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Numerical Study of Convective Heat Transfer in Flat Tube Heat Exchangers Operating in Self-Sustained Oscillatory Flow RegimesFullerton, Tracy 2011 December 1900 (has links)
Laminar, two-dimensional, constant-property numerical simulations of flat tube heat exchanger devices operating in flow regimes in which self-sustained oscillations occur were performed. The unsteady flow regimes were transition flow regimes characterized by cyclic variations of flow parameters such as stream-wise or cross-stream velocity.
A computer code was developed to perform the numerical simulations. Spatial discretization was based upon a Control Volume Finite Element Method (CVFEM). Temporal discretization was based upon a semi-implicit Runge-Kutta method. Double Cyclic conditions were used to limit the numerical domains to one repeating geometric module.
Nine geometric domains representing flat tube heat exchanger devices were tested over a range of Reynolds numbers. A maximum Reynolds number (Re) of 2000 was established to keep the study within the transition range. For each domain, a critical Reynolds number (Re_crit) was found such that for Re < Re_crit the flow was steady, laminar flow and for Re > Re_crit the flow exhibited cyclic oscillations. For the cases tested, the variation in longitudinal pitch had little impact on the Re_crit value for a fixed transverse pitch. However, for a fixed longitudinal pitch, the Re_crit was increased for decreasing transverse pitch.
The results demonstrate the importance of using unsteady simulation methods for these cases. Nusselt numbers predicted by the unsteady method were on the order of 65% higher than predicted by steady methods for the same Reynolds numbers.
Data for required pumping power versus resultant Nusselt number were collected which showed four distinct operating regions for these devices spanning the low Reynolds number, steady flow region through the self-sustained oscillating flow region. Based on the data, the recommended operating region is the region of self-sustained oscillations as this region is characterized by the highest increase in Nusselt number per increase in required pumping power.
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Numerical performance evaluation of a delugeable flat bare tube air-cooled steam condenser bundleAngula, Ester 03 1900 (has links)
Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: In this study, one and two-dimensional models are developed for the evaluation of
the thermal performance of a delugeable flat tube bundle to be incorporated in the
second stage of an induced draft hybrid (dry/wet) dephlegmator (HDWD) of a
direct air-cooled steam condenser (ACSC). Both models are presented by a set of
differential equations. The one-dimensional model is analysed analytically by
using three methods of analysis which are: Poppe, Merkel, and heat and mass
transfer analogy. The two-dimensional model is analysed numerically by means
of heat and mass transfer analogy method of analysis whereby, the governing
differential equations are discretised into algebraic equations using linear upwind
differencing scheme. The two-dimensional model’s accuracy is verified through a
comparison of the two dimensional solutions to one dimensional solutions.
Satisfactory correlation between the one and two-dimensional results is reached.
However, there is a slight discrepancy in the solutions, which is mainly due to the
assumptions made in one-dimensional model. The effect of tube height, tube
pitch, tube width, deluge water mass flow rate, frontal air velocity, steam, and air
operating conditions on the heat transfer rate and air-side pressure drop for both
wet and dry operating modes are investigated. The long tube height, large tube
width, small tube pitch, and high frontal air velocity are found to increase the tube
bundle’s performance. However, this performance is associated with a high airside
pressure drop. The performance of the deluged flat tube bundle is found to be
less sensitive to the changes in the deluge water mass flow rate and air operating
conditions. Furthermore, the best configuration of a delugeable flat tube bundle is
identified through a comparison to round tube bundle presented by Anderson
(2014). The performance of the round tube bundle is found to be around 2 times,
and 1.5 times of that of flat tube bundle, when both bundles operate as an
evaporative and dry air-cooled condenser respectively. / AFRIKAANSE OPSOMMING: In hierdie studie is een en twee-dimensionele modelle ontwikkel vir die
evaluering van die termiese prestasie van 'n benatbare plat buis bundel in die
tweede stadium van 'n geïnduseerde ontwerp hibriede (droë / nat ) deflegmator
van 'n direkte lugverkoelde stoom kondensator. Beide modelle is aangebied deur
'n stel van differensiaalvergelykings. Die een-dimensionele model is analities
ontleed deur die gebruik van drie metodes van analise wat: Poppe, Merkel, en die
hitte en massa-oordrag analogie. Die twee-dimensionele model is numeries
ontleed deur middel van hitte en massa-oordrag analogie metode van analise
waardeur , die regerende differensiaalvergelykings gediskretiseer in algebraïese
vergelykings met behulp van lineêre windop differensievorming skema. Die tweedimensionele
model se akkuraatheid is geverifieer deur 'n vergelyking van die
twee dimensionele oplossings te een dimensionele oplossings. Bevredigende
korrelasie tussen die een en twee-dimensionele resultate bereik word. Maar daar is
'n effense verskil in die oplossings, wat is hoofsaaklik te wyte aan die aannames
wat gemaak in een-dimensional model. Die effek van buis hoogte, buis
toonhoogte, buis breedte, vloed water massa-vloeitempo, frontale lug snelheid,
stoom, en in die lug werktoestande op die hitte oordrag snelheid en lug - kant
drukval vir beide nat en droë maatskappy modi word ondersoek. Die lang buis
hoogte, groot buis breedte, klein buisie toonhoogte, en 'n hoë frontale lug snelheid
gevind die buis bundel se prestasie te verhoog. Tog is hierdie prestasie wat
verband hou met 'n hoë lug - kant drukval. Die prestasie van die oorstroom plat
buis bundel gevind word minder sensitief vir die veranderinge in die vloed water
massa-vloeitempo en lug werktoestande. Verder is die beste opset van 'n
benatbare plat buis bundel geïdentifiseer deur 'n vergelyking met ronde buis
bundel aangebied deur Anderson (2014). Die prestasie van die ronde buis bundel
gevind word om 2 keer, en 1.5 keer van daardie plat buis bundel , wanneer beide
bundels funksioneer as 'n damp en droë lugverkoelde kondensor onderskeidelik.
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A numerical study of heat and momentum transfer over a bank of flat tubesBahaidarah, Haitham M. S. 01 November 2005 (has links)
The present study considers steady laminar two-dimensional incompressible flow over both in-line and staggered flat tube bundles used in heat exchanger applications. The effects of various independent parameters, such as Reynolds number (Re), Prandtl number (Pr), length ratio (L/Da), and height ratio (H/Da), on the pressure drop and heat transfer were studied.
A finite volume based FORTRAN code was developed to solve the governing equations. The scalar and velocity variables were stored at staggered grid locations. Scalar variables (pressure and temperature) and all thermophysical properties were stored at the main grid location and velocities were stored at the control volume faces. The solution to a one-dimensional convection diffusion equation was represented by the power law. The locations of grid points were generated by the algebraic grid generation technique. The curvilinear velocity and pressure fields were linked by the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm. The line-by-line method, which is a combination of the Tri-Diagonal Matrix Algorithm (TDMA) and the Gauss-Seidel procedure, was used to solve the resulting set of discretization equations.
The result of the study established that the flow is observed to attain a periodically fully developed profile downstream of the fourth module. The strength increases and the size of the recirculation gets larger as the Reynolds number increases. As the height ratio increases, the strength and size of the recirculation decreases because the flow has enough space to expand through the tube passages. The increase in length ratio does not significantly impact the strength and size of the recirculation.
The non-dimesionalized pressure drop monotonically decreased with an increase in the Reynolds number. In general, the module average Nusselt number increases with an increase in the Reynolds number. The results at Pr = 7.0 indicate a significant increase in the computed module average Nusselt number when compared to those for Pr = 0.7. The overall performance of in-line configuration for lower height ratio (H/Da = 2) and higher length ratio (L/Da = 6) is preferable since it provides higher heat transfer rate for all Reynolds numbers except for the lowest Re value of 25. As expected the staggered configurations perform better than the in-line configuration from the heat transfer point of view.
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Strömungssimulation und experimentelle Untersuchung für innovative Verflüssiger auf Basis neuartiger Rohre / CFD simulations and experimental investigation of an innovative condenser on the basis of novel tubesSchaake, Katrin, Manzke, Sebastian 09 December 2009 (has links) (PDF)
In dieser Arbeit werden neuartige Flachrohre für die Verwendung als Rückwandverflüssiger in der Haushaltskältetechnik mit numerischen und dynamischen Simulationen sowie Experimenten untersucht. Dabei kommen unterschiedliche überströmte Längen sowie der Einfluss horizontaler Abstände auf den Wärmeübergang durch freie Konvektion zur Betrachtung. Realisiert wird die numerische Strömungssimulation mit der Software Fluent 3.6.26, wobei das RNG-k-epsilon- als Turbulenzmodell und diskrete Ordinaten zur zusätzlichen Modellierung des Strahlungswärmeübergangs verwendet werden. Zur Verifizierung werden experimentelle Untersuchungen mit natürlicher Konvektion durchgeführt. Ebenso kommt ein kompakter Verflüssiger bei erzwungener Konvektion zur experimentellen Analyse. Mit einem neuen Verflüssigermodell wird außerdem ein Haushaltskühlschrank in Modelica 2.2.1 dynamisch simuliert. Diese Arbeit zeigt, dass die Verwendung eines Flachrohrverflüssigers großes Potenzial einer konkurrenzfähigen Alternative zu konventionellen Verflüssigern besitzt. / In this work novel flat tubes used as rear panel condensers in the household refrigeration technology are examined with numerical and dynamic simulations as well as experiments. Therefore different overflowed lengths and the influence of horizontal spacing on the heat transfer by free convection are taken into consideration. The CFD calculations are realized with the software Fluent 3.6.26, where the RNG-k-epsilon turbulence model and discrete ordinates for an additional modelling of radiation heat transfer are applied. For the verification, experimental studies with natural convection are carried out. Likewise, a compact condenser is experimentally analysed in forced convection. With a new model for the liquefier a domestic refrigerator is also dynamically simulated in Modelica 2.2.1. This work shows that the use of a flat tube condenser has a great potential of a competitive alternative to conventional liquefiers.
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Strömungssimulation und experimentelle Untersuchung für innovative Verflüssiger auf Basis neuartiger RohreSchaake, Katrin, Manzke, Sebastian 09 December 2009 (has links)
In dieser Arbeit werden neuartige Flachrohre für die Verwendung als Rückwandverflüssiger in der Haushaltskältetechnik mit numerischen und dynamischen Simulationen sowie Experimenten untersucht. Dabei kommen unterschiedliche überströmte Längen sowie der Einfluss horizontaler Abstände auf den Wärmeübergang durch freie Konvektion zur Betrachtung. Realisiert wird die numerische Strömungssimulation mit der Software Fluent 3.6.26, wobei das RNG-k-epsilon- als Turbulenzmodell und diskrete Ordinaten zur zusätzlichen Modellierung des Strahlungswärmeübergangs verwendet werden. Zur Verifizierung werden experimentelle Untersuchungen mit natürlicher Konvektion durchgeführt. Ebenso kommt ein kompakter Verflüssiger bei erzwungener Konvektion zur experimentellen Analyse. Mit einem neuen Verflüssigermodell wird außerdem ein Haushaltskühlschrank in Modelica 2.2.1 dynamisch simuliert. Diese Arbeit zeigt, dass die Verwendung eines Flachrohrverflüssigers großes Potenzial einer konkurrenzfähigen Alternative zu konventionellen Verflüssigern besitzt. / In this work novel flat tubes used as rear panel condensers in the household refrigeration technology are examined with numerical and dynamic simulations as well as experiments. Therefore different overflowed lengths and the influence of horizontal spacing on the heat transfer by free convection are taken into consideration. The CFD calculations are realized with the software Fluent 3.6.26, where the RNG-k-epsilon turbulence model and discrete ordinates for an additional modelling of radiation heat transfer are applied. For the verification, experimental studies with natural convection are carried out. Likewise, a compact condenser is experimentally analysed in forced convection. With a new model for the liquefier a domestic refrigerator is also dynamically simulated in Modelica 2.2.1. This work shows that the use of a flat tube condenser has a great potential of a competitive alternative to conventional liquefiers.
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