无机传热元件启动性能实验研究

为了研究无机传热元件的启动过程,采用无机传热元件实验装置,测试了不同倾斜角度、加热水入口温度和加热段冷却段长度比下光管式无机传热元件的启动性能。实验结果表明:无机传热元件在倾角为30°、60°和90°时均具有良好的启动性能,且倾角越大,稳定工作时绝热段管壁温度越高;加热水入口温度越高,无机传热元件稳定工作时的工质温度越高;加热段与冷却段长度比越大,最小启动角越大,启动越困难,在实际应用中,为保证无机传热元件的顺利启动,其安装角度确保大于2°。     

Heat transfer performance in 3D internally finned heat pipe

An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 °C–95 °C, heat flux 5.0 kw/m²–40 kw/m², inclination angle 2–90°), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50–100% and 100–200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance.     

倾角对环状热管蒸发器性能的影响

针对生产过程中低品位能量回收,设计了带有环状管蒸发器的不锈钢水工质重力型分离式热管,环状管由31.6 mm管径内管热水加热,环空间隙为15.0 mm,可视化地研究了26 kPa蒸发压力,0~90 °倾斜角度下多个充注率环状管蒸发器的壁温特性。结果表明：该类热管的环状管蒸发器运行时存在一高温区,随倾角增加而扩大;环状管内蒸发侧平均表面换热系数随倾角增大先增后减、再增大,与沸腾流型随角度发生转变有密切关联;与一些相似文献进行了对比,发现环状管蒸发器与普通重力型热管在换热性能均在10~20 °倾角达到极大值,而环状管蒸发器则在90 °时达到了另一极大值。     

Heat Transfer Measurements,Flow Pattern Maps,and Flow Visualization for Non-Boiling Two-Phase Flow in Horizontal and Slightly Inclined Pipe

Local heat transfer coefficients and flow parameters were measured for air-water flow in a pipe in the horizontal and slightly upward inclined (2°, 5°, and 7°) positions. The test section was a 27.9 mm stainless steel pipe with a length to diameter ratio of 100. For this systematic experimental study, a total of 758 data points were taken for horizontal and slightly upward inclined (2°, 5°, and 7°) positions by carefully coordinating the liquid and gas superficial Reynolds number combinations. These superficial Reynolds numbers were duplicated for each inclination angle. The heat transfer data points were collected under a uniform wall heat flux boundary condition ranging from about 1,800–10,900 W/m². The superficial Reynolds numbers ranged from about 740 to 26,000 for water and about 560 to 48,000 for air. A comparison of heat transfer data and flow visualization revealed that the heat transfer results were significantly dependent on the superficial liquid and gas Reynolds numbers, inclination angle, and flow pattern. The experimental data indicated that even in a slightly upward inclined pipe, there is a significant effect on the two-phase heat transfer of air-water flow. Flow pattern maps and flow visualization results for different inclination angles are also presented and discussed.     

Flow and Heat Transfer Characteristics of Two‐Phase Fluid in Inclined Pipes on a Rotation Platform

A rotating platform was used to create dynamic load, and the mixture air–water two‐phase flow and boiling steam–water two‐phase flow were obtained in an inclined test pipe. By changing the parameters, such as inclination of the test pipe, rotational speed, inlet temperature, flow rate, and so on, the experiments for two‐phase flow in the pipe at inclination of 0°, 45°, and 66° were conducted, respectively. The effects of acceleration and inclination on their flow and heat transfer characteristics were investigated. The two‐phase flow patterns in inclined pipes under rotation conditions were caught with a video camera. The images show that the impact mixed flow and churn flow were found in this research. The results show that the acceleration and pipe inclination significantly influence the flow characteristic and heat transfer of the two‐phase pipe flow. As the directions of the dynamic load and the gravity are opposite to the flow direction, the greater the dynamic load and inclination, the higher the pressure drop and the heat emission, and the lower the flow rate, the void fraction, and the fluid temperature. Therefore, the dynamic load and gravity will improve the flow resistance, enhance heat emission and reduce the heat gained by the fluid.     

Thermal performance of inclined screen mesh heat pipes using silver nanofluids

This study presents the effect of silver nanofluid on thermal performance of inclined screen mesh heat pipe in cooling applications. Four cylindrical copper heat pipes containing two layers of screen mesh were fabricated and tested with distilled water and water based silver nanofluids with mass concentrations of 0.25%, 0.5% and 0.75% as working fluids. The experiments were performed at four inclination angles of 0°, 30°, 6° and 90°. The main focus of this study is to investigate inclined heat pipe performance with nanofluid. Experimental results indicate that the thermal performance of heat pipes was improved with nanofluids compared to water and thermal resistance of the heat pipes decreased with the increase of nanoparticle concentration. Moreover, the thermal performance of the heat pipes at inclination angle of 60° is found to be higher than other tested inclination angles, which shows the effect of gravity on heat pipe performance.     

Analysis of Flow and Heat Transfer in a Parallelogram Non‐Uniformly Heated Enclosure Filled with Porous Medium

The flow and heat transfer in a parallelogram enclosure filled with a porous medium is analyzed numerically. The heated bottom wall has a sinusoidal temperature distribution and side walls cooled isothermally while the upper wall is well insulated. Dimensionless Darcy law and energy equations are solved using the finite difference method along with the corresponding boundary condition. Computations were carried out for four inclination angles of side walls (γ = 45°, 60°, 75°, 90°) with different Rayleigh numbers (100≤Ra≤1000) and their effects on the flow field and heat transfer are discussed. It is found that the inclination angle has a significant effect on flow pattern and heat transfer and an increase in the angle leads to a decrease in the strength of the right vortex. The study also revealed that as the Rayleigh number increases at γ = 45°, another (third) vortex develops along the left wall and its strength enhances with Rayleigh number. At the end, a correlation is extracted from the numerical data which represents the relation between the Nusselt number, inclination angle, and the Rayleigh number. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; 39(7): 497–506, 2010; Published online in Wiley Online Library ( wileyOnlinelibrary.com ). DOI 10.1002/htj.20312     

Heat transfer in gas–solid fluidized bed with various heater inclinations

The study explored the heat transfer properties in an air-fluidized bed of sand, heated with an immersed heat transfer tube positioned at several angles of inclination. Operating with fluidizing velocity up to 0.5 m/s; and particles of 150–350 μm diameter, the effect of air velocity and particle size on the average and maximum achieved heat transfer coefficient was examined for the heat transfer tube at angles of inclination in the range 0–90°. Experimental results showed that the angle of inclination altered the bubble size and behavior close to the heat transfer tube hence the expected heat transfer coefficient, with the influence of tube inclination being less pronounced for smaller particles. The optimum angle of inclination was in the range of 10–15° relative to the direction of the flow, while the heat transfer coefficient had its lowest values at the angle of 45°, and thereafter improved upon transition to 90°. Upon comparison with existing correlations, a correction factor is proposed to account for the impact of the angle of inclination on the heat transfer coefficient calculated by the Molerus–Wirth semi-empirical correlation.     

Free Convective Heat Transfer in Tilted Longitudinal Open Cavity

Heat transfer experiments were performed to investigate the effects of inclination and channel height-to-gap ratio on free convection in a simulated fin-passage with a strategic aim of devising a criterion for selecting the optimal fin length that could provide the maximum free convective capability. The ranges of parameters investigated include the Grashof number, up to 500,000; channel height-to-gap ratios of 1, 2, and 3; and tilt angles of 0°, 30°, 60°, 90°, 120°, 150°, and 180°. Selections of local and spatially averaged Nusselt number results demonstrate the manner by which the Grashof number, tilt angle, and channel height-to-gap ratio interactively affect the heat transfer. In conformity with the experimentally revealed heat transfer physics, the correlation of a spatially-averaged Nusselt number over two parallel walls and the bottom surface of an open-ended channel is derived that permits the individual and interactive effects of the Grashof number, tilt angle, and channel height-to-gap ratio on heat transfers to be evaluated. A criterion for selecting the optimal height-to-gap ratio of the fin channel is subsequently formulated as a design tool for maximizing the convective capability of a free convective fin assembly.     

Feasibility of using multiport minichannel as thermosyphon for cooling of miniaturized electronic devices

The feasibility of using multiport minichannel (MPMC) as thermosyphon for cooling miniaturized electronic products is experimentally investigated with acetone as the working fluid. A detailed analysis on thermal performance and entropy generation due to heat transfer and pressure drop with the effects of heat load (10-50 W), filling ratio (FR; 40%, 50%, and 60%), and inclination angle (45°, 60°, and 90°) has been carried out. The results showed a reduction of 22.2% and 9.31% in thermal resistance and evaporator wall temperature at optimum filling ratio (OFR) of 50%. Reduction in entropy generation due to heat transfer and pressure drop of 16.6% and 12.3%, respectively, was observed at OFR. Internal fins in MPMC increase the surface area and evaporation rate by enhancing heat transfer leading to a decrease in the rate of entropy generation. Multiport increases surface tension of condensate at right angles to the flow direction along with the effects of gravity and enhancing rate of condensation. A new correlation is developed to predict evaporator wall temperature as a function of heat load and FR. The proposed correlation agrees well with a deviation of ±20% with present experimental results and also with the published literature. Thus, the obtained results will be useful in cooling miniaturized electronic devices.     

Biofouling Control with Ultrasound

The flow field in shell-and-tube heat exchangers with helical baffles was measured using laser Doppler anemometry (LDA). The influence on the velocity distribution, impulsive velocity by helix inclination angle, and flow rate was investigated. The influence on heat exchanging capability and flow resistance on velocity distribution was also investigated. The dimensions of the heat exchanger shell used in these experiments were 200 2 6 2 3,000 mm (inner diameter 2 wall thickness 2 length). The heat exchanger was made of organic glass and the tube bundle consisted of 52 tubes with external diameter of 15 mm. Six different inclination angles were designed in double-helix style: 30°, 35°, 40°, 42°, 45°, and 50°. The working flow medium under normal temperature was service water. Generally, the linear velocity and impulsive velocity will increase with decreasing helix inclination angle, which promotes the heat exchanging capability. With flow volume increasing, the velocity distribution along the diameter increases on average. The pressure drop increases with decreasing helix inclination angle. For all of the helix inclination angles tested, the minimum pressure loss took place at a certain Reynolds number; and at different helix inclination angles, the Reynolds number at which the minimum pressure loss occurs is different. In general, it was concluded that the optimum helix inclination angle depends on the Reynolds number of the working fluid on the shell side of heat exchanger.     

Thermal performance of inclined grooved heat pipes using nanofluids

An experimental study was performed to investigate the thermal performance of an inclined miniature grooved heat pipe using water-based CuO nanofluid as the working fluid. This study focused mainly on the effects of the inclination angle and the operating pressure on the heat transfer of the heat pipe using the nanofluid with the mass concentration of CuO nanoparticles of 1.0 wt%. The experiment was performed at three steady sub-atmospheric pressures. Experimental results show that the inclination angle has a strong effect on the heat transfer performance of heat pipes using both water and the nanofluid. The inclination angle of 45° corresponds to the best thermal performance for heat pipes using both water and the nanofluid. The present investigation indicates that the thermal performance of an inclined miniature grooved heat pipe can be strengthened by using CuO nanofluid.     

Numerical Analysis of Baffle Cut on Shell Side Heat Exchanger Performance with Inclined Baffles

In this work, an attempt has been made to decrease the pressure drop and to increase the heat transfer rate in a shell and tube heat exchanger (STHX) by tilting the baffle angle and by varying the baffle cut. The process of solving the simulation includes modeling, meshing, and analyzing the geometry of the STHX by using Pro-E, hypermesh, and computational fluid dynamics package of ANSYS Fluent, respectively. The objective of this study is to find a suitable baffle inclination and baffle cut for the efficient performance of the STHX. The baffle inclinations of 25°, 30°, 35°, and 40° were considered for three different baffle cuts of 25%, 30%, and 35% of shell inside diameter and the results were compared with segmental baffle of inclination angle 0°. The shell side flow with different inclination angles and baffle cuts results in a significant variation in heat transfer rate and pressure drop in the STHX. The results provide a clear idea that the heat transfer rate is maximum in inclined baffle heat exchanger compared to that of segmental baffle heat exchanger. Further it is found that the STHX with the configuration of 35º baffle inclination angle and baffle cut of 30% of shell inside diameter provides higher heat transfer rate with minimum pressure drop compared to all other configurations.     

Experimental study on heat transfer augmentation in a double pipe heat exchanger utilizing jet vortex flow

This paper examines experimentally the effect of jet vortex technology on enhancing the heat transfer rate within a double pipe heat exchanger by supplying the heat exchanger with water at different vortex strengths. A vortex generator with special inclined holes with different inlet angles was designed, manufactured, and integrated within the heat exchanger. In this study, four levels of Reynolds number for hot water in the annulus (Re_h) were used, namely, 10,000; 14,500; 18,030; and 19,600. Similarly, four levels of Reynolds number for cold water in the inner tube (Re_c) were used, namely, 12,000; 17,500; 22,500; and 29,000. As for the inlet flow angle (θ), four different levels were selected, namely, 0°, 30°, 45°, and 60°. The temperature along the heat exchanger was measured utilizing 34 thermocouples installed along the heat exchanger. It was found that increasing the inlet flow angle (θ) and/or the Reynolds number results in an increase in the local Nusselt number, the overall heat transfer coefficient, and the ratio of friction factor. It is revealed that the percentage increase in the average Nusselt number due to swirl flow compared to axial flow was 10%, 40%, and 82% for an inlet flow angle of 30°, 45°, and 60°, respectively.     

Investigation on the heat transfer characteristics of propylene glycol–water mixture in the shell side of a spiral‐wound heat exchanger

The heat transfer characteristics of propylene glycol–water (PG–W) mixture (10%, 20%, and 30% propylene glycol) on the shell side of a spiral‐wound heat exchanger (SWHE) were investigated experimentally. Among the SWHE selected, there are 18 twined tubes with a diameter of 8 mm. PG–W mixture is on the shell side and water is on the tube side. The results show that the heat transfer coefficient of PG–W mixture flowing downwards is higher than upwards under countercurrent conditions. The heat transfer coefficient decreases with the increasing of concentration of PG–W mixture. When the inclination angle of the SWHE is 90°, the heat transfer coefficient of PG–W mixture is the largest; and when the inclination angle is less than 90°, the heat transfer coefficient decreases with the decrease of inclination angle. The inclination angle has a great effect on the heat transfer coefficient at a high concentration. The fitting correlation equations between Nu, Re, Pr, and inclination angles of SWHE are established.     

Numerical Analysis of the Effects of Selected Geometrical Parameters and Fluid Properties on MHD Natural Convection Flow in an Inclined Elliptic Porous Enclosure with Localized Heating

Magnetohydrodynamic (MHD) natural convection flow and associated heat convection in an oriented elliptic enclosure has been investigated with numerical simulations. A magnetic field was applied to the cylindrical wall of the configuration, the top and bottom walls of the enclosure were circumferentially cooled and heated, respectively, while the extreme ends along the cross‐section of the elliptic duct were considered adiabatic. The full governing equations in terms of continuity, momentum, and energy transport were transformed into nondimensional form and solved numerically using finite difference method adopting Gauss–Seidel iteration technique. The selected geometrical parameters and flow properties considered for the study were eccentricity (0, 0.2, 0.4, 0.6, and 0.8), angle of inclination (0°, 30°, 60°, and 90°), Hartmann number (0, 25, and 50), Grashof number (10⁴, 10⁵, and 10⁶), and Darcy number (10^?3, 10^?4, and 10^?5). The Prandtl number was held constant at 0.7. Numerical results were presented by velocity distributions as well as heat transfer characteristics in terms of local and average Nusselt numbers (i.e., rate of heat transfer). The optimum heat transfer rate was attained at e value of 0.8. Also, the heat transfer rate increased significantly between the angles of inclination 58° and 90°. In addition, Hartmann number increased with decreased heat transfer rate and flow circulation. A strong flow circulation (in terms of velocity distribution) was observed with increased Grashof and Darcy numbers. The combination of the geometric and fluid properties therefore can be used to regulate the circulation and heat transfer characteristics of the flow in the enclosure.     

Effects of duct inclination angle on thermal entrance region of laminar and transition mixed convection

Experimental investigation was performed on the mixed convection heat transfer of thermal entrance region in an inclined rectangular duct for laminar and transition flow. Air flowed upwardly and downwardly with inclination angles from ?90° to 90°. The duct was made of duralumin plate and heated with uniform heat flux axially. The experiment was designed for determining the effects of inclination angles on the heat transfer coefficients and friction factors at seven orientations (θ = ? 90°, ?60°, ?30°, 0°, 30°, 60° and 90°), six Reynolds numbers (Re ≈ 420, 840, 1290, 1720, 2190 and 2630) within the range of Grashof numbers from 6.8 × 10³ to 4.1 × 10⁴. The optimum inclination angles that yielded the maximum heat transfer coefficients decreased from 30° to ?30° with the increase of Reynolds numbers from 420 to 1720. The heat transfer coefficients first increased with inclination angles up to a maximum value and then decreased. With further increase in Reynolds numbers, the heat transfer coefficients were nearly independent of inclination angles. The friction factors decreased with the increase of inclination angles from ?90° to 90° when Reynolds numbers ranged from 420 to 1290, and independent of inclination angles with higher Reynolds numbers.     

An Experimental Study on the Performance of a Stainless Steel-Water Loop Heat Pipe under Natural Cooling Condition简

Aiming to improve the thermal characteristics of modern electronics, we experimentally study the performance of a stainless steel/water loop heat pipe （LHP） under natural cooling condition. The LHP heat transfer performance, including start-up performance, temperature oscillation and total thermal resistance at different heat loads and with different incline angles have been investigated systematically. Experimental results show that at an optimal heat load （i.e. 60 W） and with the LHP being inclined 60~ to the horizontal plane, the total thermal resistance is lowered to be -0.24 K/W, and the temperature of evaporator could be controlled steadily at around 90~C.     

Investigation of the Influence of Inclination Angle and Diffusion Angle on the Film Cooling Performance of Chevron Shaped Hole

The film cooling performance of chevron holes with different inclination angles and exit lateral diffusion angles has been studied experimentally and numerically. The inclination angles include 35° and 55°. The exit lateral diffusion angles include 20° and 25°. The film cooling effectiveness, heat transfer coefficient and discharge coefficient were measured on a flat plate model by transient liquid crystal measurement technique under four blowing ratios. The results show that the large inclination angle reduces the film cooling effectiveness. The influence of diffusion angle has two aspects: the large diffusion angle leads to mainstream ingestion and decreases film cooling effectiveness at M=1.0 and 1.5; however, the large diffusion angle increases the film cooling effectiveness at high blowing ratio of 2.0, because the larger hole exit area decreases the normal momentum component of the film jet. The large inclination angle decreases the heat transfer coefficient in the right downstream region at M=0.5 and 1.0. The large diffusion angle enhances the heat transfer in the right downstream of the holes in M=0.5～1.5 conditions. The chevron hole with large inclination angle generally has the highest discharge coefficient.     

Analysis of mixed convection in an inclined square cavity using nanofluids with Vajjha and Das' nanofluid model

In this article, the effects of angle of inclination on heat transfer by mixed convection have been analyzed numerically in a square cavity packed with a CuO nanofluid. Cavity boundaries are constructed by having sinusoidal varying temperature on sidewalls, inactive horizontal walls, and the hot passing plate at the center of the cavity. The transport equations for fluid and heat are solved using the finite-volume method with SIMPLE algorithm. The Richardson number (Ri) varying from 0.01 to 100, inclination angle (γ) from 0° to 90°, wall speed ratios (λ) from 0 to 3 and volume fraction of nanoparticles (φ) from 0.0 to 0.1 are given and represented in the form of flow fields, temperature fields, and mean heat transfer graphs. It is detected that the principal flow constraints have a substantial impact on the flow lines and thermal lines. Specifically, the structures of cavity inclination, existence of copper nanoparticles, and the hot wall in motion at the midpoint of the cavity are established to enrich the overall rate of heat transfer. Correspondingly, in the present study, the Vajjha and Das model is taken into account for the effective thermal conductivity and viscosity of the nanofluid; application of this model is beneficial for the industries working in a high-temperature environment.