Channel Transfer Research Articles

The effect of using staggered and partially tilted perforated baffles on heat transfer and flow characteristics in a rectangular channel

Experimental and numerical studies have been carried out to study the effect of using baffles on the flow and heat transfer characteristics in a rectangular channel with an aspect ratio of 3:1. The baffles were mounted on the top and the bottom surfaces in a staggered arrangement with a constant pitch (P=100 mm). Uniform heat flux was applied on the upper and lower surfaces while the side walls were insulated. Over a range of Reynolds numbers from 12,000 to 42,000, the effect of tilting angles (θ=0°,30°,45°and60° ) and perforation ratios from 10% to 40% on the heat transfer enhancement and flow resistance features have been reported in the form of flow structure contours, temperature contours, local Nusselt numbers, average Nusselt numbers, Nusselt number ratios, friction factors, friction factor ratios and thermal enhancement factor (TEF). Additionally, based on the study outcomes, correlations for the Nusselt number and the friction factor have been developed. Among all the studied cases, baffles with tilting angle (θ=0°) and perforation ratio (β=10%) showed the highest heat transfer enhancement and the greatest flow resistance while the case of the lowest heat transfer augmentation and the minimum friction factor is the baffle configuration with tilting angle (θ=60°) and perforation ratio (β=40%). Moreover, it is noticed from the predicted flow field that the baffles with lower tilting angles and lower perforation ratios provide stronger reattachment and recirculating flow on the heating surfaces. In addition, the insertion of the proposed baffles leads to higher heat transfer augmentation compared to the smooth channel such that the Nusselt number ratio (Nurs/Nuss) reaches 2.6. The TEF showed that the optimum condition has been achieved at the baffle configuration with tilting angle (θ=60°) and perforation ratio (β=10%) at Re=12,000 where it reached the peak (about 0.7).

Friction factor and heat transfer evaluation of cross-corrugated triangular flow channels with trapezoidal baffles

Heat recovery ventilator (HRV) with cross-corrugated triangular flow channels has the advantages of high heat transfer coefficient and high mechanical strength, but it also has the disadvantages of uneven internal temperature distribution and large pressure loss. In this paper, a kind of cross-corrugated triangular flow channel with trapezoidal baffle is proposed, and the influence of the trapezoidal baffle on the flow and heat transfer characteristics of the cross-corrugated triangular flow channel is comprehensively studied by numerical simulation. The Nusselt number (Nu), friction factor (f) and Performance Evaluation Criteria (PEC) value of the flow channel with trapezoidal baffles and the original flow channel were compared. The synergistic effect of the trapezoidal baffle on the flow and heat transfer in the flow channel was analyzed. The results show that the trapezoidal baffle can promote fluid mixing and momentum exchange, thus enhancing the field synergistic effect and improving the comprehensive heat transfer performance of the flow channel. The Nu and f of flow channels with trapezoidal baffles are more than 1.7 and 3.5 times of those without baffles, and the PEC value can be increased by 30%. The data and empirical formulas of this study can provide a guideline for HRVs design.

Adaptive channel estimation for cognitive fully adaptive radar

The paper addresses channel state information estimation for a cognitive radar system. The underlying radar signal model considered in this paper is based on a stochastic Green's function approach. In the radar model, the clutter returns can be expressed as a convolution of the stochastic Green's function impulse response with the transmit radar waveform. The paper proposes and solves constrained channel estimation algorithms that exploit the cosine similarity constraint and the inner product constraint to improve conventional least squares solutions. Using the fact that adjacent channel transfer functions have a close relationship in terms of a cosine similarity metric, the cosine similarity constraint can be exploited in the least squares problem. It turns out that the derived optimisation problem under the cosine similarity constraint is non-convex. However, since strong duality holds for the non-convex optimisation problem, it can be solved using the semi-definite relaxation (SDR) approach. The paper also proposes a new optimisation problem with an additional inner product constraint, which results in a convex second-order cone programming (SOCP) and is robust to the signal-to-noise ratio (SNR) in terms of the cosine similarity measurement. Numerical simulations are provided to verify the performance of the proposed methods on a challenge dataset generated using the high-fidelity modelling and simulation tool, RFView®.

Steady Magnetohydrodynamic Micropolar Fluid Flow and Heat and Mass Transfer in Permeable Channel with Thermal Radiation

The present work is devoted to the study of magnetohydrodynamic micropolar fluid flow in a permeable channel with thermal radiation. The Rosseland approximation for thermal radiation is taken into account in the modelling of heat transfer. The governing equations are expressed in non-dimensional form. The Homotopy Perturbation Method (HPM) is briefly introduced and applied to derive the solution of nonlinear equations. The effects of various involved parameters like Reynolds number, microrotation parameter and Prandtl number on flow and heat transfer are discussed. Further, their effects on Nusselt and Sherwood numbers are also investigated from the physical point of view. Analytic solutions of the problem are obtained by HPM and a numerical technique bvp4c package MATLAB is applied to predict the graphs between different parameters.

Experimental studies of turbulent pulsating flow and heat transfer in a serpentine channel with winglike turbulators

Particle Image Velocimetry and Infrared Thermography measurements are applied to explore the effect of quasi-triangular pulsatile flow inlet on active thermal-fluidic performance in a serpentine channel with innovative winglike turbulators. The novel turbulators are mounted in-line on two sidewalls of the channel with the attack angle (α), thickness to chord line ratio (t/C), and pitch to the channel hydraulic diameter ratio (Pi/DH) respectively fixed at 20°, 0.2, and 0.7. The Strouhal number (St) varies from 0 to 0.67 at a fixed Reynolds number (Re) of 10,000. Relative to the steady-state case (St = 0), the flow pulsation not only strengthens the Fujiwhara co-rotating vortices but also eliminates the corner vortex streams, leading to the overall Nusselt number (Nu¯/Nuo) enhancement by 13%. Moreover, both the Nu¯/Nuo and friction factor (f¯/fo) reveal the tilde-like trends with St, resulting in the highest Nu¯/Nuo of 5.2 and thermal performance factor of 1.39 at St = 0.67, which are higher than the previous best results for 10≤f¯/fo≤100 in the Nu¯/Nuo-f¯/fo diagram. Finally, by combing the present data with our previous steady state results, composite correlations of Nu¯/Nuo and f¯/fo versus Re, α, t/C, Pi/DH, and St are proposed.

Augmentation of Heat Transfer in a Circular Channel with Inline and Staggered Baffles

This numerical investigation presents the effects of the position of baffles in the shape of a circle’s segment placed inside a circular channel to improve the thermal and flow performance of a solar air heater. Three different baffles’ positions with Reynolds number varying between 10,000 to 50,000 were investigated computationally. The k-omega SST model was used for solving the governing equations. Air was taken as the working fluid. Three pitch ratios (Y = 3, 4, and 5) were considered, while the height of the baffles remained fixed. The result showed an enhancement in Nusselt number, friction factor, j-factor, and thermal performance factor. Staggered exit-length baffles showed maximum enhancement in heat transfer and pressure drop, while inline inlet-length baffles showed the least enhancement. For a pitch ratio of Y = 3.0, the enhancement in all parameters was the highest, while for Y = 5.0, the enhancement in all parameters was the least. The highest thermal performance factor of 1.6 was found for SEL at Y = 3.0.

Experimental investigation of thermal performance in a rectangular channel using compound structure

An experimental study was conducted to measure the heat transfer and pressure drop in a rectangular channel emphasizing a compound structure to improve the cooling performance of gas turbine blades. W shaped, semicircular, and multi semicircular shaped ribs with dimples are studied and applied to a lower surface of a channel. The experiment was carried out at a Reynolds number ranging from 10,000 to 32,000 and the ratio of pitch (P) to height (e) of the rib was 10. Also, the ratio of rib height (e) to channel hydraulic diameter (Dh) was 0.187 and the dimple depth (δ) to dimple diameter (D) ratio was 0.2. It was observed that the combination of ribs and dimple channel (compound channel) has an average of 23 % more heat transfer than the ribbed channel. W rib compound channel shows the highest thermal performance and enhanced up to 30 % more heat transfer than semi and multi-semicircular compound channel. friction rise was observed in the compound channel compared to the ribbed channel.

Numerical study of optimized three-dimension novel Key-shaped flow field design for proton exchange membrane fuel cell

Key-shaped three-dimension (3D) flow field channel is designed to improve the performance and mass transfer of proton exchange membrane fuel cell (PEMFC). This study comprehensively analyses the impacts on the performance and mass transfer of the flow channel from multiple dimensions such as the size, shape, and placement of the blocks. In comparison with the conventional straight single flow field channel, the new channel with rectangular blocks can effectively improve performance by 30%. Semi-elliptical and quarter-elliptical blocks are designed to make forced convection and increase the diffusion area of oxygen. The results indicate that the flow velocity in the Z-axis direction can be increased to 0.08–0.2 m/s due to the narrow space formed by variable cross-sections. In conclusion, the Key-shaped design has a potential to improve the performance of mass transfer in the cathode channel, providing a new strategy for the development of flow field design in PEMFC field.

Effect of the attack angle of the ribs on the flow and heat transfer in a flat channel

The results of numerical simulation of a turbulent flow in a flat channel with periodic inclined ribs by the RANS method are presented. The Reynolds number, calculated from the rib height and the superficial velocity, is Re = 12600. The obtained data are analyzed in order to determine the influence of the inclination angle on heat transfer. It is shown that the optimal angle of inclination, at which the average heat transfer in the channel is maximum, is 60°.

Lattice Boltzmann simulation of fluid flow and heat transfer in a micro channel with heat sources located on the walls

This study investigated a numerical study of fluid flow and heat transfer in micro channel with four heat sources that located on upper and lower walls. Four heat sources are located in the upper and lower walls symmetrically respect to centerline. Lattice Boltzmann method is used to solve related equations of flow and temperature of fluids, this method is included two steps such as collision and streaming steps. Reynolds number is varied from 0.1 to 10 and Knudsen number is changed from 0 to 0.1 for air. The slip velocity and temperature jump boundary condition are used for micro channel simulation with Knudsen numbers related to slip velocity flow. Bounce-back boundary conditions were applied on all solid boundaries, which means that incoming boundary populations are equal to out-going populations after the collision. A comparison with other study is done and a good result is gained. The results show that the Knudsen number has important role in heat transfer and the highest mean temperature at outlet occurs at highest Knudsen number and heat transfer convection is more significant for first heat source comparing second heat source.

Numerical simulation of flow dynamics and heat transfer in a rectangular channel with periodic ribs on one of the walls

The result of numerical simulation of a turbulent flow in a flat channel with a periodic transverse rib by the RANS and LES methods is presented. The Reynolds number, calculated from the rib height and the superficial velocity, is Re = 12600. The data obtained as a result of the study demonstrate the influence of the modeling method and the turbulence model on the quality of heat transfer prediction. The optimal model for this type of problems is presented.

Mathematical Model of Fluid Flow and Solute Transfer in a Permeable Channel with Slip Velocity at the Boundaries

Mathematical Model of Fluid Flow and Solute Transfer in a Permeable Channel with Slip Velocity at the Boundaries

The influence of high-porosity nickel foam on the transition flow regime for heat transfer and pressure drop characteristics in a rectangular channel

The influence of high-porosity nickel foam on the transition flow regime for heat transfer and pressure drop characteristics in a rectangular channel

An electrochemically switched ion exchange α-ZrP/PPy film as a synergistically catalytic and anchoring material towards lithium-sulfur battery design

The commercial application of lithium-sulfur (Li-S) batteries is largely hindered by their insufficient rate performance and cycle stability because of the shuttle effect and sluggish kinetic conversion of lithium polysulfides (LiPSs). Herein, an electrochemically switched ion exchange film of heterostructured α-zirconium phosphate/polypyrrole (α-ZrP/PPy), which combined the virtues of conductive PPy with highly adsorptive α-ZrP was investigated to be able to synergistically restrain the shuttle effect and facilitate the redox reaction kinetics of LiPSs by employing the density functional theory (DFT) calculation. The mechanism of redox reaction and the microcosmic-properties of electron were explored in detail. Adsorption calculations revealed that the captured LiPSs by α-ZrP/PPy with a moderate adsorption ability and the electrocatalytic activity of the anchoring material would effectively restrain the shuttle effect and accelerate the decomposition of LiPSs back to sulfur during charging process. Furthermore, α-ZrP and PPy provided unique ion migration channel and electronic transfer channel, respectively, and the rapid redox reactions were induced by excellent surface diffusion of lithium ion in the interface of α-ZrP/PPy. Particularly, the charge differential density uncovered that LiPSs served as an accurate regulable-switch tune the electronic transfer in the redox reactions relating to α-ZrP and PPy. Thus, the α-ZrP/PPy based cathode owned outstanding adsorption capacity as well as electric catalytic properties in the Li-S batteries.

Application of biomachining on copper for a minichannel heat exchanger

Studies have established a random surface roughness found on the surface of copper after immersion in an Acidithiobacillus ferooxidans-containing culture medium during the biomachining process. In this study, a heat exchanger system was built by employing the biomachining process to manufacture the copper-based minichannel parts for the system. This study investigates the effect of the surface roughness of the minichannel obtained from the biomachining process upon the pressure drop and heat transfer of a fluid flowing through the system. These parameters were also compared to another system possessing minichannel parts with a lower surface roughness made by conventional milling. The result shows that the system containing the biomachined minichannels tends to increase the pressure drop and heat transfer rate of the fluid flow compared to the milled example due to the effect of the roughness average (Ra), which consistently occurred with flow rates (Q) of 32 and 64 mL/min. A novel finding revealed in this study is that the biomachining method can contribute to fluidic channel and heat transfer applications, due to its naturally unique surface roughness.

Method for Solving the Problem of Heat Transfer in a Flat Channel

In power engineering, studies related to the distribution of temperatures and velocities in fluids that move, for example, in pipelines or channels, are of theoretical and practical importance. The presented work displays the results of the development of an approximate analytical method for mathematical modeling of the process of heat transfer in laminar flows. By the example of solving the problem of heat transfer in a flat channel with a Couette flow, the main provisions of the method are considered. The combined use of the integral heat balance method and the collocation method made it possible to obtain an analytical solution that is simple in form. The obtained accuracy of solutions depends on the number N of points of the spatial variable at which the original differential equation is exactly satisfied.

Numerical study on heat transfer characteristics of molten salt in annular channel with wire coil

• Annular channel with wire coil is firstly proposed to heat molten salt. • Wire coil inserts are used to enhance the heat transfer in annular channel. • Overall performance is evaluated using the thermal performance factor. • The correlations for Nusselt number and friction factor are presented. In order to develop a heat transfer enhancement technique for molten salt heat exchanger, Fluent simulation is carried out to investigate the molten salt flow and heat transfer performances in annular channels with wire coil. Two experimental set-ups were built to measure the temperature distribution of the test section, and the maximum relative deviation between the simulated temperature and experimental one is 3.27%. The effects of the wire width and helical pitch on heat transfer and flow characteristics, and the mechanism of heat transfer enhancement in wire coil inserted annular channels are discussed in the light of numerical results. It is found that adding coiled wire inserts in an annular channel can enhance the heat transfer performance by up to eight times as compared to a smooth annular channel, at a cost of higher flow resistance. The thermal performance factor is evaluated, which is greater than 3.7 at a constant pumping power. Finally, the heat transfer and friction factor correlations for annular channels with wire coil are proposed.

Heat transfer and skin-friction in channel with flow behind a cylinder

The paper presents the results of an experimental study of the parameters of the boundary layer, distribution of static pressure, heat transfer and friction coefficients of smooth surface located in the wake behind the cylinder in the channel. Cylinders of various diameters were placed in a slotted channel with a height of 30 mm on its axis. In all experiments, the flow velocity at the inlet was 50 m/s. The cylinder was made unheated. The friction coefficients of the smooth model were determined both from the velocity profile in the boundary layer and by direct weighing of the model on a one-component strain-gage balance. The local values of the heat transfer coefficients were determined by transient heat-transfer method using a thermal imager. The values of the heat transfer and friction coefficients in the wake behind the cylinder, referred to the values on the smooth wall in the undisturbed flow, varied in the range 1.15–1.65 and 1.3–1.75, respectively. The value of the Reynolds analogy factor for all cylinder diameters turned out to be less than unity.

Dispersion From Diffuse Reflectors and Its Effect on Terahertz Wireless Communication Performance

This article investigates the temporal dispersion of a wireless terahertz communication signal caused by reflection from a rough (diffuse) surface, and its subsequent impact on symbol error rate versus data rate. Broadband measurements of diffuse reflectors using terahertz time-domain spectroscopy were used to establish and validate a scattering model that uses stochastic methods to describe the effects of surface roughness on the phase and amplitude of a reflected terahertz signal, expressed as a communication channel transfer function. The modeled channel was used to simulate a quadrature phase shift keying (QPSK) modulated wireless communication link to determine the relationships between symbol error rate and data rate as a function of surface roughness. The simulations reveal that surface roughness from wall texturing results in group delay dispersion that limits achievable data rate with low errors. A distinct dispersion limit in surface roughness is discovered beyond which unacceptable numbers of symbol errors begin to accrue for a given data rate.

Asymmetrical Uplink and Downlink Transceivers in Massive MIMO Systems

Even if massive multiple-input multiple-output (MIMO) can theoretically bring huge benefits, it incurs substantial hardware complexity and expensive hardware costs. To address these issues while maintaining the system performance simultaneously, we develop an asymmetrical transceiver architecture for massive MIMO systems in this paper by releasing the shackles on radio frequency (RF) chains. Specifically, we first develop the architecture for the asymmetrical transceiver where the number of receive RF chains is different from that of the transmit RF chains. Due to this unequal number of RF chains, channel inconsistency appears. To overcome the channel inconsistency and thus fully harness the system's downlink data transmission capability, we propose two uplink-to-downlink channel transfer algorithms. The cost and power consumption models for the asymmetrical transceiver are also developed and the uplink signal-to-noise loss due to the channel inconsistency is investigated. Through analyzing system spectral, cost, and energy efficiency, we demonstrate that the proposed asymmetrical transceiver-based massive MIMO system can achieve excellent downlink spectral efficiency while maintaining a reasonable energy efficiency.