approach from macro- to micro-scale is numerically investigated. Thermally as well as hydrodynamically fully developed flow of water through the fixed length channels with constant total heat flow rate and total mass flow rate is considered. The effects of k(T) variation and \mu (T) variation on entropy generation are analyzed individually as well as collectively. It is observed that in the case of Constant Property Solutions (CPS) {S_{\mathit{gen},\mathit{tot}}} is maximum at the macro-level; however, in the case of combined k(T) and \mu (T) variations it is maximum at the micro-level. The Bejan number (\mathit{Be}) and irreversibility distribution ratio (φ) are also calculated for asserting the dominance of frictional irreversibility and conduction heat transfer irreversibility. Additionally, the optimum diameter ({D^{\a...

High pressure drop and high length to hydraulic diameter ratios yield significant compressibility effects in microchannel flows, which compete with rarefaction phenomena at the smaller scale. In such regimes, flow field and temperature field are no longer decoupled. In presence of significant heat transfer, and combined with the effect of viscous dissipation, this yields to a quite complex thermo-fluid dynamic problem. A finite volume compressible solver, including generalized Maxwell slip flow and temperature jump boundary conditions suitable for arbitrary geometries, is adopted. Roughness geometry is modeled as a series of triangular shaped obstructions, and relative roughness from 0% to 2.65% were considered. The chosen geometry allows for direct comparison with pressure drop computations carried out, in a previous paper, under adiabatic conditions. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 300 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to around 0.05, and the competing effects of rarefaction, compressibility and roughness are investigated in detail. Compressibility is found to be the most dominant effect at high Mach number, yielding even inversion of heat flux, while roughness has a strong effect in the case of rarefied flow. Furthermore, the mutual interaction between heat transfer and pressure drop is highlighted, comparing Poiseuille number values for both cooled and heated flows with previous adiabatic computations.

This study uses a semi-theoretical steady state computer simulation of an automotive air conditioning system to evaluate design options. The simulation has been validated with experimental data. Influence coefficients are used to combine energy and cost data to provide a reasonable basis for comparison. Influence coefficients are provided for evaluating seven different design changes. Four of these are used in an example which illustrates the use of influence coefficients in making design choices. For the system modeled it is found that enhancing the internal surface of the condenser coil is the best option for increasing capacity, increasing efficiency and decreasing head pressure. The other three design changes included in the example were increasing the condenser length, evaporator length and compressor displacement.

In microchannel flow boiling, bubble nucleation, growth and flow regime development are highly influenced by channel cross-section and physical phenomena underlying this flow boiling mechanism are far from being well-established. Relative effects of different forces acting on wall-liquid and liquid-vapor interface of a confined bubble play an important role in heat transfer performances. Therefore, fundamental investigations are necessary to develop enhanced microchannel heat transfer surfaces. Force analysis of nucleating bubble and bubble dynamics in flow boiling Silicon Nanowire microchannels has been performed based on theoretical, experimental and visualization studies. The relative effects of different forces on flow regimes, instabilities and heat transfer performances of flow boiling in Silicon Nanowire microchannels have been identified. Inertia, surface tension, shear, buoyancy, and evaporation momentum forces have significant importance at liquid-vapor interface as discussed earlier by other researchers. However, no comparative study has been done for different surface properties till date. Detail analyses of these forces including contact angle effect, channel dimension effect, heat flux effect and mass flux effect in flow boiling microchannels have been conducted in this study. A comparative study between Silicon Nanowire and Plainwall microchannels has been performed based on force analysis in the flow boiling microchannels. Compared to Plainwall microchannels, enhanced surface rewetting and CHF are owing to higher surface tension force at liquid-vapor interface and Capillary dominance resulting from Silicon Nanowires. Whereas, low Weber number in Silicon Nanowire helps maintaining uniform and stable thin film and improves heat transfer performances. Moreover, results from these studies are compared with literature and great agreements have been observed.

In this work, the effects of dimensionless parameters on velocity field, thermal field and nanoparticle concentration field have been studied. In this regard, the governing partial differential equations are transformed into ordinary differential equations by using similarity transformations. These transformed equations are then solved numerically using the function bvp4c of MATLAB for different values of the parameters.

The present study proposes an efficient small scale vapour compression refrigeration system to actively cool a high performance computer. This system includes several miniaturized components like a compressor, evaporator, and condenser which are part of a refrigeration system designed to fit in smaller scale power electronics. An array of micro- channels is used for the evaporator unit. Here various micro channels designs while miniaturizing the evaporator are consider for incorporating in the system and the performance is evaluated to maximize the heat transfer coefficient. This system is capable of maintaining a device temperature below 25˚C while dissipating heat of 100 W/cm 2 by incorporating a micro channel heat sink as an evaporator using 134 a refrigerant. In this study numerical results are used to get the thermal performance of a microchannel evaporator in relation to the performance of the refrigeration cycle. This study shows the effect of channel geometry on evaporator p...

A promising approach to reducing the >870,000 deaths/year globally from unsafe water is flow-through solar water pasteurization systems (SWPs). Unfortunately, demonstrated systems have high capital costs, which limits access for the poor. The most expensive component of such systems is the heat exchanger (HX). Thus, this study focuses on cost optimization of HX designs for flow-through SWPs using high-effectiveness polymer microchannel HXs. The theoretical foundation for the cost optimization of a polymer microchannel HX is provided, and outputs are plotted in order to provide guidelines for designers to perform HX optimizations. These plots are used in two case studies: (1) substitution of a coiled copper HX with polymer microchannel HX, and (2) design of a polymer microchannel HX for a 3-D printed collector that can fit in an arbitrary build volume. The results show that substitution of the polymer expanded HX reduced the overall expenditure for the system by a factor 50, which aids in making the system more economical. For the second case study, the results show how future system designers can optimize an HX for an arbitrary SWP geometry. The approach of distributed manufacturing using laser welding appears promising for HX for SWP.

This article first reviews non-traditional heat exchanger geometry, laser welding, practical issues with microchannel heat exchangers, and high effectiveness heat exchangers. Existing microchannel heat exchangers have low material costs, but high manufacturing costs. This article presents a new expanded microchannel heat exchanger design and accompanying continuous manufacturing technique for potential low-cost production. Polymer heat exchangers have the potential for high effectiveness. This article discusses one possible joining method – a new type of laser welding named ‘forward conduction welding’, used to fabricate the prototype. The expanded heat exchanger has the potential to have counter-flow, cross-flow, or parallel-flow configurations, be used for all types of fluids, and be made of polymers, metals, or polymer–ceramic precursors. The cost and ineffectiveness reduction may be an order of magnitude or more, saving a large fraction of primary energy. The measured effectiven...

In the present study, the mixed convection heat transfer for a nanofluid flow inside solar cell cooling system was numerically investigated. To this end, an inclined two-dimensional channel with insulated walls was taken under study. The solar cells were assumed to locate on one side of channel where the nanofluid enters at constant temperature. The study has been carried out for the Richardson numbers in the range 0.01 6 Ri 6 20, with solid volume fraction 0 6 u 6 0.04, Reynolds number 100 and for Prandtl number 7.02. The effects of solid volume fraction and solar cells location in terms of Richardson number on the flow and temperature fields and also the heat transfer rate were studied. The results showed that the presence of nanoparticles led to an increase in temperature gradient and heat transfer near the solar cells. It was also observed that the maximum heat transfer in solar cells occurs in minimum inclination angle of channel at a certain relative distance between the cells.

Flow boiling in microchannels promises high heat transfer due to the combined effect of latent heat of vaporization and forced convection in confined spaces. However, flow boiling based miniaturized thermal management devices are limited due to instability induced dryout. While several efforts have been made to delay instabilities via advanced surface modification techniques, there is a need to expand the scope of applications by developing low-cost and scalable fabrication technologies for commonly used heat exchanger materials. In this paper, we use a facile and self-limiting chemical oxidation technique for fabricating sharp needle-like superhydrophilic CuO nanostructures within six parallel 500 × 250 µm 2 microchannels spread uniformly over a 1x1 cm 2 area in a copper heat sink. We demonstrate heat transfer enhancement with nanostructured microchannels (NSM) without any appreciable change either in the average pressure drop or the fluctuations in comparison to baseline plain wall microchannels (PWM). Analysis of the high-speed images was performed to attribute the enhancement with NSM to the presence of a capillarity-fed thin-film evaporation regime, which otherwise was absent in PWM. We believe that these results are encouraging and suggest that the heat sink geometry can be optimized to investigate the true potential of nanostructured microchannels.

In this paper, the thermal performance of nanostructured (CuO nanostructures) copper multiple parallel microchannels for single-phase flow is studied. The test section consists of 6 parallel channels and each channel is 350 µm wide and 605 µm deep with an aspect ratio of 1.729. Microchannels are covered on the top surface with an acrylic sheet. A cartridge heater is inserted at the bottom of the microchannels for supplying the desired heat flux and the position of the heater is determined numerically. Deionized water (DI) is used as the working fluid in the experiment. Single-phase pressure drop and heat transfer coefficient in multiple nanostructured microchannels are measured. A maximum enhancement of ~21% in Nusselt number is reported without any observable increase in pressure drop upon the incorporation of nanostructured in microchannels. The current study could provide a suitable low-cost framework for investigating the potential of nanostructures in further enhancement of single phase heat transfer coefficients in microchannel geometry.

A 2-D numerical investigation was carried out to study the effect of spacing between ribs on nanofluid flow and heat transfer inside a horizontal micro-channel. Two identical ribs were placed at the lower wall of micro-channel with variable spacing between them. The alumina oxide nanoparticles was suspended in water as based fluid at different volume fraction 0, 2 and 4%. The finite volume method was used to solve the continuity, momentum and energy equations. The effects of different parameters such as nanoparticles volume fraction, Reynolds number, and the spacing between ribs has been evaluated. The results showed that increasing nanoparticles volume fraction and Reynolds number significantly enhanced the heat transfer and the Poiseuille number. The presence of ribs improves the heat transfer. However, increasing the spacing between ribs leads to decrease the heat transfer rate.   

This paper investigates forced convection of water/Al 2 O 3 nanofluid flow in a microchannel with inclined cross-flow injection on the lower wall. There are a number of injecting holes on its lower wall. Other walls are insulated. The effect of different parameters including injection angle and number of injections on slip flow and heat transfer is investigated. It is concluded that increasing the injection angle up to 94° lead to an enhancement of the slip velocity and the Nusselt number on the microchannel walls. The results demonstrate that maximum heat transfer corresponds to the injection angle of 94°. It is revealed that the variation of Nusselt number decreases with the number of injection. As the volume fraction of nanoparticles increases, the heat transfer rate increases.

Researchers commonly assume that the effect of constant temperature and constant heat flux thermal boundary condition on forced convection from a circular cylinder is negligible. Such claim was assessed using Computational Fluid Dynamics for Reynolds number, Re, 3,000. The flow was directly solved without modeling for Re 300, and Large Eddy Simulation was used for Re ¼ 1,000 and Re ¼ 3,000. Constant property simulations were done for Prandtl number of 0.7. Fluid dynamics parameters: drag coefficient, Strouhal number, separation angle and recirculation length, were used to assess the accuracy of the simulations. The main parameter of interest, the Nusselt number, is higher locally for constant heat flux than constant temperature for 60 Re 3,000. In addition, the ratio of overall Nusselt number for constant heat flux to that for constant temperature increases for Re < 30 and is $1.15 between 30 Re 3,000, showing that thermal boundary condition has significant effect on heat transfer from circular cylinder. Finally, the study developed empirical correlations relating overall Nusselt number to studied Reynolds number range.

Response surface methodology is used to investigate an active method for flow boiling heat transfer enhancement by means of fluid flow pulsation. The flow pulsations are introduced by a flow modulating expansion device and compared with the baseline continuous flow provided by a stepper-motor expansion valve. Two experimental designs (data point sets) are generated using a modified Central Composite Design for each valve and their response surfaces are compared using the quadratic model. Statistical information on the significant model terms are used to clarify whether the effect of fluid flow pulsations is statistically significant in terms of the time-averaged flow boiling heat transfer coefficient. The cycle time range from 1 s to 9 s for the pulsations. The results show that the effect of fluid flow pulsations is statistically significant, disregarding the lowest heat flux measurements. The response surface comparison reveals that the flow pulsations improves the time-averaged heat transfer coefficient by as much as 10 % at the smallest cycle time compared with continuous flow. On the other hand, at highest cycle time and heat flux, the reduction may be as much as 20 % due to significant dry-out when the valve is closed. These values are higher than reported in part 1 of the paper, but evaluated more consistently at equal heat flux using the response surfaces.

A CFD study of bubbles growing in a mini-channel with a diameter of 0.64 mm has been done. Coupled level set and volume of fluid (CLSVOF) method is applied to capture the two phase interface. Georeconstruct method is used to reconstruct the two-phase interface. A constant velocity inlet boundary with mass flux 335 / 2 and a heated boundary wall with constant heat flux (10 / 2) is applied. Both saturated and sub-cooled inlet condition are studied. The growth of bubbles and the transition of flow regime differs each other under these two conditions. Sub-cooling significantly lowers the bubble growth rate. However, it does not affect the heat transfer coefficient at the same level due to its complicated heat transfer mechanism.

Three dimensional steady state mixed convection in a lid driven cubical cavity heating from below has been investigated numerically. Two sided walls are maintained at a constant ambient temperature Ttop &lt; Tbottom, while the vertical walls are thermally insulated. Governing equations expressing in a dimensionless form are solved by using finite element method. The Reynolds number is fixed at Re=100, while the Richardson number is varied from 0.001 to 10. Parametric studies focusing on the effect of the Richardson number on the fluid flow and heat transfer have been performed. The flow and heat transfer characteristics, expressed in terms of streamlines, isotherms and average wall Nusselt number are presented for the entire range of Richardson number considered. Multiple correlations in terms of the heat transfer rate and Richardson number has been established. Mots clefs : Convection mixte, le nombre de Richardson, cavité entrainée et gradients de température 22 ème Congrès França...

Pour atteindre des champs magnétiques plus intenses que ceux produits commercialement avec des aimants supraconducteurs, des bobines de champs à base d'alliage de cuivre sont développées dans quelques installations de champs magnétiques intenses. A Grenoble, une installation d'une puissance électrique de 24 MW permet de produire des champs continus jusqu'à 37 T dans un diamètre de 34 mm. Une augmentation de puissance à 30 MW est prévue pour être opérationnelle en 2022. Un aimant pour champs magnétiques intenses est un archétype d'échangeur compact aux caractéristiques extrêmes. Des flux de chaleur atteignant 10 MW.m-2 sont traités par un refroidissement par eau monophasique avec des vitesses de passages atteignant 30 m.s-1. Les canaux de refroidissement sont caractérisés par des diamètres hydrauliques de 200 à 1800 µm et des longueurs de 30 mm à 430 mm. Une perte de pression de 20 bars se développe entre l'entrée et la sortie de l'aimant pour champs intenses, le coefficient de transfert à la paroi des bobines atteint h=10 5 W.m-2 .K-1. En 2018, 15 GWh d'énergie électrique ont été injectés dans les aimants pour champs intenses de Grenoble. Cette énergie est intégralement transformée en chaleur fatale rejetée dans la rivière voisine. Un des projets du laboratoire est d'injecter une partie de ces calories dans le réseau de chaleur de l'agglomération grenobloise. La température de sortie de l'aimant est aujourd'hui au maximum de 40°C, température au-dessous de celle requise par le réseau de chaleur. L'utilisation d'une pompe à chaleur pour relever la température est donc nécessaire. Pour améliorer le modèle économique et les performances environnementales du projet, une solution est d'augmenter la température de sortie de l'aimant pour minimiser les coûts électriques associés aux pompes à chaleur. Pour cela nous devons diminuer le débit d'alimentation de l'aimant qui est actuellement 300 l.s-1 pour 24 MW et augmenter Mots clefs: transfert thermique, hydrodynamique, mini canaux , ébullition nucléée, transfert de chaleur intense, récupération de chaleur fatale, champs magnétiques intenses, réseau de chaleur urbain.

Pour atteindre des champs magnétiques plus intenses que ceux produits commercialement avec des aimants supraconducteurs, des bobines de champs à base d'alliage de cuivre sont développées dans quelques installations de champs magnétiques intenses. A Grenoble, une installation d'une puissance électrique de 24 MW permet de produire des champs continus jusqu'à 37 T dans un diamètre de 34 mm. Une augmentation de puissance à 30 MW est prévue pour être opérationnelle en 2022. Un aimant pour champs magnétiques intenses est un archétype d'échangeur compact aux caractéristiques extrêmes. Des flux de chaleur atteignant 10 MW.m-2 sont traités par un refroidissement par eau monophasique avec des vitesses de passages atteignant 30 m.s-1. Les canaux de refroidissement sont caractérisés par des diamètres hydrauliques de 200 à 1800 µm et des longueurs de 30 mm à 430 mm. Une perte de pression de 20 bars se développe entre l'entrée et la sortie de l'aimant pour champs intenses, le coefficient de transfert à la paroi des bobines atteint h=10 5 W.m-2 .K-1. En 2018, 15 GWh d'énergie électrique ont été injectés dans les aimants pour champs intenses de Grenoble. Cette énergie est intégralement transformée en chaleur fatale rejetée dans la rivière voisine. Un des projets du laboratoire est d'injecter une partie de ces calories dans le réseau de chaleur de l'agglomération grenobloise. La température de sortie de l'aimant est aujourd'hui au maximum de 40°C, température au-dessous de celle requise par le réseau de chaleur. L'utilisation d'une pompe à chaleur pour relever la température est donc nécessaire. Pour améliorer le modèle économique et les performances environnementales du projet, une solution est d'augmenter la température de sortie de l'aimant pour minimiser les coûts électriques associés aux pompes à chaleur. Pour cela nous devons diminuer le débit d'alimentation de l'aimant qui est actuellement 300 l.s-1 pour 24 MW et augmenter Mots clefs: transfert thermique, hydrodynamique, mini canaux , ébullition nucléée, transfert de chaleur intense, récupération de chaleur fatale, champs magnétiques intenses, réseau de chaleur urbain.

Our work focuses on the numerical study of two-dimensional and transient natural convection in a fluid confined within a crescent-shaped space delimited by two horizontal cylinders. The upper wall is subjected to a non-uniform heat flux, while the lower wall experiences a uniform heat flux, thereby generating thermal natural convection. The transfer equations are solved in a bi-cylindrical coordinate system using the formalism of stream function and vorticity, and then integrated using the finite difference method. Subsequently, these transfer equations are integrated using S.V Patankar's finite difference method with an implicit scheme. The computational program is implemented using Maple V Release Student software. The discretization of the equations highlights the following parameters: the Prandtl number (Pr), the modified Grashof number (Gr), and the aspect ratio (r2/r1). The Prandtl number is fixed at 0.7. The results include temperature distributions, local and average Nusselt values, as well as graphs illustrating variations in various parameters based on slice indices.

The miniaturization trend of transistors and increase in packing density of electronic devices has resulted in high heat flux generation, which has created a need for efficient heat removal systems. The present research is an experimental study of pool boiling using plain copper chip and microchannel chip with boiling surface of 34.5mm x 32mm. Three dielectric fluids, Perfluoro-2-methylpentane (PP1), perfluoro-methyl-cyclopentane (PP1C), and fluorocarbon (FC-87) were used in a closed loop pool boiling system to determine their performance at atmospheric pressure. The pool boiling results have been compared with literature for a boiling surface of 10 mm x 10 mm to study the effect of heater size. To improve the performance of the pool boiling system, we desire high critical heat flux and low surface temperatures. In the current study, we introduced two external structures fitted on the test surfaces for regulating the flow of vapor through specific structures and generating independent liquid-vapor pathways without any deposition and/or chemical surface modifications of the test surface. Firstly, an array of hollow conical structures (HCS) called volcano manifold are printed using additive manufacturing technique. A critical heat flux (CHF) of 28.1 W/cm², 38.3 W/cm² and 32.5 W/cm² was achieved for volcano manifold with plain copper chip using PP1, PP1C and FC87 respectively giving 19%, 33% and 6.5% enhancement in CHF respectively as compared to a plain chip without volcano manifold. Secondly, dual taper manifold having taper angle of 15° is printed using a stereolithography (SLA) additive manufacturing technique. Plain chip with dual taper manifold gave the CHF of 25.6 W/cm², 31.7 W/cm² and 32.3 W/cm² for PP1, PP1C and FC-87, respectively. These results indicate a deterioration in CHF caused by vapor constriction. In addition, the heater size effect was studied by comparing the pool boiling performance of a plain copper boiling surface of 34.5 mm x 32 mm (Large heater) with 10 mm x 10 mm (Small heater) from published literature for all three refrigerants. It was noted that 31%, 66% and 104% increment in maximum heat High temperature polylactide HTPLA

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"A numerical simulation of heat transfer and pressure loss analysis is presented for variously shaped turbulators inserted into a rectangular channel. A constant heat flux is applied to the external side of the channel. Numerical simulations were conducted using COMSOL Multiphysics. The shapes of the turbulators vary, and the effects of turbulator height are studied.

The obtained numerical results clearly show that the dynamic and thermal performances depend on the dimensions and shape of the turbulators. The development of swirling structures behind the turbulators is observed, which seems to be responsible for the increase in heat transfer. Pressure drops are primarily attributed to the flow blockage caused by these elements."

In this paper we demonstrate how ferrofluids (magnetic nanofluids) can be used to significantly enhance heat transfer rates in microchannels to help meet the ever-growing demand for high heat flux removal. There are a few mechanisms that can be used to aid mixing and to enhance heat transfer in inherently low Reynold number flow in microchannels. Firstly, the addition of a small amount of the thermally conductive paramagnetic particles can increase the thermal conductivity of the base fluid. For example, a solution of 5% particles can increase the thermal conductivity by approximately 10%, with only a 4% increase in dynamic viscosity. With the application of a magnetic field, the effective thermal conductivity can be increased further in the direction of the field, due to the formation of particle chains. Heat transfer can be additionally enhanced by applying a magnetic field to disrupt the flow, but a non-uniform magnetic field is required. Two-phase slug flow using a plug of ferrofluid in an immiscible liquid can have a large and controllable effect on the heat transfer rate.

Purpose-Currently, human hearts destined for transplantation can be used for 4.5 hours which is often insufficient to test the heart, the purpose of this paper is to find a compatible recipient and transport the heart to larger distances. Cooling systems with simultaneous internal and external liquid cooling were numerically simulated as a method to extend the usable life of human hearts. Design/methodology/approach-Coolant was pumped inside major veins and through the cardiac chambers and also between the heart and cooling container walls. In Case 1, two inlets and two outlets on the container walls steadily circulated the coolant. In the Case 2, an additional inlet was specified on the container wall thus creating a steady jet impinging one of the thickest parts of the heart. Laminar internal flow and turbulent external flow were used in both cases. Unsteady periodic inlet velocities at two frequencies were applied in Case 3 and Case 4 that had four inlets and four outlets on walls with turbulent flows used for internal and external circulations. Findings-Computational results show that the proposed cooling systems are able to reduce the heart temperature from +37°C to almost uniform +5°C within 25 min of cooling, thus reducing its metabolic rate of decay by 95 percent. Calculated combined thermal and hydrodynamic stresses were below the allowable threshold. Unsteady flows did not make any noticeable difference in the speed of cooling and uniformity of temperature field. Originality/value-This is the pioneering numerical study of conjugate convective cooling schemes capable of cooling organs much faster and more uniformly than currently practiced.

Thermal performance enhancement of microchannel heat sinks for electronics cooling is becoming more and more necessary. To this end, microchannels with non-circular cross-sections conveying immiscible droplets have been employed in the present study and geometric manipulations are applied to enhance fluid mixing and consequently achieve a higher rate of heat removal. Three-dimensional numerical simulations are performed using volume of fluid method for channels of 100 µm hydraulic diameter. Constant wall temperature is chosen as the boundary condition for heating section of the channels. Effects of parameters such as adding curvature to the side walls and varying the entering velocity of the base liquid on heat transfer rate are studied. A performance coefficient is used to evaluate the relative impact of increase in both the Nusselt number and pressure drop as a result of adding curvature to the channel walls. Results of the study showed that slug flow in curved channels is capable of improving the thermal performance in comparison with single liquid flow in straight channels and in best case, can improve the performance up to 50%.

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This paper presents an experimental and numerical investigation of the flow maldistribution caused by the pressure drop in headers and its impact on the performance of a microchannel evaporator with horizontal headers and vertically oriented tubes. Experimental results show that the flash gas bypass method almost eliminates the quality induced maldistribution. However, refrigerant flow maldistribution caused by the header pressure drop still exists. This is mainly because the pressure drop along the headers results in uneven pressure difference and therefore non-uniform liquid refrigerant mass flow rate across each microchannel tube. A microchannel evaporator model validated by experimental results is employed to quantify header pressure drop induced flow maldistribution. Parametric analysis reveals that such maldistribution impact is significantly reduced by enlarging the outlet header size, increasing heat exchanger aspect ratio, or reducing the microchannel size while other parameters are kept constant. When ratio of outlet header to the total evaporator pressure drop is less than 30%, the cooling capacity reduction is limited below 3%.

This paper proposes a novel solution to reduce the impacts of periodic reverse flow and induced boiling fluctuations on the performance of a microchannel evaporator used in an airconditioning or other similar refrigeration system. Reverse vapor trapped within the inlet header is continuously vented bypassing microchannels. Simultaneous flow visualizations and measurements quantified the effects of venting the reverse vapor on the evaporator performance. The vapor-liquid interface in the inlet header is elevated above all the microchannel inlets, resulting in more uniform liquid distribution among channels. Evaporator surface temperatures measured by an infrared camera oscillate with reduced amplitude (less than 1°C) compared to the situation without venting, indicating more stable boiling heat transfer and twophase flow within the microchannels. The evaporator pressure drop is reduced up to about 15% due to removal of the reversed vapor flow. In addition, the reverse vapor flow is characterized for the first time through this method. Both its average flow rate and oscillation amplitude increase with average heat flux, while the oscillation period is reduced. Compared to the total refrigerant flow rate supplied to the evaporator, the average reverse vapor flow is in the range of 2-8% at the conditions explored.

This work reports results on the drag and heat transfer from an in-line array of three isothermal spheres falling in a cylindrical confinement filled with Bingham plastic fluids. The effects of dimensionless parameters, such as the Reynolds number (1 Re 100), Prandtl number (1 Pr 100), Bingham number (0 Bn 100), blockage ratio (2 b 4) and sphere-to-sphere distance (1.5 t 6) have been elucidated. The flow and heat transfer characteristics were analysed in terms of yielded/unyielded regions, streamline and isotherm contours, drag coefficient, pressure coefficient, and local and average Nusselt number. Broadly, the drag coefficient shows a positive dependence on Bn and sphere-to-sphere distance (t) while it exhibits an inverse dependence on Re and b. On the other hand, the Nusselt number shows a positive dependence on Re, Pr, Bn and b; and a complex dependence on t for each sphere. Simple predictive expressions for the average Nusselt number for each sphere are formulated, thereby enabling its prediction in a new application.

Experimental and numerical analysis of heat transfer and fluid flow in the compact heat exchanger has been done in this paper. In an open circuit wind tunnel, developed on purpose for this investigation, the measurement of working media temperatures and mass flow rates for heat exchanger with microchannel coil has been accomplished. In accordance with the heat exchangers used for experiments, numerical 3D simulation of adequate geometry shapes has been done. With utilization of air/water side numerical simulation, more detailed results have been achieved in relation to the simulation that assumes constant temperature or constant heat flux on the pipe wall. Good agreement between experimentally measured and numerically calculated results has been accomplished. The influence of different microchannel shapes on heat transfer effectiveness and