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1.
Thermalhydraulic performances of membrane helical-coil heat exchangers in in-line and staggered arrangements, are numerically investigated. The influences of Reynolds number, dimensionless pitch, and arrangement on heat transfer and flow are discussed. Moreover, the axial and tangential velocity distributions were presented for various pitches and arrangements. The results show that the membrane helical-coil arrangement has a profound effect on the thermalhydraulic performance. For the smaller radial pitch (s 1/d < 1.8), the heat transfer coefficient in in-line arrangement is higher than that in staggered arrangement. Meanwhile, the in-line arrangement gives a significantly higher friction factor than the staggered arrangement. However, for the larger radial pitch (s 1/d ≥ 1.8), the coil arrangement appears to have no effect on the heat transfer and friction factor. In addition, the average Nusselt number Nu and the friction factor f for in-line and staggered arrangements were calculated and correlated against the Reynolds number and structural parameters. According to the thermalhydraulic performance evaluation criterion, the staggered arrangement is recommended as the optimal coil arrangement.  相似文献   

2.
ForcedConvectiveHeatTransferinaPlateChannelFilledwithSolidParticlesForcedConvectiveHeatTransferinaPlateChannelFilledwithSolid...  相似文献   

3.

Phosphoric acid is a weak electrolyte with complex physical properties. This complexity combined with its industrial importance has necessitated intensive studies into its heat transfer behavior. In this investigation, pool boiling heat transfer coefficients of phosphoric acid solutions have been measured over a wide range of acid concentrations. The effects of various operating parameters such as heat flux, temperature, and acid concentration have been investigated. Also, the bubble departure diameter and the number of active nucleation sites for phosphoric acid solutions are compared with those for pure water under identical conditions. A model was developed for pool boiling heat transfer of phosphoric acid solutions, which can easily be adapted for other weak electrolyte solutions. In this model, the correct boiling temperature at the vapor/liquid interface is determined rather than applying an arbitrary correction to the boiling heat transfer coefficient. The proposed model is confirmed by comparison between calculated and experimental data.  相似文献   

4.
为了获得翅片间距Pf对平直翅片管换热器的传热与阻力特性的影响规律,根据相似模化原理对3种不同Pf的平直翅片管换热器进行了试验研究。结果表明:雷诺数Rea在4 000~8 000范围内,努赛尔数Nua数随Rea的增大而增加,欧拉数Eua随Rea的增大而降低;同一Rea下,Nua随Pf减小而增加,但增加不明显,Eua随Pf增大而降低;同一Rea下,Pf越大,综合流动传热性能越好,但实际换热面积会减小,需综合考虑。研究成果可为汽轮发电机平直翅片管换热器的结构和性能优化提供依据。  相似文献   

5.
The electrochemical limiting diffusion current technique was used to evaluate free convective mass transfer rates at free-standing circular disk electrodes of varying inclination and diameter. The electrochemical system used was copper deposition from acidified cupric sulphate solution. Experiments were carried with the single faces active and also with both faces simultaneously active. A Toepler-Schlieren system was used to photovisualise the ascending convection streams in order to obtain flow pattern information. For the upward-facing surface, the flow separates at a certain distance from the leading edge of the inclined disk, whereas the flow on the downward-facing surface always stays attached. The data correlations for the doubled disks show an exponent in the Sh-Ra correlation ranging from 0.25 (vertical case), indicating laminar flow, to 0.294 (horizontal case), indicating a mixed effect of turbulent separated flow on the upward-facing side and attached flow on the downward-facing surface. A universal correlation Shd=fA(Rad)fB was obtained for disk diameters d from 5 to 80 mm and inclination angle from 0° to 90° (from the vertical) where fA and fB are functions of inclination.  相似文献   

6.
Three-dimensional conjugate numerical simulations using the inlet, average and variable thermal properties respectively were performed for the laminar water flow and heat transfer in rectangular microchannels with Dh of 0.333 mm at Re of 101–1775. Both average and variable properties are adopted in data reduction. The calculated local and average characteristics of flow and heat transfer are compared among different methods, and with the experiments, correlations and simplified theoretical solution data from published literatures. Compared with the inlet property method, both average and variable property methods have significantly lower fapp, but higher convective heat transfer coefficient hz and Nuz. Compared with the average property method, the variable property method has higher fappReave and lower hz at the beginning, but lower fappReave and higher hz at the later section of the channel. The calculated Nuave agree well with the Sieder-Tate correlation and the recently reported experiment, validating the traditional macroscale theory in predicting the flow and heat transfer characteristics in the dimension and Re range of the present work.  相似文献   

7.
The influences of the performance parameters and the heat transfer characteristics of the absorption heat pump using ammonia–water mixture are theoretically carried out. There is a pronounced effect of the ammonia concentration ξ after rectifier on the temperature glides that has been investigated. At ξ = 0.9000 and saturation pressures of 75 and 0.5 bar, the temperature glides are 64.4°C and 81.21°C, respectively, whereas these glides are 0°C and 16.1°C at ξ = 0.9999 and at the same pressures. This mixture property considerably affects the absorption system performance and the design of the rectifier as well as other absorption components. A correlation of the Nusselt number, Nu, is developed and compared with some published work in the literature for plate type heat exchanger. The effects of ammonia concentration ξ, mass fraction spread Δξ, specific solution circulation ratio f, and pressure ratio Rp on the refrigerant mass flow rate, the pressure drop, and the heat transfer coefficients during the condensation, the evaporation, and the absorption processes are investigated. It was found that increasing ammonia mass fraction spread Δξ results in both specific circulation ratio f and Rp that have insignificant effects on the refrigerant mass flow rate. Mounting Δξ at constant f reduces the pressure drop gradually and subsequently starts to increase as Δξ escalates. The ammonia concentration ξ has insignificant effect on the evaporation heat transfer coefficient but has a little effect on the condensation and the absorber heat transfer coefficients. The ammonia mass fraction spread Δξ and f have considerable effects on the heat transfer coefficient for different absorption heat pump components. Rp has a pronounced effect on the evaporation heat transfer coefficient, although it has a slight effect on the condensation and the absorber heat transfer coefficients. The effect of Rp on the heat transfer coefficient may be eliminated in the absorber for Δξ > 0.18. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

8.
Three-dimensional turbulent flow and heat transfer in an internally finned tube with a blocked core-tube have been numerically studied by the realizable k ? ε turbulence model with the wall-function method. The numerical method is validated by comparing the calculated results with experimental data. The range of ratio of blocked core-tube outside diameter to outer-tube inside diameter (d 0/D i ) is from 0.25 to 0.75. The computational results demonstrated that there exists an optimal ratio of (d 0/D i ) under both identical mass flow rate and identical pressure drop. The optimal ratio of (d 0/D i ), which is reduced with the increase of mass flow rate, is approximately 0.5 to 0.625 at given mass flow rate for both constant wall temperature and uniform wall heat flux. The optimal ratio of (d 0/D i ) at a given pressure drop is from 0.44 to 0.50, which is also slightly reduced with the increase of pressure drop. Furthermore, the optimal ratio of (d 0/D i ) is not sensitive to the number of cross-section wavy fins of an internally longitudinal finned tube, in the range of a fin wave number of 15–25.  相似文献   

9.
The present study deals with the turbine casing radiation effect on the impinging cooling encountered in the blade tip active clearance control (ACC) system of aero-engine turbine. To this end, numerical simulations are carried out for a simplified model, that is, a pipe with a single row of impinging jets. The effects of the nozzle-to-plate distance to the diameter of the impinging hole (H/d?=?2–8), the number of the holes (n?=?17–68), the impinging wall temperature (Tp?=?400–800?K), and the inlet Reynolds number (Re?=?5,000–20,000) on the flow and heat transfer are investigated. Analysis is performed on the radiation heat transfer effects on the corresponding flow fields and surface heat flux distributions. The results indicate that, with the radiation included in the simulations, the mass flow rate of the cooling jet near the end of the pipe decreases significantly under the conditions of high wall temperature and small nozzle-to-plate distance. Radiation heat transfer should be accounted for in the numerical study for the casing cooling as it affects the flow and heat transfer remarkably. When the nozzle-to-plate distance is relatively large, such as H/d is larger than 8, the radiative heating leads to uniform heat flux and the radiative heating can suppress the uneven distributions of the heat flux.  相似文献   

10.
The general objective of this study is to contribute to a better understanding of heat transfer in a nucleate boiling regime. The aim is to determine the heat transfer characteristics under controlled operating conditions (thermodynamics of the fluid, noncondensable gas, surface state). Experimental investigations have been carried out in natural convection and nucleate boiling regimes. An experimental device was realized to perform boiling experiments using a boiling-meter, allowing investigations for different orientations of the wall. The boiling-meter is designed to investigate boiling for single and multiple nucleation sites. The purpose of this paper is to detail the experimental setup as well as the boiling-meter. This device allows the determination of the temporal heat transfer characteristics evolutions. In particular, this new device allows bringing to light the phenomenon of nucleation, growth, and detachment of generated vapor bubbles on a single artificial nucleate site, as well as for multiple natural nucleation sites. First results of the influence of the orientation of the heating wall for multiple and single nucleation sites on heat transfer are presented and analyzed.  相似文献   

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