Flow Transfer Characteristics Research Articles

Numerical study on heat and flow transfer characteristics in rectangular mini-channel with S-shaped turbulator inserted

The mini-channel heat exchanger has better heat and flow transfer characteristics than conventional heat exchanger. In this study, we use the S-shaped turbulator inserted, which is simple to install and inexpensive, to improve the heat exchange. The fluid-flow and heat transfer characteristics in smooth and inserted S-shaped turbulator rectangular mini-channels are simulated by numerical simulation under constant wall temperature heating in the Reynolds number 198.77-1987.67. The results of the numerical simulation show that compared with the smooth mini-channel, the pressure drop of the three rectangular mini-channels with different inserted S-shaped turbulators with radii on the horizontal axis of 1 mm, 1.5 mm, and 2 mm increase by 334.8-774.3%, 275.9-606.4%, and 234.6-500.4%. The average Nusselt numbers grow by 28.7-98.6%, 18.8-92% and 11.1-88.5%. The total thermal resistances reduce by 22.43-50.15%, 15.91-48.40%, and 10.08%-47.45%. The field coordination numbers increase by 30.36%-115.29%, 19.72-106.94%, and 19.72-104.72%. Moreover, the non-linear regression method establishes the prediction formulas of the pressure drop and average Nusselt number. In most cases, the deviation between predicted and simulated values is between ?4.6% and ?14%.

NUMERICAL INVESTIGATION OF FLOW AND HEAT TRANSFER CHARACTERISTICS OF SUPERCRITICAL CO2 IN HIGH-SPEED ROTATING ANNULUS

NUMERICAL INVESTIGATION OF FLOW AND HEAT TRANSFER CHARACTERISTICS OF SUPERCRITICAL CO2 IN HIGH-SPEED ROTATING ANNULUS

Security Margin Assessment Method of Centralized Spot Market Based on Information Entropy

In the centralized spot market, each participant pursues the maximization of economic benefits, which leads to the operation condition of the power grid closer to the security boundary. Thus, in this paper, a novel approach to security margin assessment of the centralized spot market is proposed. By proceeding from the mechanism of influence of load variation on power flow, the relationship between power flow transfer characteristics and the security margin of the power grid is explored. Then, on the basis of information entropy theory, the power flow transfer balancing evaluation index of the power grid is established. And this index is applied for security margin assessment of the power grid in the environment of the centralized spot market. Results of an IEEE 14-bus system case study have validated the effectiveness and quickness of the proposed method. Meanwhile, in accordance with the analysis results, it is proved that the proposed method is capable of accurately identifying the critical power transmission and transformation equipment affecting the security margin of the power grid, which is helpful to identify the potential risks of the operation of the power grid in advance.

Effects of homogeneous-heterogeneous reactions and thermal radiation on magneto-hydrodynamic Cu-water nanofluid flow over an expanding flat plate with non-uniform heat source

This study presents the effect of non-uniform heat source on the magneto-hydrodynamic flow of nanofluid across an expanding plate with consideration of the homogeneous-heterogeneous reactions and thermal radiation effects. A nanofluid’s dynamic viscosity and effective thermal conductivity are specified with Corcione correlation. According to this correlation, the thermal conductivity is carried out by the Brownian motion. Similarity transformations reduce the governing equations concerned with energy, momentum, and concentration of nanofluid and then numerically solved. The influences of the effective parameters, e.g., the internal heat source parameters, the volume fraction of nanofluid, the radiation parameter, the homogeneous reaction parameter, the magnetic parameter, the heterogeneous parameter and the Schmidt number are studied on the heat and flow transfer features. Further, regarding the effective parameters of the present work, the correlation for the Nusselt number has been developed. The outcomes illustrate that with the raising of the heterogeneous parameter and the homogeneous reaction parameter, the concentration profile diminishes. In addition, the outcomes point to a reverse relationship between the Nusselt number and the internal heat source parameters.

Heat transfer characteristics of latent functionally thermal fluid flows in the pin-fin Micro channels

In view of the cooling problem of high power lasers, the existing traditional working fluids are replaced by the latent functionally thermal fluid. The laminar flow and heat transfer characteristics of the latent heat functionally thermal fluid and that of deionized water in a piece of pin-fin micro channels with height of 4 mm, width 2 mm and spacing 1 mm were experimentally studied. The results show that when Reynolds Number ranges 625~1125, the latent functionally thermal fluid shows better cooling performance and lower wall temperature than the deionized water, and it has an optimum concentration value; Its average Nusselt number is greater than that of the deionized water, and the average Nusselt number increases with the Reynolds Number. To clearly demonstrate the relationship between the average Nusselt number，Reynolds Number，mass concentration and Prandtl number，an experimental formula was further fitted based on the experimental data，the maximum relative deviation is 16.9%， which is acceptable for experiments. Along the length in the direction of flow, there exists an optimal length for a stable local heat transfer enhancement section.

Experimental and Numerical Studies of Heat Transfer Characteristics for Impinging Jet on Dimple Surfaces

Experimental and numerical works have been carried out to investigate the effect of impingement dimples surface on heat transfer characteristics in a circular test plate. Impingement jet technique and dimple shape can enhances the heat transfer rate caused by the high turbulence intensity. The cylindrical shape of dimple was used in this work. The simulation results were validated with the experimental data obtained from 4 types test plates. The simulate data displayed the flow structure and contour temperature surface on the test plate which provided the complex flow and heat transfer characteristics. The experimental results showed that the case of the dimple diameter (d) equal to the jet diameter (Dj), and the distance between jet and the test plate (B) is 2 times of Dj, yielded the maximum heat transfer rate. The heat transfer rate enhanced up to 200 % compared to a flat plate at Rej = 14,500.

Investigation of the effect of a bend in a transfer line that separates a pulse tube cold head and a pressure wave generator

A transfer line between a pulse tube cold head and a pressure wave generator is usually required to isolate the cold head from the vibrations of the compressor. Although it is a common practice to use a thin and narrow straight tube, a bent tube would allow design flexibility and easy mounting of the cold head, such as in a split Stirling type pulse tube cryocooler. In this paper, we report a preliminary investigation on the effect of the bending of the tube on the flow transfer characteristics. A numerical study using commercial computational fluid dynamics model is performed to gain insight into the flow characteristics in the bent tube. Oscillating flow experiments are performed with a straight and a bent tube at a filling pressure of 15 bar and an operating frequency of 40, 50 and 60 Hz. The data and the corresponding numerical simulations point to the hypothesis that the secondary flow in the bent tube causes a decrease in flow at a fixed pressure amplitude.

微通道内气-液弹状流动及传质特性研究进展 (Review on flow and mass transfer characteristics of gas-liquid slug flow in microchannels)

Gas-liquid slug flow (also termed as Taylor flow) is a flow pattern characterized by the alternate movement of elongated bubbles and liquid slugs. Gas-liquid slug flow operation in microchannels has been found important implications in the enhancement of gas-liquid reactions due to its advantages such as easy control, uniform bubble and slug size, narrowed residence time distribution as well as enhanced radial mixing. This review presents the basic conceptions and recent research progress on flow and mass transfer characteristics during the gas-liquid slug flow in microchannels. The gas bubble formation mechanisms, the corresponding bubble and liquid lengths, and mass transfer during bubble formation are summarized. For regular slug flow in the main section of microchannels, several important aspects are addressed including bubble cross-sectional shape and liquid film profile, internal liquid recirculation and leakage flow through the gutters, gas-liquid mass transfer coefficients and coupling phenomena between flow and mass transfer in physical and chemical absorption processes. Finally, an outlook is given for future research directions in this field.

Flow and Heat transfer Characteristics in a Square Cavity at High Rayleigh Number

Numerical simulations are performed to investigate the flow and heat transfer characteristics in a air-filled (Pr = 0.71), two-dimensional (2-D) cavity where the flow is induced by a buoyancy force due to differential heating of vertical walls at high Rayleigh numbers (10^9, 1.58 × 10^9, 10^(9.5), and 10^(10)). The governing equations are discretized spatially into a fourth-order-accurate compact form and integrated temporally by the second-order-accurate alternating direction implicit (ADI) method. Numerical results of instantaneous and time-averaged flow structures are presented. The time-averaged temperature and vertical velocity near the hot wall are used to generate the mean temperature and velocity profiles in the turbulent boundary layer, and to illustrate the required spatial resolution for simulations of turbulent natural convection. A universal structure is found to exist in the mean velocity and temperature turbulent boundary layer profiles for x^+ ＜ 10. This finding is also in good agreement with reported experimental data.

사인 웨이브 핀과 타원관으로 구성된 핀-관 열교환기의 공기측 습표면 성능

본 논문은 12.7mm 원관을 변형하여 만든 세장비 0.6(짧은 직경 10.0mm, 긴 직경 16.5mm)의 타원관이 적용된 사인 웨이브 핀-관 열교환기의 습표면 공기측 성능실험에 관한 것이다. 핀 핏치와 열 수를 변화시켜 총 12개의 시료에 대하여 실험을 수행하였다. 타원관에서 핀 핏치 변화에 따른 j, f 인자의 영향은 미미 하였으나, 열수에 따라서는 1row에서 j인자가 가장 낮았다 (가장 높은 2row의 81%). 하지만 원관에서는 1row에서 가장 높게 나타나는 차이점을 보였다. 이는 타원관에 적용된 사인 웨이브 채널의 유동 특성에 기인한 것으로 판단된다. 원관 열교환기와 $j/f^{1/3}$ 값을 비교한 결과 1열을 제외하고는 타원관 열교환기의 전열성능이 우수함을 알 수 있다. 즉, 타원관 열교환기의 $j/f^{1/3}$ 값이 원관의 값보다 2열에서 1.6~2.5 배, 3열에서 1.4~2.4 배, 4열에서 1.2~1.8배 크게 나타났다. 【Experiments were conducted on sine wave fin-and-tube heat exchangers having oval tubes under wet condition. Oval tubes having an aspect ratio of 0.6 were made, by deforming 12.7mm round tubes. Twelve samples, having different fin pitch and tube row, were tested. Results showed that, for oval tube samples, the effect of fin pitch on j and f factor was not significant. As for the effect of tube row, the lowest j factor was obtained for one row configuration(81% of two row configuration), which is clear contrast to round tube samples, where the highest j factor was obtained for one row configuration. Possible reasoning is provided considering the flow and heat transfer characteristics of sine wave channel combined with connecting oval tubes. Comparison of $j/f^{1/3}$ with plain fin-and-tube heat exchanger having 15.9mm O.D. round tube reveals that present oval fin-and-tube heat exchanger shows superior thermal performance except for one row configuration. In other words, $j/f^{1/3}$ of the two row oval tube heat exchanger was 1.6~2.5 times larger than those of round tube heat exchanger, 1.4~2.4 times larger for three row configuration and 1.2~2.8 times for four row configuration.】

Effects of Radiation and Magnetic Field on Unsteady Boundary Layer Flow of a Nanofluid Past an Exponentially Stretching Sheet in the Presence of Heat Source/Sink

An analysis has been carried out to investigate an unsteady two-dimensional hydromagnetic nanofluid flow past an exponentially stretching sheet in the presence of thermal radiation and heat generation/absorption. The governing time-dependent partial differential equations are transformed into ordinary differential equations, using an appropriate similarity transformation. The resulting ordinary differential equations are solved numerically using the shooting method. The effects of the governing parameters on the flow and heat transfer characteristics are analyzed and presented graphically. Different types of nanoparticles, namely, Cu, Ag, CuO, Al2O3 and TiO2 with water as the base fluid, are studied. It is found that the flow field and temperature are significantly influenced by the volume fraction of nanoparticles, radiation, heat generation, heat absorption and the magnetic effects. Comparisons with previously published works are performed in some special cases, and found to be in good agreement.

Comparison of turbulence models for transient cool-down natural convection in a cylindrical cavity

By using standard k-e and low Reynolds number k-ω model, we compared fluid flow and heat transfer characteristics generated by cooled surface in a cylindrical enclosure at a relatively high Rayleigh number. It was shown that although initialisation of flow is very alike in both models, sinking of convective plumes are mainly located around the periphery of the cylinder with k-e model. The low Reynolds k-ω model produced flow in the core region and smoother vertical temperature variation. Convection lines are developed in both models.

Effect of Pulsations on Flow and Heat Transfer Characteristics of an Impinging Jet

Experiments are carried out to investigate the effect of pulsations on the flow and heat transfer characteristics of an axisymmetric impinging jet on a flat plate heated by using a gold coated aim. Vertex motion in the impinging jet is visualized using a fog generator, and a thermochromatic liquid crystal (TLC) technique is used to measure the time averaged local temperature distributions on the impingement plate. In addition, the quantitative data for mean velocity and turbulence intensity are obtained employing hot-wire anemometer. Parameters such as pulsating frequency (f = 0, 10 and 20 Hz) and the nozzle-to-palate spacing (H/D = 2, 10) are considered at the jet Reynolds number of 20,000. Consequently, the significant changes of flow structure and local Nusselt number distribution due to pulsations are observed. In the case of H/D = 2, the enhanced heat transfer coefficient exceeding 30 % is observed at the stagnation point. At the high H/D, heat transfer rate increases with pulsation frequency.

A Study of Natural Convection Heat Transfer in a Composed Rectangular-Parallelogrammic Enclosure with a Guide Vane

The present experimental and numerical study investigate flow and natural convection heat transfer characteristics of a composed rectangular-parallelogrammic enclosure with a guide vane. The governing equations for the two-dimensional, laminar, natural convection process in an enclosure are discretized by the control volume approach which insures the conservative characteristics to be satisfied in the calculation domain, and solved by a modified SIMPLE algorithm. The momentum and energy equations are coupled through the buoyancy term. In this results of experimental study, the natural convection heat transfer characteristics was well coincided with conclusions of other earlier experimental researches and numerical analysis.

Influence of the Unsteady Wake on the Flow and Heat Transfer in a Linear Turbine Cascade

The influence of unsteady wake on the flow and heat transfer characteristics in a four-vane linear cascade was experimentally investigated. The unsteady wake was generated with four rotating rectangular plates located upstream of the cascade. Tested inlet Reynolds number based on chord length was set to 66,000 by controlling free-stream velocity. A hot-wire anemometer system was employed to measure turbulent velocity components. For the convective heat transfer coefficients measurement on turbine blade surface, thermochromic liquid crystal and gold film Intrex were used. It was found that the unsteady wake enhances the turbulent motion in the cascade passage and accordingly promotes the development and transition of boundary layer. It was found that the heat transfer coefficients on the blade surface increase as the plate rotating speed increases. However, the increasing of heat transfer coefficients is not significant in the case that Strouhal number is higher than 0.503.

Local Heat/Mass Transfer Measurement on the Effusion Plate in Impingement/Effusion Cooling Systems

The present study is conducted to investigate the local heat/mass transfer characteristics for flow through perforated plates. A naphthalene sublimation method is employed to determine the local heat/mass transfer coefficients on the effusion plate. Two parallel perforated plates are arranged in two different configurations: staggered and shifted in one direction. The experiments are conducted for hole pitch-to-diameter ratios of 6.0, for gap distance between the perforated plates of 0.33 to 10 hole diameters, and for Reynolds numbers of 5000 to 12,000. The result shows that the high transfer region is formed at stagnation region and at the midline of the adjacent impinging jets due to secondary vortices and flow acceleration to the effusion hole. For flows through the perforated plates, the mass transfer rates on the surface of the effusion plate are about six to ten times higher than for effusion cooling alone (single perforated plate). In general, higher heat/mass transfer is obtained with smaller gap distance between two perforated plates.

Heat Transfer Characteristics on Effusion Plate in Impingement/Effusion Cooling for Combustor

The present study is conducted to investigate the local heat/mass transfer characteristics for flow through perforated plates. A naphthalene sublimation method is employed to determine the local heat/mass transfer coefficients on the effusion plate. Two parallel perforated plates are arranged for the two different ways: staggered and shifted in one direction. The experiments are conducted for hole pitch-to-diameter ratios of 6.0, for gap distance between the perforated plates of 0.33 to 10 hole diameters, and for Reynolds numbers of 5,000 to 12,000. The result shows that the high transfer region is formed at stagnation region and at the mid-line of the adjacent impinging jets due to secondary vortices and flow acceleration to the effusion hole. For flows through the perforated plates, the mass transfer rates on the surface of the effusion plate are about six to ten times higher than for effusion cooling alone (single perforated plate). More uniform and higher heat/mass transfer characteristic is obtained in overall region with small gap between two perforated plates.

Flow and Heat Transfer Characteristics in a Slot Film Cooling with Various Flow Inlet Conditions

An experimental investigation is conducted to improve a slot film cooling system which can be used for the cooling of gas turbine combustor liner. The tangential slots are constructed of discrete holes with different injection types which are the parallel, vertical, and combined to the slot lip. The investigation is focused on the coolant supply systems of normal-, parallel-, and counter-flow paths to the mainstream direction. A naphthalene sublimation technique has been employed to measure the local heat/mass transfer coefficients in a slot with various injection types and coolant feeding directions. The velocity distributions at the exit of slot lip for the parallel and vertical injection types are fairly uniform with mild periodical patterns with respect to the hole positions. However, the combined injection type increases the nonuniformity of flow distribution with the period equaling twice that of hole-to-hole pitch due to splitting and merging of the ejected flows. The secondary flow at the lip exit has uniform velocity distributions for the parallel and vertical injection types, which are similar to the results of a two-dimensional slot injection. In the results of local heat/mass transfer coefficient, the best cooling performance inside the slot is obtained with the vertical injection type among the three different injection types due to the effect of jet impingement. The lateral distributions of Sh with the parallel- and counter-flow paths are more uniform than the normal flow path. The averaged Sh with the injection holes are times higher than that of a smooth two-dimensional slot path

Numerical Simulation of Flow and Heat Transfer Characteristics of Impinging Jet Using $k-{\varepsilon}-{\overline{v^{'2}}}$ Model

This study deals with jet impingement, which is extensively used in the process industries to achieve intense heating, cooling or drying rates and also widely employed as a test flow for turbulent models due to its complex flow configuration, on a flat plate by numerical methods. In this calculation, the finite volume method was employed to solve the Navier-stokes equation based on the non-orthogonal coordinate with non-staggered variable arrangement. To get a better understanding for the fluid flow and heat transfer characteristics of the turbulent jet impingements, turbulent model was adapted and compared with the experimental data and the result of standard turbulent model. Numerical calculations were carried out with various flow rates, nozzle to plate distances. In the case of the axisymmetric jet impingement on a flat plate, turbulent model showed better agreement with the experimental data than the standard turbulent model in the prediction of the mean velocity profiles, the turbulent velocity profiles. the turbulent shear stress and the heat transfer rate. The highest heat transfer rate can be obtained when the impingement occurs within the potential core..

Air-side flow and heat transfer for a two-row lanced finned tube heat exchanger

A numerical study has been performed to obtain the air-side flow and heat transfer characteristics for a two-row lanced finned tube heat exchanger with 7 mm tube outer diameter. The increases of dimensionless local heat flux at the leading edge of slit and bottom surface of the fin were noticed. The temperature of air at downstream of the 2nd row of the lanced fin becomes more uniform than that of the plain fin because the mixing of energy increases by the slit and the side-slit. As the inlet velocity increases, the contribution of the 1st row to heat transfer decreases and that of 2nd row increases.