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1 
A study of heat transfer from cylinders in turbulent flows by using thermochromic liquid crystalsWiberg, Roland January 2004 (has links)
<p>In gas quenching, metal parts are rapidly cooled from hightemperatures, and the convection heat transfer coefficientdistributions are of importance for the hardness and thedistortion (the shape nonuniformities) of the quenched parts.Thermochromic liquid crystals (TLC) and a thin foil techniques,were investi gated and used for studies of a circular cylinderin axial flows, affected and not affected by upstream owmodifying inserts. Quadratic prisms in cross ows were alsostudied, a single prism, two prisms arranged inline, and forfour prisms arranged in a square pattern. In this study,particle image velocime try (PIV) was used for visualizationof the flow, giving physical insight to the convection heattransfer data. Further, relations of the type<i>Nu</i>=<i>CRe</i><i>e</i>were established. The TLC and thin foil techniques werealso used to indicate the dimensions of separated flowregions.</p><p><b>Descriptors:</b>Fluid mechanics, windtunnel, turbulence,gas quenching, con vection heat transfer, thermochromic liquidcrystals, calibration, temperature measurement errors, thinfoils, particle image velocimetry, cylinder in axial flow, flowmodifying inserts, quadratic prisms in cross flow</p>

2 
Natural convection in a vertical channel related to passive solar systemsAlAzzawi, Abdul Rassol H. January 1987 (has links)
Heat transfer and fluid flow characteristics for natural convection of air in a vertical parallel channel system have been examined. The results of both theoretical and experimental work are reported in this thesis. The system of differential equations governing the fluid flow was solved using the PHOENICS code program. The PHOENICS solution gave velocities, temperatures and pressures throughout the field and velocity and temperature profiles are presented at different vertical locations for three channel heights, 1,2 and 3 m, each for three different channel widths, 50,100 and 150 mm. A further PHOENICS subroutine was written to obtain relevant dimensionless parameters such as the Nusselt Number, which characterises the heat transfer to the air, the Rayleigh Number, the dimensionless air flow rate and dimensionless channel length. The results obtained have been compared with experimental results and with existing data for channels with constant wall temperatures. Nonisothermal wall cases were also considered and the resulting velocity and temperature profiles are presented. The channel used in the experimental work was formed from two vertical plates, one of which was electrically heated, while the other was glass. Heated plates of height 1m and 2m were used and combination of these formed a plate of height 3 m. The width of the plates was 1 m. A double glazed cover plate of the same dimensions could be adjusted to give spacings between the heated plate and glass from 25 mm to 150 mm. The electrically heated plate could be controlled to give the required constant plate temperature (range 35125°C) and heating could be augmented by a solar simulator consisting of 50 Tungsten Halogen Lamps of 150 Watts each. From the experimental results, relationships between the Nusselt number, Rayleigh number, dimensionless air flow rate and dimensionless channel height have been obtained. In addition, the effect of diffuser sections at the channel inlet and outlet and transient operating conditions were investigated experimentally. Effects of atmospheric pressure and humidity were also considered. The experimental results are compared with those from the PHOENICS solution and with existing data for constant wall temperature conditions and they show good agreement. A discussion of the use of the correlating equations for the heat transfer coefficient and air flow rate in the design of passive solar heating systems, such as the Trombe wall, is also included.

3 
A study of heat transfer from cylinders in turbulent flows by using thermochromic liquid crystalsWiberg, Roland January 2004 (has links)
In gas quenching, metal parts are rapidly cooled from hightemperatures, and the convection heat transfer coefficientdistributions are of importance for the hardness and thedistortion (the shape nonuniformities) of the quenched parts.Thermochromic liquid crystals (TLC) and a thin foil techniques,were investi gated and used for studies of a circular cylinderin axial flows, affected and not affected by upstream owmodifying inserts. Quadratic prisms in cross ows were alsostudied, a single prism, two prisms arranged inline, and forfour prisms arranged in a square pattern. In this study,particle image velocime try (PIV) was used for visualizationof the flow, giving physical insight to the convection heattransfer data. Further, relations of the typeNu=CReewere established. The TLC and thin foil techniques werealso used to indicate the dimensions of separated flowregions. Descriptors:Fluid mechanics, windtunnel, turbulence,gas quenching, con vection heat transfer, thermochromic liquidcrystals, calibration, temperature measurement errors, thinfoils, particle image velocimetry, cylinder in axial flow, flowmodifying inserts, quadratic prisms in cross flow

4 
Forced Convective Critical Heat Flux Modeling for Tubes and Rod BundlesDahlquist, Joseph E. 01 January 1983 (has links) (PDF)
This thesis presents a model for predicting the forced convective critical heat flux (CHF) for water over a wide range of thermalhydraulic conditions which might be encountered during normal and accident operations of a light water nuclear reactor. The model is primarily composed from existing steadystate CHF correlations for tubes or tube and rod bundle geometries, and encompasses the following parametric ranges:
0.3 ≤ P (MPa) ≤ 16.0
6.0 ≤ D (mm) ≤ 30.0
100.0 ≤ G (kg/m2s) ≤ 8000.0
0.30 ≤ X ≤ 1.0
The correlations used as the foundation of this model are the
1) Westinghouse3
2) Biasi correlation, and the
3) Modified Barnett correlation
The mode 1 presented is comp a red with available data, and the resultant model is illustrated as a 3D surface in mass flux, quality, and CHF space to represent general CHF behavior.

5 
Characteristics of multimode heat transfer in a differentiallyheated horizontal rectangular ductWangdhamkoom, Panitan January 2007 (has links)
This study presents the numerical analysis of steady laminar flow heat transfer in a horizontal rectangular duct with differential heating on the vertical walls. Three heating configurations: one uniform wall temperature (CS1) and two linearly varying wall temperature cases (CS2 and CS3) are analysed. The study considers the combined effects of natural convection, forced convection and radiation heat transfer on the overall heat transfer characteristics. Air, which is assumed to be a nonparticipating medium, is chosen as the working fluid. A computational fluid dynamics solver is used to solve a set of governing equations for a range of parameters.For chosen duct aspect ratios, the numerical model simulates the flow and heat transfer for two main effects: buoyancy and radiation heat transfer. Buoyancy effect is represented by Grashof number, which is varied from 2,000 to 1,000,000. The effect of radiation heat transfer is examined by choosing different wall surface emissivity values. The weak and strong radiation effect is represented by the emissivity values of 0.05 and 0.85 respectively. Three duct aspect ratios are considered  0.5, 1 and 2. The heat transfer characteristics of all the above heating configurations  CS1, CS2, and CS3 are analysed and compared. The numerical results show that, for all heating configurations and duct aspect ratios, the overall heat transfer rate is enhanced when the buoyancy effect increases. Since buoyancy effect induces natural circulation, this circulation is therefore the main mechanism that enhances heat transfer. Radiation heat transfer is found to significantly influence convection heat transfer in high Grashof numbers.

6 
A supercritical R744 heat transfer simulation implementing various Nusselt number correlations / Philip van Zyl Venter.Venter, Philip van Zyl January 2010 (has links)
During the past decade research has shown that global warming may have disastrous effects on our planet. In order to limit the damage that the human race seems to be causing, it was acknowledged that substances with a high global warming potential (GWP) should be phased out. In due time, R134a with a GWP = 1300, may probably be phased out to make way for nature friendly refrigerants with a lower GWP. One of these contenders is carbon dioxide, R744, with a GWP = 1.
Literature revealed that various Nusselt number (Nu) correlations have been developed to predict the convection heat transfer coefficients of supercritical R744 in cooling. No proof could be found that any of the reported correlations accurately predict Nusselt numbers (Nus) and the subsequent convection heat transfer coefficients of supercritical R744 in cooling.
Although there exist a number of Nu correlations that may be used for R744, eight different correlations were chosen to be compared in a theoretical simulation program forming the first part of this study. A watertotranscritical R744 tubeintube heat exchanger was simulated. Although the results emphasise the importance of finding a more suitable Nu correlation for cooling supercritical R744, no explicit conclusions could be made regarding the accuracy of any of the correlations used in this study.
For the second part of this study experimental data found in literature were used to evaluate the accuracy of the different correlations. Convection heat transfer coefficients, temperatures, pressures and tube diameter were employed for the calculation of experimental Nusselt numbers (Nuexp). The theoretical Nu and Nuexp were then plotted against the length of the heat exchanger for different pressures. It was observed that both Nuexp and Nu increase progressively to a maximal value and then decline as the tube length increases. From these results it were possible to group correlations according to the general patterns of their Nu variation over the tube length.
Graphs of Nuexp against Nus, calculated according to the Gnielinski correlation, generally followed a linear regression, with R2 > 0.9, when the temperature is equal or above the pseudocritical temperature. From this data a new correlation, Correlation I, based on average gradients and intersects, was formulated. Then a modification on the Haaland friction factor was used with the Gnielinski correlation to yield a second correlation, namely Correlation II. A third and more advanced correlation, Correlation III, was then formulated by employing graphs where gradients and yintercepts were plotted against pressure. From this data a new parameter, namely the turning point pressure ratio of cooling supercritical R744, was defined. It was concluded that the employed Nu correlations under predict Nu values (a minimum of 0.3% and a maximum of 81.6%). However, two of the correlations constantly over predicted Nus at greater tube lengths, i.e. below pseudocritical temperatures. It was also concluded that Correlation III proved to be more accurate than both Correlations I and II, as well as the existing correlations found in the literature and employed in this study. Correlation III Nus for cooling supercritical R744 may only be valid for a diameter in the order of the experimental diameter of 7.73 mm, temperatures that are equal or above the pseudocritical temperature and at pressures ranging from 7.5 to 8.8 MPa. / Thesis (M.Ing. (Mechanical Engineering))NorthWest University, Potchefstroom Campus, 2010.

7 
A supercritical R744 heat transfer simulation implementing various Nusselt number correlations / Philip van Zyl Venter.Venter, Philip van Zyl January 2010 (has links)
During the past decade research has shown that global warming may have disastrous effects on our planet. In order to limit the damage that the human race seems to be causing, it was acknowledged that substances with a high global warming potential (GWP) should be phased out. In due time, R134a with a GWP = 1300, may probably be phased out to make way for nature friendly refrigerants with a lower GWP. One of these contenders is carbon dioxide, R744, with a GWP = 1.
Literature revealed that various Nusselt number (Nu) correlations have been developed to predict the convection heat transfer coefficients of supercritical R744 in cooling. No proof could be found that any of the reported correlations accurately predict Nusselt numbers (Nus) and the subsequent convection heat transfer coefficients of supercritical R744 in cooling.
Although there exist a number of Nu correlations that may be used for R744, eight different correlations were chosen to be compared in a theoretical simulation program forming the first part of this study. A watertotranscritical R744 tubeintube heat exchanger was simulated. Although the results emphasise the importance of finding a more suitable Nu correlation for cooling supercritical R744, no explicit conclusions could be made regarding the accuracy of any of the correlations used in this study.
For the second part of this study experimental data found in literature were used to evaluate the accuracy of the different correlations. Convection heat transfer coefficients, temperatures, pressures and tube diameter were employed for the calculation of experimental Nusselt numbers (Nuexp). The theoretical Nu and Nuexp were then plotted against the length of the heat exchanger for different pressures. It was observed that both Nuexp and Nu increase progressively to a maximal value and then decline as the tube length increases. From these results it were possible to group correlations according to the general patterns of their Nu variation over the tube length.
Graphs of Nuexp against Nus, calculated according to the Gnielinski correlation, generally followed a linear regression, with R2 > 0.9, when the temperature is equal or above the pseudocritical temperature. From this data a new correlation, Correlation I, based on average gradients and intersects, was formulated. Then a modification on the Haaland friction factor was used with the Gnielinski correlation to yield a second correlation, namely Correlation II. A third and more advanced correlation, Correlation III, was then formulated by employing graphs where gradients and yintercepts were plotted against pressure. From this data a new parameter, namely the turning point pressure ratio of cooling supercritical R744, was defined. It was concluded that the employed Nu correlations under predict Nu values (a minimum of 0.3% and a maximum of 81.6%). However, two of the correlations constantly over predicted Nus at greater tube lengths, i.e. below pseudocritical temperatures. It was also concluded that Correlation III proved to be more accurate than both Correlations I and II, as well as the existing correlations found in the literature and employed in this study. Correlation III Nus for cooling supercritical R744 may only be valid for a diameter in the order of the experimental diameter of 7.73 mm, temperatures that are equal or above the pseudocritical temperature and at pressures ranging from 7.5 to 8.8 MPa. / Thesis (M.Ing. (Mechanical Engineering))NorthWest University, Potchefstroom Campus, 2010.

8 
Numerical investigations of heat and mass transfer in a saturated porous cavity with Soret and Dufour effectsAlFarhany, Khaled Abdulhussein Jebear January 2012 (has links)
The mass and thermal transport in porous media play an important role in many engineering and geological processes. The hydrodynamic and thermal effects are two interesting aspects arising in the research of porous media. This thesis is concerned with numerical investigations of doublediffusive natural convective heat and mass transfer in saturated porous cavities with Soret and Dufour effects. An inhouse FORTRAN code, named ALFARHANY, was developed for this study. The DarcyBrinkmanForchheimer (generalized) model with the Boussinesq approximation is used to solve the governing equations. In general, for high porosity (more than 0.6), Darcy law is not valid and the effects of inertia and viscosity force should be taken into account. Therefore, the generalized model is extremely suitable in describing all kinds of fluid flow in a porous medium. The numerical model adopted is based on the finite volume approach and the pressure velocity coupling is treated using the SIMPLE/SIMPLER algorithm as well as the alternating direction implicit (ADI) method was employed to solve the energy and species equations. Firstly, the model validation is accomplished through a comparison of the numerical solution with the reliable experimental, analytical/computational studies available in the literature. Additionally, transient conjugate natural convective heat transfer in twodimensional porous square domain with finite wall thickness is investigated numerically. After that the effect of variable thermal conductivity and porosity investigated numerically for steady conjugate doublediffusive natural convective heat and mass transfer in twodimensional variable porosity layer sandwiched between two walls. Then the work is extended to include the geometric effects. The results presented for two different studies (square and rectangular cavities) with the effect of inclination angle. Finally, the work is extended to include the Soret and Dufour effects on doublediffusive natural convection heat and mass transfer in a square porous cavity. In general, the results are presented over wide range of nondimensional parameters including: the modified Rayleigh number (100 ≤ Ra* ≤ 1000), the Darcy number (106 ≤ Da ≤ 102), the Lewis number (0.1 ≤ Le ≤ 20), the buoyancy ratio (5 ≤ N ≤ 5), the thermal conductivity ratio (0.1 ≤ Kr ≤ 10), the ratio of wall thickness to its height (0.1 ≤ D ≤ 0.4), the Soret parameter (5 ≤ Sr ≤ 5), and the Dufour parameter (2 ≤ Df ≤ 2).

9 
Experimental Investigation Of R134a Flow In A 1.65 Mm Copper MinitubeTekin, Bilgehan 01 February 2011 (has links) (PDF)
This thesis investigates the refrigerant (R134a) flow in a minitube experimentally. The small scale heat transfer is a relatively new research area and has been in favor since the end of 1970&rsquo / s. Refrigerant flow in mini and microscale media is a potential enhancement factor for refrigeration technology in the future. For the forthcoming developments and progresses, experimental studies are invaluable in terms of having an insight and contributing to the establishment of infrastructure in the field in addition to leading the numerical and theoretical approaches. The studies in the literature show that low mass flow rate and constant wall temperature approach in minitubes and minichannels were not among the main areas of interest. Therefore, an experimental setup was prepared in order to perform experiments of twophase refrigerant flow in a 1.65 mm diameter copper minitube with the constant wall temperature approach. The design, preparation, and modifications of the experimental setup are explained in this thesis. Twophase flow and quality arrangements were done by preheating the refrigerant at saturation pressure and the constant wall temperature was achieved by a secondary cycle with water and ethylene glycol mixture as the working fluid. The heat transfer coefficient and the pressure drop for the twophase flow with varying quality values and saturation temperatures of the refrigerant were calculated and compared with the results available in literature.

10 
温度分布を規定する強制熱対流場の形状同定片峯, 英次, KATAMINE, Eiji, 織田, 恭平, ODA, Kyohei, 畔上, 秀幸, AZEGAMI, Hideyuki 03 1900 (has links)
No description available.

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