Rectangular Heat Research Articles

Simultaneous Optimization of Geometric and Nanofluids Parameters in a Rectangular Microchannel Heat Sink

The use of nanofluids as coolant fluid in the microchannel heat sink (MCHS) is an effective technique for improving the thermal performance of electronic devices. A comparative study is performed with the help of a multi-objective genetic algorithm (MOGA) to find the optimal geometric variables of the MCHS and choose the appropriate nanofluid. For that purpose, four practical nanofluids, including Al2O3-water, Cu-water, SiO2-water, and carbon nanotube (CNT)-water, are thoroughly investigated. Simultaneously minimization of the total thermal resistance and pumping power consumption (POW) are considered as the optimization goal, and the MOGA is employed to achieve the optimal solution. To check the accuracy of the thermal resistance modeling and assessing the optimization algorithm, several case studies with a different number of optimization variables are defined to investigate the capability of the algorithm in finding the soptimal microchannel design variables and choosing the suitable nanofluid. The optimization variables consist of the channel aspect and wall ratios, base thickness, nanoparticles volume concentration, nanoparticles diameter, and the volume flow rate. Compared to other nanofluids, CNT provides better thermal performance. Furthermore, increasing the volume concentration of nanoparticles enhances thermal performance, which can also be achieved through the reduction of nanoparticles diameter.

Numerical investigation of flow boiling characteristics in cobweb-shaped microchannel heat sink

In recent years, researchers have conducted extensive studies on the improvement of heat transfer performance of microchannels, however, exploiting biomimetic microchannels to enhance the flow boiling heat transfer performance is still relatively rare. In this study, inspired by the cobweb structures in nature, cobweb-shaped microchannels with horizontal inlet and outlet (CMHS-H) and cobweb-shaped microchannels with inclined inlet and outlet (CMHS-I) are proposed. Flow boiling simulations are carried out with the inlet temperature of 300 K at the bottom heat flux of 75–125 W/cm2 under different mass fluxes by utilizing volume of fluid (VOF) model. The flow boiling characteristics of the CMHS-I and CMHS-H are studied and compared with those of the rectangular microchannel heat sink (RMHS). The results demonstrate that both the CMHS-H and CMHS-I can enhance the heat transfer coefficient and reduce the wall temperature due to the flow disturbance and increased heat transfer area; whereas the CMHS-H is effective to reduce the pressure drop and suppress the flow instability at high mass flux. At high heat flux, the CMHS-H shows the best heat transfer performance and the most stable flow boiling behavior. This study provides a promising approach of using biomimetic microchannels to dissipate high heat flux associated with advanced electronics.

Effects of Parameters on Heat Transfer Characteristics of a Rectangular Cross Section Heat Pipe with Mesh Wick

This research investigated effects of parameters on heat transfer characteristics of a rectangular cross section heat pipe with mesh wick (RHP/MW). The working substance used Distilled Water and Refrigerant R-11. Heat loads at the evaporator were set at 20, 40, 60 and 80 W. Results revealed that, throughout the test, RHP/MW/ R-11 gave the highest heat transfer rate and average heat flux of 26.91 W and 2054.57 kW/m2 respectively. Heat Load of 80 W produced the lowest total thermal resistance of 0.4388 °C/W with the highest average thermal efficiency of 52.91%, the highest efficiency among the test cases. This is due to an increase of heat receiving areas of RHP/MW. More heat is transferred through the pipe wall to the wet perimeter of the working substance which caused the working substances in the evaporator to boil faster. The mesh wick also helped the condensed working substance to disperse around the inner tube wall and resulted in a thin film of the condensate liquid. This resulted in a higher heat transfer rate and heat flux for the RHP/MW. Mesh wick also generated capillary pressure that drew the condensed liquid back to the evaporator. The maximum capillary pressure obtained from the test was 93.29 N/m2 when the heat load of 80 W was applied to the RHP/MW/R-11. RHP/MW/R11 greatly enhances the heat transfer characteristics. Thus, it is suitable for application as an alternative to specific work sites that require smooth contact with heat receiving areas.

Constructal Optimization of Rectangular Microchannel Heat Sink with Porous Medium for Entropy Generation Minimization

A model of rectangular microchannel heat sink (MCHS) with porous medium (PM) is developed. Aspect ratio of heat sink (HS) cell and length-width ratio of HS are optimized by numerical simulation method for entropy generation minimization (EGM) according to constructal theory. The effects of inlet Reynolds number (Re) of coolant, heat flux on bottom, porosity and volume proportion of PM on dimensionless entropy generation rate (DEGR) are analyzed. From the results, there are optimal aspect ratios to minimize DEGR. Given the initial condition, DEGR is 33.10% lower than its initial value after the aspect ratio is optimized. With the increase of Re, the optimal aspect ratio declines, and the minimum DEGR drops as well. DEGR gets larger and the optimal aspect ratio remains constant with the increasing of heat flux on bottom. For the different volume proportion of PM, the optimal aspect ratios are diverse, but the minimum DEGR almost stays unchanged. The twice minimized DEGR, which results from aspect ratio and length-width ratio optimized simultaneously, is 10.70% lower than the once minimized DEGR. For a rectangular bottom, a lower DEGR can be reached by choosing the proper direction of fluid flow.

Optimization of porous fin location and investigation of porosity and permeability effects on hydro-thermal behavior of rectangular microchannel heat sink

Rectangular microchannel heat sink incorporated with porous fin is studied to improve the thermal performance of conventional solid fin microchannel. Three-dimensional microchannel with fixed aspect ratio and hydraulic diameter is considered and porous fin with three different thickness have been analyzed by mounting at various locations. Copper is used as porous medium with a porosity of 0.4 to 0.8 and permeability of 10−8 - 10−12 m2 are considered. The working fluid is water and the simulations were carried out for the Re range, 500 ≤ Re ≤ 1000. The performance improvement of the porous fin over solid fin is presented with the help of slip length. Pressure reduction efficiency is observed by comparing pressure drop values with and without slip effect. The parametric optimization of fin location has been done with the help of combined hydro-thermal performance. Further, the effect of porous parameters like porosity, permeability and quadratic drag factor on pressure drop and heat transfer coefficient is studied for various fin thicknesses. It is observed that the porous fin with porosity 0.4 and permeability 10−8 m2 has shown better heat transfer characteristics.

Discriminating BTX Molecules by the Nonselective Metal Oxide Sensor-Based Smart Sensing System.

Discriminating structurally similar volatile organic compounds (VOCs) molecules, such as benzene, toluene, and three xylene isomers (BTX), remains a significant challenge, especially, for metal oxide semiconductor (MOS) sensors, in which selectivity is a long-standing challenge. Recent progress indicates that temperature modulation of a single MOS sensor offers a powerful route in extracting the features of adsorbed gas analytes than conventional isothermal operation. Herein, a rectangular heating waveform is applied on NiO-, WO3-, and SnO2-based sensors to gradually activate the specific gas/oxide interfacial redox reaction and generate rich (electrical) features of adsorbed BTX molecules. Upon several signal preprocessing steps, the intrinsic feature of BTX molecules can be extracted by the linear discrimination analysis (LDA) or convolutional neural network (CNN) analysis. The combination of three distinct MOS sensors noticeably benefits the recognition accuracy (with a reduced number of training iterations). Finally, a prototype of a smart BTX recognition system (including sensing electronics, sensors, Wi-Fi module, UI, PC, etc.) based on temperature modulation has been explored, which enables a prompt, accurate, and stable identification of xylene isomers in the ambient air background and raises the hope of innovating the future advanced machine olfactory system.

Numerical analysis of the solidification process of water used as PCM in a rectangular latent heat thermal storage unit

A numerical analysis of the solidification process of water used as phase change material (PCM) has been carried out in a rectangular latent heat thermal storage unit. The major heat transfer phenomena involved in such a process were numerically characterized using the CFD code Star CCM+. During the solidification process, the flow and heat transfer were analysed through vector field, temperature and solid fractions contours. Quantitative global results such as the temporal evolution of the average temperature of the PCM were also provided during the solidification process. The present study shows that the natural convection plays an important role in heat transfer kinetics during solidification process.

Enhanced heat dissipation of truncated multi-level fin heat sink (MLFHS) in case of natural convection for photovoltaic cooling

The increase in module operating temperature above 25 °C can be detrimental to the electrical output performance of photovoltaic (PV) modules. The study introduces a novel truncated multi-level fin heat sink (MLFHS) subject to natural convection for photovoltaic (PV) cooling. The proposed fin geometry aims to improve the heat transfer by inducing abrupt change within the boundary layer in the fin confined region through the multi-level fin height. The effect of fin spacing, fin height, and fin thickness on the heat transfer properties of the MLFHS was investigated numerically using finite element analysis (FEA). The parametric study was conducted to compare the thermal performance of the MLFHS with the conventional rectangular plate-fin heat sinks by varying the fin spacing (p = 10–35 mm), fin height (h = 60–120 mm), and fin thickness (t = 0.5–1.5 mm). The results indicate that the average temperature decreases first and increases with fin height, fin spacing, and fin thickness. The findings were promising with an optimized fin heat sink showing a decrease in the average PV module temperature as high as 6.13% and improved power output by 2.87%. The proposed fin geometry provides a promising heat transfer approach for PV cooling applications.

Analytical closed-form solution for fluid flow through microchannel heat sinks including axial conduction

An analytical study on the convective heat transfer through a rectangular microchannel heat sink including solid axial conduction, has been performed. The system has been split into two coupled boundary value problems, each addressing the heat transfer to the fluid through different routes (namely, the sidewall or fin and the base wall or substrate). The results are derived in the form of a net wall and fluid temperature rise. The validation of the proposed analytical model has been done using existing experimental data and a three-dimensional conjugate computational model. The results show excellent agreement in terms of solid and fluid temperature distributions. The parametric studies show an increasingly non-linear behaviour in the solid and fluid temperature profiles in the microchannel, if axial conduction is dominant. The simulated results are consistent with the CFD and experimental outcome. It has also been noted that a uniform heat flux applied at the bottom of the microchannel (substrate) cannot ensure the transfer of heat uniformly to the fluid. If axial conduction is high, a significant fraction of the applied heat influx is transmitted to the fluid near the channel entry region with a corresponding reduction in the flux distribution at the channel exit.

Numerical research on the solidification heat transfer characteristics of ice thermal storage device based on a compact multichannel flat tube-closed rectangular fin heat exchanger

In this study, a compact ice thermal storage device that combines multichannel flat tube and closed rectangular fins is designed to improve solidification performance. To analyze the heat transfer characteristics and visualize the ice formation process, a numerical model was formulated and solved using enthalpy-porosity method. The temperature distribution, liquid fraction, and ice front evolution were studied during solidification process. The effect of heat transfer fluid on the performance of storage device at different inlet temperatures and flow rates was numerically investigated. Results show that when the inlet temperature was reduced from -3 °C to -6 °C, the ice packing factor increased by 32.2% and the storage power forms a linear relationship with the inlet temperature. When the flow rate was increased from 0.1 m3/h to 0.2 m3/h, the ice packing factor increased by 8.7% but the effect of flow rate was less obvious than the inlet temperature. Furthermore, the heat transfer characteristics of closed rectangular fins were compared with traditional longitudinal fins. We found that the former can achieve more uniform temperature distribution and a higher ice formation rate. The average solidification enhancement ratios of the both are 2.28 and 2.08, respectively, the former being 9.6% higher than the latter.

Heat transfer factor j and friction factor f correlations for offset strip fin and wavy fin of compact plate-fin heat-exchangers

A unique model of heat transfer factor "j" and friction factor "f" are developed for different types of fins, which are rectangular offset strip fin, rectangular wavy fin compact heat exchangers by considering the geometric variables such as plate space (Hp), fin pitch (Pf), hydraulic diameter (Dh), wave amplitude (Aa), fin length (λ), and Reynolds number (Re). The proposed j and f models provided 98% of experimental data points within ±20% and 82.3% of experimental data points within ±10% accuracy for OSF fins. Also, they provided 100% of Kays and London's experiential data points within ±5% accuracy for wavy fins. These proposed j and f models for a wide range (120–10000) of Reynolds number flows, including laminar and turbulent flows. j model provides maximum RMSPE 15% and 2% accuracy values for PFHE offset strip fins and wavy fin geometries, respectively. f model provides maximum RMSPE 17% and 3% accuracy values for PFHE offset strip fins and wavy fin geometries, respectively. Based on RMSPE and experimental data analysis, proposed j and f models were validated and better than existing models for offset strip fins and wavy fins.

Study of Bridge Rectangular Fin Heat Sink and the Effect of Angle Change on CPU Cooling

Study of Bridge Rectangular Fin Heat Sink and the Effect of Angle Change on CPU Cooling

Numerical study on the thermal performance of high-temperature latent heat packed-bed with a rectangular heat storage unit

As one of the most effective methods of thermal energy storage, packed-bed (PB) systems using phase change material (PCM) as heat storage media have been vigorously developed. To obtain the thermal performance of brick structure heat storage unit (HSU) in high-temperature latent heat PB, a 3-D PBsystem based on Navier-Stokes equations for heat transfer fluid, phase change model for PCM, and surface-to-surface (S2S) radiation model is established. The idea of changing the geometric structure of the brick HSU is proposed to improve the thermal performance of PB system. The influence of three-dimensional size of carbonate-based composite PCM with brick structure on the thermal performance of high-temperature PB is studied systematically. The numerical analyses show that the influence of three-dimensional size of brick-type HSU in different directions on the thermal performance of the PB system is independent. And the system has a higher energy efficiency (66.5%) and lower heat loss when the width and height of HSU are around 25 mm. The research results can guide the optimization of high-temperature composite PCM PB system.

Heat transfer enhancement in mini channel heat sinks utilizing corona wind: A numerical study

A full-scale three-dimensional model of a rectangular channel mini heat sink is developed to numerically investigate the feasibility of heat transfer enhancement utilizing an external electric field. Heat transfer mechanism in the presence of corona wind induced by a wire electrode and its favorable impact on the hydrodynamic and thermal characteristics of working fluid coolant are elaborately discussed. Effects of various parameters such as number of electrodes, electrode radius, electrode location, and electrode-flow configuration as well as the external applied voltage and Reynolds number on heat transfer enhancement are investigated. This study indicates that the electric field creates a vortex which in turn causes flow mixing in the vicinity of the heated surface, disturbs the thermal boundary layer and consequently increases the heat transfer rate. Results show that heat transfer enhancement is directly and inversely proportional to the voltage and Reynolds number, respectively. Furthermore, it is indicated that increasing the number of electrodes leads to a significant reduction in the heat transfer rate unfavorably impacting the heat transfer enhancement. It is attributed to the simultaneous interaction between the adjacent induced vortices. Eventually, it is shown that perpendicular configuration of the flow-electrode layout performs better than its parallel counterpart where an induced lateral air flow reduces the heat transfer augmentation. A maximum heat transfer enhancement of 439% compared to that of without any electrical field case is achieved by using a single electrode in the perpendicular electrode-flow layout.

Dynamic Character of Thermal Analysis Where Thermal Inertia Is a Real and Not Negligible Effect Influencing the Evaluation of Non-Isothermal Kinetics: A Review

The development of instrumentation has allowed thermal analysis to become a widely used method not only in calorimetry but also in the field of non-isothermal kinetics that, however, provides a simplified philosophy of measurements. From the beginning, a methodology is used describing the course of reaction in a simplified temperature regime measured in an inert sample. In a most common case of DTA, the degree of reaction is subtracted from the partial areas of the as-cast peak in the unified mode of the peak linear background. Usually, the effect of thermal inertia, resulting from the reality of heat transfer and changing the peak background to a non-linear s-shaped form, is not incorporated. Therefore, the question of whether or not to include this effect of thermal inertia has become a current underlying problem of thermo-analytical kinetics. The analysis of the rectangular input heat pulses and their DTA responding fundamentally point to the need to include it thus becoming essential and not negligible. In the case of parallel evaluations, the effect of inertia can be partially compensated for each other such as in the Kissinger evaluation method. The study presents a broad overview of the thermo-analytical methodology used and points to the often-neglected literature. However, standard mainstream kinetics procedures need be fixed, and an improved solution found to account for the effect of heat transfer and dissipation, which is becoming the focus of thermal analysis methods of future and also the intention of this review.

Rectangular heat sink filled with PCM/ hybrid nanoparticles composites and cooled by intruded T-shaped cavity: Numerical investigation of thermal performance

The thermal storage efficiency of the ice within a rectangular system, which is charged from three sides and is cooled by an intruded T-shaped cavity, is low. It is due to the low thermal conductivity of the ice. The use of metallic nanoparticles inside ice to boost the thermal diffusion is limited due to the stability and chemical reaction. To remove this shortcoming, water-based hybrid nanofluids (NFs), which possess a lower thermal conductivity with inertness, have been used.In this study, the influences of the inclination angle and hybrid nanopowders, Ag 25 nm-MgO 40 nm (50:50 weight proportion), Al2O3-Cu 17 nm (90:10) and SiO2-MWCNT 10 nm (50:50) with the volumetric concentration ≤ 0.02 on the evolution of the convection-controlled melting in this novel thermal energy storage unit have been evaluated. An iterative explicit enthalpy-based double distribution function-lattice Boltzmann model within the single-phase framework has been applied where there is the thermal equilibrium condition between solid NPs and the base PCM. It is revealed that silica-MWCNT NPs/water composite with a volume fraction of 0.01 inside the case (I) is the best configuration to obtain the least melting time with a higher thermal storage capacity despite the high price of NPs.

Hydromagnetic mixed convection in a triangular shed filled by nanofluid and equipped with rectangular heater and rotating cylinders

Nanofluid mixed convection in a triangular shed equipped with rotating cylinders subjected to a heat source is numerically investigated in this study. The shed is heated and cooled respectively from a rectangular heat source at the bottom wall and inclined top walls. Two rotating cylinders are placed over the heat source. The shed is permeated by an external magnetic field. The conservation equations are solved using finite element method. The code is verified by comparisons with previously published results. The numerical results of flow and temperature fields are demonstrated via streamlines, isotherms and bar charts for the variation of key parameters: Reynolds number ( 0 ≤ Re ≤ 100 ), Hartmann number (0 ≤ Ha ≤ 50), nanoparticle volume fraction (0% ≤ φ ≤ 5%), rotational speed of cylinders (10 ≤ U c ≤ 100) and different positions of heat source. The strength of flow circulation is found accelerating with increasing Reynolds number and rotational velocity of cylinders but it declines for the effects of magnetic field and nanoparticle volume fraction. The thermal field is significantly influenced due to the variation in Reynolds number, cylinders rotational speed and the position of heat source. Maximum heat transfer is found at the corner positions of heat source, and it is 13.70% more than heat transfer for the case of centered position. Optimum heat transfer performance is taken place at higher rotational speed of the cylinders whereas reverse trend for higher magnetic strength. The best heat transfer rate is achieved in nanofluid with maximum concentration of nanoparticles (5%), which is 94.18% than heat transfer for base fluid water.

Temperature prediction of heat sources using machine learning techniques

AbstractThis paper explores the use of machine learning algorithms, such as XGBoost, random forest regression, support vector machine regression, and artificial neural network (ANN), which are employed for predicting temperatures of rectangular silicon heaters with dummy elements. A combination of these machine learning algorithms can predict better results over individual algorithm. Silicon heaters are equipped on an FR4 substrate board for cooling under forced convection in a horizontal channel. COMSOL Multiphysics 5.4 software is used for all the three‐dimensional numerical simulations. Heat transfer at the solid and fluid interface is studied using a module based on conjugate heat transfer and nonisothermal fluid flow. Dummy elements are coupled with heated sources to evaluate heat transfer and analyze the flow of fluid. The study is performed with 2.5 m/s velocity and a uniform heat flux of 5000 W/m2. The study is aimed at predicting and comparing results of support vector regression (SVR), ensemble learning with ANN to explore optimal configuration. Results indicate an agreement of less than 10% between the simulated and predicted temperatures. It is also found that SVR has given the best results compared with XG Boot and ANN when analyzed individually. The programming for these algorithms is performed using the Python programming language.

Numerical investigation of the melting process in a half horizontal cylinder with radial fins

In the present study, the melting phenomenon in a half horizontal cylinder cavity with rectangular heat sources is investigated using numerical lattice Boltzmann method. The cylinder is modeled two dimensionally and its boundaries are insulated. Also, the high temperature rectangular heat sources with constant area are located in radial position on cavity. The enthalpy-based lattice Boltzmann method is used for simulating the phase change problem in the cavity. Melting of lead with Stefan number of 0.86 and Prandtl number of 0.0236 is simulated in the cavity. The effects of pertinent parameters such as the aspect ratio of the fins 1,2.5,5,7.5, the Rayleigh number 103,104,105 and the angular position of the heat sources 0,15,45 are studied on the melting phenomenon. The results show that increasing the aspect ratio of the fin with constant area reduces the melting time and increases the liquid fraction at the same time. Also higher value of the Rayleigh number improves the natural convection effect and raises the rate of melting. Finally, it is observed that the highest rate of melting is obtained when two fins are positioned at 45 degrees with aspect ratio of 7.5 at Rayleigh number of 105.

Natural convection improvement of PCM melting in partition latent heat energy storage: Numerical study with experimental validation

This study examines an innovative design to enhance the natural convection in a rectangular latent heat energy storage unit (LHSU). The new design divides the rectangular plane into different small partitions (1, 2, 4, 8, 16 and 32) to trap the melted phase change material (PCM) and create several melting fronts which accelerate the melting process than with a normal 1-partition LHSU. The number of these partitions was varied by changing the PCM cavity aspect ratio with values 2, 1, 0.5, 0.25, 0.125 and 0.0625 taken. To validate the numerical model developed herein, LHSUs with one and two partitions were tested experimentally. The numerical model using the ANSYS FLUENT, extended the analysis to other partition numbers. Results showed that the natural convection was significantly strengthened by partitioning the LHSU, which shortens the time required for PCM melting. However, an optimal number of partitions were obtained after which improvement ratio was declined. An LHSU with 8 partitions (0.25 aspect ratio partitions) was found as an optimum. In addition, the comparison with a normal 1-partition LHSU found that the PCM melting process was enhanced by 31.2%, 53.6%, 65% and 68% for the LHSUs with 2, 4, 8, and 16 partitions, respectively.