Duct Height Research Articles

Thermohydraulic performance of double pass solar thermal collector with inline, staggered and hybrid fin configurations

Abstract The present experimental and numerical investigations are aimed to increase the thermal performance of a finned absorber plate solar air heater. A commercial CFD tool is used to perform the numerical investigation and the obtained results are validated with the experimental data. The geometrical parameters such as fin angle (30o ≤ θ ≤ 90o), fin length (0.007 m ≤ L1 ≤ 0.028 m), fin height (0.007 m ≤ L2 ≤ 0.014 m), fin arrangements (inline, staggered, and hybrid), and hydraulic diameter (0.029 m ≤ Dh ≤ 0.057 m) of the heater duct are considered to obtain the optimum values using numerical simulation, while Reynolds number (3000 ≤ Re ≤ 20, 000) as a flow parameter is considered. The obtained CFD results revealed the best thermohydraulic performance for the hybrid staggered fins configuration and found to be about 3.5% and 3.8% in comparison to the staggered and inline fins configuration, respectively. Moreover, CFD investigation revealed the interesting results about the staggered fins configuration, which is the generation of the wavy motion inside the heater duct that confers the significant heat transfer enhancement. Whereas, the experimental thermohydraulic efficiency of the double pass solar air heater design is found to be 79% which is about 13% higher than that of similar design of single air pass.

Heat Transfer Augmentation in Solar Air Heater using Trapezoidal Winglets

In this paper we will investigate about the thermal effectiveness of a solar air heater which consists of trapezoidal type vortex generators by experimentation and validating with numerical simulation. The trapezoidal type vortex generator is placed inside the duct having a test section of 400*300*30mm. Trapezoidal vortex generators are placed in 3 different arrangements inside the duct, and having an array of 5 winglets in each row. The vortex generator is categorized as a winglet and the height of the winglet is half the height of the air duct. The analysis of trapezoidal winglet is done for wide range of Reynolds Number ranging from 3500 to 25000. Thermal performance factor is calculated by Webb’s correlation factor equation for roughness of surface, which considers Nussult Number and friction Factor as a parameter for thermal effectiveness. Numerical simulation was carried out using Ansys Fluent Software. Comparison of experimental and numerical results was done. Experimental and Numerical results showed that an increase in heat transfer rate of upto 65% was obtained when compared to smooth channel duct.

PDD_GBR: Research on Evaporation Duct Height Prediction Based on Gradient Boosting Regression Algorithm

AbstractEvaporation duct is a special atmospheric stratification that always exists at sea, which has an important influence on electromagnetic wave propagation. Accurate prediction of evaporation duct height is of great significance for radio information system. In the era of big data, machine learning has been widely used in various research work and achieved a series of excellent results. Based on the observation data method, this paper studies the prediction model using Gradient Boosting Regression algorithm (GBR) and proposes the pure data‐driven GBR (PDD_GBR), GBR_Paulus‐Jeske (GBR_PJ), and GBR_Babin‐Young‐Carton (GBR_BYC) evaporation duct prediction models. Simultaneously, traditional Paulus‐Jeske (PJ) model, Babin‐Young‐Carton (BYC) model, and existing SVR_PJ model are introduced into the experiment to make a comparison. The comprehensive performance of PDD_GBR model is optimal among these models with high stability and strong generalization ability, whose prediction accuracy has a great promotion compared with other models.

Effect of duct aspect ratio on normal shock wave/boundary layer interaction

Experiments have been conducted in a supersonic rectangular duct (Mach 1.4), with a normal shock wave/boundary layer interaction. The duct is designed in a modular fashion so that its aspect ratio can be varied without a change in the flow geometry. The shock location, duct height, and Mach number are kept constant, while varying the aspect ratio. Conventional and inclined schlieren techniques have been used to visualize the normal shock. The height and width of the “normal part” of the normal shock have been measured. A parameter termed area fraction of the normal shock is used to quantify the extent of shock bifurcation, and the effect of the duct aspect ratio on this parameter is studied. It has been found that a nearly square duct is the preferred geometry for maximizing the area fraction of the normal shock.

Effect of inertial lift on a spherical particle suspended in flow through a curved duct

We develop a model of the forces on a spherical particle suspended in flow through a curved duct under the assumption that the particle Reynolds number is small. This extends an asymptotic model of inertial lift force previously developed to study inertial migration in straight ducts. Of particular interest is the existence and location of stable equilibria within the cross-sectional plane towards which particles migrate. The Navier–Stokes equations determine the hydrodynamic forces acting on a particle. A leading-order model of the forces within the cross-sectional plane is obtained through the use of a rotating coordinate system and a perturbation expansion in the particle Reynolds number of the disturbance flow. We predict the behaviour of neutrally buoyant particles at low flow rates and examine the variation in focusing position with respect to particle size and bend radius, independent of the flow rate. In this regime, the lateral focusing position of particles approximately collapses with respect to a dimensionless parameter dependent on three length scales: specifically, the particle radius, duct height and duct bend radius. Additionally, a trapezoidal-shaped cross-section is considered in order to demonstrate how changes in the cross-section design influence the dynamics of particles.

Peculiarities of low-Reynolds-number supersonic flows in long microchannel

The characteristics of low-Reynolds-number supersonic flows in a long microchannel having a rectangular cross section are investigated computationally. The channel is composed of a Laval nozzle and a straight duct. The design Mach number of the nozzle is 2.0 and the Reynolds number calculated at the nozzle exit is 3100. The length of the straight duct is changed from 2 to 18 h, where h is the duct height. In the computations, the Navier–Stokes equations are numerically solved. The computational code is validated using the experimental data measured by the laser-induced fluorescence (LIF) technique. The computational results demonstrate that neither a normal shock wave nor a pseudo-shock wave, which corresponds to the starting shock wave in a supersonic wind tunnel, appears in microchannel flows. Namely, a low-Reynolds-number supersonic flow is created in a channel without the starting shock wave passing along the duct, although it has been believed that a supersonic internal flow should have been formed through the starting shock wave. In addition, it is found that the microchannel flow changes gradually its supersonic state with the channel length under an underexpanded condition, although a starting shock wave for high-Reynolds-number flows suddenly appears in a channel just as its length exceeds a certain specific length. Such unexpected phenomena originate from the peculiarity that the low-Reynolds-number flows can expand (accelerate) along a straight duct at supersonic speeds, although the high-Reynolds-number flows cannot.

Electric field intensity in cross-section of evaporation duct

This paper is devoted to the investigation of an evaporation duct in the context of its impact on microwave links communication stability. The work contains the numerical results for previously developed mathematical model based on using the dyadic Green’s functions. The electric field intensity dependencies on the receiving antenna suspension height and height of evaporation duct have been obtained for two variants of antenna polarization: horizontal and vertical. These results make it possible to formulate recommendations on how to place the transceiver antennas of microwave links to ensure 24-hour radio communication stability.

Atmospheric Duct Detection Using Wind Profiler Radar and RASS

AbstractA method of detecting atmospheric ducts using a wind profiler radar (WPR) and a radio acoustic sounding system (RASS) is proposed. The method uses the RASS to measure the virtual temperature profile and calculate the Brunt–Väisälä frequency; it also uses the WPR to measure the spectral width of the atmosphere and the atmospheric refractive index structure constant. Then the profile of the atmospheric refractive index gradient and modified refractivity are calculated using virtual temperature, spectral width, and the atmospheric refractive index structure constant. Finally, the height and intensity of the atmospheric duct are calculated to achieve continuous monitoring of the atmospheric duct. To verify the height and intensity of the atmospheric duct, comparison experiments between WPR-RASS and radiosondes were carried out from June 2014 to June 2015 in Dalian, Liaoning Province, China. The results show that the profile of modified refractivity by WPR-RASS has exactly the same trend as the radiosondes, the two methods have a good consistency, and the atmospheric duct value from WPR-RASS is in good agreement with that from radiosondes.

Range and Height Measurement of X-Band EM Propagation in the Marine Atmospheric Boundary Layer

An X-band vertical array system is developed for measuring and characterizing electromagnetic (EM) propagation in the marine atmospheric boundary layer. In particular, the evaporation duct that commonly forms over water is investigated as part of the Coupled Air-Sea Processes and Electromagnetic ducting Research (CASPER) at-sea experimental campaign conducted off the coast of Duck, NC, USA, during October–November of 2015. Monte Carlo simulations are first used to develop an optimal array of four vertically spaced receiving antennas. In the experiment, the antennas are mounted on the stern A-frame of a research vessel and measure EM signals transmitted from beacons mounted on another research vessel and on the pier at the Army Field Research Facility. While the propagation loss versus range provides a data set similar to previous work, the vertical array can provide sampling of modes in the leaky waveguide formed in the duct. Combining both the range and height sampling results in a robust inversion method for evaporation duct estimation. In this paper, the efficacy of the four-element array is demonstrated by estimation of the evaporation duct height through comparison with a library of precomputed propagation curves generated using the parabolic wave equation. Low model error gives confidence in the estimates, which are consistent with the concurrent environmental measurements performed by the CASPER team. It is found that the evaporation duct can vary significantly with range and time over the duration of a data collection run.

Mean Offshore Refractive Conditions during the CASPER East Field Campaign

AbstractIn this study, we use observational and numerical model data from the Coupled Air Sea Processes and Electromagnetic Ducting Research (CASPER) field campaign to describe the mean refractive conditions offshore Duck, North Carolina. The U.S. Navy operational numerical weather prediction model known as the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) performed well forecasting large-scale conditions during the experiment, with an observed warm bias in SST and cold and dry biases in temperature and humidity in the lowest 2000 m. In general, COAMPS underpredicted the number of ducts, and they were weaker and at lower height than those seen in observations. It was found that there is a noticeable diurnal evolution of the ducts, more over land than over the ocean. Ducts were found to be more frequent over land but overall were stronger and deeper over the ocean. Also, the evaporative duct height increases as one moves offshore. A case study was chosen to describe the electromagnetic properties under different synoptic conditions. In this case the continental atmospheric boundary layer dominates and interacts with the marine atmospheric boundary layer. As a result, the latter moves around 80 km offshore and then back inland after 2 h.

Effect of Geometric Configurations on the Starting Transients in Vacuum Ejector

This paper investigates the starting transients in vacuum ejector and its dependence on various geometric configurations. Various geometric parameters investigated were the ratio of diffuser to primary duct height (), diffuser length–to–diffuser height ratio (), and secondary chamber height–to–secondary jet thickness ratio (). It is seen that the vacuum ejector start-up is associated with a gradual initial vacuum generation stage, which is followed by a rapid vacuum generation. The gradual vacuum generation is found to be caused by the mass supply by the recirculation bubble (existing in the diffuser) to the secondary chamber exit plane. The rapid evacuation happens when the initial recirculation bubble splits up, which results in a rapid reduction of the amount of reverse flow mass reaching the secondary chamber. It is found that with reduction in the ratio and increase in the ratio, the rapid evacuation stage reduces and eventually vanishes. The present investigation shows that both these configuration changes reduce the mass supply by reverse flow to the secondary chamber and this is attributed to the cause of the reduction in rapid evacuation stage. The present study also shows that the nature of vacuum generation does not vary with change in secondary chamber height.

Ray Tracing and Applications to an Evaporation Duct Model Based on Data from Oceanographic Buoy Sensors

The two-dimensional ray tracing method allows an easy and fast modeling of tropospheric propagation in the microwave frequency range. A version of this method that determines ray trajectories, amplitudes and delays of the electromagnetic field, as well as the propagation loss in a two-dimensional inhomogeneous environment will be described. The implemented algorithm may consider generalized maps of modified refractivity (or refractive modulus), including not only the vertical gradients, but also their horizontal variations along the path between transmitter and receiver. Next, the present paper will discuss the results from the application of a model of evaporation duct height to data from instruments installed in sea buoys located along the Brazilian coast. Finally, the results from the application of the ray-tracing model to evaporation ducts will be presented, to analyze the propagation of microwave signals in the maritime environment. These results will also be compared with corresponding ones from the software Advanced Refractive Effects Prediction System 3.6, based on the numerical solution of a parabolic equation.

C band transhorizon signal characterisations in evaporation duct propagation environment over Bohai Sea of China

Evaporation duct is an anomalous mechanism of transhorizon propagation in marine and coastal regions. In order to better understand the characteristics of signal fading, a C band signal transhorizon propagation experiment was carried out with a 107km path length over the Bohai Sea of China. Using an iron tower with meteorological and oceanographic (METOC) sensors at multiple altitudes, a direct diagnostic method of evaporation duct is presented. The dependence of signal strength on evaporation duct height, antenna height, and METOC parameters is analysed. The experimental results indicate that the signal strength tends to be linearly dependent on the evaporation duct height when the transmitter and receiver antennas are submerged within the duct layer. Normally, the signal strength is stronger when the relative humidity is lower. Stronger signal strength tends to occur under near-neutral or stable atmospheric conditions or when the air-sea temperature difference is higher. The results indicate that the signal strength distribution of fast fading is close to Rayleigh distribution, and the probability density of fade slope is symmetric at 0dB/s. These results have important implications for the design of ducting communications system.

Buoyant finite-size particles in turbulent duct flow

Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) have been employed to investigate the dynamics of finite-size spherical particles, slightly heavier than the carrier fluid, in a horizontal turbulent square duct flow. Interface resolved Direct Numerical Simulations (DNS) have also been performed with the Immersed Boundary Method (IBM) at the same experimental conditions, bulk Reynolds number $Re_{2H}$ = 5600, duct height to particle size ratio $2H/d_p$ = 14.5, particle volume fraction $\Phi$ = 1% and particle to fluid density ratio $\rho_p/\rho_f$ = 1.0035. A good agreement has been observed between experiments and simulations in terms of the overall pressure drop, concentration distribution and turbulent statistics of the two phases. Additional experimental results considering two particle sizes, $2H/d_p$ = 14.5 and 9 and multiple $\Phi$ = 1, 2, 3, 4 and 5% are reported at the same $Re_{2H}$. The pressure drop monotonically increases with the volume fraction, almost linearly and nearly independently of the particle size for the above parameters. However, despite the similar pressure drop, the microscopic picture, the fluid velocity statistics, differs significantly with the particle size. This one-to-one comparison between simulations and experiments extends the validity of interface resolved DNS in complex turbulent multiphase flows and highlights the ability of experiments to investigate such flows in considerable details, even in regions where the local volume fraction is relatively high.

Wireless Channel Models for Over-the-Sea Communication: A Comparative Study

Over the past few years, the modeling of wireless channels for radio wave propagation over the sea surface has drawn the attention of many researchers. Channel models are designed and implemented for different frequencies and communication scenarios. There are models that emphasize the influence of the height of the evaporation duct in the marine environment, as well as models that deal with different frequencies (2.5, 5, and 10 GHz, etc.) or the impact of various parameters, such as antenna height. Despite the increasing literature on channel modeling for the over-the-sea marine environment, there is no comprehensive study that focuses on key concepts that need to be considered when developing a new channel model, characteristics of channel models, and comparative analysis of existing works along with their possible improvements and future applications. In this paper, channel models are discussed in relation to their operational principles and key features, and they are compared with each other in terms of major characteristics and pros and cons. Some important insights on the design and implementation of a channel model, possible applications and improvements, and challenging issues and research directions are also discussed. The main goal of this paper is to present a comparative study of over-the-sea channel models for radio wave propagation, so that it can help engineers and researchers in this field to choose or design the appropriate channel models based on their applications, classification, features, advantages, and limitations.

Effect of Dimple Intrusions and Curvature Radius of Rounded Corner Triangular Duct on Fluid Flow and Heat Transfer

The sharp corner significantly affects the flow through triangular duct. In the corners, flow gets stagnant, which results in poor heat transfer. Therefore, in the present study, one corner of the duct is kept rounded with variable curvature radius values (Rc). The curvature radius is selected in such a way that it varied from the minimum value (i.e., Rc = 0.33 times duct height; h) to a maximum value (i.e., Rc = 0.67h,which named as conventional duct in the work). In addition to this, the combined effect of both rounded corner and dimple-shaped intrusion has also been studied on flow of air and heat transfer and for this purpose; the relative streamwise distance (z/e) is varied from 6 to 14 with constant relative transverse distance (x/e) that is10. Steady-state, turbulent flow heat transfer under thermal boundary conditions is analyzed for Reynolds number from 5600 to 17,700. ANSYS (Fluent) 12.1 software is used to perform numerical simulations and good match has been observed between the simulated and experimental results. Due to rounded corner and dimple intrusions, velocity near the corner region has higher value in comparison to the conventional duct. The uniform temperature distribution is seen in the case of dimple intruded duct as compared to conventional and rounded corner duct (with Rc value of 0.33h). In comparison to conventional duct, the heat transfer increased about 21–25%, 13–20%, and 5–8%, for the Rc value of 0.33h, 0.49h, and 0.57h, respectively, but the combination of rounded corner and dimple-shaped intrusion augments heat transfer by 46–94%, 75–127%, 60–110%, for the z/e value of 6, 10, and 14, respectively, with the Reynolds number increase from 5600 to 17,700.

An Investigation of Natural Convection of Air Flow through A Vertical Duct with Rectangular Cross-Section Heated from A Singled-Side.

An experimental investigation is made to study the effect of different parameters affecting the process of natural air flowing through vertical ducts. The experimental setup has been designed and installed in the Air Conditioning Laboratory, Faculty of Engineering, Mansoura University. The test rig is consisted of mainly of rectangular cross-section duct of 25 cm width with different heights and gap thicknesses. The air gap thickness ranges from 3 to 20 cm while the height is ranged from 50 to 200 cm. All duct sides are well insulated with one side heated under heat flux density ranges from 191.04 to 440.75 ⁄ . Experimental results are plotted to indicate the relation between air volume flow rate through the vertical duct and different parameters discussed, namely, gap thickness, duct height and heat flux density. Based on measured results the air volume flow rate increases with increasing both heat flux density and duct height. It is concluded also that, the air volume flow rate increases with increasing the gap thickness until it reaches maximum values, namely at gap thickness of 10 cm and then decreases with further increase in gap thickness. The results of the present study could be useful in the design and application of buoyancy-assisted natural ventilation systems.

Study on the key structure parameters of a gravity settling chamber based on a flow field simulation

A gravity settling chamber plays an important role in improving the effect of the dust removal when it is selected as a dust collector in a pneumatic cleaning system for cleaning a subway tunnel. The settling rate and the indexes evaluating the secondary blowing are studied to assess the effect of the dust removal in this paper. Regarding the length of the settling chamber, the height of the inlet duct, and the diameter and height of the outlet duct as the variables of the key structures, a flow field simulation analysis of the gravity settling chamber with different heights of the dust accumulation is carried out, then the key structure parameters are optimized by means of the experimental method of uniform design. Finally, the settling chamber with an excellent dust removal effect is obtained. Major conclusions can be drawn as follows. (1) The maximum velocity and the mean velocity ratio of the airflow near a dust accumulation surface are important evaluation indexes for judging the secondary blowing. (2) The maximum velocity near a dust accumulation surface usually doesn’t appear on the vertical plane through the axis of the inlet duct or the outlet duct. (3) An excellent settling effect can be ensured during the whole cleaning process only when the maximum design height of the dust-accumulation surface is considered as a key working condition. (4) An excellent settling effect can be realized if the settling chamber without any filtering device and affiliated baffle structure are long enough. (5) The secondary blowing can be avoided if the inlet and outlet of the setting chamber are far away from the dust-accumulation surface. This study has a good guiding significance for the design of gravity settling chamber.

Impact of Typhoon on Evaporation Duct in the Northwest Pacific Ocean

Evaporation duct is a layer above the ocean surface due to the inherent humidity inversion at the air-sea boundary layer. The formation of the duct depends on meteorological factors near the ocean surface and is affected by weather process above the ocean. The presence of this duct can have profound effects on over-water electromagnetic propagation at microwave bands. However, the information on evaporation duct features during the period of typhoon is limited. Here we show the impact of a typhoon on evaporation duct using the typhoon Parpiroon occurred in 2012 as an example. It is very interesting to find that there is an “evaporation duct eye” with very low evaporation duct height in the eye of typhoon. The trajectory of the typhoon center is almost consistent with that of the evaporation duct height minimum center during the typhoon Parpiroon. This is also confirmed by 19 more typhoons over northwest Pacific Ocean in 2018. Furthermore, we found that the low wind speed in the typhoon center is the primary cause of this interesting phenomenon. Our results demonstrate the characteristics of evaporation duct distribution during the typhoon process and illustrate the correlation between typhoon trajectory and evaporation duct height minimum center trajectory.

Research on the Airflow and Thermal Performance in a Large Forced Air-Cooled Permanent Magnet Synchronous Machine

In this paper, the airflow and temperature distribution characteristics of a 1.65MW permanent magnet synchronous machine (PMSM) adopting a totally enclosed self-circulation (TESC) axial ventilation system (AVS) are studied by numerical methods. Base on the 3-D finite volume method, the ventilation performances of the PMSM with different ventilation hole numbers on the frame and stator axial ventilation duct heights are compared to determine the optimal ventilation hole number and stator axial ventilation duct height. Then, based on the 3-D finite element method (FEM), the thermal sensitivity analysis is performed by considering the variations of the mesh and thermal conductivity of the winding insulation, and the mesh and size of the assembling clearance. The results can be benefit for identifying the most important factors affecting the accuracy of 3-D FEM thermal calculation of PMSMs. The temperature distribution of the 1.65MW PMSM with the TESC AVS is obtained in the conditions of the optimal ventilation system. At last, a 1.65MW PMSM prototype with the presented TESC AVS is developed according to the airflow and temperature calculation results, and the temperature rise test performed on the prototype validates the accuracy of the simulation results.