Technology & Engineering
Forced Convection
Forced convection refers to the movement of fluid, such as air or water, through a system due to an external force, such as a fan or pump. This process enhances heat transfer and is commonly used in various engineering applications, such as cooling systems in electronics, HVAC systems, and industrial processes. Forced convection is characterized by a higher heat transfer rate compared to natural convection.
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7 Key excerpts on "Forced Convection"
eBook - PDFHeat Transfer Basics
A Concise Approach to Problem Solving
- Jamil Ghojel(Author)
- 2023(Publication Date)
- Wiley(Publisher)
285 Heat Transfer Basics: A Concise Approach to Problem Solving, First Edition. Jamil Ghojel. © 2024 John Wiley & Sons, Inc. Published 2024 by John Wiley & Sons, Inc. Companion website: www.wiley.com/go/ghojel/heat_transfer 8 Forced Convection – Internal Flows Any heating or cooling process in a flowing fluid inside conduits of various shapes and sizes is known as internal Forced Convection heat transfer or, in short, internal Forced Convection. Examples of this heat transfer mode can be seen in Figure 8.1, which is an expanded reproduction of Figure 1.1 in Chapter 1. Heat is transferred from the combustion products in the cylinder as the piston moves downwards during the expansion (power) stroke, albeit with complex motion of the gases whose pressure and temperature vary within wide ranges. The cooling water in the water jacket surrounding the cylinder is flowing at reasonably high velocity in the annular conduit, picking up most of the heat from the hot cylinder wall and transferring it to the cooling water that is circulated with the aid of a pump (not shown) through a pipe to a heat exchanger, popularly known as a radiator, and back to the engine at a lower temperature. The reduction of the temperature in the radiator is the result of the combined effect of internal Forced Convection in the tube bank and external Forced Convection to the surroundings. 8.1 Forced Convection Inside Tubes Analysis of the process of heat transfer by Forced Convection in tubes is a prerequisite to the design of equipment where forced internal flows are involved. In addition to the cases of forced convec- tion illustrated in Figure 8.1, there are many more applications such as air-conditioning, heating, and refrigeration equipment, and power plants. The effectiveness of the forced convective process is evaluated in terms of the Nusselt number, which depends primarily on the Reynolds number.
eBook - PDFProcess Heat Transfer
Principles, Applications and Rules of Thumb
- Thomas Lestina, Robert W. Serth(Authors)
- 2010(Publication Date)
- Academic Press(Publisher)
2 CONVECTIVE HEAT TRANSFER Contents 2.1 Introduction 44 2.2 Combined Conduction and Convection 44 2.3 Extended Surfaces 47 2.4 Forced Convection in Pipes and Ducts 53 2.5 Forced Convection in External Flow 62 2.6 Free Convection 65 2 / 44 CONVECTIVE HEAT TRANSFER 2.1 Introduction Convective heat transfer occurs when a gas or liquid flows past a solid surface whose temperature is different from that of the fluid. Examples include an organic heat-transfer fluid flowing inside a pipe whose wall is heated by electrical heating tape, and air flowing over the outside of a tube whose wall is chilled by evaporation of a refrigerant inside the tube. Two broad categories of convective heat transfer are distinguished, namely, Forced Convection and natural (or free) convection. In Forced Convection, the fluid motion is caused by an external agent such as a pump or blower. In natural convection, the fluid motion is the result of buoyancy forces created by temperature differences within the fluid. In contrast to conductive heat transfer, convective heat-transfer problems are usually solved by means of empirical correlations derived from experimental data and dimensional analysis. The reason is that in order to solve a convection problem from first principles, one must solve the equations of fluid motion along with the energy balance equation. Although many important results have been obtained by solving the fundamental equations for convection problems in which the flow is laminar, no method has yet been devised to solve the turbulent flow equations entirely from first principles. The empirical correlations are usually expressed in terms of a heat-transfer coefficient, h , which is defined by the relation: q = hA T (2.1) In this equation, q is the rate of heat transfer between the solid surface and the fluid, A is the area over which the heat transfer occurs, and T is a characteristic temperature difference between the solid and the fluid.
eBook - PDF- Albert Ibarz, Gustavo V. Barbosa-Canovas(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
331 17 Heat Transfer by Convection 17.1 INTRODUCTION Convective heat transfer, often referred to simply as convection, is the transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and gases. If a fluid is in contact with a solid that is at a greater temperature, the fluid receives heat that is transferred within it by movement of the fluid particles. This movement causes heat transport by convection, and it can occur by means of natural or forced forms. The first case occurs when there is no mechanical agitation, and it is attributed to density differences at different points of the fluid caused by the effect of temperature. On the contrary, Forced Convection occurs when the movement of the fluid is produced mechanically using devices such as agitators and pumps, among others. Heat transfer by convection is very important when studying the heat exchange between two flu-ids separated by a wall in such a way that one of them gives up heat to the other one, so that the first fluid cools while the second one heats up. The devices in which this heat transmission is performed are called heat exchangers. 17.2 HEAT-TRANSFER COEFFICIENTS 17.2.1 I NDIVIDUAL C OEFFICIENTS If we consider a fluid that circulates through a solid conduit or around a solid surface, the heat transfer from the solid to the fluid (or vice versa) depends on the fluid–solid contact area and on the temperature difference. Thus, for a system such as the one shown in Figure 17.1, in which a solid of differential area dA at a temperature T S is in contact with a fluid at temperature T f , then T S > T f .
eBook - PDFHeat Transfer
Thermal Management of Electronics
- Younes Shabany(Author)
- 2009(Publication Date)
- CRC Press(Publisher)
255 11 Forced Convection Heat Transfer: Internal Flows Forced Convection internal flows have many engineering applications including those in electronic thermal management. Examples are airflow between two adjacent cards of a rack-mount telecommunication or networking equipment, airflow between two closely spaced memory modules, airflow inside a notebook, liquid flow between tightly spaced fins of a cold plate, and water flow inside pipes of a heat exchanger. Convection heat transfer in these flows will be discussed in detail in this chapter. 11.1 MEAN VELOCITY AND MEAN TEMPERATURE External flows are usually characterized by a free-stream velocity and tempera-ture. These are fluid velocity and temperature outside the corresponding velocity and thermal boundary layers where the fluid is not affected by a surface. However, every fluid particle in an internal flow is somehow affected by the bounding surfaces. Therefore, concepts such as free-stream velocity and temperature are not applicable in internal flows. Instead, the mean velocity and the mean temperature are used to characterize internal flows. These will be defined in this section. Consider fluid flow inside a pipe as shown in Figure 11.1a and b. Fluid veloc-ity changes from zero at the pipe surface to a maximum value at the center of the pipe. The infinitesimal mass flow rate through an infinitesimal cross section area dA c where fluid velocity and density are U and ρ is given by ρ UdA c . The total mass flow rate through any cross section of the pipe is obtained by integrating this over the entire cross section of the pipe, A c m UdA c A c = ∫ ρ . (11.1) Now, consider a pipe with a uniform velocity U m , as shown in Figure 11.1c, and the same total mass flow rate m . If fluid is incompressible and therefore its density is constant across the cross section of the pipe, m U A m c = ρ . (11.2) Equating Equations 11.1 and 11.2 for m gives U A UdA m c c A c = ∫ 1 .
- Allan D. Kraus, James R. Welty, Abdul Aziz(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
22 Forced Convection—Internal Flow Chapter Objectives • To evaluate axial temperature distributions for fluids flowing inside closed con-duits with either constant wall temperature or constant wall heat flux. • To generate applicable dimensionless parameters for internal Forced Convection using dimensional analysis. • To catalog the empirical correlations for laminar, transition, and turbulent internal flow. • To use the empirical expressions for h , along with the energy equation, to evaluate the heat transfer behavior of representative systems. 22.1 Introduction Heat transfer by convection occurs through the exchange of thermal energy between a sur-face and an adjacent fluid. Fluid motion is frequently involved in practical energy exchange processes. In this chapter, we will build upon the knowledge acquired in earlier chapters involving fluid mechanics to include the situations in which temperature differences exist between a fluid and the surface it contacts. Initially, we will treat Forced Convection leaving the subject of natural or free condition until Chapter 24. In this chapter, we will address the situation where the fluid, being either heated or cooled, is flowing in a closed conduit such as a pipe, tube, or duct. Fluid flow issues associated with internal flow were discussed in Chapter 16. Many of the concepts and much of the terminology introduced earlier will be used in this treatment without additional discussion. The basic rate equation for convective heat transfer, as introduced earlier, is the Newton rate equation . This relationship is ˙ Q = hS ( T H − T C ) (22.1) where ˙ Q is the rate of heat transfer in watts, S is the surface area, T H and T C are the higher and cooler temperatures, respectively, and h is the convective heat transfer coefficient in W/m 2 -K. Equation 22.1 is, in a practical sense, the definition of h , the heat transfer coefficient.
- W.H. Somerton(Author)
- 1992(Publication Date)
- Elsevier Science(Publisher)
119 Chapter Vlll HEAT TRANSFER WITH FLOWING FLUIDS In previous chapters, the tacit assumption has been made that all fluids present in the rock pore spaces are stationary. This may be true in low permeability reservoir cap-rocks or in quiescent, completely undisturbed subsurface reservoirs. However, whenever production/ injection processes are in operation, fluid motion does occur and this affects the heat transfer character of the rocWfluid system. Several types of fluid motion are possible in porous media. The simplest of these is natural convection in which fluid movement is caused by temperature-gradienvfluid-density differences and the resulting effect of gravity. So-called convection cells may be set up by temperature gradients in which the hotter fluid rises and the cooler fluid moves downwards. Conditions necessary for this type of fluid motion to occur will be discussed in a later section. A second and more complex mechanism causing fluid motion is the condition where fluid phase changes occur. Under appropriate temperature and fluid pressure conditions, liquids may vaporize at the higher temperature zone and be driven in vapor phase to the cooler zone by the action of a vapor pressure gradient. At the location where the temperature drops to the appropriate level for the prevailing fluid pressure, the vapor will condense, giving up its latent heat to the surroundings. The condensed liquid is then caused to move towards the higher temperature zone, which is low in liquid saturation, under a capillary pressure gradient. This vaporization-condensation-capillary (VCC) effect is equivalent to the so-called heat-pipe effect and can have a profound influence on the apparent heat transfer characteristics of the rocWfluid system. The third mechanism causing fluid motion is that due to natural or imposed pressure gradients, the former being moving ground water in a hydrodynamic environment and the latter being fluid motion in productionlinjection operations.
eBook - PDFHeat Transfer
Evolution, Design and Performance
- Adrian Bejan(Author)
- 2022(Publication Date)
- Wiley(Publisher)
Phys. Fluids A 25: 741–742. 14 Prandtl, L. (1969). Essentials of Fluid Dynamics , 117. London: Blackie & Son. 15 Reynolds, O. (1874). On the extent and action of the heating surface for steam boilers. Proc. Manchester Lit. Philos. Soc. 14: 7–12. 16 Colburn, A.P. (1933). A method for correlating Forced Convection heat transfer data and a comparison with fluid friction. Trans. Am. Inst. Chem. Eng. 29: 174–210; reprinted in (1964). Int . J . Heat Mass Transfer 7: 1359–1384. 17 Churchill, S.W. and Bernstein, M. (1977). A correlating equation for Forced Convection from gases and liquids to a circular cylinder in crossflow. J. Heat Transfer 99: 300–306. 18 Nakai, S. and Okazaki, T. (1975). Heat transfer from horizontal circular wire at small Reynolds and Grashof numbers—I Pure convection. Int. J. Heat Mass Transfer 18: 387–396. 19 Bejan, A. (1982). Entropy Generation through Heat and Fluid Flow . New York: Wiley. 20 Simiu, E. and Scanlan, R.H. (1986). Wind Effects on Structures , 2e, 143–152. New York: Wiley. 21 Whitaker, S. (1972). Forced Convection heat transfer correlations for flow in pipes, past flat plates, single cylinders, single spheres, and flow in packed beds and tube bundles. AlChE J. 18: 361–371. 22 Yovanovich, M.M. (1988). General expression for Forced Convection heat and mass transfer from isopo-tential spheroids. Paper No. AIAA 88-0743, AIAA 26th Aerospace Sciences Meeting , Reno, Nevada (11–14 January 1988). 23 Zukauskas, A.A. (1987). Convective heat transfer in cross flow. In: Handbook of Single-Phase Convective Heat Transfer , Chapter 6 (eds. S. Kakac, R.K. Shah and W. Aung). New York: Wiley. 24 Martin, H. (1977). Heat and mass transfer between impinging gas jets and solid surfaces. Adv. Heat Transfer 13. 25 Bejan, A. (2016). The Physics of Life: The Evolution of Everything . New York: St. Martin’s Press. 26 Bejan, A. and Lorente, S. (2008). Design with Constructal Theory . Hoboken, NJ: Wiley.
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