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  3. Numerical investigation of fluid flow and heat transfer on grid plates with cross pores under discontinuous suction

Numerical investigation of fluid flow and heat transfer on grid plates with cross pores under discontinuous suction

Number of pages: 108 File Format: word File Code: 32342
Year: 2012 University Degree: Master's degree Category: Facilities - Mechanics
Tags/Keywords: Cross pores - fluid flow - Grid pages - heat transfer - Solar collectors - under continuous suction
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  • Summary of Numerical investigation of fluid flow and heat transfer on grid plates with cross pores under discontinuous suction

    Master's Thesis in Mechanical Engineering-Energy Conversion

    Abstract

    Numerical investigation of fluid flow and heat transfer on grid plates

    Cross section under discontinuous suction

     

    Introduction

     

     

    1-1- Fluid flow on mesh plates

    rtl;">The urgent need of today's human to use different fuels and the problem of environmental pollution caused by the improper and indiscriminate use of these fuel sources has forced today's generation to think of ways to solve this problem before the end of energy reserves. In this regard, solar energy is prioritized in order to provide part of the energy needed in the future of human societies. One of the most recent applications of this energy is to use it in the air conditioning of large buildings through preheating the air by mesh collectors without glass. Heat transfer from grid plates by means of fluid suction has recently found a new application in industry. The performance of solar collectors without glass that use mesh absorber plates is based on this: air suction causes the boundary layer formed by the wind on the plate to remain in a calm state and the heat transfer from the plate to the air is done in a tight manner, and therefore the efficiency of these collectors is more than ordinary collectors.

    The technology of mesh solar collectors is very simple. A mesh metal wall is placed on the south side and at a distance of 15 cm from the wall of the building. This wall converts the energy from radiation into heat. Fans are installed on top of the wall and suck the outside air in through the holes.

     

     

    Glassless collectors with grid plates (UTC) [1] are a relatively new technology in solar energy applications. These collectors are used in buildings in many countries such as Canada, European countries and America.  In addition to air conditioning, they can also be used to dry vegetables and fruits. It should be noted that the temperature of the hot air produced is dependent on various parameters, and the temperature of the air produced can be controlled by changing these parameters.

    The way of fluid flow and heat transfer on mesh plates is such that when the fluid is sucked through the perforated plate, the degree of porosity of the plate, the geometry of the grooves and the suction power all affect the fluid flow. If one of these two environments (fluid or wall) has a higher temperature, heat transfer occurs in the direction of reducing the temperature difference. Because in reality, wind blows in different directions, grooved plates are more efficient than plates with circular holes, so plates with perpendicular grooves were used in this research. The study of heat transfer and fluid movement related to the boundary layer of a flow parallel to a porous plate has been a topic of interest in the past decades.  In the early 1960s, when keeping the surface temperature constant at a non-harmful and normal level for turbine blades was considered a challenging issue, the investigation of the boundary layer on mesh plates became particularly important and the study and research in this field expanded greatly.  At the same time, the use of suction as an effective factor to control the flow and keep it calm on the wings of high-speed airplanes led to extensive studies [1]. Suction, which was proposed for the first time in the early 20th century by Prantel [1], was considered a very suitable solution to reduce the thickness of the boundary layer, which reduced the tendency of the flow to become confused. In fact, this caused the drag coefficient to remain low, because basically the drag force caused by the smooth flow is less than the drag force caused by the turbulent flow. This control method, in order to keep the flow calm, was first proposed by Griffiths [2] and Meredith [3] [2], and its result can also be seen in Gol Nishan's research [3].

    Controlling the fluid flow to keep it calm and cooling are two engineering applications that are directly related to the present research, so the works that have been done around these two issues will be reviewed.  First, the work done on the control of slow fluid flow and then the cooling will be explained. Then, the work done for uncovered grid solar collectors will be considered. 2-1- Fluid flow control to keep it calm. The results of the research on the control of the calm flow during the years 1940-1960 are well summarized in Lachman's book[4]]1. is  The science and effectiveness of reducing the drag force caused by viscosity in airplanes through laminar flow control has been established by a series of extensive researches by Fenninger and his colleagues that lasted for more than forty years.  This issue is summarized in Fenninger's notes [5] [4].

    The most common form of research on laminar flow control is the laminar boundary layer stability problem [5 and 6]. It has been shown that for the flow on a flat plate with zero collision angle and uniform suction, the critical Reynolds number defined based on the thickness of the displacement boundary layer is more than 130 times greater than the critical Reynolds number corresponding to a similar flat plate without suction [2]. But this issue is true for a highly polished and completely porous plate on which uniform suction is applied.  In practice, it is very difficult to make a porous plate with small and closely spaced holes with sufficient strength.  Therefore, all the plates somehow have separate holes or grooves that lead to discontinuous suction in them. Here, the mesh screen means a screen that has small and very close holes. Although the use of suction for such a grid plate brings the studies very close to ideal continuous suction, it has been shown that in some conditions the suction causes three-dimensional disturbances to the boundary layer [7,8]. New in the field of technology are solar collectors that were introduced in the last decade of the 20th century for the purpose of air conditioning. Mesh collectors are a good alternative to flat collectors with glass cover and have different applications in agriculture and air conditioning industries. At the same time, it is still difficult to design and predict their performance due to the existence of many geometric parameters. For this reason, numerical simulations are currently used to predict their thermal efficiency and thermal efficiency. Numerical simulations can provide detailed information about fluid flow and heat transfer in these types of collectors. In this research, three-dimensional simulation of turbulent flow on mesh plates with cross pores under discontinuous suction is presented. A commercial code in the field of computational fluid dynamics has been used to solve the governing equations, i.e. Navier-Stokes and energy equations. The change of parameters such as plate thickness, porosity, suction speed, wind speed and its direction relative to the plate have been investigated. The results show that the thermal efficiency of the plate and its thermal efficiency decrease when the wind direction changes from perpendicular to the grid plate to parallel. Also, the thermal efficiency of the plate decreases with the increase of parameters such as the hydraulic diameter of the grooves, the Reynolds number related to the groove and the ratio of the suction speed to the wind speed.

  • Contents & References of Numerical investigation of fluid flow and heat transfer on grid plates with cross pores under discontinuous suction

    List:

    Chapter One: Introduction

    1-1- Fluid flow on grid plates. 2

    Chapter Two: A review of past research

    2-1- Controlling the fluid flow to keep it calm. 7

    2-2- Cooling .. 8

    2-3- solar collectors of grid air heater without cover (UTC). 9

    2-4- Fluid movement with continuous uniform suction. 12

    2-5- Fluid movement with discontinuous uniform suction. 13

    2-5-1- Gol Nishan et al.'s research. 13

    2-5-2-Kao's research. 17

    2-5-3- Research by Arulanandam and Van Decker. 20

    2-6- Kotcher's research on UTCs (Heat Loss theory). 20

    2-6-1-Overall heat balance for UTC. 21

    2-6-2- radiation heat loss for the collector. 22

    2-6-3- displacement heat loss. 22

    2-6-4- forced displacement and slow flow. 22

    Title

    Page

    2-6-4-1- Speed ??profile. 22

    2-6-4-2- thickness of boundary layer. 23

    2-6-4-3- temperature profile. 23

    2-6-4-4- displacement heat loss. 24

    2-6-5- Free movement and smooth flow. 24

    2-6-5-1- speed profile. 24

    2-6-6- turbulent flow. 26

    2-6-7- heat exchange efficiency. 26

    2-7- Thermal analysis of Augustus about UTC. 27

    2-7-1- Assumptions. 27

    2-7-2- Energy balance equation. 28

    2-7-2-1- absorbent plate. 29

    2-7-2-2- air distance. 29

    2-7-2-3- back page. 30

    2-7-3- Radiant heat transfer. 30

    2-7-3-1- absorbent plate to the environment. 30

    2-7-3-2- absorbent plate to the back plate. 30

    2-7-3-3- the back plate to the surroundings. 30

    2-7-4- pressure drop. 31

    2-7-5- Augustus simulation results. 31

     

     

     

    Title                                                                                 page

    Chapter 3: Problem definition

    3-1- Introduction.. 33

    3-2- History of Computational Fluid Dynamics (CFD). 34

    3-2-1- Types of networks and CFD solution methods. 34

    3-2-2- Steps to solve computational fluid dynamics problem. 36

    3-3- Introduction of Gambit software. 37

    3-4- Introduction of Fluent software. 37

    5-3- Capabilities of Fluent software. 37

    3-6- Defining the problem. 38

    3-6-1- governing equations in parallel flow on the groove. 38

    3-6-2- Dimensionalization of the governing equations in the parallel flow on the groove. 40

    3-6-3- Boundary conditions in parallel flow on a grooved plate. 42

    3-6-4-Governing equations in vertical and oblique flow on a row of grooves. 44

    3-6-5- Equations related to turbulent flow modeling in vertical and inclined flow

    a row of grooves. 45

    3-6-6- Asymptotic boundary layer. 46

    3-6-7- Parameters related to UTC efficiency in parallel flow. 46

    3-6-8- Range of changes of variables. 48

    3-6-9- Solution theory and selected network in parallel flow on the groove. 48

    3-6-10-Solution theory, selected grid and boundary conditions in vertical and inclined flow

    on a row of grooves. 50

    3-6-10-1- Boundary conditions for the problem in the general state. 51

    3-6-10-2 Geometry production. 53

    3-6-10-3- Network production. 57

    Title

    Chapter IV: Results

    4-1- Results related to parallel flow on a groove. 65

    4-1-1- The effect of Reynolds number. 66

    4-1-2- Effect (wind speed/suction speed) 68

    4-1-3- The effect of the porosity coefficient of the plate. 70

    4-1-4- The effect of dimensionless thermal conductivity (Admittance). 70

    4-1-5- Dimensionless thickness effect. 72

    4-1-6- Effect of wind angle. 73

    4-1-7 - The effect of free movement. 74

    4-2-Perpendicular flow results on a row of grooves. 75

    4-2-1- The effect of Reynolds number. 75

    4-2-2- Effect of the porosity coefficient of the plate. 76

    4-2-3- The effect of dimensionless thermal conductivity. 77

    4-2-4- Dimensionless thickness effect. 78

    4-2-5- The effect of radiation. 79

    4-2-6-Effect of plenum width.80

    4-2-7- The effect of changing the wind angle on mesh plates with limited dimensions. 81

    Chapter Five: Results and Suggestions

    5-1- Results. 84

    5-2- Your suggestion. 85

     

    List of references. 86

     

    Source:

     

    1- Lachmann, G. V. (1961); Boundary Layer and flow control vol.  1&2, Pergamon Press Ltd. , London, England.

    2- Schlichting, H. (1979) "Boundary Layer Theory" Seventh Edition, McGraw-Hill Book, New York.

    3- Golneshan, A. A. (1994) ; Forced convection Heat transfer from low porosity slotted transpired plates, PhD Thesis, University of Waterloo, Waterloo, Ontario, Canada.

    4- Pfenninger, W. (1977); "Laminar flow Control- Laminarization, in Special Course in concept for Drag Reduction", AGARD, Report No.  654.

    5- Braslow, A. L. and Fischer, M. C. (1985); "Design consideration for Application of Laminar flow Control Systems to Transport Aircraft, in Aircraft Drag Prediction and Reduction", AGARD, Report No.  723.

    6- Saric, W. S. (1983); "Laminar Flow Control with Suction: Theory and Experiment", AGARD, Report No.  723.

    7-Arnal, D. (1983); "Description and Prediction of Transition in Two-Dimension Incompressible Flow, in Special Course on Stability and Transition of Laminar Flow", AGARD, Report No.  709.

    8- Braslow, A. L., Maddalon, D. V., Bartlett, D. W., Wagner, R. D. and Collier Jr.  F. S. (1990); "Applied Aspects of Laminar Flow Technology", in Bushnell, D. M. (editor), Viscous Drag Reduction in Boundary Layers, Vol.  123, Progress in Astronautics and Aeronautics, American Ins.  Of Aeronautics and Astronautics Inc. , Washington, D.C., USA.

    9- Wilkinson, S. P. and Ash, R. L. (1980); "Hybrid Suction Surface for Turbulent Boundary Layer Flow", PP.  233-248, in Hough, G. R. (editor), Viscous flow drag reduction, Vol.  72 Progress in Astronautics and Aeronautics, American Ins.  Of Aeronautics and Astronautics, New York, USA. 10- Hartnett, J. P. and Echert, E. R. G. (1957); "Mass transfer cooling in a Laminar Boundary Layer with constant Fluid Properties", Trans.  ASME, vol. 79, pp.  247-254.

    11- Libby, P. A. and Chen, K. (1965); "Laminar boundary layer with uniform injection, Phys.  Fluids", Vol.  8, pp.  568-574.

    12- Moffat, R. J. and Kays, W. M. (1984); A Review of Turbulent - Boundary Layer Research at Stanford, 1958-1983, Advanced in Heat Transfer, vol.  16, Academic Press, Orlando, Florida.

    13- Kutscher, C. F., Christensen, C. B. and Barker, G. M. (1991); "Unglazed Transpired solar collectors: Heat Loss Theory", ASME International Solar Energy Conference Reno, USA.

    14- Wieneke, K. (1981); "Solardach Absorber", Patent No.  29 29 219, Federal Republic of Germany.

    15- Hollick, J. C., and Peter, R. W. (1990); "Method and Apparatus for preheating ventilation Air for a building", Patent No.  9, 934 338, USA.

    16- Golneshan, A. A. (1991); "Forced convection Heat Transfer from Transpired-plate having slotted perforations with application of solar air Heaters", Research proposal, Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada.

    17- Cao, S. (1993): Numerical Investigation on an Unglazed Transpired plate solar collector, MSc Thesis, University of Waterloo, Waterloo, Ontario, Canada.

    18- Kutscher, C. F. (1992); An Investigation of Heat Transfer for Air Flow Through Low Porosity Perforated Plates, PhD Thesis, Department of Mechanical Engineering, University of Colorado, USA.

    19- Kutscher, C. F. (1994); "Heat Exchange Effectiveness and Pressure Drop for Air Flow Through Perforated Plates with and without Crosswind", Transactions of the ASME J. of Heat Transfer, vol. 116, pp. 391-399

    [20]- Shah Hosseini, Ibrahim

    • Numerical investigation of fluid flow and heat transfer on grid plates with cross pores under discontinuous suction
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