Translate the following attributive constructions.

heat transfer surface, bulk fluid motions, bulk fluid flow streaming, average convection heat transfer coefficient, changed density, density change, fluid density, buoyancy forces, free-stream value, artificially induced convection current, boundary layer theory, fluid mass transport.

 

3. You are going to read Text 2A. Before reading try to answer the following questions. Share your answers with your fellow students. 1. Do you know that the word ‘Convection’ is of Latin origin? 2. What does this word mean?3. Why do you think convective heat transfer has grown to the status of a contemporary science? 4. Have you got any idea about advection? 5. Сan you give examples from real life where advection occurs?

Text 2 A

 Convective Heat Transfer

      Convective heat transfer, or simply, convection, is the study of heat transport processes effected by the flow of fluids. The very word convectionhas its roots in the Latin verb convecto-are, which means to bring together or to carry into one place. Convective heat transfer has grown to the status of a contemporary science because of our need to understand and predict how a fluid flow acts as a ‘‘carrier’’ or ‘‘conveyor belt’’ for energy and matter.

 Convective heat transfer, or convection, is the transfer of heat from one place to another by the movement of fluids, a process that is essentially the transfer of heat via mass transfer. Bulk motion of fluid enhances heat transfer in many physical situations, such as, for example, between a solid surface and the fluid. Convection is usually the dominant form of heat transfer in liquids and gases. Although sometimes discussed as a third method of heat transfer, convection is usually used to describe the combined effects of heat conduction within the fluid (diffusion) and heat transference by bulk fluid flow streaming. The process of transport by fluid streaming is known as advection, but pure advection is a term that is generally associated only with mass transport in fluids, such as advection of pebbles in a river. In the case of heat transfer in fluids, where transport by advection in a fluid is always also accompanied by transport via heat diffusion (also known as heat conduction) the process of heat convection is understood to refer to the sum of heat transport by advection and diffusion/conduction.

 Free, or natural, convection occurs when bulk fluid motions (streams and currents) are caused by buoyancy forces that result from density variations due to variations of temperature in the fluid. Forcedconvection is a term used when the streams and currents in the fluid are induced by external means - such as fans, stirrers, and pumps - creating an artificially induced convection current.

 Convective heating or cooling in some circumstances may be described by Newton's law of cooling: "The rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings." However, by definition, the validity of Newton's law of cooling requires that the rate of heat loss from convection be a linear function of ("proportional to") the temperature difference that drives heat transfer, and in convective cooling this is sometimes not the case. In general, convection is not linearly dependent on temperature gradients, and in some cases is strongly nonlinear. In these cases, Newton's law does not apply.

 Convective heat transfer is clearly a field at the interface between two older fields: heat transfer and fluid mechanics. To study the interdisciplinary is valuable, but it must come after one possesses the disciplines, not the other way around. For this reason, the study of any convective heat transfer problem must rest on a solid understanding of basic heat transfer and fluid mechanics principles.

 It is worth reexamining the historic relationship between fluid mechanics and heat transfer. During the past 100 years, heat transfer and fluid mechanics have enjoyed a symbiotic relationship in their development, a relationship where one field was stimulated by the curiosity and advance in the other field. Examples of this symbiosis abound in the history of boundary layer theory and natural convection. The field of convection grew out of this symbiosis, and if we are to learn anything from history, important advances in convection will continue to result from this symbiosis. Thus, the student and the future researcher would be well advised to devote equal attention to fluid mechanics and heat transfer literature.

  The convection mode of heat transfer actually consists of two mechanisms operating simultaneously. The first is the energy transfer due to molecular motion, that is, the conductive mode. Superimposed upon this mode is energy transfer by the macroscopic motion of fluid parcels. The fluid motion is a result of parcels of fluid, each consisting of a large number of molecules, moving by virtue of an external force. This extraneous force may be due to a density gradient, as in natural convection, or due to a pressure difference generated by a pump or a fan, or possibly to a combinationof the two.

 The principal difference is that in forced convection the velocity far from the surface approaches the free-stream value imposed by an external force, whereas in natural convection the velocity at first increases with increasing distance from the heat transfer surface and then decreases. The reason for this behavior is that the action of viscosity diminishes rather rapidly with distance from the surface, while the density difference decreases more slowly. Eventually, however, the buoyant force also decreases as the fluid density approaches the value of the unheated surrounding fluid. This interaction of forces will cause the velocity to reach a maximum and then approach zero far from the heated surface. The temperature fields in natural and forced convection have similar shapes, and in both cases the heat transfer mechanism at the fluid-solid interface is conduction.

  Convection heat transfer depends on the density, viscosity, and velocity of the fluid as well as on its thermal properties (thermal conductivity and specific heat). Whereas in forced convection the velocity is usually imposed on the system by a pump or a fan and can be directly specified, in natural convection the velocity depends on the temperature difference between the surface and the fluid, the coefficient of thermal expansion of the fluid (which determines the density change per unit temperature difference), and the body force field, which in systems located on the earth is simply the gravitational force.

 The evaluation of the convection heat transfer coefficient is difficult because convection is a very complex phenomenon. It is sufficient to note that the numerical value of a system depends on the geometry of the surface, on the velocity as well as the physical properties of the fluid, and often even on the temperature difference. In view of the fact that these quantities are not necessarily constant over a surface, the convection heat transfer coefficient may also vary from point to point. For this reason, we must distinguish between a local and an average convection heat transfer coefficient. For most engineering applications, we are interested in average values.

4. Answer the questions:

1. What is convective heat transfer? 2. What enhances heat transfer in many physical situations? 3. What kind of process is known as advection? 4. When does free, or natural, convection occur? 5. When is the term ‘forced convection’ used? 6. What does Newton's law of cooling state? 7. What must the study of any convective heat transfer problem rest on? 8. What is the principal difference between forced convection and natural convection? 9. What does сonvection heat transfer depend on? 10. Why is the evaluation of the convection heat transfer coefficient difficult? 11. What does the numerical value of a system depend on?