Heat Transfer

Mechanisms of Heat Transfer

Prepared by: Ms. Ana Antoniette C. Illahi

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Conduction
• conduction (or heat conduction) is the transfer of thermal energy between regions of matter due to a temperature gradient. Heat spontaneously flows from a region of higher temperature to a region of lower temperature, and reduces temperature differences over time, approaching thermal equilibrium.

Prepared by: Ms. Ana Antoniette C. Illahi

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(Heat Current in Conduction)
• • • • • • • • H - Heat Current dQ – Quantity of Heat dt – Time dQ/dt – the rate of heat flows A – Cross sectional area (TH - TC) – Temperature difference L – Length k – constant (thermal conductivity)
Prepared by: Ms. Ana Antoniette C. Illahi

H = dQ/dt = kA (TH - TC)/L

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Conduction
H = dQ/dt = -kA (dT/ dx) H = A(TH - TC) / R R = L/ k R – thermal resistance
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Thermal Conductivities k (W/m oC)
Metals Aluminum Brass Copper Lead Mercury Silver Steel 205.0 109.0 385.0 34.7 8.3 406.0 50.2
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Solids (representative values) Brick. Insulating 0.15 Brick. red 0.6
Concrete 0.8

Cork Felt Fiberglass Glass Ice Rock wool Styrofoam Wood

0.04 0.04 0.04 0.8 1.6 0.04 0.01 0.12-0.04
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Gases Air Argon Helium Hydrogen Oxygen

0.024 0.016 0.14 0.14 0.023

Prepared by: Ms. Ana Antoniette C. Illahi

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Example
1. A Styrofoam box used to keep drinks cold at a picnic has total wall area (including the lid) of 0.80 m2 and wall thickness 2.0 cm. It is filled with ice, water, and cans of Omni-Cola at O°C. What is the rate of beat flow into the box if the temperature of the outside wall is 30oC? How much ice melts in one day?

Prepared by: Ms. Ana Antoniette C. Illahi

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Example
2. A steel bar 10.0 cm long is welded end to end to a copper bar 20.0 cm long. Both bars are insulated perfectly on their sides. Each bar bas a square cross section, 2.00 cm on a side. The free end of the steel bar is maintained at l00oC by placing it in contact with steam, and the free end of the copper bar is maintained at 0oC by placing it in contact with ice. Find the temperature at the junction of the two bars and the total rate of heat flow.
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Convection
• Convection is one of the major modes of heat transfer and mass transfer. Convective heat and mass transfer take place through both diffusion and by advection, in which matter or heat is transported by the largerscale motion of currents in the fluid

Prepared by: Ms. Ana Antoniette C. Illahi

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Radiation
• Radiation describes a process in which energetic particles or waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing. The word radiation is commonly used in reference to ionizing radiation only (i.e., having sufficient energy to ionize an atom), but it may also refer to non-ionizing radiation (e.g., radio waves or visible light). The energy radiates (i.e., travels outward in straight lines in all directions) from its source.
Prepared by: Ms. Ana Antoniette C. Illahi 11

• H = AeT4 (heat current in radiation) •  = 5.670400(40) X 1O-8W/m2 ·oC4

Prepared by: Ms. Ana Antoniette C. Illahi

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Example
1. A thin square steel plate, 10 cm on a side, is heated in a blacksmith's forge to a temperature of 800oC. H the emissivity is 0.60, what is the total rate of radiation of energy? 2. If the total surface area of the human body is 1.20 rrland the surface temperature is 30oC find the total rate of radiation of energy from the body. If the surroundings are at a temperature of 20°C, what is the net rate of heat loss from the body by radiation? The emissivity of the body is very close to unity, irrespective of skin pigmentation.
Prepared by: Ms. Ana Antoniette C. Illahi 13