Flow Resistance Characteristics Research Articles

Effect of flow direction on circumferential velocity, radial velocity, and flow resistance characteristics of rotating gap structures

A rotating gap structure is a type of viscous flow resistance using two disks where the rotation of one disk drives the fluid within the gap, generating rotational inertia. This inertia, combined with viscous friction, determines the flow resistance characteristic curve (pressure drop vs flow rate, or Δp-Q curve). By adjusting the disk's rotational speed, the rotational inertia and the Δp-Q curve can be modified. This paper examines how the radial flow direction (positive and negative) affects the circumferential velocity, radial velocity, and the Δp-Q curve of the rotating gap structure through theoretical modeling and experiments. Results show that radial flow direction and rate influence the symmetric distribution of radial velocity and the linear distribution of circumferential velocity, altering the main components of the Δp-Q curve: the viscous flow resistance curve (Δpvis-Q) and the rotational inertia flow resistance curve (Δprot-Q). The study found that the slope of the Δpvis-Q curve is smaller for positive flow than for negative flow due to differences in radial velocity distribution. Additionally, the circumferential velocity is weakened in positive flow and enhanced in negative flow, resulting in a smaller slope of the Δprot-Q curve for positive flow. These factors cause the Δp-Q curve to deviate from linearity, with greater deviation at higher rotational speeds. Finally, experimental verification was conducted, and the measured Δp-Q curve closely matched the theoretical calculations.

Development of a liquid metal subchannel code applied to ocean conditions and study on the effect of core geometry parameters and ocean motions on flow and heat transfer characteristics

The Generation IV International Forum (GIF) proposes a type of liquid metal reactor, due to its thermal properties and efficient heat transfer in a relatively small system. This special heat transfer characteristic allows the liquid metal reactor to be modular in design and flexible in installation on a nuclear-powered ship. Accurate prediction of the coolant temperature and flow distribution between subchannels is important to ensure nuclear safety in the complex ocean environment. In this study, an ocean version of subchannel code is developed by adding the motion terms in the momentum equations for the liquid metal reactor core based on the previously land-based subchannel code. Since the research on the characteristics of flow resistance and heat transfer mechanism under ocean conditions is not mature, the original empirical model is still used in the ocean version code. The constitutive models of the liquid metal are analyzed and incorporated into the ocean code. The code has been verified and validated by comparing with Computational Fluid Dynamics (CFD) simulation results, and experimental data. The effect of geometric parameters such as rod Pitch to Diameter ratio (P/D), Rod to Wall gap (RTW) is studied in the liquid metal lead 7 rod bundles. In addition, the effect of heaving start-up and shutdown motions between liquid metal lead and water is discussed in the 5 × 5 bare rod bundles. In general, the modified subchannel analysis code can well complete the calculation of liquid metal reactor under ocean conditions, as well as provide support for the design and safety analysis of a liquid metal cooled fast reactor.

Experimental and numerical study on the flow and heat transfer of ring-ribbed tank in deep-sea manned submersible

The ring-ribbed tank is the key equipment of cooling system of deep-sea manned submersible. The heat transfer characteristics of the outer wall and the heat transfer and flow characteristics of the inner flow channel of the ring-ribbed tank for different inlet and outlet temperatures and flow rates were studied through a water tank test. The results indicate that Nusselt number for the inner flow channel correlates with Reynolds and Prandtl numbers, while for the outer wall, it correlates with Grashof and Prandtl numbers with an overall error within 5%. A numerical study using the Realizable k-ε turbulence model and Boussinesq approximation of the flow-solid conjugate heat transfer is conducted, which can accurately determine the heat exchange capacity and the import and export differential pressure with an average relative error of 1.98% for hot water temperature and 7.73% for shell wall temperature. Additional simulations for determining the heat transfer capacity and flow resistance characteristics of the ring-ribbed tank in open water results demonstrate that the differential pressure and its coefficient are basically consistent with model test results. The cooling system can achieve a heat transfer rate of up to 17 kW with an external seawater flow velocity of approximately 2 knots.

Modeling and Evaluation of Fluid Flow Resistance Characteristics of the Cooling Channel in the Aircraft Oil-Cooled Electric Machine

Modeling and Evaluation of Fluid Flow Resistance Characteristics of the Cooling Channel in the Aircraft Oil-Cooled Electric Machine

Analysis of film cooling and flow resistance characteristics of turbine blades with thermal barrier coatings

This paper employs numerical simulation to investigate the influence of thermal barrier coatings' (TBCs) thickness and surface roughness on the cooling and flow resistance characteristics of turbine blades. The results indicate that the application of TBCs significantly enhances the surface cooling efficiency of the blades. Turbine blades coated with a 0.4 mm thickness of TBC compared to blades without thermal barrier coatings, the average cooling efficiency of the blade surface increases by 12.3 %, and the maximum temperature drop at the leading edge(LE) is 317.8 K. However, the small increment in TBCs thickness leads to an increase in aerodynamic losses in the vane passage. The static pressure coefficient continuously decreases in the suction side(SS) within the interval 0.3 < x/C < 1.0. The average flow coefficient of the film holes exhibits distinct variations in different regions of the blade. As surface roughness increases, the cooling efficiency on the SS and pressure side(PS) of the blade decreases, while the heat transfer enhancement at the LE and cooling efficiency improve. Compared to a smooth coated surface, when the surface roughness height increases to 20 μm, the blade surface cooling efficiency decreases by 1.12 %, and the average temperature rise is 10.3 K. Simultaneously, the energy loss coefficient and total pressure loss coefficient in the vane passage rise with the increase in surface roughness, while the variation in the average flow coefficient of the film cooling holes remains relatively small.

Influences of thermal and mass stratification on unsteady magnetohydrodynamics parabolic flow along an infinite vertical plate with periodic temperature variation and exponential mass diffusion in porous medium

AbstractThis study explores the dynamics of unsteady magnetohydrodynamics (MHD) parabolic flow along an infinite vertical plate, emphasizing the effects of thermal and mass stratification in a porous medium subjected to periodic temperature variation and exponential mass diffusion. Utilizing the Laplace transform technique to obtain precise solutions, this study effectively integrates the impacts of both thermal and mass stratification without dependence on approximations. The main goal is to assess how thermal and mass stratification impact MHD flow dynamics, temperature, and concentration profiles under varying conditions. The study provides a thorough comparison of these findings with traditional nonstratified scenarios, presenting a comprehensive analysis of fluid behavior under diverse conditions. The conclusions reveal that thermal and mass stratifications considerably diminish velocity and stabilize temperature distributions, which suggests a damping influence on fluid movement and improved management of diffusion processes. Enhanced Grashof numbers improve heat and mass transfer efficiency, while magnetic and Darcy parameters significantly influence flow resistance and heat transfer characteristics. These conditions also result in higher Nusselt and Sherwood numbers, indicating increased efficiency in heat and mass transfer. In contrast, scenarios without stratification display higher velocities and more unstable temperature and concentration profiles. The findings highlight the critical role of stratification in improving fluid dynamics and increasing the efficiency of heat and mass transfer processes, offering valuable insights for engineering and environmental applications in similar conditions. The main novelty of the research is being the first to use the Laplace transform for exact solutions on combined thermal and mass stratification in MHD flows, enhancing prediction accuracy and process control.

Experimental study on heat storage characteristics of an air-sand moving bed heat exchanger

The air-sand moving bed heat exchanger, designed for storing fluctuating medium-temperature gas waste heat, offers significant potential for energy conservation and carbon reduction in industrial settings. However, its operational characteristics are insufficiently explored. This study investigates the heat storage process involving direct contact and cross-flow of air and quartz sand within a rectangular heat exchanger chamber. Comprehensive analyses were conducted to explore flow resistance characteristics, temperature response, and heat storage performance. Results indicate that pressure drop increases with higher air velocity and temperature, and lower air pressure, particle diameter. The relative discrepancy between fitted pressure drops and experimental data is below 10%. The primary heat transfer zone exists during particle descent. As the air-particle mass flow ratio increases, the descent rate of this zone along the airflow direction slows. Additionally, higher air temperatures elevate the optimal air-particle mass flow ratio. With inlet temperatures for air and particles set at 350 °C and 30 °C respectively, and a mass flow ratio of 0.81, an exergy efficiency of 58.93% and a power density of 600.64 kW·m−3 are achieved. These findings offer crucial insights for air-sand moving bed heat exchangers for medium-temperature gas waste heat recovery.

Experimental study on flow and heat transfer of High-Pressure helium flow in compact helical tube heat exchanger in HTGR

In this study, experiments were conducted on helium flow within a compact helical tube heat exchanger. High-temperature helium and deionized cooling water, flowing in opposite directions, were allocated to the shell side and tube side, respectively. The Reynolds number of helium ranged from 3 × 103 to 1.6 × 104 under an operational pressure of 2.1 MPa. Measurements were taken for helium temperature, tube outer-surface temperature distribution, pressure, and mass flow rate to illustrate the flow and heat transfer characteristics. Sensitivity analysis of thermal–hydraulic parameters, including heat transfer power and efficiency under various operational conditions, was performed. The convective heat transfer coefficient and pressure drop were calculated. The transition point (Re = 9000) between laminar and turbulent flow of helium on the shell side was identified, and empirical correlations for the Nusselt number and friction coefficient were proposed. The experimental results indicate that the heat transfer performance of helium in the compact helical tube heat exchanger exceeds that of water in large-scale helical tube heat exchanger by approximately 20.75 % under fully developed turbulence, while the flow resistance characteristics remain essentially consistent.

Research on the erosion resistance characteristics of liquid-solid two-phase flow in high-pressure manifolds

During on-site throttling spray operations, the ground high-pressure manifold is subjected to the erosive effects of high-speed liquid-solid two-phase flow, which poses a high risk of erosive damage and eventual failure. In response to the erosive conditions caused by high-speed sand-carrying liquid-solid two-phase flow on the high-pressure manifold, an experimental apparatus for erosive testing was designed, taking into account key factors such as internal pressure loads. Microscopic erosive morphologies were observed in the experiments. Subsequently, numerical simulation methods were employed to investigate the identification of erosive danger zones and the influence of different installation angles, sand content, pipe flow velocities, and viscosity on erosive areas of the manifold.The research findings indicate that the microscopic morphological changes in erosive experiments align with the micro-cutting theory, which was successfully applied in simulation. Erosive danger zones are mainly distributed on the outer arch side, the curved arc connecting section, and the initial part of the outlet straight pipe. The erosive areas do not vary with changes in installation angles, sand content, and flow velocities. As viscosity increases, the erosive area shifts. The erosive rate is positively correlated with sand content and pipe flow velocity, and with increasing viscosity, the erosive rate on the first outer arch side shows an initial decrease followed by stabilization, while the second outer arch side exhibits an initial increase followed by stabilization. As the installation angle increases, the erosion rate on the second outer arch side first shows a stable trend and then decreases rapidly. At 75°, the erosion rate reaches the lowest.

Experimental Dust Absorption Study in Automotive Engine Inlet Air Filter Materials.

The purpose of this study was to empirically evaluate the performance of fibrous materials that meet the criteria for inlet air filtration in internal combustion engines. The characteristics of filtration efficiency and accuracy, as well as the characteristics of flow resistance, were determined based on the mass of dust accumulated in the filter bed during the filtration process. Single-layer filter materials tested included cellulose, polyester, and glass microfiber. Multilayer filter media such as cellulose-polyester-nanofibers and cellulose-polyester were also examined. A new composite filter bed-consisting of polyester, glass microfiber, and cellulose-and its filtration characteristics were evaluated. Utilizing specific air filtration quality factors, it was demonstrated that the composite is characterized by high pre-filtration efficiency (99.98%), a short pre-filtration period (qs = 4.21%), high accuracy (dpmax = 1.5-3 µm) for the entire lifespan of the filter, and a 60-250% higher dust absorption coefficient compared to the other tested materials. A filtration composite bed constructed from a group of materials with different filtration parameters can be, due to its high filtration efficiency, accuracy, and dust absorption, an excellent filter material for engine intake air. The composite's filtration parameters will depend on the type of filter layers and their order relative to the aerosol flow. This paper presents a methodology for the selection and testing of various filter materials.

Variable-dimension optimization study and design of internally finned helically coiled tubes heat exchangers based on numerical simulation and experiment

To improve the heat transfer in the helically coiled tubes, a novel type of internally finned helically coiled tube is proposed. The heat transfer and flow resistance characteristics of internally finned helically coiled tubes are investigated numerically. The empirical correlations to calculate the Nusselt number and friction factor of internally finned helically coiled tubes are summarized. The maximum relative error is not exceeding 18 %. To validate the empirical correlations, a test bench to evaluate the performance of internally finned helically coiled tubes heat exchangers is constructed, with a smooth helically coiled tubes heat exchanger as a comparison. The experimental results indicate that 94 % of the data exhibit a relative error of less than 30 % when compared to the fitting correlations. The heat transfer coefficient of the tube side is increased by approximately 30 %. Based on the validated correlations, a generalized variable-dimension optimization approach is proposed to solve the complex problem of optimizing the design of helically coiled tubes heat exchangers. Taking the area margin, heat transfer area and total cost, the structure of internally finned helically coiled tubes heat exchangers is optimized with different limitation. The optimization results show that the internally finned helically coiled tubes heat exchangers have better performance than smooth helically coiled tubes heat exchangers. A helically coiled tubes heat exchanger has been designed in an engineering project and significant improvements have been achieved in various performance indicators.

Unlocking the Thermal Efficiency of Irregular Open-Cell Metal Foams: A Computational Exploration of Flow Dynamics and Heat Transfer Phenomena

An open-cell metal foam has excellent characteristics such as low density, high porosity, high specific surface area, high thermal conductivity, and low mass due to its unique internal three-dimensional network structure. It has gradually become a new material for enhanced heat transfer in industrial equipment, new compact heat exchangers, microelectronic device cooling, etc. This research established a comprehensive three-dimensional structural model of open-cell metal foams utilizing Laguerre–Voronoi tessellations and employed computational fluid dynamics to investigate its flow dynamics and coupled heat transfer performance. By exploring the impact of foam microstructure on flow resistance and heat transfer characteristics, the study provided insights into the overall convective heat transfer performance across a range of foam configurations with varying pore densities and porosities. The findings revealed a direct correlation between convective heat transfer coefficient (h) and pressure drop (ΔP) with increasing Reynolds number (Re), accompanied by notable changes in fluid turbulence kinetic energy (e) and temperature (T), ultimately influencing heat transfer efficiency. Furthermore, the analysis demonstrated that alterations in porosity (ε) and pore density significantly affected unit pressure drop (ΔP/L) and convective heat transfer coefficient (h). This study identified an optimal configuration, highlighting a metal foam with a pore density of 20 PPI and a porosity of 95% as exhibiting superior overall convective heat transfer performance.

Thermo-hydraulic performance of solar air heater with built-in one-eighth sphere vortex generators

The primary objective of optimizing solar air heaters is to enhance their thermal efficiency, and the incorporation of perturbing elements into the absorber plate has proven to be an effective approach. The present study employed an electric heating plate to simulate solar heating and proposed a novel one-eighth sphere vortex generator to create strong longitudinal vortices and weak transverse vortices, thereby achieving enhanced overall performance. The experimental investigation focused on examining the impact of parameters, including deflection angle (α), relative transverse pitch (S/H), relative longitudinal pitch (L/H), relative height (e/H), and Reynolds number (Re), on both heat transfer and flow characteristics. The results show that new vortex generator performs best at deflection angles of 180°. The maximum thermo-hydraulic performance parameter of 2.03 was observed in this study. Additionally, it was found that the maximum Nusselt number (Nu) and friction factor (f) were 2.45 and 1.96 times higher than those obtained for a smooth duct, respectively. New correlations of Nusselt number (Nu) and friction factor (f) were developed to predict the heat transfer and flow resistance characteristics of solar air heater with built-in one-eighth sphere vortex generators. This study proposes a new idea to enhance the performance of solar air heaters.

Numerical investigation on the flow resistance characteristics of balance block in the rotary compressor

With the increasing demand for energy saving, the air conditioner with variable frequency becomes the mainstream of the market, which leads to a high requirement for the dynamic balance characteristics of the rotary compressor at variable speed conditions. The balance block is the key part in the eccentric shaft system to balance the rotational inertia force brought by the eccentric wheel, and its flow resistance during rotating process has a large impact on the power consumption. In this paper, CFD simulations will be carried out to study the flow characteristics of several different compressor balance blocks and the results can provide reference for the optimization design of the balance block in the rotary compressor.

Study on the resistance characteristics of layered vegetation to overland flow

AbstractDifferent types of plants in the vegetation community near the surface of the basin coexist, forming a layered vegetation distribution with high and low plants in the morphology. In order to research the characteristics of flow resistance generated by layered vegetation on slope runoff, a flume experiment was carried out by simulating layered vegetation. The Manning roughness coefficient n was used to characterize the flow resistance of vegetation. Three kinds of vegetation with height combinations of 5 and 7 cm, 6 and 8 cm, and 7 and 9 cm were used for this experiment. By studying the relationship between Manning roughness coefficient n and water depth, it is found that the change of flow resistance of layered vegetation is closely related to the submerged state. The distribution of n shows an inverted “J” type with the increase of water depth. Under the condition of nonsubmerged state and transitional submerged state, n increases with the increase of water depth. The critical point n values of the two states will plummet, and the growth rate of n in the nonsubmerged state is greater than that in the completely submerged state. In the transitional submerged state and completely submerged state, the higher the plant height below the water surface, the higher the corresponding n value and the greater the change rate of n. Formula for predicting overland flow resistance of layered vegetation was established by considering the influence of combined vegetation height and Reynolds number.

Theoretical and experimental study on a novel evaluation criteria of heat exchangers applied in aero engines

For the application of heat exchangers in the aeronautic field, an effective evaluation method is required to support the satisfaction of the multi-aspect design restrictions including the thermal and hydraulic performance, occupied volume, total weight, etc. To weigh the benefit and cost for both heat transfer units and heat exchangers, this study thus proposes three evaluation indexes Rp, RV, and Rm defined by the product of heat transfer coefficient and heat transfer area of unit windward area times pressure drop (hA/(AfrontΔp)), that of unit volume (hA/V) and that of unit weight (hA/m). Considering the factors h and A comprehensively and excluding the influence of mean differential temperature, the three indexes can reflect the low flow resistance, compactness, and lightness characteristics under fixed operating conditions and heat transfer duty requirement accordingly. A validation experiment is carried out and the deviation between the evaluation and experiment results is less than 10.88% for the three indexes. To show the applicability, a detailed evaluation of six different heat transfer units and five types of heat exchangers is conducted. A heat transfer unit selection method based on the new indexes is provided finally for heat exchanger design.

Vapor flow resistance characteristics in ultra-thin flat heat pipes

Miniaturization of electronic devices drives the development of ultra-thin flat heat pipes (UTFHPs). As the thickness of the heat pipe decreases, the microchannel effect of the vapor chamber exacerbates the deterioration of the vapor flow. The vapor flow resistance, which is often neglected in the conventional theory due to its extremely small effect compared to the liquid flow resistance, becomes a critical factor limiting the heat transfer performance of UTFHPs. To provide a theoretical basis for the design and performance evaluation of UTFHPs, this paper investigated the effect of the wicking structure and working fluid on the vapor flow resistance of UTFHPs based on theoretical derivations, numerical calculations, and experiments. The results indicate that conventional analytical theory is no longer applicable when the total thickness of the heat pipe is less than 1 mm. The vapor flow resistance obtained using the conventional equation is significantly smaller than the experimental results, which is caused by several factors: raised momentum loss at the wall and gas-liquid interface due to the growth of the velocity gradient in the boundary layer; exacerbated flow instability due to the mixing between the vapor and the working fluid; enhanced fluctuating effects on the flow channel due to the presence of the wicking structure. A dimensionless flow resistance coefficient equation was proposed with the vapor Reynolds number and the channel height ratio. An experiment was conducted to verify the accuracy of the flow resistance correlation. The difference between the experimental results and the calculated values is less than 15 %.

The influence of tube diameter parameters on the flow resistance and heat transfer characteristics of supercritical CO2 in horizontal tubes

The diameter parameters of tubes are crucial for the flow and heat transfer characteristics of supercritical fluids. however, the diameter effect has not been fully understood. The existing literatures only investigate the effect of different tube diameters on the flow and heat transfer characteristics within a narrow parameter range, lacking specific theoretical analyses and verification in a wide range of parameters. Therefore, experimental research was conducted to investigate the influence of tube diameter parameters on the flow resistance and heat transfer of supercritical carbon dioxide(sCO2) in horizontal tubes and an explanation of the tube diameter effect mechanism is provided based on pseudo-boiling theory. Stainless steel smooth circular tubes with diameters of 8 mm, 10 mm, and 12 mm were used as the test section, and various experiments were carried out under the range of mass flux (G): 509.23–1267.34 kg/m2s, heat flux (qw): 97.91–410.43 kW/m2, and operating pressure (P): 5.54–20.32 MPa. From the pseudo-boiling theory analysis, the influence of tube diameter parameters has a greater influence on the heat transfer in the two-phase-like (TPL) regime and is almost negligible when the P exceeded 1.5Pc, indicating the interaction between liquid-like (LL) and vapor-like (VL) phases is found to be the dominant reason for the tube diameter effect on heat transfer and the correlations for predicting the maximum wall temperature difference between the top and bottom generatrix (ΔTmax) are established. The present work can significantly enhance the understanding on sCO2 flow boiling mechanism and provide a new research direction for supercritical fluid.

Parametric study and optimization of cast Al-Si alloy heat exchanger using Taguchi method

A 3-D numerical model of staggered pin fin array in cast Al-Si alloy heat ex-changer was established. For further structural optimization, seven geometric parameters were studied by Taguchi method, including diameter, width, transverse spacing, longitudinal spacing, offset distance, slit length, and row number. The 32 cases with different geometric parameters were simulated numerically. The heat transfer characteristics, flow resistance characteristics, and overall performance were fully discussed. The diameter and transverse spacing have important influence on heat transfer characteristics, and the structural parameters that affect the resistance characteristics are diameter, longitudinal spacing and slit length. Then analysis of variance shows that diameter and transverse spacing have greater effects on the overall performance than other five parameters. At last, the optimized structure of staggered pin fin array was obtained, and j/f1/3 increased by 1% to 3% for volume flow rate in the range from 0.03 to 0.11 m3/s.

FLOW AND HEAT-TRANSFER CHARACTERISTICS IN SMALL-DIAMETER TUBE BUNDLES WITH A STAGGERED LAYOUT: AN EXPERIMENTAL STUDY

While past research has primarily focused on heat transfer in larger diameter tubes, the capabilities of small-diameter tubes have often been estimated using correction factors. However, with the evolution of compact heat exchangers, conducting dedicated studies on small-diameter tube bundles has become increasingly crucial to achieve more precise conclusions. An experimental research on flow and heat-transfer characteristics of staggered tube bundles with different tube diameters (2, 3, and 5 mm) is conducted. In the experiment, the number of rows (4-12), the mass rate of the air (0.06-0.18 kg/s), and the transverse tube pitch (&lt;i&gt;S&lt;/i&gt;&lt;sub&gt;1&lt;/sub&gt;/&lt;i&gt;d&lt;/i&gt; &amp;#61; 2, &lt;i&gt;S&lt;/i&gt;&lt;sub&gt;1&lt;/sub&gt;/&lt;i&gt;d&lt;/i&gt; &amp;#61; 3) are variables to study the characteristics of the airside flow resistance and heat transfer. The three main conclusions of the experimental results are as follows: (1) Under the same conditions, the smaller tube diameter leads to the larger airside convective heat-transfer coefficient. Besides, the deviation between the Nusselt number of the experiment and the empirical correlation of &amp;Zcaron;ukauskas is in the range between -14 and -10&amp;#37;; (2) The effect of transverse distance on heat transfer is not obvious, but the convective heat-transfer coefficient increases significantly with the increase of row number; (3) The external pressure drop of the tube exhibits an exponential increase with the air-flow rate. Particularly in the experimental samples with smaller diameters, the outflow resistance of the tube is noticeably higher compared to other tubes. Finally, new empirical correlations of the airside convection heat transfer for the small-diameter staggered tube bundles are fitted according to the experimental data, and it is hoped to provide a reference for the more accurate design of tube-bundle heat exchangers.