Discrete Heat Sources Research Articles

Analysis of grain tribology and improved grinding temperature model based on discrete heat source

A high grinding temperature will cause thermal damage to a workpiece surface and deterioration of surface integrity, which is the bottleneck of grinding. The present grinding temperature theoretical model is based on grain homogeneity and the continuous heat source distribution in the grinding zone. However, the random change in interference between the effective grains and a workpiece during the machining process causes a change in the grain tribological properties, resulting in varying transient grinding temperatures. Based on the current situation, the grain tribological mechanism and an improved temperature model based on a discrete heat source are proposed to reveal the temperature variation law of a workpiece in an actual grinding process. First, the surface topography model of a grinding wheel is established based on the geometric characteristics of grains, and the determination mechanism of effective grains is revealed. Furthermore, the interference mechanical behavior of the grains and workpiece is analyzed according to the kinematic law of grains in the sliding, plowing, and cutting stages. The mechanical model and specific grinding energy model at different stages are established, and the thermal distribution mechanism of effective grains is revealed. Finally, a temperature field mathematical model of a discrete heat source is established, and numerical simulation is performed to demonstrate the dynamic temperature change process of different grains. A new experimental method for measuring temperature at different positions of the workpiece with a bipolar thermocouple array is designed, and a regional numerical simulation and experimental temperature comparison method is innovatively proposed. Experimental results show that the grinding temperature measured under different cutting depth conditions is in good agreement with the numerical results, and the variation law is consistent. The minimum error in 64 groups of experimental measuring and numerical calculation comparison zones can reach 4.9%, and the proportion of zones with errors less than 10% can approach 86%. This study will provide a theoretical basis for the accurate suppression of workpiece surface thermal damage and the development of precision grinding in engineering discipline and machinery industry.

Conjugate free convection from an array of discrete heat sources with water and nano-encapsulated phase change particle in a cold enclosure

The present works investigates the conjugate free convection in a cold enclosure containing a hot circular cylinder. An array of discrete heat sources are attached to the cylinder surface. The free space between the enclosure and the cylinder was filled with a heterogeneous mixture of water and nano-encapsulated phase change particle (NEPCP). This nanoparticles have an outer shell and a core inside. Specifically, the NEPCP core can absorb thermal energy on charging state or releases energy on discharging state. The governing parameters considered are the number of heated segment, 1≤n≤4, the heat source thickness, 0≤A≤0.1, the fusion temperature, 0.1≤Θf≤0.5, the heating intensity, 104≤Ra≤106, and the NEPCP concentration, 0≤ϕ≤0.05. The results shows that the Θf, n, A, and Ra modify the position and size of the area of phase change zone. The maximum Nusselt number was found at different angular positions depending on the heat source arrangement. The maximum value increases by increasing the NEPCP concentration, but its position almost unchanged by adjusting the concentration. There is critical A=Ac and max(ϕ) for the optimum arrangement of discrete heat source.

Numerical Investigation on the Heat and Mass Transfer in Microchannel with Discrete Heat Sources Considering the Soret and Dufour Effects.

Heat-transfer enhancement in microchannel heat sinks (MCHS) has been a hot topic in the last decade. However, most published works did not focus on the heat sources that are discrete, as in most microelectronic devices, and the enhancement of heat and mass transfer (HMT) due to the Soret and Dufour effects being ignored. Based on a heterogeneous two-phase model that takes into consideration the Soret and Dufour effects, numerical simulations have been performed for various geometries and heat sources. The numerical results demonstrate that the vortices induced by a heat source(s) can enhance the heat transfer efficiency up to 2665 W/m2·K from 2618 W/m2·K for a discrete heat source with a heat flux q = 106 W/m2. The Soret effect can affect the heat transfer much more than the Duffour effect. The integrated results for heat transfer due to the Soret and Dufour effects are not sampled superpositions. Discrete heat sources (DHS) arranged in microchannels can enhance heat transfer, especially when the inlet velocity of the forced flow is less than 0.01 m/s. This can provide a beneficial reference for the design of MCHS with DHS.

Characterization of a Jet Impingement Heat Sink for Power Electronics Cooling

• Heat transfer correlations for a jet impingement heat sink for cooling power electronics are presented. • Pressure drop across the jet impingement heat sink investigated. • Heat transfer to pressure trade-off is critical for heat sink design. • Jet impingement provides good cooling uniformity. • Heat transfer is enhanced with decreasing stand-off distance and increasing jet-to-jet spacing. Ongoing demand to improve power electronic converters in terms of efficiency, power density, reliability, cost and packaging calls for optimal thermal management. This paper investigates the effectiveness of a jet impingement heat sink consisting of an array of jets impinging discrete heat sources. A mixture of 60 – 40% (by volume) ethylene glycol – water is the coolant used. Through computational simulations and experimentation on a laboratory prototype, the heat transfer and pressure drop of the heat sink is characterized in terms of Reynolds number, Prandtl number, stand-off distance and jet-to-jet spacing. Correlations for area averaged Nusselt numbers and the Euler number are presented. A trade-off between heat transfer and pressure drop performance is further studied. Higher heat transfer rates were obtained with smaller stand-off distances and larger jet-to-jet spacings at the cost of higher pressure drop. Over the range of parameters tested, heat transfer was found to be more sensitive to design changes (maximum variation of 17.3%) compared to pressure drop (<1% variation). An optimal design will consist of a low stand-off distance as this will provide maximum convective heat transfer rates at the cost of minor pressure drop increases and will ensure the heat sink is compact.

MHD natural convective flow in a porous corrugated enclosure: Effects of different key parameters and discrete heat sources

Present work deals with an unsteady two-dimensional magneto-hydrodynamics natural convection flow problem in a porous-corrugated enclosure. The vertical side walls are insulated, and the top wall is non-uniformly heated; while square-shaped undulations at the bottom wall are discretely heated. Five different cases are considered depending on the discrete heat sources and various key parameters. A transformation-free higher-order compact (HOC) scheme is used to discretize the non-linear coupled transport equations, and an advanced iterative solver, like the hybrid-bi-conjugate-gradient stabilized technique, is used to solve the system of algebraic equations generated from the numerical discretization. Present higher-order compact scheme is fourth-order accurate in space coordinates and second-order accurate in the time variable. At first, the developed code is validated with existing experimental and numerical computed results for the case of a square enclosure associated with and without a heat source. Then, the computed results are analysed over a range of key parameters, like Rayleigh number (103≤Ra≤106), Darcy Number (10−5≤Da≤10−1), Hartmann number (25≤Ha≤150), with a fixed Prandtl number (Pr=6.1), to study the effects of these key parameters on the fluid flow behaviour and isotherm patterns in the porous-corrugated enclosure. It is found that at high Rayleigh–Darcy numbers with different Hartmann numbers, the effect of the permeability of the porous medium is significant; whereas the flow resistance from the boundary friction is gradually reduced, and the flow behaviour is similar to the pure buoyancy-induced flow. These simulated results are presented in the form of streamlines, isotherms, local and averaged Nusselt number plots, etc. The presented results show various flow features that have not been analysed before.

Entropy generation and natural convection on a cubic cavity with a pair of heat source at different configurations

Entropy generation and natural convection in a discretely heated cubic air-filled cavity has been numerically investigated in this study. A pair of discrete heat sources located at the bottom wall is at first considered for three different configurations. Both side walls are fixed at ambient condition, while the remaining walls are kept insulated. The three-dimensional, time-dependent continuity, momentum and thermal energy equations are selected as the governing equations of the present analysis. Galerkin finite element method is employed to solve those equations in non-dimensional forms. Parametric investigation is carried out by varying Rayleigh number within the range of 103–106 at fixed Prandtl number of 0.71. Results are presented in terms of qualitative plots of 3D velocity and thermal fields, and the quantitative evaluation of average Nusselt number and total entropy generation. A functional dependence of Nusselt number on the Rayleigh number and the distance between the symmetrically positioned heaters has also been presented in terms of empirical correlation. This comparative study shows that the configuration of the heaters has a great impact on the thermal characteristics, and thus provides one the opportunity to obtain an optimum arrangement for the system.

Thermal Effect on the Bioconvection Dynamics of Gravitactic Microorganisms in a Rectangular Cavity

In this work, the dynamics of the bioconvection process of gravitactic microorganisms enclosed in a rectangular cavity, is analyzed. The dimensionless cell and energy conservation equations are coupled with the vorticity-stream function formulation. Then, the effects of the bioconvection Rayleigh number and the heating source on the dynamics of microorganisms are discussed. The results based in streamlines, concentration and temperature contours are obtained through numerical simulations considering eight different configurations of symmetrical and asymmetrical heat sources. It is concluded that microorganisms accumulate in the warmer regions and swim through the cooler regions to reach the surface. They form cells for each heat source, but at high concentrations, they form a single stable cell. The results presented here can be applied to control and to understand the dynamics of microorganisms with discrete heat sources.

Mixed Convection inside a Duct with an Open Trapezoidal Cavity Equipped with Two Discrete Heat Sources and Moving Walls

The current research presents a numerical investigation of the mixed convection inside a horizontal rectangular duct combined with an open trapezoidal cavity. The region in the bottom wall of the cavity is heated by using two discrete heat sources. The cold airflow enters the duct horizontally at a fixed velocity and a constant temperature. All the other walls of the duct and the cavity are adiabatic. Throughout this study, four various cases were investigated depending on the driven walls. The effects of the Richardson number and Reynolds number ratio are studied under various cases related to the lid-driven sidewalls. The results are presented in terms of the flow and thermal fields and the average Nusselt number. The yielded data show that the average Nusselt number rises as the Richardson number and Reynolds number ratio increases. Furthermore, the Reynolds number ratio and the movement of the cavity sidewall(s) have a significant effect on the velocity and temperature contours. By the end of the study, it is shown that the maximum rates of heat transfer are related to Case 1 where the left sidewall moves downward and heater 2, which is placed near the left sidewall.

A novel design of discrete heat and cold sources for improving the thermal performance of latent heat thermal energy storage unit

The low thermal conductivity of phase change materials (PCMs) severely limits the operating efficiency and flexibility of the latent heat thermal energy storage (LHTES) unit. In this study, a novel LHTES unit with discrete heat and cold sources was proposed and investigated numerically, including symmetrical and staggered arrangements with side-heat and bottom-heated LHTES units. The consecutive and simultaneous heat storage and release processes of the seven developed models were comprehensively considered and compared for the first time. The results show that with the discrete arrangement of source, the additional heat conduction effect and natural convection in the side-heated LHTES unit would be significantly improved. More importantly, the heat storage and release cycle of the side-heated and symmetrical arrangement was shortened by 49.9%, the steady-state time was advanced by 46.6%, the steady-state liquid fraction was increased by 4.2%, and the heat transfer rate was increased by 22.3%. The discrete source arrangement had no obvious improvement on the performance of the bottom-heated LHTES unit. In addition, energy analysis showed that the side-heated LHTES unit stored more sensible heat. The discrete heat and cold sources effectively improved the thermal performance and flexibility of the LHTES unit.

Effect of Heat Source Position in Fluid Flow, Heat Transfer and Entropy Generation in a Naturally Ventilated Room

In this study, a 3D numerical study of free ventilated room equipped with a discrete heat source was performed using the Finite Volume Method (FVM). To ensure good ventilation, two parallel openings were created in the room. A suction opening was located at the bottom of the left wall and another opening was located at the top of the opposite wall; the heat source was placed at various positions in order to compare the heating efficiency. The effects of Rayleigh number (103 ≤ Ra ≤ 106) for six heater positions was studied. The results focus on the impact of these parameters on the particle trajectories, temperature fields and on the heat transfer inside the room. It was found that the position of the heater has a dramatic effect on the behavior and topography of the flow in the room. When the heat source was placed on the wall with the suction opening, two antagonistic behaviors were recorded: an improvement in heat transfer of about 31.6%, compared to the other positions, and a low Rayleigh number against 22% attenuation for high Ra values was noted.

The Heat Distribution Mechanism and Improved Temperature Model Based on Discrete Heat Source

The Heat Distribution Mechanism and Improved Temperature Model Based on Discrete Heat Source

Thermal Analysis of Melting Occurring Inside a Finned Rectangular Enclosure Equipped with Discrete Pulsed Protruding Heat Sources

This paper numerically investigates the effect of the location of a horizontal fin on the melting of a phase change material (PCM) inside a rectangular enclosure heated by multiple discrete pulsed protruding heat sources. The fin and the phase change material filling the enclosure store the thermal energy extracted from the heat sources, in sensible and latent forms. The heat sources are assumed to simulate electronic components undergoing a superheating technical issue. By extracting heat from the electronics, the PCM plays the role of a heat sink. To analyze the thermal behavior and predict the cooling performance of the proposed cooling system, we derive a nonlinear mathematical model based on mass, momentum and energy conservation laws. Several numerical investigations are conducted to quantify the influence of the fin position on the thermal behavior and the cooling performance of the heat sink. Predictions include the transient maximum temperature occurring inside the heat sources and the liquid volume. A comparison between our numerical results and experimental data selected from the literature shows a good agreement. The main conclusion is that the presence of the fin leads to a slight increase in the melting time.

EFFECT OF LOCATION OF DISCRETE HEAT SOURCES ON A WAVY-WALL MICROCHANNEL FOR LIQUID COOLING

EFFECT OF LOCATION OF DISCRETE HEAT SOURCES ON A WAVY-WALL MICROCHANNEL FOR LIQUID COOLING

Geometry Design of Square Column Heating Devices in Jet Channels

Based on the constructal theory, a heat dissipation optimization model for discrete square column heating devices on thermal conduction bases in 2D jet channels was established. With the total longitudinal section area and the discrete heat source height as constraints, the maximum temperature and entropy production rate of the system were taken as optimization objectives, and the length ratio of each heating device was taken as the optimization variables in the geometry design. The effects of the jet velocity and the heating device spacing on the constructal optimization of the heating device were analyzed. When the jet velocity and the heating device spacing are fixed, there will exist optimal length ratios to minimize the maximum temperature and the entropy production rate of the system, but the optimal length ratios corresponding to different jet velocities and different heating devices spacings are different. The results provide a theoretical guidance for the thermal design of square column heating devices.

A Fast Computational Method for the Optimal Thermal Design of Anisotropic Multilayer Structures with Discrete Heat Sources for Electrified Propulsion Systems

This work investigates the effect of the anisotropy on the heat transfer properties of layered composite materials with discrete heat sources, that are used in the power electronics of hybrid-electric propulsion systems. An analytical method, based on closed-form expressions structured on Fourier expansion series, is proposed for the solution of the Laplace’s steady-state anisotropic heat equation. The physical presence of electrical components is replaced by externally applied power sources. The method provides an accurate prediction of the temperature distribution and of the heat transfer across perfect layer-to-layer adhesion or finite conductance interfaces; its use is therefore encouraged for the optimization of composite substrates. Code verification has been employed on test cases for which the analytical solution was available.

Multi-objective optimization of a composite phase change material-based heat sink under non-uniform discrete heating

This paper presents a novel heat sink that integrates paraffin/copper foam composite phase change material. The heat sink base is subject to non-uniform heating with multiple discrete heat sources on a circuit board. Numerical simulations are performed using FLUENT software to investigate its thermal transfer characteristics and performance. Three types of paraffin, including RT35HC, RT44HC, and RT54HC, are compared. An optimization scheme that combines Artificial Neural Network and Non-Dominated Sorting Genetic Algorithm-II is proposed to (a) maximize the operation time and (b) minimize the heat sink mass. Operation time is defined as the time for simulated chips to reach set point temperature, 80 °C. The result shows that local overheating caused by non-uniform heating limits operation time. The optimization method shows great robustness and effectiveness. Three sets of non-dominated solutions are obtained, and optimal configurations are determined with further analysis. A figure of merit is defined as the ratio of charging time to heat sink mass. Optimal configuration's figure of merit increases by 1.2–44.0% comparing to original designs.

Galerkin finite element analysis of Darcy–Brinkman–Forchheimer natural convective flow in conical annular enclosure with discrete heat sources

This numerical study is intended for the analysis of thermal convection induced by buoyancy forces generated within a conical annular porous gap. The annulus was vertically positioned, it contains a discrete heat source and is filled with a Single-Walled Carbon Nanotubes-Water (SWCNT-H2O) nanoliquid exposed to a Lorentz force. To describe the porous medium in question, we have adopted the Darcy–Forchheimer model. Galerkin Finite Element Method (GFEM) has been used in this study to predict both thermal and hydrodynamic fields in the physical model. An extensive range of parameters are explored, i.e., the Rayleigh number (103 to 106), Hartman number Ha (1 to 100), and the volume fraction of nanoparticles (0 ≤ϕ≤0.08). For the purpose of exterminating the effects of heat source location on thermal and hydrodynamic fields, three locations (bottom, middle and upper) have been considered. Findings include current lines, isotherms, and Nusselt number evolution according to the previously stated variables. Predictably, our findings prove that heat transfer rate exhibits a decreasing function of Ha and an increasing function of Da and ϕ. Also, it was revealed that the convective regime is preponderant when the heat source was located in the bottom wall.

Numerical Investigation on Two-Phase Flow Heat Transfer Performance and Instability with Discrete Heat Sources in Parallel Channels

With the rapid development of integrated circuit technology, the heat flux of electronic chips has been sharply improved. Therefore, heat dissipation becomes the key technology for the safety and reliability of the electronic equipment. In addition, the electronic chips are distributed discretely and used periodically in most applications. Based these problems, the characteristics of the heat transfer performance of flow boiling in parallel channels with discrete heat source distribution are investigated by a VOF model. Meanwhile, the two-phase flow instability in parallel channels with discrete heat source distribution is analyzed based on a one-dimensional homogeneous model. The results indicate that the two-phase flow pattern in discrete heat source distribution is more complicated than that in continuous heat source distribution. It is necessary to optimize the relative position of the discrete heat sources, which will affect the heat transfer performance. In addition, compared with the continuous heat source, the flow stability of discrete heat sources is better with higher and lower inlet subcooling. With a constant sum of heating power, the greater the heating power near the outlet, the better the flow stability.

Characteristics of natural convection in n-eicosane in a square cavity with discrete heat source

The natural convection of phase change material (PCM) in a two-dimensional square cavity is numerically analyzed. The cavity consists of heat surface with a constant total heated area and adiabatic wall. The Grashof and Prandtl numbers for the PCM (n-eicosane with the melting temperature, Tmelt=36°C) in basic LHS system are 9 × 10^5 and 62.7, respectively, at 350.15K. The mass, momentum and energy balance equations of the system were considered. Three basic heating surface strategies were considered (discrete heat sources): single, side-side and side-bottom heating surface. The results show that the transient Nusselt number, mean kinetic energy at the surface and energy storage rate of the fluid are effectively enhanced by proper arrangement of the discrete heat source location and heating from the bottom half of the left and right sides requires the least time for 300 kJ energy storage. The results indicate that optimally placed discrete heat sources in PCM could be a promising alternative for high-efficient thermal energy storage.

MHD double-diffusive mixed convection and entropy generation of nanofluid in a trapezoidal cavity

Double diffusive, Magnetohydrodynamics (MHD), mixed convection flow of Al2O3-water nanofluid inside a trapezoidal enclosure with discrete heat and mass source at the bottom wall of the cavity has been discussed numerically. Together with these entropy generations due to fluid friction, heat transfer, mass transfer and magnetic field are discussed. The upper wall of the cavity is considered to move with a fixed velocity U0 towards positive X-direction. Different inclination angles and different aspect ratios are considered in the present study. Second and fourth order finite difference approximations are used to solve the governing equations by using biconjugate gradient stabilized method (BiCGStab). It is observed that, entropy generation mostly depends on heat transfer and lower values of Richardson number (Ri=0.01) reduces the entropy generation. The highest quantitative value of |Sθ|max is 21,664 for Ri=100,Re=100andϕ=450 and lowest quantative value of |Sθ|max is 5,824 for Ri=0.01,Re=1000andϕ=600 throughout the present study. Also the highest quantitative values of average Nusselt and Sherwood numbers are 0.2597 and 0.4208 for Ri=10,Re=100andϕ=45∘. The main objective of our present study is to minimize entropy generations due to combined effects of fluid flow, heat transfer, mass transfer and the effect of magnetic field so that the loss of energy can be reduced. This study observed that mixed convection flow, low magnetic field and low aspect ratio cavity of rectangular shape is always preferable to reduce total entropy generation and the middle part of the lower wall of the cavity is highly acceptable for heat and mass source for sprinkle of heat and mass throughout the whole of the cavity.