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Experimental Investigation of Micro Heat Pipes of Different Cross-Sections Having Same hydraulic diameter

Journal of Thermal Science 2008年第3期17卷 247-252页

作者： S.L.Mahmood M.A.R.Akhanda Department of Mechanical and Chemical Engineering Islamic University of Technology

Effects of micro heat pipe （MHP） cross-sections and orientations on its thermal performance are experimentally investigated in this study. Tests are conducted using five different cross-sections （circular, semicircular, elliptical, semi-elliptical and rectangular） of micro heat pipes having same hydraulic diameter of 3 rnm placed at three different inclination angles （0°, 45°, 90°）, where water is used as the working fluid. Evaporator section of the MHP is heated by an electric heater and the condenser section is cooled by circulation of water in an annular space between condenser section and the water jacket. Temperatures at different locations of the MHP are measured using five calibrated K type thermocouples. Heat supply is varied using a voltage regulator which is measured by a precision ammeter and a voltmeter. It is found that thermal performance tends to deteriorate as the MHP is flattened. Thus among all cross-sections of MHP, circular one exhibits the best thermal performance in terms of heat flux dissipation followed by semi-elliptical, semi-circular, elliptical and rectangular cross-sections. Moreover, its heat transfer capability also decreases with decreasing of its inclination angle. Finally, a correlation is developed which covers all the experimental data within ＋7%.

关键词： Micro heat pipe cross-section hydraulic diameter inclination.

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The Solely Use of the “hydraulic-diameter” Concept Is Not Sufficient to Describe Correctly Non-Symmetric Flow in Conduits

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Open Journal of Fluid Dynamics 2021年第4期11卷 210-250页

作者： Christos Krimizis-Tsatsoulis Alchimica S.A. R&D department Athens Greece

A well-known cornerstone in fluid mechanics is the equations that relate the friction factor to the Reynolds number obtained from the measurements in cylindrical cross-sectional tubes. The extension of these equations to different geometries failed to give reliable results. The introduction of the hydraulic diameter has fixed this issue particularly for the square ducts. However, for non-symmetric flows, as in concentric annuli, the discrepancies were unacceptable. Several attempts have been made to fix these problems with finally the introduction of a new concept like, “Laminar Equivalent hydraulic diameter” or “Efficient hydraulic diameter” provided satisfactory results. This approach seems to have fixed the problem and hence has been widely accepted. Nevertheless, it is based on a non-robust theoretical argument. In the present paper, it has been demonstrated that the solely use of the “hydraulic diameter” concept is insufficient to describe non-symmetric flows as in concentric annuli. It appears the need to use the Z axis component of the skew driving force for the laminar flow and the parameter λ for the turbulent one. At the same time, instead, it has been shown that in the case of flow in square and rectangular ducts, the “hydraulic diameter” is sufficient to describe it. In this case, the flow is practically symmetric. Moreover, several new straightforward equations are provided, which simplify a lot dealing with non-cylindrical cross-sectional conduits. In doing so, the concept of “Eigenvectors-Eigenvalues” has been implemented. This theoretical approach could help to simplify other non-symmetric cases in fluid dynamics. To mention, “Flow past immersed non-symmetric bodies”, “Flow in curved conduits” etc.

关键词： Fluid Mechanics hydraulic diameter Flow in Annuli Flow in Ducts Flow of Two Adjacent Immiscible Fluids Eigenvectors-Eigenvalues

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Geometric effects of fuel regression rate in hybrid rocket motors

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Science China(Technological Sciences) 2016年第5期59卷 807-813页

作者： CAI GuoBiao ZHANG YuanJun WANG PengFei HUI Tian ZHAO Sheng YU NanJia School of Astronautics Beihang University Beijing 100191 China Beijing Institute of Aerospace Systems Engineering Beijing 100076 China

The geometric configuration of the solid fuel is a key parameter affecting the fuel regression rate in hybrid rocket motors. In this paper, a semi-empirical regression rate model is developed to investigate the geometric effect on the fuel regression rate by incorporating the hydraulic diameter into the classical model. The semi-empirical model indicates that the fuel regression rate decreases with increasing hydraulic diameter and is proportional to dh?0.2 when convective heat transfer is dominant. Then a numerical model considering turbulence, combustion, solid fuel pyrolysis, and a solid–gas coupling model is established to further investigate the geometric effect. Eight motors with different solid fuel grains are simulated, and four methods of scaling the regression rate between different solid fuel grains are compared. The results indicate that the solid fuel regression rates are approximate the same when the hydraulic diameters are equal. The numerical results verify the accuracy of the semi-empirical model.

关键词： geometric effect regression rate hydraulic diameter hybrid rocket motor

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Effect of nozzle geometry on pressure drop in submerged gas injection

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Journal of Central South University 2019年第8期26卷 2068-2076页

作者： XIAO Jun-bing YAN Hong-jie LIU Liu School of Energy Science and Engineering Central South University Changsha 410083 China Institute of Fluid Dynamics Helmholtz-Zentrum Dresden-Rossendorf Dresden 01328 Germany Chair of Imaging Techniques in Energy and Process Engineering Technische Universitat Dresden 01062 Germany

Submerged gas injection into liquid leads to complex multiphase flow, in which nozzle geometries are crucial important for the operational expenditure in terms of pressure drop. The influence of the nozzle geometry on pressure drop between nozzle inlet and outlet has been experimentally studied for different gas flow rates and bath depths. Nozzles with circular, gear-like and four-leaf cross-sectional shape have been studied. The results indicate that, besides the hydraulic diameter of the outlet, the orifice area and the perimeter of the nozzle tip also play significant roles. For the same superficial gas velocity, the average pressure drop from the four-leaf-shaped geometry is the least. The influence of bath depth was found negligible. A correlation for the modified Euler number considering the pressure drop is proposed depending on nozzle geometric parameter and on the modified Froude number with the hydraulic diameter of the nozzle do as characteristic length.

关键词： submerged gas injection nozzle geometry hydraulic diameter pressure drop modified Euler number

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Limitations in the Use of the Equivalent diameter

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World Journal of Nuclear Science and Technology 2016年第1期6卷 53-62页

作者： Licia Del Frate Fabio Moretti Giorgio Galassi Francesco D’Auria GRNSPG (San Piero a Grado Nuclear Research Group) University of Pisa Pisa Italy

This paper deals with the inaccuracy assessment of the friction pressure loss estimation based on Darcy formula combined with an equivalent hydraulic diameter and a friction factor valid for circular pipes when applied to a square rod bundle. The assessment has been done by comparing the analytical and semi-empirical predictions with two different CFD codes results: CFX and NEPTUNE_CFD. Two different analytical approaches have been considered: the whole-bundle and sub-channel approaches, both for laminar and turbulent flow conditions. Looking at results, it is reasonable to assume that an error in the range of 11% - 23% is likely when using equivalent diameter in the laminar regime. In the case of turbulent regime, the equivalent diameter works better and the error is in the range between a few percent and ~12%.

关键词： hydraulic diameter Pressure Losses Accuracy

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