Convectional Heat Transfer from Heated Wires Sigurds Arajs Iowa State College
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Chapter 6 Natural Convection in a Large Channel with Asymmetric
Chapter 6 Natural Convection in a Large Channel with Asymmetric Radiative Coupled Isothermal Plates Main contents of this chapter have been submitted for publication as: J. Cadafalch, A. Oliva, G. van der Graaf and X. Albets. Natural Convection in a Large Channel with Asymmetric Radiative Coupled Isothermal Plates. Journal of Heat Transfer, July 2002. Abstract. Finite volume numerical computations have been carried out in order to obtain a correlation for the heat transfer in large air channels made up by an isothermal plate and an adiabatic plate, considering radiative heat transfer between the plates and different inclination angles. Numerical results presented are verified by means of a post-processing tool to estimate their uncertainty due to discretization. A final validation process has been done by comparing the numerical data to experimental fluid flow and heat transfer data obtained from an ad-hoc experimental set-up. 111 112 Chapter 6. Natural Convection in a Large Channel... 6.1 Introduction An important effort has already been done by many authors towards to study the nat- ural convection between parallel plates for electronic equipment ventilation purposes. In such situations, since channels are short and the driving temperatures are not high, the flow is usually laminar, and the physical phenomena involved can be studied in detail both by means of experimental and numerical techniques. Therefore, a large experience and much information is available [1][2][3]. In fact, in vertical channels with isothermal or isoflux walls and for laminar flow, the fluid flow and heat trans- fer can be described by simple equations arisen from the analytical solutions of the natural convection boundary layer in isolated vertical plates and the fully developed flow between two vertical plates. -
Heat Transfer in Flat-Plate Solar Air-Heating Collectors
Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X Heat transfer in flat-plate solar air-heating collectors Y. Nassar & E. Sergievsky Abstract All energy systems involve processes of heat transfer. Solar energy is obviously a field where heat transfer plays crucial role. Solar collector represents a heat exchanger, in which receive solar radiation, transform it to heat and transfer this heat to the working fluid in the collector's channel The radiation and convection heat transfer processes inside the collectors depend on the temperatures of the collector components and on the hydrodynamic characteristics of the working fluid. Economically, solar energy systems are at best marginal in most cases. In order to realize the potential of solar energy, a combination of better design and performance and of environmental considerations would be necessary. This paper describes the thermal behavior of several types of flat-plate solar air-heating collectors. 1 Introduction Nowadays the problem of utilization of solar energy is very important. By economic estimations, for the regions of annual incident solar radiation not less than 4300 MJ/m^ pear a year (i.e. lower 60 latitude), it will be possible to cover - by using an effective flat-plate solar collector- up to 25% energy demand in hot water supply systems and up to 75% in space heating systems. Solar collector is the main element of any thermal solar system. Besides of large number of scientific publications, on the problem of solar energy utilization, for today there is no common satisfactory technique to evaluate the thermal behaviors of solar systems, especially the local characteristics of the solar collector. -
Heat Transfer (Heat and Thermodynamics)
Heat Transfer (Heat and Thermodynamics) Wasif Zia, Sohaib Shamim and Muhammad Sabieh Anwar LUMS School of Science and Engineering August 7, 2011 If you put one end of a spoon on the stove and wait for a while, your nger tips start feeling the burn. So how do you explain this simple observation in terms of physics? Heat is generally considered to be thermal energy in transit, owing between two objects that are kept at di erent temperatures. Thermodynamics is mainly con- cerned with objects in a state of equilibrium, while the subject of heat transfer probes matter in a state of disequilibrium. Heat transfer is a beautiful and astound- ingly rich subject. For example, heat transfer is inextricably linked with atomic and molecular vibrations; marrying thermal physics with solid state physics|the study of the structure of matter. We all know that owing matter (such as air) in contact with a heated object can help `carry the heat away'. The motion of the uid, its turbulence, the ow pattern and the shape, size and surface of the object can have a pronounced e ect on how heat is transferred. These heat ow mechanisms are also an essential part of our ventilation and air conditioning mechanisms, adding comfort to our lives. Importantly, without heat exchange in power plants it is impossible to think of any power generation, without heat transfer the internal combustion engine could not drive our automobiles and without it, we would not be able to use our computer for long and do lengthy experiments (like this one!), without overheating and frying our electronics. -
(CFD) Modelling and Application for Sterilization of Foods: a Review
processes Review Computational Fluid Dynamics (CFD) Modelling and Application for Sterilization of Foods: A Review Hyeon Woo Park and Won Byong Yoon * ID Department of Food Science and Biotechnology, College of Agricultural and Life Science, Kangwon National University, Chuncheon 24341, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-33-250-6459 Received: 30 March 2018; Accepted: 23 May 2018; Published: 24 May 2018 Abstract: Computational fluid dynamics (CFD) is a powerful tool to model fluid flow motions for momentum, mass and energy transfer. CFD has been widely used to simulate the flow pattern and temperature distribution during the thermal processing of foods. This paper discusses the background of the thermal processing of food, and the fundamentals in developing CFD models. The constitution of simulation models is provided to enable the design of effective and efficient CFD modeling. An overview of the current CFD modeling studies of thermal processing in solid, liquid, and liquid-solid mixtures is also provided. Some limitations and unrealistic assumptions faced by CFD modelers are also discussed. Keywords: computational fluid dynamics; CFD; thermal processing; sterilization; computer simulation 1. Introduction In the food industry, thermal processing, including sterilization and pasteurization, is defined as the process by which there is the application of heat to a food product in a container, in an effort to guarantee food safety, and extend the shelf-life of processed foods [1]. Thermal processing is the most widely used preservation technology to safely produce long shelf-lives in many kinds of food, such as fruits, vegetables, milk, fish, meat and poultry, which would otherwise be quite perishable. -
Recent Developments in Heat Transfer Fluids Used for Solar
enewa f R bl o e ls E a n t e n r e g Journal of y m a a n d d n u A Srivastva et al., J Fundam Renewable Energy Appl 2015, 5:6 F p f p Fundamentals of Renewable Energy o l i l ISSN: 2090-4541c a a n t r i DOI: 10.4172/2090-4541.1000189 o u n o s J and Applications Review Article Open Access Recent Developments in Heat Transfer Fluids Used for Solar Thermal Energy Applications Umish Srivastva1*, RK Malhotra2 and SC Kaushik3 1Indian Oil Corporation Limited, RandD Centre, Faridabad, Haryana, India 2MREI, Faridabad, Haryana, India 3Indian Institute of Technology Delhi, New Delhi, India Abstract Solar thermal collectors are emerging as a prime mode of harnessing the solar radiations for generation of alternate energy. Heat transfer fluids (HTFs) are employed for transferring and utilizing the solar heat collected via solar thermal energy collectors. Solar thermal collectors are commonly categorized into low temperature collectors, medium temperature collectors and high temperature collectors. Low temperature solar collectors use phase changing refrigerants and water as heat transfer fluids. Degrading water quality in certain geographic locations and high freezing point is hampering its suitability and hence use of water-glycol mixtures as well as water-based nano fluids are gaining momentum in low temperature solar collector applications. Hydrocarbons like propane, pentane and butane are also used as refrigerants in many cases. HTFs used in medium temperature solar collectors include water, water- glycol mixtures – the emerging “green glycol” i.e., trimethylene glycol and also a whole range of naturally occurring hydrocarbon oils in various compositions such as aromatic oils, naphthenic oils and paraffinic oils in their increasing order of operating temperatures. -
Forced and Natural Convection
FORCED AND NATURAL CONVECTION Forced and natural convection ...................................................................................................................... 1 Curved boundary layers, and flow detachment ......................................................................................... 1 Forced flow around bodies .................................................................................................................... 3 Forced flow around a cylinder .............................................................................................................. 3 Forced flow around tube banks ............................................................................................................. 5 Forced flow around a sphere ................................................................................................................. 6 Pipe flow ................................................................................................................................................... 7 Entrance region ..................................................................................................................................... 7 Fully developed laminar flow ............................................................................................................... 8 Fully developed turbulent flow ........................................................................................................... 10 Reynolds analogy and Colburn-Chilton's analogy between friction and heat -
THERMODYNAMICS, HEAT TRANSFER, and FLUID FLOW, Module 3 Fluid Flow Blank Fluid Flow TABLE of CONTENTS
Department of Energy Fundamentals Handbook THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW, Module 3 Fluid Flow blank Fluid Flow TABLE OF CONTENTS TABLE OF CONTENTS LIST OF FIGURES .................................................. iv LIST OF TABLES ................................................... v REFERENCES ..................................................... vi OBJECTIVES ..................................................... vii CONTINUITY EQUATION ............................................ 1 Introduction .................................................. 1 Properties of Fluids ............................................. 2 Buoyancy .................................................... 2 Compressibility ................................................ 3 Relationship Between Depth and Pressure ............................. 3 Pascal’s Law .................................................. 7 Control Volume ............................................... 8 Volumetric Flow Rate ........................................... 9 Mass Flow Rate ............................................... 9 Conservation of Mass ........................................... 10 Steady-State Flow ............................................. 10 Continuity Equation ............................................ 11 Summary ................................................... 16 LAMINAR AND TURBULENT FLOW ................................... 17 Flow Regimes ................................................ 17 Laminar Flow ............................................... -
Forced Convection Heat Transfer Convection Is the Mechanism of Heat Transfer Through a Fluid in the Presence of Bulk Fluid Motion
Forced Convection Heat Transfer Convection is the mechanism of heat transfer through a fluid in the presence of bulk fluid motion. Convection is classified as natural (or free) and forced convection depending on how the fluid motion is initiated. In natural convection, any fluid motion is caused by natural means such as the buoyancy effect, i.e. the rise of warmer fluid and fall the cooler fluid. Whereas in forced convection, the fluid is forced to flow over a surface or in a tube by external means such as a pump or fan. Mechanism of Forced Convection Convection heat transfer is complicated since it involves fluid motion as well as heat conduction. The fluid motion enhances heat transfer (the higher the velocity the higher the heat transfer rate). The rate of convection heat transfer is expressed by Newton’s law of cooling: q hT T W / m 2 conv s Qconv hATs T W The convective heat transfer coefficient h strongly depends on the fluid properties and roughness of the solid surface, and the type of the fluid flow (laminar or turbulent). V∞ V∞ T∞ Zero velocity Qconv at the surface. Qcond Solid hot surface, Ts Fig. 1: Forced convection. It is assumed that the velocity of the fluid is zero at the wall, this assumption is called no‐ slip condition. As a result, the heat transfer from the solid surface to the fluid layer adjacent to the surface is by pure conduction, since the fluid is motionless. Thus, M. Bahrami ENSC 388 (F09) Forced Convection Heat Transfer 1 T T k fluid y qconv qcond k fluid y0 2 y h W / m .K y0 T T s qconv hTs T The convection heat transfer coefficient, in general, varies along the flow direction. -
Low Outgassing of Silicon-Based Coatings on Stainless Steel Surfaces for Vacuum Applications
Low Outgassing of Silicon-Based Coatings on Stainless Steel Surfaces for Vacuum Applications David A. Smith, Martin E. Higgins Restek Corporation, 110 Benner Circle Bellefonte, PA 16823, www.restekcoatings.com Bruce R.F. Kendall Elvac Laboratories 100 Rolling Ridge Drive, Bellefonte, PA 16823 Performance Coatings This presentation was developed through the collaborative efforts of Restek Performance Coatings and Bruce Kendall of Elvac Labs. Restek applied the coatings to vacuum components and Dr. Kendall developed, performed and interpreted the experiments to evaluate the coating performance. lit. cat.# RPC-uhv3 1 Objective Evaluate comparative outgassing properties of vacuum components with and without amorphous silicon coatings Performance Coatings lit. cat.# RPC-uhv3 2 Theoretical Basis – Heat Induced Outgassing • Outgassing rate (F) in monolayers per sec: F = [exp (-E/RT)] / t’ t’ = period of oscillation of molecule perp. to surface, ca. 10-13 sec E = energy of desorption (Kcal/g mol) R = gas constant source: Roth, A. Vacuum Technology, Elsevier Science Publishers, Amsterdam, 2nd ed., p. 177. • Slight elevation of sample temperature accelerates outgassing rate exponentially Performance Coatings The design of the first set of experiments allowed the isolation and direct comparison of outgassing rates with increasing temperature. By applying heat, the outgassing rates are exponentially increased for the purpose of timely data collection. These comparisons with experimental controls will directly illustrate the differences incurred by the applied coatings. lit. cat.# RPC-uhv3 3 Experimental Design – Heated Samples • Turbo pump for base pressures to 10-8 Torr – pumping rate between gauge and pump: 12.5 l/sec (pump alone: 360 l/sec) – system vent with dry N2 between thermal cycles • Comparative evaluation parts – a-silicon coated via CVD (Silcosteel®-UHV); 3D deposition – equally treated controls without deposition Performance Coatings The blue sample (left) was a standard coating commercially available through Restek. -
Connected in Various Ways and Pressure Environment
https://ntrs.nasa.gov/search.jsp?R=19700015462 2020-03-23T19:09:28+00:00Z X-327-69-524 PREPRI NT INTERNAL PRESSURES OF A SPACECRAFT OR OTHER SYSTEM OF COMPARTMENTS, CONNECTED IN VARIOUS WAYS AND INCLUDING OUTGASSING MATERIALS, IN A TIME-VARYING 1 PRESSURE ENVIRONMENT J. I. SCIALDONE / INTERNAL PRESSURES OF A SPACECRAFT OR OTHER SYSTEM OF COMPARmMENTS, CONNECTED IN VARIOUS WAYS &W INCLUDING OUTGASSING MATERIALS, IN A TLME-VARYING PRESSURE ENVIRONMENT John 3. Scialdone Test and Evaluation Division Systems Reliability Directorate August 1969 Goddard Space Flight t errter Greenbelt, Mary1ar.d. INTERNAL PRESSURES OF A SPACECMFT OR QTHER SYSTEM OF COMPARTMENTS, CONNECTED IN VARIOUS WAYS AND INCLUDING OUTGASSING MATERIALS, IN A TIME-VARYING PRESSURE E1'IRONMENT Prepared by: fohn Vcialdone Research and Technology Office Approved by hn H. Boeckel eputy Chief, Test and Evaluation Division PROJECT STATUS This report describes a method for calculating the pressure versus time inside each of a system of volumes containing outgassing materials. Authorization Test and Evaluation Charge No. 327-124-12-06-01 iii PRECEDlNG PAGE BLANK NOT FILMED. INTERNAL PRESSURES OF A SPACECRAFT OR OTFIER SYSTEM OF CBMPAR?%lEN'I'S, CONNECTED IN VARIOUS WAYS AND INCLUDING OUTGASSING MATERIAIS, IN A TIME-VARYING PRESSURE ENVIRONMENT John J. Scidd~ne Test and Evaluatim Division SUMMARY A method od calculating the pressure profile for eac:h com- partment of a multi-compartment system such as a spacecraft is presented. Thc method is applicable to volumes connected in series or in parallel or bothby passages of various sizes and geometries, where each volume may include several materials outgassing ac- cox~dingto known or assumed characteristics. -
Thermodynamics Vs. Heat Transfer
Thermodynamics vs. Heat Transfer Thermodynamics is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another, Thermodynamics gives no indication about how long the process takes, Heat Transfer determines how fast heat can be transferred to or from a system and thus the times of cooling or heating. Spring 2003 ECE309: Chapter 8 1 Heat Transfer Mechanisms Conduction: ∆T dT Q& = kA or Q& = −kA (W) cond ∆x cond dx Convection: Q&convvection = hA (Ts − T∞ ) (W) Radiation: 4 4 Q& rad = εσA(Ts −Tsurr ) (W) Spring 2003 ECE309: Chapter 8 2 1 Example Determine the rate of heat transfer between the plates per unit surface area assuming the gap between the plates is 1. filled with atmospheric air, 2. evacuated, 3. filled with urethane insulation, and 4. filled with superinsulation with k = 0.00002 W/m.°C. Spring 2003 ECE309: Chapter 8 3 Steady Heat Conduction in Plane Walls Energy Balance: rate of rate of rate of change of the heat transfer − heat transfer = energy content into the wall out of the wall of the wall dE Q& −Q& = wall in out dt dT Q& = −kA (W) cond ,wall dx T − T Q& = kA 1 2 (W) cond,wall L Spring 2003 ECE309: Chapter 8 4 2 Thermal Resistance T T T − T s − ∞ 1 2 Q&conv = (W) Q&cond ,wall = (W) Rconv Rwall L 1 R = (°C/W) R = (°C/W) wall kA conv hA 4 4 Ts −Tsurr Q& rad = εσA(Ts −Tsurr ) = hrad A(Ts −Tsurr) = (W) Rrad 1 Rrad = (K/W) hrad A 2 2 2 hrad = εσA(Ts −Tsurr )(Ts −Tsurr ) (W/(m .K)) Spring 2003 ECE309: Chapter 8 5 Thermal Resistance Network T − T Q& = ∞1 ∞2 (W) Rtotal 1 L 1 Rtotal = Rconv,1 + Rwall + Rconv,2 = + + (°C/W) h1A kA h2 A Spring 2003 ECE309: Chapter 8 6 3 Mutilayer Plane Wall T − T Q& = ∞1 ∞2 Rtotal Rtotal = Rconv,1 + Rwall,1 + Rwall,2 + Rconv ,2 1 L L 1 = + 1 + 2 + h1 A k1 A k1 A h2 A Spring 2003 ECE309: Chapter 8 7 Thermal Contact Resistance Perfect Thermal Contact Actual Thermal Contact The thermal resistance due to the roughness at the contact areas is called thermal contact resistance. -
Chaotic Flow in a 2D Natural Convection Loop
International Journal of Heat and Mass Transfer 61 (2013) 565–576 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt Chaotic flow in a 2D natural convection loop with heat flux boundaries ⇑ William F. Louisos a,b, , Darren L. Hitt a,b, Christopher M. Danforth a,c a College of Engineering & Mathematical Sciences, The University of Vermont, Votey Building, 33 Colchester Avenue, Burlington, VT 05405, United States b Mechanical Engineering Program, School of Engineering, The University of Vermont, Votey Building, 33 Colchester Avenue, Burlington, VT 05405, United States c Department of Mathematics & Statistics, Vermont Complex Systems Center, Vermont Advanced Computing Core, The University of Vermont, Farrell Hall, 210 Colchester Avenue, Burlington, VT 05405, United States article info abstract Article history: This computational study investigates the nonlinear dynamics of unstable convection in a 2D thermal Received 13 August 2012 convection loop (i.e., thermosyphon) with heat flux boundary conditions. The lower half of the thermosy- Received in revised form 1 February 2013 phon is subjected to a positive heat flux into the system while the upper half is cooled by an equal-but- Accepted 3 February 2013 opposite heat flux out of the system. Water is employed as the working fluid with fully temperature dependent thermophysical properties and the system of governing equations is solved using a finite vol- ume method. Numerical simulations are performed for varying levels of heat flux and varying strengths Keywords: of gravity to yield Rayleigh numbers ranging from 1.5 Â 102 to 2.8 Â 107.