# Chapter 18. Heat Transfer Objectives: After finishing this unit, you should be able to: Heat Transfer by Conduction Heat Transfer by Convection

## Chapter 18. Heat Transfer Objectives: After finishing this unit, you should be able to: Heat Transfer by Conduction Heat Transfer by Convection

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Heat Transfer by Conduction Conduction is the process by which heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move. Heat Transfer by Convection Convection is the process by which heat energy is transferred by the actual mass motion of a heated fluid. Convection Heated fluid rises in addition to is then replaced by cooler fluid, producing convection currents. Convection is significantly affected by geometry of heated surfaces. (wall, ceiling, floor) Heat Transfer by Radiation Radiation is the process by which heat energy is transferred by electromagnetic waves. No medium is required !

Kinds of Heat Transfer Consider the operation of a typical coffee maker: Think about how heat is transferred by: Conduction Convection Radiation Heat Current The heat current H is defined as the quantity of heat Q transferred per unit of time t in the direction from high temperature to low temperature. Typical units are: J/s, cal/s, in addition to Btu/h Thermal Conductivity The thermal conductivity k of a material is a measure of its ability to conduct heat.

The SI Units as long as Conductivity Taken literally, this means that as long as a 1-m length of copper whose cross section is 1 m2 in addition to whose end points differ in temperature by 1 C0, heat will be conducted at the rate of 1 J/s. In SI units, typically small measures as long as length L in addition to area A must be converted to meters in addition to square meters, respectively, be as long as e substitution into as long as mulas. Older Units as long as Conductivity Taken literally, this means that as long as a 1-in. thick plate of glass whose area is 1 ft2 in addition to whose sides differ in temperature by 1 F0, heat will be conducted at the rate of 5.6 Btu/h. Older units, still active, use common measurements as long as area in ft2 time in hours, length in seconds, in addition to quantity of heat in Btus. Glass k = 5.6 Btu in./ft2h F0 Thermal Conductivities Examples of the two systems of units used as long as thermal conductivities of materials are given below:

Examples of Thermal Conductivity Comparison of Heat Currents as long as Similar Conditions: L = 1 cm (0.39 in.); A = 1 m2 (10.8 ft2); Dt = 100 C0 Example 1: A large glass window measures 2 m wide in addition to 6 m high. The inside surface is at 200C in addition to the outside surface is at 120C. How many joules of heat pass through this window in one hour Assume L = 1.5 cm in addition to that k = 0.8 J/s m C0. A = (2 m)(6 m) = 12 m2 Q = 18.4 MJ Example 2: The wall of a freezing plant is composed of 8 cm of corkboard in addition to 12 cm of solid concrete. The inside surface is at -200C in addition to the outside surface is +250C. What is the interface temperature ti Note:

Example 2 (Cont.): Finding the interface temperature as long as a composite wall. Rearranging factors gives: Example 2 (Cont.): Simplifying, we obtain: 0.075ti + 1.50C = 250C – ti From which: ti = 21.90C Knowing the interface temperature ti allows us to determine the rate of heat flow per unit of area, H/A. The quantity H/A is same as long as cork or concrete: Example 2 (Cont.): Constant steady state flow. Over time H/A is constant so different ks cause different Dts Cork: Dt = 21.90C – (-200C) = 41.9 C0 Concrete: Dt = 250C – 21.90C = 3.1 C0 Since H/A is the same, lets just choose concrete alone:

Example 2 (Cont.): Constant steady state flow. Note that 20.7 Joules of heat per second pass through the composite wall. However, the temperature interval between the faces of the cork is 13.5 times as large as as long as the concrete faces. If A = 10 m2, the heat flow in 1 h would be — 745 kW Radiation The rate of radiation R is the energy emitted per unit area per unit time (power per unit area). Rate of Radiation (W/m2): Emissivity, e : 0 > e > 1 Stefan-Boltzman Constant : s = 5.67 x 10-8 W/m·K4 Example 3: A spherical surface 12 cm in radius is heated to 6270C. The emissivity is 0.12. What power is radiated A = 0.181 m2 T = 627 + 273; T = 900 K P = 808 W Power Radiated from Surface:

Summary: Heat Transfer Convection is the process by which heat energy is transferred by the actual mass motion of a heated fluid. Conduction: Heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move. Radiation is the process by which heat energy is transferred by electromagnetic waves. Summary of Thermal Conductivity Summary of Radiation

Summary of Formulas CONCLUSION: Chapter 18 Transfer of Heat

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