Channel Transfer Research Articles

Numerical investigation on the impact of protrusions mounted on sidewalls of double layered micro channel heat sink

Several passive cooling methods are utilized in electronics cooling. for the sake of their more reliability than active cooling and microchannel heat sink is the device suitable in such present trends of technology. With the evolution of microchannel, researchers predicted that microchannel with double layered pattern is more beneficial in terms of performance. The modification of channel geometry is implemented to get enhanced Nusselt number and heat transfer coefficient. The present work has the objective to implement protrusions on sidewalls of double layered microcahnnel. The influence of protrusions on thermal characteristics along with hydraulic profiles is investigated in the work. Two cases with protruded channel are evaluated and compared with a base smooth sink. Performance parameters are interestingly found superior for both protruded cases. These cases exhibit better Nusselt number and heat transfer coefficient than that of smooth-channel sink at all range of Re. For example, Nu of Case 1 is calculated maximum as 11.13 at the highest Re. In the study of bottom wall temperature, it has been found that convection heat transfer in channel occurs at higher rate due to the presence of protrusions.

EFFECTS OF VARIOUS COOLANTS ON FLOW AND HEAT TRANSFER CHARACTERISTICS IN A ROUND-TIP PIN-FINNED INTERNAL CHANNEL

This paper performs a numerical study regarding the effects of different kinds of coolants on flow and heat transfer in a pin-finned internal channel. A new pin-fin structure detached from one endwall with a round-tip is proposed which aims to increase the heat transfer enhancement. As a contrast, the investigation of the typical fully-attached pin-fin and the flat-tip detached pin-fin is also considered. In order to evaluate the reliability of the numerical method, an experiment of an internal channel with round-tip pin-fin arrays is conducted to serve as the reference, and all cases are carried out with the shear stress transport (SST) turbulence model. The effects of coolants including air, mist/air, steam, and mist/steam are analyzed with the Reynolds number ranging from 15, 000 to 50, 000. The results reveal that the round-tip pin-fin outperforms in heat transfer enhancement and pressure loss deterioration among all three pin-fin structures. With respect to Reynolds numbers, the averaged Nusselt numbers increase gradually for all coolants, and cases of mist/steam achieve the highest values, followed by cases of steam, mist/air, and air, respectively, but the differences between the mist coolants and pure coolants are not obvious as the Reynolds number is lower than 30, 000. In conclusion, cases of mist/steam obtain the best thermal performances, and the internal channel with round-tip pin-fin arrays using mist/steam as coolant is most favorable. (Less)

Numerical study of the effects of pressure gradient on the heat transfer in the KMS heat exchanger channel

The work is devoted to studying the influence of the pressure gradient on the heat transfer intensity in the channel of the KMS air heater. The aim of the work is to develop methods for improving the heat engineering characteristics of a heat exchanger using a pressure gradient. Numerical simulation is implemented based on the RANS approach using the Code Saturne software package using the k-w-sst turbulence model. The simulation conditions were set in accordance with the manufacturer’s instructions for the KMS-2 type air heat exchanger. In the work, the heat transfer coefficient was studied in the presence of a longitudinal pressure gradient in the heat exchanger channel from the gas path. As a result of the simulation, data were obtained on the effect of the pressure gradient on the heat transfer intensity for different working fluids in the channel of an advanced heat exchanger.

Моделирование тепловых режимов в канале воздушного охлаждения аппарата восстановления титана

Numerical modeling of thermophysical processes in the air cooling channel of titanium sponge reduction apparatus is performed. The cooling channel is an area bounded by a retort filled with liquid magnesium and the apparatus wall, on which heating elements are mounted. Air flows inside the channel, cooling the retort. A mathematical model is constructed based on unsteady Navier-Stokes equations in axisymmetric formulation using a k-ω SST turbulence model. The model takes into account the radiation heat transfer between the retort and reactor walls. Four variants of thermal boundary conditions are considered. The aim of this work is to create a mathematical model of conjugate heat transfer in an air channel. Based on this model, the temperature conditions of the retort wall are calculated and the profiles of the heat transfer coefficient along the retort wall for a wide interval of air flow rates are obtained. It is shown that temperature distributions along the retort are heterogeneous and strongly depend both on external boundary conditions and on the cooling intensity. Heat transfer coefficient distributions from its outer wall for different retort heating conditions are plotted and an empirical formula for calculating the profile of this coefficient is proposed.

Numerical Calculation of Two-dimensional Steady Counterflows and Heat Transfer in H-shaped Channel

Numerical Calculation of Two-dimensional Steady Counterflows and Heat Transfer in H-shaped Channel

Condensing Heat Transfer and Flow Characteristics in Horizontal Mini Triangular Channel

Condensing Heat Transfer and Flow Characteristics in Horizontal Mini Triangular Channel

Numerical analysis of roughened solar air heater with arc and fan shape roughness to understand heat transfer and fluid flow characteristics

The computational model of rectangular solar air heater is analyzed by using computational fluid dynamics (CFD). In this work, we investigate the heat transfer, friction factor, and fluid flow characteristics in the SAH channel with mounted fan and arc shape roughness along the total length (L = 1200 mm), and these results are carried out from the RNG k-ɛ model for flowing fluid with 3000 ≤ Re ≤ 21,000. The combination of fan and arc shape roughness are mounted on SAH channel to improve heat transfer rate with parameters radius of fan shape Rf = 8.5 mm, the radius of arc shape roughness Ra = 14 mm, Gaps between fan and arc shape roughness G = 1.5, 3, 4.5 and 6 mm, angle of combine shape roughness ϕ = 115°, 120°, and 125°. The relative gaps and angle parameters are discussed for heat transfer, friction factor, the thermal-hydraulic performance of the SAH channel. The maximum thermal performance factor of 2.869946 and also an increase of 4.842202696 times heat transfer rate as compared to the smooth channel has been observed.

Numerical analysis of forced convection heat transfer in a rectangular micro-channel totally filled with Ag/ water nano fluid in slip flow regime using the lattice Boltzmann method

Numerical simulation reported on heat transfer and fluid flow in a two-dimensional rectangular micro channel totally filled with Ag/water. The first –order slip/jump boundary conditions were uniformly imposed to the up and bottom walls. The governing conservation equations are translated in dimensionless form using the thermal Single Relaxation Time (T-SRT) modified Lattice Boltzmann Method (LBM) with double distribution functions (DDFs). The viscous dissipations effects are adopted into the energy equation. Effects of nanoparticle volume fraction φ, slip coefficient, B, on the flow of Nano fluid and heat transfer were studied. The results were interpreter in terms of slip velocity; temperature jump and Nusselt number. Based on the results found, it can be concluded that decreasing the values of slip coefficient enhances the convective heat transfer coefficient and consequently the Nusselt number (Nu) but increases the slip velocity at the wall and temperature jump values.

A Trajectory-Driven 3D Non-Stationary mm-Wave MIMO Channel Model for a Single Moving Point Scatterer

This paper proposes a new non-stationary three-dimensional (3D) channel model for a physical millimeter wave (mm-Wave) multiple-input multiple-output (MIMO) channel. This MIMO channel model is driven by the trajectory of a moving point scatterer, which allows us to investigate the impact of a single moving point scatterer on the propagation characteristics in an indoor environment. Starting from the time-variant (TV) channel transfer function, the temporal behavior of the proposed non-stationary channel model has been analyzed by studying the TV micro-Doppler characteristics and the TV mean Doppler shift. The proposed channel model has been validated by measurements performed in an indoor environment using a MIMO radar kit operating at 24 GHz. For the measurement campaign, we used a single swinging pendulum as a model for a moving point scatterer. The trajectory of the pendulum has been captured by an inertial measurement unit attached to the pendulum and by a motion capture camera system. The measured trajectories are fed into the proposed mm-Wave MIMO channel model. The results obtained for the micro-Doppler characteristics show an excellent agreement between the proposed MIMO channel model and real-world measured channels in the presence of a moving point scatterer. We believe that our model can serve as a basis for the development of novel non-stationary MIMO channel models capturing the effects caused by moving objects and people.

A Trajectory-Driven 3D Channel Model for Human Activity Recognition

This paper concerns the design, analysis, and simulation of a 3D non-stationary channel model fed with inertial measurement unit (IMU) data. The work in this paper provides a framework for simulating the micro-Doppler signatures of indoor channels for human activity recognition by using radio-frequency-based sensing technologies. The major human body segments, such as wrists, ankles, torso, and head, are modelled as a cluster of moving point scatterers. We provide expressions for the time variant (TV) speed and TV angles of motion based on 3D trajectories of the moving person. Moreover, we present mathematical expressions for the TV Doppler shifts and TV path gains associated with each moving point scatterer. Furthermore, a model of the non-stationary time variant channel transfer function (TV-CTF) is provided, which takes into account the effects caused by a moving person as well as fixed objects, such as furniture, walls, and ceiling. The micro-Doppler signatures of the moving person is extracted from the TV-CTF by employing the concept of the spectrogram, whose expression is also provided in closed form. Our model is confirmed by channel state information (CSI) measurements taken during walking, falling, and sitting activities. The proposed channel model is fed with IMU data that has been collected. We evaluate the micro-Doppler signature of the model and CSI measurements. The results show a good agreement between the spectrograms and the TV mean Doppler shifts of our IMU-driven channel model and the measured CSI. The proposed model enables a paradigm shift from traditional experimental-based approaches to future simulation-based approaches for the design of human activity recognition systems.

Effect of rib arrangements on the flow pattern and heat transfer in internally ribbed rectangular divergent channels

In this article, a numerical investigation is conducted to analyze three-dimensional incompressible Navier-strokes flows through the rectangular divergent square ribbed channels with different Reynolds number value 20,000, 40,000 and 60,000 having five different configurations of square ribs (p/e = 2.5, 3.33, 5, 10, and 20) has been used as roughness element. The commercial finite-volume based CFD code ANSYS FLUENT is used to simulate turbulent airflow through the channels. The RNG k-ε turbulence model is used to solve the transport equations for turbulent flow energy and dissipation rate. Detailed results about average heat transfer and fluid friction in a rectangular divergent square channel are presented and discussed.

Fouling and cleaning of plate heat exchangers: Dairy application

Plate heat exchangers (PHEs) used in milk thermal treatments are subject to rapid fouling, while Cleaning-in-Place (CIP) produces large amounts of wastes. Up to 80% of production costs in the dairy industry have been attributed to the effects of fouling and cleaning.In spite of decades of research, a detailed model for simulation, monitoring, control and optimisation of full heating and cleaning cycles for PHEs is still not available. Mechanistic simulation models based on differential equations typically address only fouling but not cleaning. More detailed models based on Computational Fluid Dynamics (CFD) are computationally very expensive and impractical to use for optimization, scheduling and control of complete PHEs.Here, a dynamic 2D model of PHEs is presented that enables optimizing milk thermal treatment operations, taking into account both fouling and cleaning. The model balances predictive accuracy and computational feasibility.It integrates: (i) various mechanisms and kinetics for fouling and cleaning; (ii) a detailed moving boundary model of deposit growth that captures its spatial distribution; (iii) a dynamic thermo-hydraulic model of mass and heat transfer in a single PHE channel; (iv) the flexible assembly of channels into a variety of PHE configurations, and (v) the flexible definition of heating-cleaning cycles.The fouling model has been validated for two PHE configurations against experimental data, with excellent results. Alternative fouling mechanism (due to aggregate proteins or denatured proteins, and with/without deposit re-entrainment) have been explored. Results show that the fouling observed in the two arrangements is best fitted by distinct fouling models, and that the performance of the two PHE arrangements is quite different.Dynamic cleaning models have been integrated with the deposit moving boundary model and validated. This has enabled for the first time the seamless, detailed simulation of individual and multiple heating-cleaning cycles, where each phase starts from the detailed deposit distribution at the end of the previous phase. The models detail enables the introduction of sophisticated condition-based logic in the operation of each phase and overall cycle. Using such condition-based logic it is shown that cleaning time could potentially be reduced by ∼50%. Finally, it is shown that the heating/cleaning cycle can be optimized for maximum productivity, balancing fouling and cleaning trade-offs. This is demonstrated for one of the PHE arrangements.

Experimental and numerical investigations of heat transfer and fluid flow in a rectangular channel with perforated ribs

The present work concentrates on perforated 90° ribs to improve the thermal performances in a rectangular cooling channel with an aspect ratio of 4:1. Three different types of perforated ribbed channels are designed and compared. Steady-state Liquid Crystal Thermography (LCT) is employed to measure surface temperature and derive heat transfer coefficients over the ribbed surfaces in the tested channels. The turbulent flow details are presented by numerical calculations with two established turbulence model, i.e., the k-ω SST (Shear Stress Transportation) model and the DES (Detached Eddy Simulation) model. Compared with the normal rib (Case 1), the low heat transfer behind the ribs is improved by the perforated ribs with slightly reduced pressure drop. This phenomenon is more obvious when the perforated ratio is larger (Case 4). The local heat transfer is enhanced by about 12%–24% and the overall heat transfer is enhanced by about 4%–8%. The overall thermal performance is also improved by the perforated ribs with a slightly reduced pressure drop. The recirculating flows behind the ribs are reduced by the perforated cases. The reduced recirculation flows enhance the local heat transfer in this region. However, the flow reattachment region is disturbed by the perforated ribs and the local heat transfer in this region is slightly decreased. As a perforated rib can improve the overall thermal performance and provide more uniform heat transfer fields, it is promising for applications in internal cooling of turbine blades.

Effect of asymmetrical wall heat flux and wall temperature ratio on mixed convection in a vertical micro-porous-channel with internal heat generation

Mixed convective heat transfer in a vertical parallel plate micro-porous channel with internal heat generation and viscous dissipation, varying wall heat flux ratio and wall temperature ratio at the boundaries is investigated using the Darcy-Brinkman model under local thermal non-equilibrium assumption. Numerical solution for both fluid and solid temperature distributions are obtained by applying the finite element method. The effect of pertinent parameters such as Brinkman number, Rayleigh number, Darcy number, inter-phase heat transfer coefficient, porosity scaled thermal conductivity ratio and solid internal heat generation are discussed. The results indicate that the Nusselt number increases with the increase in the solid internal heat generation as well as Rayleigh number in both wall heat flux ratio and wall temperature ratio boundary conditions. It is observed that with the quantitative increase in viscous dissipation parameter Br, Nusselt number Nu increases in the presence of internal heat generation and it decreases in the absence of internal heat generation, for a specific range of values of wall heat flux ratio and wall temperature ratio. Beyond this range Nu increases with the increase in Br regardless of internal heat generation. For the cases, constant wall temperature and wall heat flux ratios, good correlation is observed in the results obtained with that of available in the literature.

Analyzing the Impact of Molecular Re-Radiation on the MIMO Capacity in High-Frequency Bands

In this paper, we show how the absorption and re-radiation energy from molecules in the air can influence the Multiple Input Multiple Output (MIMO) performance in high-frequency bands, e.g., millimeter wave (mmWave) and terahertz. In more detail, some common atmosphere molecules, such as oxygen and water, can absorb and re-radiate energy in their natural resonance frequencies, such as 60 GHz, 180 GHz and 320 GHz. Hence, when hit by electromagnetic waves, molecules will get excited and absorb energy, which leads to an extra path loss and is known as molecular attenuation. Meanwhile, the absorbed energy will be re-radiated towards a random direction with a random phase. These re-radiated waves also interfere with the signal transmission. Although, the molecular re-radiation was mostly considered as noise in literature, recent works show that it is correlated to the main signal and can be viewed as a composition of multiple delayed or scattered signals. Such a phenomenon can provide non-line-of-sight (NLoS) paths in an environment that lacks scatterers, which increases spatial multiplexing and thus greatly enhances the performance of MIMO systems. Therefore in this paper, we explore the scattering model and noise models of molecular re-radiation to characterize the channel transfer function of the NLoS channels created by atmosphere molecules. Our simulation results show that the re-radiation can increase MIMO capacity up to 3 folds in mmWave and 6 folds in terahertz for a set of realistic transmit power, distance, and antenna numbers. We also show that in the high SNR, the re-radiation makes the open-loop precoding viable, which is an alternative to beamforming to avoid beam alignment sensitivity in high mobility applications.

Fast and Accurate Optical Fiber Channel Modeling Using Generative Adversarial Network

In this work, a new data-driven fiber channel modeling method, generative adversarial network (GAN) is investigated to learn the distribution of fiber channel transfer function. Our investigation focuses on joint channel effects of attenuation, chromic dispersion, self-phase modulation (SPM), and amplified spontaneous emission (ASE) noise. To achieve the success of GAN for channel modeling, we modify the loss function, design the condition vector of input and address the mode collapse for the long-haul transmission. The effective architecture, parameters, and training skills of GAN are also displayed in the paper. The results show that the proposed method can learn the accurate transfer function of the fiber channel. The transmission distance of modeling can be up to 1000 km and can be extended to arbitrary distance theoretically. Moreover, GAN shows robust generalization abilities under different optical launch powers, modulation formats, and input signal distributions. Comparing the complexity of GAN with the split-step Fourier method (SSFM), the total multiplication number is only 2% of SSFM and the running time is less than 0.1 seconds for 1000-km transmission, versus 400 seconds using the SSFM under the same hardware and software conditions, which highlights the remarkable reduction in complexity of the fiber channel modeling.

Two-phase MHD Flow through Porous Medium with Heat Transfer in a Horizontal Channel

The present study deals with two layered MHD immiscible fluid flow through porous medium in presence of heat transfer through parallel plate channel. The fluids are incompressible, and flow is fully developed. The fluids are of different viscosities and thermal conductivities so flowing without mixing each other. Two different phases are accounted for study and are electrically conducting. Temperature of the walls of parallel plate channel is constant. Rheological properties of the immiscible fluids are constant in nature. The flow is governed by coupled partial differential equations which are converted to ordinary differential equations and exact solutions are obtained. The velocity profile and temperature distribution are evaluated and solved numerically for different heights and viscosity ratios for the two immiscible fluids. The effect of magnetic field parameter M and porosity parameter K is discussed for velocity profile and temperature distribution. Combined effects of porous medium and magnetic fields are accelerating the flow which, can be helpful in draining oil from oil wells.

A model of heat pulse induced limit-cycle-oscillations in the edge of magnetically confined plasmas

Limit-cycle-oscillation (LCO) is a ubiquitous feature in low to high confinement mode transition. We propose a double-source (heat and turbulence) prey-predator model of heat pulse induced LCOs in the edge plasmas. It is shown that the development of the radial electric field is a combined process of turbulent thermal and momentum transports. The causality relation between the turbulence intensity and the radial electric field reverses during the transport channel transfer. A numerical study reveals that the appearance of LCOs can be explained as a transition from a ‘forced’ oscillation to an ‘intrinsic’ oscillation of the edge plasmas. It is also found that there exists a critical equilibrium pressure gradient, above which the transient LCOs become stationary ones, i.e. a steady I-phase may be sustained.

Effect of the channel divergence ratio on heat transfer in a turbulent flow around the rib-step system

The article presents the result of the numerical simulation of the turbulent flow for a rib-step system by the RANS method at the Reynolds number, calculated by the step height and average flow rate velocity, Re = 5000. The data obtained in these studies showed the influence of divergence ratio on heat transfer in a flat channel with a transverse rib acting as a vortex generator. It is shown that with an increase in the divergence ratio, the length of the recirculation zone increases, while the position of the heat transfer maximum varies slightly.

Unsteady conjugate convective heat transfer in a vertical channel at a sudden heating of the bottom

The processes of unsteady heat transfer in the mode of thermogravitational convection in a vertical channel with solid walls of finite thermal conductivity after a sudden heating from bottom was studied numerically by the finite element method in three-dimensional problem statement. The system of thermogravitational convection equations was solved in the Boussinesq approximation in the variables of vortex–vector potential of the velocity field and temperature. The calculations were performed with the Prandtl number of 10, the Grashof number of 5000, and the ratio of the thermal conductivity of solid walls to the thermal conductivity of the liquid equal to 6.29. Distributions of the unsteady temperature field into the liquid and solid walls, temperature gradient fields, and velocity fields in the liquid were obtained. The process of formation, loss of axial symmetry and subsequent development of the rising stream was shown.