16 Chapters
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11

Condensation and Boiling

11.1. Condensation—Filmwise condensation—Dropwise condensation. 11.2. Laminar Film Condensation on a Vertical Plate.

11.3. Condensation on a Single Horizontal Tube—Condensation on horizontal tube banks—Calculation of reynolds number. 11.4. Turbulent

Filmwise Condensation. 11.5. Condensate Number. 11.6. Dropwise Condensation. 11.7. Film Condensation Inside Horizontal Tubes.

11.8. Boiling—Boiling modes. 11.9. Pool Boiling Regimes—Critical heat flux—Leidenfrost point. 11.10. Mechanism of Nucleate Boiling—

Critical diameter of a bubble. 11.11. Pool Boiling Correlations—Correlation for nucleate boiling—Correlation for critical heat flux—Pool film boiling—Minimum heat flux. 11.12. Forced Convection Boiling. 11.13. Summary—Review Questions—Problems—References and

Suggested Reading.

The condensers and boilers are widely used heat transfer equipments in the industries. The condensation and boiling involve convection processes associated with change of phase of fluid. Because there is a phase change during the process, the fluid transfers the latent heat only at its saturation temperature.

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4

Steady State Conduction with Heat Generation

4.1. The Plane Wall—Specified temperatures on both sides—Plane wall without heat generation—Plane wall with insulated and convective boundaries—Plane wall exposed to convection environment on its both boundaries—The maximum temperature in the wall. 4.2. The

Cylinder—Solid cylinder with specified surface temperature—Solid cylinder exposed to convection environment. 4.3. Hollow Cylinder with

Heat Generation and Specified Surface Temperatures—Hollow cylinder insulated at its inner surface—The location of maximum temperature in the cylinder—4.4. The Sphere—Solid sphere with convective boundary—Solid sphere with specified surface temperature—4.5. Summary—

Review Questions—Problems—References and Suggested Reading.

Most of the engineering applications involve heat generation in the solids, such as nuclear reactors, resistance heaters etc. In this chapter, we will consider one dimensional steady state heat conduction with heat generation and determination of temperature distribution and heat flow in solids of simple shapes such as plane wall, a long cylinder and a sphere. Such type of problems cannot be solved with electrical analogy concept presented in previous chapter.

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10

Natural Convection

10.1. Physical Mechanism. 10.2. Definitions—Buoyance force—Volumetric expansion coefficient—Grashof number. 10.3. Natural

Convection Over a Vertical Plate. 10.4. Empirical Correlations for External Free Convection Flow—Vertical plate—Horizontal surfaces

—Inclined plates—Free convection on a long cylinders—Free convection on a spheres. 10.5. Simplified Equations for Air. 10.6. Natural

Convection in Enclosed Spaces. 10.7. Summary—Review Questions—Problems—References and Suggested Reading.

10.1.

PHYSICAL MECHANISM

In natural convection, the fluid motion is due to buoyancy forces within the fluid. The buoyancy forces are developed due to density variation in the fluid caused by temperature difference between the fluid and adjacent surface. The larger the temperature difference in adjacent fluid, the larger the buoyancy force and stronger natural convection currents and higher the heat transfer rate. Whenever a heated object for an example a hot egg, is exposed to atmospheric air, the air adjacent to the hot egg gets heated and becomes lighter (less dense) and thus rises up as shown in Fig. 10.1. This motion leads to the formation of the boundary layer on the surface of the egg and the heat is transferred from the warmer boundary layer to outer atmospheric air by natural convection. The velocity of air is zero at the boundary surface and it is significant outside the boundary layer.

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8

External Flow

8.1. Laminar Flow Over a Flat Plate—Approximate analysis of momentum equation—Approximate analysis of energy equation. 8.2. Reynolds

Colburn Analogy : Momentum and Heat Transfer Analogy for Laminar Flow Over Flat Plate. 8.3. Turbulent Flow Over a Flat Plate.

8.4. Combined Laminar and Turbulent Flow. 8.5. Flow Across Cylinders and Spheres—Drag coefficient—Heat transfer coefficient.

8.6. Summary—Review Questions—Problems—References and Suggested Reading.

When a fluid flows over a body such as plate, cylinder, sphere etc., it is regarded as an external flow. In such a flow, the boundary layer develops freely without any constraints imposed by adjacent surfaces. Accordingly, the region of flow, outside the boundary layer in which the velocity and temperature gradients are negligible is called the free stream region.

In an external flow forced convection, the relative motion between the fluid and the surface is maintained by external means such as a fan or a pump and not by buoyancy forces due to temperature gradients as in natural convection.

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13

Radiation Exchange between Surfaces

13.1. Radiation View Factor—View factor integral—The view factor relations—The cross string method. 13.2. Black body Radiation

Exchange. 13.3. Radiation from Cavities. 13.4. Radiation Heat Exchange between Diffuse, Gray Surfaces—The net radiation exchange by a surface—Radiation exchange between two gray surfaces—Radiation heat exchange between two parallel infinite planes. 13.5. The

Radiation Exchange between Three Surfaces Enclosure. 13.6. Radiation Heat Transfer in Three Surface Enclosure. 13.7. Radiation

Shields. 13.8. Temperature Measurement of a Gas by Thermocouple: Combined Convective and Radiation Heat Transfer

13.9. Summary—Review Questions—Problems—References and Suggested Reading.

In the previous chapter, our discussion was restricted to radiation properties, physical relation, and radiation processes that occur at a single surface. In this chapter, we will consider the radiation heat exchange between two or more surfaces. Such type of radiation exchange depends on the surface geometries, surface orientation as well as their temperatures and radiative properties.

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