Channel Heat Research Articles

Optimization of parameters and fabrication of micro channel heat sinks using micro scanning EDM and grey relational analysis

Micro-Channel Heat Sinkisan effective and efficient device that can circumvent excess heat from the batteries of an electrical car. Coolants with less resistances are used as carriers of heat from the batteries through the channels. In this research, a novel approach of fabricating micro channels on copper using Micro-Scanning Electrical Discharge Machining (MSEDM) through a multi-functional transistor and capacitance based micromachine is done. The total thickness of the heat sinkis1.2 mm with0.5 mm of channel depth and channel width. A full factorial design of experiments with the input parameters of Voltage(V), Capacitance(pf)and(μs) are considered to get 32 experiments in total. Grey Relational Analysis (GRA) is used to optimize the response parameters MRR, TWR and Channel dilation. The multiple responses are converted into one single Grey Relational Grade and the optimum parameters are selected. Results show that the capacitance is the most influential parameter followed by voltage and then Ton. Also, a parameter combination of Ton = 90 µs, Voltage = 130 V and Capacitance = 100000pf reduces the Channel dilation, MRR andTWR.

Numerical study on double layered micro channel heat sink with partly diverged channel in top layer

Innovative techniques are essential for heat transfer enhancement in microchannel heat sink to meet the demand of present trends of technology. The performance of the modern electronics system is directly dependent on the cooling capacity of microchannel. The present work has the objective to compare a new microchannel with simple straight channel of double layered pattern. A novel type cross-section of channel in the double layer heat sink has been proposed. Thermal characteristics along with hydraulic profiles are investigated in the work. The addition of bigger flow area at upper section of top layer due to diverged portion helps in decreasing pressure drop remarkably. Significant improvement is observed at higher Reynolds number. The novel cross-sectional sink exhibits lower pressure drop of 52.47 kpa when smooth straight channel shows pressure drop of 57.08 kpa at largest Re.

CFD-Based Simulation and Analysis of Hydrothermal Aspects in Solar Channel Heat Exchangers with Various Designed Vortex Generators

CFD-Based Simulation and Analysis of Hydrothermal Aspects in Solar Channel Heat Exchangers with Various Designed Vortex Generators

Gradient wick channels for enhanced flow boiling HTC and delayed CHF

Liquid supply to wick structures of flow boiling surfaces is fundamentally restricted by the capillary limit at which the pressure drop of the wicked liquid surpasses texture-amplified capillary forces. Gradient wick structures partially decouple permeability and capillary pressure, thereby delaying the capillary limit. In this study, gradient wick channels facilitating out-of-plane liquid delivery are introduced to postpone the capillary limit and thus enhance the two-phase flow boiling heat transfer coefficient (HTC) and delay critical heat flux (CHF). Here, the permeability of the gradient wick channels is augmented by large-pore-size meshes employed near the bulk fluid while capillary pressure is maximized by small-pore-size meshes utilized near the hot boiling surface. This combination of wick structures enables to preferentially guide the cooling liquid, deionized water, from the far-field cold liquid toward the bottom hot substrate. The spatial distribution of individual gradient wick channels promotes separate liquid-vapor pathways, thus facilitating the vapor escape process. Experiments conducted here reveal that the flow boiling performance metrics of the proposed heat sink leveraging the gradient wick channels outperform those of the homogenous wick channels and solid fin channels. The proposed heat sink demonstrates a strong liquid mass flux dependency due to a combination of convective boiling and amplified wickability effects. The enhanced convective boiling could be related to surface roughness, a high number of active nucleation sites, and a large surface area available through tortuous passages of wick channels. At higher mass flow rates, effective capillary pressure available for out-of-plane wicking action also increases, thereby further boosting the wickability effect and associated heat transfer processes. This could meaningfully delay the CHF. In fact, the CHF limit was not observed on the gradient wick channel surfaces in the mass flux and wall superheat range studied. The experimental results indicated a maximum heat flux of 870 W/cm2 with a gradient wick channel heat sink, a 60% improvement compared with a plain copper surface. Furthermore, a maximum HTC of 1000 kW/m2-K at a wall superheat of 3 °C was observed, a three-fold enhancement compared with that of the plain surface. The proposed gradient wick channel topology offers new pathways for designing innovative surface technologies with high heat removal capabilities, thereby potentially improving the energy economy in myriad modern energy applications.

Analysis of flow pattern change in horizontal mini-channel under electric field force

In this paper, the phenomenon of the flow pattern in the channel changes when the electric field force exists. Through the visual observation in the experiment, it is found that the study found that the bubble behavior was suppressed by the electric field force. When the electric field is large enough, the working fluid in the channel will always maintain a single-phase liquid flow. However, When there is enough superheat at the bottom of the mini-channel, the liquid film at the bottom of the liquid phase changes to a vapor phase, and the dryout appears in the channel, and it rapidly changes into a mist flow as the heating power increases. This phenomenon of flow pattern change due to electric field force is referred to herein as a flow pattern special phenomenon. Due to the existence of this special flow phenomenon, the temperature difference between the inlet and outlet of the working fluid in the channel has a large change. Through calculation, it is found that the dimensionless temperature difference between the inlet and outlet at a voltage of 200 V is 1.67 times that of 0 V. At the same time, the channel heat transfer coefficient increases with the increase of voltage.

Enhancement of the turbulent convective heat transfer in channels through the baffling technique and oil/multiwalled carbon nanotube nanofluids

An attempt is made to improve the overall performances of channel heat exchangers. The techniques of baffles and nanofluids are combined to enhance the dynamic and thermal behaviors within the channel exchanger. Baffles under various attack angles are used as vortex generators. In addition, oil/multiwalled carbon nanotubes (MWCNT) is used as a working fluid. Both inclinations in the upstream and downstream directions were considered, referenced as Case A (UIB) and Case B (BIB), respectively. While the channel equipped with vertical baffles is referenced as Case C. The proposed models with combined techniques allowed a considerable enhancement in the overall efficiency. The comparison between the three cases revealed that the most significant value of thermal enhancement factor (TEF) of 5.634 was reached with vertical baffles (Case C) at the highest value of Reynolds number. When using inclined baffles, the 75° upstream attack angle (Case A) allowed the highest TEF of 4.814, compared with Case B.

Numerical Simulation of Frost Formation in Interrupted Micro Channel Heat Sinks Considering Microfluidic Effects in Slip Regime

Frost formation is a renowned phenomenon in HVAC, aeronautical and refrigeration industries. In this paper, numerical modeling and parametric study of the frost formation in the interrupted Micro Channels Heat sinks (MCHS) is investigated considering microfluidic effects in slip flow regime. For numerical modeling, basic equations of humid air and frost including continuum, momentum, energy and phase change mechanism are numerically solved and results are compared with reported data. Knudsen number (Kn) is changed so that slip flow regime requirement is accomplished. This requirement is also considered for setting boundary conditions. The effect of different parameters like cold surface temperature, time and Kn are studied on the frost formation and details of the flow field. Results revealed that with an increase in time and a decrease in Kn and cold surface temperature, weight and thickness of the frost increase. Moreover, with thicker frost maximum flow velocity rises in the microchannel. The details of frost formation and flow field, revealed by the numerical results can remarkably assist designing interrupted microchannel.

Numerical and experimental investigation of high concentration aqueous alumina nanofluids in a two and three channel heat exchanger

Abstract Nanofluids are often able to provide a method to increase the possible heat transfer of a system, with relatively few detrimental factors created by its inclusion. The use of nanofluids and their optimal concentrations has become an area of great interest as of late, with different nanofluid concentrations being key to a systems success or hindrance. The aim of this work is to examine the impact of nanofluid concentration on the possible heat transfer for different heat sink types containing porous media. In this work, both a numerical and experimental investigation into the impact of an aluminum oxide nanofluid on a fluid flow-based system comprised of porous open-cell aluminum foam to examine their impact on the thermal performance of the system was conducted. The porous media implemented in the experimental work was made of a 6061-T6 aluminum with a porosity of 0.91 and a permeability of 2.3869 × 10−7 m2. The experimental work was carried out for a large range of applied heat flux values varying from 3.8328 W cm-2 to 10.3737 W cm−2. The nanofluid examined was also subjected to varying flow rates. The nanofluid concentration was varied from 1% vol and 2% vol Al2O3–water mixture. The nanofluid was suspended in distilled water and mixed using a magnetic mixer. The performance of the work was examined through the Nusselt number to determine the possible thermal enhancement presented by the nanofluid. The use of high concentration (1% vol) nanofluid paired with the use of two different channel designs, both containing porous media resulted in an average thermal enhancement 15% and a maximum enhancement of 24.5% when compared to that of the 2% vol alumina nanofluid.

АКТУАЛЬНОСТЬ МОДЕЛЕЙ ЗАГРЯЗНЕНИЯ ДЛЯ ДИАГНОСТИКИ СОСТОЯНИЯ ПЛАСТИНЧАТЫХ ТЕПЛООБМЕННИКОВ

The article analyzes various theoretical approaches to describing the processes of formation of scale layers on the working surfaces of heat exchangers. The main tasks include analyzing existing models of contamination of heat exchange channels, determining the main mechanism of salt deposition on the heating surface of plate heat exchangers of heat supply systems, determining the main factors that determine the intensity of salt deposition on the working plate in accordance with the dynamics of heat and hydraulic processes in heat exchange channels formed by corrugated plates, as well as forming trends for further research. The article presents the main results of research devoted to the study of contamination processes on heating surfaces. Inaccuracies in the proposed approaches to describing the nature of the formation of salt deposition layers are identified. By generalizing existing approaches to the mathematical description of pollution processes, the main assumptions are revealed when describing the processes of salt deposition on working plates. A hypothesis is proposed about the influence of the location of channels relative to the inlet pipe on the uniformity of the flow distribution between parallel channels in the device. There is a fairly large gap between the existing computational methods for modeling pollution processes and the actual distribution of scale layers during the operation of heat exchange equipment of heat supply systems

Simulation of conjugate heat and moisture transfer in multilayer porous materials with ventilated channels

A physical and mathematical model for calculating the thermal and moisture state of the facade of a building with ventilated channels has been developed. The model is based on the joint solution of a system of non-stationary differential equations of heat and moisture transfer in multilayer porous materials. In addition, equations describing heat and mass transfer in ventilated air channels are included in the model. The calculations of the thermal and moisture state of the brick wall of the building, insulated from the outside with heat-insulating panels with ventilated channels, have shown that the selected geometry of the ventilated channels provides a low level of moisture content in the facade materials.

Numerical simulation of heat transfer during the liquid cooling of complex surfaces at unsteady heat release

The presented mathematical model allows simulating the heat transfer process during the liquid cooling of complex surfaces under conditions of unsteady heat release. The numerical simulation can be provided for the case of film cooling as well as for channel heat exchangers. The presented simulation approach is based on the Finite Volume Method (FVM) which makes it possible to perform the calculations for heat exchangers with complex geometry. Usage of the Volume of Fluid (VOF) method allows simulating the heater cooling by the liquid film with a free surface. The presented mathematical model was tested on a set of specific problems. The simulation results are in satisfactory agreement with the known solutions.

Experimental investigation of mini-channel heat sink with nano-enhanced phase change materials

An experimental investigation is conducted to study the potential of enhancement cooling performance for mini channel heat sink by using different cooling mediums which are air, pure paraffin wax, and nano-enhanced phase change material (NEPCM). Paraffin wax used as phase change material (PCM) and mixed with three types of nanoparticles of alumina (Al2O3 ) and titanium dioxide (TiO2) to improve the thermal conductivity of PCM. Volume fraction values for each type of nanoparticles are (0.1, 0.2, 0.3, 0.4, and 0.5)% which dispersed through PCM. A constant heat flux had been applied to the heat sink base with values (449, 963, 1839, and 4946)W/m2. The results showed enhancement in cooling performance when dispersion the nanoparticles through the PCM which mean reducing the temperature of heat sink base as compared with air and pure paraffin wax. The experimental results also indicated that the cooling performance enhancement of NEPCM and reduction the time of melting process continue with increasing the concentrations of nanoparticles for all material types but any surplus addition may cause negative effect due to sedimentation. Optimum cooling performance of mini heat sink is achieved with Al2O3 –PCM then TiO2-PCM as compared with air with percentage of temperature reduction of 23.306 and 22.069% respectively as compared to air.

Exergetic performance assessment of magnesium oxide–water nanofluid in corrugated minichannel heat sinks: An experimental study

To keep the temperature of miniature devices in the safe threshold, cooling of minichannel heat sinks via nanofluid has emerged as an effective technique. The prime focus of present work is to analyze the impact of channel configuration, nanoparticle concentration, Reynolds number, and heating power on the thermal and exergetic performances of the corrugated minichannel heat sinks by employing distilled water and MgO–water nanofluid as coolants. The heat sinks used in this study are made of aluminium. The volume fractions of nanoparticles used in the experimentation are 0.006%, 0.008%, and 0.01%. From experimental results, comparisons of thermal performance were made between the heat sinks employing different coolants. The heat sink with minimum wavelength (5 mm) and amplitude (0.5 mm) displayed the lowest wall temperature of 33.86°C, while 28.75% enhancement in Nusselt number for 0.01 vol.% nanofluid as compared to distilled water. The enhancement in nanoparticle concentration and Reynolds number showed an increase in exergy efficiency and outlet exergy of coolants, whereas a reduction in thermal resistance and logarithmic mean temperature difference of heat sinks.

Three dimensional unsteady heat and mass transport from six porous moist objects in a channel under laminar forced convection

Three dimensional unsteady heat and mass transport features for six identical porous moist objects are studied in a channel with hot dry air under laminar flow conditions. Direct coupling between the channel flow and porous moist objects is considered while the governing equations for flow in channel domain and heat and mass transport in porous moist objects are solved simultaneously. In this case, specification of local heat and mass transfer coefficients on the porous object boundaries is not needed and it is accurate since these coefficients are time dependent and spatially varying along the objects when multiple porous objects in three dimensional unsteady configuration are considered. Numerical simulations are performed with Galerkin weighted residual finite element method. The simulation is performed for varying values of hot air velocities (between 0.15 m/s and 0.5 m/s), inlet temperatures (between 303 K and 343 K) and spacing between the moist objects in the flow direction (between 0.4H and 0.9H). The impact of the spacing on the heat and mass transfer features is profound for the last block in the array (B5) while it has slight impact on the first two blocks (B1 and B2) in the array. The variation in the reduced moisture content is 13% for block B5 while it is only 2% for block B1 when cases with the lowest and highest distances are compared. Hot dry air temperature has the most impact on the moisture reduction for the first two blocks B1 and B2 while the velocity impact is very influential for the last block B5. As the lowest and highest hot air temperatures are compared, 33% rise in the reduced moisture content is obtained for block B2, but it is only 12% for block B5. An efficient modeling strategy based on proper orthogonal decomposition is used for the approximation of temperature and vapor concentration in the computational domain with 10 modes for temperature and 45 modes for vapor concentration.

Experimental study of R744 heat pump system for electric vehicle application

In this paper, two types of refrigerant, R134a and R744, are studied with different heat pump (HP) structures. Gas cooler, evaporator and indoor gas cooler (IGC) are all micro channel heat exchangers. One four-way valve and a combination of accumulator with internal heat exchanger (ACCU/IHX) are applied in R744 HP system, which can make the heat pump system structure simpler. In addition, one bidirectional throttle valve is applied before the evaporator to obtain the cooling and heating functions for R744 HP system. And the indoor gas cooler and evaporator are in series structure not only for the heating mode but also for the cooling mode in order to increase the heat transfer surface in the R744 system. The system charging test is carried out first to determine the optimal charge amount for the whole system, since it is essential for the stability of the system. Then the R744 and R134a heat pump system cooling and heating performance are studied by the experiment at different ambient working conditions. Test results show that the R744 system proposed in this paper can achieve similar cooling capacity and COP to that of the R134a heat pump. For R744 system, under 40℃ ambient working condition with external air circulation, the cooling capacity can reach 7524 W with COP of 1.36. And the R744 heat pump system can provide higher heating capacity and higher COP than the R134a heat pump system. Especially in the very cold weather, such as the −20℃ and −25℃, the R744 heat pump still can provide enough heating capacity with higher COP. Compared to the traditional R134a heat pump system, the heating capacity of R744 system increases by 83%, reaching 7378 W (R134a, 3994 W) at 6000 revolutions per minute (RPM) under −10℃ ambient temperature conditions. Under −20℃ ambient temperature with external air circulation, the maximum heating capacity is 7502 W with COP of 2.16.

Equal Channel Angular Pressing Consolidation and Heat Treatment of Blended Elemental Powders of Fe-Mn-Al

In this work, the consolidation of blended elemental powders of iron, manganese and aluminum (Fe-25Mn-15Al wt.%) was performed by Equal Channel Angular Pressing (ECAP). Samples were consolidated at room temperature in a Φ = 120° die by a single pass and a second pass in route A. Both samples were heat treated at 650 °C and water cooled. Prior to heat treatment, samples presented a dense but chemically inhomogeneous structure. Fe and Al particles were highly deformed, whereas, Mn was almost undeformed. Mn particles were partially shattered by friction with Fe and Al particles. After heat treatment, the samples were characterized by SEM-EDX and presented substantial interdiffusion along the particles interfaces. It is believed that higher deformations by ECAP may improve the sinterability of consolidated samples in order to densify and chemically homogenize it.

Wall Heat Flux Evaluation in Regeneratively Cooled Rocket Thrust Chambers

In the present Paper, different wall heat flux evaluation methods for rocket engines are analyzed and compared. The test case for the comparison is a supercritical load point of an upper-stage engine. The evaluation of the heat flux using existing gradient methods shows unacceptable deviations, whereas significant improvements are found with an inverse heat transfer method. Using a Nusselt number correlation for the modeling of the cooling channel heat transfer coefficient, good agreement with the experimental profiles is achieved, while the number of installed thermocouples is held at a minimum. However, the choice of the Nusselt number correlation directly influences the obtained results and strongly depends on the fuel and geometry of the system. Using a simultaneous optimization of the heat flux and heat transfer coefficient, on the other hand, seems to eliminate the need for additional modeling and leads to great agreement with the experiment. A new optimization method proposed in this Paper based on a preprocessed Jacobi matrix significantly speeds up the estimation of the free parameters. With the obtained results, the placement of the thermocouples before the design of the hardware can be optimized, leading to reduction of manufacturing costs.

Investigation and Modeling of the Magnetic Nanoparticle Aggregation with a Two-Phase CFD Model

In this paper the magnetic nanoparticle aggregation procedure in a microchannel in the presence of external magnetic field is investigated. The main goal of the work was to establish a numerical model, capable of predicting the shape of the nanoparticle aggregate in a magnetic field without extreme computational demands. To that end, a specialized two-phase CFD model and solver has been created with the open source CFD software OpenFOAM. The model relies on the supposed microstucture of the aggregate consisting of particle chains parallel to the magnetic field. First, the microstructure was investigated with a micro-domain model. Based on the theoretical model of the particle chain and the results of the micro-domain model, a two-phase CFD model and solver were created. After this, the nanoparticle aggregation in a microchannel in the field of a magnet was modeled with the solver at different flow rates. Measurements with a microfluidic device were performed to verify the simulation results. The impact of the aggregate on the channel heat transfer was also investigated.

Combination of baffling technique and high-thermal conductivity fluids to enhance the overall performances of solar channels

In the present study, two-phase flow and forced-convection heat transfer of hydrogen gas (H2) in a solar finned and baffled channel heat exchanger (SFBCHE) is studied numerically. The effect of different obstacles in the channel is addressed. A H2 heat transfer fluid (HTF) having a high thermal conductivity with the baffling technique is implemented to enhance the overall performance of a solar channel. In the initial step, the results from the proposed numerical model were compared with the experimental data of a smooth channel, and then against data with a baffled channel. After checking the validity of our model, the same numerical approach was used for studying thermal-fluid characteristics of the channel with the new fluid. A hydrothermal analysis is presented for a range of Reynolds number (Re) from 5000 to 25,000. At the lowest Re = 5000, the thermal enhancement factor (TEF) is about 1.25. This value increases to 2.16, or 73.46%, when Re = 10,000. This increase in the TEF values continues as Re increases. The largest Re = 25,000 gives the highest TEF value, as it is about 4.18, which is 2.75 times greater than that given for the case of using the conventional gaseous fluid (air). Therefore, our proposed structure for the SFBCHE with high H2 HTF flow velocity leads to improve the values of dynamic pressure (Pd) and heat transfer (Nu), while reducing the skin friction (f) values, which increases the overall TEF of the channel. In addition, all performance values are greater than unity (or 1.00). This reflects the importance of the H2 HTF baffling and finning technique in improving the hydrodynamic thermal-energy performance of solar heat exchangers. The suggested model of SFBCHE filled with an H2 HTF having a high thermal conductivity allows a considerable enhancement in the overall thermal performances which can be employed in various thermal types of equipment, such as solar energy receivers, automotive radiators, and cooling in chemical industries.

Thermo-hydraulic analysis of compact heat exchanger for a simple recuperated sCO2 Brayton cycle

Supercritical carbon dioxide (sCO2) Brayton cycle-based power plants are being extensively explored as viable alternatives to traditional Rankine based steam power plants. Higher temperatures at the turbine exit in a sCO2 cycle provide better opportunities for heat recuperation, thus improving overall cycle efficiency. Among the various heat exchanger configurations available, compact microchannel heat exchangers commonly known as Printed Circuit Heat Exchangers (PCHE's) are typically used in sCO2 power plants. In the current work, a hybrid approach comprising of a Thermal Resistance Network (TRN) model coupled with a CFD model for estimating local heat transfer and pressure drops is presented for a PCHE core with straight and zigzag channel configurations. Full-scale TRN model is used to optimize the overall stack dimensions based on minimum rate of heat loss from the external surfaces of the PCHE core. The TRN model accounts for the thermo-physical property variations of sCO2 along the channel length to effectively capture the channel pressure drop and heat transfer. This is achieved by discretizing the heat transfer domain comprising of alternatively stacked hot and cold streams into sub-heat exchangers to account for variations in thermophysical properties while calculating the nodal friction factors and local heat transfer coefficients. CFD simulations are performed for a full length of a single stack of hot and cold fluid streams to arrive at corrected heat transfer and pressure drop correlations. Thermo-hydraulic analysis is performed for a range of channel hydraulic diameters and channel mass flow rates for both straight and zig-zag configurations to deduce optimum stack dimensions. The efficacy of the hybrid model is demonstrated with a case study of a counterflow recuperator used in a simple recuperated 1 MW sCO2 Brayton power plant.